A BSRIA Guide
www.bsria.co.uk
The Illustrated Guide to
Mechanical Cooling
By Kevin Pennycook
BG 1/2010
ACKNOWLEDGEMENTS
The guide has been compiled by BSRIA’s Kevin Pennycook with
additions from Roderic Bunn, designed by Ruth Radburn and
produced by Alex Goddard.
BSRIA would like to thank the following organisations who
kindly provided photographs, diagrams and information:
TROX UK Ltd
Clivet Air Conditioning Ltd
Toshiba Air Conditioning
Mitsubishi Electric
JS Humidifiers plc
Voyant Solutions
Dravo Environmental Services
Max Fordham
We would also like to thank the reviewers of the document:
Les Smith, Cudd Bentley
Nick Cullen, Hoare Lea
Richard Tudor, WSP
Their input has been invaluable but the responsibility of the final
document remains entirely that of BSRIA.
This publication has been printed on Nine Lives Silk recycled paper, which is
manufactured from 100% recycled fibre.
All rights reserved. No part of this publication may be reproduced, stored in a
retrieval system, or transmitted in any form or by any means electronic or
mechanical including photocopying, recording or otherwise without prior written
permission of the publisher.
©BSRIA 2010
May 2010
ISBN 978 0 86022 675 8
Printed by ImageData Ltd.
ILLUSTRATED GUIDE TO MECHANICAL COOLING
© BSRIA BG 1/2010
1
INTRODUCTION
BG 1/2010 The Illustrated Guide to Mechanical Cooling starts with a
general overview of the various cooling systems and their purpose
in maintaining comfortable conditions in buildings. It then
describes the main refrigeration systems and their application
principles, the types of refrigerants available, and the various ways
in which renewable forms of cooling energy can be used. The
guide goes on to explain the various ways in which cooling can be
delivered to an occupied space.
The use of buildings is intensifying. More people are using more
IT equipment and the internal heat loads are growing. In addition,
expectations are increasing with almost every new car being sold
with air conditioning. And climate change is resulting in more
extremes of weather. Hardly surprising that the demand for
cooling our buildings is also growing.
As concerns over our impact on the environment escalate, we
need to maximise every opportunity to reduce cooling loads
before we consider how to remove the remaining unwanted heat.
Traditionally we have used refrigeration based cooling but for the
lay person, what is it?
Essentially, it’s where the water in hydraulic circuits or the air in
ventilation systems is cooled by some form of powered
refrigeration cycle. It can either be gas-powered or electricallypowered, and some or all of the cooling work can be done by
recourse to natural resources, such as the use of ground water. At
the more complex end, equipment known as absorption chillers
can utilise hot water to create cold water. The absorption cycle
enables waste heat from combined heat and power machines or
any other source of high grade waste heat such as exhaust steam
from a laundry to be used to produce cooling. But all that, of
course, just begs another question: “what is combined heat and
power”?
Non-technical people struggle with these concepts on a regular
basis. Even technical people can have difficulty with explaining
how systems work – the absorption refrigeration cycle being a
classic example. This is why BSRIA has created a series of
illustrated guides that explain and demystify complex
environmental engineering systems. The various technologies are
described in straightforward language that non-technical people
can understand. Simple illustrations also provide a deeper insight
to the workings of often arcane concepts. It’s important to appreciate that cooling systems can be both
augmented and/or boosted by passive design measures, such as
thermally heavyweight and well-insulated building structures.
Some systems, such as ground-coupling, can provide what is
known as free cooling. This can significantly reduce or even
eliminate the electrical energy required to cool air or water.
2
ILLUSTRATED GUIDE TO MECHANICAL COOLING
© BSRIA BG 1/2010
It is vital for everyone involved in considering a cooling system –
clients and designers alike - to ensure that a building’s cooling
loads are reduced as far as is practicable. Whatever equipment is
installed must be as efficient as possible to reduce waste. Those
two principles are inviolate – they’re not negotiable. Clients need
to accept them, and designers need to uphold them. Once cooling
loads have been driven down, and the equipment efficiencies
driven up, sources of on site or off site renewable energy can be
used to offset the remainder. This sequence is important – just
because renewable energy is clean (and often free), doesn’t mean it
is acceptable to waste it. In fact, wasting renewable energy is
arguably a greater crime than wasting fossil fuel energy, as there is
so little of it to go round. Twenty five years ago, cooling systems tended to rely on simple
mechanical refrigeration based on chlorofluorocarbons (CFCs).
Today, cooling involves far greater complexity, and often requires
more than one system. Commissioning, controlling and
maintaining these systems places a greater burden on both the
construction team and the client’s premises management team.
This publication therefore provides some key commissioning and
maintenance guidance, along with key design checks for each
technology described.
There is much more that can be said, but for more detailed
guidance on commissioning and operation, readers are urged to
consult other BSRIA guides that go into these topics in far greater
detail. A list of these guides is provided in the appendix.
This guide is chiefly but not exclusively concerned with central
systems. It covers all of the most popular types of central
mechanical cooling systems and other important types such as
absorption cooling, even though the purists might argue that this
is not mechanical cooling.
Whatever cooling system is being considered, clients and designers
are urged to keep things simple, install it well, plan for
commissioning well in advance, and fine-tune it during the initial
period of operation. It must be easy to maintain, and
straightforward to control. A provision in the budget for seasonal
commissioning may also show dividends. Occupants of buildings
like stable conditions - they don’t like disruption, and they don’t
like unreliable or unmanageably complex control. And of course
comfortable people are productive people. When selecting a
cooling system, that’s a good place from which to start.
ILLUSTRATED GUIDE TO MECHANICAL COOLING
© BSRIA BG 1/2010
3
4
ILLUSTRATED GUIDE TO MECHANICAL COOLING
© BSRIA BG 1/2010
CONTENTS
Page
INTRODUCTION
2
ALPHABETICAL LIST
6
OVERVIEW OF COOLING SYSTEMS
7
CENTRAL SYSTEMS
12
RENEWABLE COOLING
TECHNOLOGIES
34
CENTRALISED AIR SYSTEMS
43
LOCAL SYSTEMS
51
COMMISSIONING
57
MAINTENANCE AND UPKEEP
58
STANDARDS AND REQUIREMENTS
60
REFERENCES AND BIBLIOGRAPHY
62
GLOSSARY OF TERMS
64
ILLUSTRATED GUIDE TO MECHANICAL COOLING
© BSRIA BG 1/2010
5
ALPHABETICAL LIST
Absorption chillers
15, 39
59
10
Absorption chillers and CHP systems
17
Lighting
Adiabatic processes
19
Local systems
Air movement
45
Open loop systems
Air-side fan coil units
47
Partially centralised air/water system
Centralised air systems
8, 43
Centralised plant
8
Centrifugal compressors
14
Plate heat-exchanger
8, 51
36, 42
8
21, 36
Reciprocating compressors
13
Recirculation air systems
27
Chilled beams
48, 55
Refrigerants
31
Chilled ceilings
50, 56
Room-based heat pumps
51
Chillers
12, 39
Run-around coil system
23
Closed loop systems
37, 41
Screw compressors
13
Constant volume systems
43
Scroll compressors
14
Control of ventilation rates
10
Simultaneous air and water free-cooling
29
Cooling towers
28, 40
Solar shading
9
Desiccant cooling systems
29
Split systems
52
Diffusers
45
Steam humidifiers
19
Direct heat recovery
26
Surface water cooling
DX systems
12
Thermal storage
11
Fan-assisted VAV
44
Thermal wheel
22
38, 42
Fan coils
46, 55
Thermolabyrinths
35
F-gas Regulation
33, 58
Thermosyphon systems
29
Full fresh-air systems
27
Vapour generators
19
Gas turbine combined heat and power (CHP)
17
Variable air volume
44, 54
Ground water cooling
6
Legionnaires’ disease
36, 41-42
Variable geometry supply diffusers
45
Heat pipes
24
Variable refrigerant flow systems
Heat recovery
21
Variable speed pumping
30
Heat rejection techniques
18
Water atomising
20
Humidifiers
19
Water spray
19
Improved airtightness
11
Water-side fan coil units
47
Innovative and ground-coupled cooling
34
Wetted media
20
Isothermal processes
19
Zoning and space use
10
ILLUSTRATED GUIDE TO MECHANICAL COOLING
© BSRIA BG 1/2010
53, 56
OVERVIEW OF COOLING SYSTEMS
A full air conditioning system provides complete control of air
temperature, humidity, air freshness and cleanliness. In practice, the term
air conditioning is often mis-applied to describe systems that do not
provide full control of humidity in the occupied space. Systems without
humidity control are more correctly known as comfort cooling systems.
This guide uses the term cooling to cover both air conditioning and
comfort cooling systems.
The decision to cool a building requires consideration of many factors,
including the following:
Cost. Both initial costs and life cycle costs
Comfort. The level of thermal comfort required. Clients and
their designers need to determine whether internal conditions
can be relaxed, allowing internal conditions to rise to say 25oC in
peak summertime conditions instead of maintaining say 21 oC in
order to save energy, reduce the size of the air conditioning
plant, or even forgo air conditioning altogether
Control. The level and types of control required
Complexity. Clients and designers need to determine what type
of system will be appropriate and how difficult it will be to
operate and maintain. A full air conditioning system provides
close control of air temperature and humidity, but this comes at a
price
Noise levels. Some air conditioning systems adversely affect
noise levels in occupied areas. The amount of acceptable
mechanical noise will need to be determined
Adaptability and flexibility. To meet possible future
requirements
Energy use. The amount of energy required to operate the
plant. A refrigeration and air-handling plant can account for a
major part of a building’s electrical load
Global warming potential. The environmental effects of
chillers can be determined using the Total Equivalent Warming
Impact (TEWI). This is a measure of the global warming impact
of equipment based on the total related emissions of greenhouse
gases during the operation of the equipment and the disposal of
the operating fluids at the end of its life. This takes into account
both direct fugitive emissions, and indirect emissions produced
through the energy consumed in operating the equipment.
TEWI is measured in units of mass of carbon dioxide (CO2)
equivalent
Plant space. Air conditioning systems can require a large
amount of space to accommodate the refrigeration and airhandling plant. Access for operation, maintenance and
replacement must be considered.
The most common types of cooling system can been classified as
centralised or partially centralised air/water systems, or local systems.
ILLUSTRATED GUIDE TO MECHANICAL COOLING
© BSRIA BG 1/2010
7
Types of systems
Centralised plant
As the name suggests, a centralised cooling system has all the plant
located in single area, for example in a basement or roof-top plant room.
One or more air-handling units (AHUs) condition the air which is then
supplied by ductwork to the floors/spaces within the building.
The air-handling units typically contain heating and cooling coils, a
humidifier, filters, and fans to move the air.
One or more chillers will typically be located nearby to provide chilled
water for the cooling coil(s). Hot water for the heating coil(s) is provided
by a heat-raising system (such as gas boilers or heat pumps).
Centralised air systems
Centralised air systems can be categorised as:
Constant volume (CV)
Variable air volume (VAV).
Partially centralised air/water systems
In a partially centralised air/water system the bulk of the cooling/heating
is carried out within the occupied space by individual room units such as
fan coils. These are supplied with hot/chilled water from a central plant
area via a pipework system.
A centralised air system showing cold and hot water
supply from chiller and boiler and distribution of air
through VAV units.
Partially centralised air/water systems consist of:
Fan coils
Chilled beams
Chilled ceilings
Room-based heat pumps.
Central plant also supplies outside air throughout the building by means
of ductwork for ventilation dilution of odours and prevent build up of
CO2. The size of the ductwork installation and associated air-handling
plant is smaller than that required by the centralised air system. Unlike a
centralised air system, air is only required for ventilation. Consequently,
the high volume of air necessary to carry the building’s heating/cooling
requirements is avoided.
Local systems
Local systems are not linked to any centralised plant and only provide
cooling in the immediate space where they are located. They may or
may not provide ventilation depending on their level of complexity.
The common type of local systems are local systems
Split units
Variable refrigerant flow (VRF).
This guide is structured to take the reader from an understanding of
central plant first, (such a chillers and heat pumps) then to the systems
that are supported with mechanical cooling
8
ILLUSTRATED GUIDE TO MECHANICAL COOLING
© BSRIA BG 1/2010
A partially centralised air/water system (the
example is a fan coil system).
Minimising cooling loads
Before deciding which systems to use it is important to reduce the
cooling load on the system. Some of the steps that can be taken to
minimise cooling loads are as follows.
Solar shading
In the absence of shading, solar gains into buildings will be very high.
Cooling loads can therefore be reduced by the introduction, or
improvement, of solar shading. The appropriate type, size and
positioning of any shading device will depend on climate, building use
and the source of the light to be excluded (high or low angle direct
sunlight, diffuse sky light or reflected light). A range of shading options
are possible:
Overhangs (including light shelves) and awnings, such as fixed
finned external shading
External blinds and brise soleil
Glazing films and special low-energy glass
Coated glazing (often used where windows are being replaced)
Mid-pane blinds
Internal blinds. These are more glare-control devices and do not
stop heat from reaching inside the building.
Interior shades protect occupants against the immediate effects of direct
sunlight and against glare. When infrared radiation penetrates the glazing
most of it is trapped in the room and must be dissipated. Mid-pane blinds
are often a partial solution, and while more expensive do tend to require
less maintenance and cleaning.
Example of fixed finned external shading.
Horizontal shading elements are effective in reducing peak summer solar
gain where high solar attitudes are experienced, primarily on southern
façades. Vertical elements are effective for restricting solar gain at lower
solar attitudes as the sun tracks round from the east to west during the day.
Fixed external shading devices include permanent façade features such as
overhangs and deep window reveals. Unlike external blinds, the shading
effect cannot be adjusted and the obstruction to daylight is permanent.
Different forms of shading will be appropriate for different points on the
compass. Façades that receive sunlight from the west in the afternoon
will benefit from shading that can reduce low angle solar gain.
Plants and trees can be used to screen the solar heat and glare in the
summer and filter light in winter. Planting can sometimes solve the
problem of reflected light from neighbouring structures, water or ground
finishes.
The use of deep overhangs for solar shading at the
Rivergreen headquarters in Durham.
Table 1: Minimising cooling loads.
Glazing orientation in relation to heat gain
South facing
Beneficial gains in winter
Shading for high sun
angles
North facing
Building envelope as a climate modifier
In cold weather
Good day lighting
without gains
Reduce heat loss through
fabric
No need for shading
Maximise benefits of
solar and internal heat
gains
Locate spaces where
overheating is critical
In warm weather
Minimise solar heat gain
Avoid overheating
Use window shading and
thermal mass to attenuate
heat gain
Reduce losses associated
with uncontrolled air
filtration
Note: The building envelope should not be considered as a sole means of excluding external conditions.
ILLUSTRATED GUIDE TO MECHANICAL COOLING
© BSRIA BG 1/2010
9
Zoning and space use
Buildings are often divided into a number of zones for the purposes of
system control. The way in which a building is zoned depends on:
Varying solar loads At certain times of the day the south side
of a building can have high solar gains and require more cooling
than the north side
Varying internal conditions Zoning may be used when two
or more internal spaces require different conditions
Internal partitions Internal work areas or full height
partitioning can determine the boundaries for control zones.
An example of zoning with an office printer set up.
Heat-generating office equipment should be located in groups, served by
dedicated extract systems, or if located in separate rooms, by local air
conditioning units.
Control of ventilation rates
Cooling loads can be reduced by effective control of ventilation air
required to satisfy occupant comfort. It is normal practice for a
ventilation system to be designed on the basis of either a known or
anticipated level of occupancy. In conjunction with the requirements of
Approved Document F of the Building Regulations this will determine a
required design ventilation rate. If excessive amounts of ventilation are
supplied during periods of low occupancy, energy consumption relating
to the ventilation system will be unnecessarily high.
One approach to ventilation control is through the use of demandcontrolled ventilation. This involves the measurement of metabolic
carbon dioxide. With this approach, inferring occupancy numbers based
on levels of carbon dioxide enables the ventilation rate to be varied.
Lighting
The heat emitted from electric lighting can impose a significant cooling
load. Building designs should:
Make the most effective use of natural daylight (poor design, or
cost-cutting on shading, can create conflicts with efforts to
minimise solar gain)
Avoid unnecessary levels of high illuminance and/or daylight
asymmetry where the perceived creation of gloomy spots forces
the use of electric lighting
Incorporate the most efficient luminaires, control gear and lamps
Include effective lighting controls. Best practice for control of
electric lighting is manual on, and auto and manual off.
10
ILLUSTRATED GUIDE TO MECHANICAL COOLING
© BSRIA BG 1/2010
A carbon dioxide controller. Above, the blue lights
signal when the room is over-ventilated. Below, the
red lights signal when the ventilation system is
needed.
Thermal storage
Cooling loads that otherwise would be met by active cooling systems can be
reduced or even eliminated by thermal storage techniques. Thermal storage
describes the use of the building’s exposed mass (whether frame, ceilings or
walls) to absorb heat energy. Concrete is a commonly used material,
rammed-earth a newer innovation (if a very traditional, indeed historic)
material. Night ventilation can enable the structure to reject the heat gained
during the day.
There are various ways to use thermal mass. The simplest is to expose the
building’s structure to the occupied spaces, rather than clad it or hide it
behind the finishes. A flat slab can provide a cooling capacity of
2
2
approximately 65 W/m while a profiled slab can provide 80 W/m .
Even lightweight timber buildings can have mounted or free standing
heavyweight elements that perform the same function.
Interior cladding materials, such as plastic-based boards, can incorporate
phase-change materials that perform the same function.
A proprietary Swedish originated system called Termodeck effectively
combines ventilation and heat recovery with the building’s structure.
The Termodeck system comprises precast, hollowcore concrete slabs
where the hollowcores are used as routes for mechanical ventilation.
Ventilation air is passed through the cores at low velocities allowing
prolonged contact between the air slabs for good heat transfer.
Termodeck can provide a cooling capacity of around 40 W/m2 without
recourse to mechanical cooling.
Rammed-earth wall providing some thermal mass.
Cooling capacity can be increased with water cooling via polybutylene
pipework embedded in the structure. The use of water rather than air to
cool the slabs enables higher cooling capacities to be achieved. The water
is circulated at approximately 14-20oC depending on the required room
air temperature. Elevated flow temperatures allow the use of water from
boreholes rather than from refrigerant circuits.
Improved airtightness
Uncontrolled air infiltration through leaks in the building’s fabric will
cause heat loss in winter and heat gain in summer. In the summer, the
heat gained from hot air leaking into the building will increase the
cooling load on the building’s cooling system.
There are four main leakage paths that result in air infiltration:
The main meeting room in NG Bailey’s Solais
House. Acrylic-based phase-change materials have
been incorporated into the glass reinforced plastic
(grp) wave-form ceiling, sandwiched between two
layers of conventional grp. This provides around 2
kW of thermal storage before the chilled beams are
required.
Joints around components such as windows
Gaps between one element and another, such as wall to eaves
junctions
Gaps around pipes and cables passing through the building fabric
Permeable building materials, such as blockwork.
A wide range of sealing materials can be used, including:
Gun-applied sealants (elastic and elastomer types), including
mastics, polyurethane and silicone sealants
Expanding foam sealants
Gaskets for movement joints, including solid and foam-strip
Schematic of the Termodeck system which
combines ventilation and heat recovery with the
building’s structure.
types
Draught stripping
Sealing fibre
Membranes or films.
For information on ground and water cooling systems (see section on
Renewable Cooling Technologies, page 34).
ILLUSTRATED GUIDE TO MECHANICAL COOLING
© BSRIA BG 1/2010
11
CENTRAL SYSTEMS
At the heart of many cooling systems is a piece of
refrigeration equipment called a chiller. This
typically produces chilled water. The way in which
the chilled water is used depends on the type of air
conditioning system served by the chiller. For
example, in a centralised air system chilled water
will be pumped around one or more cooling coils
located in central air-handling units. In a partially
centralised air/water system (for example a fan coil
installation), it will be supplied to both a central airhandling unit and the individual fan coils located
around the building. In the process of chilling the
water the chiller generates heat which can be
directly air cooled or water cooled using an
evaporative cooling tower dry cooler (see pictures
opposite and more detail on page 18).
A schematic of an air-cooled chiller.
Some compact systems provide refrigerant directly
to a cooling coil in an air-handling unit, and
consequently avoid the need for a chilled water
circuit (these are known as Direct Expansion or DX
systems). The refrigeration plant is known as a
condensing unit and air-handling unit in a DX
system and can be purchased as an integrated unit.
Vapour compression
The vast majority of central plants are based on a
vapour compression cycle. The type of compressor
used usually defines the type of chiller. The four
main types of compressor used are: reciprocating,
screw, scroll and centrifugal.
A schematic of a water-cooled chiller.
Table 2: Overview of vapour compression chillers.
Type
Cooling range kW
Refrigerant type and
typical operating range
Capacity control
Semi-hermetic
o
o
Reciprocating (2,4,6,9,10 &12
cylinders)
20-1000
All types (-25 C to +10 C)
Cylinder, unloaded
Single screw
200-2000
HFCC and HFC
Moving plate
Twin screw
200-3000
HCFC and HFC
Slider system
Twin screw
200-600
HFCC and HFC
Slider system, variable speed
Scroll
5-250
HCFC and HFC
Reciprocating (single-stage)
2-400
All types (-25 C to +10 C)
Hermetic
o
100%
o
o
Reciprocating (two-stage)
2-150
All types (-25 C to +10 C)
50/100% speed control
Centrifugal (multi-stage)
300-15000
HFC
Inlet guide vanes (all cases)
Variable speed (in some cases)
Open-type reciprocating
(2,4,6,8,10 and 12 cylinders)
100-1000
HFC and ammonia
Cylinder unloading
Open-type screw
200-300
HFC and ammonia
Slider system, variable speed
Source: CIBSE Guide B4
Key: HFC = Hydrofluorocarbons
HCFC = Hydrochlorofluorocarbons
12
o
ILLUSTRATED GUIDE TO MECHANICAL COOLING
© BSRIA BG 1/2010
Reciprocating compressors are positive displacement-type piston
machines, where the refrigerant is compressed by pistons moving in their
respective bores (in a similar manner to an internal combustion car
engine). This type of compressor can operate over a wide range of
conditions and is available in a wide range of sizes and number of
cylinders. Capacity control is normally provided by cylinder unloading in
steps, or by switching multiple compressors and refrigerant circuits.
Speed regulation and a technique called hot gas bypass can also be used
to provide control over refrigeration capacity.
A schematic of a reciprocating compressor.
Screw compressors are high speed, positive displacement machines
with compression produced by rotating helical screws. They can operate
over a wider pressure ratio range than reciprocating compressors and can
be used with a wide range of refrigerants. Screw compressors are
available as open hermetic or semi-hermetic machines. They have the
advantages of little vibration and low noise levels.
A schematic of a screw compressor.
ILLUSTRATED GUIDE TO MECHANICAL COOLING
© BSRIA BG 1/2010
13
Scroll compressors are hermetically sealed and based on the
compression effect obtained by one fixed and one orbiting scroll that
progressively compresses the refrigerant. Scroll compressors can have
higher efficiencies than reciprocating compressors, along with lower
noise and vibration levels.
A schematic of a scroll compressor.
Centrifugal compressors can meet a wide range of cooling duties
(300 kW to 15 MW) and can be either hermetic or open hermetic.
Advantages of this type of compressor include:
Saving of space compared with screw and reciprocating machines
Low vibration
Reduced maintenance due to no wearing or reciprocating parts
Efficient part-load operation (down to 10 per cent).
A schematic of a centrifugal compressor.
14
ILLUSTRATED GUIDE TO MECHANICAL COOLING
© BSRIA BG 1/2010
Absorption chillers
In a conventional vapour-compression chiller an electric motor is used to
drive a compressor. In an absorption chiller a heat source drives the
cooling process. Absorption chillers are a very small proportion of the
market (In 2009 in the UK, the volume of sales of absorption chillers was
81 units, valued at £2 million, about 3% of the market by volume), but
are covered in detail here as they are a product with a growing market
share. Heat sources can include: hot water, steam, hot air or hot products
of combustion (exhaust gases) from the burning of fuel.
Absorption cooling can be considered as an alternative to traditional
chillers if one of the following factors is applicable:
An existing combined heat and power (CHP) unit is present and
An absorption chiller.
at least some of the waste heat generated can be used to power
the absorption cycle
A new CHP installation is being considered
Waste heat is available from a process
Renewable fuel sources can be used, such as landfill gas.
Absorption chillers have a number of advantages:
They can utilise spare heat
The refrigerants used do not damage the atmosphere and have
no global warming potential (whereas some refrigerants used in
vapour compression chillers have very high global warming
potential)
The equipment does not require lubricants
Absorption machines are quiet and vibration-free.
In a conventional mechanical vapour compression chiller the refrigerant
evaporates at a low pressure and produces a cooling effect. A compressor
is then used to compress the vapour to a higher pressure where it
condenses and releases heat. In an absorption chiller the compressor is
replaced by a chemical absorber, generator and a pump. The pump
consumes much less electricity than a comparable compressor
(approximately nine per cent of that for a vapour compression plant).
The majority of the energy required to drive the cooling process is
provided by the external supply of heat.
Absorption cycles use two fluids, the refrigerant and the absorbent. The
most common fluids are water for the refrigerant and lithium bromide or
ammonia/water for the absorbent. These fluids are separated and recombined in the absorption cycle.
The low-pressure refrigerant vapour is absorbed into the absorbent,
releasing heat. The liquid refrigerant/absorbent solution is pumped to a
generator with a high operating pressure. Heat is then added at the highpressure generator which causes the refrigerant to desorb from the
absorbent and vapourise. The vapours flow to a condenser, where it is
condensed to a high-pressure liquid and the heat is rejected. The liquid is
then throttled through an expansion valve to the lower pressure in the
evaporator where it evaporates by absorbing heat (this absorbing of heat
is used to provide a useful cooling effect). The remaining liquid
absorbent in the generator passes through a valve where its pressure is
reduced and the absorbent is then re-combined with the low-pressure
refrigerant vapours returning from the evaporator. The cycle is then
repeated (see schematic on page 16).
ILLUSTRATED GUIDE TO MECHANICAL COOLING
© BSRIA BG 1/2010
15
The basic workings of an absorption chiller.
Absorption chillers can be categorised by the type of heat source, the
number of effects and the chemicals used in the absorption process.
Indirect-fired absorption chillers use waste or heat rejected from another
process to drive the absorption cycle. Typical heat sources include steam,
hot water or hot gases. Direct-fired chillers include an integral burner,
usually operating on natural gas.
In a single-effect absorption chiller, the heat released during the chemical
process of absorbing refrigerant vapour into the liquid stream is rejected
as waste heat. In a double-effect absorption chiller, some of this energy is
used to generate high-pressure refrigerant vapour. Using this heat of
absorption reduces the demand for heat and boosts the efficiency of the
chiller system.
Double-effect chillers use two generators paired with a single condenser,
absorber and evaporator. Although they operate with a greater efficiency,
they require a higher temperature heat input compared with a singleeffect chiller. The minimum heat source temperature for a double effect
o
chiller is 140 C. Double-effect chillers are more expensive than singleeffect chillers. Triple-effect chillers are under development.
In a lithium bromide/water mixture, the lithium bromide (a salt) is the
absorbent and the water is the refrigerant. Lithium bromide systems are
the most commonly used absorption system, especially for commercial
cooling. In an ammonia system, the water is the absorbent and the
ammonia is the refrigerant. Ammonia systems are typically used when
low temperature cooling or freezing is required. As ammonia is toxic, the
plant room will need to be well ventilated.
Lithium bromide water systems are widely available as packaged units,
with refrigeration capacity ranging from 100 kW to several thousands of
kW. A practical limitation associated with this type of system is that the
o
minimum chilled water temperature produced is approximately 5 C.
Ammonia water systems are available in small (30-100 kW), medium
(100-1000 kW) and large (greater than 1000 kW) sizes. Cooling
temperatures down to –60oC are possible.
16
ILLUSTRATED GUIDE TO MECHANICAL COOLING
© BSRIA BG 1/2010
The operating efficiency of chillers can be expressed in terms of the
coefficient of performance (COP), which is the cooling effect by the
energy input of the system. A conventional vapour compression chiller
can have a COP of four or above (for cooling it is expressed as energy
efficiency ratio (EER)) which compares to an EER of around 0·7 for a
single-effect absorption chillers and around 1·2 for a double-effect
absorption chiller.
Absorption chillers and CHP systems
The poor system efficiency associated with absorption chillers is
mitigated when they are used in conjunction with what would otherwise
be waste heat. For direct-fired chillers this is not applicable. However,
when comparing total carbon dioxide (CO2) emissions between
absorption chillers and conventional chillers, account must be taken of
the CO2 emissions attributable to the generation and transmission of
electrical power.
Typical gas engine CHP.
Gas turbine combined heat and power (CHP)
The exhaust gas from the gas turbine is used to raise steam in a waste
heat boiler. The high-pressure steam available is suitable for supplying a
double-effect absorption unit. The overall efficiency of the CHP can be
enhanced if second stage heat recovery using the exhaust gases is used to
heat water for domestic hot water needs and/or space heating uses.
From a reciprocating engine CHP
Reciprocating engine CHP units typically provide hot water at 85-90oC.
This can be used for a single-effect absorption chiller, although the
performance of the chiller will have to be down-rated (single-effect
absorption chillers normally work on a heat source at 102oC and above).
Some CHP engines can produce water at higher temperatures, in which
case the performance of the absorption chiller will be improved.
Other waste heat
Waste heat from other sources such as industrial processes can also be
used to drive absorption chillers. Low-pressure steam and water can be
used with single-effect absorption chillers while higher-pressure steam (79 bar) can be used to drive double-effect chillers.
Hot water/steam from existing boilers
In instances where boilers provide space heating and are required to
supply a small load in summer, or where a large ring-main is used to
supply a few users, the efficiency of the boiler system can be improved by
utilising the heated water/steam to drive an absorption chiller. In practice
it may be more efficient to reconsider the heating strategy and install a
number of small local boilers.
ILLUSTRATED GUIDE TO MECHANICAL COOLING
© BSRIA BG 1/2010
17
Heat rejection techniques
In order for a chiller to cool the water used in a cooling system, it must
first extract heat from the water and then discharge it to an available
cooling medium. This process is the same as the operation of a domestic
fridge, which takes heat from the food inside and then rejects it at the
back of the unit via the black serpentine coil which warms the
surrounding air. In the same way, a commercial chiller must reject the
heat it picks up.
Heat rejection can be achieved in several ways. The simplest approach is
to combine the heat rejection system and chiller into a single unit called a
packaged chiller. This is located outside and incorporates one or more
fans which draw fresh air through the unit to carry away the heat. Large
chillers often have a separate heat rejection system linked by pipework,
enabling the chiller to be located in a plant room.
A heat rejection system can take several forms. The most efficient is the
evaporative cooling tower which uses the cooling effect of evaporating
water to boost the cooling provided by fresh air. This approach has
become less popular during the last 10 to 15 years as a result of the risk of
Legionnaires’ disease associated with poor maintenance. However, for
some building applications, properly maintained cooling towers remain
the favoured method of heat rejection due to their high efficiency
(which also enables a small footprint).
An example of a dry cooler.
A more widely used system for providing separate heat rejection is the
dry cooler. This consists of a low profile unit containing one or more
fans that drive fresh air across a serpentine coil. The coil contains hot
water from the chiller which is cooled and pumped back to the chiller.
Alternatively, the coil can contain hot refrigerant directly from the
refrigeration process, which is cooled in the same way and then travels
back to the chiller.
Table 3: The main types of heat rejection equipment commonly used in buildings.
System type
Description
Air-cooled
condenser
Fans induce air flow
over finned tubing in
which refrigerant
condenses.
Convenient and common for chillers up to a few
100 kW. Free of hygiene risks and does not
require water piping. Can be adapted to provide
free cooling with thermosyphon systems.
Dry-air cooler
Similar to an air-cooled
condenser but aqueous
glycol solution or water
is passed through the
tubes instead of
refrigerant.
Less efficient than an air-cooled condensor
because an additional heat transfer process, and
pumps, are required to reject heat from
refrigeration plant. May cool water sufficiently in
winter to avoid the need to operate a
refrigeration plant (free cooling). Requires a larger
plant area than other options. Adiabatic sprays can
be added to improve their performance.
Cooling tower
Water is sprayed over a
packing material. Airflow
over the packing
evaporates some of the
water, causing the water
to be cooled.
More efficient than air-cooled condenser or dryair cooler because less air is required and water is
cooled to a few degrees above the wet bulb
temperature. May cool water sufficiently to avoid
the need to operate a refrigeration plant, known
as free cooling. High maintenance requirement.
Evaporative
condenser
Water is sprayed over
tubing in which
refrigerant condenses.
Airflow across the
tubing evaporates some
of the water, causing the
water and the tubes to
be cooled.
The most efficient method of rejecting heat from a
refrigeration plant. Has similar maintenance
requirements as cooling tower. Can be adapted to
provide free cooling with thermosyphon systems.
Source: CIBSE Guide B4
18
ILLUSTRATED GUIDE TO MECHANICAL COOLING
© BSRIA BG 1/2010
Heat rejection system
Notes
Humidifiers
An important variable that affects occupant comfort is the level of
moisture in the air. Air that is too dry can cause respiratory problems,
skin and hair dryness, and eye irritation. Humidifiers can be used to
actively control the level of humidity in the occupied spaces of a
building. A wide range of humidifiers is available and they can be
categorised depending on whether they use adiabatic or isothermal
processes.
Adiabatic processes
An adiabatic process is one in which no heat is added or taken out of a
closed thermodynamic system. There are three basic types of adiabatic
humidifier: water spray, water atomising, and wetted media.
Water spray
These were traditionally used in large air-handling systems for
commercial and industrial buildings. Their popularity has dropped since
the risks of micro-biological contamination, especially Legionnaires’
disease have become apparent.
A spray humidifier has a grid of nozzles arrayed in a chamber with a
waterproof tank or reservoir within the air passage. Water is pumped
from the reservoir to the nozzles and sprayed to form a curtain of water
droplets through which the ventilation air must pass. Baffles are arranged
inside the duct to ensure that the air onto the spray washer is uniformly
distributed.
A variant of the spray humidifier is the wetted-cell type. In these, water
is sprayed over a number of cells packed with a fibrous material. As air
passes through them, the evaporation from the wetted surfaces enhances
the humidification effect, such that the quantity of water required is
approximately half of that required for a similar performance in a basic
spray unit. Another type involves spraying water onto cooling coils
located immediately downstream, the extended surface of the coil
providing an increased contact area between the water and the air.
Isothermal processes
In this context, the term isothermal means a process occurring at a
constant temperature. Isothermal humidifiers can be grouped into two
categories: steam humidifiers and vapour humidifiers. Steam humidifiers
include systems that deliver steam produced remotely to the air stream.
Vapour generators convert the heat energy to water vapour within the
apparatus itself.
Steam humidifiers take steam generated by a boiler and apply it either
by an injection system to a ducted ventilation system or by direct release
into the occupied space to be humidified. The boiler is usually an
electrode or electrical resistance boiler. Humidifiers designed for direct
applications are called ‘area type’ while those intended for duct
applications are called ‘injection type’.
Vapour generators include devices such as the heat pan and infrared
evaporator. The heated-pan humidifier is constructed from a copper or
stainless-steel pan containing water. Water vapour is produced in the pan
by providing the necessary energy for evaporation using electric heating
elements, steam or hot water tubes. Infrared evaporators use infrared
lamps to evaporate water contained in reservoirs or pans. Parabolic
reflectors are used to reflect and focus the infrared radiation downward
onto the water.
Further information on humidification is given in the BSRIA guide AG
10/94.1 Efficient Humidification in Buildings.
ILLUSTRATED GUIDE TO MECHANICAL COOLING
© BSRIA BG 1/2010
19
Water atomising
There are three basic types of water atomiser: atomising nozzle,
mechanical atomising, and ultrasonic atomisation. Atomising nozzles may
be based on water pressure or air pressure. Both types use pressure
nozzles that produce a fine mist of water particles. A finer mist is
produced by the air atomising system.
Mechanical atomisation systems normally take the form of spinning disks
or drums onto which water droplets impinge. A fine mist is produced
and carried into the air stream.
Ultrasonic humidification relies on the principle of ultrasonic
nebulisation brought about by a rapidly oscillating crystal submerged in
water. The crystal, a piezo-electric transducer, converts the electrical
frequency into a mechanical oscillation. During the rapid oscillations a
cavity is formed between the crystal and the water, creating a partial
vacuum. At this instant the water is able to boil, creating a low-pressure
gas. This is then followed by a positive oscillation creating a highpressure wave which is able to expel the pocket of gas through to the
surface of the water. Condensation occurs, but the net result is the release
of finely atomised water that is readily able to evaporate.
Source: JS Humidifiers
Wetted media
This type is distinct from the wetted-cell type outlined above, in that it
does not rely on the generation of water droplets by spraying. Instead,
water is either trickled over fibrous media or, more unusually, the media
is wetted by capillary action from water in a reservoir in which the media
is partially submerged. Air passing through the media gains moisture by
evaporation from the wetted surfaces.
These three images show a
JetSpray air and water
atomising humidifier. The
JetSpray nozzles combine
compressed air and water to
achieve total atomisation within
a minimum distance.
20
ILLUSTRATED GUIDE TO MECHANICAL COOLING
© BSRIA BG 1/2010
Heat recovery
Heat recovery is not in itself a cooling technology, more a means of
reducing loads. This is because a heat recovery unit is equally capable of
recovering coolth as well. Where ventilation is provided by an airhandling unit, heat recovery can be used to transfer energy from the
extract air to the supply air. In the summer when the internal air
temperature is below the exterior temperature, the extract air can be used
to cool the supply air. The most common types are:
Plate heat-exchanger
Thermal wheel
Run-around coil system
Heat pipes.
Plate heat-exchanger
Plate heat-exchangers are relatively simple devices with no moving parts.
They consist of a framework supporting a number of thin plates spaced
apart with air passages in-between. The plates are normally of metal but
can be made from other materials and the plates may have flat,
corrugated or finned surfaces. A typical plate spacing is between 2 and
12 mm.
An advantage of this type of exchanger is that a wide range of
combinations of plate surface types and finishes and of plate spacings is
available to suit many applications. Most manufacturers offer their heat
exchangers in modular form so that the appropriate number of modules
may be selected to suit the air flow rates to be handled.
A cross-flow plate heat-exchanger. This is part of an airhandling unit.
Advantages of plate heat-exchangers include:
No moving parts, except for controlling the rate of heat recovery
or de-frost through by-pass dampers where fitted
Little or no possibility of cross contamination of air streams if
properly constructed
Plate material (including protective surface coating) and plate
spacings can be selected to suit a wide range of applications
Easily cleaned if the exchanger can be quickly withdrawn from
the duct.
Disadvantages of plate heat-exchangers include:
Static pressure differences between fresh air and exhaust air
streams is limited, depending on construction
A by-pass may be needed to avoid overheating fresh air in
summer and to reduce fan power.
ILLUSTRATED GUIDE TO MECHANICAL COOLING
© BSRIA BG 1/2010
21
Thermal wheel
A thermal wheel (also known as a rotary regenerator) consists of a matrix
in the shape of a wheel rotating slowly between adjacent fresh air and
exhaust air ducts. The wheel rotates at between 8 to 15 rev min. As it
does so the matrix material in the airstream absorbs heat as the warmer
air stream, and releases the heat again on re-entering the cooler air
stream, which flows through the wheel in the opposite direction.
The rotation of the wheel allows for a continuous transfer of heat from
one air stream to the other due to the heat storage capacity of the matrix
medium.
Advantages of thermal wheels include:
A relatively high heat-transfer efficiency compared to other types
of air-to-air heat recovery devices
The energy consumption of the electric motor used to rotate the
wheel is very low compared with heat energy savings
Matrix material and density can suit a wide range of applications
Some types of thermal wheel can transfer latent heat as well as
Schematic of
a thermal
wheel.
sensible heat
Non-metallic matrices may use a desiccant coating (a material
that readily absorbs and desorbs moisture) to achieve latent heat
transfer, which significantly improves their effective heat
capacity.
Disadvantages of rotary regenerators include the following:
Regular air filter maintenance/replacement is essential as the
matrix or a thermal wheel is difficult to clean, especially in larger
units
Static pressure in the fresh air stream must be higher than that in
the exhaust air stream to limit cross-contamination and for
successful operation of a purge unit (where fitted)
A thermal wheel will occupy a relatively large space in the plant
room
The large ratio of surface area to volume of matrix material
makes this type of heat exchanger particularly susceptible to
corrosion (depending on the material used)
The thermal capacity of the matrix and its resistance to flow may
create a tendency to clog.
22
ILLUSTRATED GUIDE TO MECHANICAL COOLING
© BSRIA BG 1/2010
A thermal wheel with an air-handling
unit.
Run-around coil system
A run-around coil system consists of one or more coils located in the
exhaust air duct connected to one or more coils in the fresh air intake
when heating is required. It can work in both heating and cooling
modes.
A heat transfer fluid is pumped between the two sets of coils to provide a
continuous transfer of heat from the exhaust to fresh air during the
heating season. Only sensible heat is transferred by the fluid, although
latent heat can be transferred from the warmer air stream to the heat
transfer fluid if the coil temperature falls below the air dew-point
temperature, in which case condensation (and heat transfer) will occur.
Schematic of a run-around coil system.
A run-around coil system can be assembled using commercially-available
items of equipment. Alternatively, some manufacturers offer a preassembled unit comprising coils, a circulating pump set, a thermal
expansion vessel, a condensate collection tray below the exhaust coil, a
mixing valve for the control bypass circuit of heat transfer fluid, and
control sensors and actuators.
Advantages of run-around coils include:
Suitable for an existing ductwork system as the technology does
not require adjacent fresh air and exhaust ducts
Relatively low capital cost compared to other heat recovery
systems
Coils are standard items of equipment similar to cooling coils,
and are therefore well-proven components
No possibility of cross-contamination of air streams
The number of rows and fin spacing can be selected to suit the
required heat transfer rate, and the permissible air pressure-drop
and level of exhaust air contaminants.
Disadvantages of run-around coils include:
Sensible heat transfer only (except when condensation occurs on
the coil)
Relatively low-heat transfer efficiency
The circulating pump and additional fan energy (or running cost)
must be offset against heat recovery savings.
ILLUSTRATED GUIDE TO MECHANICAL COOLING
© BSRIA BG 1/2010
23
Heat pipes
A heat pipe is a passive device that, in effect, has an unusually high ability
to transfer heat. A heat pipe heat-exchanger used in a ventilation system
consists of a bundle of externally-finned heat pipes contained in a frame
and arranged in staggered rows, typically between four and eight, in the
direction of flow. A central partition plate at right angles to the heat
pipes separates the heat exchanger into two halves.
The heat exchanger is installed with one half in the warm waste air
stream, from which heat is to be recovered, and the other in the adjacent
fresh air stream. The waste air and fresh air streams normally flow in
opposite directions. The heat pipe itself is a hollow tube, sealed at both
ends, containing an easily vapourised fluid and wick. In use, the liquid
boils at the warm end of the tube and condenses at the coil end, where
the condensate is wicked back to the liquid reservoir. There is a very
high heat transfer rate with a small temperature difference. A heat pipe
operating with the evaporator (hot end) below the condenser (cold end)
is capable of transferring typically ten times as much heat as one having
the evaporator above the condenser. This is because gravity assists the
return of condensate in the former case and hinders it in the latter.
A schematic of a heat-pipe.
In ventilation and air conditioning systems, the heat pipes are normally
installed at a slight angle to the horizontal so that the lower end is in the
warmer airstream. Heat pipes can also be installed vertically, with the
warm air duct below the cold air duct, to maximise heat transfer. In this
case the direction of heat transfer cannot be reversed.
A schematic of a vertical heat pipe.
Advantages of heat pipes include:
Robust construction. No moving parts except a tilt mechanism
(where fitted)
A relatively high-pressure difference between airstreams is
possible, limited by the baffle/separating-plate
Little or no possibility of cross-contamination of airstreams when
the system is properly constructed
A relatively high heat transfer rate
May be designed for easy removal and cleaning of the heat
exchangers
The number of heat pipe rows can be selected to suit the
required heat transfer rate.
24
ILLUSTRATED GUIDE TO MECHANICAL COOLING
© BSRIA BG 1/2010
Disadvantages of heat pipes include the following:
Relatively high capital cost
Sensible heat transfer only (except when condensation occurs on
heat pipe surfaces in the warmer air stream and is removed as
liquid)
A by-pass duct may be needed in summer to avoid over-heating
the incoming fresh air (in systems without air cooling). Control
by tilt mechanism may be too expensive to justify use except in
large installations.
Cost
The potential effects on capital and operating costs associated with the
installation of any air-to-air heat recovery system are listed in Table 4.
Table 4: Cost issues of heat recovery systems
Capital costs
Increased
Design cost
The heat exchanger
Running costs
Decreased
Possible reduction in
ventilation plant size
Additional air filters
Possible reduction in
refrigeration plant size
Pipework and pumps (for
run-around coils)
Marginal cost of heating
coils
Fan (increased motor
size)
Marginal cost of cooling
coils (in air conditioning
systems)
Additional plant room
space requirements (if
any)
Automatic controls
Installation costs
Possible reduction of
heat distribution
equipment, such as pipes
and ducts
Increased
Additional fan power
needed to drive air
through the heat
exchanger and filter(s)
Pump power (run-around
coils)
Inspection, maintenance
and cleaning
Decreased
Sensible or total heat
recovered from exhaust air
reduces the quantity of
energy required to heat or
humidify fresh air
In air conditioning systems,
the total heat removed
from fresh air by the heat
exchanger reduces the
quantity of energy required
to cool or dehumidify fresh
air
Commissioning and
testing
ILLUSTRATED GUIDE TO MECHANICAL COOLING
© BSRIA BG 1/2010
25
Direct heat recovery
Heat recovery direct from cooling systems is also possible. Air
conditioning and comfort cooling rely on the rejection of heat removed
from areas within a building that are cooled. This removed heat can be
used to provide heating in other areas of a building.
The most direct form of heat recovery is with the use of variable
refrigerant flow (VRF) comfort cooling systems (see page 53). These
systems are based on heat pump technology and usually comprise an
externally-mounted unit and a number of internal units that serve
individual rooms or zones. The external and internal units are linked via
a network of pipes that transfer refrigerant between them.
An advantage of simultaneous VRF systems is their ability to allow the
simultaneous operation of indoor units in either cooling or heating
mode. This allows for direct heat recovery to be performed. Where
different indoor units are in cooling and heating mode, heat removed
from those operating in cooling mode can be used to supply heat to
those in heating mode.
Heat recovery using VRF technology can be further enhanced where the
indoor units are combined with a ducted air supply/exhaust. In this
situation air-to-air heat recovery can be used.
Room-based heat pumps (see page 51) provide another form of roomto-room heat recovery, such as only the balancing loads need to be
served by cooling plants (see page 51).
Heat can also be recovered from conventional chiller-based cooling
systems where the chiller is capable of producing a relatively high
condenser water temperature. Where this is the case, the recovered heat
can be used to pre-heat domestic hot water or provide heated water for
space heating requirements. One possible approach is to use two chillers,
one of which is dedicated to producing the desired amount of hot water
and part of the cooling load, and the other used to meet the required
cooling load.
The combination of a dedicated heat recovery chiller operating at an
elevated condenser water temperature, and a main, high-efficiency chiller
operating at the most efficient condenser water temperature, allows for
optimum loading of the heat recovery chiller to provide the heating load.
The more efficient main chiller is set up to meet the bulk of the cooling
load.
Schematic of heat recovery from a cooling system.
26
ILLUSTRATED GUIDE TO MECHANICAL COOLING
© BSRIA BG 1/2010
Free cooling
Free cooling is a term used to describe the operation of a conventional
system where maximum use is made of ambient conditions before
energising chillers. In other words if it is cold, turn the chiller off.
Free cooling takes advantage of favourable weather conditions to enable
the chiller plant to run without refrigeration. With the right control
software, chillers can be shut down for periods. The incorporation of a
free cooling capability to a cooling system can significantly reduce the
annual energy requirement for refrigeration.
Two kinds of free cooling exist: direct and indirect.
Indirect free-cooling. Indirect free cooling is generally used in water
chillers: water is cooled exploiting the external air during the period of
low external ambient temperature. This allows a drastic reduction of the
compressor operating hours with a consequent energy saving from 30 up
to 60%.
Direct free-cooling. Cold external ambient air, after being
appropriately filtered, is injected in the locals to cool, generally data
centers. The external air is drawn directly from the unit. This is done
using a dumper controlled by the microprocessor.
The viability of free cooling is increased if:
There is a significant cooling load during winter months,
typically greater than 20 per cent of the full design cooling load
There is a continuous 24-hour demand for cooling
Chilled water can be circulated at higher temperatures without
compromising comfort
The building has a high performance envelope.
The following outlines some of the most common solutions for
achieving free cooling (note that many of the options could, depending
on the circumstances, be a low-cost refurbishment feature).
Recirculation air systems
In a system where air is conditioned at a central air-handling unit and
distributed via a constant volume ductwork system, it is likely that the
total volume of air required to cool the space is greater than the amount
needed to provide fresh air for occupants. This means that a proportion
of the extracted air can be re-circulated back into the occupied space. To
achieve free cooling, modulating dampers can be used to increase the
ratio of fresh air to re-circulated air when the outside air temperature is
less than the required internal space temperature.
Full fresh-air systems
In systems where the re-circulation of extracted air is not required or not
possible (for example due to smoke or fumes), it is possible to install
some form of heat recovery device to transfer heat from extracted air to
incoming air during winter, or from incoming air to extracted air during
summer. Typical heat recovery devices include run-around coils, thermal
wheels, and plate heat-exchangers.
ILLUSTRATED GUIDE TO MECHANICAL COOLING
© BSRIA BG 1/2010
27
Systems with cooling towers
A common cooling solution is to circulate chilled water to room
terminal units such as fan coil units or chilled beams. Free cooling is
achievable where heat rejection from the chillers is via cooling towers.
Free cooling works by rejecting heat from the returning chilled water
straight into the atmosphere, by-passing the chiller completely.
Free cooling can be achieved during periods when the outside wet bulb
temperature is less than the required chilled water temperature. For
chilled beam systems in particular, where chilled water temperatures of
around 14-15oC are acceptable, free cooling is available for a significant
proportion of the year.
Basic configuration of a direct tower-based free cooling system.
Basic configuration of an indirect tower-based free cooling system.
28
ILLUSTRATED GUIDE TO MECHANICAL COOLING
© BSRIA BG 1/2010
Desiccant cooling systems
The free cooling capacity of a full fresh air system can be further
improved by the addition of desiccant cooling. Desiccant material is
commonly a coating on the matrix of a heat recovery wheel that readily
absorbs and desorbs moisture. Although the process of cooling avoids the
need for refrigeration plant, energy is still consumed due to the need to
dry and re-generate the desiccant material. Where possible, this heat
could be provided by waste heat from some other process or solar
energy. Nevertheless, even with electric power regeneration, cooling
costs can be up to 30 per cent less than for mechanical chiller plant.
Systems with evaporative coolers
Water-side systems with evaporative coolers are able to take advantage of
the same free cooling method as for systems with cooling towers. For an
evaporative cooler, condenser water is circulated through a finned coil
across which air is drawn to carry away the heat. To improve heat
transfer, the coils are kept wet by water spray nozzles. Evaporative
coolers have an advantage over cooling towers in that there is no need
for a collection sump, and less water is in contact with the air.
Simultaneous air and water free-cooling
Where fresh air is supplied by a central air-handling unit, chilled beam or
fan-coil systems can take advantage of simultaneous air-side heat recovery
and water-side free cooling. During periods when the outside air
temperature is less than the required chilled water temperature, a chiller
can be shut down so that returning chilled water is circulated through the
cooling coil on the air-handling unit.
Under this condition, instead of providing cooling to the entering fresh
air, the entering fresh air cools the chilled water and is slightly heated as a
result. An evaporative humidifier could be used to further increase the
time when free cooling is available by lowering the dry bulb temperature
of the entering air before it reaches the cooling coil.
Main types of water-side free cooling (blue line
denotes free cooling flow).
Thermosyphon systems
Thermosyphon systems can offer a packaged solution for water-side free
cooling by incorporating free cooling within the chiller operation itself.
When external conditions permit, the difference in temperatures inside
the condenser and evaporator can encourage natural circulation of
refrigerant around a circuit without the need to run a compressor. In this
mode, refrigerant by-passes the compressor and enters the condenser
where it is cooled. The resulting condensate then passes straight back to
the evaporator, by-passing the expansion valve.
Further information on free cooling is given in the BSRIA publication
BG 8/2004 Free Cooling Systems.
ILLUSTRATED GUIDE TO MECHANICAL COOLING
© BSRIA BG 1/2010
29
Variable speed pumping
The advent of variable speed motors and drives has enabled the
widespread use of variable speed drives for pumps. These are systems
where pump speed is varied in response to a changing cooling demand.
The outputs from terminals (such as fancoil units) are normally varied by
the throttling action of two-port control valves. Pump speed is then
controlled to match the chilled water flow requirements as closely as
possible to the cooling needs.
The use of variable speed pumping for chilled water circuits can result in
the following benefits:
Reduced pumping energy
Pump duties can be matched to system requirements without the
need to adjust main regulating valves or change pump pulleys or
impellers
There is an opportunity to make allowance for diversity of
cooling loads around a building, and therefore reduce mains flow
rates and pipe sizes
There are potential capital savings on pumps if the duty can be
shared across a number of pumps and standby capacity is
minimised to reflect the anticipated load diversity
There is potential to leave out flow-regulating devices in parts of
the system, thereby reducing installation costs and commissioning
costs
Chilled water temperature differentials will tend to remain
roughly constant, thereby providing a constant load for the
chillers and increased efficiency
The heat gains from chilled water pumps will be reduced, so that
less heat goes into the water
Primary plant and terminals can be added to the system more
easily than in the case of constant flow systems.
Further information on variable speed-pumping is given in the BSRIA
guide AG 14/1999 Variable Speed-Pumping in Heating and Cooling Circuits.
30
ILLUSTRATED GUIDE TO MECHANICAL COOLING
© BSRIA BG 1/2010
A typical combination
valve.
A typical differential
pressure control valve.
Refrigerants
A wide range of refrigerants are used in central and local cooling systems,
as indicated in Table 5.
The Ozone Depleting Substances Regulation came into force in 2000 and
has resulted in the prohibition of ozone-depleting HCFC refrigerants
such as R22 in new systems. R22 is a common refrigerant in existing
cooling systems. The Regulation will ban the use of R22 as a top-up
refrigerant for maintenance between 2010 (for virgin fluid) and 2015 (for
recycled fluid the date is under review and may be brought forward to
2012). Users of R22 and other HCFC systems will need to consider
alternative refrigerants or the installation of new cooling equipment by
the due dates (see Table 6).
Table 5: Refrigerants used in mechanical cooling systems.
Type
Refrigerant
examples
Ozone
F-gas
Comments
HCFC
Pure fluids: R22, R123,
R124, R141b, R142b
9
x
R22 is very common in air conditioning plant and food
factories
HCFC blends with
HFCs
Blends: R403A, R403B,
R408A, R411B
9
9
HCFC blends were introduced in the mid-1990s to
help with CFC phase out. Most HCFC blends also
contain HFCs, so these refrigerants are affected by the
regulations
HCFC blends with no
HFCs
R406A, R409A, R409B
9
x
These uncommon HCFC blends do not contain any
HFC components, so are only subject to the Ozone
Regulation
HFC
Pure fluids R134a, R32,
R125
x
9
HFCs have been used since 1995 as alternatives for
CFCs and HCFCs
HFC blends
Blends: R404A, R407C,
R410A
x
9
HFC blends are used because the properties of pure
HFCs do not suit all refrigeration applications
Other
Ammonia (R717), CO2
x
x
Ammonia is quite common in the food industry and is
not affected by the regulations
Source: IOR Guidance Note 15.
Table 6: Options for the phase-out of R22 and other HCFC refrigerants.
Option
Advantages
Disadvantages
Replace whole plant
New plant can be designed to have the best
energy efficiency
New plant can meet current and future cooling
requirements and use the latest technology
New plant will have 20 to 30 years life span
An alternative refrigerant can be used, such as
ammonia, hydrocarbons or CO2.
The most expensive option in terms of initial
costs
The longest implementation time
Modify plant to use a
new refrigerant
Fairly quick implementation
Efficiency might get worse
Probably much lower capital cost than plant
replacement
Cooling capacity might fall
Not applicable to all plant designs
Some risks of reliability problems
Plant life not extended
Use existing plant
with recycled
HCFCs
Easy, zero-capital cost option
Efficiency not being improved
This option only delays response – either of
the first two options must be adopted by the
end of 2014
The 2014 date could be changed to an earlier
phase-out date
There is no guarantee of recycled HCFCs
being available at reasonable cost
Source: IOR Guidance Note 15.
ILLUSTRATED GUIDE TO MECHANICAL COOLING
© BSRIA BG 1/2010
31
The choice of the best refrigerant for a specific application is complex
and involves the evaluation of a number of competing characteristics (see
table 8, opposite). An ideal refrigerant would:
Have excellent global environmental characteristics, such as zero
ozone depletion potential (ODP) and zero or very low global
warming potential (GWP) – see table 7
Be non-toxic and non-flammable
Have excellent thermodynamic properties for the given
application. This means that the efficiency of the refrigeration
cycle should be as high as possible
Be a practical fluid to incorporate in the plant design. This
includes factors such as materials compatibility (it is helpful if the
refrigerant is compatible with a wide range of metals and other
materials such as seals and gaskets), lubricating oil compatibility,
and operating pressure level (evaporating pressure must not be
too low and condensing pressure must not be too high)
Be low cost, widely available and familiar to designers, installers
and maintenance contractors.
Table 7: Examples of refrigerant ozone depletion potential (ODP),
and global warming potential (GWP).
Refrigerant
GWP
ODP
Comments
CFC 12
8100
1
Banned in the EU since
2000
HCFC 22
1500
0·05
Being phased out in the
EU, 2010 to 2015
HFC 134a
1300
0
HFC 404A
3300
HFC 410A
1725
Various HFCs used since
mid-1990s as alternatives
to ozone-depleting CFCs
and HCFCs in a wide
variety of refrigeration
and air conditioning
applications. Three
examples are given here –
around 20 others are
available
New
fluorocarbons
10
(approx)
0
New refrigerants. Very
low GWP. Not yet
commercially available
CO2
1
0
Operates at very high
pressure
Hydrocarbons
3
0
Widely used in very small
systems. Highly flammable
Ammonia
0
0
Used in large industrial
systems. Toxic and
flammable
Source: F-gas Information Sheet RAC 7 – Alternatives.
32
ILLUSTRATED GUIDE TO MECHANICAL COOLING
© BSRIA BG 1/2010
Table 8: Comparison of refrigerant characteristics.
Refrigerant
Hydrofluorocarbons
Hydrocarbons
GWP
88
9
99
99
9
Toxicity
99
99
88
9
99
Flammability
99
88
8
99
8
Efficiency
9
9
9
9
9
Materials
9
9
8
9
9
Pressure
9
9
9
88
9
Cost
9
99
99
99
88
99
9
9
9
88
9
9
8
8
Availability
99
Familiarity
Key:
Very poor 88
Poor 8
Good 9
Ammonia
CO2
Low GWP
fluorocarbons
Very good 99
Note than as set out in the RAC 7 sheet, 88 against toxicity for Ammonia means high toxicity and 99 against Cost for Ammonia means low cost.
Source: F-gas Information Sheet RAC 7 – Alternatives.
Note that all refrigerants have been characterised as ‘Good’ in terms of
efficiency. All these refrigerant types have the potential to have ‘Very
good’ efficiency if the system design is carefully optimised. However,
poor design could lead to ‘Poor’ or even ‘Very poor’ efficiency.
The EU F-gas Regulation imposes obligations on operators and
contractors relating to the use of fluorinated greenhouse gas-based
refrigerants (see section on Maintenance and Upkeep, page 58).
ILLUSTRATED GUIDE TO MECHANICAL COOLING
© BSRIA BG 1/2010
33
RENEWABLE COOLING
TECHNOLOGIES
Innovative and ground-coupled cooling
Ground-coupled air systems, also referred to as earth-coupled systems,
are primarily used for pre-conditioning incoming supply air in summer.
Ground-coupled air systems typically comprise a length of piping placed
underground. The network of piping is connected to an outdoor air
intake and to the building ventilation system at the other end.
During the summer, heat transfer to the surrounding ground cools the
incoming air, while during the winter the colder outside air is warmed
by heat transfer from the ground. At depths greater than two metres
ground temperature is constant at 12-13oC all year round. The tempering
effect provided by the thermal mass of the ground can significantly
reduce (or even remove) the need for mechanical cooling. Groundcoupled cooling systems can also be run at night to purge buildings of
daytime heat.
While a ground-coupled air system may be capable of completely
removing the need for mechanical cooling, close temperature control
within the building will not be possible. This puts greater emphasis on
insulation and airtightness.
Ground-coupled systems can be in the form of earth pipes or labyrinths.
Earth pipes or ducts are typically constructed from a range of materials
including concrete drain sections, or corrugated galvanised ductwork.
The best materials are those with good thermal transfer properties.
Earth pipes or labyrinths used in ground-coupled
systems.
The effectiveness of a ground-coupled system is dependant on a range of
factors including the following:
Soil temperature
Soil type – thermal conductivity
Soil moisture levels – wet and heavy soils are an advantage in
terms of thermal performance
The number of bends in the duct run (most of the thermal
transfer occurs at bends)
The entry point of a large ground-coupled air
system.
Source: Consultant Atelier Ten
Incoming air temperature
Mass flow rate of air, and air velocity
Degree of air turbulence at the inside surface of the pipe or duct.
A schematic showing a ground-coupled system designed for an office project.
34
ILLUSTRATED GUIDE TO MECHANICAL COOLING
© BSRIA BG 1/2010
The positioning of the air intakes will influence the temperature and air
quality of the supply air. The following good practice steps can be taken:
Avoid air intakes over areas exposed to direct sunshine, or
macadamised surfaces that absorb solar heat
Raising the intake will result in cooler air entering the system
Placement of vegetation around the intake can reduce intake
temperatures
Schematic of a ground-coupled air thermal storage
system.
Coarse filters can be fitted to remove large particulates, with
finer filters in the air-handling unit.
The distribution ducts should be large enough to ensure that the pressure
losses for all air paths through the network are of similar magnitude. The
distribution ductwork should also be large enough to allow access for
inspection and cleaning. At the other end of the pipe network, a
collection duct is used to equalise the pressure between the ducts. To
ensure that condensate and any ground water can drain off, the pipes
should be inclined towards the intake or the plant room where it will be
visible. A means of drainage should be provided.
Thermolabyrinths
Labyrinths consist of a network or maze of interconnecting passages,
tunnels or stone-filled chambers below the building. They are usually
part of a building’s construction or the foundations.
Being located beneath a building means that labyrinths, (or thermolabyrinths) differ from earth ducts in that neighbouring land is not
required for a network of trenches to house the ductwork. Space for a
labyrinth can also be excavated at the same time as the ground is being
dug for the building’s basements or foundations.
The downsides of labyrinths include the need for an extra basement,
which may compete with any requirements for underground car parking.
The biggest operational problem of labyrinths is that they are part of the
building structure. Unlike ducts buried in neighbouring land, labyrinths
can build up heat from the building above. This stored heat needs to be
periodically purged, and that requires extra fan energy. Labyrinths can be
appropriated for extra storage, but only where air movement is not
obstructed.
As air travels through a thermolabyrinth, it picks up
heat or is cooled depending on season and the
temperature of the incoming air.
The thermal effects of bends in specific zones of duct length.
ILLUSTRATED GUIDE TO MECHANICAL COOLING
© BSRIA BG 1/2010
35
Ground water cooling sources
Ground water can be used as a cooling medium. Typically, the water can
be obtained by drilling a borehole to the water table or aquifers. Aquifers
are water-yielding rock strata, either of unconsolidated (gravel, sand and
other friable rocks) or consolidated rocks like chalk. At a depth of around
50 m the ground temperature is approximately the same as the mean
ambient air temperature.
Water from aquifers can be supplied directly or indirectly to cooling
systems, and as a heat source/sink for heat pumps. It can also be
indirectly connected to a building’s ventilation system via heat
exchangers in order to temper the supply air; pre-heating in winter and
cooling in summer.
There are two types of aquifer-based systems:
Schematic showing how aquifiers are used for
cooling.
Open loop
Closed loop.
Open loop systems
Open loop ground-water cooling systems comprise two boreholes – a
supply well and a return well. When cooling is required in the building,
water is extracted from one part of the aquifer system and transferred to a
heat exchanger, then returned to the aquifer at a different location.
The borehole water will flow in a separate piped circuit (the primary
circuit). The building’s cooling system will be a secondary circuit. The
two circuits will interface with one or more heat exchangers..
On the secondary side of the heat exchanger, the building’s cooled water
circuit will be chilled by contact with the primary circuit. The chilled
water is used to supply space-cooling systems such as fan coil units,
chilled ceilings and chilled beams. The chilled water can also be used to
supply slab-cooling systems that comprise a pipe network embedded in
the floor or ceiling slab of a building. In some areas of the UK a second
borehole may not be a requirement imposed by the Environment
Agency. This means that the extracted water can be re-used for toilet
flushing, for irrigation, or discharged directly to the drain (this will
require a discharge consent). In some locations it may be possible to
discharge waste aquifer to a stream or river.
Return water boreholes can gradually store the heat rejected from the
building. This characteristic can be turned to advantage in winter by
using the return borehole as the supply. The elevated temperature of the
supply water will help to pre-heat the building’s ventilation system until
the rock surrounding the borehole is exhausted of heat built up from the
previous season.
For open borehole circuits particularly, the build-up of heat may create
other problems, such as microbial or algal growth that may block pipes
or foul heat exchangers. A qualified hydrologist will be able to advise on
the degree of risk and any appropriate counter measures.
36
ILLUSTRATED GUIDE TO MECHANICAL COOLING
© BSRIA BG 1/2010
Source: Max Fordham LLP
Typically, ground water can be extracted at a temperature of around
o
6-10 C giving a chilled water supply temperature from the heat
exchanger of approximately 12oC. Where additional cooling is required,
this water can be cooled further using vapour compression or absorption
cooling techniques. The supply of chilled water from the heat exchanger
can also be used to provide condenser cooling.
Plate heat-exchangers exchange heat between the
ground water and the chilled water circuit.
Cooling coils can be incorporated in floor slabs, fed
by a secondary chilled water circuit. The system
could be cooled with water from natural sources,
such as boreholes.
In locations where there is no ground water movement, extraction of
heat during the winter provides a thermal balance between heat and cool
extraction from the aquifer. Without this switching, the performance of
the system would degrade over time.
When considering the use of groundwater, the starting point is to
investigate the site’s geological characteristics and potential suitability for
a ground water system. The initial investigation can be achieved by
engaging the services of a hydrogeological consultancy (the Geological
Society produces a Geologist’s Directory), which provide a water
borehole prognosis report for the site. The next stage is likely to involve
the services of a specialist consultant, who will undertake exploratory
drilling and testing to establish the depth/suitability of the aquifer, the
water quality, and the most appropriate drilling technique.
Once enough information has been gathered, the consultant will be able
to calculate the groundwater cooling potential and establish the borehole
requirements.
Closed loop systems
Closed loop systems do not extract water from the ground. They
comprise a continuous loop of piping (high density polyethylene pipe)
which is installed underground. Water is circulated through the loop and
into the building where it can be used for space cooling. There are two
types of closed loop systems: vertical boreholes and horizontal loops.
Vertical loops are inserted as U-tubes into boreholes that are backfilled
with thermally conductive grout. This provides good thermal contact
between the aquifer and loop. Vertical loops provide better performance
than horizontal loops because of the lower and more stable temperature
of water at greater depth; less horizontal space is also required. The main
disadvantage with vertical loops is the higher installation cost due to the
requirement for boring. Boreholes themselves can be problematic to drill.
A typical closed-loop ground-source system for a
domestic dwelling. These systems are common in
Germany and Austria.
Horizontal loops consist of single, or pairs, of pipes laid in trenches
usually around 2 m in depth. The trench is then backfilled with fine
aggregate. Better performance can be achieved by laying the pipes at
greater depth but the cost of excavation will be higher. Coiled pipes
(sometimes referred to as slinkies) are a variation of horizontal loops and
can be used to increase the length of the installation. However, the
performance of a slinkie-based system will be less than that of a noncoiled system with the same length of pipe due to the overlapping nature
of the slinkie.
If there is a low temperature difference between the aquifer and the
water in the pipework loop, the cooling output will be reduced, unless
the cooling output can be increased through the use of a heat pump.
As the ground water will not be used directly, closed loop systems suffer
fewer of the operational problems of open loop systems. In addition,
approval for water extraction is not required from the Environment
Agency.
Horizontal loops, known as slinkies.
ILLUSTRATED GUIDE TO MECHANICAL COOLING
© BSRIA BG 1/2010
37
Benefits
Surface water cooling
Surface water cooling involves pumping water from the sea, lakes or
water courses to provide a building’s cooling requirements. Such
systems are inherently open loop. Water is extracted, passed through a
heat exchanger, then returned to the water source. The resultant
cooled water from the heat exchanger can be used for a range of
cooling puposes, including:
Used directly to supply cooling plant functions such as fan
coils, chilled ceilings/beams, or to supply slab cooling systems
Pre-cooling return chilled water prior to further cooling by
other techniques
As a source of cooled water for the condenser of vapour
compression or absorption chiller
Reduces the requirement for mechanical
cooling
Can be used to pre-heat ventilation during
the heating season
Can be used as a heat source or heat sink
for a heat pump.
Limitations
Relatively few buildings are close to suitable
water sources
Water needs to be deep enough to provide
water that remains sufficiently cool
Cost of piping may be prohibitive
Filtration may be required to prevent heatexchanger fouling.
Used as a heat source/sink for a heat pump.
At Sydney Opera House (above) sea water is used as
the heat transfer mechanism for the building’s
cooling system. The image below is the seawater
sump.
Here the system consists of two main loops. In the first loop, pumps draw
cold seawater from the bottom of the harbour, and then circulate the
seawater through heat exchangers. The warmed water is then returned to
the harbour floor. The second loop carries the building’s cooling water,
which is chilled as heat is transferred to the seawater.
38
ILLUSTRATED GUIDE TO MECHANICAL COOLING
© BSRIA BG 1/2010
Schematic of a surface water cooling system.
COOLING DESIGN AND APPLICATION ISSUES: KEY
DESIGN AND APPLICATION CHECKS
Chillers
Check at an early stage that the type of refrigerant is approved by the client
Consider product availability, reliability, past performance and spares availability
Check that the chiller type selected is appropriate for the application, location and space
available
Consider the method of heat rejection
Check that the design takes into account ambient and extreme conditions at design location,
for example temperatures at roof level may be higher than ambient design temperatures
Check that noise and vibration generated by the chiller(s) are acceptable, taking into account
any locally occupied areas
Consider part-load operation when selecting chiller(s) to optimise performance and
maximise part-load efficiency
Check that heat can be properly rejected and that recirculation of rejected heat cannot
occur during all prevailing wind conditions
Provide adequate filtration of chilled water – particularly if plate heat-exchangers are used
External air cooled condensers should be corrosion-resistant and have weatherproof motors
Check that the floor/roof loadings are acceptable
Check that there is adequate space around air-cooled condensers to permit unimpeded
airflow
Check the requirement for a refrigerant leakage alarm system.
Absorption chillers
A lower heat source temperature, a higher condenser water temperature or lower chiller
water temperature will reduce the cooling output. This means that a larger, (more
expensive) machine will be required
The heat rejection from an absorption chiller will be greater than a conventional chiller with
the same cooling capacity. This will require larger heat rejection units (for example dry air
coolers or wet cooling towers) for absorption chillers, with consequences of space and
weight
Absorption chillers are slower to start than vapour compression chillers. They are also
slower to respond to changing loads. Therefore, frequent starting and stopping of absorption
chillers should be avoided
Consider the use of an absorption chiller to meet the base load cooling demand in a building.
Peak cooling loads can be met by a conventional chiller
Consider the requirements for a standby heat source, in case the normal heat source (such
as a combined heat power unit) is not available
Consider whether it is more appropriate to size the absorption chiller based on the available
heat source, or on the building’s cooling demand
The temperature of the heat source will determine whether a single or double effect chiller
is appropriate
The use of an absorption chiller in conjunction with a CHP unit will raise the viability and
cost effectiveness of the CHP unit. Most CHP installations are sized on the basis of heat
demand.
This is an abridged list of design checks. A more detailed list of chillers design checks for design
engineers can be found on page 126 in BG 4/2007 Design Checks for HVAC.
ILLUSTRATED GUIDE TO MECHANICAL COOLING
© BSRIA BG 1/2010
39
Cooling towers
Select the appropriate type of cooling tower for the application, such as forced/induced
draught, or open/closed circuit
Check that the installation complies with the relevant Legionella legislation and that the
design follows good practice guidelines
Check that the system is designed in accordance with HSC ACOP L8 – The Control of
Legionella Bacteria in Water Systems and CIBSE TM13 – Minimising the Risk of Legionnaires’
Disease
Select the location of the cooling tower carefully, so that the air discharge - which could be
contaminated - is not carried into air intakes into openable windows, or across public access
routes
Check whether any local industry creates significant amounts of dust or fumes that could
collect in the cooling tower. This may mean that open-circuit cooling towers will not be
appropriate
Cooling towers should be sited away from air intakes and flue outlets
Check that adequate space is available to prevent recirculation
Check that the design noise levels from the cooling tower(s) are acceptable for the
proposed location
Consider the use of plate heat-exchangers to hydraulically isolate chillers from cooling
towers, and to limit the volume of glycol required in the cooling system.
This is an abridged list of design checks. A more detailed list of cooling towers design checks can
be found on page 128 in BG 4/2007 Design Checks for HVAC.
Direct water-side free cooling
See design checks for cooling towers
Check whether the load profile indicates sufficient demand for cooling during months when
free cooling is possible
Consider designing chilled water systems and selecting plant and equipment to operate at
higher chilled water temperatures, in order, to maximise free cooling potential
System design should aim to maximise the number of hours each year that free cooling is
possible
Consider the use of overcooling outside normal hours, in order to cool the building
structure and reduce loads
Design to avoid fouling of the chilled water circuit, such as by water treatment, strainers and
filters
Non-ferrous components should be used wherever possible. The thorough mixing of air and
water in open circuit cooling towers results in a significant amount of air becoming entrained
and held in solution. At points of low static pressure in the chilled water circuit, oxygen can
come out of solution and combine with metal to form oxides. This can lead to corrosion
problems
Consider and specify water treatment requirements. A strict water treatment regime is
needed to protect the chilled water system.
This is an abridged list of design checks. A more detailed list of direct water-side free cooling
design checks can be found on page 134 in BG 4/2007 Design Checks for HVAC.
40
ILLUSTRATED GUIDE TO MECHANICAL COOLING
© BSRIA BG 1/2010
Indirect water-side free cooling
See design checks for cooling towers
Check whether the load profile indicates sufficient demand for cooling during the months
when free cooling is available
Check the highest chilled water temperature that can be used by the chilled water systems
and equipment
Allow for the additional resistance of plate heat-exchangers when sizing pumps, as this can
be substantial. The additional pumping costs incurred should be considered against potential
free cooling when assessing system economics
Overall efficiency of the chilled water system can be improved by operating the plate heatexchanger only when the system is in free cooling mode, and isolating it during conventional
chiller operation
Design to compensate for the effects of fouling. Cooling tower water can cause fouling in
plate heat-exchangers due to a combination of crystallisation, sedimentation and organic
material growth
Check the approach to water treatment is suitable.
This is an abridged list of design checks. A more detailed list of indirect water-side free cooling
design checks can be found on page 136 in BG 4/2007 Design Checks for HVAC.
Ground-coupled cooling
In general, the following considerations are applicable to the UK:
At a soil depth of between 2 – 5 m, the ground temperature is relatively stable at around
12oC all year round
To limit the pressure drops in the piping network, the air velocity in the pipe should be
about 2 m/s
The optimum pipe length is a function of pipe diameter and air velocity. Pipes over 40 m in
length perform efficiently only when of a larger diameter
Creating and maintaining turbulent airflow along the duct length can increase heat transfer
by between 3-8oC for an incoming air temperature range of –1oC to 5OC
Note that most heat transfer occurs at significant bend angles, so introduce bends wherever
possible
A cooling effect of 45 W/m
2
from ground coupling can be obtained with an outside air
2
of ground coupling can be obtained with an outside air
O
temperature of 32 C
A heating effect of 45 W/m
O
temperature of –5 C
Typically, outside air at a temperature of 28 C can be expected to be cooled down to
o
o
around 17 C
Pipes should be installed 1 m apart. This distance prevents thermal interference between
pipes
Minimising fan power will enhance the energy performance of the system
Access should be provided for inspection and cleaning.
Ground water cooling sources – closed-loop systems
The thermal performance of a closed-loop system will be less than that of an open loop
system
As water is not extracted from the aquifer, Environment Agency approval is not required
Systems will not suffer from problems relating to blockage of the loops or boreholes
Glycol/antifreeze required in external closed circuit.
ILLUSTRATED GUIDE TO MECHANICAL COOLING
© BSRIA BG 1/2010
41
Ground water cooling sources – open-loop systems
Supply and return boreholes should be between 100 m and 150 m apart
Borehole depths are typically between 30 and 200 m
Based on an extraction rate of 25 l/s, a peak cooling of 900 kW is possible. This can provide
a typical design cooling load of 50 – 100 W/m2
While capital costs are relatively high, energy operating costs are low compared to
conventional HVAC
The ratio of cooling produced to energy consumed is approximately 10
Favourable ground conditions for aquifers consist of sand or limestone, bounded by tight
layers of clay or similar soil materials (for thermally balanced two-bore systems)
Aim to balance the cooling and heating extracted for systems that are intended to be
thermally balanced (situations with low levels of ground water movement)
Open-loop systems can be susceptible to blockages caused by silt and corrosion from
dissolved salts. Filtration of the extracted water will be required along with possible water
treatment
Drilling costs are a significant factor and problems can be encountered when drilling through
sand layers, pebble beds, gravels and clay
Problems can arise with boreholes silting up due to the settling of suspended solids and algae
growth
The cost and availability of extraction licenses could vary
Changes in local ground conditions could affect water quality and the amount that can be
extracted
Approval for abstraction from and discharge to an aquifer will be required by the
Environmental agency
Plate heat exchangers are required between the open loop circuit and the cooling circuit
serving the building to satisfy EA requirements
There is no requirement for glycol to be introduced into the systems.
Surface water cooling
The viability of a proposed system will depend on the proximity of the building to the cold
water source
The source of water needs to be deep enough to provide water that is sufficiently cool.
Water from rivers may not be cool enough
Effective direct cooling occurs only when intake temperature from the water source is
below 10oC. Water will be cooler at greater depths, however this will incur greater pump
energy
Indirect cooling of condensers in conjunction with mechanical cooling is effective provided
the intake temperature remains below 13oC
Filtration will be required to prevent fouling of the heat exchanger
Salinity of sea water/brackish water may cause corrosion problems. A titanium heatexchanger many be necessary
Cathodic protection can be used to impede marine growth and corrosion in the system
Contact the Environment Agency to determine any restrictions on the extraction and use of
surface water. There may be ecological consequences from raising water temperatures.
42
ILLUSTRATED GUIDE TO MECHANICAL COOLING
© BSRIA BG 1/2010
Constant volume
Benefits
Suited to certain types of public spaces and
foyers; commonly used to provide fresh air
in air/water systems such as fan coil and
chilled beam installations
Can be used for single zone applications,
where only one set of internal conditions
need to be satisfied
Simple and relatively easy to maintain.
Limitations
Not generally suitable for multiple-zone
applications (such as offices), as each zone
will have varying cooling requirements.
CENTRALISED AIR SYSTEMS
Constant volume systems
Constant volume cooling is a simple system often used to provide
tempered fresh air in multi-zone buildings containing partially centralised
air/water systems (such as a fan coil installation). Constant volume
systems can also be used to cool single zones, such as clean rooms, and
operating theatres in hospitals.
As the name suggests, constant volume systems provide a fixed volume of
air at a temperature and humidity determined by the conditions of the
space being served. They are therefore not generally suitable as the
primary cooling system for buildings with multiple zones, as each zone
will have varying cooling requirements. The exception to this is a
constant volume system that incorporates re-heaters in each zone: if the
supply air is too cold for the conditions in a zone, the re-heater will raise
its temperature slightly, thereby providing local control. However, reheaters are no longer widely used since this approach is generally wasteful
of energy.
A major application for constant volume systems is to provide fresh air in
partially centralised air/water systems, such as fan coil or chilled beam
installations. For these applications, a constant volume system would
provide tempered ventilation air, and may meet a small proportion of the
heating or cooling load. The primary heating and cooling requirement
would be met by the room units, such as fan coils or chilled beams.
A simple constant volume air conditioning system.
Some of the supply and extract ductwork for a
constant volume system providing the ventilation in
a building cooled by chilled beams. The small
diameter round ductwork shown is typical of this
type of constant volume system and can be found in
the ceiling voids of many buildings with partially
centralised air/water systems.
ILLUSTRATED GUIDE TO MECHANICAL COOLING
© BSRIA BG 1/2010
43
Variable air volume
Benefits
Variable air volume
Used in buildings with multiple zones to
match the particular cooling/heating
demands of each zone
Variable air volume (commonly known as VAV) cooling is an all-air
system which can satisfy the individual cooling requirements of multiple
zones, typically within office buildings. This is achieved by supplying air
at a constant temperature from a central plant to one or more VAV
terminal units in each zone.
Can be relatively energy efficient due to the
ability to reduce the speed of the
supply/extract fan(s) during periods of low
to moderate loads
The terminal units contain thermostatically-controlled dampers which
regulate the amount of air entering the zone in response to the
requirement for cooling. For example, the volume of air (and hence the
cooling) supplied to a south facing zone on a sunny day will be higher to
offset the heat gain.
A correctly designed and commissioned
system will give good temperature control
VAV systems are particularly suited to
buildings with a year-round cooling load.
Limitations
The primary benefit of VAV over constant volume is its ability to
simultaneously provide the required level of cooling to any number of
zones within a building. VAV systems can be particularly energy efficient
as a result of their ability to operate the main supply/extract fan(s) at
reduced speeds for much of the year, when the overall volume of air
required by the various zones is low (fans are generally the most
significant user of energy in a centralised air system).
Space requirements are high in both the
plant room and ceiling voids
Design and commissioning is particularly
important if good system performance is to
be achieved in terms of comfort and energy
efficiency
The design of some VAV systems is
simplified by allowing the terminal units to
bypass air that is not required. However
this approach can result in oversized plant,
wasteful fan power, and increased capital
cost
When the cooling load is low, the VAV terminal unit will throttle the
supply air down to a minimum level of around 40 per cent of the
maximum volume. There are two reasons why the volume cannot be
allowed to go lower. First, the minimum requirement for fresh air must
be maintained, and second, the velocity of air leaving the diffuser must
not drop too low if an acceptable level of air circulation is to be ensured
within the space.
Fan-assisted terminal units generally have
higher capital and maintenance costs and
the potential for increased noise levels in
the occupied space.
Most types of VAV terminal unit can incorporate a heating device,
which can boost the temperature of the supply air if conditions within
the zone require it. Alternatively, perimeter zones can be heated by
radiators or convectors. In many buildings, perimeter heating is the
preferred option as it helps counter the effect of cold down-draughts
from windows.
Fan-assisted VAV
The use of fan-assisted terminal units can ensure that dumping of the
supply air will not occur. These units contain a small fan which mixes
the supply air with re-circulated room air, and provides a virtually
constant volume supply to the occupied space. The mixing of the two
streams is controlled to achieve an air temperature that satisfies the
cooling load for the zone being served. The disadvantages of fan-assisted
terminal units are generally higher capital and maintenance costs,
increased total fan power, and the potential for high noise levels in the
occupied space.
Source: TROX UK Ltd
At low velocities the Coanda effect (that which causes the air to adhere
to and move along the underside of the ceiling) will be lost and the air
will dump from the diffusers, leading to cold draughts. It is particularly
important that VAV systems are designed and commissioned to a high
standard to ensure that a satisfactory balance between cooling needs and
ventilation rate is achieved.
Example of a VAV terminal unit.
Configuration of a basic VAV system for a single
zone.
44
ILLUSTRATED GUIDE TO MECHANICAL COOLING
© BSRIA BG 1/2010
Variable-geometry supply diffusers
Variable-geometry supply diffusers can be used to ensure adequate air
movement when the supply volume is low. These contain a mechanism
which varies the size of the outlet aperture in response to the volume of
air delivered. For low volumes the aperture is throttled so that the air
velocity leaving the diffuser is maintained at a sufficient level to ensure
good air distribution.
Air movement
For air-based cooling systems to be effective, it is important that the air is
distributed evenly within the occupied space. Air diffusers are used to
supply air to a space as they provide control of airflow and direction. The
system designer will select the most appropriate type(s) of diffuser for the
space, taking into account the following points.
A schematic of a variable-geometry supply diffuser.
Use of space and required aesthetics of diffuser
Constraints imposed by layout and structure
Partitioning of space
Volume flow rate of air
Maximum noise levels
Length and type of throw required.
Some examples of common diffuser types are pictured here.
Example of a cone type ceiling diffuser.
Source: Dravo Environmental Services
An example of a linear diffuser with a single slot
outlet.
An example of jet type diffusers, usually used in factory or shed applications
A four slot linear diffuser installed as a continuous
span along the perimeter wall.
An enhanced ceiling appearance and simpler installation is possible by
combing the diffuser with other overhead surfaces.
ILLUSTRATED GUIDE TO MECHANICAL COOLING
© BSRIA BG 1/2010
45
Fan coil systems
Fan coil systems
Fan coils come in a variety of shapes and sizes, but can be broadly
divided between the perimeter under-window type, and overhead units
mounted above a false ceiling.
A typical fan-coil unit comprises a fan, a heating coil, a cooling coil and
an air filter, all housed in a metal casing. The fan draws a combination of
room air and fresh air through the filter and across the heating and
cooling coils. The air then passes into a plenum which, for units
mounted above false ceilings, has multiple outlets for connection to one
or more supply diffusers. Flexible ductwork is often used to make this
connection. Perimeter fan coils discharge air directly into the space
through a linear grille at the top of the unit.
Perimeter fan coils take up floor space, but are easily accessible for
maintenance. They are also ideally placed to counter cold downdraughts
from glazing.
Benefits
Ideally suited to buildings with multiple
zones
Excellent temperature control and quick
response to changes in heat gains to the
space and/or control settings
Compared with an all-air system, fan coils
have a relatively small diameter ductwork,
as they only have to carry air for
ventilation. This can help keep the depth of
the ceiling void to a minimum, to maximise
the height of the occupied space
Compared with all air systems, fan coil
systems need a smaller air-handling plant for
fresh air. This reduces the space required in
the plant room
Enclosures for perimeter fan coils can be
designed by an interior designer to achieve
a particular appearance, and constructed as
part of the builder’s work.
Fan coil systems can satisfy the individual heating and cooling
requirements of multiple zone buildings and enable good building
flexibility. A zone may be served by one or more fan-coil units
depending on the level of heating/cooling required. Fan coil systems are
only partially centralised, as fresh air is ducted to each unit from the
central plant, along with hot or chilled water.
Limitations
Although fan coil systems generally require a ducted fresh air supply, the
associated air-handling plant and ductwork occupies relatively little space
compared to an all-air system like VAV. This requires a much greater
quantity of air to satisfy heating, cooling, and ventilation segments
together.
Systems operating with a low chilled-water
temperature will create condensation on
the cooling coils and will consequently
require each unit to be connected to a
condensate drain
While fan coils can provide good environmental control and air
movement, the maintenance requirements should not be underestimated.
Each unit contains a filter which requires regular cleaning/changing.
Accessing the units in occupied groups can be time consuming, and
disruptive to occupants.
Generally, high quality fan coils have good acoustic alternation. This
makes them very quiet, but noise can be a problem when the fan is
operating on a high-speed setting. However, this may be required to
achieve the required output from a unit in terms of its cooling duty
and/or air volume.
Each fan coil unit incorporates a filter which
requires regular cleaning/changing and can
be difficult to access
Internal fans can be noisy when operating at
high-speed settings
General maintenance requirements can be
more onerous than that for an all-air
system.
A separate ducted air system is required to
provide air for ventilation (see section on
Constant Volume Systems page 43)
There is a risk of water leaking from
overhead fan coils into the space below
Perimeter fan coils can occupy valuable
floor space.
Two main types of fan coil units are available:
Water-side fan coil units
Air-side fan coils.
Basic configuration of a perimeter fan-coil unit.
46
ILLUSTRATED GUIDE TO MECHANICAL COOLING
© BSRIA BG 1/2010
Water-side fan coil units
Water-side fan coil units are by far the most popular type in the UK.
With this type of unit the heating or cooling provided is modulated by
varying the flow rate of hot water or chilled water.
Most water-side fan coil units are four pipe units that allow units to be
heated or cooled independently in different areas of the building. Either
separate heat exchangers are used for heating or cooling, or a single heat
exchanger is used in conjunction with a changeover valve. The simpler
two-pipe system uses a single supply of water to the unit. It is therefore
not possible to have different units heating or cooling simultaneously
unless they are equipped with separate hydronic circuits.
Basic configuration of a high-level fan-coil unit.
Air-side fan coil units
Air-side fan coil units control their output by varying the flow of air
passing over either a heating coil or a cooling coil. Dampers within the
unit are adjusted to provide the necessary flow path. In heating mode the
dampers are adjusted to provide a flow of air over the heating coil. In
cooling mode the air flow is directed over the cooling coil.
Water-side fan coil units are generally more energy efficient than air-side
units and are physically smaller, although they are more time consuming
to commission and have higher maintenance requirements due to the
control valves. The higher energy consumption associated with air-side
units relates to carryover of heat, as it is difficult to fully isolate each coil.
Also, as each unit is simultaneously provided with both hot and chilled
water, standing heat losses and gains occur, which is also inefficient.
Basic configuration of a high-level fan-coil unit. A
tray collects condensation that can form on the
chilled water pipework. The condensate can either
be pumped to a local drainage point using a small
electric pump, or allowed to flow by gravity. The
gravity option requires sufficient fall in the drain
pipe, which can be a problem in shallow ceiling voids.
The output from a fan coil unit can also be varied by altering the fan
speed. Traditionally this has been achieved through the use of a multitapped auto-transformer with typically two or three speed settings. An
energy efficient alternative to this approach is to use direct current (DC)
motors with electronic commutation. In addition to providing variable
speed control, DC motors provide significantly lower levels of energy
consumption compared to alternating current (AC) motors.
These two images show a perimeter
overhead fan-coil unit before and after the
ceiling grid is installed. Flexible ductwork
from each unit supply air to linear diffusers is
located above the glazing. The round duct
running parallel to the rear of the fan-coil
units provides fresh air to the fan-coil units.
ILLUSTRATED GUIDE TO MECHANICAL COOLING
© BSRIA BG 1/2010
47
Chilled beams
Chilled beams
Chilled beams are simple devices. Mounted at high level within a space,
they cool the surrounding air, causing it to travel downwards into the
occupied area below.
Chilled beams comprise long rectangular units containing a finned tube
through which chilled water is pumped. Warm air rising up in the space
passes over the beams, where it is cooled and falls back into the space due
to its negative buoyancy. The simplest type of chilled beam, without
moving elements, is sometimes referred to as a passive beam.
Beams are typically arranged at regular intervals above, or partly below, a
suspended ceiling, and usually require a minimum ceiling void depth of
around 300 mm. Some beams are suitable for suspension below the
ceiling. As good air flow is essential, any ceiling tiles positioned directly
below beams must have openings within the tiles equivalent to at least 50
per cent of their area.
Active chilled beams are also available which incorporate small fans to
assist air movement. These types of chilled beam systems require a
separate ventilation system to supply fresh air to the space (see section on
Constant Volume Systems, page 43).
Multi-service chilled beams (sometimes abbreviated to MSCB) can
combine a range of building services functions within a single unit,
including:
Cooling and heating
Ventilation supply
Electric uplighting, downlighting and emergency lighting
Sensors for computerised controls, control valves and
condensation detectors
Fire alarms and sprinkler systems
Passive infrared detectors for occupancy control or daylight
Benefits
Provide a quiet, draught-free operation
Chilled beams require a relatively small
depth of ceiling void, which can free up
space for raised floors (which is in a building
with low floor to ceiling heights)
Maximum cooling outputs are in the order
of 100 W/m2 - 160 W/m2, which is
significantly higher than chilled ceilings (see
section on Chilled Ceilings, page 50)
Elevated chilled water temperatures can be
used, offering the potential for sources of
chilled water other than generated by a
refrigeration cycle, such as water from
rivers, evaporative and dry cooling systems,
and ground water
Minimal maintenance requirements
Good at coping with perimeter heat gains
Some types of chilled beam are designed to
allow ventilation air to be ducted directly
into the unit, which can increase the
system’s cooling output.
Limitations
A separate ducted ventilation system is
likely to be required (see section on
Constant Volume Systems, page 43)
Control of water flow temperatures can be
relative to room dewpoint temperature,
provided a temperature differential (around
2qC) is maintained to avoid condensation
Insufficient cooling capacity for spaces with
2
heat gains greater than160 W/m , although
top-up cooling can be provided separately
via a ventilation system.
sensing
Public announcement and voice alarm speakers
Acoustic insulation
Pipework, ductwork and trunking for electrical cables.
Typical operation of a passive chilled beam.
48
ILLUSTRATED GUIDE TO MECHANICAL COOLING
© BSRIA BG 1/2010
The benefits of multi-service chilled beams will depend on the specific
installation but can comprise the following:
Can use either active or passive chilled beam technologies
They can be used where floor-to-slab height is low
A range of services can be provided in a single unit. This
provides benefits including: a single source of contractual
responsibility, reduced installation costs, factory assembly and
commissioning of several systems in one integrated component
The aesthetic appearance of a chilled beam can be customised to
meet client requirements.
Source: TROX UK Ltd
A view looking up at newly installed chilled beams
that will be hidden behind a perforated suspended
ceiling, as shown in the centre of this picture.
Example of a completed chilled beam installation.
A row of chilled beams prior to installation of the suspended ceiling.
ILLUSTRATED GUIDE TO MECHANICAL COOLING
© BSRIA BG 1/2010
49
Chilled ceilings
Benefits
Chilled ceilings
Can be accommodated in a very shallow
ceiling void of around 60 mm – 70 mm. This
can free up space for raised floors in
buildings with low floor-to-ceiling heights
Chilled ceilings are simple devices mounted at high level within a space.
They are supplied with chilled water to provide a combination of radiant
and convective cooling to the space below.
Each unit typically comprises a small bore chilled-water pipe arranged in
a serpentine pattern and attached to the upper surface of a thin metallic
ceiling panel. Alternatively, the pipe may be embedded within the panel,
in which case it is likely to be made from polypropylene. The panel is
cooled through contact with the chilled-water pipework which, in turn,
cools the space with a combination of convective and radiant cooling (up
to 40 per cent radiative).
Provide quiet, draught-free comfort cooling
A key benefit of chilled ceilings is that they can be accommodated in a
very shallow ceiling void and are therefore suited to buildings with
minimal floor to ceiling heights. However, the limited cooling output
can preclude them from use in environments with heat gains greater than
2
70 W/m C.
Limitations
Chilled-ceiling systems require a separate ventilation system to supply
fresh air to the space.
Control of water flow temperatures can be
relative to room dewpoint temperature,
provided a suitable temperature (around
2qC) differential is maintained to prevent
condensation.
Elevated chilled water temperatures are
used, offering the potential for sources of
non-refrigerated chilled water from: lakes,
rivers, evaporative and dry cooling systems,
and ground water
Low maintenance requirements.
Maximum cooling output is around only
2
70 W/m . However, chilled-ceiling systems
are often installed with a displacement
ventilation system which provides a further
2
25 - 65 W/m of cooling
Source: TROX UK Ltd
Example of a chilled-ceiling panel showing the
serpentine chilled-water pipe attached to a
perforated metal ceiling panel.
Accessing a chilled-ceiling panel. The serpentine
chilled-water pipe is attached to the top side of the
hinged panel.
50
ILLUSTRATED GUIDE TO MECHANICAL COOLING
© BSRIA BG 1/2010
Room-based heat pumps
Benefits
In buildings with unbalanced heating/cooling
loads, waste heat can be transferred from
one area and used to heat another area
Occupant control for individual units can be
provided, which is a useful feature in
buildings such as hotels and offices
Quick response
Overall reliability is good, as the cooling
process is spread across many room units
and the failure of a single unit does not have
a major effect on total cooling capacity.
Limitations
A separate ducted air system will be
required to provide ventilation (see section
on Constant Volume Systems, page 43)
Room units are relatively heavy
Room units can require specialist
maintenance
Careful system design is required to avoid
excessive noise.
LOCAL SYSTEMS
Room-based heat pumps
Cooling systems are available that are capable of also working in heating
mode.
Individual room units, which can be floor standing or concealed in a
void such as the ceiling, are linked by a piped water circuit that runs
around the building. Each unit operates independently and is able to heat
or cool the air in the immediate area. This is achieved by means of a
small heat pump in the unit which takes low grade heat from the water
circuit and uses it to heat the room. To cool the room, the heat pump
works in reverse to remove heat from the space and transfer it to the
water circuit.
A key benefit of this type of system is the ability to save energy by
transferring heat from one area to another by virtue of the piped water
system. Any additional heating required by the system is provided by
either a small boiler, electric heater or a link to a separate hot water
circuit in the building. Any additional cooling is performed by one or
more dry coolers or cooling towers (see section on Central Systems –
Cooling towers, page 28) which remove waste heat from the water
circuit. As with fan coil systems, a separate ventilation system may be
required (see section on Constant Volume Systems, page 43).
A variation is to use another heat pump on the water loop to add or
remove heat.
A simple schematic showing a typical local cooling
system.
ILLUSTRATED GUIDE TO MECHANICAL COOLING
© BSRIA BG 1/2010
51
Split systems
Split systems
Split systems provide a convenient way to cool small buildings or specific
areas within a building. Typical applications include shops, garages,
restaurants and office areas. They are sold as a package making them
quick to install with minimal disruption to building occupants. Another
key benefit of split systems is that they do not require any form of
centralised plant space within the building.
As the name suggests, split cooling systems are made up of two basic
components: one or more indoor room cooling units, and an outdoor
refrigeration unit which dumps heat taken from the building. The indoor
and outdoor units are linked by pipes which transport refrigerant
between the units. There are four basic options for locating the indoor
units, and these are illustrated in the schematic on this page.
The cooling capacity of split systems ranges from approximately 2 kW to
30 kW. The higher capacity systems can incorporate several indoor units,
or a concealed fan-coil unit can be installed which has one or more
ducted outlets.
Some split systems can operate as a heat pump, whereby they are able to
provide heating by reversing the refrigeration process. The benefit of this
is that a building’s heating and cooling needs can be provided by one
system.
Benefits
Relatively quick and easy to install
Do not require any plant room/area within
a building
Heat pump systems can provide heating and
cooling
The indoor unit can be concealed if
required
Simple occupant control can be provided,
with the option of an infrared remote
control
Some concealed indoor fan coil-type units
can be configured to provide fresh air in
addition to re-circulating the room air.
Limitations
Only suitable for relatively small spaces
Typically require a specialist service
operative for repairs and maintenance
Can only service a single internal zone;
systems with multiple indoor units cannot
provide simultaneous heating and cooling in
different areas (see section on VRF
systems)
Simple split systems only re-circulate room
air and cannot provide ventilation
Outdoor units can be unsightly
Noise.
An example of a ceiling-mounted cassette unit in a meeting room.
A schematic showing the four basic option for
locating the units for a split system.
Typical wall mounted outdoor condenser units. The
same units could alternatively be located on the
ground or on a rooftop.
52
ILLUSTRATED GUIDE TO MECHANICAL COOLING
© BSRIA BG 1/2010
VRF systems
Benefits
Variable refrigerant flow systems
Capable of providing simultaneous heating
and cooling
A variable refrigerant flow (VRF) cooling system is essentially a more
complex split system (see section on Split Systems, page 52). The
difference is that VRF systems can provide heating or cooling from each of
the indoor units on an individual basis. The internal units can be located in
the same basic positions as described for split systems (see section on Split
Systems). This is particularly useful in applications such as office blocks,
hotels and large retail stores which may need cooling in some areas and
heating in others. Split systems are not capable of simultaneous heating and
cooling unless a dedicated system is installed for each zone.
Relatively quick and easy to install
Does not require any form of plant
room/area within the building
Simple occupant control provided, with the
option of an infrared remote control.
Alternatively, a central control system can
be specified
VRF systems contain complex microprocessor-based electronics, which
are needed to ensure efficient operation. Central to VRF control is the
ability to automatically vary the flow refrigerant from the outdoor unit in
response to the heating/cooling load of the building. Occupant control is
very simple, with easy to use wall-mounted keypads or hand-held remote
controllers allowing individual control of room units. Alternatively,
overall control can be performed by a computer, which itself can be
linked to the building’s central building management system.
Some concealed indoor fan-coil type units
can be configured to provide fresh air in
addition to re-circulating room air
Low noise levels in operation.
Limitations
Significant amount of refrigerant passes
through occupied spaces. This could
potentially cause a problem if a leak occurs
VRF systems typically require a specialist
service technician for repairs and
maintenance
VRF ceiling cassettes serving an office environment.
Source: Toshiba Air Conditioning
Example of a perimeter VRF fan-coil unit concealed
in an architectural enclosure. This approach could
equally be applied to a split system, and the other
options for indoor units depicted for split systems
are equally relevant to VRF systems.
Source: Mitsubishi Electric
The system must be installed to a high
standard to ensure good performance and
reliability.
The outdoor condensing units for a relatively large
VRF system.
Many VRF systems can provide simultaneous heating and cooling to match
the comfort requirements in different parts of the building.
ILLUSTRATED GUIDE TO MECHANICAL COOLING
© BSRIA BG 1/2010
53
TYPES OF COOLING KEY DESIGN AND
APPLICATION CHECKS
Constant air volume systems
Check the variation between zone loads and review whether a constant air volume system is
the most appropriate and energy efficient choice
Check that adequate outside air will be provided for each zone under all operating
conditions
Check minimum and maximum acceptable supply temperatures to the space
Check that the air system is balanced, and that other systems do not interfere, such as a
toilet extract system
Check for any simultaneous heating and cooling requirements, and zone the system
accordingly
Check system velocities are appropriate to the application, considering noise and energy
efficiency
Check enough space is available within the ceiling void to house the distribution system
Choose a suitable control strategy for the system
Check whether free cooling can be used to save energy
Consider the use of heat recovery devices wherever possible
Check that specific fan power does not exceed that stated in the Building Regulation
Approved Document L2.
This is an abridged list of design checks. A more detailed list for constant volume systems design
checks can found on page 94 in BG 4/2007 Design Checks for HVAC.
Variable air volume systems
Consider the choice of VAV system with respect to system requirements and energy
efficiency, such as fan-assisted terminals, terminal reheat, and induction VAV
VAV control needs very careful consideration to ensure that fresh air requirements, heating
and cooling requirements and adequate room air diffusion can be achieved
Check that the fresh air requirements for all zones are met under all possible operating
conditions
Check that room air diffusion patterns are acceptable at low volume flows, with no stagnant
areas in the occupied zone
Reheat coils may be needed on internal zones and other areas with low relative heat gains,
to prevent overcooling
The use of variable geometry diffusers should be considered
Check that the noise output is acceptable at high volume flows
Consider any known future needs or flexibility requirements when positioning VAV.
This is an abridged list of design checks. A more detailed list of VAV design checks can be found
on page 102 in BG 4/2007 Design Checks for HVAC
54
ILLUSTRATED GUIDE TO MECHANICAL COOLING
© BSRIA BG 1/2010
Fan coils
Check that the manufacturer’s thermal and acoustic data is applicable to the conditions at
which the fan coils will be operating
Check performance is acceptable for both sensible and latent cooling requirements
Check that the noise level is acceptable for the usage of the space. High fan-speed settings
can create noise problems
Check the supply air temperature off the fan coil under both heating and cooling. Too low a
temperature gives dumping and draughts, too high a temperature creates stratification and
discomfort at foot level
Check that the throw from diffusers is satisfactory under both heating and cooling
conditions
Check whether air-side or water-side control is appropriate
Gravity-fed condensate drainage systems require sufficient fall in the pipework for adequate
runoff. Shallow ceiling voids may not permit long drainage pipe runs. A pumped condensate
system may be required
Allow adequate access to fan coil units for cleaning, filter replacement, and general
maintenance.
This is an abridged list of design checks. A more detailed list of fan coil design checks can be found
on page 86 in BG 4/2007 Design Checks for HVAC.
Chilled beams
Check the selection data meets design requirements
Decide whether passive or active beams are the most appropriate choice
Check that there is space available in the ceiling void – some passive chilled beams require a
clear space of some 300 mm above the beam for adequate air circulation. This can give a
total required ceiling depth in excess of 600 mm
Check that the operating conditions and chilled water temperatures will not lead to
condensation forming on the beams
Consider providing condensation control for chilled beams
If open chilled beams are used, particular care is needed to prevent these being
overwhelmed by perimeter solar-driven updraughts
If used in conjunction with displacement ventilation, control of both systems needs careful
consideration and scheduling.
This is an abridged list of design checks. A more detailed list of chilled beams design checks can be
found on page 98 in BG 4/2007 Design Checks for HVAC.
ILLUSTRATED GUIDE TO MECHANICAL COOLING
© BSRIA BG 1/2010
55
Chilled ceilings
See key checks for chilled beams
Check client and architect approval for the type of suspended ceiling
Check the panel design selected has an acceptable balance between acoustic performance
and thermal performance
Chilled ceilings are best when a separate fresh air supply system is provided, such as lowlevel supply or displacement ventilation
Check that the temperature gradient along long panel runs gives an acceptable horizontal
space temperature gradient
Allowance should be made for the higher radiant cooling that will be achieved at the room
perimeter, as warm surfaces will give a higher temperature difference and thus a higher total
cooling effect
Perimeter outer zones will require separate control and possibly the use of chilled beams
where heat gains and losses are markedly different to adjacent inner zones
Hinged ceiling access panels work well, but ensure that good flexible connections are used
for the pipework.
This is an abridged list of design checks. A more detailed list of chilled ceiling design checks can be
found on page 100 in BG 4/2007 Design Checks for HVAC.
Variable refrigerant flow systems (VRF)
Consider the choice of VRF system with respect to system requirements and energy
efficiency, such as cooling only, heating and cooling, or simultaneous heating and cooling
Check that the refrigerant to be used complies with the latest regulations
Ensure that the fresh air requirements will be met for spaces served by VRF systems
Consider the best position for indoor units, for example above ceiling, ceiling suspended,
wall or floor installation
Check noise levels are acceptable at high outputs
Check that external units do not recirculate air or exhaust air to other unit inlets
Consider refrigerant pipework routes carefully to minimise runs through occupied spaces
Check maximum permissible vertical and total refrigerant pipework runs
Check that the energy efficiency ratio and the controls system meets the requirements of
the Approved Document L2 second-tier document Non-Domestic, Heating, cooling and Ventilation
Compliance Guide
Indoor unit control requirements should be established, such as room sensors or return air
sensor controls.
This is an abridged list of design checks. A more detailed list of variable refrigerant flow system
design checks can be found on page 104 in BG 4/2007 Design Checks for HVAC.
56
ILLUSTRATED GUIDE TO MECHANICAL COOLING
© BSRIA BG 1/2010
COMMISSIONING
It is important that a cooling system is properly commissioned in order
for the desired comfort conditions to be provided without excessive
energy consumption. The proper commissioning of a cooling system is a
requirement of Approved Documents L2A and L2B.
Depending on the type of cooling system, commissioning procedures can
include:
Flushing of water-based systems to remove debris
Commissioning of chilled and condenser cooling water systems
Commissioning of air-handling systems and distribution
ductwork
Commissioning of the refrigeration plant
Commissioning of electrical supply/equipment
Commissioning of associated control system
Performance of combined pressure and refrigerant
Setting to work and adjusting.
Guidance on the commissioning of cooling systems is given in the
following publications:
CIBSE Commissioning Code R: Refrigeration Systems
CIBSE Commissioning Code W
CIBSE Commissioning Code A: Air Distribution Systems
BSRIA AG2/89.3: Commissioning Water Systems – Application
BSRIA AG3/89.3: Commissioning Air Systems – Application
Procedures.
Guidance on the cleaning of pipework systems is given in the BSRIA
publication AG1/2001.1: Pre-commissioning Cleaning of Pipework Systems.
ILLUSTRATED GUIDE TO MECHANICAL COOLING
© BSRIA BG 1/2010
57
MAINTENANCE AND UPKEEP
Inspection of air conditioning systems
Cooling systems are generally reliable but can be inefficient. This often
goes unnoticed until a very hot day when it becomes apparent, however
the Energy Performance of Buildings (Certificates and Inspections) (England and
Wales) Regulations 2007 requires regular inspection of all cooling systems
with rated outputs over 12 kW at intervals not greater than five years.
The main aim of the inspections is to give building owners and operators
information about the performance of their cooling system and to
identify opportunities to save energy and cut operating costs.
CIBSE TM44 Inspection of Air Conditioning Systems gives practical
guidance concerning the implementation of the Regulations.
As the organisation responsible for the regulations, the Department of
Communities and Local Government have reported on their first air
conditioning inspection, and the consequent improvement of 12-6% energy performance (Building Services and Environment Engineer, July
2009).
F-gas Regulation
The EU F-gas Regulation imposes obligations on operators and
contractors relating to cooling systems that use fluorinated greenhouse
gas-based refrigerants. F-gases include HFC refrigerants such as R134a,
R407C, and R410A.
For systems with over 3 kg of refrigerant charge (6 kg if hermetic),
operators must:
Prevent leakage, and repair any leaks as soon as possible
Arrange proper refrigerant recovery by certified personnel during
servicing and disposal
Carry out leak checks to a defined schedule
Ensure that only certified competent personnel carry out leakage
checks
Maintain records of refrigerants and of servicing.
The requirements for refrigerant leakage checking varies depending on
the amount of refrigerant charge as follows:
At least annually for applications with 3 kg or more of F-gases
(6 kg if hermetically sealed)
At least once every six months for applications with 30 kg or
more of F-gases
At least once every three months for applications with 300 kg or
more of F-gases
Leakage detection systems must be installed on applications with
300 kg or more of F-gases; when these are in place, checking
requirements are halved
If a leak is detected and repaired, a further check must be carried
out within one month to ensure that the repair has been
effective.
58
ILLUSTRATED GUIDE TO MECHANICAL COOLING
© BSRIA BG 1/2010
Operators of systems containing 3 kg or more of F-gases must maintain
records including:
Quantity and type of F-gas installed, added or recovered
Identification of the company or technician carrying out the
servicing
Dates and results of leakage checks, specifically identifying
separate pieces of equipment containing 3 kg or more of
refrigerant.
Contractors are affected by minimum requirements for training and
certification of companies and personnel involved in installation,
maintenance, servicing, containment, and recovery activities.
Legionnaires’ disease
Cooling towers and evaporative condensers can have the potential to be
a source of Legionnaires’ disease. As such the Notification of Cooling Towers
and Evaporative Condensers Regulations 1992 requires that persons in
control of non-domestic premises notify their local authority in writing
of any “notifiable devices” situated on the premises. Notifiable devices
include all cooling towers or evaporative condensers except where the
water is not exposed to air.
The HSC Approved Code of Practice L8: Legionnaires’ Disease – the Control
of Legionella Bacteria in Water Systems provides a strategy and detailed
guidance on minimising the risk of Legionnaires’ disease.
Supplementary guidance is given in the CIBSE publication TM13
Minimising the Risk of Legionnaires’ Disease.
ILLUSTRATED GUIDE TO MECHANICAL COOLING
© BSRIA BG 1/2010
59
STANDARDS AND REQUIREMENTS
Cooling systems can consume considerable amounts of energy. In response to
this, Approved Document L2A – Conservation of Fuel and Power in New Buildings
Other Than Dwellings and L2B Conservation of Fuel and Power in Existing Buildings
Other Than Dwellings along with the associated document The Non-Domestic
Heating, Cooling and Ventilation Compliance Guide impose energy conservation
requirements on mechanical cooling systems. Specific requirements are:
A minimum energy efficiency ratio (the ratio of the cooling energy
delivered into the cooling systems, divided by the energy input to the
cooling plant)
A controls package providing independent control of each terminal unit,
along with the avoidance of simultaneous heating and cooling in a zone.
In addition, multiple cooling modules require controls to provide the
most efficient operating modes for the combined plant.
In order to ensure the energy efficient operation of existing air conditioning
systems the Energy Performance of Buildings Directive requires the regular inspection
of air conditioning systems with rated outputs over 12 kW.
Further operational requirements are covered by the F-gas Regulation. The
Regulations aim to minimise emissions of fluorinated greenhouse gases that are
used in some cooling refrigerants. Further details of the Energy Performance of
Buildings Directive and the F-gas Regulation are given in the section on
maintenance and upkeep.
Cooling systems that incorporate evaporative cooling devices such as cooling
towers can present a risk of Legionnaires’ disease. The HSC’s Approved Code of
Practice L8: Legionnaires’ Disease – the Control of Legionella Bacteria in Water Systems
provides a strategy and detailed guidance on minimising the risk of incubating
the Legionella doctrine in cooling towers and reducing the risk of Legionnaires’
disease.
Standards
BS EN 378-1:2008 Refrigerating Systems and Heat Pumps. Safety and Environmental
Requirements. Basic Requirements, Definitions, Classification and Selection Criteria
BS EN 378-2:2008 +A1:2009 Refrigerating Systems and Heat Pumps. Safety and
Environmental Requirements. Design, Construction, Testing, Marking and
Documentation
BS EN 378-3:2008 Refrigerating Systems and Heat Pumps. Safety and Environmental
Requirements. Installation Site and Personal Protection
BS EN 378-4:2008 Refrigerating Systems and Heat Pumps. Safety and Environmental
Requirements. Operation, Maintenance, Repair and Recovery
BS EN 12102:2008 Air Conditioners, Liquid Chilling Packages, Heat Pumps and
Dehumidifiers with Electrically Driven Compressors for Space Heating and Cooling.
Measurement of Airborne Noise. Determination of the Sound Power Level
BS EN 12309-1:2000 Gas-fired Absorption and Adsorption Air-Conditioning and/or
Heat Pump Appliances with a Net Heat Input not exceeding 70kW. Safety
BS EN 12309-2:2000 Gas-fired Absorption and Adsorption Air-Conditioning and/or
Heat Pump Appliances with a Net Heat Input not exceeding 70kW. Rational Use of
Energy
BS EN 12599:2000 Ventilation for Buildings. Test Procedures and Measuring Methods
for Handing Over Installed Ventilation and Air Conditioning Systems
60
ILLUSTRATED GUIDE TO MECHANICAL COOLING
© BSRIA BG 1/2010
BS EN 12693:2008 Refrigerating Systems and Heat Pumps. Safety and Environmental
Requirements. Positive Displacement Refrigerant Compressors
BS EN 13053:2006 Ventilation for Buildings. Air Handling Units. Rating and
Performance for Units, Components and Sections
BS EN 13771-1:2003 Compressor and Condensing Units for Refrigeration. Performance
Testing and Test Methods. Refrigerant Compressors
BS EN 13771-2:2007 Compressor and Condensing Units for Refrigeration. Performance
Testing and Test Methods. Condensing Units
BS EN 13779:2007 Ventilation for Non-Residential Buildings. Performance
Requirements for Ventilation and Room-Conditioning Systems
BS EN 14240:2004 Ventilation for Buildings. Chilled Ceilings. Testing and Rating
BS EN 14276-2:2007 Pressure Equipment for Refrigerating Systems and Heat Pumps.
Piping. General Requirements
BS EN 14511-1:2007 Air Conditioners, Liquid Chilling Packages and Heat Pumps with
Electrically Driven Compressors for Space Heating and Cooling. Terms and Definitions
BS EN 14511-2:2007 Air Conditioners, Liquid Chilling Packages and Heat Pumps with
Electrically Driven Compressors for Space Heating and Cooling. Test Conditions
BS EN 14511-3:2007 Air Conditioners, Liquid Chilling Packages and Heat Pumps with
Electrically Driven Compressors for Space Heating and Cooling. Test Methods
BS EN 14511-4:2007 Air Conditioners, Liquid Chilling Packages and Heat Pumps with
Electrically Driven Compressors for Space Heating and Cooling. Requirements
BS EN 14518:2005 Ventilation for Buildings. Chilled Beams. Testing and Rating of
Passive Chilled Beams
BS EN 15116:2008 Ventilation in Buildings. Chilled Beams. Testing and Rating of
Active Chilled Beams
BS EN 15218:2006 Air Conditioners and Liquid Chilling Packages with Evaporatively
Cooled Condenser and Electrically Driven Compressors for Space and Cooling. Terms,
Definitions, Test Conditions, Test Methods and Requirements
BS EN 15232:2007 Energy Performance of Buildings. Impact of Building Automation,
Controls and Building Management
BS EN 15240:2007 Ventilation for Buildings. Energy Performance of Buildings.
Guidelines for Inspection of Air Conditioning Systems
BS EN 15243:2007 Ventilation for Buildings. Calculation of Room Temperatures and of
Load and Energy for Buildings with Room Conditioning Systems
BS EN 15255:2007 Energy Performance of Buildings. Sensible Room Cooling Load
Calculation. General Criteria and Validation Procedures
BS EN 15423:2008 Ventilation for Buildings. Fire Precaution for Air Distribution
Systems in Buildings
BS EN 1886:2007 Ventilation for Buildings. Air Handling Units. Mechanical
Performance
EN 15500:2008 Control for Heating, Ventilating and Air-Conditioning Applications.
Electronic Individual Zone Control Equipment
SI 2009/216 Ozone Depleting Substances (Qualifications) Regulations 2009
ILLUSTRATED GUIDE TO MECHANICAL COOLING
© BSRIA BG 1/2010
61
REFERENCES/BIBLIOGRAPHY
Building Log Book Toolkit, TM31, CIBSE, 2006, ISBN 1 903287 71 5
Business-Focussed Maintenance – Guidance and Sample Schedules,
BG 3/2004, BSRIA, 2004, ISBN 0 86022 604 2
Chilled Beam Application Guidebook, Guidebook No5, REHVA, 2004,
ISBN 2 9600468 3 8
Commissioning Management, CIBSE Commissioning Code M, 2003,
ISBN 1 903287 33 2
Controls for End Users – A Guide for Good Design and Implementation,
BSRIA/UBT/BCIA 2007, ISBN 978 0 86022 6703
nd
Design Checks for HVAC, 2 Edition, BG4/2007, BSRIA, 2007,
ISBN 978 0 86022 669 7
Design for Improved Solar Shading Control, TM 37, CIBSE, 2006,
ISBN 1 903287 57 5
Efficient Humidification in Buildings, AG 10/94.1, BSRIA, 1995,
ISBN 0 86022 392 2
Free Cooling Systems, BG 8/2004, BSRIA, 2004, ISBN 0 86022 642 5
Guidance for Stationary Refrigeration & Air-Conditioning,
Information Sheet RAC 6 – Practical Guidance, F-Gas Support, 2009
Guidance on Minimising Greenhouse Gas Emissions from Refrigeration, Air
Conditioning and Heat Pump Systems, Information Sheet RAC 7 –
Alternatives, F-Gas Support, 2009
Handover, O&M manuals, and Project Feedback: A Toolkit for Designers and
Contractors, BG 1/2007, BSRIA, 2007, ISBN 978 0860 22 6673
Heat Recovery Systems, BSRIA Guidance Document, 2009
th
Heating & Air Conditioning of Buildings, 10 edition, Martin P L, Oughton
D R, Faber & Kell’s, 2008, ISBN 978 0 7506 8365 4
Hygiene Requirement for Ventilation and Air-Conditioning, Guidebook No 9,
REHVA, 2007, ISBN 2 9600468 8 9
Inspection of Air Conditioning Systems, TM 44, CIBSE, 2007,
ISBN 978 1 903287 85 9
Legionnaires’ Disease – The Control of Legionella Bacteria in Water Systems:
Approved Code of Practice & Guidance, L8, HSE, 2000,
ISBN 0 7176 1772 6
Low Temperature Heating and High Temperature Cooling, Guidebook No7,
REHVA, 2007, ISBN 2 9600468 6 2
Minimising the Risk of Legionnaires’ Disease, TM 13, CIBSE, 2002,
ISBN 1 903287 23 5
R22 Phase Out – Guidance for Owners and Users of Refrigeration Equipment,
Guidance Note 15, October 2007, Institute of Refrigeration
Refrigeration, Knowledge series KS13, CIBSE, 2008,
ISBN 978 1 903287 92 7
62
ILLUSTRATED GUIDE TO MECHANICAL COOLING
© BSRIA BG 1/2010
Refrigeration and Heat Rejection, Guide B4, CIBSE, 2003,
ISBN 1 903287 19 7
Refrigeration Systems, CIBSE Commissioning Code R, 2002,
ISBN 1 903287 28 6
Refurbishment for Improved Energy Efficiency: an Overview, Knowledge series
KS 12, CIBSE/BSRIA, 2007, ISBN 978 1 903287 88 0
Sustainable Low Energy Cooling: An Overview, KS3, CIBSE, 2005,
ISBN 1 903287 62 6
The Effective BMS: A Guide to Improving System Performance,
AG 10/2001, BSRIA 2001, ISBN 086022 580 1
The Illustrated Guide to Renewable Technologies, BG 1/2008, BSRIA, 2008,
ISBN 978 0 86022 672 7
Utilisation of Thermal Mass in Non-Residential Buildings. Guidance on System
Design, Floor Types, Surface Finish and Integration of Services, The Concrete
Centre, 2006, ISBN 1 904482 30 9
Variable-Flow Water Systems – Design, Installation and Commissioning
Guidance, AG 16/2002, BSRIA 2002, ISBN 0 86022 607 7
Variable Speed Pumping in Heating and Cooling Circuits, AG 14/99, BSRIA
1999, ISBN 0 86022533 X
Useful websites
Air Conditioning and Refrigeration Industry Board, Website information,
www.acrib.org.uk
F-Gas Information Sheet RAC 7 – Alternatives, Website information and free
download,
http://www.defra.gov.uk/environment/quality/air/fgas/documents/fgassupportrac7.pdf
ILLUSTRATED GUIDE TO MECHANICAL COOLING
© BSRIA BG 1/2010
63
GLOSSARY OF TERMS
Term
Definition
Building management
system (BMS)
A microprocessor based system that is connected to devices, plant and systems to enable remote monitoring
and control.
Central plant
The siting of plant items such as boilers, chillers and air handling units in a centralised plant room or area,
generally serving all or large parts of the building.
Condensation
pipework
Pipework required to drain condensation from devices such as fan coil units. Condensation pipework needs to
be laid to fall to enable the condensate to be drained.
Constant volume
system
A simple system used to provide a fixed volume of tempered air in multi-zone buildings often associated with a
partially centralised air/water system, such as fan coil units.
Decentralised plant
Plant items located at strategic points throughout the building, serving the local area.
Exhaust air
Air which is exhausted to atmosphere.
Extract air
Stale air removed from a space. Some of this air may be re-circulated and some exhausted.
Fan coil unit
A device mounted in the ceiling void or floor mounted often at the perimeter of a building which comprises a
fan, a heating coil, a cooling coil and an air filter housed in a metal casing. The fan coil unit is supplied with
fresh air via a ductwork distribution network from a central plant. The fan draws a combination of room air
and fresh air through the filter and across the heating and cooling coils. The air then passes into a plenum
which, for units mounted above false ceilings, has multiple outlets for connection to one or more supply
diffusers. Low pressure hot water and chilled water is distributed via pipework to each fan coil unit.
Free cooling
Cooling that can be obtained without operating chillers, for example by using outside air directly when it is at a
low enough temperature.
Natural ventilation
Ventilation air that enters a building by natural means, due to temperature difference and/or wind.
Occupied zone
The volume of space occupied by people - usually from floor level to a height of 1·8 m. In spaces where the
occupants are seated, such as an auditorium, the occupied zone will be smaller.
Plant
Large items of machinery and apparatus. In the case of building services this term is usually used to describe
major pieces of equipment such as boilers and chillers.
Re-circulation air
Often more air is required to heat or cool than is needed to provide ventilation. The excess can sometimes be
re-circulated providing a very effective method of energy recovery. Re-circulation may not be possible when
the air is contaminated with dangerous or unpleasant pollutants such as cigarette smoke.
Relative humidity
A term often used to specify the internal design condition for humidity within a space. A ratio, usually
expressed as a percentage, indicating the humidity of the air. Literally the actual vapour pressure of the air at a
given dry bulb temperature divided by the saturation vapour pressure of the air at the same temperature.
Resultant
temperature
A temperature often used to specify a design condition for a space, it combines air temperature, surface
temperature and air velocity in a single index.
Solar gain
Heat gain caused either by sunshine directly entering a space or by sunshine incident on building fabric, which
absorbs the heat and then transmits it to the space.
System
An organised arrangement of plant and equipment that works together to provide a function such as heating
or cooling.
Terminal
The end point of a system run. An air terminal device is the end point of a ductwork system, such as a grille or
diffuser.
Terminal unit or
device
A unit at the system outlet which usually provides local control, such as a VAV terminal unit. Its primary role is
to supply and direct air via diffuser(s) into the occupied zone at the desired temperature and location.
Thermal response
The time taken for a system or emitter such as a radiator to warm up. Also the time taken for the fabric of a
building to respond to a change in temperature. It can vary from minutes to hours.
Variable air volume
(VAV)
An air conditioning system consisting of centralised plant connected to supply air ductwork distributed in the
ceiling void, which carries variable amounts of air at a given temperature to terminal devices called VAV boxes.
These boxes regulate the amount of air entering the occupied space to suit the varying loads.
Ventilation
Ventilation is primarily the supply of fresh air to a building to meet the needs of the occupants - to provide
oxygen, dilute carbon dioxide and odours to acceptable levels and remove contaminants. The stale (vitiated)
air must also be extracted.
Zoning
The division of a building into a number of distinct zones for the purposes of system control.
64
ILLUSTRATED GUIDE TO MECHANICAL COOLING
© BSRIA BG 1/2010
BSRIA ⳮ the built
environment experts
BSRIA gives you confidence in design, added value in
manufacture, competitive advantage in marketing,
profitable construction, and efficient buildings
Whatever your building
services requirement
contact BSRIA:
T: +44 (0)1344 465600
F: +44 (0)1344 465626
E:
bsria@bsria.co.uk
W:
www.bsria.co.uk
Old Bracknell Lane West,
Bracknell, Berkshire,
RG12 7AH, UK
Offices in Bracknell, Beijing,
St Helens, Stuttgart and Toulouse.
Associates in Armagh and Cadiz
Testing
Troubleshooting
Modelling
Information
Research
Training
Consultancy
Publications
Instrument hire,
Market research and
sales and calibration
intelligence
Membership is the foundation of BSRIA’s
expertise and independence