Building Services
By Dr David Johnston and Alan Newall– licensed under the Creative Commons Attribution – NonCommercial – Share Alike License
http://creativecommons.org/licenses/by-nc-sa/2.5/
ENVIRONMENTAL SCIENCE & SERVICES LEVEL 1
Air conditioning
Alan Newall & Dr David Johnston
Air conditioning
The term”air conditioning” is commonly used to describe any system where
refrigeration is included to provide cooling. Cooling may be required to:

Provide a healthy and comfortable environment for the occupants.
 Preserve goods.

Precisely control various industrial processes.
A wide range of air conditioning systems are available. The various systems can
be categorised according to their function:
 Full air conditioning systems - Provide all of the ventilation and fresh air
requirements of the building and control both temperature and humidity within
predetermined limits. The air supply is filtered, heated or cooled to the required
temperature, and is humidified or dehumidified to maintain acceptable levels of
relative humidity.

Close control air conditioning systems - Similar to full systems, but are
capable of maintaining temperature and relative humidity within close control
limits, typically within ±0.5ºC and ±5% RH.

Comfort cooling systems - Use refrigeration to reduce indoor temperatures
when required to produce more comfortable conditions for occupants.
Ventilation and fresh air are not always supplied from a comfort cooling system,
and may need to be provided separately.
Air conditioning
Air conditioning systems do not have to be applied to a whole building. They can
be applied to specific areas of a building in order to provide a healthy and
comfortable environment for the building occupants.
The building can incorporate:

Full air conditioning - Where all parts of the building are treated, except for
toilets, staircases and utility or storage areas.

Partial air conditioning - Where only certain specific areas of the building are
treated.

Mixed-mode system - Where parts or all of the building are provided with air
conditioning for use at times when natural ventilation or mechanical ventilation
alone are insufficient (e.g. summer peak conditions).
(MoD, 1996)
The use of partial or mixed-mode systems instead of a full air conditioning
system can reduce capital installation and operating costs.
Energy and environmental cost of
air conditioning
Air conditioned buildings have previously consumed 1½ to 2 times as much energy as
an equivalent naturally ventilated building (MoD, 1996), resulting in an increase in
CO2 emissons.
Introducing full air conditioning into a design can often add significantly to the eventual
running costs of the building (CIBSE, 1998).
Air conditioned buildings are more complex and costly to build and maintain.
In the UK, there is a limited set of circumstances in which a building will require air
conditioning. It can be necessary in certain circumstances due to pollution, external
noise and high heat gains (CIBSE, 1998). Climate change is leading to increased
heat gains and peak summertime temperatures, which is in turn increasing the
demand for air conditioned buildings.
In a large number of buildings, full mechanical air conditioning is not always
necessary and large savings can be made by using mechanical or natural ventilation.
Building heat gains
Air conditioning systems are normally specified to prevent overheating of certain
buildings in summer.
Buildings suffer heat gains from a number of sources, predominantly:

Solar heat gain through glazing.

Internal heat gains from:
- People.
- Lighting.
- Equipment.

Ventilation from warm outside air.
These heat gains peak in summer and must be offset by the air conditioning system to
maintain thermal comfort conditions.
Determining the need for air conditioning
The figure below provides useful guidance in assessing the need for full air
conditioning, comfort cooling or mechanical ventilation.
What are your requirements [Source: BRECSU, 1993]
Types of air conditioning system
There are three generic types of air conditioning system:

Centralised all-air systems - Air is heated or cooled in a central plant room and
is then distributed to the rooms via ductwork.

Partially centralised air/water systems - Air may be heated or cooled in a
central plant room and is then further heated or cooled at entry to the rooms.

Local systems (self-contained) - All of the operations are performed locally.
Many local systems provide comfort cooling only. Other functions such as
fresh air supply, humidity control and heating are often not available.
Centralised all-air systems
These systems are typically based around a packaged air handling unit (AHU) which
consists of a fan, combinations of heating and cooling coils, filters,humidifiers and
control dampers. They may also include the facility to recycle exhaust air back into the
building.
AHU [Source: Flakt Woods, 2003]
The AHU would be located within a plant room with chillers and boilers located nearby,
which supply hot and cold water to the heater battery and cooling coil.
Centralised all-air systems
These comprise:
 Constant volume single zone systems.

Variable air volume (VAV) systems.
Constant volume single zone systems






Heating and cooling loads are met by changing the temperature of the supply
air.
A constant volume air supply at a single temperature is delivered to all areas
of the building.
Humidification may be provided in winter by a steam humidifier, included within
the air handling unit.
Separate systems may be required to serve different zones, increasing capital
costs and plant room space.
These systems are simple, relatively low cost and easy to commission, but
they cannot provide adequate control for zones which have different
heating or cooling needs.
The system is suitable for large factory spaces, supermarkets and assembly
halls.
Centralised all-air systems
Single duct variable air conditioning system [Source: Chadderton, 1993].
Centralised all-air systems
Variable air volume (VAV) system
The VAV system overcomes some of the problems associated with zones which
have different heating or cooling needs.

They provide zone control of the temperature from the central plant. Humidity
is still controlled centrally and therefore humidity conditions may vary widely
between different zones.

Air is supplied at constant temperature and relative humidity to all parts of
the building.

Different cooling requirements are achieved by varying the volume of air
supplied to each zone from the central plant. Control of the air volume is
performed by a thermostatically controlled damper box (VAV box), which is
fitted in the air supply ducts. Heating is provided either by a separate heating
system or by providing reheat coils in each VAV box.

The air flow rate may be controlled by an energy efficient variable speed fan.

VAV systems can be energy efficient and are common in new open plan
buildings.
Centralised all-air systems
Fan-powered variable air volume terminal unit installed in a false ceiling
[Source: Chadderton, 1993].
Centralised all-air systems
Variable air volume (VAV) system
VAV unit [Source: Barcol-air, 2003]
VAV terminal unit [Source: Hassan, 1996]
Partially centralised air/water systems
The common factor with these systems is that a central AHU is used, with the air
being further heated or cooled to the individual room requirements by additional
heating or cooling coils.
Partially centralised systems generally consist of:

Centralised air systems with re-heat.

Fan coil unit systems.

Chilled ceilings/beams.
 Unitary reversible heat pumps.
Partially centralised air/water systems
Centralised air systems with re-heat






Both constant volume and VAV systems are available.
In constant volume systems this greatly improves controllability and the
ductwork can be configured to serve zones with quite different heating and
cooling requirements (referred to as Constant volume multi-zone systems).
Heating coils which provide reheat are normally located in ductwork within
ceiling voids serving each zone (partially centralised system).
In VAV systems reheating coils are provided located within the VAV box (VAV
with reheat). This has the disadvantage that hot water must be piped to the
VAV boxes and there is some potential for leaks within the occupied areas.
An advantage of a VAV with reheat system is that reheat need only be applied
to the VAV boxes where heating needs are greatest.
These systems, which first cool and then reheat the supply air, are often
responsible for excessive energy use in air conditioned buildings.
Partially centralised air/water systems
Fan coil unit systems



Fan coil units are similar to fan convector heaters. The major difference is that
chilled water is fed to the units in summer, as well as hot water in winter.
Fan coil units have two coils in each unit, supplied with hot and cold water from
a central boiler and refrigeration plant (4-pipe system).
Fan coil units provide a flexible comfort cooling system most commonly used in
multi-zone buildings, e.g. hotels and cellular offices.
Fan coil unit systems




Each room (or zone) is provided with a fan coil unit, located within a suspended
ceiling or floor mounted.
The units recirculate heated or cooled room air and can introduce fresh air to
maintain air quality. Air filtration can be included.
Fresh air may be provided from a separate system, ducted to the inlet of the fan
coil unit, or supplied directly to the space.
Fan coil systems can provide multi-zone control of temperature without the need
for reheat, and can cope with situations where simultaneous heating and cooling
may be required.
Fan-coil unit air conditioning system (SDFC) [Source: Chadderton, 1993].
Chilled ceilings/beams
Conventional air conditioning systems provide cooling almost entirely by
convective heat transfer. Cooling can also be provided by a combination of
radiation and convection. This can be achieved using chilled ceilings and/or
beams.
Chilled ceiling/beams use chilled or cooled water (normally between 13 to 18°C)
as the cooling medium (CIBSE, 2001). “Free cooling” may be utilised if a suitable
source of groundwater is available.
There are three generic types of chilled ceiling/beams:

Radiant ceiling panels - Cooling is principally achieved via the process of
radiant exchange. Typically, 60% of the cooling output is achieved through this
heat transfer mechanism. Serpentine chilled water pipework is incorporated
within the ceiling panels.

Passive chilled beams - Cooling is achieved by natural convection, by passing
chilled water through finned elements (similar in form to a heat exchanger). The
beams can be installed flush with most false ceilings or can be fully exposed in
aesthetic casings.

Active chilled beams - Similar to passive chilled beams, except that the air
moving through the beam is mechanically assisted. The air is supplied to the
room through diffusers built into the beam.
Chilled ceilings/beams
Chilled ceiling panels [Source: SAS, 2003]
Chilled beams [Source: SAS, 2003]
Chilled ceilings/beams
Chilled ceilings and beams only provide comfort cooling. Filtration, heating and
humidification are not available.
With active chilled beams ventilation is provided from the central plant, and
filtration may be provided.
With passive chilled beams and ceiling panels, ventilation has to be introduced
separately. This is normally achieved by displacement ventilation.
The avoidance of condensation on the surface of chilled beams and ceilings has
been a major design issue in the UK (CIBSE, 2001).
Chilled ceilings and beams must be controlled to prevent the risk of
condensation, otherwise ‘office rain’ could occur.
This can be achieved by incorporating condensation detection into the chilled
beam control system.
Partially centralised air/water systems
Unitary reversible heat pumps




Heating or cooling is provided from a constant temperature water (two pipe)
loop.
The water loop is heated or cooled according to demand, in a central plant
room by means of a boiler and central refrigeration plant.
Advantageous if several zones of a building require significantly different
temperature conditions, or if there is a need for simultaneous heating and
cooling in different zones.
A separate ventilation system is normally required to maintain indoor air quality.
Calorex heat pumps [Source: Calorex, 2003]
Local systems
Local systems normally only provide comfort cooling, although filtration and
heating can also be provided. Humidification is not normally available.
They are often used as a refurbishment option.
Local systems may be less effective than centralised plant but can provide
energy savings through reduced distribution losses, simpler heat rejection
equipment, greater control over operating periods, and their ability to be more
readily confined to the areas of greatest need (CIBSE Guide B2), e.g. partial air
conditioning.
Local systems generally consist of:

Through-the-wall packaged units.

Split packaged units.

Variable refrigerant flow units.
Local systems
Through-the-wall packaged units – small scale domestic/commercial application
providing comfort cooling




Generally consist of a small refrigeration unit with an integral air circulation
fan. Air is drawn from the room, cooled and returned.
They require wall mounting, can be noisy and are generally not very efficient.
Some units offer heating by electric resistance heaters which can be
expensive to run.
Maintenance is carried out in situ, which could be difficult and expensive if
there are a large number of units.
Single duct packaged unit [Source: Greeno, 1997]
Local systems
Split packaged units
 Divided into two components:
 the indoor fan coil unit. This contains a filter, a fan, an evaporator coil and
the expansion or regulating valve.
 the outdoor refrigeration unit. This contains the condenser coil, a fan and
a compressor.
 The units are connected to each other directly by pipes containing the refrigerant
gas/liquid.
 Multi-split packaged units are available, where one outdoor unit can service
several indoor units (around 6 to 8).
 Noise problems can be avoided in the space since the noisy condenser and
compressor are located outside and can be detached from the occupied area
of the building.
Multi-split packaged unit [Source: Daikin, 2003]
Local systems
Variable refrigerant flow units – alternative system to fan coil units.




A special type of multi-split system where the refrigerant flow rate can be
controlled to match the cooling load through a variable speed compressor.
Most systems act as heat pumps and can switch between heating and cooling
modes.
Most advanced systems allow the heat removed from areas that require
cooling to be supplied to areas that require heating. This results in energy
savings.
Maintenance costs can be relatively high.
Variable refrigerant flow system [Source: Daikin, 2003]
Air conditioning considerations
Fundamental to the design of air conditioning systems are the following
considerations:

Systems which rely entirely on ducted air to distribute heat are bulky
because air has a low specific heat capacity and high volumes are
required.

Water provides a very compact medium for heat distribution, but water
circuits may leak and must therefore be separated from electrical systems.

Refrigerants themselves provide an effective distribution system in most
local systems.

Systems which condition large areas from central plant minimise the
number of some components but involve bulkier distribution systems, and
may make separate control of different zones within a building more
difficult.

Local/decentralised systems facilitate zone control and reduce or
eliminate distribution systems. They may impose higher maintenance
costs due to the large number of separate heating systems and chillers
involved.
Refrigeration systems
Refrigeration equipment is used to chill air or water in all mechanical air
conditioning systems. There are two types of system commonly in use:

Vapour compression type.

The absorption type.
In the UK, a mechanical vapour compression system is generally used,
although absorption refrigeration systems should be considered where waste
heat is available (e.g. from CHP).
Refrigeration systems
Vapour compression system
(Electrical)
Compression refrigerator cycle [Source: McMullan, 1998]
Stage A - Liquid refrigerant evaporates at a low temperature, absorbing heat and providing
cooling.
Stage B - The pressure of the refrigerant vapour is increased by means of a compressor. The
pressurisation raises the temperature of the refrigerant and requires an input of energy into the
system.
Stage C - The refrigerant vapour condenses. The latent heat released is emitted by the
condenser to the surrounding air.
Stage D - The liquid refrigerant is passed through an expansion valve and its temperature and
pressure drops for a repeat of the cycle.
Coefficient of performance
The efficiency of a vapour compression system is expressed as a Coefficient of
Performance (C.o.P.), where:
C.o.P,
(cooling)


=
Heat absorbed by the evaporator (cooling provided)
Electrical energy used by the compressor
Refrigeration plant should be selected with as high a possible C.o.P. to minimise the
environmental impact of the air conditioning system.
Typical values of C.o.P. (cooling) are between 2 to 3.0.
Heat pump technology
Heat pumps use the vapour compression cycle to provide both heating and cooling for a
building as required.
In winter, when heating is required, the functions of the evaporator and the condenser are
reversed.
The evaporator absorbs low grade heat from a low temperature source and high grade
heat (at around 40 to 50°C) is released to the building from the condenser.
Heat pump technology
Heat sources for heat pumps include ambient air, and the ground, where buried coils are
used to extract low grade heat effectively.
Heat pumps perform well with low temperature heating systems, e.g. warm air or
underfloor heating.
For heating the C.o.P. is defined as:
C.o.P,
(heating)
=
Heat supplied to the building
Electrical energy used by the compressor
C.o.P.’s of around 4 to 5.0 can be achieved using ground source heat pumps, which
provides a significant reduction in environmental impact, relative to conventional heating
systems.
Refrigeration systems
Absorption system – used where a source of waste heat is available, e.g. from CHP
systems.
(Waste heat from
process/CHP)
Absorption refrigerator cycle [Source: McMullan, 1998]
Stage A - The evaporator coils are placed in the low temperature heat source and heat is
absorbed by the evaporating refrigerant.
Stage B - The temperature of the refrigerant and water solution is increased, separating the
refrigerant from the water.
Stage C - The refrigerant vapour is cooled and condensed to a liquid, releasing heat.
Stage D - The refrigerant is expanded and it evaporates for a repeat of the cycle.
Selection of refrigeration systems
There are a number of key issues which need to be addressed when selecting
refrigeration equipment. These are:

Can a refrigeration plant be avoided, or can its use be limited by using a
‘free cooling’ technique (e.g. outside air or groundwater).

If a refrigeration plant is necessary, then it must be efficient. The
difference between the running costs of a good and bad system can be as
much as 50%.
References
BARCOL-AIR (2003) Variable Air Volume Systems (VAV). Available from:<
http://www.barcol-air.co.uk/variable_air-index.htm > [Accessed February 18th 2003].
BRECSU (1993) Selecting Air Conditioning Systems. Good Practice Guide 71. Department of
the Environment, London, HMSO.
CALOREX (2003) Calorex Heat Pumps. Available from:< http://www.calorex.com/ > [Accessed
February 19th 2003].
CHADDERTON, D. (1993) Air Conditioning – A Practical Introduction. E & FN Spon.
CIBSE (2001) Ventilation and Air Conditioning: CIBSE Guide B2. London, Chartered Institute of
Building Services Engineers.
CIBSE (1998) Energy Efficiency in Buildings: CIBSE Guide. London, Chartered Institute of
Building Services Engineers.
DAIKIN (2003) Daikin Industries Ltd. Available from:< http://global.daikin.com/ > [Accessed
February 19th 2003].
FLAKT WOODS (2003) Air Handling Unit EC 2000. Available from:< http://www.flaktwoods.com/
standard/sitemanagerview.aspx?strSiteID=60&strLanguageID=1&strMenuID=23&strModuleObje
ct=&strModuleID=&strResourceID=> [Accessed February 18th 2003].
References (continued)
GREENO, R. (1997) Building Services, Technology and Design. Harlow, UK, Addison Wesley
Longman Limited.
HASSAN, G. (1996) Building Services. London, UK, MacMillan Press Ltd.
MoD (1996) Justifying the Provision of Air Conditioning. Defence Works Function Standard
Design and Maintenance Guide 07, Ministry of Defence, London, HMSO.
SAS (2003) SAS International Chilled Ceilings. Available from:< http://www.sasint.co.uk/index.html >
[Accessed February 19th 2003].