HVAC
Heat Transfer
Heat is a form of energy. Every object on earth has some heat energy. Cooling in the process of
transferring heat from one object to another. When an air-conditioning system cools, it is actually
removing heat and transferring its somewhere else.
There are two forms of heat energy:
i.
ii.
Sensible heat
Latent heat
Sensible heat is the form of heat energy which is most
commonly understood because it in sensed by touch
on measured directly with a thermometer. When
weather reporters say it will be 90 degrees, they are
referring to sensible heat.
Latent heat can’t be sensed by touch or measured with
a thermometer. Latent heat causes an object to change
its properties. When enough latent heat is removed
from water vapor (steam in humidity), it condenses
into water (liquid). If enough latent heat is removed
from water (liquid) it will freeze, this process is
reversed when latent heat is added.
Air Conditioning:
Air conditioning is the process of treating air to control its temperature, humidity, quality, movement and
pressure simultaneously to meet the requirements of the conditional space.
Refrigeration system:
If we take a liquid refrigerant, confine it in a container and place this near a warm object, the liquid will
absorb heat from the warm object. If enough heat is absorbed by the liquid refrigerant, it will boil and
vaporize. If the vaporized refrigerant gas is sufficiently Compressed, it will give up the heat that absorbed
from the warm object and the gas will condense back to liquid.
•The process of alternatively vaporizing and condensing a refrigerant is called a refrigeration cycle.
•When the cycle accomplished continuously through the use of machinery, it is called mechanical
refrigeration and also called vapor compression cycle.
1
Vapor Compression cycle
The basic components required to make up a
refrigeration system are:
I.
II.
III.
IV.
An evaporator the cooling coil
A pump the compressor
A Condenser the heat disposer
A liquid metering device - expansion valve or
capillary tube.
General Provisions
Air Conditioning, Heating and Ventilation (HVAC) system is a combined process that performs
many functions simultaneously and provide comfortable conditions within and enclosed space
without complaining on health and safely of occupants or for the purpose of the product
processing.
HVAC system controls and optimizes the following factors in the building
a)
b)
c)
d)
e)
f)
g)
Temperature
Relative humidify
Air quality
Air movement
Noise and vibration
Energy efficiency and
Fire safety.
HVAC system shall be designed, constructed, installed, operated and maintained in accordance
with good engineering practice such as described in the ASHRAE Handbooks, HI manuals and
relevant chapters of latest BNBC.
General Design Aspect of Air-Conditioning System
7.5.1
2
Select internal conditions required, temperature and humidity, whether for comfort or for process
or for equipment/ storage area.
7.5.2
Calculate cooling load required, floor wise/zone wise based on Design-criteria given in ASHRAE
Hand Book/ Carrier’s Hand Book/ Trane’s Hand Book. Design parameters like natural heat gain,
occupancy, room orientation, smoking/ no smoking zones and corridor space should be taken into
account. Add ventilation load to the cooling load. Consider use of Air-Curtains at certain areas to
reduce load requirement.
7.5.3
Select the system of Air-conditioning depending on the type of Building and its use. By system,
we mean All-Air, All-Water and Air/ Water mixed.
7.5.4
Once the system is selected, the duct/ pipe layout must be carefully designed to have required air
flow/ water flow in conditioned area and return air/ water system. Short circulating between
supply air outlets and return air outlets should be avoided.
7.5.5
While designing Air-conditioning system in Bangladesh, the comfort temperature/ humidity for
people, the dust and moisture content of local air must be getting in mind. Also fresh airrequirement of the local occupants must seriously be taken into account. Since higher fresh air
requirement increases the cost of the system, a compromise should be done with the inside
temperature/ humidity to maintain fresh air requirement. Lesser the fresh air, greater is the chance
of fungus growth in the air-conditioned area from which persons working in the air-conditioned
area is attacked with Fungus Allergy. Once the cooling load is determined, equipment selection
made, the next important point to be considered is the machine room layout. It is a hard task to
extract enough space from the building for a comfortable layout of the plant-room requirement.
No compromise should be made on space requirement which is very vital for equipment
operation maintenance.
7.5.6
In air-conditioning system be it in the plant room or in the conditioned area sound level should be
kept at as minimum as possible- especially for the office areas.
Special consideration is to be given to Heat shock i.e. movement from cold area to normal area
(in the summer season specially) would cause the persons to catch cold and therefore, the
temperature between air-conditioning area, corridor and outside condition to be carefully
considered for health reason.
In designing air-conditioning system for this country, one must remember that we live in a natural
moderate temperature excepting for a short period of high temperature. So what is comfort to a
person in cold countries or very hot countries is not applicable to Bangladesh. Room temperature
should be so selected that a local person can use the room without coat and tie. Should certain
areas/ rooms are to be so maintained that use of coat and tie is a must those areas should be
designed specially and that would keep the cost of the system own.
7.5.7
7.5.8
7.5.9
In order to have a long life of the Central plants, due consideration should be given to water
treatment, in case of water-cooled chillers.
Living in air-conditioned area is hazardous if enough fresh-air is not introduced. In residence,
where rooms are full of pot plants, fresh air requirement is to be increased. In room air cooler, the
fresh air inlet should always be kept open.
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7.5.10 The split-air coolers do not have any provision for fresh air-inlet and the rooms polluted air full of
carbon-dioxide and human odor is being circulated. In case use of split-air coolers is a must, extra
arrangement must be made to introduce fresh air of adequate quantity.
7.6
INSIDE DESIGN CONDITIONS
7.6.1 For comfort air-conditioning the inside design conditions shall be selected with an objective to
reduce energy consumption in the operation of the air-conditioning system. Acceptable values of inside
design conditions for summer is provided in Table-A unless otherwise specially required the design
calculations shall be based on the normal practice value of Table-A
7.6.2 To avoid thermal shock, the difference between the dry bulb temperatures of outdoor air and
indoor air shall not exceed 11oC. If it is absolutely necessary to have a difference more than 11oC, there
shall have adequate provision for ante-room to reduce the effect of thermal shock.
7.6.3 For air-conditioning systems other than comfort air-conditioning, design conditions required by
the specific processes involved or applications may be adopted. When required, proper protective
measures shall be taken for persons working therein.
7.6.4 Velocity of air in an air-conditioned space, in the zone between the floor level and the 1.5 m
level, shall be within 0.12 m/s and 0.25 m/s for comfort applications for commercial buildings, and for
other applications it shall not exceed 0.5 m/s.
7.7
OUTSIDE DESIGN CONDITIONS
7.7.1 The outside design conditions for summer months for different cities are provided in Table-B.
Selection of outside design conditions from this table shall be based on requirements of the application
and the percent of time the outside air temperature is allowed to exceed the outside design conditions.
7.7.2 In case of stringent design conditions a meteorologist with experience in applied climatology may
be consulted to evaluate conditions such as; the formation of heat sinks in urban areas; the duration of
extreme temperatures; project sites located remotely from reporting stations.
7.8
VENTILATION AIR
7.8.1
Every space served by the air-conditioning system shall be provided with outside fresh air not
less than the minimum amount mentioned in Table-C. If adequate temperature regulation along
with efficient filtration of air and absorption of odour and gas are provided, the amount of fresh
air requirement may be reduced. However, in no case the outdoor air quantity shall be lower than
2.5 l/s per person.
7.8.2
In hospital operation theaters, a large quantity of outdoor air supply is recommended to overcome
explosion hazard of anesthetics and to maintain sterile conditions. However, if adequate filtration
with efficient absorption of anesthetics and laminar flow of supply air is provided, outside air
requirement may be substantially reduced
4
Table -A Inside Design Conditions of Some of Applications for Summer
a
Indoor Design Conditions
SI No
Use Category of Space
Dry Bulb
Temperature (ºC)
1.
2.
3.
4.
5.
6.
7.
8.
9.
10.
11.
12.
13.
14.
15.
16.
17.
18.
19.
20.
21.
22.
Relative Humidity
(%)
Restaurants, Cafeteria and Dining Hall
23 ~ 26
Kitchens
28 ~ 31
--
Office buildings
23 ~ 26
50 ~ 60
Bank/Insurance/ Commercial building
23 ~ 26
45 ~ 55
Hotel guest rooms
23~ 26
55 ~ 60
50 ~ 60
Ball room/meeting room
23 ~ 26
40 ~ 60
Class rooms
23 ~ 26
50 ~ 60
Auditoriums
23 ~ 26
50 ~ 60
Recovery rooms
24 ~ 26
45 ~ 55
Patient rooms
24 ~ 26
45 ~ 55
Operation theatres
17 ~ 27
45 ~ 55
Delivery room
ICU/CCU
New born Intensive care
Treatment room
Trauma room
Endoscopy / Bronchoscopy
20~ 23
45 ~ 55
20~ 23
30 ~ 60
22.5~ 25.5
30 ~ 60
23~ 25
30 ~ 60
17 ~ 27
45 ~ 55
20~ 23
30 ~ 60
X-ray (diagnostic & treatment)
25.5 ~ 27
40 ~ 50
X-ray (surgery/critical area and
21~ 24
30 ~ 60
22.5~ 24.5
30 ~ 60
Art Galleries/Museums
17~ 22
40 ~ 55
Libraries
20~ 22
45 ~ 55
Laboratory (diagnostics)
Note:
a
The room design dry bulb temperature should be reduced when hot radiant panels are adjacent to the occupant and increased
when cold panels are adjacent, to compensate for the increase or decrease in radiant heat exchange from the body. A hot or cold panel
may be un‐shaded glass or glass block windows (hot in summer, cold in winter) and thin partitions with hot or cold spaces adjacent. Hot
tanks,
furnaces,
or
machines
are
hot
panels.
5
Table: Outside design Conditions of Different Stationsa
Cooling, DB /MWBb
0.4%
Station
Evaporator, DB /MWBc
1%
2%
0.4%
1%
Range
of
2%
DB
MWB
DB
MWB
DB
MWB
WB
MDB
WB
MDB
WB
MDB
DBd
(2)
(3)
(4)
(5)
(6)
(7)
(8)
(9)
(10)
(11)
(12)
(13)
(14)
Barishal
35.5
28.5
34.5
28.5
34.0
28.0
30.0
33.0
29.5
33.0
29.0
32.0
5.0
Bogura
36.5
28.5
35.5
28.5
34.5
28.0
31.5
33.5
30.0
33.0
29.0
33.0
5.5
Chandpur
35.5
28.5
34.5
28.5
34.0
28.0
29.5
33.0
29.5
33.0
29.0
32.5
5.0
Chattogram
34.0
28.5
33.5
28.5
33.0
28.0
29.5
32.5
29.0
32.5
28.5
32.0
4.5
Comilla
35.0
27.5
34.5
27.5
34.0
27.5
29.0
33.0
28.5
32.5
28.0
32.0
5.5
Cox’s Bazar
34.0
27.5
33.5
27.5
33.0
27.5
29.0
31.0
29.0
30.0
29.0
30.0
5.0
Dhaka
35.55
27.5
35.0
27.00
34.0
27.0
29.0
33.0
28.5
29.0
28.5
29.0
5.0
Dinajpur
36.0
28.5
35.5
28.0
34.5
28.0
30.0
32.5
29.0
33.0
29.0
33.0
5.5
Faridpur
36.5
28.5
35.5
28.0
34.5
28.0
29.5
33.0
29.0
33.0
29.0
33.0
5.0
Ishurdi
37.5
27.0
36.5
27.0
35.5
28.0
30.0
34.5
29.0
33.5
29.0
33.5
6.0
Jashore
38.0
28.5
37.0
28.5
36.0
28.0
30.0
35.0
30.0
35.0
29.5
34.0
6.5
Khulna
36.5
29.0
36.0
28.0
35.5
28.0
30.0
34.0
30.0
34.0
29.5
33.5
5.0
Mongla
37.0
31.0
36.0
30.5
35.0
30.0
33.0
34.0
32.5
34.0
32.0
33.5
5.0
Mymensingh
35.0
28.0
34.0
28.0
33.5
27.5
29.5
33.0
29.0
32.56
28.5
32.0
4.5
Patuakhali
35.5
28.5
35.0
28.5
34.5
28.5
30.0
33.0
30.0
33.0
29.5
32.5
5.0
Rajshahi
35.5
296.0
34.5
28.5
34.0
29.0
31.5
33.0
31.0
32.5
30.0
31.5
5.0
Rangamati
35.5
26.0
34.5
27.0
34.0
27.5
28.5
33.5
28.0
32.5
28.0
32.5
6.0
Rangpur
35.5
28.5
34.5
28.0
34.0
28.0
29.5
30.5
29.5
30.0
29.5
30.5
5.0
Sylhet
35.5
27.0
34.5
27.0
34.0
26.5
28.5
32.5
28.0
32.5
28.0
32.5
5.5
Tangail
36.0
27.5
35.5
27.5
34.5
27.5
29.0
33.5
28.5
33.0
28.5
33.0
5.5
(1)
Notes:
a) This table has been prepared by statistical analysis of weather data of ten years, recorded three hourly by trained observers of
Bangladesh Meteorological Department
b) The dry bulb (DB) temperatures presented in column‐2, 4 & 6 represent values which have equaled or exceeded by 0.4%, 1%, and
2% of the total hours during the summer months of May through September. The coincident mean wet bulb temperatures (MWB)
listed in columns 3, 5 & 7 are the mean of all wet bulb temperatures occurring at the specific design dry bulb temperatures. These
values shall be used for cooling load calculation.
c) Wet bulb (WB) temperatures presented in column‐8, 10 & 12 represent values which have been equaled or exceeded by 0.4%, 1%
and 2% of the total hours during the summer months of May through September. The coincident mean dry bulb temperatures
(MDB) listed in columns 9, 11 & 13 are the mean of all dry bulb temperatures occurring at the specific design wet bulb
temperatures. These values shall be used for selection of Cooling Tower, evaporative cooling equipment, fresh air cooling and
other similar equipment.
d) Mean daily range temperatures presented in column‐14 are the difference between the average daily maximum and average daily
minimum temperatures during the warmest months at each station
6
Table: Required Minimum Air Circulation Rate for Mechanical Ventilation of Non-AirConditioned Space
Sl
Application
Air Change
Sl
Application
Air Change
No.
per Hour
No.
per Hour
1 Assembly Rooms
4-8
28 Hospitals-sterilizing
15-25
2 Bakeries
20-30
29 Hospitals-Wards
6-8
3 Bank/Building societies
4-8
30 Hospitals Domestic
15-20
4 Bathrooms
6-10
31 Laboratories
6-15
5 Bedrooms
2-4
32 Launderettes
10-15
6 Billiard Rooms
6-8
33 Laundries
10-30
7 Boiler Rooms
15-30
34 Lavatories
6-15
8 Cafes and Coffee Bars
10-12
35 Lecture Theatres
5-8
9 Canteens
8-12
36 Libraries
3-5
10 Cellars
3-10
37 Living Rooms
3-6
11 Churches
1-3
38 Mushroom House
6-10
12 Cinemas and Theaters
10-15
39 Offices
6-10
13 Club Rooms
12, min
40 Paint Shops (not cellulose)
10-20
14 Compressor Rooms
10-12
41 Photo and X-Ray Darkroom
10-15
15 Conference Rooms
8-12
42 Public House Bars
12, min
16 Dairies
8-12
43 Recording Control Rooms
15-25
17 Dance Halls
12, min
44 Recording Studios
10-12
18 Dye Works
20-30
45 Restaurants
8-12
19 Electroplating Shops
10-12
46 School Rooms
5-7
20 Engine Rooms
15-30
47 Shops and Supermarkets
8-15
21 Entrance Halls
3-5
48 Shower Baths
15-20
22 Factories and Work Shops
8-10
49 Stores and Warehouses
3-6
23 Foundries
15-30
50 Squash Courts
4, min
24 Garages
6-8a
51 Swimming Baths
10-15
25 Glass Houses
25-60
52 Toilets
6-10
26 Gymnasium
6, min
53 Utility Rooms
15-20
27 Hair Dressing Saloon
10-15
54 Welding Shops
15-30
Note: The Ventilation Rates may be increased by 50 percent where heavy smoking occurs or if the room
is below ground.
a
Only outdoor air and no recirculation shall be done.
7
Design Considerations for cooling load calculation
To Calculate the space cooling load; detailed building information, location, site and weather
data, internal design information and operating schedules are required.
Information regarding the outdoor the outdoor design conditions and desired indoor condition are
the standing point for the load calculation.
Building Characteristics
To calculate space heat gain, the following
information on building envelope is required.
a) Architecture plans, sections and elevations for
estimating building dimensions/ area/ volume.
b) Building orientation (N, S, E, W, NE, SE, SQ,
BW etc.) location etc.
c) External/ Internal shading, ground reflectance etc.
d) Material of construction for external walls, roofs,
windows, doors, internal walls, partitions, ceiling,
insulating materials and thickness, external wall
and roof colors select and/ or compute u-vastness
for walls, roof, windows, doors, partitions etc.
e) Amount of glass, type and shading on window.
Internal load and operating nature
i.
ii.
iii.
iv.
Number of occupants, duration of occupancy & nature of activity.
Lighting total wattage at peak and duration of operators.
Appliances rated power input or heat generator data.
Nature of operation continuous or intermittent.
Others
▪
▪
Infiltration: Movement of air into indoor space from the outside.
Exfiltration: Movement of air escaping outside the building,
The air conditioner’s efficiency, performance, durability and cost depend on metering its size to
the above factors.
7.9
NOISE AND VIBRATION
7.9.1 General: Air-conditioning, heating and ventilation system design and installations shall
consider all the aspects of noise and vibration control related to the system. Selection and
installation of equipment for air-conditioning, heating and ventilation system shall be such that
8
noise and vibration transmitted to the space served by the system shall not exceed the
recommended value for the space served.
7.9.2 Equipment room: Equipment room for installation of air handling units, refrigeration
machinery, pumps, boilers, blowers and other equipment, which produce noise and vibration,
shall not preferably be located adjacent to any acoustically sensitive area. Location of the
equipment room shall be such that direct transmission of noise and vibration from the equipment
room to acoustically sensitive areas do not occur. Appropriately designed sound barriers shall be
used to restrict transmission of noise from equipment room to any acoustically sensitive areas,
wherever necessary. Similarly, adequate measures shall be taken to restrict transmission of
vibration from equipment room to other rooms.
7.9.3 Selection of equipment: Where possible, the equipment shall be selected which produce
low sound power level consistent with the required performance and ensuring operation at
maximum efficiency. Noise levels shall be reduced by appropriate shrouding of the equipment, if
necessary. Equipment shall be so oriented that the noise will be radiated away from the likely
areas of complaint.
7.9.4
Noise control:
7.9.4.1 Air Ducts: Air ducts shall be so designed and installed to avoid any transmission of
noise and vibration which may be picked up by the duct system from equipment room or
adjoining rooms. Duct system shall not allow cross talk or noise transfer from one occupied
space to another. Duct system shall be appropriately designed, constructed and installed to obtain
adequate attenuation of noise required to maintain recommended noise level in the airconditioned space. Duct construction and installation shall be such that drumming effect of duct
walls and noise transmission through the duct walls can be minimized to the approved level.
7.9.4.2
Plenum Chamber: If required, properly designed plenum chamber, lined with
approved sound absorbed material, and/or sound attenuators shall be used for attenuation of
noise.
7.9.4.3
Flow Control Devices: Air dampers and other flow control devices shall be so
selected that noise generation does not exceed approved levels.
7.9.4.4 Air Terminals: Air terminals shall be selected for the approved noise generation
characteristics.
7.9.4.5 Piping: Velocity of fluids in piping shall be so selected that noise generation does not
exceed approved levels.
7.9.4.6 Chiller and Refrigeration Equipment: Chiller(s) and refrigeration equipment(s) shall be so
selected and installed that the combined effect of noise level does not exceed 65 dBA or
approved levels at the property boundary line. Where ever possible refrigerant compressors may
be encased in acoustically treated enclosures to reduce noise transmission. Similarly, low speed
condenser fans may be used to have reduced noise generation. Fan cylinders may be acoustically
treated to reduce noise transmission.
9
7.9.4.7 Cooling Tower: Cooling Towers(s) shall be so selected and installed that the combined
effect of noise generation does not exceed 65 dBA or approved levels at the proper boundary
level. Where ever possible, fan cylinders shall be acoustically treated to reduce noise
transmission. Floating mats may also be used to reduce water droplet noise.
7.9.5
Vibration Control:
7.9.5.1 Appropriately designed vibration isolators shall be installed under the machinery to
restrict vibration transmission to structures. Similarly vibration isolators shall also be used
between machinery and all pipe work and duct work including the supports when applicable.
7.9.5.2 Where ever necessary “Inertia Block” with spring vibration isolators shall be used to
restrict vibration transmission to structures.
7.9.5.3 Spring vibration isolators shall be earthquake restraint type.
7.10.
FIRE DAMPER
Fire damper shall be provided at locations where air distribution systems penetrate
assemblies that are required to be fire resistance rated by the latest Bangladesh National Building
Code.
Types of Air Conditioning system
Air Conditioning effect can be achieved with various types of air conditioning units and system.
The common of them are listed below.
i) Room air-conditioning unit
ii) Split air-conditioning unit
iii) Single package air-conditioning unit
iv) DX or Direct Expansion air-conditioning unit
v) Central air-conditioning system
vi) Variable refrigerant flow (VRF) air-conditioning system
Window (Unitary) Air Conditioner
A window air conditioner is a packaged unit controlling of a compressor, condenser, evaporator,
expansion valve, fan, filter appropriate control and a housing. It is generally and for cooling and
general dehumidification.
10
Installation
Window (unitary) air conditioner is suitable for bedroom, office cabin, general office area, hotel room
and similar applications, where normal comfort conditions are required to be maintained. It is also not
suitable for areas that require close control of both the indoor temperature and relative humidity.
Application
Window (unitary) air conditioner should be mounted preferably at the window sill level on an external
wall, where hot air from air-cooled condenser can be discharged without causing nuisance. There
should not be any obstruction for the inlet air to, and discharge air from, the condenser. While deciding
location of the window air conditioner, care shall be taken to ensure that the condensate drain water is
piped to the ground level and does not drip, causing nuisance
Split Air Conditioner
A split system is an air conditioning system that has separate cabinets for the evaporator and the
condenser. The cabinets are connected by refrigerant lines, plumbing and electrical conductors.
The evaporator of a split system is located inside a building, called indoor unit and the condenser
in located outdoor, caller outdoor unit. The indoor unit may be , mounted on wall or ceiling or
floor. It also may be exposed type or concealed type.
Installation
Wall mounted unit and similar exposed indoor unit are provided with installation plate for ease in
installation. Care shall be taken to ensure that enough clearance space is available below the ceiling in
order to have free intake of return air.Outdoor unit is mounted on an epoxy-coated steel frame in an open
area so that the fan of the air cooled condenser can discharge hot air to the atmosphere, without any
obstruction.
Application
Split air conditioner is suitable for wide range of applications including residences, small office, club,
restaurant, showroom, departmental store, and others.
Variable Refrigerant Flow (VRF)
Variable Refrigerant flow (VRF) is an air-conditioning system where There is a one outdoor
condensing unit and multiple indoor units. VRF can be explained as a multiple Split Airconditioning system wing Principle of control of flow/quality of Refrigerant through the indoor
unit to control cooling and heating medium. This Refrigerant in compressed and liquefied by a
single Outdoor Condensing Unit (ODU) and is circulated within the building through copper
refrigerant pipes to multiple Fan Coil Units (FCU’s) called Indoor Unit (IU’s).
VRF outdoor units are typically provided with rectifier-inverter power system, which provided a
Variable Voltage & Variable Frequency (VVVF) supply to compressor motor, in order to
support variable speed. This in turn provided variable Refrigerant flow though the Refrigerant
lines meeting the demand of cooling/heating. The speed of motor is controlled through a
feedback system sensing the refrigeration demand from the indoor units.
11
Applications
 VRF systems are typically distributed system the outdoor unit is kept at a far-off location like
the top of the building of remotely at grade level and all the evaporator units are installed at
various location inside the building.
 There system is basically extension of split type air conditioners and much less efficient as
compared to central air conditioning plant.
 It is suitable for the existing building, requiring central AC but the space for providing AC
plant, height of ceiling for during, water supply for skilled water lines is unavailable.
 Such system is normally suitable where high diversity in demand is available include small
officers, small hotels, great houses, art galleries etc.
Prohibition of application: Operation theatres where provisions for 100 percent fresh air and
high-quality filtration of air are required.
System Selection Criteria
There are various types of air conditioning system. Suitability of system cannot be generalized, It
needs to be building specific decision consideration, initial cost, efficiency, maintenance, effect
on building aesthetics, noise, service life and other factors.
 When low initial cost and simplicity are primary concern, designers may select some-bysome distributed system. This approach tends to be used for smaller buildings as well as for
larger building with multiple tenancy and sufficient loop area distributed system includes
unitary air-conditioners (window & split air-conditioners), packed roof top air conditioners &
VRF.
 When indoor air quality humidity, lower operating cost, central maintenance and control are
primary concerns, designers may select centralized systems that may be DX (Direct
Expansion) type central plant or chilled water type central plant. These systems often require
a greater initial cost than distributed systems but often result in annual energy savings
because additional benefits also include the ability to reduce installed capacity by using load
diversity. Centralized cooling systems usually prove to be more cost effective when the
building load is large, depending on climate and patterns of occupancy use.
Comparative advantages, disadvantages and constraints of is option should be carefully
evaluated before final HVAC system selection.
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Packaged Air Conditioner
It is a self-contained unit suitable for floor mounting, designed
to provide conditioned air to moderate rise conditioned spaces.
It includes prime source of refrigeration for cooling and
dehumidification, facility for drawing fresh air and mixing
with return and fresh air and generally provision for external
air distribution ducting for uniform air distribution in the
conditioned spaces.
There are two types of packaged air condition
i) Single packaged unit
ii) Split packaged unit
When the machine is equipped with compressor, evaporator,
expansion device and condenser in a single housing it is
called single packaged unit. When the machine is
equipped with compressor, evaporate & expansion
device in a one cabinet/housing and remote condenser in a
separated housing and two are interconnected with copper
refrigerate pricing, it is called split packaged unit.
Installation
Packaged unit is normally mounted on resilient pads which prevent vibration of the unit from being
transmitted to the floor.
Application
Packaged unit is suitable for wide range of applications including office, club, community hall, banquet
hall, showroom, departmental store other.
Central Air Conditioning System
Central Air Conditioning System are generally deviated into three basic types, classified by the methods
by which final spaces cooling in affined.
These are as follows:
i) All Air System
ii) All Water System
iii) Air Water System
All Air System
The refrigeration unit (chiller) is located in a remote plant room from which secondary refrigerant (chilled
water) is supplied to the air-handling units Air handling units also remotely located in separate
room/places from the conditioned spaces. Only the final cooling or heating medium (air) is brought and
distributed into the conditioned spaces through duct network and air outlets.
Application
Single purpose occupancies, office buildings, place of assembly, Industries, large departmental store,
special requirement etc.
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Benefits
Quiet operation, central operational control, centralized maintenance & better environment control.
All Water System
The refrigeration unit (chiller) is located in a remote plant room. The secondary refrigerant (i.e. chilled
water) in supplied from a remote refrigeration unit and circulated through coil of air circulating unit
(FCU) installed within the conditioned spaces. Outdoor unit is supplied directly from the outside into the
remote air plenum fitted with the unit.
Application
Multi-room building with separate central such as hotel, hostel etc. building where adequate space for
installation of AHU and space for ductwork not available building where inside air quality is not of prime
importance.
Benefits
Flexibility of operation, room wise operational control, unit wise (FCU) maintenance etc.
Air Water System
The refrigeration unit and air handling unit (AHU) are located in separate places from the conditioned
spaces. Cooling or heating is done partly by air brought through the duct work from separate AHU/after
filtering and cooling. The remaining cooling or heating is done by cooled water or worm heater circulated
through a coil of fan coil unit (FCU) installed within conditioned space.
Application
Multi-room building with separate control such as hotel, hostel etc. & building where adequate space for
installation AHU and space for ductwork not available everywhere.
Benefits
Same as those of all water system. In addition, this provides filtered and clean outdoor air from central
AHU.
Chillers
Chiller are the heart of the chilled-water central air conditioning system. They serve the pivotal faction of
creating the cooling effect required to maintain comfort conditions. Chillers utilize the vapor compression
(mechanical) or absorption refrigeration cycle to produce chilled water.
Chillers use a refrigerant gas to move the unwanted heat between the evaporator and the condenser. The
chilled water is generated in evaporator and this is sent around the building by a pump to collect the
unwanted heat and bring it back to the evaporator to be cooled down. The refrigerant collects this heat
and moves it to the condenser. The condenser parts this unwanted heat into another loop which is sent by
a pump to the cooling towers to sent it into atmosphere or it will send it to a cooling cool where a fan
blower the heat away into the atmosphere.
There are generally two different types of chillers in central air conditioning system:
i) Air Cooled
ii) Water Cooled
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Air Cooled Chiller
Air Cooled condensers are used in this system. Air-cooled condensers employ ambient air as the
condensing medium and use a fan to move the air over the coil.
Water Cooled Chiller
Water cooled condenser are used in this chiller.
Water cooled condenser employ water as the condensing medium and uses a pump to circulate the water
through the condenser and out to a cooling tower that rejects the heat to the atmosphere.
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Table: HVAC system Analysis and Selection Matrix
(1)
i)
(2)
Temperature
Fixed Speed Unitary Systems
(Window ACs/Split ACs/Package
ACs)
(3)
No uniform and effective control
ii)
Capacity
requirements
Capacity to suit zone peak, no
diversity
iii)
Special
requirements
No plant/equipment room required.
Compromise building elevation
iv)
Electric supply
Distributed electric supply required
v)
Initial cost
Minimum initial cost
Moderate initial cost but marginally higher
than variable speed unitary product
vi)
Operating cost
Strategic scheduling of equipment can save
operating cost better than unitary product, but
higher peak energy requirement
vii)
Maintenance
cost
Higher operating cost. Strategic
scheduling of multiple pieces of
equipment can save marginal operating
cost, but equipment is less efficient
Comparatively less maintenance cost
vii)
Reliability
Reliable equipment but low service life
Reliable equipment but moderate service life
ix)
Flexibility
Has to be placed at fixed locations
Can be placed at distributed locations
x)
Noise and
vibration
Noise and vibration within/adjacent to
occupied spaces for unitary window
type air conditioners which is
substantially reduced in case of split
system
Noise and vibration on roof terrace or ground,
away from occupied spaces; however, the
same can be reduced with good installation
practices
SL
No.
System
Characteristic
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Variable Refrigerant Flow Systems (VRF)
(4)
Reasonably uniform and effective control
possible
Capacity to suit zone peak, limited diversity
can be considered
No plant/equipment room required, Outside
units can be located on roof or on adjacent
ground. Very small shaft desirable for
refrigerant piping
Zone-wise distributed electric supply required
Maintenance cost higher than both fixed and
variable speed unitary system but less than
central system
Central Systems
(5)
Uniform and effective control possible
Allows the design engineer to consider HVAC
load diversity factors, accordingly reduce
installed equipment capacity
Equipment rooms/spaces and accessible shaft
required for chilled water piping
Minimal distribution cost by centralized supply
near the substation
Even the HVAC diversity, a central system may
not be less costly than decentralized HVAC
systems
More energy-efficient primary equipment and
multiple pieces of HVAC equipment allow
staging of operation to match building loads
while maximizing operational efficiency
Comparatively higher since centralized
equipment room requires operator with no
access to occupant workspace, but with fewer
pieces of HVAC equipment to service
Reliable equipment with much longer service
life
Flexibility available in terms of alternative
locations
Noise and vibration away from occupied spaces
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