building services iii
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building servicesTRANSCRIPT
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V Semester Building Services III
MSAJ Academy of Architecture 1
BUILDING SERVICES - III
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V Semester Building Services III
MSAJ Academy of Architecture 2
CONTENT:
UNIT I AIR CONDITIONING: BASIC REFRIGERATION PRINCIPLES 9
Thermodynamics Heat Temperature Latent heat of fusion evaporation, saturation temperature, pressure temperature relationship for liquid refrigerants, refrigeration cycle
components vapour compression cycle compressors evaporators Refrigerant control devices electric motors Air handling Units cooling towers
UNIT II AIR CONDITIONING: SYSTEMS AND APPLICATIONS 12
Air conditioning system for small buildings window types, evaporative cooler, packaged terminal units and through the wall units split system b) Systems for large building Chilled water plant All Air system, variable air volume, and all water System Configuring/ sizing of mechanical equipment, equipment spaces and sizes for chiller
plant, cooling tower, Fan room, Circulation Pumps, Pipes, ducts
UNIT III AIR CONDITIONING: DESIGN ISSUES AND HORIZONTAL
DISTRIBUTION OF SYSTEMS 6
Design criteria for selecting the Air conditioning system for large building and energy
conservation measures - Typical choices for cooling systems for small and large buildings -
Horizontal distribution of services for large buildings - Grouped horizontal distribution over
central corridors, Above ceiling, In floor, Raised access floor, Horizontal distribution of
mechanical services
UNIT IV FIRE SAFETY: DESIGN AND GENERAL
GUIDELINES OF EGRESS DESIGN 10
Principles of fire behaviour, Fire safety design principles _ NBC Planning considerations in
buildings Non- Combustible materials, egress systems, Exit Access Distance between exits, exterior corridors Maximum travel distance, Doors, Smoke proof enclosures General guidelines for egress design for Auditoriums, concert halls, theatres, other building types,
window egress, accessibility for disabled- NBC guidelines lifts lobbies, stairways, ramp design, fire escapes and A/C, electrical systems.
UNIT V FIRE SAFETY: FIRE DETECTION AND FIRE FIGHTING INSTALLATION 8
Heat smoke detectors sprinkler systems Fire fighting pump and water requirements, storage wet risers, Dry rises Fire extinguishers & cabinets
Fire protection system CO2 & Halon system Fire alarm system, snorkel ladder
Configuring, sizing and space requirements for fire fighting equipments
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UNIT I
Control Devices
To maintain correct operating conditions, control devices are needed in a refrigeration system.
Metering Devices Metering devices, such as expansion valves and float valves, control the flow of liquid refrigerant between the high side and the low side of the system. It is at the end
of the line between the condenser and the evaporator. These devices are of five different
types: an automatic expansion valve (also known as a constant-pressure expansion valve), a
thermostatic expansion valve, low-side and high-side float valves, and a capillary tube.
Automatic Expansion ValveAn automatic expansion valve (fig. maintains a constant pressure in the evaporator. Normally this valve is used only with direct expansion, dry type of
evaporators. In operation, the valve feeds enough liquid refrigerants to the evaporator to
maintain a constant pressure in the coils. This type of valve is generally used in a system
where constant loads are expected. When a large variable load occurs, the valve will not feed
enough refrigerant to the evaporator under high load and will over feed the evaporator at low
load. Compressor damage can result when slugs of liquid enter the compressor.
Thermostatic Expansion Valve.before discussing the thermostatic expansion valve, lets explain the term SUPERHEAT. A vapor gas is superheated when its temperature is higher
than the boiling point corresponding to its pressure. When the boiling point begins, both the
liquid and the vapor are at the same temperature. But in an evaporator, as the gas vapor moves
along the coils toward the suction line, the gas may absorb additional heat and its temperature
rises. The difference in degrees between the saturation temperature and the increased
temperature of the gas is called superheat.
A thermostatic expansion valve (fig. 6-22)
keeps a constant superheat in the refrigerant
vapor leaving the coil. The valve controls the
liquid refrigerant, so the evaporator coils
maintain the correct amount of refrigerant at all
times. The valve has a power element that is
activated by a remote bulb located at the end of
the evaporator coils. The bulb senses the
superheat at the suction line and adjusts the
flow of refrigerant into the evaporator. As the
superheat increases (suction line), the
temperature, and therefore the pressure, in the
remote bulb also increases. This increased
pressure, applied to the top of the diaphragm,
forces it down along with the pin, which, in
turn, opens the valve, admitting replacement
refrigerant from the receiver to flow into the
evaporator. This replacement has three effects. First, it provides additional liquid refrigerant
to absorb heat from the evaporator.
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Second, it applies higher pressure to the bottom of the diaphragm, forcing it upward, tending
to close the valve. And third, it reduces the degree of superheat by forcing more refrigerant
through the suction line
Low-Side Float Expansion Valve. The low-side float expansion valve (fig. 6-23) controls the liquid refrigerant flow where a flooded evaporator is used. It consists of a ball float in
either a chamber or the evaporator on the low-pressure side of the svstem. The float actuates a
needle valve through a lever mechanism. As the float lowers, refrigerant enters through the
open valve; when it rises, the valve closes.
High-Side Float Expansion Valve.In a high-side float expansion valve (fig. 6-24), the valve float is in a liquid receiver or in an auxiliary container on the high-pressure side of the
system. Refrigerant from the condenser flows into the valve and immediately opens it,
allowing refrigerant to expand and pass into the evaporator. Refrigerant charge is critical. An
overcharge of the system floods back and damages the compressor. An undercharge results in
a capacity drop.
Capillary Tube.The capillary tube consists of a long tube of small diameter. It acts as a constant throttle on the refrigerant. The
length and diameter of the tube are
important; any restrictions cause trouble in
the system. It feeds refrigerant to the
evaporator as fast as it is produced by the
condenser. When the quantity of
refrigerant in the system is correct or the
charge is balanced, the flow of refrigerant
from the condenser to the evaporator stops
when the compressor unit stops. When the
condensing unit is running, the operating
characteristics of the capillary tube equipped evaporator are the same as if it were equipped
with a high-side float.
The capillary tube is best suited for household boxes, such as freezers and window air-
conditioners, where the refrigeration load is reasonably constant and small horsepower motors
are used.
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UNIT II
Factors Affecting Air- Conditioning
Outdoor Design Conditions Not in our control Building Orientation Indoor Design Temperature External Glass & Skylights External Walls Exposed Roof Internal Walls, Ceilings & Floors Occupancy Not in our control - defined by the usage Lighting Equipment inside Fresh Air
Air Conditioning:
It can be defined as the process of transferring heat from a low temperature region to a high
temperature region. In other words it is the process of cooling a substance. This can be
achieved only if the heat is removed from that substance.
Principle of refrigeration:
The principle of refrigeration is based on second law of thermodynamics. It sates that heat
does not flow from a low temperature body to a high temperature body without the help of an
external work. In refrigeration process, since the heat has to be transferred from a low
temperature body to a high temperature body some external work has to be done according to
the second law thermodynamics. This external work is done by means of compressor,
condenser etc.
Types of Cycle:
1 . V apo r C o mp ress i on Cy c l e
2 . V apo r A bs orpt io n Cy c le
Vapor-Compression Cycle:
The Vapor Compression Cycle uses energy input to drive a compressor that increases the
pressure and pressure of the refrigerant which is in the vapor state. The refrigerant is then
exposed to the hot section (termed the condenser) of the system, its temperature being higher
than the temperature of this section. As a result, heat is transferred from the refrigerant to the
hot section (i.e. heat is removed from the refrigerant) causing it to condense i.e. for its state to
change from the vapor phase
to the liquid phase (hence
the term condenser). The
refrigerant then passes
through the expansion valve
across which its pressure
and temperature drop
considerably. The
refrigerant temperature is
now below that existing in
the cold or refrigerated
section (termed the
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evaporator) of the system, its temperature being lower than the temperature in this section. As
a result, heat is transferred from the refrigerated section to the refrigerant (i.e. heat is absorbed
by the refrigerant) causing it to pass from the liquid or near-liquid state to the vapor state
again (hence the term evaporator). The refrigerant then again passes to the compressor in
which its pressure is again increased and the whole cycle is repeated.
The four basic components of the vapour compression refrigeration system are thus:
1 . C o m p r e s s o r :
The function of the compressor is to compress the input refrigerant of low pressure and low
temperature. As a result the pressure and the temperature of the refrigerant increases.
Generally reciprocating compressors are used in refrigeration system. An external
motor is used to drive the compressor.
2 . C o n d e n s e r :
The condenser is a coil of tubes, which are made of copper. This issued to
condense the refrigerant which is in the form of vapor. And convert into liquid.
3 . E x p a n s i o n V a l v e :
This is otherwise called throttle valve. This valve is used to
control the flow rate of refrigerant and also to reduce the pressure of the refrigerant.
4 . E v a p o r a t o r :
This is the part in which the cooling takes place. This is kept in the space where cooling is
required. It is a coil of tubes made up of copper.
Compressor
The purpose of the compressor is to circulate the refrigerant in the system under pressure; this
concentrates the heat it contains.
At the compressor, the low pressure gas is changed to high pressure gas.
This pressure build up can only be accomplished by having a restriction in the high
pressure side of the system. This is a small valve located in the expansion valve.
The compressor has reed valves to control the entrance and exit of refrigerant gas during the
pumping operation. These must be firmly seated.
An improperly seated intake reed valve can result in gas leaking back into the low side
during the compression stroke, raising the low side pressure and impairing the cooling
effect.
A badly seated discharge reed valve can allow condensing or head pressure to drop as
it leaks past the valve, lowering the efficiency of the compressor.
Two service valves are located near the compressor as an aid in servicing the system.
One services the high side, it is quickly identified by the smaller discharge hose routed
to the condenser.
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One is used for the low side, the low side comes from the evaporator, and is larger
than the discharge hose
The compressor is normally belt-driven from the engine crankshaft. Most manufacturers use a
magnetic-type clutch which provides a means of stopping the pumping of the compressor
when refrigeration is not desired.
Condenser
The purpose of the condenser is to receive the high-pressure gas from the compressor
and convert this gas to a liquid.
It does it by heat transfer, or the principle that heat will always move from a warmer
to a cooler substance.
Air passing over the condenser coils carries off the heat and the gas condenses.
The condenser often looks like an engine radiator.
Condensers used on R-12 and R-134a systems are not interchangeable. Refrigerant-134a has a
different molecular structure and requires a large capacity condenser.
As the compressor subjects the gas to increased pressure, the heat intensity of the refrigerant
is actually concentrated into a smaller area, thus raising the temperature of the refrigerant
higher than the ambient temperature of the air passing over the condenser coils. Clogged
condenser fins will result in poor condensing action and decreased efficiency.
A factor often overlooked is flooding of the condenser coils with refrigerant oil. Flooding
results from adding too much oil to the system. Oil flooding is indicated by poor condensing
action, causing increased head pressure and high pressure on the low side. This will always
cause poor cooling from the evaporator.
Expansion valve
The expansion valve removes pressure from the liquid refrigerant to allow expansion or
change of state from a liquid to a vapour in the evaporator.
The high-pressure liquid refrigerant entering the expansion valve is quite warm. This may be
verified by feeling the liquid line at its connection to the expansion valve. The liquid
refrigerant leaving the expansion valve is quite cold. The orifice within the valve does not
remove heat, but only reduces pressure. Heat molecules contained in the liquid refrigerant are
thus allowed to spread as the refrigerant moves out of the orifice. Under a greatly reduced
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pressure the liquid refrigerant is at its coldest as it leaves the expansion valve and enters the
evaporator.
Pressures at the inlet and outlet of the expansion valve will closely approximate gauge
pressures at the inlet and outlet of the compressor in most systems. The similarity of pressures
is caused by the closeness of the components to each other. The slight variation in pressure
readings of a very few pounds is due to resistance, causing a pressure drop in the lines and
coils of the evaporator and condenser.
Evaporator
The evaporator works the opposite of the condenser; here refrigerant liquid is converted to
gas, absorbing heat from the air in the compartment.
When the liquid refrigerant reaches the evaporator its pressure has been reduced, dissipating
its heat content and making it much cooler than the fan air flowing around it. This causes the
refrigerant to absorb heat from the warm air and reach its low boiling point rapidly. The
refrigerant then vaporizes, absorbing the maximum amount of heat.
This heat is then carried by the refrigerant from the evaporator as a low-pressure gas through
a hose or line to the low side of the compressor, where the whole refrigeration cycle is
repeated.
The evaporator removes heat from the area that is to be cooled. The desired temperature of
cooling of the area will determine if refrigeration or air conditioning is desired. For example,
food preservation generally requires low refrigeration temperatures, ranging from 40F (4C)
to below 0F (-18C).
A higher temperature is required for human comfort. A larger area is cooled, which requires
that large volumes of air be passed through the evaporator coil for heat exchange. A blower
becomes a necessary part of the evaporator in the air conditioning system. The blower fans
must not only draw heat-laden air into the evaporator, but must also force this air over the
evaporator fins and coils where it surrenders its heat to the refrigerant and then forces the
cooled air out of the evaporator into the space being cooled.
Vapor Absorption Refrigeration System:
The compressor in the vapor compression refrigeration system consumes lot
of energy. To avoid this, the vapor absorption refrigeration system has been developed. In
this system, the compression process of vapor compression cycle is eliminated. Instead of that
the three following process are introduced.
Ammonia vapour is absorbed into water
This mixture is pumped into a high pressure cycle
This solution is heated to produce ammonia vapor. Construction:
The vapor absorption refrigeration system has the following components
Generator:
The generator receives the strong solution of aqua-ammonia from the absorber and heats it.
Because of this heating, the aqua-ammonia solution gets separated into ammonia Vapor at
high pressure and hot weak ammonia solution which contains mostly water.
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Condenser:
The condenser converts the high pressure ammonia vapor received from the generator into
high pressure ammonia liquid. This condensation is done by means of circulating cool water.
Expansion valve:
This valve is otherwise called the throttling valve since the expansion,
which takes place here, is throttling. While passing through this valve, the liquid
ammonia gets expanded and gets converted into low pressure and low temperature ammonia.
Evaporator:
The evaporator is otherwise known as cold chamber. Here the refrigerant absorbs the heat
from the material which is to be cooled and gets evaporated. It has many coils made
of copper.
Absorber:
The absorber receives the low pressure ammonia vapor from the evaporator and the weak
ammonia solution from the generator and mixes them well to form a strong solution of aqua-
ammonia.
Working Principle:
The working fluid in vapor absorption refrigeration system is normally ammonia. The
ammonia vapor and water is mixed to form a strong solution of aqua-ammonia in the
absorber. This aqua-ammonia solution is then pumped into the generator. In the generator,
this solution is heated. Because of heating, ammonia gets evaporated at high pressure and
leaves behind the weak ammonia solution, which mostly contains water.The high pressure
ammonia vapor produced by the generator is condensed in the condenser and it becomes
ammonia liquid, which is at high pressure. This high pressure liquid ammonia is allowed to
pass through the expansion valve or throttling valve where it expands and becomes a low
pressure and low temperature ammonia which mostly contains liquid ammonia and a little
vapor ammonia.
Ammonia at low pressure
and low temperature then
passes through the
evaporator where it absorbs
the heat from the material
which is to be cooled and
gets evaporated. The
evaporator is where the real
cooling takes place. Because
of the heat absorbed by
ammonia, it gets evaporated
and becomes low pressure
ammonia vapor. The low
pressure ammonia vapour is then sent into the absorber and the cycle is repeated.
Comparison between VCRS and VARS:
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Vapor Compression Refrigeration System - Vapor Absorption Refrigeration System
It is more noise and wear and tear because of
more moving parts.
The system is comparatively quieter.
Mechanical energy is utilized by means of
compressor
Heat energy is utilized
Refilling of refrigerant is easy Refilling of refrigerant is difficult
During partial loading conditions the perform
ance is poor
The performance is not affected even at the
partial loading
The liquid refrigerant accumulated in the
cylinder may damage the cylinder. So
preventive measures are needed.
Liquid refrigerants do not affect the performa
nce of the system. They do not produce any
bad effect.
Air Conditioning:
It is the process of controlling and maintaining the properties of air like temperature,
humidity, purity, direction of flow etc in a closed space. One can have the desired condition
around him using air conditioning.
Terms in Air Conditioning:
Psychrometry:
It is the study of the properties of moist air. The properties of the air and water vapor mixture
are called psychometric properties.
Dry Air:
Atmospheric air without presence of water vapor is called dry air. It is combination of 79% of
nitrogen and 21% of oxygen by weight.
Moist Air:
It is the mixture of dry air and water vapor. The amount of water vapor present varies
according to the temperature.
Dry Bulb Temperature (DBT):
It is the temperature of the air measured using an ordinary thermometer. This temperature is
not affected by the water vapor present in the air.
Wet Bulb Temperature (WBT):
It is the temperature measured by ordinary thermometer when its bulb is covered with wet
cloth and exposed to air. It is always less than DBT.
Wet Bulb Depression (WBD):
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It is the difference between the dry bulb temperature and the wet bulb temperature. If the air is
fully saturated then the wet bulb depression is zero.
Dew Point Temperature (DPT):
The temperature at which the water Vapor in the air begins to condense when the temperature
of the air is continuously reduced.
Humidity:
The quantity of water vapor present in the air is known as humidity. It depends on the
temperature of the air and is independent of the pressure of the air.
Relative Humidity:
It is defined as the ratio of mass of water vapor present in a given volume of air at a given
temperature to the mass of water vapor present in the same volume and temperature of the air
when it is fully saturated.
Air handling unit (AHU) a central unit consisting of a blower, heating and cooling elements, filters, etc. that is in direct contact with the airflow.
To improve air quality circulating air is mixed with fresh air
Usually equipped with a heat recovery unit for energy saving purposes
Supply air temperature kept constant so that temperature can be adjusted locally with thermostats.
Chillers a device that removes heat from a liquid. The cooled liquid flows through pipes and passes through coils in air handling units, FCUs, etc
Damper a plate or gate placed in a duct to control airflow
Fan coil unit (FCU) a small terminal unit that is often composed of only a blower and a cooling coil
Variable air volume (VAV) an HVAC system that has a stable supply air temperature and varies the airflow rate with dampers and adjusting fan speeds to meet the temperature
requirements
Working of a Air conditioning system
It consists of dampers, air filter, cooling coil, spray type humidifier, heating coil and a fan.
Atmospheric air flows through the dampers. The quantity of air depends upon the load and the dampers control it. Air then passes through the Air filter. The filter removes dirt, dust and
other impurities. The air now passes over a cooling coil. So when air is cooled below its dew
point temperature, the water vapour is removed from the air in the form of water droplets. The
surface temperature of the cooling coil has to be maintained below the dew-point temperature
of the atmospheric air to accomplish dehumidification. The quantity of water removed from
air is collected in the sump and is drained. The temperature of air leaving the cooling coil is
lower than the ambient temperature for comfort. During the dry weather the spray type
humidifier is used to increase the humidity of the conditioned air. During wet weather
condition the relative humidity of the air is high, is controlled by the heating coil. For the
comfort condition required is DBT around23 degree c and relative humidity 60%. So the air is
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to be cooled and humidified to the comfort condition. Now the conditioned air is supplied to
the conditioned space by a fan and ducts.
Working of a Window Air conditioning system
It is called a window air conditioner because it is usually fixed in a window. The Window or
Room air conditioner is used to cool a single room or a large space. This window room air
conditioner system has four main components. They are
An entire cooling system, which includes a condenser, compressor and an evaporator.
A fan and adjustable grills to ensure proper circulation of air.
A filter, which is made of fibre, mesh or glass wool to remove the impurities in the air.
Controlling equipments to regulate the properties of the air.
The working of the window air conditioner shown in Figure is described asunder: The
refrigerant vapour leaving the compressor is at high pressure and temperature. It then passes
through the condenser. Outside air is drawn in by the fan and it cools the refrigerant in the
condenser, the refrigerant then becomes liquid. The high pressure, low temperature liquid
refrigerant enters the expansion valve. The pressure and temperature of the refrigerant falls
when it leaves the valve. The cold refrigerant from the valve passes through the evaporator
(the evaporator side of the air conditioner faces the room to be cooled). The warm air from the
room is drawn in by blower. The evaporator cools this air and the liquid inside the evaporator
tube gets vaporized by absorbing the heat from the warm air. The cool air is again sent to the
room through the opening at the top of the air conditioning unit. The liquid and vapour
refrigerant from the evaporator passes to the compressor and is compressed to high-pressure,
high temperature liquid. The operation hereafter is carried out in cycle as the same manner as
explained.
The amount of air circulated into the room can be controlled by the dampers provided. When
air flows over the cooling coil or the evaporator coil, the moisture in the air gets condensed
and they are made to drip into the trays provided below the coils. This water evaporates to
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some extend and thus helps in cooling the compressor and condenser. For every cycle, the
temperature of the air keeps on reducing. The unit automatically stops with the help of
thermostat and control panel, when the required temperature is reached inside the room.
Split Air Conditioner:
A Streamlined and light-weight air handler is mounted on the inside wall. Refrigerant and
condensate lines run through a small hole in the wall to the outside unit. Initial power is to the
outside unit and then relayed to the air handler. Extremely quiet as the compressor and
condenser coil are outside. Full electronic and remote control. The compressor (6) in the
exterior unit compresses the refrigerant into a high-temperature, high-pressure gas. When this
gas flows along the cooling fins of the condenser (7), heat is exuded and the gas is led to the
evaporator (1) in the interior unit. The liquid expands into a gas at a low temperature and low
pressure. This gas absorbs the warmth of the air in the room, the cooled air is blown back into
the room and the heat is led to the compressor along with the gas.
A fan (3) draws the air (a) over the filter (2) and blows the cooled air (b) back into the room.
A fan (8) draws air over the condenser and blows warm air (d) away. As with cooling, the
moisture in the air condenses on the cold evaporator at room temperature.
Evaporative Cooling: As the name indicates, evaporative cooling is the process of reducing the temperature of a
system by evaporation of water. Human beings perspire and dissipate their metabolic heat by
evaporative cooling if the ambient temperature is more than skin temperature. Animals such
as the hippopotamus and buffalo coat themselves with mud for evaporative cooling.
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Evaporative cooling has been used in India for centuries to obtain cold water in summer by
storing the water in earthen pots. The water permeates through the pores of earthen vessel to
its outer surface where it evaporates to the surrounding, absorbing its latent heat in part from
the vessel, which cools the water. It is said that Patliputra University situated on the bank of
river Ganges used to induce the evaporative-cooled air from the river. Suitably located
chimneys in the rooms augmented the upward flow of warm air, which was replaced by cool
air. Evaporative cooling by placing wet straw mats on the windows is also very common in
India. The straw mat made from khus adds its inherent perfume also to the air. Now-a-days coolers are being used in hot and dry areas to provide cooling in summer.
Direct evaporative cooling (open circuit) is used to lower the temperature of air by using
latent heat of evaporation, changing liquid water to water vapor. In this process, the energy in
the air does not change. Warm dry air is changed to cool moist air. The heat of the outside air
is used to evaporate water.
Indirect evaporative cooling (closed circuit) is similar to direct evaporative cooling, but
uses some type of heat exchanger. The cooled moist air never comes in direct contact with the
conditioned environment.
Advantages and disadvantages of evaporative cooling systems:
Compared to the conventional refrigeration based air conditioning systems, the evaporative
cooling systems offer the following advantages:
1. Lower equipment and installation costs
2. Substantially lower operating and power costs. Energy savings can be as high as 75%
3. Ease of fabrication and installation
4. Lower maintenance costs
5. Ensures a very good ventilation due to the large air flow rates involved, hence, are very
good especially in 100 % outdoor air applications
6. Better air distribution in the conditioned space due to higher flow rates
7. The fans/blowers create positive pressures in the conditioned space, so that infiltration of
outside air is prevented
8. Very environment friendly as no harmful chemicals are used
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Compared to the conventional systems, the evaporative cooling systems suffer from the
following disadvantages:
1. The moisture level in the conditioned space could be higher, hence, direct evaporative
coolers are not good when low humidity levels in the conditioned space is required. However,
the indirect evaporative cooler can be used without increasing humidity
2. Since the required air flow rates are much larger, this may create draft and/or high noise
levels in the conditioned space
3. Precise control of temperature and humidity in the conditioned space is not possible
4. May lead to health problems due to micro-organisms if the water used is not clean or the
wetted surfaces are not maintained properly.
Depending upon the following factors the conditioning systems has its own advantages
and disadvantages:
Ducts
Portability
Aesthetics
Flexible Sizing
Affordability
Air conditioning systems for large buildings:
Selection criteria for air conditioning systems:
Selection of a suitable air conditioning system depends on:
Capacity, performance and spatial requirements
Initial and running costs
Required system reliability and flexibility
Maintainability
Architectural constraints
The relative importance of the above factors varies from building owner to owner and may
vary from project to project. The typical space requirement for large air conditioning systems
may vary from about 4 percent to about 9 percent of the gross building area, depending upon
the type of the system. Normally based on the selection criteria, the choice is narrowed down
to 2 to 3 systems, out of which one will be selected finally.
Classification of air conditioning systems:
Based on the fluid media used in the thermal distribution system, air conditioning systems can
be classified as:
1. All air systems 2. All water systems 3. Air- water systems 4. Unitary refrigerant based systems
All air systems:
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As the name implies, in an all air system air is used as the media that transports energy from
the conditioned space to the A/C plant. In these systems air is processed in the A/C plant and
this processed air is then conveyed to the conditioned space through insulated ducts using
blowers and fans. This air extracts (or supplies in case of winter) the required amount of
sensible and latent heat from the conditioned space. The return air from the conditioned space
is conveyed back to the plant, where it again undergoes the required processing thus
completing the cycle. No additional processing of air is required in the conditioned space. All
air systems can be further classified into:
1. Single duct systems, or
2. Dual duct systems
The single duct systems can provide either cooling or heating using the same duct, but not
both heating and cooling simultaneously. These systems can be further classified into:
1. Constant volume, single zone systems
2. Constant volume, multiple zone systems
3. Variable volume systems
The dual duct systems can provide both cooling and heating simultaneously.
These systems can be further classified into:
1. Dual duct, constant volume systems
2. Dual duct variable volume systems
Single duct, constant volume, single zone systems:
Figure 36.2 shows the classic, single duct, single zone, constant volume systems. As shown
in the figure, outdoor air (OD air) for ventilation and re circulated air (RC air) are mixed in
the required proportions using the dampers and the mixed air is made to flow through a
cooling and dehumidifying coil, a heating coil and a humidifier using a an insulated ducting
and a supply fan. As the air flows through these coils the temperature and moisture content of
the air are brought to the required values. Then this air is supplied to the conditioned space,
where it meets the building cooling or heating requirements. The return air leaves the
conditioned space, a part of it is re circulated and the remaining part is vented to the
atmosphere. A thermostat senses the temperature of air in the conditioned space and controls
the amount of cooling or heating provided in the coils so that the supply air temperature can
be controlled as per requirement. A humidistat measures the humidity ratio in the conditioned
space and controls the amount of water vapour added in the humidifier and hence the supply
air humidity ratio as per requirement.
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This system is called as a single duct system as there is only one supply duct, through which
either hot air or cold air flows, but not both simultaneously. It is called as a constant volume
system as the volumetric flow rate of supply air is always maintained constant. It is a single
zone system as the control is based on temperature and humidity ratio measured at a single
point. Here a zone refers to a space controlled by one thermostat. However, the single zone
may consist of a single room or one floor or whole of a building consisting of several rooms.
The cooling/ heating capacity in the single zone, constant volume systems is regulated by
regulating the supply air temperature and humidity ratio, while keeping the supply airflow
rate constant. A separate sub-system controls the amount of OD air supplied by controlling
the damper position.
Since a single zone system is controlled by a single thermostat and humidistat, it is important
to locate these sensors in a proper location, so that they are indicative of zone conditions.
The supply air conditions are controlled by either coil control or face-and-bypass control.
In coil control, supply air temperature is controlled by varying the flow rate of cold and hot
water in the cooling and heating coils, respectively. As the cooling season gradually changes
to heating season, the cooling coil valve is gradually closed and heating coil valve is opened.
Though coil control is simpler, using this type of control it is not possible to control the zone
humidity precisely as the dehumidification rate in the cooling coil decreases with cold water
flow rate. Thus at low cold water flow rates, the humidity ratio of the conditioned space is
likely to be higher than required.
In face-and-bypass control, the cold and hot water flow rates are maintained constant, but
the amount of air flowing over the coils are decreased or increased by opening or closing the
by-pass dampers, respectively. By this method it is possible to control the zone humidity
more precisely, however, this type of control occupies more space physically and is also
expensive compared to coil control.
Applications of single duct, single zone, constant volume systems: 1. Spaces with uniform loads, such as large open areas with small external loads e.g. theatres,
auditoria, departmental stores.
2. Spaces requiring precision control such as laboratories
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The Multiple, single zone systems can be used in large buildings such as factories, office
buildings etc.
Single duct, constant volume, multiple zone systems: For very large buildings with several zones of different cooling/heating requirements, it is not
economically feasible to provide separate single zone systems for each zone. For such cases,
multiple zone systems are suitable. Figure 36.3 shows a single duct, multiple zone system
with terminal reheat coils. In these systems all the air is cooled and dehumidified (for
summer) or heated and humidified (for winter) to a given minimum or maximum temperature
and humidity ratio. A constant volume of this air is supplied to the reheat coil of each zone. In
the reheat coil the supply air temperature is increased further to a required level depending
upon the load on that particular zone. This is achieved by a zone thermostat, which controls
the amount of reheat, and hence the supply air temperature. The reheat coil may run on either
electricity or hot water.
Advantages of single duct, multiple zone, constant volume systems with reheat coils: a) Relatively small space requirement
b) Excellent temperature and humidity control over a wide range of zone loads
c) Proper ventilation and air quality in each zone is maintained as the supply air amount is
kept constant under all conditions
Disadvantages of single duct, multiple zone, and constant volume systems with reheat
coils: a) High energy consumption for cooling, as the air is first cooled to a very low temperature
and is then heated in the reheat coils. Thus energy is required first for cooling and then for
reheating. The energy consumption can partly be reduced by increasing the supply air
temperature, such that at least one reheat coil can be switched-off all the time. The energy
consumption can also be reduced by using waste heat (such as heat rejected in the condensers)
in the reheat coil.
b) Simultaneous cooling and heating is not possible.
Single duct, variable air volume (VAV) systems: Figure 36.4 shows a single duct, multiple zone, and variable air volume system for summer
air conditioning applications. As shown, in these systems air is cooled and dehumidified to a
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required level in the cooling and dehumidifying coil (CC). A variable volume of this air is
supplied to each zone. The amount of air supplied to each zone is controlled by a zone
damper, which in turn is controlled by that zone thermostat as shown in the figure. Thus the
temperature of supply air to each zone remains constant, whereas its flow rate varies
depending upon the load on that particular zone.
Compared to constant volume systems, the variable air volume systems offer advantages such
as:
a) Lower energy consumption in the cooling system as air is not cooled to very low
temperatures and then reheated as in constant volume systems.
b) Lower energy consumption also results due to lower fan power input due to lower flow
rate, when the load is low. These systems lead to significantly lower power consumption,
especially in perimeter zones where variations in solar load and outside temperature allows
for reduced air flow rates.
However, since the flow rate is controlled, there could be problems with ventilation, IAQ and
room air distribution when the zone loads are very low. In addition it is difficult to control
humidity precisely using VAV systems. Balancing of dampers could be difficult if the airflow
rate varies widely. However, by combining VAV systems with terminal reheat it is possible to
maintain the air flow rate at a minimum required level to ensure proper ventilation and room
air distribution. Many other variations of VAV systems are available to cater to a wide variety
of applications.
Dual duct, constant volume systems: Figure 36.5 shows the schematic of a dual duct, constant volume system. As shown in the
figure, in a dual duct system the supply air fan splits the flow into two streams. One stream
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flow through the cooling coil and gets cooled and dehumidified to about 13o
C, while the other
stream flows the heating coil and is heated to about 3545o
C. The cold and hot streams flow
through separate ducts. Before each conditioned space or zone, the cold and hot air streams
are mixed in required proportions using a mixing box arrangement, which is controlled by the
zone thermostat. The total volume of air supplied to each zone remains constant, however, the
supply air temperature varies depending upon load.
Advantages of dual duct systems: 1. Since total airflow rate to each zone is constant, it is possible to maintain proper IAQ and
room air distribution.
2. Cooling in some zones and heating in other zones can be achieved simultaneously
3. System is very responsive to variations in the zone load, thus it is possible to maintain
required conditions precisely.
Disadvantages of dual duct systems:
1. Occupies more space as both cold air and hot air ducts have to be sized to handle all air
flow rate, if required.
2. Not very energy efficient due to the need for simultaneous cooling and heating of the air
streams. However, the energy efficiency can be improved by completely shutting down the
cooling coil when the outside temperature is low and mixing supply air from fan with hot air
in the mixing box. Similarly, when the outside weather is hot, the heating coil can be
completely shut down, and the cold air from the cooling coil can be mixed with supply air
from the fan in the mixing box.
Dual duct, variable air volume systems:
These systems are similar to dual duct, constant volume systems with the only difference that
instead of maintaining constant flow rates to each zone, the mixing boxes reduce the air flow
rate as the load on the zone drops.
Outdoor air control in all air systems: Outdoor air is required for ventilation purposes. In all air systems, a sub-system controls the
amount of outdoor air by controlling the position of exhaust, re-circulated and outdoor air
dampers. From mass balance, since the outdoor airflow rate should normally be equal to the
exhaust airflow rate (unless building pressurization or de-pressurization is required), both the
exhaust and outdoor air dampers open or close in unison. Again from mass balance, when the
outdoor air damper opens the re-circulated air damper closes, and vice versa. The control
system maintains a minimum amount of outdoor air (about 10 to 20% of supply air flow rate
as required for ventilation) when the outdoor is too cold (30oC) or too warm ( 24oC). For energy conservation, the amount of outdoor air can be increased gradually as the outdoor air
temperature increases from 30oC to about 13oC. A 100 percent outdoor air can be used when the outdoor air temperature is between 13oC to about 24oC. By this method it is
possible to reduce the annual energy consumption of the air conditioning system significantly,
while maintaining the required conditions in the conditioned space.
Advantages of all air systems: 1. All air systems offer the greatest potential for energy conservation by utilizing the outdoor
air effectively.
2. By using high-quality controls it is possible to maintain the temperature and relative
humidity of the conditioned space within 0.15o
C (DBT) and 0.5%, respectively.
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3. Using dual duct systems, it is possible to provide simultaneous cooling and heating.
Changeover from summer to winter and vice versa is relatively simple in all air systems.
4. It is possible to provide good room air distribution and ventilation under all conditions of
load.
5. Building pressurization can be achieved easily.
6. The complete air conditioning plant including the supply and return air fans can be located
away from the conditioned space. Due to this it is possible to use a wide variety of air filters
and avoid noise in the conditioned space.
Disadvantages of all air systems: 1. They occupy more space and thus reduce the available floor space in the buildings. It could
be difficult to provide air conditioning in high-rise buildings with the plant on the ground
floor or basement due to space constraints.
2. Retrofitting may not always be possible due to the space requirement.
3. Balancing of air in large and particularly with variable air volume systems could be
difficult.
Applications of all air systems: All air systems can be used in both comfort as well as industrial air conditioning
applications. They are especially suited to buildings that require individual control of multiple
zones, such as office buildings, classrooms, laboratories, hospitals, hotels, ships etc. They
are also used extensively in applications that require very close control of the conditions in the
conditioned space such as clean rooms, computer rooms, operation theatres, research
facilities etc.
All water systems:
In all water systems the fluid used in the thermal distribution system is water, i.e., water
transports energy between the conditioned space and the air conditioning plant. When cooling
is required in the conditioned space then cold water is circulated between the conditioned
space and the plant, while hot water is circulated through the distribution system when
heating is required. Since only water is transported to the conditioned space, provision must
be there for supplying required amount of treated, outdoor air to the conditioned space for
ventilation purposes. Depending upon the number of pipes used, the all water systems can be
classified into a 2-pipe system or a 4-pipe system.
A 2-pipe system is used for either cooling only or heating only application, but cannot be used
for simultaneous cooling and heating. Figure 36.6 shows the schematic of a 2-pipe, all water
system. As shown in the figure and as the name implies, a 2-pipe system consists of two pipes
one for supply of cold/hot water to the conditioned space and the other for the return water. A cooling or heating coil provides the required cold or hot water. As the supply water flows
through the conditioned space, required heat transfer between the water and conditioned space
takes place, and the return water flows back to the cooling or heating coil. A flow control
valve controls the flow rate of hot or cold water to the conditioned space and thereby meets
the required building heating or cooling load. The flow control valve is controlled by the zone
thermostat. As already mentioned, a separate arrangement must be made for providing the
required amount of ventilation air to the conditioned space. A pressure relief valve (PRV) is
installed in the water line for maintaining balanced flow rate.
A 4-pipe system consists of two supply pipelines one for cold water and one for hot water; and two return water pipelines. The cold and hot water are mixed in a required proportion
depending upon the zone load, and the mixed water is supplied to the conditioned space. The
return water is split into two streams, one stream flows to the heating coil while the other
flows to the cooling coil.
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Heat transfer between the cold/hot water and the conditioned space takes place either by
convection, conduction or radiation or a combination of these. The cold/hot water may flow
through bare pipes located in the conditioned space or one of the following equipment can be
used for transferring heat:
1. Fan coil units
2. Convectors
3. Radiators etc.
A fan coil unit is located inside the conditioned space and consists of a heating and/or cooling
coil, a fan, air filter, drain tray and controls. Figure 36.7 shows the schematic of a fan coil unit
used
for
cooling applications. As shown in the figure, the basic components of a fan coil unit are:
finned tube cooling coil, fan, air filter, insulated drain tray with provision for draining
condensate water and connections for cold water lines. The cold water circulates through the
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finned tube coil while the blower draws warm air from the conditioned space and blows it
over the cooling coil. As the air flows through the cooling coil it is cooled and dehumidified.
The cold and dehumidified air is supplied to the conditioned space for providing required
conditions inside the conditioned space. The water condensed due to dehumidification of
room air has to be drained continuously. A cleanable or replaceable filter is located in the
upstream of the fan to prevent dust accumulation on the cooling coil and also to protect the
fan and motor from dust. Fan coil units for domestic air conditioning are available in the
airflow range of 100 to 600 l/s, with multi-speed, high efficiency fans. In some designs, the
fan coil unit also consists of a heating coil, which could be in the form of an electric heater or
steam or hot water coil. Electric heater is used with 2-pipe systems, while the hot water/steam
coils are used with 4-pipe systems. The fan coil units are floor mounted, window mounted or
ceiling mounted. The capacity of a fan coil unit can be controlled either by controlling the
cold water flow rate or by controlling air flow rate or both. The airflow rate can be controlled
either by a damper arrangement or by varying the fan speed. The control may be manual or
automatic, in which case, a room thermostat controls the capacity. Since in the fan coil unit
there is no provision for ventilation, a separate arrangement must be made to take care of
ventilation. A fan coil unit with a provision for introducing treated ventilation air to the
conditioned space is called as unit ventilator.
A convector consists of a finned tube coil through which hot or cold fluid flows. Heat
transfer between the coil and surrounding air takes place by natural convection only, hence no
fans are used for moving air. Convectors are very widely used for heating applications, and
very rarely are used for cooling applications.
In a radiator, the heat transfer between the coil and the surrounding air is primarily by
radiation. Some amount of heat is also transferred by natural convection. Radiators are widely
used for heating applications, however, in recent times they are also being used for cooling
applications.
Advantages of all water systems: 1. The thermal distribution system requires very less space compared to all air systems. Thus
there is no penalty in terms of conditioned floor space. Also the plant size will be small due to
the absence of large supply air fans.
2. Individual room control is possible, and at the same time the system offers all the benefits
of a large central system.
3. Since the temperature of hot water required for space heating is small, it is possible to use
solar or waste heat for winter heating.
4. It can be used for new as well existing buildings (retrofitting).
5. Simultaneous cooling and heating is possible with 4-pipe systems.
Disadvantages of all water systems: 1. Requires higher maintenance compared to all air systems, particularly in the conditioned
space.
2. Draining of condensate water can be messy and may also create health problems if water
stagnates in the drain tray. This problem can be eliminated, if dehumidification is provided by
a central ventilation system, and the cooling coil is used only for sensible cooling of room air.
3. If ventilation is provided by opening windows or wall apertures, then, it is difficult to
ensure positive ventilation under all circumstances, as this depends on wind and stack effects.
4. Control of humidity, particularly during summer is difficult using chilled water control
valves.
Applications of all water systems:
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All water systems using fan coil units are most suitable in buildings requiring individual room
control, such as hotels, apartment buildings and office buildings.
Air-water systems:
In air-water systems both air and water are used for providing required conditions in the
conditioned space. The air and water are cooled or heated in a central plant. The air supplied
to the conditioned space from the central plant is called as primary air, while the water
supplied from the plant is called as secondary water. The complete system consists of a
central plant for cooling or heating of water and air, ducting system with fans for conveying
air, water pipelines and pumps for conveying water and a room terminal. The room terminal
may be in the form of a fan coil unit, an induction unit or a radiation panel. Figure 36.8 shows
the schematic of a basic air-water system. Even though only one conditioned space is shown
in the schematic, in actual systems, the air-water systems can simultaneously serve several
conditioned spaces.
Normally a constant volume of primary air is supplied to each zone depending upon the
ventilation requirement and the required sensible cooling capacity at maximum building load.
For summer air conditioning, the primary air is cooled and dehumidified in the central plant,
so that it can offset the entire building latent load. Chilled water is supplied to the conditioned
space to partly offset the building sensible cooling load only. Since the chilled water coil kept
in the conditioned space has to take care of only sensible load, condensation of room air
inside the conditioned space is avoided thereby avoiding the problems of condensate drainage
and related problems in the conditioned space. As mentioned, the primary takes care of the
ventilation requirement of the conditioned space, hence unlike in all water systems, there is no
need for separate ventilation systems. In winter, moisture can be added to the primary air in
the central plant and hot water is circulated through the coil kept in the conditioned space. The
secondary water lines can be of 2-pipe, 3-pipe or 4-pipe type similar to all water systems.
As mentioned the room unit may be in the form of a fan coil unit, an induction unit or in the
form of a radiant panel. In an induction unit the cooling/heating coil is an integral part of the
primary air system. The primary air supplied at medium to high pressure to the induction unit,
induces flow of secondary air from the conditioned space. The secondary air is sensibly
cooled or heated as it flows through the cooling/heating coil. The primary and secondary air
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are mixed and supplied to the conditioned space. The fan coil units are similar to the ones
used in all water systems.
Advantages of air-water systems: 1. Individual zone control is possible in an economic manner using room thermostats, which
control either the secondary water flow rate or the secondary air (in fan coil units) or both.
2. It is possible to provide simultaneous cooling and heating using primary air and secondary
water.
3. Space requirement is reduced, as the amount of primary supplied is less than that of an all
air systems.
4. Positive ventilation can be ensured under all conditions.
5. Since no latent heat transfer is required in the cooling coil kept in the conditioned space, the
coil operates dry and its life thereby increases and problems related to odours or fungal
growth in conditioned space is avoided.
6. The conditioned space can sometimes be heated with the help of the heating coil and
secondary air, thus avoiding supply of primary air during winter.
7. Service of indoor units is relatively simpler compared to all water systems.
Disadvantages of air-water systems: 1. Operation and control are complicated due to the need for handling and controlling both
primary air and secondary water.
2. In general these systems are limited to perimeter zones.
3. The secondary water coils in the conditioned space can become dirty if the quality of filters
used in the room units is not good.
4. Since a constant amount of primary air is supplied to conditioned space, and room control
is only through the control of room cooling/heating coils, shutting down the supply of primary
air to unoccupied spaces is not possible.
5. If there is abnormally high latent load on the building, then condensation may take place on
the cooling coil of secondary water.
6. Initial cost could be high compared to all air systems.
Applications of air-water systems: These systems are mainly used in exterior buildings with large sensible loads and where
close control of humidity in the conditioned space is not required. These systems are thus
suitable for office buildings, hospitals, schools, hotels, apartments etc.
Unitary refrigerant based systems:
Unitary refrigerant based systems
consist of several separate air
conditioning units with individual
refrigeration systems. These systems
are factory assembled and tested as per
standard specifications, and are
available in the form of package units
of varying capacity and type. Each
package consists of refrigeration
and/or heating units with fans, filters,
controls etc. Depending upon the
requirement these are available in the
form of window air conditioners, split
air conditioners, heat pumps, ductable systems with air cooled or water cooled condensing
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units etc. The capacities may range from fraction of TR to about 100 TR for cooling.
Depending upon the capacity, unitary refrigerant based systems are available as single units
which cater to a single conditioned space, or multiple units for several conditioned spaces.
Figure 36.9 shows the schematic of a typical window type, room air conditioner, which is
available in cooling capacities varying from about 0.3 TR to about 3.0 TR. As the name
implies, these units are normally mounted either in the window sill or through the wall. As
shown in the figure, this type of unit consists of single package which includes the cooling
and
dehumidification coil, condenser coil, a hermetic compressor, expansion device (capillary
tube), condenser fan, evaporator fan, room air filter and controls. A drain tray is provided at
the bottom to take care of the condensate water. Both evaporator and condensers are plate fin-
and-tube, forced convection type coils. For rooms that do not have external windows or walls,
a split type room air conditioner can be used. In these air conditioners, the condensing unit
comprising of the condenser, compressor and condenser fan with motor are located outside,
while the indoor unit consisting of the evaporator, evaporator fan with motor, expansion valve
and air filter is located inside the conditioned room. The indoor and outdoor units are
connected by refrigerant piping. In split type air conditioners, the condensed water has to be
taken away from the conditioned space using separate drain pipes. In the room air
conditioners (both window mounted and split type), the cooling capacity is controlled by
switching the compressor on-and-off. Sometimes, in addition to the on-and-off, the fan speed
can also be regulated to have a modular control of capacity. It is also possible to switch off the
refrigeration system completely and run only the blower for air circulation. Figure 36.10
shows a typical package unit with a remote condensing unit. As shown, in a typical package
unit, the remote condensing unit consists of the compressor and a condenser, while the indoor
unit consists of the plate fin-and-tube type, evaporator, a blower, air filter, drain tray and an
arrangement for connecting supply air and return air ducts. These units are available in
capacities ranging from about 5 TR to up to about 100 TR. The condenser used in these
systems could be either air cooled or water cooled. This type of system can be used for
providing air conditioning in a large room or it can cater to several small rooms with suitable
supply and return ducts. It is also possible to house the entire refrigeration in a single package
with connections for water lines to the water cooled condenser and supply and return air
ducts. Larger systems are either constant air volume type or variable air volume type. They
may also include heating coils along with the evaporator.
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Condensing unit Cold air Most of the unitary systems have a provision for supplying outdoor air for ventilation
purposes. The type of control depends generally on the capacity of the unit. The control
system could be as simple as a simple thermostat based on-off control as in room air
conditioners to sophisticated microprocessor based control with multiple compressors or
variable air volume control or a combination of both.
Advantages of unitary refrigerant based systems: 1. Individual room control is simple and inexpensive.
2. Each conditioned space has individual air distribution with simple adjustment by the
occupants.
3. Performance of the system is guaranteed by the manufacturer.
4. System installation is simple and takes very less time.
5. Operation of the system is simple and there is no need for a trained operator.
6. Initial cost is normally low compared to central systems.
7. Retrofitting is easy as the required floor space is small.
Disadvantages of unitary refrigerant based systems: 1. As the components are selected and matched by the manufacturer, the system is less
flexible in terms of air flow rate, condenser and evaporator sizes.
2. Power consumption per TR could be higher compared to central systems.
3. Close control of space humidity is generally difficult.
4. Noise level in the conditioned space could be higher.
5. Limited ventilation capabilities.
6. Systems are generally designed to meet the appliance standards, rather than the building
standards.
7. May not be appealing aesthetically.
8. The space temperature may experience a swing if on-off control is used as in room air
conditioners.
9. Limited options for controlling room air distribution.
10. Equipment life is relatively short.
Applications of unitary refrigerant based systems: Unitary refrigerant based systems are used where stringent control of conditioned space
temperature and humidity is not required and where the initial cost should be low with a small
lead time. These systems can be used for air conditioning individual rooms to large office
buildings, classrooms, hotels, shopping centers, nursing homes etc. These systems are
especially suited for existing building with a limitation on available floor space for air
conditioning systems.
Chilled water system:
The supply air, which is approximately 20 F cooler than the air in the conditioned space,
leaves the cooling coil through the supply air fan, down to the ductwork and into the
conditioned space. The cool supply air picks up heat in the conditioned space and the warmer
air makes its way into the return air duct back to the air handling unit. The return air mixes
with outside air in a mixing chamber and goes through the filters and cooling coil. The mixed
air gives up its heat into the chilled water tubes in the cooling coil, which has fins attached to
the tubes to facilitate heat transfer. The cooled supply air leaves the cooling coil and the air
cycle repeats.
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The chilled water circulating through the cooling coil tubes, after picking up heat from the
mixed air, leaves the cooling coil and goes through the chilled water return (CHWR) pipe to
the chiller's evaporator. Here it gives up the heat into the refrigeration system. The newly
"chilled" water leaves the evaporator and is pumped through the chilled water supply
(CHWS) piping into the cooling coil continuously and the water cycle repeats.
The evaporator is a heat exchanger that allows heat from the CHWR to flow by conduction
into the refrigerant tubes. The liquid refrigerant in the tubes "boils off" to a vapor removing
heat from the water and conveying the heat to the compressor and then to the condenser. The
heat from the condenser is conveyed to the cooling tower by the condenser water. Finally,
outside air is drawn across the cooling tower, removing the heat from the water through the
process of evaporation.
The figure above provides a conceptual view of chilled water air-conditioning system with
water cooled condenser.
The main equipment used in the chilled water system is a chillers package that includes
1) A refrigeration compressor (reciprocating, scroll, screw or centrifugal type),
2) Shell and tube heat exchanger (evaporator) for chilled water production
3) Shell and tube heat exchanger (condenser) for heat rejection in water cooled configuration
(alternatively, air cooled condenser can be used, where water is scarce or its use is prohibited)
4) A cooling tower to reject the heat of condenser water
5) An expansion valve between condenser and the evaporator
The chilled water system is also called central air conditioning system. This is because the
chilled water system can be networked to have multiple cooling coils distributed through out a
large or distributed buildings with the refrigeration equipment (chillers) placed at one base
central location.
Central Systems Are Complex
Central Systems comprise one or more large mechanical spaces
Sizable distribution trees are used.
Central Systems are generally Direct Expansion (DX) or Chilled Water systems.
Chilled water systems are marginally more efficient than DX systems.
Chilled water systems cool water, instead of air, and pass it through heat exchangers to cool the air.
Water treatment may be required to control corrosion and scaling.
Chillers cool the building by removing heat from water which has passed through the evaporators.
Cooling towers are installed in large systems to increase efficiency.
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Water cooled systems pass water over the condenser coils.
The water may be recycled after cooling in an atmospheric cooling tower.
Water cools as it falls through the air.
Large blowers may also be used to increase cooling by forcing air through the falling water.
DX systems directly cool air and distribute it by air-handling units.
Air-handling units can be combined with the cooling equipment or separate.
When separate, air-handling units can be centrally located in a building or distributed on each floor.
Central cooling and local air-distribution takes advantage of benefits attributed to both the central and local systems
Distribution trees are large because air has low heat-capacity. Chilled water systems use all-water or air-and-water
Chilled water systems are frequently used in the perimeter zones.
A two-pipe system is used for cooling alone
Chillers for 2-pipe systems occupy 0.2 - 1.0% of the gross floor area.
All-water fan-coil units can be located against an exterior wall.
Air-and-water induction systems use two coils.
Induction systems are well suited for multi-zone applications.
All-Air Systems Require More Space
o Single-duct Variable-Air-Volume systems require smaller distribution trees. Air-handling units supply a cooled stream of air at normal velocity and
pre-determined temperature.
Automatic volume controls connected to a zones thermostat adjust the volume of air admitted.
A zone needing more cooling received more air, and vice-versa.
o Size of Air-handling equipment and their rooms are smaller for lower rates of air flow.
Fan speeds should be reduced so that temperature-sensitive thermostats will permit sufficient de-humidification to take place.
EEBC-94 requirements different ventilation rates. The ventilation rate for non-smoking occupants is 3.5 L/s.
The rate for smoking occupants is 11.8 L/s. Low-pressure ductwork is larger.
o Cooling equipment for a low velocity single-zone system requires 0.2 - 1% of the gross floor area.
o Air-handling units for a low velocity single-zone system require 2.2 - 3.5% of the gross floor area.
Adequate space is required for maintenance. Rooms should be centrally located to minimize ductwork.
o Air-handling room requires careful detailing. Special acoustical treatment is required if rooms are adjacent to sound-
sensitive areas (eg. Conference Rooms).
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Air-intake and exhaust should be located on different walls, where possible, or no closer than 3m apart when located on the same wall.
Baffles may be used to provide the separation required. Configuring of Mechanical Equipment.
A method and system of managing a configuration of mechanical equipment provides a
structured procedure for managing information on parameters of the mechanical equipment to
facilitate the maintenance of safety, legal compliance, performance, and reliability of the
mechanical equipment. A desired configuration of the mechanical equipment is defined based
on a design objective, such as safety, reliability, performance, or any combination of the
foregoing objectives. An actual configuration of the mechanical equipment is determined
based on an evaluation of the mechanical equipment. Upgrade requirements are planned for
upgrading the actual configuration to the desired configuration if the actual configuration is
noncompliant with the desired configuration.
Chillers
Larger buildings and multiple building campuses usually use a chiller plant to provide cooling. In
such systems, chilled water is centrally generated and then piped throughout the building to air
handling units serving individual tenant spaces, single floors, or several floors. Ductwork then
runs from each air handler to the zones that are served. Chilled water-based systems result in far
less ductwork than all-air systems because chilled water piping is used to convey thermal energy
from the point of generation to each point of use. Whereas the all-air systems used to cool smaller
buildings usually contain all of their components packaged within a single cabinet (ergo the term
packaged cooling unit), a chiller plant is a collection of individual components that have been selected to work together as a system. Though more costly to install and more complicated to
operate, a chiller plant offers a number of benefits over simple packaged cooling units, including
greater energy efficiency, better controllability, and longer life. Additionally, a chiller-based
system can be much more efficient in terms of space utilization within the building because
components need not be located within the same space.
Chiller plants are usually used to cool large buildings because their components require much less
space within the building than all-air systems. One reason that less space is needed is that the size
of pipes that convey chilled water throughout the building is much smaller than the size of air
ducts that would deliver cold air to provide the same cooling effect. Water is a more space-
efficient heat transfer medium than air, and therefore works
well in space-constrained applications such as high-rise buildings.
Characteristics of an Efficient Chiller Plant
There are three key characteristics of an efficient chiller plant. Severe shortcomings in any one of
these areas cannot necessarily be overcome by excellence in the others:
An efficient design concept. Selecting an appropriate design concept that is responsive to the anticipated operating conditions is essential to achieving efficiency. Examples
include using a variable-flow pumping system for large campus applications, and
selecting the quantity, type, and configuration of chillers based upon the expected load
profile.
Efficient components. Chillers, pumps, fans, and motors should all be selected for stand-alone as well as
systemic efficiency. Examples include premium efficiency motors, pumps that have
high efficiency at the anticipated operating conditions, chillers that are efficient at both
full and partial loads, and induced-draft cooling towers.
Proper installation, commissioning, and operation.
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A chiller plant that meets the first two criteria can still waste a lot of energyand provide poor comfort to building occupantsif it is not installed or operated properly. For this reason, following a formal commissioning process that functionally tests the
plant under all modes of operation can provide some assurance that the potential
efficiency of the system will be realized.
Air-Handling Components
The major components in an air-handling system are its fans, filters, ducts, and dampers. Each
component performs a task critical to the proper operation of the system: Fans circulate the air and
provide the pressure required to push it through filters, coils, ducts, transitions, fittings, dampers,
and diffusers. Filters clean the air, protecting occupant health, inhibiting bacteria and mold growth,
and keeping coil surfaces clean. Ducts convey the conditioned air throughout the building,
distributing the air to occupants and then returning it to be conditioned and circulated again.
Dampers control the flow and mix of returned and outside air through the ducts to the various parts
of the building. All of these components must function well both individually and together to
ensure efficient system operation and occupant comfort.
Fans
Fans are the heart of a buildings air-handling system. Like a heart that pumps blood
Figure 8.4: Centrifugal and axial fans
through a body, they distribute the conditioned (heated or cooled) air throughout the building.
There are two main types of fans: centrifugal and axial (Figure 8.4).
Centrifugal fans (A) are the most common fans used in HVAC applications. They are often
cheaper but usually less efficient than axial fans (B).
Centrifugal fans. Centrifugal fans are by far the most prevalent type of fan used in the HVAC
industry today. They are usually cheaper than axial fans and simpler in construction, but they
generally do not achieve the same efficiency. Centrifugal fans consist of a rotating wheel, or
impeller, mounted inside a round housing. The impeller is driven by a motor, which is usually
connected via a belt drive.
Axial fans. Axial fans consist of a cylindrical housing with the impeller mounted inside along the
axis of the housing. In an axial fan, the impeller consists of blades mounted around a central hub
similar to an airplane propeller. As with an airplane, the spinning blades force the air through the
fan. Axial fans are typically used for higher-pressure applications (over 5 inches total static
pressure) and are more efficient than centrifugal fans.
The motor of an axial fan can be mounted externally and connected to the fan by a belt. However,
axial fans are often driven by a motor that is directly coupled to the impeller that is mounted
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within the central hub. As a result, all heat due to motor electrical losses is added to the airstream
and must be removed by the cooling system.
Filters
Air filtration occupies an increasingly important role in the building environment.
Filters work by capturing particles through gravity or through centrifugal collection, screening,
adhesion, impingement, and/or adsorption. The efficiency of a filter refers not to energy efficiency,
but to how well it removes particles from the airstream.
Regular filter maintenance is essential to
keeping ductwork and coils clean. Dirt
accumulation in ductwork can facilitate the
growth of bacteria and mold, particularly if
condensation occurs within the ducts. Dirt
accumulation on coils impedes heat transfer,
reducing system efficiency and increasing
HVAC costs. Dirty filters will also reduce
airflow, and may therefore reduce occupant
comfort.
Visual inspection is not always an adequate
way to determine whether filters cleaning or
replacement is necessary. A sure-fire way to
determine when filter maintenance is
necessary is to install a device that measures
pressure drop across the filter bank. A signal
from such a device can be an input to a
building automation system to alert operators when filter maintenance is required.
Commonly found filter types in commercial buildings include dry filters, bag filters, high-
efficiency particulate air (HEPA) filters, electrostatic precipitators, and carbon filters.
Dampers
Dampers modulate the flow of air through the ducts to the various parts of the building, reducing
or increasing the airflow depending upon conditions. Dampers also regulate the quantity of outside
air that is allowed to enter the air-
handling unit and mix with return air
for ventilation purposes. Dampers
can be difficult to maintain and can
affect occupant comfort as the space
requirements change and as the air-
handling system ages.
A typical commercial HVAC system
has numerous dampers that alter the
flow of outside air, return air,
exhaust air, and supply air. An
efficient air-handling system
minimizes the number of dampers
necessary overall and eliminates dampers or uses low-loss dampers at branch takeoffs, reducing
the fan power needed to blow air past them but maintaining the capability for minor balancing
adjustments. Using variable-speed drives for fan regulation can eliminate the need for fan inlet or
discharge dampers.
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Circulator Pump
A circulator pump is used to circulate gases, liquids or slurries in a circuit. Most often, these
pumps are found circulating water in a hydronic heating or cooling system. The circulator's
job is to move hot water from the boiler to the radiators, and then return the cooled water for
another injection of heat.
Types of Circulator Pumps
While the function of circulator pumps is generally the same, there are many different kinds.
Among the designs are bronze sweat end pumps, stainless steel/bronze circulator pumps, cast
iron pumps, pre-wired models and in-line pumps. Circulator pumps also vary based on
horsepower, flow range (expressed in gallons per minute), head range (expressed in
submersible feet of depth), motor type, and the maximum and minimum liquid temperatures
they can be used in.
Design of piping systems:
Types of piping system: The piping systems are divided into two types:
Closed system: In a closed system chilled or hot water flowing through the coils, heater ,
chillers, boiler or other heat exchanger forms a closed re circulating loop as shown in the
figure below. In close system water is not exposed to the atmospheric during its flowing
process. The purpose of re circulating is to save water and energy.
Open system: In an open system the water ix expose to the atmosphere as shown in the
Figure below. For example, chilled water come directly into contact with the cooled and
dehumidified air in the air washer and condenser water is exposed to atmosphere in the
cooling tower. Recirculation of water is used to save water and energy.
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The close systems are consists of the following components:
Load unite which represents the terminal unite as cooling or heating coils or radiators
Source unites which represent the chiller in cooling system or the boiler and furnace in heating systems.
Distribution systems which represents the piping and fitting of the piping systems.
Pump that used to circulate the water in the cooling or heating systems. It is usually of centrifugal types with constant flow rates (0.3 l/s with 20 kPa up to
hundreds of l/s and appropriate pressures.
Expansion tanks which are of two types.
Types of closed systems:
One pipe system: A single pipe connects all the system components i. e. the pipe started from
the source unit through the pump to the load units and then returns to the source. The
disadvantage of this system is that the efficiency of the last units are low because the return
cold or hot water of all units is added to the same pipe that supply the end units.
Two pipe system: This system has a two pipes one to the supply water and the other to the
return water. In this system the disadvantage of the one pipe system is overcome. This is the
most popular system in use because it is simple and cheep.
Three pipe system: This system can be use in central air conditioning units that used
for cooling and heating in the same time . It has one pipe to supply hot water, the other to
supply cold water and the third is a common return pipe i. e. the third pipe is used to return
cold and hot water to the chillers and