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  • V Semester Building Services III

    MSAJ Academy of Architecture 1

    BUILDING SERVICES - III

  • 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