11 Airconditioning

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Air-conditioning

11AIR-CONDITIONING

Introduction

The primary function of an air-conditioning system is to maintain conducive conditionsfor human comfort. Comfort air-conditioning, is defined as the process by which the

temperature, moisture content, movement and quality of air are maintained withinrequired limits.

An air-conditioning system has to perform the following functions:

Cooling and dehumidification for summer conditioning.

Heating and humidification for winter conditioning.

Air filteration and proper ventilation round the year.

ecessarily, these functions involve control of temperature, humidity, purity andmovement of air.

Comfort Conditions

There is nothing li!e a "best temperature# for human comfort since it depends on severalfactors. These factors are:

$eason of the year

Temperature differential between outside and inside

%sychological condition of the human being

%hysiological condition of the human being

&ressing style

However, the universally accepted temperature range for comfort is '() to '*) duringsummer and +() to +*) during winter. The humidity for human comfort should be

between and + per cent.

Unit of Air-conditioning Capacity

The capacity of air-conditioning plants is always epressed as so many Tons of/efrigeration 0T/1. 2ne ton of refrigeration is defined as the amount of cooling yielded

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Induction Course (General)

by melting one ton of ice 0assumed as (, lbs1 at 6(7 ) to water at 6(7 ) over a (-hour period. 8hen one pound of ice at 6(7 ) melts, it absorbs 9 T; 0ritish Thermal;nits1. $o, when one ton of ice at 6(7 ) melts, it absorbs (, < 9 = (**, T;.Therefore, one ton of refrigeration 0T/1 is equal to (**, T; over a (-hour period or9(, T; over an hour or ( T; over a minute. The metric equivalent is 6,!Cal4hour.

Refrigeration Cycle

The refrigeration cycle line diagram is shown in )ig. 9. >echanical refrigeration isachieved by alternately compressing and epanding the refrigerant with the help of acompressor and pressure reducing device 0such as epansion valve1.

Compressor serves two purposes: first, it draws the refrigerant from the evaporator0cooling coil1 and forces it into the condenser and secondly, it increases the pressure ofthe refrigerant. y suc!ing the refrigerant, the compressor reduces the pressure in thecooling coil and maintains it at a level low enough to permit the refrigerant to boil orvapori?e and consequently absorb heat in the process.

Fig. 1: Refrigeration Cycle Line Diagram

The compressed gas hot vapour flows to the condenser, where it is condensed intoliquid at high pressure giving up heat to atmospheric air or water depending uponwhether the condenser is air cooled or water cooled.

The liquid refrigerant then passes through a pressure reducing device li!e an epansionvalve 0or capillary tube1, where its pressure is reduced enabling it to start boiling atthe evaporator, resulting in refrigerating effect.

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Air-conditioning

System ypes

asically, all air-conditioning systems can be divided into two broad categories:

i! D" type System: 3n &< system, air is cooled by the refrigerant flowing inside theevaporator coil.

ii! C#illed $ater System: 3n chilled water system, air is cooled by means of chilledwater flowing inside the coil.

&< type can be further classified as follows:

0a1 $elf Contained

8indow type 0or room air-conditioners1, commonly available in 9, 9.@ and ( T/

capacities. These are invariably air-cooled.

%ac!age type, commonly available either as air-cooled or water-cooled models.

0b1 $plit type

ither air-cooled or water-cooled. Available capacities are (., 6.@, '.@ and 9T/.

0c1 Central or uilt up Type. ither air-cooled or water-cooled, commonly availableabove 9 T/ upto ( T/.

System Components

Compressor

The equipment used for compression of the refrigerant forms the heart of therefrigeration unit. The positive displacement refrigerating compressor is thecompression system most commonly used today.

All reciprocating compressors consist of one or more cylinders, suitable valves forsuction and discharge of the refrigerant gas and reciprocating pistons for compression.The design of the cylinders is of multi-cylinder type with automatic capacity controlmechanism. Compressors are usually equipped with forced lubrication system involvinga separate oil pump to maintain circulation.

Condenser

3t is a heat echanger. 3t is in the condenser that the refrigerant must give up the heatabsorbed in the evaporator plus the heat added by the compressor. $ome amount ofsub-cooling of the liquid refrigerant is also provided before it leaves the condenser. Thepressure of the refrigerant in the condenser is little-less than at the compressordischarge, where it is higher than at any other part in the system. Hence, the condenser

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and other equipment in the system between the compressor discharge and epansionvalve are often referred to as high pressure side equipment.

There are three types of refrigeration condensers:

air-cooled condenser,

water-cooled condenser and

a combination of air and water-cooled condenser 0evaporative condenser1.

Air-Cooled Condenser: ;nits upto 9 T/ are predominantly air-cooled. >ainadvantages are simplicity, easy maintenance and low installation cost. vensome units having 6 T/ capacity are air-cooled. The disadvantages are highnoise level and reduced efficiency at high ambient temperatures.

$ater-Cooled Condensers: 8ater-cooled condensers are used with compressors of9 H% or larger. This, usually, constitutes the most economical and efficientchoice of condenser, where an adequate supply of clean water of least chemicalcomponents cause scale formation and corrosion is available.

%&aporati&e Condenser: This type of condenser was designed to combine thefunctions of a condenser and a cooling tower and is used where high cost of water is aserious obBection and use of cooling tower is not practical.

Cooling o'er and Spray (onds

Cooling towers may be classified as atmospheric draught or natural and mechanicaldraught.

atural draught cooling towers are either installed away from buildings or at the top ofthe buildings to enable free flow of atmospheric air through the louvres fitted inside.atural draught cooling towers are installed in places where reasonable flow of airmovement is there throughout the year for better efficiency. atural draught coolingtower requires more space for installation compared to mechanical one.

>echanical draught cooling towers can be classified as follows:

a. )orced draughtb. 3nduced draught

3n forced draught tower, water from the condenser output is pumped to cooling towerand is allowed to reach the sump through headers, spreaders and louvers throughgravity. Air from atmosphere is forcibly flown inside the tower with the help of a fan andescapes bac! to atmosphere through the top of the tower after absorbing the heat fromthe water. )orced draught cooling towers are normally used for lower tonnage capacityplants.

3n induced draught, atmospheric air is suc!ed by a fan fitted on the top through the airinlet opening provided in the tower. Air passing through the louvres ta!es away the heatfrom the water. ssentially, heat is given away by evaporation of water droplets and,

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Air-conditioning

consequently, the quantity of water in circulation is reduced. This is compensated bydrawing water from the ma!e-up tan! through a floor valve.

8hen air is used to cool water, the role of heat transfer depends upon

the difference between wet bulb temperature of air and the water temperature.

the area of the water surface eposed to the air.

the relative velocity of air and water.

3n order to provide a greater area of eposed water surface, ponds are developed. )orthis, no??les are located to spray the discharge water into the air. They should brea! upthe water into droplets, but not into a mist which would easily drift off. A normal coolingtower should typically cool the water to within 'o ) of the wet bulb temperature of theambient air.

%&aporator )Cooling coil!

This component is common to both the air cycle and refrigerant cycle. 3t is the coolingcoil where heat from circulating air is absorbed by the refrigerant. The cooling coil servesas a heat transfer device. Heat is transferred from the air to the coil surface and then tothe refrigerant in the coil. 3n the process, the liquid refrigerant boils and turns intovapour.

%*pansion +al&e

The epansion valve reduces the pressure of the refrigerant liquid and in doing so, coolsthe liquid. The refrigerant enters the valve under pressure. As it passes through thesmall valve port, it enters the low pressure area of the cooling coil. The valve port actsas metering device between a high pressure area 0condenser1 and low pressure area

0evaporator1.

Refrigerant

The refrigerant performs the function of etracting heat from the air in the evaporator andreBecting it to water or air in the condenser. $everal chemicals can be used asrefrigerant. A few eamples are Ammonia, Carbon-dioide, )reon, etc. 3n plants of lowcapacity )reon-9( 0CCl()(1 is used, whereas in higher capacity plants, )reon-((0CHCl)(1 is used. oth belong to the Chloro-)louro Carbon family. ut, due to its o?onedepleting property, manufacture and sale of )9(has been banned. A chemical call H)C-96a 0Tetrafluoroethane1 is slowly replacing )9(.

The desirable properties of a refrigerant are:

9. 3t should be non-irritating, non-poisonous, non-toic and non-inflammable. 3t shouldalso be free from eplosion ha?ards.

(. 3t should have an acceptable odour, so that it can be detected easily, if there is alea!age.

6. 3t should be non-corrosive, even in the presence of oil or water, to ferrous and non-ferrous materials used in the entire refrigerant path.

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. 3t should be easily miscible with the cran!case lubricant oil.

@. The vapour pressure corresponding to 6C in the condenser should not be high,

necessitating thic!er pipes, fittings, etc., thus boosting the system cost.

+. The vapour temperature 0saturation temperature1 at atmospheric pressure shouldbe sufficiently low to give the desired temperature in the evaporator.

'. 3t should have a high critical temperature so that it remains in the vapour stage at thehighest wor!ing temperature.

*. 3n the liquid stage, the specific heat should be low to reduce the quantity of vapourformed during throttling in the epansion valve. 3n the vapour stage, however, thespecific heat should be as high as possible to minimi?e the amount of superheat.

. 3t should have as large a latent heat of evaporation as possible, so that minimumquantity of refrigerant can be utili?ed for the same amount of tonnage.

9. 3ts specific volume 0volume4weight1 should be as low as possible for minimum si?e ofthe compressor.

99. 3t should have a low viscosity for improving the flow rate, and to reduce si?es ofpipes and valves and pressure drop.

9(. 3t should have high thermal conductivity for increasing the efficiency of the condenser

and evaporator.96. 3t should have a high dielectric strength, particularly when used as a refrigerant in

hermetically sealed compressor.9. 3t should be cost effective.9@. 3t should be environment friendly.

3t should, however, be remembered that none of the available gases 0including )reon1meet all the above requirements.

Air cycle

3ndoor air may be too cold, too hot, too dry, too wet, too drafty or too still. These

conditions are changed by circulating the air and these treatments are provided in theair-conditioning air cycle.

Air distribution system directs the treated air from the air-conditioning equipment to thespace to be conditioned and then bac! to the equipment.

The main components in the air cycle are:

0i1 )an 0ii1 $upply duct0iii1 $upply 2utlets 0iv1 /eturn outlets0v1 /eturn duct 0vi1 )ilter

0vii1 Cooling coil 0or heating coil1.

The total resistance of these components to the flow of the air plus the friction resistancecaused by the air passing through the duct run are maBor factors in determining the si?eof the fan and fan motor and the amount of air pressure that is required. )or aroadcast $tudio setup, this resistance is of the order of (@ mm to @ mm of watergauge.

Centrifugal fan is most commonly used in commercial and residential installations. 3tconsists of a scroll, a shaft and a wheel. The scroll is actually a housing for the shaft

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and wheel and the shaft serves as an ale for the wheel. The wheel is cylindrical inshape and has many blades. Centrifugal fans are available with forward or bac!wardcurved blades. A forward curved fan can deliver required quantity of air at low fanspeeds.

The air velocity and speed of the fan wheel 0tip speed1 not only play a maBor part in

determining the efficiency of the fan, but also affect the level of noise generated by thefan. High tip speed and high velocity usually result in more noise.

/emote location of the fan reduces the noise but the system becomes more epensive.&ucts may be circular, rectangular or square in shape. )rom the appearance andpractical point of view, rectangular ducts are generally adopted.

&ucts are fabricated from a wide variety of materials. &ucts made of sheet metal arevery common. The ducts are lined with glass wool or mineral wool slabs of (@ mmthic!ness wrapped in copper naphthanate treated cloth.

2utlets are important from the point of view of appearance, functions and performance.

The primary function of the outlets is to provide properly controlled distribution of air tothe room and removing the air from the room. Ceiling diffusers, grilles and registers areused as supply outlet and grilles are used as return outlets.

Cooling load capacity selection

The air-conditioning plant should be of adequate capacity to maintain comfort conditionduring periods of maimum outside temperature. The plant should ta!e care of all theheat load coming from different sources.

,utdoor eat Source

$olar heat enters the structure either directly through glass which is immediatelyabsorbed in the room or as conducted heat through walls and roof, which is notimmediately absorbed in the room. The effect of the conducted heat may not be felt forseveral hours depending upon the thermal properties of construction material used.

3nfiltration of air entering the conditioned space through crac!s, crevices, opening,closing of doors for dilution of unwanted odour also ma!e a significant contributiontowards the load.

Indoor eat Source

Human occupancy, lights and electrical equipments are indoor heat sources.2ccupantDs load depends upon the number of people wor!ing inside and their activities.The heat given off by incandescent lights is directly related to its wattage.

eat Load Reduction

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$everal methods of reducing the heat load are available. %roper orientation of thebuilding reduces considerable load. ;se of double pan glass, insulating walls and roofs,and false ceiling help to a large etent in heat reduction.

3t may be noted that heat load contributed by various sources do not reach pea! valuesimultaneously. )or eample, transmission heat load due to outside-inside temperature

difference attains pea! value at about 9 pm, whereas heat due to solar heat throughwalls and roof is mostly felt at about ' pm Human occupancy may be maimum at 99am. All these aspects have to be seen for proper load estimate.

,peration

efore starting the plant, ensure that proper functioning of safety controls includinginterloc! circuit have been chec!ed and correctly set, and that all motors are megger-tested, direction of rotation verified, all bearings lubricated and refrigeration system fullycharged. The cran! case heater must be energised well in advance.

%roceed step by step for operating the system as follows:

$tart the air handling unit, ensuring that dampers in the supply duct are fully

open.

2pen all water valves and start the water pump. 2bserve pressures at

condenser inlet and outlet.

2pen hot-gas valve on the condenser and the discharge service valve on the

compressor. 2pen discharge gauge valve to read the pressure.

)ollow the same procedure and read the suction pressure.

2pen liquid line valve. 2bserve standing pressure on the gauges. This should

be approimately '.6 !g4cm(09 psi1 for /-9( and 9.@ !g4cm(09@ psi1 for /-

(( to indicate that the system is tight with no lea!age.

2pen suction service valve and start the compressor. 2bserve the refrigerant

and oil pressures. Chec! the current drawn by the compressor motor, observethe oil level in the compressor sight glass. 2il should be clear without foam afteroperation has stabilised.

Compressor (ump-Do'n

3t is essential to collect the refrigerant in the condenser with isolation to prevent any lossbefore opening the compressor or any other part of the system. This is called pump-down and the operation involves the following procedure:

$hort the low pressure switch with a temporary Bumper wire so that the

compressor does not stop before the refrigerant from it is emptied.

$lowly close the suction valve with the compressor running.

8hen the suction pressure drops to about .9@ !g4cm( 0( psi1, stop the

compressor.

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ever pump the compressor below .9@ !g4cm(to prevent infiltration of moisture

and dirt into the cran! case.

After a few minutes, the dissolved refrigerant will leave the cran! case raising the

suction pressure. This additional refrigerant can be pumped to the condenser byoperating the compressor again for a short while.

/epeat the above procedure till the suction pressure does not rise above .9@!g4cm(after closing the service valves.

Remo&ing Refrigerant from t#e System

3t may be necessary to remove the refrigerant from the system into a cylinder, if there isan ecess charge or there is a lea! in the condenser. Ta!e the following steps for thisoperation:

Connect a suitable line between the angle valve provided for charging and an

empty refrigerant cylinder.

%urge the air from the connection line.

Eeep the cylinder cold by immersing it in ice cold water to ensure a faster

refrigerant flow from the system.

$tart the compressor and open the liquid line charging valve, allowing the liquid

into the empty cylinder. 3f ecess refrigerant is to be removed, hold the chargingvalve open only until the discharge pressure reaches the normal reading. Afterthis operation, remove the charging line and close the charging valve.

&o not overcharge the cylinder as ecessive pressure is dangerous.

(urging on-Condensi/le 0ases

%resence of non-condensibles gases such as air causes high discharge pressure,resulting in reduction of capacity and high power consumption. 3n case such symptomsare present, the following chec! should be done:

$hut down the system overnight, long enough for the temperature of all

components to level off.

/ead the standing pressure and compare it with the refrigerant saturation

pressure corresponding to the temperature of the system. 3f the standingpressure eceeds the saturation pressure by .'@ !g4cm( 09 psi1, the non-condensibles are ecessive and must be removed.

)or eample, if /-(( is used and the system temperature is *@o) 0(.oC1 and standingpressure is 9(.* !g4cm(09'@ psi1, then there is ecess of non-condensibles. $aturationpressure for /-(( corresponding to *@o) is 99 !g4cm(. The difference is 9.@ !g4cm(

more than .'@ !g4cm(, indicating corrective purging. )or purging, ta!e the followingsteps:

%ump down the system as described earlier.

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3mmediately after stopping the compressor, close the compressor discharge

valve.

/un the water through the condenser for complete condensation of refrigerant

vapour.

Crac! open the purge valve on the top of the condenser for an instant, shut it

again. Allow the system to stabili?e for a few minutes before reopening and closing the

purge valve. /epeated purging and closing operation should clear the system ofnon-condensibles.

/estore normal system operation, chec! the improvement in discharge pressure.

Chec! refrigerant charge and compressor oil pressure.

Refrigerant C#arging

A correct operating charge of refrigerant in the system is essential. Foss due to lea!agein the system has to be made up. 3t may be necessary to replace the entire charge. Anover charge results in unduly high temperatures, pressures and operating costs and may

damage the system components. An undercharged system leads to insufficient cooling,high operating cost, and, in hermetic system, the compressor motor may fail.

/efrigerant may be added to the system either as a vapour or liquid depending upon thelocation of charging point and quantity required. 5enerally, for adding ma!e-uprefrigerant, vapour charging method is more convenient. )or total system charge, liquidcharging at the high side followed by vapour charging at compressor low side will bequic!er.

;nder no circumstances should liquid refrigerant be allowed to enter the compressor toavoid damage to the compressor. The procedure for vapour charge method is describedbelow:

2pen the suction and discharge shut-off valves of the compressor. 3nstall agauge in the discharge gauge port and open the gauge line if a gauge port hasnot been provided.

Connect a refrigerant cylinder and the connection with a compound gauge to the

charging valve provided on compressor suction line. %urge the air from the linesand tighten the connections.

Admit the refrigerant by slowly opening the refrigerant cylinder. The cylinder

should be !ept in upright position to prevent the refrigerant from entering thecompressor in liquid state.

$tart the compressor.

As the cylinder gets emptied, its pressure will drop to the same level as thesuction pressure. The remaining refrigerant can be drawn from the cylinder byclosing the suction shut off valve and pulling a vacuum on the cylinder with thecompressor running.

Chec! the quantity of refrigerant charge by noting the difference in the weight of

the cylinder and observing the pressure.

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$ater reatment

Algae4slime scale and corrosion on the water side of the heat transfer equipment retardsheat transfer causing general loss of efficiency and brea!downs.

2ygen, Carbon &ioide, $ulphur &ioide absorbed from the air and dissolved in watercause corrosion, reducing the capacity of lines, increasing frictional losses and pumpingcost. Hard water causes scaling problem. 8hen heated, the minerals are left behind,which form a deposit on the heat echanger surface. The heat transfer rating of thescale is very much lower than metal. /etarded heat transfer results in increaseddischarge pressure causing loss in capacity and increased power consumption.

$caling of condenser tubes in a re-circulated water system is unavoidable. &escalinghas to be carried out as a preventive maintenance once every 9( months or earlierdepending on the hardness of the water. &escaling can be carried out quiteconveniently by circulating mild inhibited acid solution with the help of a small pumpconnected across the condenser inlet and the water valves are closed to confine the

circulation to the condenser only.

Chemical compounds are available, which suspend minerals of dissolved scale. Algaeattach themselves to the surfaces, and since they are living plants, they grow until theyclog the passages of the system. acteria forms slime and close the system in muchthe same way as algae. Algae4$lime is controlled by use of toic. A specialist should beconsulted to determine the algae4slime.

rou/le S#ooting

Any trouble should be diagnosed as accurately as possible before any repair isattempted. &efinite symptoms will accompany a faulty operation in the system. The

following trouble shooting chart will help in fault location and prompt correction:

ature ofrou/le

(ossi/le Causes Correcti&e Steps

Compr

essorfailstostart

>ain switch open. Close switch.

)uses blown.

Chec! electrical circuit and motorwinding for short or ground. 3nvestigatefor possible overloading. /eplace fuseafter fault is corrected.

&efective contactor. /epair or replace.

$ystem shuts downthrough safety devices.

&etermine type and cause of shut downand correct it before resetting safetyswitch.

Thermostat set too high. Chec! evaporator temperature. Fowerthe thermostat setting, if possible without

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free?e-up.

Fiquid line solenoid valvemay not be opened.

/epair or replace.

>otor 0lectrical1 trouble. Chec! motor for open, short circuit, orburnout.

Foose wiring.Chec! all wire Bunctions. Tighten allterminals4screws.

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Compressornoisy

or&i/rating

3mproper isolation. Chec! isolator.

3mproper piping support. /elocate, add, or remove hangers.

3mproper clearances. 2verhaul compressor and replacedefective parts.

)looding of refrigerant intocran! case.

Chec! rating and setting of epansionvalve.

elts4coupling loose ormisaligned.

Tighten4/ealign.

-ig#disc#argepressure

&ischarge shut-offvalve partially closed.

2pen valve.

Condenser water flowinsufficient or temperaturetoo high.

Chec! water shut-off valve. 3nvestigateways to increase water supply.

$ystem overcharged withrefrigerant.

/emove ecess refrigerant.

)ouled condenser tubes. Clean.

on-condensibles insystem.

%urge the non-condensibles.

Lo'disc#argepressure

Fow water temperature. AdBust water shut-off valve to reducewater quantity.

$uction shut-off valvepartially closed.

2pen valve.

3nsufficient refrigerant inthe system.

Chec! for lea!s, repair and add charge.

Fow suction pressure.$ee corrective steps for low suctionpressure.

Compressor operatingunloaded.

$ee corrective steps for failure ofcompressor to load.

Condenser too large.Chec! condenser rating table against theoperation.

8orn out piston rings.8orn out valve0s1.

2verhaul the compressor.

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-ig#Suction

pressure

cessive load. /educe load or add equipment.

pansion valve over-feeding.

Chec! remote bulb. /egulate superheat.Chec! valve operation. /epair orreplace, if necessary.

Compressor operatingunloaded.

$ee corrective steps for failure ofcompressor to load.

Lo'

Suctionpressur

e

Chilled water pump notoperating.

Chec! and start the pump.

Fac! of refrigerant gas. Chec! for lea!s. /epair and add charge.

vaporator dirty or icedup.

Clean or defrost.

Clogged liquid linefilter4drier /eplace cartridge0s1.

Clogged suction gasstrainers.

Clean strainers.

pansion valvemalfunctioning.

Chec! and reset for proper superheat./epair or replace, if necessary.

Condensing temperaturetoo low.

Chec! means for regulating condensingtemperature.

Compressor will notunload.

$ee corrective steps for failure ofcompressor to unload.

vaporator fan notoperating.

Chec! and start fan. 3f interloc!ed,chec! the circuit.

Co

mpressordoes

notunload

$olenoid valve in the oilline stuc! closed, notrelieving oil pressure onthe unloader mechanism.

/epair or replace the valve after rulingout physical bloc! due to foreign matter.

)aulty unloadermechanism.

/epair or replace after ruling out otherpossibilities.

&efective automaticcontrol.

Chec! setting and verify operation.

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Compressordoes

notload

3nadequate oilpressure.

/ule out a clogged oil strainer and foamin the oil, failing which chec! the oil, reliefvalve, and, as a last resort, verify the oilpump and moving parts clearance afterdisassembly.

$olenoid valve in the oilline not opening or lettingoil flow.

Chec! whether solenoid coil is gettingenergised properly and plunger lifting.

)aulty unloadermechanism.

/epair or replace ruling out otherpossibilities.

&efective automaticcontrol.

Chec! setting and verify operation.

(erformance easurement

3n order to assess the performance of an air-conditioning plant regularly, we shouldmeasure the refrigerating capacity 0tonnage1 delivered by the plant at regular intervals.

As per A3/ >anual, the tonnage is to be measured every ' days. Theoretically, thetonnage can be measured by performing measurement on any of the following:

The cooled medium 0Air1The refrigerant 05as1The intermediary medium 08ater1lectrical parameters

However, practically, it is more convenient and accurate to adopt the first option out of all

the above. This method is called air-flow method and will be eplained herein.

efore eplaining the procedure for the measurement of tonnage, it will be appropriate tounderstand the following terms:

Dry 2ul/ emperature: 3t is the temperature of air as measured by an ordinary mercurythermometer. 3t is epressed in degrees )ahrenheit.

$et 2ul/ emperature:3t is the temperature of air as measured by an ordinary mercurythermometer whose bulb is covered by a wet cloth so that water from the cloth caneasily evaporate all around. 3t is epressed in degrees )ahrenheit.

3n practice, both dry and wet bulb temperatures are measured by one compositeinstrument called $#irling #ygrometer or Sling psyc#rometer3 which is nothing but acombination of dry bulb thermometer and wet bulb thermometer mounted on a commonrotatable spindle.

Relati&e umidity: This is the amount of moisture in the air epressed as a percentageof the maimum amount of moisture that the air can hold. A related term is 0rains ofmoisture3 which indicates the absolute content of moisture in the air. 02ne pound =', grains1

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Specific +olume: This is the volume occupied by one unit of mass of air and is thereciprocal of density. 3t is epressed in terms of cubic feet per pound.

%nt#alpy: This refers to the total heat content in the air and the moisture in the air. 3nair-conditioning calculations, we are only concerned with the difference in the heatcontent between two different conditions of air, but not the absolute heat content. 3t is

epressed in terms of T; per pound.

All the above parameters are inter-related to each other. )or eample, if the dry bulband wet bulb temperatures are fairly close to each other, it means that the air is highlysaturated 0leaving little scope for evaporation of the water around the wet bulb1 andhence the relative humidity is high. )urther, as the dry bulb temperature is increased,the volume of the air increases 0for the same mass1 and, hence, the specific volumegoes up. Also, if the dry bulb temperature goes up, it leads to a higher enthalpy value.

)ortunately, we have a wonderful graphical aid to compute any of these parameters ifBust any two parameters are !nown. This is called (syc#rometric C#art. This chartsummari?es the properties of air. A psychrometric chart is shown in )ig. (.

To start with, the various components of the psychrometric chart should be understood.

All vertical lines in the chart represent constant dry bulb temperature. The readings ofdry bulb temperature are indicated in the hori?ontal ais 0increasing from left to right1.

The lines having gentle slope represent constant wet bulb temperature. They alsorepresent constant enthalpy. The values of wet bulb temperature are indicated along theoutermost curve, while the values of enthalpy are indicated in a scale by proBecting thelines beyond the outermost curve in different bloc!s.

)ig. (: %sychrometric Chart

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The hori?ontal lines indicate constant grains of moisture. The actual values are shownon the G-ais both in grains of moisture per pound of dry air and pounds of moisture perpound of dry air increasing upwards.

The parabolic curves represent constant relative humidity. The values are mar!ed on

the curves themselves and are in steps of 9. The steeper lines indicate constantspecific volume. Ialues are indicated by proBecting the lines below the


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Thus, if any two independent parameters are !nown, we can mar! a point on the chartrepresenting the state of the air corresponding to those parameters. 02f course,frequently, we may need to interpolate between the lines and ma!e Budicious selection ofthe point1. 2nce a point is plotted, we can Budge the values of all other parameters from

the chart.

ow, using the chart, try to find out all the properties of air, given the followingparameters:

9. &.. Temp 7 ) and 8.. Temp '7 ).(. &.. Temp 7 ) and /.H. @.6. 8.. Temp '7 ) and /.H. 9.. &.. Temp 7 ) and 9 grains of water vapour per lb.@. &.. Temp *7 ) and Total Heat 6 T;4lb.+. &.. Temp 7 ) and 8.. Temp 97 ).

8hat was the result for the last plot, and whyJ ow that we are in a position to find outthe heat content 0or enthalpy1 using the psychrometric chart, we can proceed with theactual tonnage calculation.

%lease recall that the actual cooling of the air ta!es place in the evaporator of the plant,through which the return air is passed by the blower of the Air Handling ;nit. Hence, ifwe measure the dry and wet bulb temperatures on either sides of the evaporator coil, weshall be in a position to find out the heat lost to the refrigerant from the air. The onlyadditional quantity required to calculate the tonnage is the quantity of air passing throughthe evaporator. This can be obtained by measuring the velocity of air with the help of avane anemometer and then multiplying it with the cross sectional area.

)or eample, let us say that the measured velocity is + feet per minute in the returnduct and the dimensions of the return duct are @ feet by ( feet, then the volume of the airis + < @ < ( = + cubic feet per minute. $ince the total heat content, enthalpy, isepressed in terms of T;4lb, we have to !now the mass of the air. However, sincespecific volume can be found out from the psychrometric chart 0cft per lb1, we can !nowthe mass of air from the volume flow rate obtained earlier by dividing it by the specificvolume.

)or eample, if the specific volume in the above eample is noted as 9. cft4lb, then thequantity of air is +49 or (*.+ lb per minute. ow, if the enthalpy difference notedacross the evaporator is, say, 9@ T;4lb, then the heat lost is 9@ < (*.+ = +( T;per minute. Again, recall that one ton of refrigeration is equivalent to ( T; per

minute. Hence, the above results lead to a tonnage of 6(.9@ T/.

3n summary, the formula for tonnage calculation can be stated as follows:

Fet H9be the enthalpy at the inlet of the evaporator and H(be the enthalpy at the outletin T;4lb. Fet A be the cross sectional area at the evaporator input in sqft. Fet I be thevelocity of air measured at the inlet in ft4minute and let $ be the specific volume as notedfrom the psychrometric for the inlet air in cft4lb.

$T30T1 %ublication 154 (43C0514(+


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Air-conditioning

3f the values were in >E$ system 0enthalpy in !ilocalories per !g, A in sqm, I in metersper minute and $ in cubic meters per !g1, then the formula is:

ow, can you calculate the tonnage for the following observationsJ

3ncoming air dry bulb temp is *@7 ) and wet bulb temp is '@7 ). 2utlet air dry bulb tempis ''7 ) and wet bulb temp is +@7 ). Ielocity of incoming air is @ feet per minute andthe cross sectional dimensions are # and 6#.

8hile measuring tonnage, the following precautions should be ta!en:

ach plant is to be tested separately. 3f, however, the load is such that normallymore than one plant is to be run, then another set of test should be performed withthe required number of plants.

As far as possible, the test should be performed during a time when the wet bulb

temperature of the outside air is between +@7 ) and +7 ), optimum being +'7 ).

$tart the plants and pumps at least one hour before the measurements.

)ilters must be cleaned before the measurements.

fforts should be made to adBust the load on the plant under test to ma!e it

approimately equal to the rated capacity by noting the suction and dischargepressure readings. 3f the load is too low, the percentage of fresh air inta!e may beincreased even to 9, if necessary. 3f it is too high, fresh air inta!e may be cutbelow normal or the supply and ehaust of a few rooms may be bloc!ed temporarily./eadings should be ta!en only after steady state is reached.

Care should be ta!en while measuring the air velocity. A number of readings should beta!en to find the most representative value.

$T30T1 %ublication 155 (43C0514(+

(--

1HH0.

$

I.Atonnage,Then (9

=

@-

1HH0.

$

I.ATonnage (9

=

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