APPLIED THERMODYNAMICS

UNIT IV

COMPRESSORS, REFRIGERATION AND AIRCONDITIONING

AIR COMPRESSORSINTRODUCTION

• Air compressor is a machine which is used to increase the pressure of air.

• By drawing a volume of air from the surrounding atmosphere then compressingand discharging it to a storage tank under high pressure are the functions of acompressor.

• Compressed air is a precious commodity in which a considerable amount ofenergy can be stored and later it can be released when required.

CLASSIFICATION OF COMPRESSORSBased on the following considerations compressors are classified as given below:

1. Based on number of stagesa. Single stage

b. Multi stage2. Based on moving parts

a. Reciprocating compressorb. Rotary compressor

3. Based on the number of cylindersa. Single cylinder

b. Multi cylinder4. Based on the method of cooling

a. Air cooledb. Water cooled

5. Based on pressure developeda. Low pressure (blowers)

b. Medium pressure (single stage)c. High pressure (multi stage)

6. Based on actiona. Single acting

b. Double actingRECIPROCATING AIR COMPRESSOR

• It consists of a piston which is enclosed within a cylinder and has suction anddelivery valves.

• This type of compressor may be single stage or multi stage and may be singleacting or double acting.

• The piston receives power from main shaft through a crank shaft and connectingrod. A fly wheel is fitted on the main shaft to ensure turning moment to besupplied throughout the cycle of operations.

• The delivery pressure of air in a single stage compressor is from 10 to 200 bar.The speed of a reciprocating compressor piston limited to about 400m/mm.

SINGLE STAGE RECIPROCATING AIR COMPRESSOR

• A diagramatic sketch of a single stage reciprocating compressor is shown in fig.1.

• The suction and delivery valves are simply check valves or they may bemechanically operated.

• In the former type the opening and closing of valves are based n pressuredifference, but in the later type it is controlled by cams.

Fig. 1. Reciprocating air compressor1. There is no pressure drop through suction and delivery valves,

2. Pressure is both suction and delivery line remain constant3. The compressor has no clearance volume

4. Entire compression is carried out in a single cylinder

• During the suction stroke of the piston, the inlet valve opens and air is suckedin the cylinder at pressure p1 N/m2 up to the end of suction stroke.

• Let line AS represent the suction line in the p- V diagram. The work is doneby the air and is represented by the rectangle under AB.

• During the return stroke of the piston the valves are closed and the air in thecylinder is com pressed along the line BC on the p - V diagram till thecompression pressure reaches the final delivery pressure p2 N/m2

Fig. 2 Ideal reciprocating compressor indicating diagram

Air is Compressed Polytropically According to the Law (pVn = Constant)Let p1 = Pressure of air in N/m2 before compression

V1 = Volume of air in m before compressionT1 = Temperature in degree Kelvin before compression

Work required per cycle = Area ABCD of p - V diagramand p2, V2, and T2 are the final conditions of air after compression.

W = Work required per cycle

But for a polytrophic process,

Substituting the value of (V2/V1) the above equation,Work required per cycle is given by

Equation gives the work required per cycle or per revolution of a single actingcompressor.

Where W = work required in joules per cycle,N = rpm of the compressor.

But p1V1 = mRT1

we have work required per cycle,

For one kg of air work required is given by

Indicated power of the compressor = W × (Mass of air delivered per second) J/s or W

Where cycle will be

For one kg of air, work required is given by

Air is Compressed isothermally (pV= Constant)

Work required per cycle = Area ABCD

For one kg of air work required is given by

CLEARANCE VOLUME AND VOLUMETRIC EFFICIENCY

• In actual compressor, a certain clearance space is provided between the extremetravel of the piston and the cylinder cover to prevent the piston from striking theend and cover of the cylinder.

• The volume, thus left unswept by the piston is known as clearance volume.Therefore at the end of every delivery stroke the amount of air filling theclearance volume remains in the cylinder.

• The clearance volume is generally expressed as the percentage of pistondisplacement. Figure 3 shows the indicator diagram for a single stage aircompressor with clearance.

• As already stated, at the end of the delivery stroke the amount of air filling theclearance volume will not be discharged but remains in the cylinder.

• At the beginning of the forward stroke, air is not sucked in but the air in theclearance space expands till the pressure becomes p and volume V and thensuction begins.

• The volume of air drawn in at the end of suction stroke is V. But the volume of airsucked in without clearance is equal to the displacement volume V.

• Thus the effect of clearance in the compressor is to reduce the amount of fresh airthat can be sucked in the cylinder during the suction stroke,

Fig. 3 Single stage air compressor with clearanceLet Vc = clearance volume

Vs = swept volume

P2= pressure of air in the clearance space in N/m2

P1= pressure of air in the clearance space at the end of expansion in N/m2

n = index of expansion.The volume of clearance air at the end of re-expansion is given by

Actual volume of air taken in, Va = V1 V=4

• The ratio (p2/p1) is called the pressure ratio and the ratio (Vc/Vs) is called theclearance ratio.

• Thus the volumetric efficiency depends upon the pressure ratio and clearanceratio. If there is no clearance, then the volumetric efficiency becomes unity.

WORK DONE OF A COMPRESSOR HAVING CLEARANCE

Let the index n of expansion curve 3-4 and compression curve 1-2 be same. Workrequired per cycle = area 1-2-3-4 = (area 1-2-6-5) - (area 3-4-5-6)

Thus it is seen that the work required to compress and deliver same volume of air Va withclearance and without clearance is same.

Indicated power of the compressor

Work required in J/s = W× (Mass of air delivered per second)

POWER AND EFFICIENCY OF A COMPRESSORIsothermal work required per cycle, of a single stage compressor without clearance,

But p1v1 = m R T1 then isothermal work required per kg of air is given by,

Adiabatic work required per cycle,

MULTI-STAGE COMPRESSOR

• The isothermal compression requires minimum work, but in actual practice it isnot possible to compress isothermally, particularly, if the delivery pressure ishigh.

• So, the compression is carried out in stages. This is called multistagecompression.

• In a two stage compressor, the air is first compressed in the first cylinder frompressure p to some intermediate pressure p The air coming out of this cylinder iscooled to initial temperature in an intercooler and then led to the second cylinderin which it is com pressed from pressure p to p

Advantages of Multistage Compression(a) Saving in work is obtained

(b) There is little chance of lubrication trouble as the maximum temperature is reduced.(c) It improves the volumetric efficiency.

(d) Leakage loss is reduced considerably.(e) It provides more uniform torque and thus smaller sized flywheel is required.

(f) Cheaper material may be used for construction as the operating temperatures arelower.

(g) Lighter cylinders.Disadvantages of Multistage Compression

(a) Unit is more complicated.(b) Initial investment is more.

TWO-STAGE AIR COMPRESSOR WITH INTERCOOLERThe following assumptions are made for a two stage air compressor with intercooler.

(a) Effect of clearance is neglected.(b) For both the cylinders the compression follows the law pV = constant.

(c) There is no pressure drop in the intercooler.(d) Suction and delivery of air takes place at constant pressure.

Imperfect Intercooling

Fig. 4 Air compressor with intercooler

Figure 4 shows the indicator diagram of a two stage air compressor with imperfectIntercooling. Let

P1 = pressure of aIr entering the low pressure (L.P) cylinder in N/m2

V1 = volume of the low pressure (L.P) cylinder in cubic metre.

P2= pressure of air leaving the L.P cylinder or entering the high pressure (H.P.)cylinder in N/m2

V2 = volume of H.P. cylinder in cubic meter.Work done in L.P. cylinder = area 1 - 2' - 5 - 6

Work done in H.P. cylinder = area 5 - 2 - 3 - 4Due to imperfect inter cooling the saving in work is shown by the shaded area 2- 2 - - 3.Work required per cycle in L.P cylinder

Work required per cycle in HP cylinder

Total work required per cycle,

ROTARY COMPRESSORS

• Rotary compressors are used for supplying large volume of air up to 3000 m at avery low pressure which rises up to 10 bar.

• The compression of air follows the law pV = constant. The index of compressionmay be as high as 1.7 if no cooling devices are used.

• It runs at a very high speed up to 40000 rpm. By using intercoolers between thestages the value of index n can be reduced which approximates adiabaticcompression.

• Rotary compressors are classified as: (a) positive displacement compressor, and(b) non-positive displacement compressors.

Positive Displacement CompressorsPositive displacement compressors are further sub-divided into roots blower and vaneblower.Roots Blower

The back flow of high pressure air from the receiver creates a rise in pressure in the rootsblower. The p-V diagram of roots blower is given in Fig. 6

The ratio of adiabatic work done to the

Fig. 6 Roots blower p-V diagram

Theoretical work done in compressing the air

Where rp is the pressure ratio

It is seen from the efficiency of the root compressor decreases with increase in pressureratio. It is used to supply air from 0.15 m to 1500 m. The pressure ratio is in the order of1 to 3.6 for single stage machines. The maximum rpm is 12500.

Non-Positive Displacement CompressorsThese compressors are also known as dynamic compressors and are sub-divided into

(a) Centrifugal compressors, and (b) axial flow compressor.Centrifugal Compressor

• It consists of a rotor in which a number of curved vanes are mounted. The rotorrotates in a casing.

• As the rotor rotates, it sucks air through its eye, increases its pressure due tocentrifugal force and forces the air to flow into the diffuser where its velocity isreduced by providing more cross-sectional area.

• Part of the kinetic energy of the air is converted into pressure energy and pressureof the air is further increased.

• Finally the air at a high pressure is delivered to the receiver.Let p1 = initial pressure of air, V1 = initial volume of air, T1 = initial temperature of air, p2V2, T2 = Corresponding values for the final condition, m = mass of air compressed perminute.

For isothermal compression the work done is given by,

For polytropic compression work done is given by,

For adiabatic compression work done is given by,

REFRIGERATION & AIR-CONDITIONINGINTRODUCTION

• A major application area of thermodynamics is refrigeration which is the scienceof producing and maintaining temperatures below that of the surroundingatmosphere, i.e. the transfer of heat from lower temperature region to highertemperature region.

• The devices that produce refrigeration are called refrigerator or heat pump and thecycles on which they operate are called as refrigeration cycle.

• The melting of ice or snow was one of the earliest methods of refrigeration. Whenice is placed in a given space which is warmer than ice s melting point 0 C, thenspace is cooled by the heat flow from the space to the ice.

• The ice changes its state from solid to liquid. This is a non-cyclic process inwhich the cooling substance is consumed and discarded. In order to overcome thisuse of cyclic process is introduced.

• The most frequently used refrigeration cycle is the vapour compressionrefrigeration cycle in which the refrigerant is used again and again by carrying outvaporization and condensation alternately.

REFRIGERATORS AND HEAT PUMPS

• From common experience it is observed that the heat flows from high temperatureregion to low temperature region.

• This heat transfer process occurs in nature without requiring any device.

• The reverse process; the heat flow from low temperature region to hightemperature region requires special devices called as refrigerator, i.e., reversedheat engine which receives heat Q2 from a low temperature T2 region, dischargesheat Q1 to a high temperature T1 region by the net inflow of work W as shown inFig.5.

• Refrigerators are cyclic devices and the working fluids used in the refrigerationcycles are called as refrigerants.

• Heat pump which follows the refrigeration is used to maintain a heated space at ahigh temperature by transferring heat from the cold environment. A heat pump isshown schematically in Fig. 6.

Fig. 5 Refrigeration or reversed heat engine Fig. 6 Heat pump

• The performance of refrigeration and heat pumps is expressed in terms of thecoefficient of performance (COP) which is defined as the ratio of desired outputeffect to the work required to produce effect.

• The amount of heat extracted in a given time is known as refrigerating effect.

• The cooling capacity of a refrigeration system is defined as the rate of heatremoved from the refrigerated space is often expressed in terms of tons ofrefrigeration.

• A tones of refrigeration is defined as the quantity of heat to be removed in orderto form one ton of ice in 24 hour and is equivalent to 210 kJ/min.

REVERSED CARNOT CYCLEThe reversed Carnot cycle is used for producing refrigeration. It is shown in Fig.7,Process : 1-2The air is expanded isentropically from I to 2 which causes temperature to fall from T1, toT2.

Process 2-3

The air is expanded isothermally to point 3, at temperature T3 during which causes heat toabsorb from the cold body.

Fig. 7 Reversed Carnot cycle

Process 3-4The air is compressed isentropically to point 4, by the help of external power whichcauses the temperature to rise to T1. During this process no heat is absorbed from orrejected by the air.

Process 4-1The air is compressed isothermally from 4 to 1. During which the heat is rejected by theair to the hot body.

VAPOUR COMPRESSION REFRIGERATION SYSTEM

• The modern refrigerating plants work on vapour compression system.

• The refrigerant used in this system alternately undergoes a change of phase fromvapour to liquid during the cycle.

• The basic operations involved in a vapour compression refrigeration plant areillustrated in the flow diagram, Fig. 8, and the property diagram, Fig.9.

Fig. 8 Flow diagram for vapour compression refrigeration systemThe assumptions made to draw the T-s diagrams are

1. Condition of the vapour leaving the evaporation and entering the compressor is drysaturated.

2.Compression of vapour in the compressor is entropic.3. There is no pressure loss in the system.

4. There is no undercooling of the refrigerant in the condenser.5, required to drive the system is equal to the difference between the heat rejected in the

condensor and heat absorbed in the evaporator.Compression

• A reversible adiabatic process 1-2 or 1- either starting with state 1 (saturatedvapour) called as dry compression or starting with state 1 (wet vapour) called aswet compression.

• Because of the liquid refrigerant being trapped in the cylinder during wetcompression (1 - ), dry compression (1-2) is always preferred.

• The liquid in the cylinder may damage the valves and wash away the lubricant oilfrom the walls of the cylinder, thus accelerating wear.

Fig. 9 p-V and T-s diagram

Cooling and CondensingA reversible constant pressure process 2-3 first desuperheated and then condensed,ending with saturated liquid. Heat Q is rejected out.Expansion

An adiabatic throttling process 3-4 for which enthalpy remains constant which is anadiabatic but not an isentropic.

Since it is irreversible it is shown in dotted line in the property diagrams.Evaporation

• A constant pressure reversible process 4-1 completes the cycle. The refrigerant isthrottled by the expansion valve to a pressure.

• The saturation temperature at this pressure being below the temperature of thesurroundings which gets cooled.

• The evaporator thus produces the cooling effect by absorbing heat Q2 from thesurroundings by evaporation.

PERFORMANCE OF VAPOUR COMPRESSION SYSTEM

• In a vapour compression refrigeration plant, when steady state has been reached,for 1 kg of refrigerant flow through the cycle, the steady flow energy equationsmay be written for each component in the cycle as follows

• Neglecting kinetic energy and potential energy changes Compressor

The mass fraction of vapour in liquid-vapour mixture or the quantity of the refrigerant atthe inlet to the evaporator in x

Equation gives the amount of heat removed from the surroundings per unit mass flow ofrefrigerant.The coefficient of performance of the cycle

From the p-h chart of the refrigerant the values of enthalpy at all the points of the cyctecan be obtained.

If is the mass flow of refrigerant in kg/s then the rate of heat removal from thesurroundings

• One tones of refrigeration is defined as the rate of heat removal from thesurroundings equivalent to the heat required for melting 1 tones of ice in one day.

• If the latent heat of fusion of ice is taken as 336 kJ/kg, then 1 tones is equivalentto heat removal at the rate of (1000 x 336) I 24 kJ/h or 14,000 kJ/h.

The rate of heat removal in the condenserQ = (h2 h3) kJ/s

• If water cooling is used in the condenser the mass flow rate of cooling water m inkgls, the rise in temperature of water is (tc2 tc1) and Cc in specific heat of coolingwater

• For the condition of heat transfer is between the refrigerant and water and there isno interaction with the surroundings.

Rate of work input to compressor

VAPOUR ABSORPTION REFRIGERATION SYSTEM

• The absorption system differs from the compression system in a way that it raisesheat energy instead of mechanical energy to perform refrigeration cycle.

• In the basic absorption system, the compressor air absorber - generator assemblyinvolving less mechanical work. Figure 10 shows a simple vapour absorptionrefrigeration system, in which ammonia is the refrigerant and water is theabsorbent. This is known as aqua-ammonia absorption system.

• The ammonia vapour at low pressure leaving the evaporator passes to theabsorber where it is dissolved in the weak ammonia solution contained in theabsorber. The absorber is cooled by the cooling water circulation.

Fig. 10 Vapour absorption refrigeration system

• From the absorber strong ammonia solution is pumped to the generator and iscirculated through the system.

• The pump increases the pressure of the solution to a minimum value of 10 bar, inorder to attain fluid flow through the condenser.

• The strong ammonia solution is heated in the generator by a heating source andvapour is driven out of the solution. Then the vapour passes to the condenser andit is condensed from the condenser.

• The high pressure liquid ammonia passes through the expansion value, there it isconverted into low pressure wet vapour (about 3 bar). Then this cold and wetammonia vapour passes through the evaporation when it extracts the latent heatfrom the brine or substance to be cooled.

COP OF AN ABSORPTION SYSTEM

• In a refrigeration plant let us assume, QG is the heat supplied to the generator froma source at T1, temperature, provides refrigeration by extracting QE from theregion at a temperature of TR.

• This is done in the evaporator and rejects heat QA from absorber and fromcompressor to the (sink) atmosphere at T2 temperature as shown in Fig. 11

Fig. 11 Energy fluxes in vapour absorption system

Linde-Hampson System

• Linde - Hampson cycle is used successfully for the liquefaction of gases which isshown schematically and on T-s diagram in Fig.11

• Make up gas is mixed with the vapour from previous cycle, the mixture at 2 iscompressed a multistage compressor to state 3.

• By this isothermal compression process gas pressure is increased. By providingIntercooling between each stage of the compression the isothermal process isperformed.

• The high pressure gas is cooled in after cooler (heat exchanger) to state 4 and it isfurther cooled in the counter flow regenerator to state 5.

• Then it is passed through the throttle valve, there it is converted into saturatedliquid - vapour mixture in state 6.

Fig. 11 Linde-Hampson system

• The desired liquid (state 7) is collected in the tank and the vapour (state 8) ispassed through the regenerator to increase the temperature to state 9. Then thegas from state 9 is mixed with fresh makeup gas and the cycle is repeated.

AIR CONDITIONING SYSTEMSThe science of air conditioning deals with supplying and maintaining desirable internal,atmospheric conditions irrespective of external conditions.The four important factors involved in a complete air conditioning installation are

(i) Temperature control (ii) Humidity control(iii) Air filtering, cleaning and purification (iv) Air movement and circulation.

• The simultaneous control of these four factors within required limits whendirected towards human comfort and health or when industrially directed towardsconditions per matting the best product yield during manufacturing and storagecan rightly be called air conditioning.

• The importance of controlling the above four factors in relation to human comfortis briefly discussed in the following sections

CLASSIFICATION OF AIR CONDITIONING SYSTEMS

The air conditioning systems may be classified in several ways as discussed belowClassification as to Major Function

Air conditioning systems are of two basic types as far as their functions are concerned:Comfort Air Conditioning Systems

• The purpose of such a system is to create atmospheric conditions conductive tohuman health, comfort and efficiency.

• For example air-conditioning systems used in homes, offices, shops, restaurants,theatres, hospitals and schools, etc.

Industrial Air Conditioning Systems

• The purpose of these air conditioning systems is to control atmospheric conditionsprimarily for the proper conduct of research and manufacturing processes.

• Examples are, air conditioning systems used in textile mills, paper mills, machinepart manufacturing plants, tool rooms, printing and photo processing plants, etc.

Classification as to Season of the YearWinter Air Conditioning System

• Such systems when properly designed and installed, maintain indoor atmosphericconditions for winter comfort.

• The major problems of winter air conditioning are to heat the air and to bringmoisture content up to an acceptable level.

• Heating is accomplished by electric heaters or furnaces and boilers fired by gas,oil or coal. Humidifier may be of the simple pan type or spray type.

Summer Air Conditioning Systems

• These systems control all the four atmospheric conditions for summer comfort.

• The major problems are to cool the air and to remove excess moisture from it.Cooling is accomplished by mechanical refrigeration.

• Removal of moisture (dehumidification) is accomplished as condensation ofwater vapour in the air occurs on cold coil surfaces.

Year Round Air Conditioning Systems

These systems are composed of heating and cooling equipment with automatic controlsand associated components to produce the four atmospheric conditions for humancomfort at all times of the year.

Classification as to Equipment ArrangementUnitary Systems

• These systems make use of air conditioners which are completely factoryassembled.

• A single air conditioner may serve if the building is a small one or, the area maybe divided into several zones, each being served by a conditioner of small tomedium capacity.

• This type of system has the advantage of moderate initial cost and also that offlexibility of operation.

Central Station Systems

When several rooms in the same building are intended for use which requires airhaving approximately the same temperature and RH, they can usually be air conditionedmore economically from a central system than from a number of self-contained units.Combination systems

• This type of system combines the features of central station and unitary systems.

• Heat energy is supplied in pipes to several unit air conditioners in the form ofsteam or hot water.

• Chilled water from the central refrigerating equipment is also piped to the airconditioner.

PSYCHROMETRYINTRODUCTION

• It is the branch of science which mainly deals with the study of mixture of dry airand water vapour.

• It is the foundation on which most of the calculations of air conditioning loads,heat transmission through structures, cooling towers, etc are based.

• The earth s atmosphere, the air we breathe, is a mixture of several gases includingnitrogen, oxygen, argon, carbon dioxide, water vapour and traces of other gases.

• But generally speaking, in refrigeration applications the atmosphere is consideredto be a mixture of dry air and water vapour.

PROPERTIES OF ATMOSPHERIC AIR

Moist AirIt is a mixture of dry air and water vapour. The quantity of water vapour present in airdepends upon the temperature of the air.Water Vapour

• Water vapour present in air is known as moisture. The determination of quantityof moisture present in air is a very important factor in all air conditioning systems.

• Moist air is said to be saturated when it contains maximum amount of watervapour that it can hold. Such air will be invisible, If we add more water to this air,drops of water will remain in suspension and will make the air foggy or misty.

• If the temperature of mixture of air and water vapour is more than the saturationtemperature of the water vapour, the. vapour will be in a superheated state.

Specific Humidity or Humidity RatioIt is the mass of water vapour per unit mass of dry air. In a vapour air mixture is denotedby w.Then specific humidity,

Where ma mass of dry air, mf = mass of water vapour associated with the above mass ofdry air in a sample of moist air of mass (ma + mf)

Absolute Humidity or Vapour DensityIt is the mass of water vapour in kg per m of air vapour mixture is denoted by .

Degree of Saturation

• It is the ratio of prevailing humidity ratio of moist air to the humidity ratio ofsaturated air at the same temperature and pressure.

• If w = kg of moisture contained per kg of any air under given conditions, w = kgof moisture required to saturate one kg of air at the same dry bulb temperature.

Relative Humidity

It is defined as the ratio of actual mass of water vapour in a given volume of air to themass of water vapour contained in the same volume at the same temperature when the airis saturated.Dry Bulb Temperature

It is the temperature recorded by a thermometer whose reading is not affected by thehumidity ratio or by thermal radiation. It is denoted by td

PSYCHROMETRIC CHARTS

• A psychometric chart is a graphical representation of various thermodynamicproperties of moist air.

• Such a chart helps us to readily measure the properties of air and eliminates manytime consuming and tedious calculations which would otherwise be necessary.

• Different air-conditioning manufactures have slightly different forms of this chartwhich may differ in the location of the information.

• But basically they are alike because they all graphically represent the properties ofair. Such as temperature, humidity ratio, relative humidity, enthalpy etc.

• One of such charts copy righted by the Carrier Corporation having dry bulbtemperature as the abscissa and humidity ratio or moisture content of air-in kg perkg dry air as the ordinate is shown in Fig. 12.

(1) Dry Bulb Temperature (td) Lines are straight and vertical lines drawn parallel to theordinate.

(2) Humidity Ratio (w) Lines are the straight and horizontal lines drawn parallel to theabcissa.

(3) Vapour Pressure (pu) Lines. These are the straight, horizontal and parallel lines withnon-uniform spacing between them. On the given psychometric chart, instead of markingthese lines a scale showing vapour pressure in mm of Hg has been given on its extremeleft.

(4) Dew Point Temperature (tdP) Lines are straight horizontal and parallel lines. The caleof dew point temperature is shown on the saturation line.

(5) Wet Bulb Temperature (t Lines are straight but inclined lines which extend diagonallyas shown on the chart. The scale of wet bulb temperature is again shown on the saturationline.(6) Enthalpy (h) Lines are the same as the wet bulb temperature lines. The scale ofenthalpy is shown on a diagonal line above the saturation line.(7) Relative Humidity ( Lines are curved lines. The saturation line shows 100% relativehumidity.(8) Specific Volume Lines are the straight - inclined lines.

The lines on the psychometric chart are drawn by assuming standard barometric pressureOf 760 mm of Hg. If pressures other than the standard pressure are given necessarycorrection shall have to be applied.

Fig. 12 Psychrometric chart

SENSIBLE HEAT FACTOR

• The process of cooling and dehumidification occurs so frequently in airconditioning that the psychometric line which represents this process has beengiven a special label.

• It is the change that takes place in sensible heat and latent heat.

Fig. 13 Sensible heat factor

Now we can define sensible heat factor as Sensible heat factor or SHF

• If the cooling process involves the removal of only sensible and no latent heat, thesensible heat factor line is horizontal and the numerical value of sensible heatfactor is 1.

• The scale on the extreme right of the psychometric chart is the sensible heat factorscale which is drawn with reference to a point shown as a dark circle on 50% RHline (near 25°C DBT).

ROOM SENSIBLE HEAT FACTOR (RSHF)

• It may be defined as the ratio of the room sensible heat to the room total heat.

• The room total heat means the sum of room sensible heat and the room latentheat. The room latent heat load is due to the moisture rejected by persons workingin the room and steam load supplied by cooker, coffee, tea pots and such othermoisture evaporating devices.

• The sensible heat load may be due to the persons, lighting, electrical andmechanical devices working in the room and solar radiation, etc.

If RSHF = room sensible heat factorRSH = room sensible heat

RLH = room latent heat

Fig. 14 Room sensible heat factor

• The conditioned air supplied to the room must have the capacity to take up bothroom sensible and latent heat load simultaneously.

• The required final condition in the room say given by point A on thepsychometric chart (Fig. 14) when joined with point B, which represents supplyair conditions, gives a line which is called room sensible heat factor line.

• The slope of this line gives the ratio of room sensible heat to room latent heat. Alittle consideration will show that supply air, having its conditions given by anypoint on this line will be able to offset the given room heat load.

• In other words supply air can have conditions marked by point 1, 2, 3, 4, etc., tosatisfy the requirement.

JAYAM COLLEGE OF ENGINEERING AND TECHNOLOGY

DHARMAPURIDEPARTMENT : EEE

YEAR / SEM : SECOND/ THIRD

SUBJECT : ME1211 / APPLIED THERMODYNAMICS

ASSIGNMENT NO 4

Unit - 4 Compressors, Refrigeration and Air conditioning

PART A

1. What is a tone of refrigeration?2. What is refrigerant?

3. How are refrigerants numbered?4. What is specific humidity?

5. What is degree of saturation?6. What is a psychrometer?

7. When do the DBT, WBT, and DPT become equal?8. When is multistage compression used?

9. Why intercooler is provided in air compressors?10. Define volumetric efficiency of a compressor?

PART B

1. A single acting air compressor compresses air from 1.5 to 8.1 bar. The clearancevolume is 2 liters. The compression and expansion follows the law pV1.3=C. if thevolumetric efficiency of a compression is 85%. Find the stroke volume and thecylinder dimension. Assume diameter of the piston is equal to stroke.

2. Derive an expression for volumetric efficiency.

3. Explain the vapour compression cycle with the help of T-s and p-h diagrams. Canthis cycle be reversible? If not why?

4. a refrigeration plant produce 0.139 kg/s of the ice at 5 C from water at 30 C. if thepower required to drive the plant is 22 kW. Determine the capacity of the ice plantin tones and the actual COP. The specific heat of ice is 2.1 kJ/kg.K.