Refrigeration and Air Conditioning(Mech)

8/13/2019 Refrigeration and Air Conditioning(Mech)

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Code No: V3214/R07 Set No. 1III B.Tech II Semester Regular Examinations, April 2010

REFRIGERATION AND AIR CONDITIONING(Mechanical Engineering)

Time: 3 hours Max Marks: 80Answer any FIVE Questions

All Questions carry equal marks

1. An air craft refrigeration plant has to handle a cabin load of 30 tonnes. The at-mospheric temperature is 17 0 C. The atmospheric air is compressed to a pressureof 0.95 bar and temperature of 30 0 C due to ram action. This air is the n furthercompressed in a compressor to 4.75 bar, cooled in a heat exchanger to 6 70 C, ex-panded in a turbine to 1 bar pressure and supplied to the cabin. The air l eaves the

cabin at a temperature of 270

C. The isentropic efficiencies of both compressor andtur bine are 0.9. Calculate the mass of air circulated per minute and the C O P forair, c p =1.004kJ/kg K and c p /c v = 1.4. [16]

2. (a) Draw the refrigeration cycle on T-S diagram. When the refrigerant is dry andsaturated at the end of the compression and nd an expression for th e C.O.P.in terms of (i) Temperature and entropies(ii) Enthalpy.

(b) An ammonia vapour compression refrigerating machine works betw een 250 Cand - 200 /C. The ammonia leaves the compressor in dry and satura ted con-dition. Liquid ammonia is under cooled to 21.5 0 C, before passing throughthrottle valve. The average specic heat of liquid ammonia is 4.75 kj/kg- 0 C.Find the theoretical C.O.P. of the machine?If the net refrigeration required is 400 103 kj/hr 1 nd the mass of ammoniacalculated per minute. Assume relative C.O.P. is 75% of theoretical C.O.P.

[8+8]

3. (a) Derive an expression for the shaft work of a reciprocating commpr essor as-suming zero clearance volume.

(b) Single stage reciprocating commpressor is required to commpress 1.5 m 3 /minif vapour refrigerant from 1 bar to 8 bar. Find the power required to drive thecommpressor, if the commpression of refrirant is (i) Isothermal; (ii) polytropicwith polytropic index as 1.12; and (iii) isentropic with inentropic index as1.31. [7+9]

4. (a) Derive an expression for the C.O.P. of an ideal vapour absorption system interms of temperature T g at which heat is supplied to the generator, the tem-perature T e at which heat is absorbed in the evaporator and the temperatureT c at which heat is discharged from condenser and absorber.

(b) In a vapour absorption refrigertion system, heating, cooling and refrigerationtakes place at the temperatures of 100 0 C, 200 C and -50 C respectively. Findthe maximum C.O.P of the system. [10+6]

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Code No: V3214/R07 Set No. 15. (a) Explain, with the help of a neat sketch, the working of a steam jet refrigeration

system.

(b) What are the advantages of barometric condenser over surface condenser insteam jet refrigeration system? [10+6]

6. (a) Explain the following terms:

i. Specic volume of humid airii. Density of humid air

iii. Enthalpy of humid air(b) Dene relative humidity, specic humidity, and dew-point tempera ture and

describe a theoretical method for determining their values. [8+8]

7. A class room is to be air-conditioned for the following given summer co nditions.

Th e data collected is given as follows:Size of class-room: 18 12 6 m, out-door conditions: 42 0 C DBT and 5 2% R.H.,required comfort condition: 18 0 C DBT and 58% R.H., seating capacity: 45 , sensibleheat in the room excluding inltrated load: 40,000 kJ/hr, sensible heat l oad fromother sources: 8000 kJ/hr, lighting load: l2 tubes of 80 watts, inltrate d air: 27m3 /min. If 35% air is taken from outside and remaining is recirculated then ndthe following:

(a) Capacity of the cooling coil in tons of refrigeration and its bypass fa ctor

(b) Capacity of the blower in m3/min. Assume DPT of the coil 6 0 C. [16]

8. (a) Differentiate between central, district and Unitary air-conditioning s ystems.

(b) Discuss relative merits and demerits of central system with District system.[8+8]

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1. (a) Explain the working of simple air evaporative cooling system use d for aircrafts?

(b) A simple evaporative air refrigeration system is used for an aero plane totake 20 tonnes of refrigeration load. The ambient air conditions are 200 C and0.9 bar. The ambient air is rammed isentropically to a pressure of 1 bar.The air leaving the main compressor at pressure 3.5 bar is rst cool ed in theheat exchanger having effectiveness of 0.6 and then in the evaporat or whereits temperature is reduced by 5 0 C. The air from the evaporator i s passedthrough the cooling turbine and then it is supplied to the cabin wh ich is tobe maintained at a temperature of 25 0 C and at a pressure of 1.05 bar. If theinternal efficiency of the compressor is 80% and that of cooling turbin e is 75%.Determine (i) Mass of air bled off the main compressor, (ii) Power requiredfor the refrigerating system, and (iii) C O P of the refrigerating syst em.[8+8]

2. (a) Sketch the T-S and p-h diagrams for the vapour compression cycles when thevapour after compression is:

i. dry saturated, andii. wet.

(b) A vapour compression refrigerator works between the pressure limits of 60 barand 25 bar. The working uid is just dry at the end of the compres sion andthere is no under cooling of the liquid before the expansion valve. De termine:

i. C O P of the cycle andii. Capacity of the refrigerator if the uid ow is at the rate of 5 k g/min.

[8+8]

Data:Pressure (bar) Saturation Temp (K) Enthalpy (kJ/kg) Entropy (kJ/jg.K)

Liquid Vapour Liquid Vapour60 295 151.96 293.29 0.554 1.033225 261 56.32 322.58 0.226 1.2464

3. (a) Explain the dry expansion evaporator with the help of a neat sketch.

(b) Write short note on:

i. Ozone depletionii. Global warming. [8+8]

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Code No: V3214/R07 Set No. 24. Draw a neat line diagram of Electrro - Lux refrigerator and explain its working

principle. What is the important role of hydrogen in this refrigeration system?[16]

5. A steam jet refrigeration system is to supply 1200 kg per minute of chilled water at6.50 C. The make up water from the mains is at 27 0 C. If the steam supply is availableat 9.5 bar and 200 0 C and nozzle, entrainment and diffuser efficiency can be assumedas 89%, 63% and 72% respectively, the quality of vapour entering the ejector maybe assumed 0.98, and condensate leaves the condenser at 33 0 C. Determine:

(a) Steam consumption in kg/hour and kg/hr per ton of refrigeration

(b) Heat rejected in the condenser in kJ/hr and kJ/hr per ton of refriger ation.[16]

6. (a) When do the DBT, WBT and DPT become equal?

(b) Air at 370 C, 44 percent relative humidity, is cooled to 23 0 C by spraying water

at 130

C into it. The mixture pressure remains constant at 101.3 kPa. Assum-ing that all of the water evaporates and that the mixing occurs in an insulatedduct, calculate the mass of water added per kilogram of air. [4+12]

7. The following data refers for a space to be air conditioned:Inside design conditions = 23 0 C DBT, 48% RH, outdoor air conditions = 450 CDBT, 28 0 C WBT, room sensible heat gain = 18 kW, room latent heat gain = 6kW, by-pass factor of the cooling coil = 0.12, the return air from the space is mixedwith the outside air before entering the cooling coil in the ratio of 5 : 1 by mass.Determine

(a) Apparatus dew point

(b) Condition of air entering and leaving the cooling coil

(c) Dehumidied air quantity(d) Fresh air mass ow and volume ow rate

(e) Total refrigeration load on the air-conditioning plant. [16]

8. (a) Describe the different methods of air conditioning duct design. Why aredampers required in some systems?

(b) What are the different methods of heat recovery and where they are used?[8+8]

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1. (a) The atmospheric air at pressure 1 bar and temperature -5 0 C is drawn inthe cylinder of the compressor of a Bell-Coleman refrigerating mach ine. It iscompressed isotropically to a pressure of 5 bar. In the cooler, the co mpressedair is cooled to 150 C, pressure remaining the same. It is then expan ded to apressure of 1 bar in an expansion cylinder, from where it is passed to the cold

chamber. Find:1. The work done per kg of air, and2. C.O.P. of the plant.For air assume law for expansion PV 1 .2 = constant. Law for compressi on PV 1 .4

= constant and specic heat of air at constant pressure = 1kj/kg.k

(b) The carnet cycle machine operates between the temperature limits of 470 Cand -300 C. Determine the C.O.P. when it operates as1. A refrigerating machine2. A heat pump and3. A heat engine [8+8]

2. (a) With a neat sketch, explain the working principle of vapour compre ssion re-frigeration system.

(b) A15TR freon22 Vapour compression system operates between a c ondensertemperature of 40 0 C and an evaporator temperature of 5 0 C.i) Determine the compressor discharge temperature.ii) Calculate the theoretical piston displacement and power consump tion f thecompressor per ton if refrigeration.Using the p-h diagram for Freon22. Assuming the specic heat of it s vapouras 0.8 kj/kg.k [8+8].

3. (a) What is the difference between dry expansion type and ooded type evapora-tor.

(b) What are the different types of commpressors? Mention the elds for the useof each in refrigeration systems giving reasons. [8+8]

4. (a) Draw a neat sketch of a practical vapour absorption refrigeration cycle. Indi-cate there on the phases of various ieds and the name of the equipments. Alsoindicate the direction of the external energy ow to or from the equipments.

(b) Mention the function of each uid in a three-uid vapour absorption system.[8+8]

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Code No: V3214/R07 Set No. 35. (a) List out the merits and demerits of thermo-electric refrigeration system over

other refrigeration systems.

(b) What are the various applications of thermo-electric refrigerator. [8+8]6. (a) Why does the enthalpy of an air-vapour mixture remain constant during an

adiabatic saturation process?

(b) An air-water vapour mixture enters a heater-humidier unit at 8 0 C, 110 kPa,45% RH. The ow rate of dry air is 0.15 kg/s. Liquid water at 15 0 C is sprayedinto the mixture at the rate of 0.003 kg/s. The mixture leaves th e unit at400 C, 110 kPa. Calculate

i. The relative humidity at the outlet, andii. The rate of heat transfer to the unit. [6+10]

7. (a) Explain how does the body attempt to compensate for a cool env ironmentwhich tends to lower the internal temperature?(b) Why ventilation is required? Explain why different ventilation stan dards for

different purposes are recommended? [6+10]

8. (a) What are the advantages of all air-conditioned system over all w ater air-conditioned system?

(b) What are the different methods of controlling the temperature? Disc uss theirrelative merits. [6+10]

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1. (a) What are the advantages of dense air refrigeration system over an open airrefrigerating system?

(b) A refrigerator works on the carnet cycle in temperature between - 70 C and270 C. It makes 500kg of ice per hour at -50 C from water at 14 0 C. Find H.P.required to drive the compressor and C.O.P. of the cycle. Take spe cic heatof ice as 2.1 kj/kg-k and latent heat as 336kj/kg? [6+10]

2. (a) Explain the construction of T-S and P- h diagrams and state why P- h diagramis more suitable for calculations over T-S diagram.

(b) A refrigerating machine using NH3 operates between the temperatu re limitsof -150 C and 300 C. Find the C O P of the system. Also nd the corre spondingvalue for a reversed carnot cycle operating between the same temper atures.The properties of NH 3 are given below:

[8+8]Pr essure(bar) Temperature 0 C Liquid Vapour

hf(Kj /Kg) Sf(Kj /Kg-K) hg(Kj /K g) Sg(Kj /Kg-K)2.41 -5 351 3.95 1667.5 9.05

11.895 30 562 4.69 1711 8.48

3. (a) Explain why reciprocating compressor cannot be used as a vaccum pump forproducing high vacuumm.

(b) A reciprocating compressor operates on 1m 3 /min of gas at 2 bar and deliversit at 12 bar. The clearance is 6%. The adia batic commpression index is 1.3%.Determine the change in work of compressor if the re-expansion index is 1.1instead of 1.31. [8+8]

4. (a) What is the function of the following components in an absorption system:

i. Absorberii. Rectier

iii. Analyseriv. Heat exchangers.

(b) Draw a neat diagram of lithiem bromide water absorption system and explainits working. [8+8]

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Code No: V3214/R07 Set No. 45. (a) What is the situation unnder which the Steam Jet Refrigeration system is

recommended? What are its limitations? Can it be used for obtaining sub-zero temperatures?

(b) Explain the various components of Steam Jet Refrigeration system and clearlydiscuss the function of each component. [6+10]

6. 1100 m3 of air per minute at 25 0 C DBT and 45% R.H. is supplied to an auditorium.The condition of the atmospheric air is 35 0 C DBT and 65% R.H. The requiredcondition is achieved rst by cooling and dehumidifying and then by heati ng. Findthe cooling capacity required in tons of refrigeration and heating capacity requiredin kW.Take air pressure as 1.03 bar. [16]

7. The following data were collected for designing an air-conditioned hall. Out-doorconditions 41 0 C DBT and 30 0 C WBT, required inside conditions: 29 0 C DBT and

230

C WBT, sensible heat load of the hall: 1,50,000 kJ/hr, latent heat lo ad of thehall:60,000 kJ/hr, inltrated air 28 m 3 /min, cooling coil DPT:18 0 C, recirculatedair from hall: 64% of total. The quantity of recirculated air is mixed with theconditioned air after the cooling coil. Find the following:

(a) The condition of air after the cooling coil and before mixing the rec irculatedair

(b) Condition of air entering the hall

(c) Weight of fresh air entering the cooler(d) Bypass factor of the cooling

(e) Capacity of the cooling coil in tons of refrigeration. [16]

8. (a) Discuss the common considerations for humidication of air in air c ondition-ing.

(b) What are the different methods of humidifying the air? Explain the workingof any one of the atomizing the water type humidier. [8+8]

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