R05410305 Refrigeration And Air Conditioning

Set No. 1

Code No: 37025

IV B.Tech I Semester Supplimentary Examinations, May/Jun 2009 REFRIGERATION AND AIR CONDITIONING (Mechanical Engineering) Time: 3 hours Max Marks: 80 Answer any FIVE Questions All Questions carry equal marks ⋆⋆⋆⋆⋆ 1. (a) The atmospheric air at pressure 1 bar and temperature -50 C is drawn in the cylinder of the compressor of a Bell-Coleman refrigerating machine. It is compressed isotropically to a pressure of 5 bar. In the cooler, the compressed air is cooled to 150 C, pressure remaining the same. It is then expanded to a pressure 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, and 2. C.O.P. of the plant. For air assume law for expansion PV1.2 = constant. Law for compression PV1.4 = constant and specific heat of air at constant pressure = 1kj/kg.k (b) The carnet cycle machine operates between the temperature limits of 470 C and -300 C. Determine the C.O.P. when it operates as 1. A refrigerating machine 2. A heat pump and 3. A heat engine [8+8] 2. The evaporator and condenser temperature in an NH3 refrigeration system are -100 C and 400 C respectively. Determine per TR basis: (a) mass flow rate (b) compressor work (c) condenser heat rejection (d) COP and (e) refrigerating efficiency. Use only the properties of NH3 given below: Saturation temp. Pressure(p) (t)(0 C) (bar) -10 40

Sp.Volume of vapour (vg)m3 /kg

2.908 15.55

0.418 0.0833

[16] Enthalpy Enthalpy (kJ/kg.K) (kJ/kg.K) Liquid Vapour Liquid Vapour 134.95 1431.41 0.5435 5.4712 371.47 1472.02 1.3574 4.8758

For superheated NH3 at 15.55 bar, the following values may be taken: SuperheatK Specific volume(v)m3 /kg Enthalpy(h) (kJ/kg) Entropy (s) (kJ/kg.K) 60 0.108 1647.9 5.3883 80 0.116 1700.3 5.5253

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Set No. 1

Code No: 37025

3. (a) Explain why reciprocating compressor cannot be used as a vaccum pump for producing high vacuumm. (b) A reciprocating compressor operates on 1m3 /min of gas at 2 bar and delivers it 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 1.1 instead of 1.31. [8+8] 4. (a) Draw a neat diagram of three fluid system of refrigeration and explain its working. (b) Draw a neat compact diagram of lithiem bromide water absorption refrigeration system and explain its working. [16] 5. (a) What is the situation unnder which the Steam Jet Refrigeration system is recommended? What are its limitations? Can it be used for obtaining subzero temperatures? (b) Explain the various components of Steam Jet Refrigeration system and clearly discuss the function of each component. [6+10] 6. (a) Explain the process of heating and humidification. . (b) The pressure, temperature and relative humidity of air at a place are 1.013 bar, 320 C and 65% respectively. Find i. The dew point ii. Specific volume of the constituent and iii. The humidity ratio. The universal gas constant, R = 8.3143 kJ/ kg mole K. [6+10] 7. (a) Explain the difference between comfort air-conditioning and industrial airconditioning. (b) Define the term “Effective Temperature” and explain its importance in airconditioning system. Describe the factors which affect effective temperature. [6+10] 8. (a) What are the purposes of automatic controls in an air conditioning system? (b) Explain the function of basic components of automatic control circuit of an air conditioning system. [6+10] ⋆⋆⋆⋆⋆

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Set No. 2

Code No: 37025

IV B.Tech I Semester Supplimentary Examinations, May/Jun 2009 REFRIGERATION AND AIR CONDITIONING (Mechanical Engineering) Time: 3 hours Max Marks: 80 Answer any FIVE Questions All Questions carry equal marks ⋆⋆⋆⋆⋆ 1. (a) A refrigerating system working on Bell-Coleman cycle receives air from cold chamber at -50 C and compresses it from 1 bar to 4.5 bar. The compressed air is then cooled to a temperature of 370 C before it is expanded in the expander. Calculate the C O P of the system when compression and expansion are i. isentropic and ii. follow the law pv1.25 =constant. (b) An air refrigerator is used for food storage, provides 50TR. The temperature of air entering the compressor is 70 C and the temperature before entering in to the expander is 270 C. Assuming a 70% of mechanical efficiency, find i. actual C O P and ii. the power required to run the compressor. The quantity of air circulated in the system is 100kg/min. The compression and expansion follow the law pv1.3 = constant. [8+8] 2. (a) Why a throttle valve is used in vapour compressor refrigerator rather than an expansion cylinder to reduce the pressure between the condenser and evaporator. (b) A water cooler using F-12 as refrigerant works between 260 C and 20 C respectively. The vapour leaves the evaporator dry and saturated. The out put of cold water is 100kg per hour cooled from 260 C to 60 C. Assume the following: Volumetric efficiency of the compressor = 80% Mechanical efficiency of the compressor = 85% Mechanical efficiency of the motor = 95% Assume 20% of the useful cooling is lost in to the water cooler. Find the following: (a) Power required to run the motor (b) Volumetric displacement of the compressor. take CP (Water) = 4.2 kj/kg-k.

[6+10]

3. (a) With the help of neat diagram, explain the working principle of shell and tube evaporators. (b) Discuss the importance of boiling point and freezing point of following refrigerants with particular reference to their applications. R-11, R-12, R-13, R-22 and R-717 [8+8] 1 of 2

Set No. 2

Code No: 37025

4. (a) Draw a neat sketch of a practical vapour absorption refrigeration cycle. Indicate there on the phases of various flieds and the name of the equipments. Also indicate the direction of the external energy flow to or from the equipments. (b) Mention the function of each fluid in a three-fluid vapour absorption system. [8+8] 5. (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 in steam jet refrigeration system? [10+6] 6. The following two indoor conditions give the same comfort when the out-door conditions are 380 C and 62% R H. (a) 200 C DBT and 60% R.H. (b) 240 C DBT and 50% R.H. The required conditions are achieved first by cooling and dehumidifying and then by heating. The dew-point temperature of the cooling coil is 80 C. If the supply of free air to the air-conditioning system is 450 m3 /min., then find the cost of the plant for 24-hours working for both comfort conditions. Also find the by-pass factors of the cooling coils in both cases if the Cost of cooling = 50 Paise per ton of refrigeration and the Cost of heating = 15 Paise per kWh. [16] 7. (a) What are the different factors which must be considered in evaluating cooling load? What are the different means by which this load can be reduced? (b) 5000 m3 of air at 150 C and in fully saturated condition enters into a room. The sensible and latent heat loads are 72000 kJ/hr and 32000 kJ/hr respectively. What condition of the air will be maintained in tile room? [8+8] 8. (a) Discuss the effect of chilled water flow control on cooling coil performance with respect to room RH for a room with varying sensible heat gain and relatively constant latent heat gain (b) Compare the use of two position valve with the use of a modulating valve. [10+6] ⋆⋆⋆⋆⋆

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Set No. 3

Code No: 37025

IV B.Tech I Semester Supplimentary Examinations, May/Jun 2009 REFRIGERATION AND AIR CONDITIONING (Mechanical Engineering) Time: 3 hours Max Marks: 80 Answer any FIVE Questions All Questions carry equal marks ⋆⋆⋆⋆⋆ 1. (a) What are the advantages of dense air refrigeration system over an open air refrigerating system? (b) A refrigerator works on the carnet cycle in temperature between -70 C and 270 C. It makes 500kg of ice per hour at -50 C from water at 140 C. Find H.P. required to drive the compressor and C.O.P. of the cycle. Take specific heat of ice as 2.1 kj/kg-k and latent heat as 336kj/kg? [6+10] 2. (a) In a vapour compression refrigerator, the working fluid is super heated at the end of compression and is under cooled in the condenser before throttling. Sketch a working cycle on temperature entropy diagram and show how theoretical C.O.P. may be calculated from this diagram. (b) A food storage requires a refrigeration system of 12 tons capacity at an evaporator temperature of -100 C and condenser temperature 250 C. The refrigerant NH3 is sub-cooled by 50 C before passing through throttle valve. The vapour leaving the evaporator coil is 0.97 dry. Find the C.O.P. and power required to run the plant. Neglect all losses. [8+8] 3. (a) Explain why reciprocating compressor cannot be used as a vaccum pump for producing high vacuumm. (b) A reciprocating compressor operates on 1m3 /min of gas at 2 bar and delivers it 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 1.1 instead of 1.31. [8+8] 4. (a) Explain with the help of neat sketches the various components and their functions for a vapour absorption refrigeration system. (b) Discuss the function of rectifier and analyser in vapour absorption refrigeration systems. [16] 5. (a) Derive the expression for finding out the quantity of steam required per ton of refrigeration. (b) A steam jet refrigeration installation is to deliver chilled water at the rate of 2300 kg per minute at 80 C from supply water at 180 C. Condenser Saturation temperature is 38 0 C, nozzle efficiency is 90%, entrainment efficiency is 68% and diffuser efficiency is 78%. Quality of flashed vapour is 0.97. The steam consumption for the motive jet is 6500 kg/hr. Estimate the pressure of the dry and saturated motive steam. [8+8] 1 of 2

Set No. 3

Code No: 37025

6. (a) Write a short note on by-pass factor for cooling coils. (b) The sensible heat load factor (SHF) of an air-conditioned room is 0.67. The condition of the air leaving the air-conditioned room is 270 C DBT and 52% R .H. The maximum permissible temperature difference between the inlet air and out-let air is 110 C. If the quantity of air flow at the inlet of the room is 180 m3 /min, then find the sensible heat load and latent heat load of the air-conditioned room. [6+10] 7. A laboratory has 27 kW sensible and 23 kW latent heat load. The inside design conditions of air are 200 C DBT and 53% R.H. and outside design conditions of air are 380 C DBT and 250 C WBT. The ventilation air used is 76 m3 /min. A cooling coil with a bypass factor of 0.06 must be used. An apparatus DPT is 80 C. Determine (a) Amount of reheat required (b) Supply air quantity (c) DBT and WBT of air entering and leaving the apparatus (d) Supply air temperature.

[16]

8. (a) What are the specific problems concerned to factory air conditioning? (b) What methods are adopted for factory air conditioning? ⋆⋆⋆⋆⋆

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[8+8]

Set No. 4

Code No: 37025

IV B.Tech I Semester Supplimentary Examinations, May/Jun 2009 REFRIGERATION AND AIR CONDITIONING (Mechanical Engineering) Time: 3 hours Max Marks: 80 Answer any FIVE Questions All Questions carry equal marks ⋆⋆⋆⋆⋆ 1. (a) A cold storage is to be maintained at -50 C while the surroundings are at 350 C. The heat leakage from the surroundings into the cold storage is estimated to be 29KW. The actual C.O.P. of the refrigeration plant is one third of an ideal plant working between the same temperatures. Find the power required to drive the plant? (b) Refrigerator working on Bell-coleman cycle operates between pressure limits of 1.05 bar and 8.5 bar. Air is drawn from the cold chamber at 100 C, compressed and, then it is cooled to 300 C before entering the expansion cylinder. The expansion and compression follows the law PV1.3 = Constant. Determine the theoretical C.O.P of the system? 2. A commercial refrigerator operates with R-12 between 1.2368 bar and 13.672 bar. The vapour is dry and saturated at the compressor inlet. Assuming isentropic compression, determine the theoretical COP of the plant. The isentropic discharge temperature is 64.860 C. If the actual C O P of the plant 80% of the theoretical, calculate the power required to run the compressor to obtain a refrigerating capacity of 1 TR. If the liquid is sub-cooled through 100 C after condensation, calculate the power required. The properties of R-12 are given below: Saturation temp. (0 C) -25 55

Saturation pressure (bar) Enthalpy Liquid 1.2368 13.33 13.672 90.28

(kJ/ kg) Entropy (kJ/kg.K) Vapour Liquid Vapour 176.48 0.0552 0.7126 207.95 0.1397 0.6774

Properties of superheated R-12. Temperature 0 C 64.86

Pressure (bar) Enthalpy (kJ/kg) Entropy (kJ/kg.K) 13.672 220.6 0.7126

Assume specific heat of liquid to be 1.055 kJ/kg.K.

[16]

3. Describe with neat sketches, the working of (a) shell and coil condenser; and (b) shell and tube condenser.

[8+8]

4. (a) Draw a neat sketch of a practical vapour absorption refrigeration cycle. Indicate there on the phases of various flieds and the name of the equipments. Also indicate the direction of the external energy flow to or from the equipments.

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Set No. 4

Code No: 37025

(b) Mention the function of each fluid in a three-fluid vapour absorption system. [8+8] 5. (a) What is the situation unnder which the Steam Jet Refrigeration system is recommended? What are its limitations? Can it be used for obtaining subzero temperatures? (b) Explain the various components of Steam Jet Refrigeration system and clearly discuss the function of each component. [6+10] 6. (a) Define specific humidity and derive an expression for specific humidity in terms of total pressure of the humid air and partial pressure of water vapour (b) The atmospheric conditions of air are 270 C dry bulb temperature and specific humidity of 0.02 kg per kg of dry air. Find i. Partial pressure of vapour ii. Relative humidity iii. Dew point temperature.

[8+8]

7. The following data were collected for designing the air-conditioning system of a small auditorium: Total Seating Capacity: 400, Out-door conditions: 350 C DBT and 76% R.H., Required comfort conditions: 200 C DBT and 53% R.H., Sensible heat given out per person:300 kJ/hr, Latent heat given out per person: 100 kJ/hr, other sensible heat load: 150,000 kJ/hr, Latent heat load of infiltration: 1,00,000 kJ/hr, Quantity of fresh air supplied: 28 m3 /hr/person, desirable temperature rise of conditioned air within the theatre 9 0 C. Calculate (a) Percentage of total air recirculated and bypassed (b) Volume capacity of the fan (c) The capacity of the cooling coil in tons of refrigeration. Take 20% safe margin. Assume that air leaves dehumidifying coil with 100% R.H. [16] 8. (a) Explain the process of desalination of sea water by using a heat pump with neat diagram. (b) Describe the use of heat pump for heating and cooing cycle with a neat sketch. [8+8] ⋆⋆⋆⋆⋆

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