Roll No. ____________________ Total No. of pages 37ME2A

B.Tech. (Sem. VII) (Main /Back) Examination December-2014Mechanical Engineering

7ME2 Refrigeration & Air Conditioning

Time: 3 Hours Maximum Marks: 80Min. Passing Marks: 24

Instructions to Candidates:Attempt any five questions selecting one question from each unit. Allquestion carry equal marks. Schematic diagrams must be shown wherevernecessary. Any data you feel missing suitably be assumed and stated clearly.Units of quantities used/calculated must be stated clearly.

Use of following supporting material is permitted during examination.(Mentioned in form No. 205)

1. Psychometric Chart 2. Steam Table

Unit -1

Q1 (a) What is difference between a refrigerator and heat pump? Derive an expression forthe performance factor for both if they are running on reversed Carnot cycle

(8)

(b) How does an actual vapour compression cycle differ from that of a theoretical cycle? (8)Or

Q1 (a) Sketch the TS and PH diagram for the vapour compression cycles, when the vaporcompression is in-

(i) Dry saturated (ii) Wet saturated (6)

(b). A refrigerator operated between 120KPa and 800KPa in ideal vapour compression cyclewith R-134 as refrigerant. The refrigerant enters the compressor as saturated vapour andleaves the condenser as saturated liquid. The mass flow rate of refrigerant is 0.2 kg/sec.

(i) Find the rate at which heat is extracted from the refrigerated space in KJ/sec(ii) Find power input in compressor (10)

Unit 2

Q2 (a) Describe with a sketch of a Regenerative air cooling system (8)(b) Explain Bell Coleman air refrigeration cycle. (8)

OR

Q2 (a) In a refrigerator working on Bell-Coleman cycle, the air is drawn into the cylinder ofthe compressor from the cold chamber at a pressure of 1.03bar and temperature 12C.After isentropic compression to 5.5 bar, the air is cooled at constant pressure to atemperature of 22C. The polytropic expansion 1.25pv = constant then follows and the airexpanded to 1.03 bar is passed to cold chamber. Determine:

(a) Work done per kg of air(b) Refrigerating effect per kg of air flow(c) C.O.P.For air take =1.4 and pc =1.003KJ/kgK

(10)(b) Explain the working of a simple air cycle cooling system used for aircrafts. (6)

Unit-3

Q 3 (a) Draw a neat diagram of lithium bromide water absorption system and explain itsworking (10)(b) A vapour absorption system works with generator, ambient and evaporator temperature of360 K, 310 K and 260 K respectively. Find maximum COP of the system. If the evaporatortemperature falls to 150 K. What should be the generator temperature in order to operate thesystem with same COP (6)

OR

Q 3 (a) What are the desirable properties of an ideal refrigerant? Write the factors consideredfor the selection of refrigerant for a system? (8)

(b) Derive the maximum COP for vapour absorption system with neat sketch (8)Unit -4

Q4 (a) In a heating application, Moist air enters a steam heating coil at10oC,50% RH andleaves at 30oC. Determine the sensible heat transfer, If mass flow rate of air is 100Kg of dryair per second. Also determine the steam mass flow rate if steam enters saturated at 100oCand condensate leaves at 80oC (8)(b) Define Specific humidity or humidity ratio. Derive the relation between specific humidity,

vapour pressure and total pressure of air (8)OR

Q4 (a) 800 m3/min of re-circulated air at 22oC dry bulb temp. and 10oC dew point temp. is tobe mixed with 300m3/min of fresh air at 30oC dry bulb temp. and 50% RH. Determine theenthalpy, specific volume, humidity ratio and dew point temp. of the mixture. (10)

(b) Define relative humidity, wet bulb temperature, dry bulb temperature, dew pointtemperature and degree of saturation (6)

Unit 5

Q5 (a) A hall is to be maintained at 24oC dry bulb temperature and 60% relative humidityunder the following conditions-

Outdoor conditions-38oC DBT and 28oC WBT, Sensible heat load in the rooms-46.4KW,Latent heat load in the rooms-11.6KW, Total infiltration air-1200m3/h, Apparatus dew pointtemperature-10oC, Quantity of re circulated air from the hall-60%

If quantity of re circulated air is mixed with the conditioned air after the cooling coil.

Find the following-

(i)The condition of air leaving the conditioner coil and before mixing with the re circulatedair

(ii) The condition of air before entering the hall.

(iii) The mass of air entering the cooler.

(iv) The mass of total air passing through the hall (12)

Q5 (b) Define sol-air temperature and equivalent temperature difference (4)

OR

Q5 (a) What is Internal heat gain? Write short notes on loads due to occupancy, lighting andappliances (8)

(b) What is the difference between winter air conditioning and summer air conditioning(8)