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2003 Course Refrigeration & Air Conditioning

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Total No. of Questions : 12] P1319 [Total No. of Pages : 6 [3764]-153 B.E. (Mech. Sand) REFRIGERATION AND AIR CONDITIONING (2003 Course) Time : 3 Hours] [Max. Marks : 100 Instructions to the candidates: 1) Answer 3 questions from Section-I and 3 questions from Section-II. 2) Answers to the two sections should be written in separate answer books. 3) Neat diagrams must be drawn wherever necessary. 4) Use of logarithmic tables, slide rule, Mollier charts, electronic pocket calculator and steam tables is allowed. 5) Assume suitable data, if necessary. SECTION - I Q1) a) Discuss the advantages of air as a refrigerant in air-craft. Explain necessity of air-craft refrigeration. [8] b) Explain Regenerative air-refrigeration system. Compare it with simple system. [8] OR Q2) a) An air craft refrigeration plant has a capacity of 30 TR. The ambient temperature is 17 C. The atm.air is compressed to 0.95 bar and 30 C due to ram action. The air is then further compressed in a compressor to 4.75 bar and then is cooled in a heat exchanger to 67 C. It then expands in a turbine to 1 bar before supply to the cabin. Air leaves the cabin at 27 C. The isentropic efficiency of the compressor and the turbine are 0.9. Determine i) mass flow rate circulated/min ii) COP [12] iii) power required. b) Explain the term Ram Efficiency . Q3) a) What are limitations of air refrigeration system? [4] [4] b) What is difference between retrofitting and drop in? What are the modifications made in R-12 system to make it suitable for HFC-134a refrigerant? [4] P.T.O. c) An ice production machine produces 20 tons of ice in 24 hours when water is supplied at 0 C. The temperature range of the machine is -15 C to 25 C. The vapour leaves the compressor in dry and saturated conditions and there is no under-cooling in the condenser. If the actual C.O.P., is 75% of theoretical, find the power required of a compressor. The properties of NH3 used in machine as refrigerant are given below: Temperature C hf (kJ/kg) sf (kJ/kg-K) hg (kJ/kg) 25 100.4 0.349 1324 -15 -54.77 -0.214 1310 [8] OR Q4) a) Discuss briefly the factors affecting the choice of refrigerants commonly used in refrigerating plants. [8] b) Explain the effect of various operating parameters on performance of vapour compression system. [8] Q5) a) What is necessity of multi staging? Explain the analysis of two stage compression system with flash gas chamber as a liquid subcooler. [8] b) A refrigeration system using R-12 as refrigerant has three evaporators of capacities 30TR at -10 C, 20 TR at 5 C and 10 TR at 10 C. The refrigerant leaving the evaporator is dry and saturated. The system is provided with compound compression, individual expansion valves. The condenser temperature is 40 C. Assuming isentropic compression in each compressor, find the power required to run the system and COP of the system when the liquid refrigerant leaving the condenser is saturated. [10] OR Q6) a) In a vapour absorption refrigeration system, the refrigeration temperature is -15 C. The generator is operated by solar heat where the temperature reached is 110 C, the temperature of heat sink is 55 C. What is the maximum possible COP of the system? [6] b) Explain with neat sketch lithium bromide refrigeration system. [6] c) Explain with neat sketch Cascade refrigeration system. Why CO2 is a suitable refrigerant for this system? [6] [3764]-153 -2- SECTION - II Q7) a) Prove Enthalpy of moist air (h) = cpm x Td + w x (hfg)at 0 C Where cpm = humid specific heat, Td = DBT, w = humidity ratio, hfg is enthalpy of evaporation. [4] b) Sketch comfort chart and show on it the comfort zone . [6] c) The air at 40 C DBT and 35% R.H. is passed through adiabatic humidifier and it comes out with 26.4 C DBT and fully saturated. Find the quantity of water vapour added per kg of dry air. Assume air pressure = 1.013 bar. [6] OR Q8) a) State and explain the factors affecting human comfort. [4] b) What is the effective temperature? What are the factors that influence the optimum effective temperature? [4] c) Room air at 26 C DBT and 50% RH is mixed with outdoor air at 35 C DBT and 23 C DPT in the ratio of 2:1. The mixture is passed through a cooling coil whose temperature is maintained constant at 10 C whose by pass factor is 0.2. Determine the following: [8] i) Condition of air before entering the cooling coil, ii) Condition of air leaving the coil and iii) Refrigeration load on cooling coil when 250m 3/min of air is supplied to the room. Q9) a) Explain RSHF, GSHF, ERSHF with illustrations. b) Explain air-water air-conditioning system with neat sketch. [10] [6] OR Q10) a) List the various equipments used in an air conditioning system. Explain the function of each. [6] b) Explain the working of an automatic expansion valve. Why it is called constant pressure expansion valve? [6] c) Explain the procedure of charging of refrigerant in the system. [3764]-153 -3- [4] Q11) a) Explain pressure losses in duct. b) Discuss the methods for sizing of ducts. [6] [4] c) Derive an expression for the equivalent diameter of circular duct corresponding to a rectangular duct of sides a and b for the same pressure loss per unit length when i) the quantity of air passing through both the ducts is same, and ii) the velocity of air flowing through both the ducts is same. [8] OR Q12) a) Discuss limitation of vapour compression systems for the production of low temperature. What are the applications of low temperature? [8] b) Explain construction and working of Claude system liquefaction of [10] gas. [3764]-153 -4- [3764]-153 -5- rrr [3764]-153 -6-

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