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INFANT JESUS COLLEGE OF ENGINEERING AND TECHNOLOGY

ME 51-THERMAL ENGINEERING

Unit III

PartA

1. Why clearance is necessary in reciprocating compressors?2. Differentiate positive and non-positive displacement compressors?3. Define dew point temperature?4. Explain adiabatic humidification of air?5. When are the dry-bulb and dew point temperature identical?6. What is the difference between Wet compression and Dry compression?7. State the conditions which lower the volumetric efficiency?8. What is the effect of inter-cooling in multi stage compressors?9. Name any four commercial Non-CFC refrigerants?10.What are the components used in winter air conditioning system?

Part-B

11.Explain with a neat sketch practical Ammonia-water vapour absorption refrigeration system.Also bring out any four important differences between vapour compression and vapour

absorption refrigeration systems?

12.A two stage double acting air compressor operating at 200rpm takes in air at 1.013bar and27

oC. The size of the L.P. cylinder is 355 x 375 mm,the stroke of HP cylinder is same as the

LP cylinder and the clearance of both the cylinders is 4%. The LP cylinder discharges the air

at a pressure of 4.52 bar. The air passes through the inter cooler so that it enters the HP

cylinder at 27oC and 3.850 bar, finally it is discharged from the compressor at 15.4 bar. The

values of n for both cylinders are 1.25. Cp = 1.0035 KJ/KgK and R = 0.28 KJ/KgK

Calculate

(i) The heat rejected in the inter-cooler(ii)

The diameter of HP cylinder and

(iii) The power required to drive HP cylinder13.The following data relate to a performance test of a single acting 14 cm x 10 cm

reciprocating compressor:

Suction pressure = 1 bar, Suction temperature =20oC, Discharge pressure = 6 bar, Discharge

temperature =180oC, Speed of the compressor = 1200 rpm, Shaft power =6.25 KW, Mass of

air delivered=1.7 Kg/min. Calculate

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(i) The actual volumetric efficiency(ii) The indicated power(iii) The isothermal efficiency(iv) The mechanical efficiency(v) The overall isothermal efficiency

14.A food storage locker requires a refrigerating capacity of 50 KW. It works between acondenser temperature of 35

oC and an evaporator temperature of -10

oC. The refrigerant is

ammonia. It is sub cooled by 5oC before entering the expansion valve and dry saturated

vapour leaving the evaporator. Assuming a single cylinder, single acting compressor

operating at 1000 rpm with stroke equal to 1.2 times the bore. Determine

(i) The power required and(ii) The cylinder dimensions. Properties of ammonia are

Saturation

temperature

Pressure Enthalpy Entropy Specific volume Specific heat

oC bar KJ/Kg KJ/KgK m / Kg KJ/KgK

Liquid Vapour Liquid Vapour Liquid Vapour Liquid Vapour

-10

2.9157 154.056 1450.22 0.82965 5.7550 - 0.417477 - 2.492

35 13.522 366.072 1488.57 1.56605 5.2086 1.7023 0.095629 4.556 2.903

15.Four cylinders, double acting air compressor is required to compress 25 m3/ min of air at 1bar and 25

oC to a pressure of 15 bar. Determine the size of motor required and the cylinder

dimensions if the following additional data is given: clearance volume=15%; L/D=1.2;

rpm=300; Mechanical efficiency=80%; Polytropic index n =1.35. Assume no pressure

changes in suction valves and that the air gets heated by 10oC during suction stroke.