Refrigeration and Air Conditioning Assignment 1

Department of Mechanical Engineering

DVR & Dr. HS MIC College of Technology

  Refrigeration and Air conditioning

Assignment – I

1) A simple evaporative air refrigeration system is used for an aero plane to take 20 tonnes of refrigeration load. The ambient air conditions are 20⁰C and 0.9 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 first cooled in the heat exchanger having effectiveness of 0.6 and then in the evaporator where its temperature is reduced by 5⁰C. The air from the evaporator is passed through the cooling turbine and then it is supplied to the cabin which is to be maintained at a temperature of 25⁰C and at a pressure of 1.05 bar. If the internal efficiency of the compressor is 80% and that of cooling turbine is 75%. 
Determine (i) Mass of air bled off the main compressor, (ii) Power required for the refrigerating system, and (iii) C O P of the refrigerating system.

2)  A vapour compression machine is used to maintain a temperature of -23⁰C in a refrigerated space. The ambient temperature is 37⁰C. The compressor takes in dry saturated vapour of F12. A minimum 10⁰C temperature difference is required at the evaporator as well as at condenser. There is no sub-cooling of liquid. If the refrigerant flow rate is 1kg/min, find:

i. Tonnage of refrigeration

ii. Power requirement

iii. Ratio of C.O.P. of this cycle of C.O.P. of carnet cycle.

3) A vapour compression refrigerator uses methyl chloride (R-40) and operates between pressure limits of 177.4 kPa . At entry to the compressor, the methyl chloride is dry saturated and after compression has a temperature of 102⁰C. The compressor has a bore and stroke of 75mm and runs at 8 rev/s with a volumetric efficiency of 80%. The temperature of the liquid refrigerant as it leaves the condenser is 35⁰C and its specific heat capacity is 1.624 kJ/kg. K. The specific heat capacity of the superheated vapour may be assumed to be constant. Determine:

(a) refrigerator C O P

(b) mass flow rate of refrigerant and

(c) cooling water required by the condenser if its temperature rise is limited to

12⁰C. Specific heat capacity of water = 4.187 kJ/kg.K.

The relevant properties of methyl chloride are as follows:

Sat.temp. 0C	Pressure kPa	Specific Volume m3/kg		Specific enthalpy kJ/kg		Specific entropy kJ/kg.K
		Liquid	Vapour	Liquid	Vapour	Liquid	Vapour
-10	177.4	0.00102	0.233	45.38	460.76	0.183	1.762
45	967.5	0.00115	0.046	132.98	483.6	0.485	1.587

4) . (a) Explain shell and coil type water cooled condensers. How do these compare with shell and tube type condensers in large capacity applications.

(b) Write short note on “plate evaporators”.

5. (a) Make a comparative list between a vapour - absorption system and a compression system.

(b) Explain the Electro-lux refrigeration system with a neat sketch. What is the purpose of hydrogen in it?

6)  (a) Describe, with a sketch, a centrifugal compressor. Where are centrifugal compressors preferred over reciprocating compressor in refrigeration system?

(b) Describe the refrigerating properties of ammonia for use in domestic and commercial type of refrigerating appliances.

7. (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.

8 (a) Derive an expression for the C.O.P. of an ideal vapour absorption system in terms of temperature Tg at which heat is supplied to the generator, the temperature Te at which heat is absorbed in the evaporator and the temperature Tc at which heat is discharged from condenser and absorber.

(b) In a vapour absorption refrigertion system, heating, cooling and refrigeration takes place at the temperatures of 100⁰C, 20⁰C and -5⁰C respectively. Find the maximum C.O.P of the system.

9) (a) What is the difference between dry expansion type and flooded type evaporator.

(b) What are the different types of compressors? Mention the fields for the use

of each in refrigeration systems giving reasons

10) 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?

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

(b) Write short note on:

i. Ozone depletion

ii. Global warming.

12) (a) With a neat sketch, explain the working principle of vapour compression refrigeration system.

(b) A15TR freon22 Vapour compression system operates between a condenser temperature of 40⁰C and an evaporator temperature of 5⁰C. i) Determine the compressor discharge  temperature.

ii) Calculate the theoretical piston displacement and power consumption f the

compressor per ton if refrigeration.

Using the p-h diagram for Freon22. Assuming the specific heat of its vapour

as 0.8 kj/kg.k .