GATE and UPSC ESE Mechanical Engineering consists of a rather ignored subject, i.e. RAC or Refrigeration and Air conditioning. It can be one of the most scoring ones, if prepared well. Here are some basic questions on the air refrigeration systems involving reversed Carnot cycle, Bell Coleman cycle and Simple air cooling system.

Q1) A Carnot refrigerator works on reversed Carnot cycle. This unit requires 1.5 kW power for every one TR of refrigeration at -23oC. Determine, (a) the COP, (b) the higher temperature of the cycle, and (c) the heat rejected in kJ/min. Also calculate the heat removal rate and COP when this device is used as a heat pump.

Q2) An aircraft refrigeration system needs a refrigeration capacity of 10 TR. At the altitude of aircraft the atmospheric pressure is 0.9 bar and 10oC respectively. The pressure of air after ramming effect in a diffuser increases to 1.013 bar. The temperature of the air is reduced by 50oC in the heat exchanger. The pressure in the cabin is 1.01 bar and the temperature of air leaving the cabin is 25oC. The pressure of the compressed air is 3.5 bar. Assume that all the expansions and compressions are isentropic. Also carry out the calculation if the compressio and expansion efficiencies are 90%. Determine the following:

• power required to take the load of cooling in the cabin
• COP of the system

Q3) An aircraft refrigeration system has a capacity of 30 TR. The ambient temperature is 17oC. The atmospheric air is compressed to 0.95 bar and 30oC due to ram action. The air is then further compressed in a compressor to 4.75 bar and is then cooled in a heat exchanger to 67oC. It then expands in a turbine to 1 bar before it is supplied to the cabin. Air leaves the cabin at 27oC. The isentropic efficiencies of the compressor and the turbine are 0.9. Determine the following:

• mass flow rate of air circulated
• COP
• specific power required

Q4) An aircraft is moving at a speed of 1000 km/h at an altitude of 8000 m, where the ambient pressure and temperature are 0.35 bar and -15oC respectively. The cabin of the plane is maintained at 25oC by using a simple air refrigeration system. The pressure ratio of compressor is 3. The air is passed through the heat exchanger after compression and cooled to its original condition entering into the plane. A pressure loss of 0.1 bar takes place in the heat exchanger. The pressure of the air leaving the cooling turbine is 1.06 and the air pressure in the cabin is 1.013 bar. Considering the total cooling load of plane to be 70 kW, determine the following:

• stagnation temperature and pressure
• mass flow rate of air circulated through the cabin
• volume handled by the compressor and the expander
• net power delivered to the refrigeration system and COP of the system

Q5) An air-cooling system for a jet plane cockpit operates on the simple cycle. The cockpit is to be maintained at 25oC. The ambient air pressure and temperature are 0.35 bar and -15oC respectively. The pressure ratio of the jet compressor is 3. The speed of the plane is 1000 km/h. The pressure drop through the cooler coil is 0.1 bar. The pressure of the air leaving the cooling turbine is 1.06 bar and that in the cockpit is 1.01325 bar. The cockpit cooling load is 58.05 kW.

Calculate the following:

• stagnation temperature and presssure of the air entering the compressor
• mass flow rate of the air circulated
• volume handled by the compressor
• net power delivered by the engine to the refrigeration unit
• COP of the system

Q6) A simple air refrigeration system is used for an aircraft to take a load of 20 TR. The ambient pressure and temperature are 0.9 bar and 22oC. The pressure of air is increased to 1 bar due to isentropic ramming process. The air is further compressed in a compressor to 3.5 bar and then cooled in a heat exchanger to 72oC. Finally the air is passed through the cooling turbine and then supplied to the cabin at 1.03 bar. The air leaves the cabin at 25oC. Assuming the isentropic efficiency of the compressor and turbine are 80% and 75% respectively. Find:

• the power required to take the cooling load to the cabin
• the COP of the system

Q2. T2=293 k T3=417k T4=367k T5=257k Mass=57.72 kg/kg Power = 107.8kw and COP= 0.326

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Q4 TS plot

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