JAWAHARLAL NEHRU TECHNOLOGICAL UNIVERSITY HYDERABAD
JAWAHARLAL NEHRU TECHNOLOGICAL UNIVERSITY HYDERABAD B.Tech IV Year REFRIGERATION AND AIR CONDITIONING Previous Question Paper (July/August - 2022 - R18)
Answer any five questions. All questions carry equal marks.
Main
Differentiate between open air and dense air refrigeration systems based on constructional and operational features.
Answer
The Open Air Refrigeration System operates with the cabin air being circulated through the system, cooled, and returned to the cabin. It is primarily used in aircraft for cabin cooling. Its construction is simpler and lighter as it uses the cabin as part of the cycle. However, it is prone to moisture accumulation and icing issues, and it requires a continuous supply of fresh air.
The Dense Air Refrigeration System operates in a closed loop where the air is continuously recirculated within the system at a pressure higher than atmospheric pressure. It is typically used in marine or industrial applications. Because the air is kept at a higher pressure, the density is higher, allowing for a smaller compressor and turbine for a given cooling capacity. It avoids moisture issues as the system is sealed, but it requires a more complex, heavier construction and a separate heat exchanger to reject heat from the closed loop.
A bootstrap air refrigeration system of 15 TR capacity is operated at ambient temperature and pressure are 8° C and 0.7 bar. The air is rammed is entropically to a pressure of 0.09 bar. The pressure of air bled off from the main compressor is 2.5 bar and this is further compressed in a secondary compressor to a pressure of 3.8 bar, the isentropic efficiency of both the compressors is 90% and that of cooling turbine is 85%. The effectiveness of the heat exchangers is 90%. If the cabin is to be maintained at 25° C and the pressure in the cabin is 1.01325 bar, then calculate (i) mass of air passing through the cabin and (ii) COP of the system.
Answer
Given: Capacity = 15 TR = 15 × 3.517 = 52.755 kW. Ambient conditions: T1 = 281 K, P1 = 0.7 bar. Ram pressure P2 = 0.79 bar. Main compressor discharge P3 = 2.5 bar. Secondary compressor discharge P4 = 3.8 bar. Cabin conditions: T_cabin = 298 K, P_cabin = 1.01325 bar. Efficiencies: η_c = 0.9, η_t = 0.85, ε = 0.9.
- Calculate ramming temperature T2 = T1 × (P2/P1)^((γ-1)/γ) = 281 × (0.79/0.7)^0.286 = 288.5 K.
- Main compressor exit temperature T3 = T2 + (T2/0.9) × ((P3/P2)^0.286 - 1) = 288.5 + (288.5/0.9) × ((2.5/0.79)^0.286 - 1) = 398.2 K.
- Heat exchanger 1 exit: T4 = T3 - ε(T3 - T2) = 398.2 - 0.9(398.2 - 288.5) = 299.5 K.
- Secondary compressor exit T5 = T4 + (T4/0.9) × ((P4/P3)^0.286 - 1) = 299.5 + (299.5/0.9) × ((3.8/2.5)^0.286 - 1) = 330.4 K.
- Heat exchanger 2 exit: T6 = T5 - ε(T5 - T_ambient_ram) = 330.4 - 0.9(330.4 - 288.5) = 292.7 K.
- Turbine exit T7 = T6 × (1 - η_t × (1 - (P_cabin/P4)^0.286)) = 292.7 × (1 - 0.85 × (1 - (1.01325/3.8)^0.286)) = 237.6 K.
- Mass flow rate m = Capacity / (Cp × (T_cabin - T7)) = 52.755 / (1.005 × (298 - 237.6)) = 0.869 kg/s.
- Work input W = m × Cp × ((T3 - T2) + (T5 - T4)) = 0.869 × 1.005 × ((398.2 - 288.5) + (330.4 - 299.5)) = 122.7 kW.
- COP = Capacity / W = 52.755 / 122.7 = 0.43.
When the regenerative cooling system is more preferable over the others? Justify the statement.
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For an air craft cabin cooling system, a turbine is to exhaust air at 0.75 bar and 6° C at a rate of 2.5 kg/s. The turbine drives a compressor which takes in air at 50° C. The air discharged from the compressor is mixed with air at the same state bled from the main engine compressor. It is then cooled to 60° C before entering the cooling turbine. Assuming compression and expansion are isentropic, determine the pressure and the flow rate of air to be bled from the main engine compressor.
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Explain the influence of sub cooling in the condenser and superheating in evaporator of a vapour compression refrigeration system.
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An ammonia refrigerating machine fitted with an expansion valve works between the temperature limits of -10° C and 30° C. The vapour is 95% dry at the end of isentropic compression and the fluid leaving the condenser is at 30° C. If the actual coefficient of performance is 60% of the theoretical, find the ice produced per kW hour at 0° C from water at 10° C. The latent heat of ice is 335 kJ/kg.
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Use p-h and T-s diagram and explain the actual cycle for the operation of vapour compression refrigeration system.
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A F-12 refrigeration machine works between the pressures of 10.1 bar and 3.6 bar. The condition of the vapour leaving the compressor is 94% dry and there is under cooling in the condenser. Determine the theoretical COP of the machine.
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Explain the working principle of evaporative condenser used in vapour compression refrigeration system with a suitable diagram.
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Describe the desirable physical and chemical properties of better refrigerant used for the operation of vapour compression refrigeration system.
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Differentiate between Azeotropes and Zeotropes and discuss the limitations of them.
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What are the advantages and limitations of hermitically sealed reciprocating air compressors? Explain.
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Describe the operational features of Lithium-Bromide vapour absorption refrigeration system with a suitable diagram.
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What are the basic components required for the operation of steam jet refrigeration system? Explain them.
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Eighteen grams of moisture per kg of dry air is removed from atmospheric air when it is passed through an air-conditioning system and its temperature becomes 26° C. The atmospheric conditions are 42° C DBT and 70% R.H. Find (i) Relative humidity, (ii) wet-bulb temperature and (iii) dew-point temperature. Take atmospheric air pressure as 1.01325 bar.
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How cooling and dehumidification process can be achieved in air conditioning system? Explain the processes on psychometric chart.
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