JNTU
JNTU B.Tech IV REFRIGERATION AND AIR CONDITIONING Previous Question Paper (February/March - 2022 - R18)
Answer any five questions
All questions carry equal marks
Main
Draw p-V and T-s diagrams of actual air refrigeration system and discuss the salient points.
Answer
The actual air refrigeration cycle (Bell-Coleman cycle) deviates from the ideal reversed Brayton cycle due to irreversibilities in the compressor and expander, as well as pressure drops in the heat exchangers. On the p-V diagram, the compression and expansion processes are polytropic (PVn = C) rather than isentropic, resulting in a larger area enclosed by the cycle compared to the ideal case. On the T-s diagram, these processes show an increase in entropy due to internal friction and heat transfer.
Salient points include:
- Polytropic Processes: Compression and expansion are non-isentropic, leading to higher work input and lower work output.
- Pressure Losses: Frictional resistance in pipes and heat exchangers causes pressure drops, reducing the effective pressure ratio.
- Heat Transfer: Heat gain from the surroundings during the cold air flow reduces the net refrigerating effect.
- Efficiency: The overall COP is significantly lower than the ideal cycle due to these losses.
In a Bell-Coleman cycle of a refrigerator system, air is taken in at 1 bar and a temperature of -8°C. The compression ratio maintained in the compressor is 4 by following the law of compression and expansion as
Answer
Given: P1 = 1 bar, T1 = 265 K (-8°C), rp = 4, n = 1.2, Tmax = T3 = 298 K (25°C). For air, Cp = 1.005 kJ/kgK, R = 0.287 kJ/kgK.
1. Temperatures:
2. Work Calculations (per kg of air):
3. Refrigerating Effect (RE):
4. COP:
5. Mean Effective Pressure (MEP):
What are the advantages, limitations and different applications of air refrigeration system? Explain.
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A simple vapour compression cycle using F-12 is designed to take a load of 10 tons. The refrigerator and ambient temperatures are 0°C and 30°C respectively. A minimum temperature of -5°C is required in evaporator and condenser for heat transfer. Find (i) mass flow rate through the system (ii) Power required in kW (iii) cylinder dimensions assuming L/D ratio as 1.2 for a single cylinder and single acting compressor running at 300 rpm with a volumetric efficiency of 0.9.
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How does the sub cooling and super heating influence the performance of vapour compression refrigeration system? Explain.
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Explain the important components required for the operation of simple vapour compression refrigeration system.
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A single stage, single acting reciprocating compressor has a bore of 200 mm and a stroke of 300 mm. It receives vapour refrigerant at 1 bar and delivers it at 5.5 bar. If the compression and expansion follows the law
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Explain the principle of operation of evaporative condenser used in the refrigeration system.
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Differentiate between Azeotropes and Zeotropes and discuss their importance with respect to Ozone depletion and global warming.
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Describe the constructional and operational features of Li-Br vapour absorption refrigeration system.
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How does the Hilsch tube refrigeration system function? Explain by drawing the suitable line diagram.
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Derive the equation for the estimation of max COP of vapour absorption refrigeration system and how does it affect with the generator temperature?
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Air at 1 bar and 29°C has a wet-bulb temperature of 23°C. Determine (i) the partial pressure of water vapour (ii) the humidity ratio, (iii) the relative humidity (iv) the dew point, and (v) the enthalpy of air, and (vi) the degree of saturation per kg of dry air. Solve the problem analytically and check the results using Psychometric chart.
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What is the need of ventilation air in the air conditioning system? Discuss the method to supply the ventilation air.
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Why does the enthalpy of an air-vapour mixture remain constant during an adiabatic saturation process?
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The following data refer to an air conditioning system for industrial process for hot and wet summer conditions: outdoor conditions = 33°C DBT and 78% RH, required conditions = 20°C DBT and 73% RH, amount of out-door air supplied = 220 m3/min, coil dew point temperature = 12°C. If the required condition is achieved by first cooling and dehumidifying and then by heating, find: (i) The capacity of the cooling coil and its by-pass factor (ii) The capacity of the heating coil and surface temperature of the heating coil if the by-pass factor is 0.18.
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