JAWAHARLAL NEHRU TECHNOLOGICAL UNIVERSITY HYDERABAD
JAWAHARLAL NEHRU TECHNOLOGICAL UNIVERSITY HYDERABAD B.Tech III REFRIGERATION AND AIR CONDITIONING Previous Question Paper (January/February - 2021 - R16)
Answer any five questions. All questions carry equal marks.
Main
A cooler pressure of 10 bar and refrigerator pressure of 1.1 bar are used in an air refrigeration system. The temperature of air leaving the cooler is 28℃ and air leaving the room is 7℃. Calculate the capacity of the system in terms of TR, power required per TR, and Expander displacement if compressor displacement is 40 m3/min.
Answer
Given data:
- Compressor pressure (P2) = 10 bar
- Refrigerator pressure (P1) = 1.1 bar
- Temperature leaving cooler (T3) = 28℃ = 301 K
- Temperature leaving room (T1) = 7℃ = 280 K
- Compressor displacement (Vc) = 40 m3/min
- Assumed properties for air: γ = 1.4, Cp = 1.005 kJ/kgK, R = 0.287 kJ/kgK
Pressure ratio (rp) = P2 / P1 = 10 / 1.1 = 9.09
Temperature after compression (T2):
Temperature after expansion (T4):
Mass flow rate (m):
Refrigeration Capacity (Q):
Power required (W):
Power per TR:
Expander Displacement (Ve):
A R-12 refrigeration machine has saturated suction temperature of 4℃ and saturated discharge temperature of 39℃.
Answer
To determine the theoretical piston displacement, we first identify the properties of R-12 at the given saturation temperatures:
- At suction temperature T1 = 4℃: hg1 = 192.5 kJ/kg, vg1 = 0.055 m3/kg
- At discharge temperature T2 = 39℃: hf2 = 73.2 kJ/kg
The refrigeration effect (RE) per kg is:
Mass flow rate for 1 TR (3.517 kW):
Theoretical piston displacement:
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Draw the actual vapour compression refrigeration system and discuss the salient points about the actual cycle.
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With the help of a neat diagram, explain the working details of a thermostatic expansion valve and give its working temperature and pressure ranges.
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In a vapour absorption type refrigerator, the heat is supplied to NH3 generator by condensing steam at 1.9 bar and 95% dry. The temperature in the refrigerator is maintained at -4℃ and the atmospheric temperature is 28℃. Find the maximum C.O.P possible. If the refrigeration load is 34 tonnes and the actual C.O.P is 75% of the maximum C.O.P., find the mass of steam required per hour.
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Outdoor air at 12℃ and 36% RH is supplied at 50 m3/min to an air conditioning system to condition it to 28℃ and 65% RH. The outdoor air is first heated to 20℃ in the heating section and then humidified by hot steam injection. The entire process takes place at 0.1 MPa. Calculate the rate of heat supplied in the heating section and the mass flow rate of steam required in the humidifying section. Represent this on the psychrometric chart.
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What do you understand by the concept of effective temperature for human comfort? Discuss the method to achieve it in summer.
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