JNTU
JNTU B.Tech III REFRIGERATION AND AIR CONDITIONING Previous Question Paper (February - 2023 - R16)
Note: i) Part A is compulsory, Answer all questions. ii) Part B, Answer any one question from each unit. iii) R&AC Data books are permitted.
Part A
Why air is used as refrigerant in aircrafts?
Answer
Air is used as a refrigerant in aircraft refrigeration systems primarily due to the following reasons:
- Safety and Non-toxicity: Air is non-toxic, non-flammable, and non-corrosive. In the event of a leakage, it poses no risk to the passengers or the aircraft structure, which is critical for pressurized cabins.
- Weight and Compactness: The Air Cycle Refrigeration System (Bell-Coleman cycle) utilizes the ambient air available in abundance at high altitudes. This eliminates the need for carrying heavy liquid refrigerants and bulky condensers, significantly reducing the overall weight of the cooling system.
- Availability: Since the aircraft operates in an atmosphere of air, the system can use the surrounding air as the working fluid, simplifying the logistics of the refrigeration cycle.
- Simplicity of Design: The system components, such as air turbines and heat exchangers, are lightweight and robust, making them ideal for the space-constrained and weight-sensitive environment of an aircraft.
Derive COP of a Bell Coleman cycle?
Answer
The Bell-Coleman cycle, also known as the Reversed Brayton cycle or Joule cycle, is used in air refrigeration systems. It consists of four processes: isentropic compression, constant pressure heat rejection, isentropic expansion, and constant pressure heat absorption.
Let the states be defined as: 1-2 (Isentropic compression), 2-3 (Constant pressure cooling), 3-4 (Isentropic expansion), and 4-1 (Constant pressure refrigeration).
The Coefficient of Performance (COP) is defined as the ratio of the refrigeration effect to the net work input:
Using the relations for an ideal gas (h = CₚT):
- Refrigeration effect = Cₚ(T₁ - T₄)
- Work of compression = Cₚ(T₂ - T₁)
- Work of expansion = Cₚ(T₃ - T₄)
Substituting these into the COP expression:
Using the isentropic relation T₂/T₁ = T₃/T₄ = (P₂/P₁)(γ-1)/γ = rp(γ-1)/γ, where rp is the pressure ratio:
This shows that the COP of a Bell-Coleman cycle depends solely on the pressure ratio and the adiabatic index of the gas.
Explain the effect of super heating with a neat diagram?
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If the efficiency of a heat engine is 0.75, what is the COP of a Carnot Refrigerator while operating between the same temperature limits?
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Why hermetically sealed type of compressor is used in R&AC applications?
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Differentiate Automatic and Thermostatic expansion valve?
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What are the desirable properties of an ideal Refrigerant and Absorbent?
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Differentiate Vapor compression and Vapor Absorption refrigeration systems?
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Represent Adiabatic saturation process on Psychrometric chart?
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In a process, Sensible Heat is 65 KW and Latent Heat is 25 KW. Determine Sensible Heat Factor for this process?
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Part B
A dense air refrigeration cycle operates between the pressures of 4 bar and 16 bar. The air temperature after heat rejection to surroundings is 37°C and air temperature at exit of the refrigerator is 7°C. Determine compressor and turbine work per TR. Take γ = 1.4 and Cp= 1.005 kJ/kg-K.
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An air refrigerator used for food storage provides 50 tons of refrigeration. The temperature of air entering the compressor is 7°C and the temperature before entering into expander is 27°C. Assuming 30 % more power is required than theoretical, find (a) Actual C.O.P of the cycle (b) KW capacity required to run the compressor.
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A Vapour compression refrigerator works between the pressure limits of 60 bar and 25 bar. The working fluid is just dry at the end of compression and there is no under cooling of the liquid before the expansion valve. Determine (a) C.O.P of the cycle (b) Capacity of the refrigerator if the fluid flow is at the rate of 5 kg/min.
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A refrigeration machine using R-12 as refrigerant operates between the pressures 2.5 bar and 9 bar. The compression is isentropic and there is no undercooling in the condenser. The vapour is in dry saturated condition at the beginning of the compression. Estimate theoretical C.O.P. If the actual C.O.P is 0.65 of theoretical value, calculate the net cooling produced per hour. The refrigerant flow is 5 kg/min.
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Explain with a neat sketch the working principle of Thermostatic expansion valve. Discuss the factors that affect the capacity of the valve.
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What problems does lubricating oil cause in the evaporator? Explain flooded evaporator with a neat sketch.
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With a neat sketch explain Aqua Ammonia Vapour Absorption Refrigeration System working details.
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With a neat sketch explain steam jet Refrigeration System working.
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The following data relates to the office of air conditioning plant having maximum seating capacity of 25 occupants: Outside design conditions: 34°C DBT, 28°C WBT; Inside design conditions: 24°C DBT, 50% RH; Solar heat gain: 9120W; latent heat gain per occupant: 105W; Sensible heat gain per occupant: 90W; Lightening load: 2300W; Sensible heat load from other sources: 11630W; Infiltration load: 14m³/min. Assuming 40% fresh air and 60% of recalculated air passing through the evaporator coil and the BPF of 0.15, find the DPT of the coil and capacity of the plant. Take the minimum temperature at the exit of the cooling coil is 11°C.
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An Air conditioned auditorium is to be maintained at 27°C DBT and 60% RH. The ambient conditions are 40°C DBT and 30°C WBT. The total sensible heat load is 1,00,000 KJ/hr and the total latent heat load is 40,000 KJ/hr. 60% of the return air is re-circulated and mixed with 40% of makeup air after the cooling coil is 18°C. Determine: a) RSHF b) Condition of air entering the auditorium c) Amount of makeup air d) ADP e) Bypass factor of cooling coil.
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