Test: Refrigeration Cycles & Systems - 3 - Mechanical Engineering MCQ

Mass flow of refrigerant through the evaporator gets reduced.

Large sized industrial air conditioning system are generally water cooled

It can be noted that the maximum possible Cofficient of Performance in any application can be obtained when:
(i) The cold body Temperature (T_c) should be as high as possible and
(ii) The hot body Temperature (T_h) should be as low as possible.
Hence most effective way to increase the COP is by increasing the temperature of cold body and at the some time decreasing the temperature of hot body.
If opposite is done i.e., decreasing the temperature of cold body and increasing the temperature of hot body, COP of a Carnot refrigeration cycle decreases drastically,

Due to low temperature in domestic refrigerator COP of domestic refrigerator < COP of domestic air-conditioner.

Hence sequence followed is
1 - 2 : Isentropic compression (in compressor)
2 - 3 : Constant pressure heat rejection (in condenser)
3 - 4 : lso - enthalpic expansion (in capillary tube)
4 - 1 : Constant pressure heat absorption (in evaporator)

(Since h₄ = h₃)

Frost decreases the cooling efficiency due to poor heat transfer rate.

At point 4 i.e. refrigerant immediately after expansion valve is wet vapour.

Effect of Sub-cooling:
1. Increase in refrigeration effect
2. Horse power per ton is reduced
3. Increase in COP

Refrigeration:

The purpose of refrigeration is to cool spaces, objects, or materials and to maintain them at temperatures below the temperature of the surrounding atmosphere.

Refrigeration cycle:

In a refrigeration cycle, the refrigerant must alternate between low temperatures and high temperatures. When the refrigerant is at low-temperature, heat flows from the space or object to be cooled to the refrigerant. When the refrigerant is at a higher temperature, heat flows from the refrigerant to a condenser. The energy supplied as work is used to raise the temperature of the refrigerant to a high enough value so that the refrigerant will be able to reject heat to the condenser.

Vapour compression cycle:

Effect of Superheating:
1. Work input increases
2. Increase in refrigeration effect

There are two method of manufacturing of solid CO₂
1. Simple vapour compression cycle
2. Pressure snow chamber method
Power requirement for the simple vapour compression cycle is of the order of 400-500 J-hr per ton of solid CO₂; one of the main reason for such high power consumption is the high pressure ratio that is nearly equal to 70.
Another problem in the process of manufacture is the blocking of expansion device by the formation of dry ice due to these difficulties, snow chamber method is most commonly used.

In aircraft refrigeration system Beil Coleman cycle is used. It is a reversed Brayton Cycle.

In aircraft, air refrigeration cycle is used due to low weight per tonne of refrigeration on the other hand COP of this cycle is very low.

Hence Bell Coleman cycle consists of two reversible isentropic process and two reversible isobaric process.

Vapour absorption refrigeration is heat operated refrigeration system and can operate by taking heat from the solar system.

In the absorption refrigeration system the compressor of the vapour compression refrigeration cycle is replaced by pump, absorber and generator.

Electrolux refrigerator also called three-fluids absorption system.
The main purpose of this system is to eliminate the pump so that absence of moving part, the machine becomes noiseless.

T₂ = lower temperature
T₁ = higher temperature

Capacity of water cooler = 1 tons = 3.5 kW
T₁ = 35°C
T₂ = 20°C
Q = mcΔT

Due to work of compression heat rejected is always greater than heat absorbed.
Hence ratio of heat absorbed to heat rejected is less than unity.