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Gas Cycle Refrigeration:
Before going to study the process of gas cycle refrigeration lets have some overview of air, vapor refrigerant used in the Reverse Carnot cycle.
Drawbacks of using Air as Refrigerant in Reverse Carnot cycle:
- Due to isentropic compression, very high pressure and due to isothermal heat rejection very high volume of gas is developed
- Isothermal heat transfer with gas is not possible practically due to the very small specific heat of the gas.
- Small irreversibilities in a gas cycle cause much increase in work due to the narrowness of the cycle.
Drawbacks of using vapor as a refrigerant in Reverse Carnot cycle:
- Liquid refrigerant may be trapped in heat of cylinder and damage the compressor valves
- Liquid droplets may wash away the lubricating oil from the walls of the compressor cylinder.
- The expander is costly and the work gained in expander is not significant
Reversed Brayton or Joule or Bell Colemen Cycle:
- In this cycle, the two isothermal processes of the Carnot cycle is replaced by two isobaric process
- These changes decrease the refrigerating effect and increase the work done.
- This cycle is used for aircraft refrigeration.
- Its COP is less than that of the vapor compression cycle.
Fig show P-V and T-S diagram of bell Coleman refrigerator. Here P1, V1, T1, S1 represents the pressure, volume, temperature, entropy of air respectively at point 1. And so on. It represents the corresponding condition of air when it passed through the component.
1-2: Isentropic Compression:
The Air is drawn from refrigerator to air compressor cylinder where it compressed isentropically (constant entropy). No heat transfer by the air. During compression, the volume decreases while the pressure and temperature of air increases.
2-3: Constant pressure cooling process:
The warm compressed air is then passed through the cooler, where it cooled down at constant pressure. The heat rejected per kg of air during this process is equal to q2-3 = Cp(T2-T3)
3-4: isentropic expansion:
No heat transfer takes place. The air expands isentropically in expander cylinder. During expansion, the volume increases, Pressure P3 reduces to P4. (P4 = atmospheric pressure). Temperature also falls during expansion from T3-T4.
4-1: Constant pressure expansion:
Heat transfer from the refrigerator to air. The temperature increases from T4 to T1. Volume increases to V4 due to heat transfer. Heat absorbed by air per kg during this process is equal to q4-1 = Cp(T1-T4)
If Maximum pressure increases or Minimum pressure decreases pressure ratio Rp increases and so COP of the cycle decreases.
Minimum pressure is generally fixed as being atmospheric pressure
If the maximum pressure increases refrigerating effect and work done both increases but COP decreases, an increase in work done is more than that of refrigerating effect.
Advantages of the Bell Coleman air refrigeration system:
- Cheap and abundant refrigerant, highly reliable: Air is used as the refrigerant, which is easily available and inexpensive.
- Charging of refrigerant is very easy.
- Design and construction are simple, No complicated parts and its maintenance cost is low.
- Refrigerant (Air) is non-toxic, non-flammable, non-corrosive. There is no danger of any kind of leakage.
- There is no phase change (liquid- gas) during the operation of the system.
- The cold air can be directly used for refrigeration; it is useful in aircraft refrigeration at high altitude.
- There would be no significant change in the performance of air refrigeration if it is operated much away from its design conditions.
- It can produce very high-temperature differences between hot and cold region. So the same system can be used for both cooling and heating effects. It also helps to achieve a very low temperature.
- Air refrigeration is used in aircraft due to the availability of cold air at high altitude and it can achieve both air-conditioning, as well as the pressurization of the cabin.
Disadvantages of the Bell Coleman air refrigeration system:
- Lower C.O.P. compared to other refrigeration cycles.
- Running cost is high.
- The mass of air required to circulate in the system is very high when compared to another type of refrigeration cycle due to low specific heat capacity.
- System components are bulky, large space per ton of refrigeration.
- The chance of frosting at expander is more due to the moisture content in the air.
- Air contains pollutant particle, so do regular cleaning of air filter in open system.
Type of Air Refrigeration Cycle:
- It can work at suction pressure higher than atmospheric pressure
- It has more COP
- It needs a heat exchanger for the refrigeration process
- Its weight and cost are more.
- It can work at atmospheric pressure at the suction pressure
- Its COP is less.
- It does not require heat exchanger for the refrigeration process
- Its weight and cost is less
Advantage of Air refrigeration cycle over Vapour Compression cycle:
- Leakage of refrigerant is tolerable
- In an open type system, no heat exchanger is required
- Refrigerant being air is available in mid-atmosphere
- Cabin pressurization and air conditioning can be done simultaneously
- Initial compression of the air is obtained by ram effect
- Ram effect (Conversion of the high kinetic energy of the ambient air into enthalpy and pressure rise in an aircraft is called ram effect).
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