Heat Pump Refrigeration Cycle (II)

The major 4 components  

it Part (I) of the Refrigeration Cycle we explain the general Idea of the Cycle now we will go more in depth. In any cycle application whether it is an air conditioner or heat pump or small refrigerator there is 4 major components;

1-     Evaporator

2-     Compressor

3-     Condenser

4-     Metering device

We will talk about each components and its function in the full cycle. I promise when we are done, you will have a lot better understanding of what is really going on. As always a picture is worth a thousand words. So we will plot the Refrigeration Cycle on the pressure enthalpy chart then we are going to go through it together.


Theoretical refrigeration cycle on pressure enthalpy chart

1-    Evaporator

We will use the same example in Part (I), but instead of the ice we will use refrigerant ( Freon )  lets say R-22 so we start at POINT (A) with low pressure  (P1) and Low Temperature (T1) and low heat (Enthalpy ) E1 , then that  liquid cold refrigerant goes  into a radiator coil placed indoor. Because your home is a lot hotter than the refrigerant, the heat from your home will go to the radiator and boil the refrigerant which will evaporate inside the radiator coil. The  refrigerant inside the “Evaporator“ will absorb the heat from the surrounding till it reaches POINT (B) as a saturated gas, note that POINT (A) and POINT (B) have the same pressure P1 and same Temperature T1 however POINT(B) has a lot more heat (Enthalpy) E2 .



2-     Compressor

Now the refrigerant become a 100%  gas and full of heat  but it is still a lot colder than the outside, but we have learned from the thermal theory section  there is a way that we can increase an object temperature without adding heat or removing heat. By increasing the pressure of that object. Yes you guessed right we use a compressor an electrical compressor to compress the refrigerant to POINT (C) at high pressure P2  and high temperature (T2) please note that POINT B and POINT C both have the same amount of heat Enthalpy (E2) we increase the pressure  and temperature without adding any heat, this process we call it an adiabatic process.. 


3-     Condenser

Now we are at POINT (C) Hi pressure (P2) high temperature (T2), that hot gas then goes to an outdoor radiator coil very similar to the indoor one but it is outdoor, and it is a lot hotter than the outdoor temperature so the heat will start to move out from the refrigerant to the outdoor, notice here that it is the same amount heat that the refrigerant absorbs from the indoor (the difference between E1- E2), when the heat leaves the refrigerant it will start to condense and reach POINT (D)  and become a liquid refrigerant but it is still very hot (T2) and still have a high pressure(P2)

4-     Metering device

The last part in the cycle is the metering device which basically it lower the pressure of the liquid refrigerant by sudden expansion to reach POINT (A) again once we lower the liquid refrigerant pressure to (P1) we lower its temperature as well to (T1) without adding or removing any heat, note that Point (D) and Point (A) have the same enthalpy (E1) ,we will end up with low pressure liquid very cold refrigerant ready to go to the evaporator again and repeat the Refrigeration Cycle again, please note that the Processes of the  Metering device it the exact reverse of the Compressor

That was the full refrigeration cycle on theory however in realty the Cycle differ due to some physical limitation of the equipment itself, and to increase and posts efficiency and it will look as shown below.


Refrigeration cycle on pressure enthalpy chart

The New Refrigeration Cycle will be (Aˊ- Bˊ- Cˊ-Dˊ) and not (A-B-C-D).

Let’s start from Bˊ, compressors cannot compress Liquids they will break , and if you notice in the original cycle Point C is not 100% gas it is in the transition state(mostly gas but it has some liquid) compressor cannot do this, so by making the evaporate coil a little bigger allows the Freon to absorb more heat from the indoor, after the saturated point B so we call it super-heat the Freon, and because it is after the saturated point the temperature also rise to T4 and we get to the Point Bˊ we call that process Superheating and it usually higher than the saturated temperature by 10-15 F, and this is very important to make sure that only gas will go to the Compressor to protect it.

If you notice in the original Cycle POINT B and POINT C (the compressor process) both point have the same amount of heat or enthalpy (E2), however in realty that compressor is an electro-mechanical device and it has an efficiency and it is not 100%, let's say 80 % that mean only 80% of the energy used to run the compressor (electricity) will actually be used to compress the Freon the rest 20% will be lost in a form of heat (so where did it go?). Do you remember the 1st law of thermodynamic , yes correct ” Energy cannot be vanished or created from nothing” the 20% heat will go to the Freon itself and heat it up,  that’s why in real life POINT Cˊ has a higher enthalpy (E5) than Point Bˊ which has enthalpy (E4.)

Also by making the condenser a little bigger we can reject more heat from the Freon after the saturation POINT D to the POINT Dˊ which has a lower temp T3 and lower Enthalpy E3, we call that sup-cooling,  the function of it is to allow the Evaporator to start from the saturation  POINT Aˊ rather than the original Transient POINT A this will allow to increase the amount of heat that the Evaporator absorb (Aˊ-Bˊ) which is larger than the original (A-B).

Note that 

  • Plotting the Refrigeration cycle on the pressure enthalpy chart can serve as a very important tool for trouble shooting the Cycle , by knowing how much is the Super-heat and the Sub-cool , and how much heat that the Compressor add we can judge the system very accurately
  • The Amount of heat we reject from the Condenser (Cˊ-Dˊ) equal to the amount of heat that we absorb from the evaporate (Aˊ-Bˊ) plus the little heat (E5 to E5) that the compressor adds to the system and 
  • The  ratio  of (A’-B’) / (Cˊ-Dˊ)  is the Refrigeration Cycle Efficiency, and it must be lower than 1.


Refrigeration Cycle allow us to move heat from one place to another against its natural flow of heat. We must pay a price and in this case it is the electricity that we pay to run the compressor, the ratio between how much energy we pay to how much energy we move we call it the Energy Efficiency Ratio (EER) we will discuss the EER and SEER and COP in a different topic .

Also notice we need a medium to carry that heat, we call it “the refrigerant” in the above example we use R-22 we could also use Propane or R-290 in fact propane is a very efficient refrigerant but we don’t use it because it is flammable so we use non flammable refrigerants like R-22 and R-410, we will discuss later on the different type of  refrigerants and its effect on the environment.

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