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HVAC – The Refrigeration Cycle

In today’s lesson on HVAC training I will be covering the refrigeration cycle and the 4 main components that make the cycle work. But before I get into this hvac training I want to go over the three ways that heat can be transferred, and yes there are only 3 ways that heat can be transferred.

The first one is radiation, this is done by absorption. A perfect example of this kind of heat transfer would be our sun, which produces heat and is absorbed by the earth.

The second way that heat can be transferred is called conduction. This is the process of heat transferring through metal. For example, if you heat up one end of a metal pipe the other end of the pipe will start to get hotter and heat up.

The third way that heat can be transferred is the most important when it comes to hvac training, and this is called convection. Convection would be a transfer of heat through a medium such as water, air, or refrigerant. That is what is used to transfer heat in a central air conditioning system, car air conditioning system, refrigerator, freezer, anything that has to do with HVAC.

basic-refrigeration-cycle-thumb26303864Now that we had a little hvac training on the basics of thermodynamics let’s move on to the four components that make up the basic refrigeration cycle.

1.The Evaporator – This is the coil that is inside of the house. Warm air will pass over the coil which contains the refrigerant, then the refrigerant absorbs the heat, then the you are left with cold air which is distributed to the rooms that you are trying to cool.

2.The Compressor – This is the life force of the refrigeration cycle, what it does is it will circulate refrigerant throughout the whole system. It will compress cold vapor into hot vapor, it also increases the low vapor pressure into high vapor pressure.

3.The Condenser – This is the coil that is located outside on a central air conditioning system. It removes the heat that is carried through the refrigerant, forcing the hot air out.

4.The Metering Device – Controls the flow of the refrigerant to the evaporator. There are different kinds of metering devices, some of them will have pressure limiting devices to protect the compressor from overloading, while some will control the evaporators pressure or superheat. Some common metering devices are thermostatic expansion valves, automatic expansion valves, capillary tubes, and fixed-bore.

All four of these components much be working, if one fails the refrigeration cycle will fail and the space you are trying to cool will not cool. Now that you have some basic hvac training on components that make up the refrigeration cycle, let’s learn how it actually works. The compressor discharges high pressure, high temperature vapor through the discharge line (the smaller line).The fan on the condenser blows the heat out over the condenser coil, then the high pressure vapor will lose some energy and turn into a high pressure liquid. The high pressure liquid will then go to the metering device, where it is turned into a low pressure liquid, and low temperature. It now passes over the evaporator coils that are inside, the heat that is being removed from the room is absorbed in the refrigerant and then turned into a vapor, at low pressure and low temperature. It is now going through the suction line (the bigger line) into the compressor where it turns the low pressure, low temperature into high pressure, high temperature liquid. Then the cycle starts all over again. Remember that compressors pump vapor only, at no point should it be sucking in liquid as this can damage the compressor.

Now I am sure that this probably confused the hell out of you, that is why I opened up photoshop and made this little diagram for everyone. I know my design might not look as pretty as others you will find on the net but it relates to the article I have written and hopefully it helps you understand all that hvac training that I am trying to pound into your head.


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{ 13 comments… add one }
  • JD October 28, 2009, 1:15 pm

    At which points in the cycle does the refrigerant become superheated and subcooled ?

    And it was stated on the tools page normal operating conditions in Florida are 250 high side 70 low side , im guessing this is for R-22 is it different for R-410a ?

  • Dave January 26, 2010, 11:22 pm

    The refrigerant gets heated inside of the compressor, it will cool down once it goes through the condenser. And yes operating pressures are very different between R-22 and R-410a

  • MAHA February 17, 2010, 9:27 am

    the refrigerant will be superheated before entering the compressor and will be sub cooled before entering the metering devise

  • Matt Nelson March 8, 2010, 4:19 am

    Great explanation Dave. For as many times as I’ve looked at the refrigeration cycle it never seems to sink in. One thing struck me with JD asking about superheat, in that superheat does not equal super “hot”. It just refers to the fact that the refrigerant has absorbed enough heat to make it a gas and then absorbed a little more before leaving the evaporator. I believe this is more obvious when you are looking at a P-T diagram. Keep up the good work.

  • tito arroyo November 15, 2010, 4:42 am

    I am currently enrolled online Ashworth hvac course. All new and interesting to me. I need to learn as much as possible to keep my job and work efficiently to keep my budget low and prove my worth at my job. This is very good information and anything else you have would be greatly appreciated.

  • dominic November 25, 2010, 6:33 pm

    it is superheated in the evaporator, just as it is going into the compressor, now it will come out of the compressor as a superheat which then needs to be DEsuperheated and condensed and turned back into a liquid, then the process begins over again

  • Aman deep gulati August 16, 2011, 6:06 am


  • Fred September 22, 2011, 7:26 pm

    what are the three ways that refrigerant give up heat

  • mark October 28, 2011, 2:04 am

    The refrigerant is super heated before entering the compressor to ensure no liquid is entering the compressor, witch will ultimately destroy it. The refrigerant is discharged through the discharge line (the smaller line) and the temp is raised due to high pressure. The outdoor fan passes over the condenser coil witch turns the high temp, high pressure vapor in to a lower temp, liquid (same psi). The liquid is then further ”sub-cooled” to ensure the refrigerant is fully condensed before flashing off through the metering device where it is turned into an even lower temp, and the pressure drops aswell into a saturated mixture.. RSI student. hope that helps you buddy.

  • jstuart August 23, 2014, 7:47 pm

    superheat and subcooling perform specific jobs in the system. Superheat is when the refrigerant picked sensible (measurable) heat to the refrigerant as it leaves the evaporator. There is a point that refrigerants are saturated with heat, or, cooling and any heat or cooling added to that temperature will either convert the refrigerant to all vapor or liquid. Superheat is necessary to avoid damaging the compressor with a “liquid slug” of refrigerant. Since you cannot compress a liquid something in the compressor has to give, usually valves. Subcooling occurs when all the hot discharge gas has condensed in the condenser, usually 90% of the condenser, and the remaining 10% of the condenser is to add additional cooling to the liquid refrigerant to bring it below the “saturation temperature” and then try to remove an additional 10 degrees more so that the liquid is ALL liquid when it enters the metering device so that you get maximum cooling from the liquid. These two concepts are critical to becoming a good mechanic and for trouble shooting a system or starting up a system and charging it with the correct amount of refrigerant. Hope this helps.

  • udioko celestime February 21, 2015, 10:11 am

    pls I will like to run an online certificate program, link me up thanks.

  • roel June 8, 2015, 6:50 pm

    it should be high pressure, high temperature vapor not liquid. it might be a typo error. the statement is located when refrigerant is going to the compressor

  • jaswal June 23, 2016, 2:10 am

    How can refirgerant is super heated without start of compressor

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