The Refrigeration Cycle

Most homeowners do not have EPA certifications or the necessary equipment to handle, recover, or charge refrigerants so this post is intended for educational purposes and to give the non-technician a basic understanding of how domestic refrigerators use the refrigeration cycle to preserve food. I will not try to explain the chemistry of refrigerants, but will explain the basic principals and thermodynamic laws that cause a refrigerant to create cold conditions inside the unit.

Cold is a term of convenience, but cold actually doesn’t exist. Heat is a form of energy. Cold is the absence of heat, although until one reaches absolute-zero even that definition is incorrect. There is nothing wrong with pronouncing that the inside of our freezer is cold as long as we know that it is less hot than something else that we are using for comparison (usually the temperature of the room where the unit is located or our body temperature).

A colder substance or object always absorbs heat unless it is perfectly insulated. A cube of ice in our drink cools the liquid because melting ice (changing states from a solid to a liquid) absorbs a lot of heat from the surrounding liquid. When the ice is melted, our drink absorbs heat from the surrounding air and begins to warm again. Most people think that an ice cube cools the drink because it melts and mixes cold water into the drink. It is true that the melted cube leaves cold water in our drink but the majority of cooling is due to the cube changing states (solid to liquid) and absorbing energy from the surrounding liquid. If we could magically gather the water from our cube and refreeze it, it would give up the energy it absorbed when it changed from a solid to a liquid. The only way we can get the absorbed heat from our water is to put it somewhere colder than 32 degrees so that the heat energy will flow from the water to the colder surroundings and cause a change of state from liquid to solid. We do this when we freeze a tray of ice cubes.

Now that we know that it takes energy to change a solid to liquid, lets consider other substances besides water. Most metals are solids at room temperature. We have all seen pictures of furnaces used to melt steel. Changing steel to a liquid takes a lot of heat energy but it is useful to mold it to the shape we desire. As soon as the liquid steel is removed from the heat source of the furnace it gives up heat to the much colder surrounding air and becomes a solid again. All substances change states depending upon the temperature (and pressure). Some solids have to be very hot to change to a liquid. Some gases have to be cooled to extremely low temperatures to make them condense (change from a gas to a liquid). Liquid oxygen and liquid nitrogen are two examples.

Water boils at 212 degrees F at sea-level. Some refrigerants boil at -30 degrees F. It may seem odd that something cold enough to freeze our skin is boiling but as the refrigerant changes from a liquid to a gas, it is absorbing heat (even if that heat feels very cold to us). When liquid refrigerant enters the evaporator of a refrigerator or freezer, it absorbs heat as it changes into a gas (evaporates). Even if the freezer is cold (0 to 10 degrees F) it is hotter than the refrigerant so heat flows to the refrigerant. When the heat inside the freezer flows to the refrigerant, heat energy is removed from the air and contents of the freezer and it gets colder.

refrigsys

Evaporators are designed so that liquid refrigerant evaporates (absorbing heat) and leaves the coil as a gas. The compressor pumps the gas out of the evaporator coil through the suction line. The compressor pumps the refrigerant gas into the condenser and raises the pressure from less than 10 psi to around 130 psi. This compresses the gas and raises its temperature (which is why condensers are hot). The refrigerant in the condenser gives up heat (absorbed from inside the refrigerator) to the surrounding air as it condenses into a liquid. Because the refrigerant is under higher pressure it condenses at a higher temperature. The liquid refrigerant is forced by the pressure of the compressor into the cap (capillary) tube for another trip to the evaporator. This cycle is constant as long as the compressor is running.

The cap tube is a small copper line with a precision internal bore measured in thousandths of an inch connecting the condenser to the evaporator. It is used in domestic freezers, refrigerators, and air conditioners. It meters liquid refrigerant into the evaporator and maintains a pressure differential between the high-side condenser and low-side evaporator.

This is a very simple and basic description of the refrigeration cycle but all mechanical refrigeration systems operate on these principles.

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5 Comments

Filed under appliance, diagnose and repair, refrigerator repair

5 responses to “The Refrigeration Cycle

  1. Nice information=D Will come back.

  2. Dionysius

    I have a question on the possible pathways between Lo and Hi sides. When the Hi and Lo pressures equalize in a normally operating system it is via the capillary tube connection one assumes.

    If the capillary tube was 100% blocked and if the pressures still equalized could it not be via the compressor head via non-sealing valves?

    Have you ever seen this condition because I now believe that is what might be wrong with my refrigerator. Why would this happen??

    If so how did the valves fail – could it be the seals were destroyed because the seats got all warped due to overheating??

    But would the Overload protection on the compressor not save this from occuring?? Surely that is one reason for its existence.

    The compressor motor is certainly good because I am measuring a normal 1.3 A run current and it sounds and vibrates just fine with a 15 A start current.

    I do not know for sure that the capillary is blocked.

    The initial failure experienced was a sudden loss of R134a resulting in no cold. The compressor when run did get very hot at least to my idea of hot. I am not a professional, just an owner trying to save my appliance.

    Now when I attempt a recharge by gas into the Lo where I added a service port and I build the Lo pressure up to 30 psi and let it sit for 10 mins the Hi side does equalize. This is with no compressor on.

    Then when I repeat the same experiment but now turn on the compressor the Lo pressure does not respond by being lowered nor is the Hi side raised. This tells me the compressor is not pumping properly. Correct or not??

    Now more info. The system does not hold the charge because after a few hours all of the pressure is back to zero on both sides. Could it be that the leak is fooling me into thinking the compressor is failing by pumping it out the leak under pressure?? This is just a theory but is it possible from your experience??

    Now you will ask me why do I not first locate the leak?? Because I am not sure how to as I do not have a detector. I am just using my eyeballs and ears but maybe not enough. It is a 1998 Kenmore Side-By-Side.

    Any help from any of you out there and certainly the host of this fine site will be great plus I have an idea as to how you can accelerate the production of your e-book if you are interested.

    • fridgeman

      Dionysius,

      Most, if not all that you say is true. With valves that seal properly, the system is forced to equalize through the cap tube. This can take a few minutes to happen. If the compressor has a bad valve, equalization is almost instant since it occurs through the compressor. Again, you are correct that the compressor’s internal thermal overloads protect the compressor from heat. Usually a thermal overload will take a couple of hours to reset.

      With a properly charged R134 system and a good compressor, I would expect the low-side pressure to pull down to <5psi. A system without refrigerant and a good compressor would pull down to 20 in Hg while running and return to zero as soon as the compressor stops running.

      If you suspect a leak, it must be found and repaired before the rest of the system can be evaluated.

  3. Dionysius

    Thank you so much. This is very helpful to me. My compressor did not ever go into an overload either current or temp but it did get hotter than a pistol after just 20 minutes. I see no reference anywhere that these compressors have temperature cutout to protect them from self destruction. In my reading it is nowhere stated that I could find and it surely is a surprise to me. I have now deduced that both valves are damaged. I posit that the seats might be warped and/or the reeds are fractured etc. To validate my theory I will have to cut into the suction and discharge tubes and see if they can hold a vacuum on hi side and contain a pressure on the lo side. If that proves true my next step has to be to cut the dome open and examine the cause as to seats or what. At that stage I am into the learning process only since cutting it open is a destructive act. Do you generally follow my logical train of thought here??

    Can I cannibalize another refrigerator and swap out the compressor?? I do like this unit I have for its features and high end capabilities. A new compressor is probably cost prohibitive??

    You have a great blog here sir and your knowledge and your ability to express it are way above average. Thank you.

    • fridgeman

      I have cannibalized countless compressors and reused them. Important considerations are to match horsepower, refrigerant type, and btu rating. Some compressor pin arrangements are different depending on manufacturer. Do not install a compressor from an r-12 system into a r-134 system as the compressor oils are different.

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