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.
Fig 1 Four basic parts of a refrigeration system
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.
