Basic Refrigeration: Thermodynamics of Heat Transfer

People not in the refrigeration business often think that refrigeration equipment produces cold air; in fact, the equipment is actually removing heat from air and leaving what’s left — the cold. When the sensor determines the target temperature has been reached, the equipment can take a pause, and when the temperature goes back up, the equipment goes on again.

It’s a fairly simple concept, but we all recognize that there is more to refrigeration equipment than that. In this article, you will learn about ways in which heat flows and factors that determine the speed of heat transfer. You will also learn about the four laws that describe how temperature and pressure changes affect the state of refrigerant in a sealed piping system. As a technician, you’ll need to know all of this as you troubleshoot equipment.

There are three different methods of heat transfer:

Now that you understand the different methods of heat transfer, you need to know what affects the rate of heat transfer:

Refrigerants are chosen for specific properties and how they behave is critical to the heat removal process. By manipulating temperature and pressure, it is possible to set up a condition that will allow refrigerant to either absorb or reject heat.

In a self-contained or sealed refrigeration system, the refrigerant piping is completely connected and not exposed to the outside air pressure, and components include the compressor, condenser, and evaporator (see Figure 1).

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FIGURE 1: Refrigeration systems contain components that include the compressor, condenser, and evaporator. (Courtesy of Heatcraft)

In liquid state, refrigerant is primed to transfer heat from the walk-in cooler, for example, through the system to the outdoor heat exchanger. Liquids can’t be compressed, so refrigerant comes into the inlet of the condenser as hot vapor and moves through the passages of the condensing coil. Because there is a temperature difference between the outside air and the hot vapor, heat will transfer and refrigerant will change state from gas to liquid when it exits the condenser outlet.

The liquid receiver shown in Figure 1 simply receives subcooled liquid, then it flows to the thermal expansion valve (TXV) or electric expansion valve (EEV). When it leaves the TXV or EEV, refrigerant goes to a distributor, which divides the flow of liquid refrigerant into all the openings of the evaporator coil. Here, refrigerant pressure will drop, lowering the temperature. These two factors are directly proportional.

In the evaporator coil, there is a significant drop in temperature because of the drop in pressure. The warmer air being blown across the coil will give up some of the heat absorbed by the colder refrigerant being drawn through the suction line back to the compressor. The refrigerant vapor enters the compressor, which discharges refrigerant as a hot gas that then enters the inlet of the condensing coil, where it is rejected or gives up the heat collected from the evaporator to the outdoor ambient. As it does, it changes state from a hot gas to a subcooled liquid. Then the cycle starts all over again.

There is a set of laws that govern these changes of state from vapor to liquid. When refrigerant is inside a sealed system, several things will influence whether it is in a liquid state or a gas (vapor) state, and there are four laws that describe how the refrigerant behaves:

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FIGURE 2: Boyle’s Law states that the pressure of an ideal gas (a gas with no contaminants in it) is inversely proportional to its volume at a constant temperature. (Courtesy of Heatcraft)

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FIGURE 3: The Perfect Gas Law states that if refrigerant is heated, its pressure will increase, and if the refrigerant is cooled, its pressure will decrease. (Courtesy of Heatcraft)

All this information can help technicians become more adept at troubleshooting. Knowing how the equipment is supposed to work provides a baseline for a means of comparison.

This is the first of a three part Basic Refrigeration series. Read the rest, which also cover:

To learn about troubleshooting refrigeration systems, technicians may want to attend in-person technical training at Heatcraft’s training facility in Stone Mountain, Georgia.