Refrigerators are a mainstay of modern life. Before refrigerators were common, food was usually preserved by curing, drying or canning. These food preservation methods were labor intensive and often the food was heavily salted. Today, fresh food is readily available and we have refrigeration to thank for it. Refrigerators are so ingrained in our lives that we seldom stop to think what life would be like without them or how they work.

Vapor-Compression Refrigeration

Modern refrigerators, freezers and air conditioners all work on the same principle. It is known as the vapor-compression cycle. The cycle works by compressing and then vaporizing a fluid with a low boiling point. The fluid, known as a refrigerant, circulates in a system consisting of a compressor, condenser, expansion valve and an evaporator. The condenser and the evaporator are both similar to a radiator in your car. The energy that powers the system enters by way of the compressor which runs on electricity.

How it cools

All fluids have a temperature at which they will turn to vapor or gas. This is the boiling point. For water it is 212°F but it is different for all liquids. For a refrigerant, the boiling point is actually below the freezing point of water. This means that if released at room temperature, the refrigerant in your refrigerator would boil away very quickly. While this may seem odd, this is actually a key to how a refrigerator works. When the refrigerant is in gas form, the compressor pressurizes it. As the pressure increases, the temperature of the vapor is also increased. The vapor is now super-heated because the temperature of the vapor is above its boiling point. This superheated refrigerant moves through the condenser. The condenser removes heat from the refrigerant by transferring it to the air. The easiest way to see this principle in action is to observe a window air conditioner. The air blowing from the back of the air conditioner is actually warmed by the heat removed from the refrigerant. Moving through the condenser cools the refrigerant enough to return it to liquid form but it is still pressurized, just as it was when it left the compressor. The refrigerant then travels through the expansion valve. As the refrigerant, in liquid form, passes through the expansion valve the pressure falls. This reduction in pressure cools the liquid even further and causes part of the liquid to vaporize. Now that the refrigerant is cool it moves through the evaporator. The evaporator is the opposite of the condenser in that instead of removing heat from the system, it adds heat to the system. Moving warm air across the evaporator removes heat from the air and adds heat to the refrigerant. The result is that the air that passes over the evaporator coils is much cooler than the surrounding air. We now have air that is cool enough to keep our food fresh or cool our homes.

Putting theory to work

Cool air, however, is not enough. Without a way to maintain the air temperature it might as well be so much smoke. This is where the body of the refrigerator plays its part. More than just a convenient place to have shelves, it is thickly insulated to contain the cool air. Add a thermostat to signal the system to run at the correct temperatures and we have basics that make the modern refrigerator. Who wants a cold drink?

This article was written by Sheldon Armstrong on behalf of Allied Heat Transfer, experts in heat exchangers in Australia.