Keeping food cold in refrigerators

How do freezers produce ice? How do food items get cold inside refrigerators? And how do air-conditioning units release cool air? All these appliances use the same process to function – refrigeration. But to know what refrigeration is, we need to understand the science of thermodynamics in the simplest way possible.

Thermodynamics has something to do with heat? Yes, it is about heat, which is ironically the opposite of coldness, which is our topic. To make it simple, let us define the word "cold" as an adjective characterized by the low degree of heat. Also, let us define refrigeration as the process of removing heat from an object to make it cold. With these twin definitions, we assume that freezers, refrigerators and air-conditioning units are machines that take away heat from an object or a surface. In other words, these machines suck heat out of an enclosed space.

Taking away heat from an object is possible through the system of evaporation. Have you ever observed that it becomes cooler after it had rained. This is because the rain, the perfect result of evaporation, takes away the heat of the soil. Have you ever felt fresher after taking a bath. This is because the water on your body takes away some of the heat from your skin.

Freezers, refrigerators and air-con units follow the same principle of evaporation, but instead of using water, they use refrigerants that vaporize at much colder temperatures. These chemicals – like ammonia, freon, and Greenfreeze – are injected into a tube or evaporator coil inside the machine where they turn from liquid into gas into liquid through evaporation via compression. As they evaporize through compression, these refrigerants absorb and take away heat from the inside surface of the refrigerator and transfer the heat to the outside surface of the refrigerator as they become liquid again while traveling through a coil mostly located at the back or bottom of the machine. The inside surface of the refrigerator thus becomes cool and the outside, hot.

A compressor is responsible for the process of evaporation. Essentially, the compressor machine, powered by an electric motor, squeezes the refrigerants into the coil that extends to the outside part of the machine. It gathers the refrigerant vapor articles inside the coil and compresses them into liquid form again, which in effect releases heat outside the body of the freezer, refrigerator or air-con unit. When the compressed gas passes through the coil on the back or bottom of the machine, the hot gas can lose its heat to the atmosphere outside the machine. Because the refrigerants travel through the tube and the coil endlessly, the whole refrigeration process is repeated over and over again, without allowing the refrigerants to leak.

Here, the common explanation is that when compressing refrigerants to a higher pressure, the temperature of the refrigerants will rise and release heat. By the time the chemicals cool off, significant amount of heat has been released from it. As the refrigerants cool down and condense into liquid form again, they flow through a device called an expansion valve, which has a small opening. The liquid refrigerants become very cold as they travel fast and pick up heat from the surface of the machine. Between the expansion valve and the compressor, there is a low-pressure area because the compressor is pulling the refrigerant gas out of that side. When the liquid refrigerants hit the low pressure area, they boil and changes into a gas. This is called vaporizing. The refrigerants then pass through the coil touching the inside surface of the machine where they cool and absorb heat at the same time, and in the process, pull the heat out of the compartment.

Cooling machines therefore are composed of a compressor, which compresses the refrigerant gas into liquid; expansion valve, where the liquid refrigerants move from a high-pressure zone to a low-pressure zone, so they can expand and evaporate; and the coils, which extend to the outside part of the machine. While evaporating, the refrigerants absorb heat from the inside surface and transfer the heat outside. The cycle repeats endlesslly until the inside surface of the refrigerator or the freezer becomes very cool. Freezers, which are more enclosed than refrigerators, allow their surface to cool to the extent of turning water into ice. In case of the air-conditioning unit, a blower pushes the cool air from the machine into the room.

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