A heat exchanger is used to carry heat from one place to another place where it is wanted more: They either heat or cool substances. In residential areas, they usually heat or cool air by moving heat. The actual heat exchanger is a simple piping system, often made of stainless steel or copper, with metal fins over which air passes.
Heat exchangers are used in many different devices, but homeowners use them most often in refrigerators and air conditioning systems. In both applications, the heat exchanger helps maintain temperature: The refrigerator must be kept at a certain low temperature, and the house temperature is raised and lowered throughout the day, depending on how the homeowner programs his thermostat.
Key to the process of heat exchange is the refrigerant gas circulating through the heat exchanger. This gas has special properties that allow it to easily hold and transfer heat. Sometimes it is compressed to concentrate heat in a focused space before it is passed along to the exchanger, and sometimes it is cooled so much it becomes a liquid before it moves on.
The refrigerant flows through the heat exchanger, which has metal fins or similar objects over which air passes at the same time. These metal fins are good at transferring heat, and if the air is being heated, the heat moves from the gas, through the fins, and into the air. Because heat always tries to reach equilibrium with its surroundings, it will move naturally to the cooler substance. If the refrigerant is cold enough, heat from the air will pass into the exchanger, cooling the air when necessary.
Heat exchangers are an important part of a system's efficiency. The goal of any heat exchange system is to change temperature using only a very small amount of energy. In a perfectly efficient system, 100 percent of the energy used in the machine would transfer to the air as heat (or the air would be cooled to the same degree). Many systems are 70 percent to 90 percent efficient, but few reach higher than that with today's heat-exchange process.