Though they may not always be seen, heat exchangers are present in a variety of different devices, machines, appliances, industrial processes, power plants, engines or vehicles. They are present primarily to salvage heat from wasted gases and increase energy efficiency. Though many different types of Heat exchangers exist, they all work on the principles of thermodynamics.
A heat exchanger is a device that passively transfers heat from one fluid to another. The fluid may be a gas or a liquid. Basically the heat exchanger is a device that has two separate chambers which are separated by a material or wall. The device could be a chamber with a coil running through it or just two chambers separated by a wall. The two fluids will each have their own chamber and the fluids will never meet or mix with each other. As the name suggests, the heat will be transferred from one fluid to another.
When both fluids are present or flowing through the heat exchange, one fluid will be cool and other will be hot. Heat will diffuse through the separating material between the two fluids and pass from one fluid to another. This follows the law of thermodynamics as one object is heated and the heat energy from that fluid will diffuse into the surroundings until the object reaches equilibrium with its surroundings.
In the case of heat exchangers, one fluid will be much hotter than the other and the heat energy will diffuse into the wall or separating material and then into the other fluid in order to try and reach equilibrium. Equilibrium may not be reached as the flow of the fluid often is fast enough that an equilibrium temperature is never reached. There isn’t necessarily enough time in most processes that the heat exchanger is used in to allow equilibrium to occur. It is also best to increase the surface area of the separating material between the fluids as much as possible. The more surface area, the more efficient and faster the heat energy will be transferred.
Use of heat exchangers
Heat exchangers are often used when the cost of chemical or refrigerant cooling is far too expensive to be practical. Instead, a cool or room temperature fluid that will need to be heated anyway is put through the heat exchange with the hot fluid. That way some of the heat energy is conserved and energy costs are reduced. Otherwise the heat energy of the hot fluid would be wasted and the cool fluid often times would still need to be heated up.
An simple example of a heat exchanger that everyone uses is their lungs. The cool fluid enters the lungs and is separated from the hot fluid (the blood) by the tissue of the lungs. The lungs have a large surface volume and hot fluid diffuses heat energy into the air that fills the lungs with each breath. The body is slightly cooled and the warmer air is expelled.