A heat exchanger is a device that transfers heat from one fluid to another. The fluids do not mingle with one another. Instead, the heat exchange takes place through a solid surface that keeps the fluids separate. The two fluids may be gaseous or liquid, and the solid surface that separates them must be an efficient conductor of heat.
In some heat exchangers, the two fluids flow in two different pipes or tubes that are in contact with one another. If a hazardous fluid flows through one of the tubes, two concentric tubes may be used to contain it instead of a simple tube. This reduces the efficiency of heat transfer but increases safety. In some heat exchangers, the two fluids flow in the same direction. In others, the two may flow in opposite directions. In other heat exchangers, only one of the fluids may be in a coiled tube, while the other fluid surrounds it on all sides. In still other heat exchangers, a metal partition separates the two fluids instead of tubes.
A car radiator is a good example of a liquid to air heat exchanger. Water, often mixed with antifreeze, circulates through the car's engine in pipes and becomes hot. The liquid then enters the coils of the radiator. The cool outside air comes in contact with the metal coils, but does not touch the fluid. The metal conducts heat from the liquid to the air. The liquid, thus cooled in the heat-exchanging radiator, then returns to the engine and repeats the cycle.
A good example of a liquid to liquid heat exchanger is a current system of using solar energy to heat water. Solar heat warms a liquid, which flows through tubes to a water tank. The tubes then coil in such a way that they are in contact with the water in the tank, and the heat is conducted from the incoming liquid to the water. The type of liquid used depends on the climate. If winters are cold, the liquid must have a low freezing point. Air may be used instead of a liquid.
A simple but effective use of heat exchangers occurs in the sun-tempered superinsulated house. When a house is so well insulated that little heat and air escape, the air will become stale. However, a system has been developed in which a certain amount of fresh air is permitted to enter the house, and the same amount is allowed to exit. The incoming and outgoing air pass each other in a heat exchanger, and some of the heat from the outgoing air is transferred to the cooler incoming air. As a result, not so much heat is lost.