The Importance of Heat Exchangers

A heat exchanger is an important device that allows heat from a liquid or gas to pass onto a second fluid without the two fluids having to come into direct contact or mix with one another. More simply put, a heat exchanger transfers heat without transferring the actual source of the heat. Throughout the aerospace industry, heat exchangers are used for both for cooling and heating. They are used in aircraft of all sizes, both fixed wing and rotary, and in many different aircraft systems. In addition to the engine, they are frequently used to cool hydraulics, ram air, auxiliary power units, gearboxes, and many other components found in an aircraft.

In an engine, exhaust gases release heat that dissipates into the open air. This heat loss is essentially wasted energy, and something that the use of a heat exchanger can minimize. Heat will still inevitably be lost, but heat exchangers are able to limit the amount. To carry out this task, the heat exchanger is usually positioned within the exhaust tailpipes or smokestacks. As the hot exhaust gases rise, they float past copper fins. These fins have water flowing through them that absorbs the gas’ heat and carries it back into the plant where it can be recycled back into the engine or fashioned for a different use.

While all heat exchangers operate under the same working principle, there are four different types of heat exchangers: shell and tube heat exchanger, plate/fin heat exchanger, recuperator, and regenerator. In a shell and tube heat exchanger, one fluid flows through an array of metal tubes while the second fluid moves through a surrounding shell. As the fluids flow past each other, the heat is exchanged. Depending on the configuration, the two fluids can flow in the same direction, opposite directions, or at right angles. Plate/fin heat exchangers work similarly, but rather than tubes and a shell, they use many thin metal plates or fins with large surface areas to transfer heat more efficiently.

Recuperators and regenerators minimize heat loss from buildings, engines, and machines. A recuperator is used to capture heat by channeling cold, incoming fluid in the opposite direction of warm, outgoing fluid. The two fluids flow through separate channels, never mixing. Because the fluids move in opposite directions, recuperators are known as counterflow heat exchangers. A regenerator is similar to a recuperator, apart from the fact that the incoming and outgoing fluids flow through the same channel. They do this in opposite directions during different intervals. The warm fluid moves through the regenerator, leaving some of its heat behind. The cold fluid follows, picking up the heat left behind by the warm fluid.

In addition to having many types, heat exchangers are made from many materials. These include metal, ceramics, and polymer plastics. Each material has its own advantages. Metal is a very quick absorber and conductor of heat. Ceramics are beneficial in extreme-temperature applications where metals or plastics would melt, and are also more resistant to corrosion and abrasion. Polymer plastics have the advantage of being lightweight, cost-effective, and the ability to be altered to have better thermal conductivity, despite their mechanical weakness.