Regenerative heat exchanger

A regenerative heat exchanger, or more commonly a regenerator, is a type of heat exchanger where heat from the hot fluid is intermittently stored in a thermal storage medium before it is transferred to the cold fluid. To accomplish this the hot fluid is brought into contact with the heat storage medium, then the fluid is displaced with the cold fluid, which absorbs the heat.

In regenerative heat exchangers, the fluid on either side of the heat exchanger can be the same fluid. The fluid may go through an external processing step, and then it is flowed back through the heat exchanger in the opposite direction for further processing. Usually the application will use this process cyclically or repetitively.

Regenerative heating was one of the most important technologies developed during the Industrial Revolution when it was used in the hot blast process on blast furnaces, It was later used in glass and steel making, to increase the efficiency of open hearth furnaces, and in high pressure boilers and chemical and other applications, where it continues to be important today.

The first regenerator was invented by Rev. Robert Stirling in 1816, and is commonly found as a component of his Stirling engine. The simplest Stirlings, and most models, use a less efficient but simpler to construct, displacer instead.

Later applications included the blast furnace process known as hot blast and the Open hearth furnace also called Siemens regenerative furnace (which was used for making glass), where the hot exhaust gases from combustion are passed through firebrick regenerative chambers, which are thus heated. The flow is then reversed, so that the heated bricks preheat the fuel.

Edward Alfred Cowper applied the regeneration principle to blast furnaces, in the form of the "Cowper stove", patented in 1857. This is almost invariably used with blast furnaces to this day.

In rotary regenerators the matrix rotates continuously through two counter-flowing streams of fluid. In this way, the two streams are mostly separated but the seals are generally not perfect. Only one stream flows through each section of the matrix at a time; however, over the course of a rotation, both streams eventually flow through all sections of the matrix in succession. Each portion of the matrix will be nearly isothermal, since the rotation is perpendicular to both the temperature gradient and flow direction, and not through them. The two fluid streams flow counter-current. The fluid temperatures vary across the flow area; however the local stream temperatures are not a function of time.

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