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.[1]
In rotary regenerators, or thermal wheels, the heat storage "matrix" in the form of a wheel or drum, that rotates continuously through two counter-flowing streams of fluid. In this way, the two streams are mostly separated. 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. The heat storage medium can be a relatively fine-grained set of metal plates or wire mesh, made of some resistant alloy or coated to resist chemical attack by the process fluids, or made of ceramics in high temperature applications. A large amount of heat transfer area can be provided in each unit volume of the rotary regenerator, compared to a shell-and-tube heat exchanger - up to 1000 square feet of surface can be contained in each cubic foot of regenerator matrix, compared to about 30 square feet in each cubic foot of a shell-and-tube exchanger.[6]
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In a fixed matrix regenerator, a single fluid stream has cyclical, reversible flow; it is said to flow "counter-current". This regenerator may be part of a valveless system, such as a Stirling engine. In another configuration, the fluid is ducted through valves to different matrices in alternate operating periods resulting in outlet temperatures that vary with time. For example, a blast furnace may have several "stoves" or "checkers" full of refractory fire brick. The hot gas from the furnace is ducted through the brickwork for some interval, say one hour, until the brick reaches a high temperature. Valves then operate and switch the cold intake air through the brick, recovering the heat for use in the furnace. Practical installations will have multiple stoves and arrangements of valves to gradually transfer flow between a "hot" stove and an adjacent "cold" stove, so that the variations in the outlet air temperature are reduced.[7]
Another type of regenerator is called a micro scale regenerative heat exchanger. It has a multilayer grating structure in which each layer is offset from the adjacent layer by half a cell which has an opening along both axes perpendicular to the flow axis. Each layer is a composite structure of two sublayers, one of a high thermal conductivity material and another of a low thermal conductivity material. When a hot fluid flows through the cell, heat from the fluid is transferred to the cell wells, and stored there. When the fluid flow reverses direction, heat is transferred from the cell walls back to the fluid.
A third type of regenerator is called a "Rothemuhle" regenerator. This type has a fixed matrix in a disk shape, and streams of fluid are ducted through rotating hoods. The Rothemuhle regenerator is used as an air preheater in some power generating plants. The thermal design of this regenerator is the same as of other types of regenerators.[citation needed]
The design of inlet and outlet headers used to distribute hot and cold fluids in the matrix is much simpler in counter flow regenerators than recuperators. The reason behind this is that both streams flow in different sections for a rotary regenerator and one fluid enters and leaves one matrix at a time in a fixed-matrix regenerator. Furthermore, flow sectors for hot and cold fluids in rotary regenerators can be designed to optimize pressure drop in the fluids. The matrix surfaces of regenerators also have self-cleaning characteristics, reducing fluid-side fouling and corrosion. Finally properties such as small surface density and counter-flow arrangement of regenerators make it ideal for gas-gas heat exchange applications requiring effectiveness exceeding 85%. The heat transfer coefficient is much lower for gases than for liquids, thus the enormous surface area in a regenerator greatly increases heat transfer.[citation needed]
The major disadvantage of rotary and fixed-matrix regenerators is that there is always some mixing of the fluid streams, and they can not be completely separated. There is an unavoidable carryover of a small fraction of one fluid stream into the other. In the rotary regenerator, the carryover fluid is trapped inside the radial seal and in the matrix, and in a fixed-matrix regenerator, the carryover fluid is the fluid that remains in the void volume of the matrix. This small fraction will mix with the other stream in the following half-cycle. Therefore, rotary and fixed-matrix regenerators are only used when it is acceptable for the two fluid streams to be mixed. Mixed flow is common for gas-to-gas heat and/or energy transfer applications, and less common in liquid or phase-changing fluids since fluid contamination is often prohibited with liquid flows.[citation needed]
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