Regenerative Heat Exchanger
In general, the heat exchangers used in regeneration may be classified as either regenerators or recuperators.
- Regenerator (Regenerative Heat Exchanger) 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. It has a single flow path in which the hot and cold fluids alternately pass through.
- Recuperator is a type of heat exchanger has separate flow paths for each fluid along their own passages and heat is transferred through the separating walls. Recuperators (e.g. economisers) are often used in power engineering, to increase the overall efficiency of thermodynamic cycles. For example, in a gas turbine engine. The recuperator transfers some of the waste heat in the exhaust to the compressed air, thus preheating it before entering the combustion chamber. Many recuperators are designed as counter-flow heat exchangers.
In steam turbines theory, significant increases in the thermal efficiency of steam turbine can be achieved through reducing the amount of fuel that must be added in the boiler. This can be done by transferring heat (e.g. partially expanded steam) from certain sections of the steam turbine, which is normally well above the ambient temperature, to the feedwater. This process is known as heat regeneration and a variety of heat regenerators can be used for this purpose. Sometimes engineers use the term economiser that are heat exchangers intended to reduce energy consumption, especially in case of preheating of a fluid.
As can be seen in the article “Steam Generator”, the feedwater (secondary circuit) at the inlet of the steam generator may have about ~230°C (446°F)and then is heated to the boiling point of that fluid (280°C; 536°F; 6,5MPa)and evaporated. But the condensate at the condenser outlet may have about 40°C, so the heat regeneration in typical PWR is significant and very important:
- Heat regeneration increases the thermal efficiency, since more of the heat flow into the cycle occurs at higher temperature.
- Heat regeneration causes a decrease in the mass flow rate through low-pressure stage of the steam turbine, thus increases LP Isentropic Turbine Efficiency. Note that at the last stage of expansion the steam has very high specific volume.
- Heat regeneration causes an increases in working steam quality, since the drains are situated at the periphery of turbine casing, where is higher concentration of water droplets.
Analysis of Heat Exchangers
Heat exchangers are commonly used in industry, and proper design of a heat exchanger depends on many variables. In the analysis of heat exchangers, it is often convenient to work with an overall heat transfer coefficient, known as a U-factor. The U-factor is defined by an expression analogous to Newton’s law of cooling. Moreover, engineers also use the logarithmic mean temperature difference (LMTD) to determine the temperature driving force for heat transfer in heat exchangers.
Special Reference: John R. Thome, Engineering Data Book III. Wolverine Tube Inc. 2004.
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