Counterflow recuperators

Fig. 5.18. Comparison of temperature patterns in parallel flow and counterflow recuperators—applicable to types other than the double pipe shown. Calculate heat transfer using LMTD, pp. 127-128 of reference 51. There may be a burnout danger at the flue gas entry with counterflow.

The heat exchanging surface needed with a cross-flow recuperator is greater than that required by a counterflow recuperator of equal heat transfer. When applied to existing recuperators, the preceding equations 5.8 and 5.9 are used to find values of the overall heat transfer coefficient, U. For new recuperators, the equations are used to determine the needed heating surface, if there are no gas, air, or heat leaks. 

A recuperator is a low- to medium-temperature (up to about ISOOT or 700°C), continuous heat exchanger that uses the sensible energy from hot combustion products to preheat the incoming combustion air. These heat exchangers are commonly counterflow where the highest temperatures for both the combustion products and the combustion air are at one end of the exchanger with the coldest temperatures at the other end. Lower temperature recuperators are normally made of metal, while higher temperature recuperators may be made of... 

Counterflow types deliver the highest air preheat temperature, but parallel flow types protect the recuperator walls from overheating. Therefore, the hot flue gases are often fed first to a parallel flow section and then to a counterflow section to benefit from both advantages. 

Fig. 5.20. Recuperator flow types, shown schematically. All but types 1 and 2 have many, many tubes. Cross-flow recuperators (types 3, 4) often have the configuration of a square shell-and-tube heat exchanger. For the same heat exchanging area, temperature levels, and type, the average heat flux rates (see glossary) of parallel flow, cross-flow, and counterflow are about proportional to 1.00 to 1.40 to 1.55, respectively.

The recuperator effectiveness (e) is the fraction of heat transferred that would be transferred in a counterflow heat exchanger of infinite heat transfer area. In this case... 

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