Parallel flow recuperators

A) Parallel-flow recuperator type and its temperature distribution. 

For gas-turbine recuperators and for mostgas-to-gas heat exchangers, the C /Cm ratio is approximately equal to unity. Show for this case that the heat-exchanger effectiveness for parallel flow is Eq. (7.48) and for counter flow is Eq. (7.49). 

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. 5.18. 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.

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.

When the last two sentences are related to heat transfer within heat recovery devices (instead of within furnaces), the low volume and velocity do present concerns with oxy-fuel firing. Heat recovery equipment with larger flow passage cross sections can benefit more from the triatomic gas radiation with oxy-fuel firing. A good example of this is the double-pipe stack or radiation type recuperator. However, they must have parallel flow at the recuperator s waste gas entrance to prevent overheating there. 

Parallel flow regenerative type Modern with recuperators such as cooler type, preheater kilns and internals Horizontal ring type, grate kilns, etc. 

However, many reactions of commercial interest have chemistry, mechanical, or system requirements that preclude the use of cross-flow reactors. Processes cannot use a cross-flow orientation primarily because of high temperatures and the need to internally recuperate heat such as steam methane reforming (SMR) [12, 13] and oxidation reactions [14]. Counter- and coflow devices require a micromanifold to dehver sufficiently uniform flow to each of the many parallel channels. 

The bypass flow concept utilizes a fractional cooling flow ( 1% of total gas flow) in parallel with the hot leg gas. The bypass flow is supplied from the recuperator outlet and is discharged into the turbine outlet. The bypass flow concept was evaluated with and without internal insulation. The un-insulated option was not viable due to excessive cold gas temperature increase, as illustrated by the results of the counter flow thermal analysis presented above. In the un-insulated bypass flow configuration, this will cause the cold gas temperature to exceed the temperature limit of the piping material. In order to maintain an outer wall temperature of 900K in the insulated bypass flow concept, the insulation thickness must approach that of the internally insulated concept. The addition of insulation will significantly increase pressure drop due to a reduction in the area available for gas flow and will also increase manufacturing complexity. 

Recuperators sized to support the operation of two 100 l e Brayton units or three 66.7 kWe Brayton units operating in parallel (may be multiple independent flow paths housed within a common pressure boundary or may be three independent recuperators each sized to service a 100 kWe Brayton)... 

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