Cross-flow recuperators

In terms of the cross-flow recuperator of the preceding example, we wish to evaluate the hot- and cold-side heat transfer coefficients from appropriate correlation formulas and select the diameter, length, number, and arrangement of the tubes. 

In a cross-flow recuperator, Tg2 is the temperature of that portion of the flue gases leaving the tubes in the center of the tube bank, and Ta2 is the temperature of the preheated air beyond the middle of the last tube. 

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. 

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.

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. 

Wire-drawing tools Diesel particulate filters Bearings and rotary seals Cross-flow heat exchangers Recuperators for ceramic kilns Wear- and corrosion-resistant machine parts BalUstic armor... 

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. 

The plant arrangement assumed for the two Brayton system heat balance is illustrated in Figure 5-27. This system has two converter loops each with one turboaltemator, recuperator, and gas cooler. No cross-strapping of converter loop components is assumed. Each turboaltemator Is sized to produce the entire 200 kWe required for full power. The recuperator and gas cooler are appropriately sized to meet this power requirement. One converter loop is normally operating while the other is an idle spare A check valve is located at the outlet of each compressor to prevent backflow through the idle loop to maximize system efficiency, minimize bypass cooling flow around the reactor, and prevent potential damage to turbomachinery due to reverse rotation. An isolation valve installed the outlet of each compressor enables loop shutdown and startup evolutions and provides Brayton overspeed protection for certain electric plant casualties. 

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