Recuperative heat exchanger

The simplest configuration for a recuperative heat exchanger is the metallic radiation recuperator (Fig. 27-57). The inner tube carries the hot exhaust gases and the outer tube carries the combustion air. The bulk of the heat transfer from the hot gases to the surface of the inner tube is by radiation, whereas that from the inner tube to the cold combustion air is predominantly by convection. 

In a recuperative heat exchanger, each element of heat-transferring surface has a constant temperature and, by arranging the gas paths in contra-flow, the temperature distribution in the matrix in the direction of flow is that giving optimum performance for the given heat-transfer conditions. This optimum temperature distribution can be achieved ideally in a con-tra-flow regenerator and approached very closely in a cross-flow regenerator. 

Typical unit operations required for this system include vaporizers/preheaters, a pyrolysis reactor, and recuperative heat exchangers. One of the challenges with this approach is the potential for fouling by the carbon formed, which is particularly important in microreactors. 

Due to its required versatility, the platform should not be dependent on a particular design of the HTS electrolyser. Beyond the different possibilities of steam and air feeding the cathodic and anodic compartments of the electrolyser (mixed or not with H2) some key components are generic as the steam generator, both low- and high-temperature (recuperative) heat exchangers, the superheater. 

The activities of heat-integrated processes with recuperative heat exchange are mainly devoted to the conversion of primary fuels (hydrocarbons or alcohols) to hydrogen, with few exceptions - for example, investigations on the dehydrogenation of light alkanes in Schmidt s group [3]. The practical relevance and vitality of... 

Figure 23. Autothermal fixed-bed reactors with recuperative heat exchange. Basic design and typical temperature profiles. A) Conventional design with separate heat exchanger B) Counter-current fixed-bed reactor.

The purpose of the regenerator is to transmit oscillating PV power, or acoustic power, from ambient temperature to some lower temperature with a minimum of losses. Just as with the recuperative heat exchangers discussed in section 5, the losses are those due to imperfect heat transfer, pressure drop AP, and conduction The gross refrigeration power available at the cold... 

The calculations for the length and the hydraulic diameter of the regenerator are the same as for the recuperative heat exchangers. Thus, equation (28) gives the optimum length and equation (39) gives the optimum hydraulic diameter. They are repeated here for convenience ... 

These furnaces are good candidates for full oxy/fuel. Recuperative heat exchanger efficiencies are much lower than with regenerative furnaces, and therefore fuel savings can help to drive the conversion. Also, recuperative furnaces operate in a continuous and steady firing mode of operation similar to oxy/fuel furnaces. 

Solutions for the heat-transfer problem, which arises in the use of packed beds as direct-contact recuperative heat exchangers, were presented by Furnas in 1930. The parameter Njj is the number of heat-transfer units. For heat transfer, the dimensionless time t is the heat capacity of the gas times the amount of gas that has passed through the bed divided by the total bed capacity. For Nu = co, the breakthrough curve of T /Tq vs. t would be a vertical line at t = 1.0, just as for mass transfer. The defining equations are... 

Develop highly effective reactors, fuel and water vaporizers, recuperative heat exchangers, and condensers broadly applicable to fuel processing and fuel cell systems. 

Achieve high efficiency through integration of steam reforming, water gas shift, and preferential oxidation reactors with microchannel recuperative heat exchangers, fuel and water vaporizers, condensers, and separators. 

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