Closed heat exchangers

Closed Heat Exchanger Exchanger in which the two fluids are separated by a solid surface. 

There are three modes of heat transfer. Conduction is the method of heat transfer within a solid. In closed heat exchangers, conduction is how thermal... 

In petrochemical plants, fans are most commonly used ia air-cooled heat exchangers that can be described as overgrown automobile radiators (see HeaT-EXCHANGEtechnology). Process fluid ia the finned tubes is cooled usually by two fans, either forced draft (fans below the bundle) or iaduced draft (fans above the bundles). Normally, one fan is a fixed pitch and one is variable pitch to control the process outlet temperature within a closely controlled set poiat. A temperature iadicating controller (TIC) measures the outlet fluid temperature and controls the variable pitch fan to maintain the set poiat temperature to within a few degrees. 

Fossil Fuel-Fired Plants. In modem, fossil fuel-fired power plants, the Rankine cycle typically operates as a closed loop. In describing the steam—water cycle of a modem Rankine cycle plant, it is easiest to start with the condensate system (see Fig. 1). Condensate is the water that remains after the steam employed by the plant s steam turbines exhausts into the plant s condenser, where it is collected for reuse in the cycle. Many modem power plants employ a series of heat exchangers to boost efficiency. As a first step, the condensate is heated in a series of heat exchangers, usually sheU-and-tube heat exchangers, by steam extracted from strategic locations on the plant s steam turbines (see HeaT-EXCHANGETECHNOLOGy). 

Flooded refrigeration systems are a version of the closed-cycle design that may reduce operating problems in some appHcations. In flooded systems, the refrigerant is circulated to heat exchangers or evaporators by a pump. Figure 11 shows the flooded cycle, which can employ any of the simple or compound closed-refrigeration cycles. 

Fig. 4. Schematic of a hemodialyzer. The design of a dialyzer is close to that of a sheU and tube heat exchanger. Blood enters through an inlet manifold, is distributed to a parallel bundle of fibers, and exits into a coUection manifold. Dialysate flows countercurrent in an external chamber the blood and dialysate are separated from the fibers by a polyurethane potting material. Housings are typically prepared from acrylate or polycarbonate. Production volume is...

The best known use of the hairpin is its operation in true counter-current flow which yields the most efficient design for processes that have a close temperature approach or temperature cross. However, maintaining countercurrent flow in a tubular heat exchanger usually implies one tube pass for each shell pass. As recently as 30 years ago, the lack of inexpensive, multiple-tube pass capability often diluted me advantages gained from countercurrent flow. 

HumidiRcation Chambers The air-cooled heat exchanger is provided with humidification chambers in which the air is cooled to a close approach to the wet-bulb temperature before entering the finned-tube bundle of the heat exchanger. 

When heat-exchange surface is being provided in the design of an absorber, the isothermal design procedure can be rendered valid by virtue of the exchanger design specifications. With ample surface area and a close approach, isotherm operation can be guaranteed. 

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