Heat exchangers, condensers Humidification

Plate and frame e.xchangers (plate heat exchangers)-used for heating and cooling in reactor off-gas quenching, vacuum condensers, desuperheating, and humidification. 

Applications include reactor off-gas quenching, vacuum condensers, cooler-condensers, desuperheating and humidification. Water-cooling towers are a particular example of direct-contact heat exchange. In direct-contact cooler-condensers the condensed liquid is frequently used as the coolant, Figure 12.65. 

Today we switch from heat-transfer operations (heat exchange, evaporation, condensation) to mass-transfer operations (distillation, extraction, absorption, humidification). Instead of sizing equipment to exchange heat between two phases we will size equipment to exchange mass between two phases. 

Air-cooled heat exchangers are employed on large scale as condensers of distillation columns or process coolers. The approach temperature - the difference between process outlet temperature and dry-bulb air temperature - is typically of 8 to 14 °C above the temperature of the four consecutive warmest months. By air-humidification this difference can be reduced to 5 °C. Air cooled heat exchangers are manufactured from finned tubes. Typical ratio of extended to bare tube area is 15 1 to 20 1. Finned tubes are efficient when the heat transfer coefficient outside the tubes is much lower than inside the tubes. The only way to increase the heat transferred on the air-side is to extend the exchange area available. In this way the extended surface offered by fins increases significantly the heat duty. For example, the outside heat transfer coefficient increases from 10-15 W/m K for smooth tubes to 100-150 or more when finned tubes are used. Typical overall heat transfer coefficients are given in Table 16.10. The correction factor Ft for LMTD is about 0.8. 

In Chapter 9 the fundamentals of humidity and adiabatic humidification were discussed. In this section the performance and design of continuous air-water contactors is considered. The emphasis is on cooling of water, since this is the most important type of process in the process industries. There are many cases in industry in which warm water is discharged from heat exchangers and condensers when it would be more economical to cool and reuse it than to discard it. 

In addition to taking the heat away from the fuel cell stack, the cooling loop also cools down the reformate gas coming from the fuel processor and provides heat, if needed, in the air humidification process. Moreover, temperature control of the preferential oxidation process is needed (as shown in Figures 9-37 and 9-38). The heat is then rejected from the system to the environment by the heat exchanger, typically of a radiator type. Some heat may be rejected from the system in the condenser just before the exhaust gases leave the system. The condenser is needed to condense and save the water in the exhaust to maintain a neutral water balance in the system. 

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