Direct contact heat exchangers

In direct-contact heat exchange the hot and cold streams are brought into contact without any separating wall, and high rates of heat transfer are achieved. 

Direct-contact heat exchangers should be considered whenever the process stream and coolant are compatible. The equipment used is basically simple and cheap, and is suitable for use with heavily fouling fluids and with liquids containing solids spray chambers, spray columns, and plate and packed columns are used. 

The design and application of direct-contact heat exchangers is discussed by Fair (1961, 1972a, 1972 ), and Chen-Chia and Fair (1989), they give practical design methods and data for a range of applications. 

The design of water-cooling towers, and humidification, is covered in Volume 1, Chapter 13. The same basic principles will apply to the design of other direct-contact exchangers. 

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Direct Contact Heat Exchangers. In a direct contact exchanger, two fluid streams come into direct contact, exchange heat and maybe also mass, and then separate. Very high heat-transfer rates, practically no fouling, lower capital costs, and lower approach temperatures are the principal advantages. 

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. 

Direct consumption sugar, 23 450-451 Direct contact heat exchangers, 13 268 Direct cooler evaporators, 21 537 Direct-coupled plasma (DCF), 25 370 Direct covalent carbon nanotube functionalization, 17 54-55 Direct current (dc) diode sputtering, 24 730-731. See also dc sensing current... 

Some industrial operations involving bubble and drop formation are extraction, direct contact heat exchange, distillation, absorption, sparger reactors, spray drying and atomization, fluidization, nucleate boiling, air lifts, and flotation. 

Heat exchanger Any device that allows two or more fluids at different temperatures to transfer heat from the hotter stream(s) to the colder stream(s). Usually the streams are separated by solid walls, but the streams are allowed to mix in direct-contact heat exchangers. 

Reboilers and Vaporizers Piate Heat Exchangers Direct-Contact Heat Exchangers Finned Tubes... 

In direct contact condensers, a physical contact of the working fluids (a saturated or superheated vapor and a liquid) occurs, allowing for the condensation to be accomplished simultaneously with the mixing process. The fluids can be subsequently separated only if they are immiscible. Direct contact is generally characterized with a very high heat transfer rate per unit volume. The classification of indirect and direct contact heat exchangers is discussed in more detail in Ref. 2. 

Work on direct-contact heat exchangers was stimulated in the last decade by the quest for economic water-desalination units. Multiphase exchange. 

Fig. 15(a). Direct-contact heat exchangers using mixer-settler contactors for immiscible liquids, after (H5a). (b). Direct-contact heat exchangers using mixer-settler contactors for miscible liquids using a third immiscible liquid, after (H5a). (c). Schematic diagram... 

The hot effluent is cooled to -1°C (in practice this very large temperature difference can only achieved by direct contact heat exchange, ie a quench system). The pressure drop across the large condenser is 50 kPa. Under these conditions, the product stream has a vapour fraction of about 0.8 and the task of recovering condensable liquid ethylene oxide begins. The cool product stream is fed into a 3-phase separator and the light liquid phase is separated from the heavy liquid phase and vapour residual. HYSYS normally puts water in the heavy phase when there is a non-zero water stream. 

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