ABOUT DIRECT-CONTACT HEAT EXCHANGERS:
In a Direct-Contact Heat Exchangers, two fluid streams come into direct contact, exchange heat, and are then separated. Common applications of a direct-contact exchanger involve mass transfer in addition to heat transfer, such as in evaporative cooling and rectification; applications involving only sensible heat transfer are rare. The enthalpy of phase change in such an exchanger generally represents a significant portion of the total energy transfer. The phase change generally enhances the heat transfer rate. Compared to indirect- contact recuperators and regenerators, in direct-contact heat exchangers,
(1) very high heat transfer rates are achievable,
(2) the exchanger construction is relatively inexpensive, and
(3) the fouling problem is generally nonexistent, due to the absence of a heat transfer surface (wall) between the two fluids. However, the applications are limited to those cases where a direct contact of two fluid streams is permissible.
DESIGN PROCESS:
The design theory for these exchangers can classified as:
1.Immiscible Fluid Exchangers
2.Gas–Liquid Exchangers.
3.Liquid–Vapor Exchangers.
Typically steam is partially or fully condensed using cooling water, or water is heated with waste steam through direct
contact in the exchanger. Noncondensables and residual steam and hot water are the outlet streams. Common examples are desuperheaters and open feedwater heaters (also known as deaeraters) in power plants.