About Direct Heat Exchangers:
Direct contact heat exchangers involve heat transfer between hot and cold streams of two phases in the absence of a separating wall.
Thus such heat exchangers can be classified as:
- Gas – liquid
- Immiscible liquid – liquid
- Solid-liquid or solid – gas
Descripition:
Most direct contact heat exchangers fall under the Gas – Liquid category, where heat is transferred between a gas and liquid in the form of drops, films or sprays.Such types of heat exchangers are used predominantly in air conditioning, humidification, industrial hot water heating, water cooling and condensing plants.
Advantages and Disadvantages in Utilizing Direct Contactors
The exchange of heat between two fluid streams can, in general, be accomplished using either direct contact or surface-type heat exchangers. There are, however, several limitations to the use of direct contactors. First, if two fluid streams are placed in direct contact, they will mix, unless the streams are immiscible. Thus, stream contamination will occur depending on the degree of miscibility. The two streams must also be at the same pressure in a direct contactor, which could lead to additional costs. The advantages in utilizing a direct contactor include the lack of surfaces to corrode or foul, or otherwise degrade the heat transfer performance. Other advantages include the potentially superior heat transfer for a given volume of heat exchanger due to the larger heat transfer surface area achievable and the ability to transfer heat at much lower temperature differences between the two streams. Still another advantage is the much lower pressure drop associated with direct contactors as compared to their tubular counterparts. A final advantage is the much lower capital cost as direct contact heat exchangers can be constructed out of little more than a pressure vessel, inlet nozzles for the fluid streams, and exit ports.
| Phases | Continuous phase | Driving force | Change of phase | Examples |
|---|---|---|---|---|
| Gas – Liquid | Gas | Gravity | No | Spray columns, packed columns |
| Yes | Cooling towers, falling droplet evaporators | |||
| Forced | No | Spray coolers/quenchers | ||
| Liquid flow | Yes | Spray condensers/evaporation, jet condensers | ||
| Liquid | Gravity | No | Bubble Columns, perforated tray columns | |
| Yes | Bubble column condensers | |||
| Forced | No | Gas spargers | ||
| Gas flow | Yes | Direct contact evaporators, submerged combustion |