Effectiveness of a Two-Phase Heat Exchanger

Heat Exchanger Types

• To understand the effectiveness of two-stage heat exchangers, you must first understand the two primary classes of heat exchangers. One is a radiator that lets heat on one side efficiently radiate through cooling fins and dissipate on the other side; the heat transfer is all one way. The other type of heat exchanger is a circuit. Rather than heat radiating through fins, tubes or other types of manifolds, heat passes from one chamber to another through a duct. It is then cooled -- often using radiators or fins described in the first type -- and passed back to the first chamber or plenum, often by the force of heat expansion.

Two-Phase Heat Exchanger

• A two-phase heat exchanger has a secondary cooling circuit. The circuit either provides additional and equivalent cooling or extra, or auxiliary, cooling. If the heat exchanger just has two cooling chambers, then it is better described as a dual-chamber system. Two-phase systems daisy-chain one to the other, so that the secondary chamber will stay cooler than the primary chamber.

Measure of Effectiveness

• A two-phase heat exchanger can be much more effective than a standard circuit-type heat exchanger. However, trade-offs exists. Adding complexity typically adds expense and may decrease reliability. You must isolate the variable in which you're measuring efficacy: cooling, money, weight. The additional phase gives you more engineering choices to optimize your mechanical objectives.

Example

• An example of a dual-heat exchanger is a heat exchange unit at each one of a race car's front disc brakes that has a centrally located master cylinder; each exchanger contributes an equal amount to cooling the pneumatic fluid. Now, consider a heat exchanger to cool the race car's water for head cooling. The primary heat exchanger is usually in the front of the car near a lot of engine heat, which diminishes its ability to cool. If, for example, you have a small scoop as found in many Formula One cars, then catching cool air away from the engine compartment, placing a small auxiliary exchanger in that air stream, using it to cool water with a radiator, then plumbing that cool water into the main plenum would optimize cooling resources. You would have one large, efficient exchanger and one smaller, high-efficiency exchanger -- an auxiliary.