Heat exchangers are an efficient tool used to increase the thermal efficiency of a system. A heat exchanger is defined by the relationship Q=UA∆T, where U is the overall heat transfer coefficient of the heat exchanger, A is the total heat transfer area, and ∆T is the entering and exiting fluid temperature difference. Improving heat exchanger performance is accomplished by increasing U, A, ∆T, or a combination of the three. 7 ways to maximize heat exchanger performance are listed below.
1) Operating pressure (increases U)
The well-known ideal gas law (PV=nRT) can be rewritten P=ρrT, where ρ represents fluid density. From that relationship, operating pressure is directly related to density. Further, the pressure drop of a fluid through a heat exchanger directly relates to [the square of] velocity, and is inversely proportional to density. As a result, for a given pressure drop, operating at higher pressures enables higher velocity, yielding higher heat transfer coefficients through higher Reynold’s number.
2) Operating temperature (increases U)
Elevating operating temperature increases heat capacity (fluid holds more heat), thermal conductivity (fluid draws more heat through it) and viscosity (less energy required for fluid to move). Increasing those properties also enables higher fluid velocities, which increase heat transfer coefficients.
3) Entering temperature difference (ETD; increases ∆T)
ETD is the driving potential for heat transfer. If U and A are fixed, the performance can be improved by driving up the difference between the cold fluid and hot fluid inlet temperatures. Pushing these values farther apart increases the heat transfer rate, which could also allow a reduction in size.
4) Enhanced surfaces (increases A)
Enhanced surfaces, such as fins, can add a significant amount of surface area in the same package space. Heat exchangers are typically held to tightly constrained package envelopes, so adding enhanced surfaces is a common practice to improve performance.
5) Flow distribution (increases U and A)
Heat exchangers are sized to ensure that sufficient area exists to transfer the required amount of heat. However, if there is excessive fluid bypass or the flow does not reach all of the active heat transfer surface, a portion of the heat exchanger is unused, reducing performance. Adding features and design practices (flow modeling in CFD) to promote flow distribution will confirm that sufficient area for heat transfer is used. Better distribution also increases the overall heat transfer coefficient as more flow contacts the active surface.
6) Fluid flow turbulation (increases U)
Utilizing enhanced surface and flow distribution methods can carry the added benefit of turbulating the flow. From fluid dynamics, a boundary layer develops at the channel wall, which insulates the center of the fluid from the heat transfer surface. This decreases the rate of heat transfer. Conversely, if the flow continuously meets resistance, it is effectively stirred. This enables a larger amount of the flow to directly contact the heat transfer surface earlier in the flow path.
7) Flow stream orientation (increases ∆T)
We saw above that it is advantageous to maximize the difference between fluid entering temperatures in a heat exchanger. However, if the fluids enter the same side of the core, the temperatures can come together, or pinch, before the exit of the heat exchanger. Thermal pinching wastes the improved driving potential created at the inlet by making some of the heat exchange area redundant, as the exit temperature of one fluid is limited by the other. Feeding the fluids to the core from opposite ends (pure counterflow) creates a “hot” and a “cold” end of the heat exchanger, and allows the hot-fluid exit temperature to drop below that of the cold fluid. This orientation increases the amount heat removed from the hot stream. As a result, pure counterflow orientation is the most effective way to configure the flows within a heat exchanger.
Heat exchangers are very effective tools to accomplish system goals. They provide many functions, such as phase-change in HVAC systems, recovery of waste heat, driving catalytic reactions and cooling oil. Knowing how to effectively maximize heat exchanger performance is critical to optimizing any system or process.
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