How Heat-Pipe Heat Exchangers Work
Heat-pipe heat exchangers are one type of air-to-air heat exchangers used to cool electronics packaged in a sealed enclosure (figure 1). These exchangers can be made to maintain an enclosure's NEMA 4 or NEMA 12 rating. The use of a flange and a closed-cell gasket prevents the introduction of contaminated exterior air into the sealed enclosure. To pass the UL 94 V0 flammability specification required for many applications, the gasket often is made from silicone. The heat pipes pass through the flange and are sealed to the flange with either grommets or brazed material. Fins attach to the heat pipes on both sides of the flange, allowing heat removal in the following manner:
- The hot air inside the cabinet passes over and warms the fins inside the enclosure.
- The warmed inner fins transfer the heat to the heat pipes.
- The heat pipes transfer the heat through the divider flange to the fins outside the cabinet.
- Cool ambient air passes over the outer fins to reject the heat to the environment.
A heat pipe is a two-phase heat transfer device with highly effective thermal conductivity. Heat pipes used in heat exchangers usually are cylindrical copper tubes with internal grooves as the wick structure (figure 2). The heat pipe is evacuated and back-filled with a small amount of working fluid such as methanol to prevent freezing and thawing.
Heat absorbed through the hot inner fins on one end of the heat pipe vaporizes the working fluid. The vapor transports heat to the other end of the pipe, where the vapor condenses, releasing the heat to the outer cooling fins. This continuous cycle transfers large quantities of heat with very low thermal gradients. A heat pipe's operation is passive, being driven only by the heat that is transferred, which results in high reliability and long life.
Practical ConsiderationsHeat pipe heat exchangers cannot cool below the ambient temperature outside the cabinet (Tc-in); therefore, when such cooling is required, air conditioning often is used. Typically, heat-pipe heat exchangers maintain a 27 to 47°F (15 to 25°C) rise above ambient (Th-in minus Tc-in). Although heat exchangers can be designed with lower overall temperature rises, such units would be large and impractical.
For outside applications, the thermal load on the enclosure needs to incorporate the solar load or flux, which causes significant heating of approximately 1,000 W/m2. Natural convection from the enclosure can help reduce the overall thermal load. When heat loads exceed approximately 1,000 W, the heat-pipe heat exchanger may not be the most cost effective solution. Plate-and-frame, cross-flow or counterflow heat exchangers are more appropriate for higher power levels.
System packaging is probably the most important factor in selecting a heat exchanger. Heat exchangers can be used with the heat pipes operating horizontally (figure 2) or vertically (figure 3).
Vertical OrientationIn figure 3, the bottom (inner) portion of the heat exchanger passes through the enclosure's top wall. The inner portion absorbs heat, and the heat pipes transfer that heat vertically to the outer portion of the heat exchanger. This orientation, in which the heat is moved vertically upward, is the preferred method for heat-pipe units. This orientation is referred to as “gravity aided” because when the vapor is condensed in the heat pipe in the outer portion of the heat exchanger, it is pulled back down to the heat-input end (inner portion) with the assistance of gravity.
With horizontal and vertical orientations, the heat-pipe heat exchanger can be mounted to the wall, door, top or side of a typical electronics enclosure.
It is challenging to make heat-pipe heat exchangers operate against gravity, where the heat is transferred from the top portion of the heat exchanger to the lower portion. For this orientation, sintered-wick heat pipes are required, which substantially increases the cost.
The fin density of a heat-pipe heat exchanger can be varied relatively easily, and for most applications the fin density is in the range of 8 to 14 fins per inch. This allows for a large surface area in a small volume, which is one of the advantages of the heat-pipe heat exchanger compared to conventional plate-and-frame or wide-gap folded-fin cores. However, the trade-off for high fin density is high pressure drop. So, the higher fin density only will provide value if the system's fans can deliver significant airflow. In general, heat-pipe heat exchangers are attractive when the volume available for the heat exchanger is small and there is ample fan power to drive the air through the dense fins.
Heat-pipe heat exchangers are an attractive solution for air-to-air heat exchangers required for sealed electronics enclosures. They allow for different mounting schemes but do pose some orientation limitations. Heat-pipe heat exchangers are particularly useful in situations where there is plenty of available airflow and a high pressure drop is acceptable. They are generally used in applications generating 1,000 W of heat or less, have a proven track record, and are extremely reliable. PCE
References1. D.S. De Lorenzo and J.B. Opdahl, “Server Design Challenges for the High-Heat-Load Internet Data Center,” Electronics Cooling, Vol. 11, No. 1, February 2005.
2. R.E. Simons, “Estimating Temper-atures in an Air-Cooled Closed Box Electronics Enclosure,” Electronics Cooing, Volume 11, No. 1, February 2005.