Heat Exchangers for Enclosure Thermal Management

In industry, electrical and electronic equipment are typically housed in an enclosure to protect the equipment from the environment.  The type environment determines an IP or NEMA type rating for the enclosure, which protects the equipment from an indoor verses outdoor environment, corrosive environments, dust, rain, snow and ice formation, hose directed water and possibly hazardous gases which could ignite. 

Because most electrical and electronic equipment is not 100% efficient, a portion of the electrical energy is converted into heat.  This heat causes a rise in the enclosure temperature which can impact the operation and life of the internal equipment.  In these cases, some type of thermal management system is required to remove this heat and maintain a safe operating temperature.  Because this thermal management system is typically mounted to the enclosure surface, it must be able to maintain the enclosures environmental integrity while in operation. 

To prevent ambient environmental contaminants from entering the enclosure, a closed loop thermal management system is typically required.  This closed loop system prevents ambient contaminants from entering the enclosure by creating a physical barrier between the two. 

There are a variety of closed loop cooling system methods that can be used.  When the enclosure’s outside ambient temperature is a few degrees less than the required enclosure internal temperature, a common type of system selected is an air-to-air heat exchanger.  These heat exchangers utilize a method of transferring the generated heat inside the enclosure to the cooler air outside of the enclosure through a heat exchanger element.  Fans are typically used to circulate the heated enclosure air and the cool ambient air to this heat transfer element.  The fan’s increased airflow allows for the use of physically smaller elements verses air flow from natural convection.  

Heat pipe coils and convoluted cores are two of the most common types of air-to-air heat exchanger elements used in the enclosure cooling industry.  Heat pipe coils are typically constructed of the thermally conductive metal (copper or aluminum) tubing with fins to enhance heat transfer.  These coils are partially filled with a working fluid, or refrigerant.  This fluid is used to transfer heat from a heated side of the coil exposed to the inside enclosure air stream to the cooled side of the coil located in the outside cool air stream. 

The convoluted core is constructed of a thermally conductive material that is formed to provide a maximum surface area for heat transfer, minimal restriction to air flow, and to maintain a sealed barrier between the enclosure and ambient environments. 

Either of these heat transfer elements will provide satisfactory results when selected and applied per manufacturer’s specifications and enclosure application.  However, the convoluted core offers a number of advantages.   

First of all, the convoluted core does not employ a working fluid to transfer heat from the enclosure to the outside ambient.  The working fluid places limitations on the temperature range that the heat exchanger can be used.  Also, the heat pipe coils use temperature difference to drive the working fluid from one side of the heat pipe to the other.  This temperature difference can cause variations in the rate of heat transfer.  Whereas the convoluted core does not utilize a working fluid, the heat transfer is more consistent based on air temperature differential through a wide range of temperatures, with small variations based on air density. 

Heat pipe cores can also be position sensitive as the working fluid can depend on gravity to assist the flow of the working fluid.  Convoluted cores are typically not position sensitive in regard to heat transfer capacity. 

Over time, the heat pipe coil’s fluid passages will leak to the surroundings.  This change in fluid composition will degrade the heat transfer capability.  Also, if the fluid is a CFC, HCFC or an HFC, this may be an environmental contaminant due to its global warming potential and/or possible ozone depletion. 

The finned-tube design of the heat pipe coils is more restrictive to air flow that the typical convoluted core.  This means more energy is required for air flow, decreasing the cooling systems operating efficiency.  Also, this restrictive air flow causes more rapid fouling of the heat pipe coils surface with air borne contaminants.  This coil fouling acts as thermal insulation inhibiting heat transfer and decreasing the overall heat transfer capability.  The increased fouling rate of the heat pipe coil also requires more frequent maintenance to assure optimum heat transfer. 

Based on aforementioned functional and design characteristics, you can see that the convoluted core heat transfer is simple, direct, and efficient with zero GWP, zero ODP.  The convoluted core also requires less frequent maintenance than the heat pipe coil and offers many advantages over the heat pipe coil.