How to Select an Air-to-Air Heat Exchanger
- Heat loss from the electronics inside the enclosure.
- Maximum ambient temperature.
- Desired internal enclosure temperature.
- Size of the enclosure to be cooled.
- Available cooling device mounting space.
- Type of environment into which the enclosure will be placed.
- Desired operating voltage of the cooling product.
When does it make sense to use an air-to-air heat exchanger for enclosure cooling rather than another cooling solution? Heat exchangers are most effective when:
- Ambient temperatures are moderate to cool (less than 90°F [32°C]).
- Enclosure cooling loads are low to moderate (less than 2,000 W).
- The ambient air is dirty, which makes filter fans a maintenance-intensive solution.
Operation and DesignThe major components in an air-to-air heat exchanger are the housing, the heat exchange cassette and two fans or blowers (figure 1). There are two separate air paths within the heat exchanger: one for the air that circulates inside the control enclosure and a second for the ambient air that passes into and out of the heat exchanger. Each air cycle is powered by its own blower. It is important to ensure that a dust and watertight seal is maintained between the internal and ambient air paths.
As the two airstreams enter the heat exchanger cassette, heat transfers from the warmer internal enclosure air, across the thin wall of the cassette via conduction, to the cooler air in the ambient airstream. The total heat exchanger capacity depends on the heat exchange cassette surface area and efficiency, air mass flow across the cassette, and temperature differential (DeltaT) between ambient and internal air temperature.
Cassette Style. Several types of heat exchanger cassettes are used in enclosure air-to-air heat exchangers, including counterflow, crossflow, and heat pipe (table 1). Cassettes typically are built of aluminum but also can be made of a thin plastic sheet material. Counterflow-style cassettes have ambient air flowing in an opposite direction from the internal air (figure 2) while crossflow-style cassettes have the two airstreams flowing perpendicular to each other. Heat pipe-style cassettes have heat pipe tubes embedded within an aluminum fin structure. One end of the heat pipe tube extends into the internal air path and the other extends into the ambient air path.
Fan/Blower Options. Several styles of fans or blowers can be used within enclosure air-to-air heat exchangers. For small enclosures with low static pressure loss across the heat exchange cassette, tubeaxial fans can be used. For larger heat exchangers with higher internal static pressure drop, higher powered, backward-curved impeller or centrifugal blowers are used. Fans can operate off of 115 or 230 VAC as well as 24 or 48 VDC. DC fans can be supplied with alarm options to signal fan failure. For outdoor applications located in especially corrosive areas, weather-hardened fans and epoxy- or polyurethane-coated heat exchange cassettes can be specified.
Control. Enclosure air-to-air heat exchangers can be set up to power both fans with a single power cord or terminal block or with separate connections for each fan. In the latter case, one connection is reserved for the internal fan and the other for the external fan. It typically is desirable to have the fan circulating air inside the enclosure run continuously to help ensure an even temperature within the control enclosure and avoid hot spots. The external fan can run continuously or be controlled with a thermostat, which can save energy and extend fan life because the fan may not be required in the cooler ambient temperature conditions during the winter months.
For applications where advanced notice of fan failure is required, fan monitoring via Hall Effect-type signaling can be used. Hall Effect signaling monitors the revolutions per minute of the fan. This requires that a DC fan with alarm output be used and that a 5 VDC transistor-transistor logic-compatible signal be generated and interpreted. TTL is a common type of digital circuit in which the output is derived from two transistors.
How to SizeBecause air-to-air heat exchangers use the ambient air to cool the internal enclosure air, the ambient air must be cooler than the desired internal enclosure air for the heat exchanger to provide the best cooling. With the maximum operating temperature for most industrial electronics ranging from 105 to 140°F (41 to 60°C), an air-to-air heat exchanger typically can be used if the maximum ambient temperature immediately outside the electrical enclosure is less than 90°F (32°C).
To calculate the required heat exchanger capacity, follow this process. First, calculate the convective heat loss through the electrical enclosure walls. This requires knowing the enclosure size, material, and internal and ambient temperatures. Use the equation
Q = A x K x DeltaT
Q is the convective heat transfer
A is the enclosure surface area (m2)
K is the coefficient of heat transfer - it is 5.5 W/m2-°C for sheet steel
DeltaT is the temperature difference between the internal temperature and the ambient temperature, in °C.
Next, subtract the convective heat transfer from the total electronics heat loss inside the enclosure. All heat loss values must be in watts. Finally, divide the remaining heat loss by the temperature differential (DeltaT) between the internal and ambient temperature in °C.
An example using real-world numbers will illustrate the method. Assume you have a sheet steel, free-standing enclosure that is approximately 79 x 24 x 20" (2,000 x 600 x 500 mm). The maximum ambient temperature is 77°F (25°C) and the maximum desired enclosure internal temperature is 95°F (35°C). The electronics heat loss inside the enclosure is 900 W (3,072 BTU/hr).
For freestanding enclosures, the calculated surface area (A) per VDE is:
A = 1.8 x H x W x (W+D) + [1.4 x W x D]
A is 4.4 m2
DeltaT is 35°C - 25°C, or 10°C
K is 5.5 W/m2-°C
Q is 242 W
Therefore, the required heat exchanger capacity is calculated by the following:
(900 W - 242 W) /10°C = 65.8 W/°C
Thermal calculation software can assist with such sizing calculations.
RatingsAir-to-air heat exchangers are rated in many ways. Some manufacturers rate the heat exchangers such that the rating is for the heat exchanger itself and not for the heat exchanger mounted to a certain size enclosure. During performance testing, the convective heat transfer is discounted, resulting in a rating for the heat exchanger only. This type of rating can be used to calculate the heat exchanger's cooling effect for use on any size enclosure.
Other manufacturers may provide a rating for a given heat exchanger when mounted to a certain size cabinet over a given temperature difference (DeltaT) between ambient and internal temperature. This results in a rating similar to an air conditioner's capacity rating - for example 2,000 BTU. One drawback of this type of rating is it cannot be applied easily to conditions other than the tested conditions.
Once it is determined that an air-to-air heat exchanger is the most effective solution, further comparison will point the user in the direction of the product that best meets their application. Finally, it is imperative to examine the application's various requirements to determine the appropriate thermal solution.