When discussing cooling options, heat exchangers are a product that is often overlooked. The reason for this is many people are simply not as familiar with them compared to other options, such as air conditioners or fans. Yet, in many cases, this option may be the most economical and efficient solution to cooling. So, what exactly is a heat exchanger? How do they work? For which applications are they best utilized? What guidelines need to be considered?
While air conditioners are required when equipment temperatures must be maintained below ambient, heat exchangers are recommended to cool equipment that can tolerate operating temperatures marginally higher than ambient. Both of these options can be considered for enclosures located in areas where airborne contaminants pose a threat to the internal electronic components, and the enclosure interior must be isolated from the external environment.
One of the most important factors in determining whether a heat exchanger will satisfy the requirements of a specific application is the temperature that needs to be maintained. For installations that can operate at above-ambient temperatures, heat exchangers provide a moderate-cost closed-loop cooling option. Available in both air-to-air and water-to-air versions, there are models covering a wide range of cabinet sizes and performance capacities. Depending upon the model selected and the heat load, near-ambient temperatures are often achievable.
For applications that can utilize heat exchangers, the advantages compared with air conditioners include:
- Lower initial, operational and maintenance costs
- Lower power consumption
- Simple construction
- Fewer operating components
- Lighter weight
There are different methods heat exchangers use to discard heat:
Advanced air-to-air heat exchanger designs for cooling enclosures include two types of heat transfer methods. One design consists of a finned-tube coil which contains liquid refrigerant. The warm air exhausted from the equipment cabinet to the heat exchanger is directed past the coil, causing the refrigerant to boil and absorb heat. The resultant refrigerant vapor rises to the upper portion of the tubes, where the heat is removed by the cooler ambient air and the refrigerant condenses back to liquid, completing the cooling cycle in a continuous process. Some heat exchanger designs employ high-efficiency heat transfer elements fabricated of embossed convoluted metal foil or thin-film polymer material, constructed into two separate air paths. The air leaving the hot enclosure is directed through one side of the exchanger, where the heat passes through the element walls into the ambient-side air stream and is dissipated. Figure 1 illustrates heat transfer in air-to-air heat exchanger applications.
If ambient air cannot be utilized directly as a cooling medium, another cost-effective method of cooling is a water-to-air system (Figure 2). In this system, water is used to remove heat from air circulated within the electrical enclosure. Cooling water is circulated through a finned-tube coil, which is installed in a compartment isolated from the enclosure to protect the contents from possible leakage. As the heat-laden air circulates through the coil, the heat is absorbed by the water and carried away, in a continuous process. Water-to-air systems are easy to install and usually require minimal maintenance. The water used must be reasonably clean and cold enough to ensure proper operation of the cooling system under the most severe anticipated conditions. In some cases where ample cold water is available, below ambient-temperature cooling can be achieved.