Several trends in electrical enclosure packaging influence enclosure cooling design, including:
- Increased heat dissipation from electronic components.
- Enclosure size reduction.
- System cost reduction.
- In-use operating costs reduction.
- Increased system reliability and maximized output.
Traditional enclosure cooling methods such as filter-fan systems and air conditioners are effective in most applications. However, these methods rely on periodic filter and coil maintenance to maintain their effectiveness. Additionally, air conditioners lose cooling effectiveness at high ambient temperatures, and they do not function when temperatures are above 130°F (54°C). Air-to-air heat exchangers require less maintenance than filter fans and air conditioners, but they are effective only when the ambient temperature is 15°F (8°C) or more below the desired internal enclosure temperature. If your application calls for a high cooling performance solution in a small space, a viable climate control option is the air-to-water heat exchanger.
Operating PrinciplesIndustries such as plastics molding, metal working, pharmaceutical and food processing already use water in the production process in close proximity to the electrical enclosure. Some of today's largest motor drives -- those in excess of 300 hp -- also are using water to cool the heat sinks attached to drives.
Because they use an existing water source or glycol coolant as the refrigerant, air-to-water heat exchangers have become increasingly popular cooling solutions in the last few years. Water is piped into the heat exchanger through a fitting. It flows through a thermostatically controlled solenoid valve, through the heat exchange coil, and then back out of the heat exchanger (figure 1). A powerful blower draws warm enclosure air continuously across the coil and pushes cooler air back into the enclosure.
Similar to an air conditioner, the air-to-water heat exchanger routes any condensate formed on the coil away from the enclosure via a drain fitting. Although unlikely, if the coil develops a leak, a deflector screen installed between the coil and the enclosure's opening would direct water to the condensate drain. Should a pipe rupture, the drain fitting is sized to remove all coolant from the heat exchanger. An overtemperature alarm contact also is available for use with external alarm devices such as signal lamps or audible alarms. The user-adjustable thermostat keeps the enclosure at an optimal temperature by controlling the solenoid valve, preventing excess condensate and minimizing water usage.
Cost and SavingsAn air-to-water heat exchanger cools loads while using little space on the enclosure wall. For example, a unit rated at 10,000 BTU when using 50°F (10°C) water and a 95°F (35°C) internal enclosure temperature can have a footprint as small as 38 x 16". In contrast, an air conditioner with similar cooling capacity could be as much as 10" taller. Because chilled water is not necessary, the same heat exchanger could be used to remove more than 6,000 BTUs with 75°F (24°C) municipal water.
Reduced maintenance requirements also minimize long-term operating costs when using air-to-water heat exchangers. For instance, in dirty environments an air conditioner or filter fan must be cleaned or replaced at least once each month at a cost of $25 per month, including parts and labor. Because the air-to-water heat exchanger is sealed from the ambient environment, filter and cleaning maintenance is not required. More importantly, because air conditioner filter maintenance often is erratic, overheating problems due to lack of maintenance are noticed only after machine shutdown or reduced performance. Using an air-to-water heat exchanger can reduce the chance of system shutdown due to overheating.
Depending on the water source, the energy consumption of an air-to-water heat exchanger system can be similar or less than an air conditioner. Water requirements are moderate, with some air-to-water heat exchangers requiring 1 to 2 gal/min of water at a minimum pressure of 15 psi. If a chilled water system is already in place in proximity to the enclosure, total system cost should be lower than an air conditioner. If a water source is not available, using a small chiller designed to dissipate 10,000 BTUs and three 3,000 BTU air-to-water heat exchangers will cost about the same as three 3,300 BTU air conditioners.
Another advantage of air-to-water heat exchangers is that they can be used in hazardous, hot and hosedown environments and can provide cooling in small spaces.
In hot environments, enclosure air conditioners typically are rated to operate at maximum ambient temperatures of 125 to 130°F (52 to 54°C). Some environments -- those near paint lines, ovens or foundries for example -- have ambient temperatures in excess of 130°F (54°C). In these situations, an air-to-water heat exchanger may be the only possible cooling solution. Air-to-water heat exchangers typically are rated to a maximum ambient temperature of 155 to 160°F (68 to 71°C). If a chiller is used, it can be located in a cooler, cleaner part of the factory, and the water can be piped to the enclosure with the air-to-water heat exchanger. In hosedown environments with no air inlet openings on the outside of the heat exchangers, air-to-water heat exchangers are suited for NEMA 4 hosedown applications. This heat exchanger design also can perform in hazardous areas. For example, if the condensate drain is plumbed to a nonhazardous area, the heat exchanger is sealed, thereby preventing explosive vapors from entering the unit. A small purge system can be attached to the heat exchanger housing for further safety.
Unlike air conditioners, air-to-water heat exchangers can be powered from a DC voltage source. This can be useful in applications where AC power is not available or magnetic motor interference must be minimized.
Air-to-water heat exchangers also are suitable for cooling in small spaces. An air conditioner, air-to-air heat exchanger or filter fan typically needs 8" of clearance between the ambient air inlet and outlet and any adjacent obstruction to ensure adequate airflow. In tight confines, this situation can present a problem. An air-to-water heat exchanger does not have this clearance issue.
Today's enclosure cooling demands require that new approaches be considered. For dirty, hot environments, an air-to-water heat exchanger is safe because it has devices that prevent it from succumbing to water or coolant damage and overtemperature conditions. Compact, energy efficient and cost effective, the heat exchanger can perform in hot or hazardous environments or confined spaces. It can be hosed down and can be powered by DC voltage. Because of these factors, and because chilled water is already used in some production processes, many factories are looking to air-to-water heat exchangers to cool their electrical enclosures. PCE