Mechanical and electrical equipment used in water and wastewater treatment systems include a range of pumps, blowers, motors, meters, and sensors for critical operations. Electrical controls for this equipment must be protected from hazards such as corrosive vapors, cleaning fluids, chemical reagents, and contaminated water, as well as environmental dust and dirt, particulates from treatment chemicals, and inclement weather.
Because the control equipment is confined inside protective electrical enclosures, waste heat given off by the equipment itself can be trapped inside the enclosure, raising its temperature by a significant amount. In addition, if the enclosure is situated near a source of heat, such as a boiler, or is located outdoors where it receives direct solar insolation, the amount of heat will be increased further.
Most electrical equipment is designed to operate within a defined temperature range, and will tend to lose efficiency above a maximum allowable temperature specified by the manufacturer. As a result, equipment inside the enclosures often requires a cooling system to protect it from excess heat, which may affect the equipment’s performance, reliability, and lifespan.
Determine NEMA type
The first step in selecting a cooling system for an electrical enclosure in a water treatment facility is to determine the correct NEMA type, based on the National Electrical Manufacturers Association rating levels for protective electrical cabinets.
Examples of NEMA types for industrial settings include NEMA 3, NEMA 4, NEMA 12, and NEMA 4X. Any of these might be appropriate for different locations in and around water and wastewater treatment plants, but because of the harsh conditions often found in these facilities, electrical enclosures should typically be specified to meet the NEMA 4X standard.
NEMA 4X Requirements
The NEMA 4X type is specified for indoor or outdoor use, and provides protection from extreme weather and climate, as well as exposure to dust and corrosive materials.
Though enclosures with a NEMA 4X rating are not fully waterproof, they must be able to withstand a water jet of 65 gallons per minute from a one-inch diameter nozzle without allowing any water ingress.
The gasket between the cooling system and the enclosure should be water tight, and the enclosure should not allow water to accumulate in places where mold could grow. It is the ideal electrical enclosure type for applications, such as wastewater treatment plants, that are subject to corrosion and wash-down.
Additional corrosion resistance
The NEMA 4X rating specifies that the enclosure will meet certain minimum conditions of corrosion resistance. However, these conditions do not necessarily replicate actual site conditions in an environment where other corrosive vapors, such as chlorine and other chemicals used for sterilizing water/wastewater treatment facilities, are present.
For this reason, it’s best to specify stainless steel construction for the enclosure construction. If it will be located in a marine environment, 316 stainless steel is preferable to 304 stainless steel, due to its increased corrosion resistance.
The resistance of seals and the enclosure materials of construction to particularly aggressive chemicals may require additional corrosion protection, for example, any exposed refrigeration tubing should be protected with a corrosion resistant coating.
Closed Loop Cooling
Enclosure cooling systems come in two basic types: open loop or closed loop. An open loop cooling system is one that allows ambient air to circulate through the enclosure, for example with a forced air fan.
A closed loop cooling system is one in which air inside the electrical enclosure has no direct contact with outside air. NEMA 4X enclosures must be sealed to prevent contaminants, corrosive gases, solids, or liquids, from reaching the electric components within. For this reason, closed loop cooling systems are required for these enclosures.
Closed loop cooling technologies include air conditioners and air-to-air heat exchangers. Both have benefits and drawbacks to be considered when selecting an enclosure cooling system for a water/wastewater treatment plant.
Air to Air Heat Exchangers
The heart of the air to air heat exchanger is a heat pipe that transfers heat from the enclosure to the ambient air. The heat pipe, fitted with aluminum fins to improve heat transfer, is evacuated and filled with a special refrigerant. The refrigerant is heated by the air circulating inside the enclosure, causing it to vaporize.
This process absorbs heat from the air and causes the hot vapor to rise to the top of the pipe, where it’s cooled by ambient air. The vapor then condenses, giving up its latent heat to the ambient air and flowing back to the bottom of the pipe. This simple process continually repeats itself.
When sized properly, a heat exchanger will cool an enclosure to a temperature slightly higher than the ambient temperature. Apart from air circulation fans, it contains no moving parts and energy consumption is extremely low, meaning annual running costs are low.
With a few limitations, an air-to-air heat exchanger is suitable for many cooling applications, including some NEMA 4X sealed enclosures. However, a heat exchanger can only approach the same temperature inside the enclosure as it is outside.
Enclosure Air Conditioners
Enclosure air conditioners represent an extremely effective method of cooling an enclosure and will work efficiently even if the ambient temperature is much higher than the enclosure’s required air temperature. They also control humidity much better than a heat exchanger, which may be important for sensitive electrical components located in or near a water treatment facility.
An enclosure air conditioner contains a refrigerant liquid under pressure, which is passed through an expansion device, creating a drop in pressure. The lower pressure causes the liquid to evaporate in the air conditioner’s evaporator coil and absorb heat, cooling the air inside an enclosure. Then the hot evaporated gas is compressed and passed through a condenser coil, where the gas liquefies again, giving up its heat to the air outside the enclosure.
Air conditioners are more effective than air-to-air heat exchangers at keeping electrical cabinets cool, but they do cost more to purchase and operate. The trade-off is that they can better keep vital equipment running efficiently, independent of ambient temperature,, preventing unscheduled downtimes and maintenance, as well as complete equipment failure, which could be disastrous for critical water treatment systems.
Selecting and Sizing a Cooling System
Selecting a properly sized enclosure cooling system is critical for optimal operation and maximum efficiency of a water or wastewater treatment plant. If the system does not have a cooling capacity high enough to maintain the electrical equipment within its specified temperature range, the equipment will quickly begin to lose efficiency. Above the maximum allowable temperature, the performance, reliability and lifespan of equipment is halved for every 10 °C (18 °F) above 70 °F.
On the other hand, specifying a cooling system that is too large will not only cost more than necessary, but can also cause problems because it will not run long enough to reduce the humidity in the air, which may damage some electrical equipment. It may also shorten the equipment life and reduce efficiency by cycling off and on too frequently and causing wide temperature swings.
Choosing the right enclosure cooling solution, including upgrades for corrosion resistance, and then sizing it correctly and maintaining it properly, will protect valuable equipment, save money and reduce downtime for a water or wastewater treatment plant.