Electronic Products & Technology

Choosing an enclosure system for electrical enclosures

May 9, 2014  EP&T Magazine

Selecting a cooling system for an electrical enclosure is an important task that is not always as simple as it might seem. The consequences of choosing the wrong system can be significant, so taking the time to make an informed choice is vital.

To choose the most efficient and cost-effective cooling system you must first consider factors such as the NEMA rating and heat load for the enclosure. These two pieces of information will help you determine whether to use an air conditioner, heat exchanger, or a filtered fan system to cool the enclosure.

However, the choices do not stop there. Once you decide which type of cooling system makes the most sense, you must determine the cooling capacity and the appropriate size to ensure optimal efficiency. Even with access to resources such as cooling capacity calculators, selecting the right cooling system can be challenging, especially if you have never gone through the process before. The financial consequences of choosing an improperly sized system are not insignificant, so it is important to make your purchase with confidence.

Common environments for enclosures

The National Electrical Manufacturers Association (NEMA) creates industry standards for the performance of electrical enclosures based on the surrounding environment. The NEMA enclosure type will help inform which kind of cooling system is appropriate for your application.

NEMA 1: A NEMA 1 rating is the least stringent designation for an electrical enclosure. The enclosure will always be indoors and the surrounding environment will be similar to normal atmospheric conditions.

NEMA 3R: An electrical enclosure with a NEMA 3R rating is intended for outdoor use so it must protect against environmental conditions.

NEMA 4 enclosures must be weatherproof for outdoor applications and able to withstand more extreme indoor conditions where water or other liquids are present.

NEMA 4X: A NEMA 4X rating is similar to a NEMA 4 rating, but the 4X designation must also include protection against corrosion.

NEMA 12: An enclosure with a NEMA 12 rating is intended for indoor use, such as manufacturing plants, and must meet certain standards for protection against dust, liquids and debris.

Enclosures of NEMA types 1 and 3R can often be cooled with a simple filtered fan system that does not provide protection against water spray, dust, or other airborne contaminants.

Determining, managing heat load

Understanding the sources that contribute to the heat load and how that heat is transferred is one of the first steps in selecting an appropriate cooling system. The two primary factors that contribute to the heat load of the enclosure are the internal and external heat sources: motor drives, transformers, communication equipment, servos, power supplies, control boards, programmable logic controllers, servers and other networking equipment,

Ambient heat

Ambient heat refers to the temperature in the environment surrounding the enclosure. In an indoor environment, the ambient temperature can be affected by factors such as: industrial ovens, kilns, furnaces. When ambient heat is high enough to impact the internal temperature of the enclosure, it must be factored into the heat load calculation.

Understanding heat transfer

The second law of thermodynamics states that heat always transfers from an object or region of higher temperature to one of a lower temperature. For example, when ice is added to a glass of water, the warmer liquid actually heats the ice, and as a result, the temperature of the liquid is lowered. The same concept can be applied to electrical enclosures in three different ways:

1. Natural convection cooling

The flow of heat from a warmer environment to a cooler environment occurs naturally when the ambient temperature surrounding an electrical enclosure is cooler than the internal temperature. The heat from the enclosure will naturally radiate through its walls and the internal temperature will be lowered accordingly. Although this method is by far the most simple, it is also the least effective because the temperature difference between most enclosures and their ambient environments is not large enough to sufficiently cool the components inside the enclosure.

2. Forced convection cooling

The amount of heat that transfers from a warmer area to a cooler area can be increased with addition of a fan or blower to decrease the thermal resistance of the barrier between the two areas.

In the case of an electrical enclosure, filtered fans can provide affordable forced convection cooling to reduce the internal temperature. But what happens when the outside air has contaminants like dust and dirt or oil? The filtered fan may provide the cooling you need, but it will deposit these contaminants on electrical components at the same time. When air contamination might be a problem, the best solution is a closed loop air to air heat exchanger.

However, just as with natural convection cooling, the amount of heat that can be transferred away from the components inside the enclosure is limited by the ambient air temperature.

3. Active cooling

When natural convection or forced convection do not provide enough heat transfer to adequately cool the components inside the enclosure, an air conditioner may be required. An air conditioner also provides a closed loop system which is needed when the components inside the enclosure must be protected from environmental factors such as dirt, dust, or liquids.

After you have identified the NEMA rating of the enclosure and calculated its heat load, you have enough information to decide whether you need a filtered fan, a heat exchanger, or an air conditioner.

AC sizing & selection

Selecting a properly sized air conditioner is critical for achieving optimal performance and efficiency. Calculating the required cooling capacity is an essential step in selecting a properly sized air conditioner. The required cooling capacity of an air conditioner,

which is expressed in BTU/hour, is based on the internal heat load and the heat load transfer.

Internal heat load – Each component in the enclosure has a maximum heat output specification, typically provided in Watts, which can be converted to BTU/hour. Adding the maximum heat output specifications for every component in the enclosure will give you the total internal heat load for the system.

Heat load transfer – The heat that transfers between the inside of the enclosure and the ambient air outside is referred to as the heat load transfer. When the temperature inside the enclosure is higher than the ambient temperature, the heat load transfer will be negative. When it is warmer outside the enclosure than it is inside, the heat load transfer will be positive.

Enclosure & AC dimensions

In addition to calculating the cooling capacity, you must also consider the physical size of both the air conditioner and the electrical enclosure to ensure that they are compatible. Enclosure air conditioners come in a variety of shapes and sizes, including narrow units that are designed to fit on enclosures as small as only seven inches deep.

Heat exchanger selection

Heat exchangers are most commonly used in relatively low ambient temperatures when a closed loop system is desired to keep contaminants out of the enclosure. They have the advantage of providing highly efficient cooling without the need for filters.

The required cooling capacity for a heat exchanger is expressed in Watts/degree C
elsius (W/°C). It is calculated in a similar way to air conditioners, but with the addition of another factor: Delta T.

*Internal heat load – Just as with air conditioners, the internal heat load is calculated by adding the maximum heat output specification for all of the components in the enclosure.

*Heat load transfer – Using the same concepts as those described for air conditioners, the heat load transfer for a heat exchanger is calculated using the square footage of the surface area and an industry standard constant that varies depending on the enclosure material.

*Delta T – Delta T is calculated by subtracting the maximum ambient temperature from the maximum allowable enclosure temperature.

Filtered Fan selection

Filtered fans are a viable solution for enclosures rated NEMA Type 1 or 3R because they do not keep particles or liquids out of electrical enclosures. Filtered fans can also only be effectively used when the ambient temperature is lower than the temperature inside the enclosure.

Calculating Cooling Capacity

The cooling capacity of a filtered fan is expressed in cubic feet per minute (CFM). The equation used to calculate the required CFM of a filtered fan includes both the internal heat load and Delta T.

*Internal heat load – Just as with an air conditioner or heat exchanger, the internal heat load of an enclosure that uses a filtered fan is calculated by adding the maximum heat output specifications for all of the components in the enclosure.

*Delta T – This is the difference between the maximum ambient temperature and the maximum internal temperature.

Conclusion

Selecting the right cooling system for an electrical enclosure is important for keeping operating costs low, protecting valuable equipment and getting the most from your investments. Choosing the wrong system could result in equipment damage, higher operating costs, or even equipment failure.

The steps for selecting the right cooling system include:

*Determining the NEMA rating of the enclosure

*Calculating the heat load of the enclosure

*Deciding which type of cooling system is appropriate

*Calculating the required cooling capacity

*Selecting a system that meets all of the above requirements and physically fits on the enclosure

Although each step in the process is clearly defined, the actual process of selecting a cooling system can be quite intimidating, especially if you have never done it before. When you consider the potential consequences of installing a cooling system that does not function as expected, it is clear that making the right choice from the beginning is extremely important.


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