Choosing a cooling system for an electrical enclosure is an important task that is not always as simple as it might seem. Even with access to resources such as cooling capacity calculators, selecting an appropriate 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 be well informed.

To choose the most efficient and cost-effective enclosure 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 determine whether to use an air conditioner, heat exchanger or a filtered fan system to cool the enclosure.

The choices do not stop there, however. 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.

The aim of this article is to help simplify the process of selecting a cooling system for an electrical enclosure. Although the details provided are comprehensive enough to help narrow the choices, it is recommended to consult those knowledgeable about these systems before making an investment in new cooling equipment for electrical enclosures.


Common Environments for Electrical Enclosures

Electrical enclosures can be found in areas as diverse as factories, windmill farms and coal mines. 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 determine which kind of cooling system is appropriate for an application.

NEMA 1. A NEMA 1 rating is the least stringent designation for an electrical enclosure. This rating designates that the enclosure will always be indoors, and the surrounding environment will be similar to normal atmospheric conditions. Although some dust protection is provided with a NEMA 1 enclosure, it is not a watertight solution; as a result, a NEMA 1 enclosure allows some dust and other airborne contaminants to enter the system.

NEMA3R. An electrical enclosure with a NEMA 3R rating is intended for outdoor use, so it must protect against rain, snow, wind, ice, rust and other environmental conditions that may be encountered. NEMA 3R enclosures also can be used in indoor environments when falling debris is a consideration.

NEMA 4 N. NEMA 4 enclosures must be weatherproof for outdoor applications and able to withstand more extreme indoor conditions where water or other liquids are present. Examples of potential applications for NEMA 4 enclosures include telecommunications or freshwater marine environments.

Both NEMA types 1 and 3R enclosures often can be cooled with a simple filtered fan system. A filtered fan system does not provide protection against water spray, dust or other airborne contaminants, but these enclosures do not require that. 

The other types of enclosures require a closed-loop system, which can be achieved with either an air conditioner or a heat exchanger. The cooling system selection depends on the ambient temperature, the heat load of the enclosure and the sensitivity of the equipment.


Determining and Managing Enclosure Heat Load

The heat load of an electrical enclosure is primarily the amount of heat generated by the equipment inside the unit. Too much heat in an enclosure can cause damage to the equipment and shorten its life. This is why it is so important to select a cooling system that has the capacity to lower the temperature adequately.

Understanding the sources that contribute to the heat load and how that heat is transferred are some 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.

Internal Enclosure Heat. Any number of heat sources might exist in an electrical enclosure. Common components include:

  • Motor drives.
  • Transformers.
  • Communication equipment.
  • Servos.
  • Power supplies.
  • Control boards.
  • Programmable logic controllers (PLCs).
  • Servers and other networking equipment.

It also is worth noting that many of these heat-generating components are the same ones that require protection from damage due to excess heat. Most components come with specifications that outline the highest expected heat output. You can use this information to help calculate the heat load of the enclosure.

Ambient Heat. Ambient heat refers to the temperature in the environment surrounding the enclosure. In an outdoor environment, the ambient temperature generally is comparable to the air temperature although solar heat gain also must be considered. In an indoor environment, however, the ambient temperature can be affected by factors such as industrial ovens, kilns and furnaces.

When ambient heat is high enough to affect the internal temperature of the enclosure, it must be factored into the heat-load calculation.

The combination of internal and external heat sources will help determine the heat load, or how much heat must be removed from the system. But, how do you actually achieve the desired cooling?


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; as a result, the temperature of the liquid is lowered. The same concept can be applied to electrical enclosures in three different ways: natural convection cooling, forced convection cooling and active cooling.

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 simplest, it also is the least effective. In general, the temperature difference between most enclosures and their ambient environments is not large enough to sufficiently cool the components inside the enclosure.

Forced Convection Cooling. The amount of heat that transfers from a warmer area to a cooler area can be increased with the addition of a fan or blower. The airflow decreases 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, dirt or oil? The filtered fan may provide the necessary cooling, but it will deposit these contaminants on electrical components at the same time. When air contamination might be a problem, the recommended solution is a closed-loop air-to-air heat exchanger.

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

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.


Selecting an Enclosure Cooling System

After identifying the NEMA rating of the enclosure and calculating its heat load, you have enough information to decide whether a filtered fan, a heat exchanger or an air conditioner is needed.

Air Conditioner Sizing and Selection. An air conditioner is necessary when the internal temperature of the enclosure must be lowered below the ambient temperature outside the enclosure. Air conditioners are suitable for enclosures of NEMA types 4, 4X and 12.

Selecting a properly sized air conditioner is critical for achieving optimal performance and efficiency. An air conditioner that has insufficient cooling capacity will not be able to adequately cool the components inside the enclosure. By contrast, an oversized air conditioner will cycle on and off too frequently. This makes it less efficient, increases operating costs and potentially could shorten the life of the equipment.

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/hr, is based on the internal heat load and the heat-load transfer.

  • With internal heat load, each component in the enclosure has a maximum heat output specification, typically provided in watts. This can be converted to BTU/hr. Adding the maximum heat output specifications for every component in the enclosure will give you the total internal heat load for the system.
  • 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. The calculation takes into account factors such as:

  • The surface area of the enclosure.
  • The enclosure material.
  • The maximum ambient air temperature.
  • The maximum temperature allowed in the enclosure.
  • Whether the enclosure is insulated.
  • The location of the enclosure.
  • Industry standard constants.

Dividing the internal heat load by the temperature differential (∆T) and then subtracting the heat-load transfer will provide the required cooling capacity for a heat exchanger. Heat exchangers are available with cooling capacities ranging from approximately 10 to 70 W/°C.

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

The cooling capacity of a filtered fan is expressed as an airflow as cubic feet per minute (cfm). The equation used to calculate the required airflow rate (cfm) of a filtered fan includes both the internal heat load and the temperature differential (∆T).

  • 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.
  • The ∆T is the difference between the maximum ambient temperature and the maximum internal temperature.

To size the filtered fan, calculate the required airflow in cfm by multiplying the industry-standard constant 3.17 by the internal heat load (in watts), and then dividing the result by the temperature diffential (∆T).

Filtered fans are available with cooling capacities ranging from approximately 80 to 750 cfm. The air resistance of the filters needs to be factored, however. A general rule of thumb is to reduce the effective fan airflow (cfm) by 33 percent for each filter in the airflow path. For example, the effective airflow rate for a 750 cfm fan with one inlet filter and one exhaust filter is 248 cfm.

In conclusion, selecting an appropriate cooling system for an electrical enclosure is important for keeping operating costs low, protecting valuable equipment and getting the most from your investments. Choosing an inappropriate 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. When you consider the potential consequences of installing a cooling system that does not function as expected, it is clear that making the best choice from the beginning is important.

Cooling capacity calculators can help simplify the selection process. Enclosure cooling suppliers also can provide insights and help you make the most cost-effective solution for your application. PC


Barry Slotnick is a product manager with Thermal Edge. The Irving, Texas-based company can be reached at 972-580-0200 or visit thermal-edge.com.