Thermal Considerations for Enclosure Design

In industry, electrical and electronic equipment are typically housed in an enclosure to protect the equipment from the environment.  The equipment and type of environment determines an IP or NEMA type rating for the enclosure, which protects the equipment from a harsh indoor or outdoor environments such as dust, rain, snow and ice formation, hose directed water, corrosive environments and possibly hazardous dusts or gases that could ignite.

Much of this equipment is not 100% efficient.  Some of the electrical power that is consumed by the equipment is converted to heat.  As this equipment is housed within an enclosure, this heat will cause an increase of the temperature inside the enclosure.  So, the protecting the equipment from air-borne contamination has caused another issue of potentially causing the equipment to overheat.  We have all probably heard the old adage regarding temperature and the relationship to electrical equipment life, “for every 10 degree Celsius increase in temperature, the life of the equipment is cut in half”.

In order to extend the life of equipment, in many cases a thermal management system is required in conjunction with the enclosure to protect this equipment.  Many times however, the heat dissipation of the equipment or the use of the thermal management system is not considered until the enclosure loaded with equipment is put into field service.  It is then discovered that the equipment is not functioning properly due to overheating.  A cooling system can be an add-on at this point.  But at times, the enclosure may be so crowded with equipment that there is not adequate space within the enclosure for proper air circulation, and to deliver the cooled air to locations inside the enclosure where it is needed to prevent equipment from overheating.  Without the proper space for air flow, the cooled air from the cooling system can be “short circuited” back to the air conditioner and never reach the equipment where the cooling is required.  In this case, even with an add-on cooling system, internal equipment will still not function properly due to overheating.

Proper enclosure design is not only protecting the equipment from environmental factors in the ambient air, but also protection from overheating.  Proper thermal design includes allowing space for air flow inside of the enclosure and selecting of the proper type of thermal management system to mitigate the threat of overheating.  This may be more expensive initially, but could greatly increase the life of the valuable equipment and avoid expensive downtime for the equipment process application.

Following are a few considerations of initial enclosure design that will assist in promoting satisfactory thermal conditions inside of the enclosure.  Please note that these are general considerations and may not apply to all enclosure designs.

  1.  Determine all equipment that will be mounted inside of the enclosure.

Think about the future.  Is it possible that additional equipment may be added in the future?  Is it possible that equipment may become outdated during the expected life of the enclosure and require an upgrade?  Again, it may be more expensive to allow for this initially, but may be less costly than an entire enclosure replacement in the future.

  1. Review equipment specifications.

Equipment manufacturers typically are aware their equipment may be mounted into an enclosure for protection.  Specifications will usually include product dimensions, mounting instructions, equipment clearances and spacing requirements, operating temperature range, air flow requirements and heat dissipation.  Heat dissipation may be listed as watts, an efficiency percentage, or BTUH.  This is all important in the final selection of the enclosure.  Failure to follow equipment manufacturer instructions may lead to premature failure, unnecessary down time, and void the equipment warranty.

  1. Determine the location of the equipment inside the enclosure.

In many instances, the top of the enclosure will be a few degrees warmer than the bottom.  This is because heated air is less dense and will rise to the top of the enclosure.  This is also true for outdoor enclosures exposed to sunlight or solar radiation.  The solar radiation will be present on the top surface of the enclosure for most of the daylight hours.  Because of this, consider locating the equipment with the highest operating ranges nearer the top of the enclosure.

Also for consideration near the top of the enclosure is the equipment with the greatest heat dissipation.  This equipment may have higher discharge air temperatures than equipment with lesser heat dissipation.  Locating near the top prevents this warmer air exposure to equipment with lower operating temperature ranges.

Another consideration is the direction of the air flow through the equipment.  For example: bottom to top, or front to back.  The warm air exhaust from one component should not be directed toward the cold air intake of another component.  With a rack enclosure, coordination of air flows can establish all cooler air intakes on one side of the enclosure, and the warmer air discharge on the opposite side.  These are sometimes referred to as a “cold aisle” and a “hot aisle”.

  1.  Determine location of enclosure cable entry and internal wire routing.

Many of the cooling systems for enclosures are designed to mount on the sides or the tops of the enclosures.  In most cases, cable entry through or near the bottom of the enclosure should not interfere with the installation or operation of the equipment or the cooling system.  Internal wire routing should not restrict equipment air flow, intake or exhaust.  It should also not restrict the air flow of the cooling system.  Check the National Electrical Code and any other codes that may apply to the wiring installation.

  1.  Select an enclosure of the proper dimensions and material construction for the environment.

An enclosure should be of the proper dimensions to house all the equipment in the enclosure, support the weight of the equipment, and provide proper spacing per equipment manufacturer’s specifications.  This includes recommended clearance for air flow.

Allow space for internal wiring conduits and raceways that will not restrict air flow.  Consider NEMA or IP seal protection requirements for the environment.  Good design also includes space above and below the equipment group at the top and bottom of the enclosure for air circulation.

  1.  Determine the thermal requirements for the enclosure.

Review all equipment specifications for operating temperature ranges and heat dissipation.  Internal enclosure design temperature should satisfy the equipment with lowest high temperature requirement of all equipment operating ranges.  Designing for this maximum temperature will thermally satisfy all other equipment.

If the enclosure is going into a cold environment, you may also want to note the highest low temperature requirement of all equipment operating ranges.  Designing for this minimum temperature will thermally satisfy all other equipment.  In many cases, the equipment heat dissipation will be sufficient to satisfy this thermal requirement.  However, in some instances a heater may be required as part of the thermal management system.

The internal heat dissipation will be required for all equipment.  This will be totaled and used as part of the calculation for the thermal management equipment.  Heat dissipation may be listed in BTUH (British Thermal Units per Hour) or Watts.

The thermal management system calculation will also require an enclosure ambient maximum temperature and a minimum temperature.  This will be determined by the location of the enclosure.  Units outdoors in direct sunlight will also receive solar radiation in the form of heat.  A stainless steel enclosure finish or a light colored paint will help to minimize the solar heat load.  In cases where there is a large temperature differential between the maximum outdoor design temperature and the maximum enclosure design, you may want to consider thermal insulation on the enclosure interior surfaces.  Both the solar reflection and insulation may result in smaller, less expensive thermal management system.  You may contact Ice Qube at 888-867-8234 or for assistance with the calculation and thermal management system selection.

  1. Select the proper thermal management system for your application.

There are many types of cooling system designs including ventilation type fan packages, heat exchangers, thermal electric and compressor driven air conditioners.  Which best suits your application will be determined by the amount of cooling and the temperature relationship between the outside ambient temperature and internal enclosure design requirements. If the ambient outside temperature is a few degrees less than the internal enclosure design, a fan system or air to air heat exchanger may be selected.  These systems typically use less power than other type systems.

If the outside ambient temperature is greater than the enclosure design, a thermal electric or vapor compression type system should be selected.  These type systems have the ability to maintain the temperature of the enclosure below the ambient temperature.  They also have the ability to lower the moisture content of the air or relative humidity.

After a system suitable for the application requirements has been selected, check that the air flow of the system will not be restricted when mounted onto the enclosure.  Repositioning of the thermal system or the equipment inside the enclosure may be necessary.  Internal diverters can also be used to assist in directing air flow inside of the enclosure.  Manufacturers of these systems also have a variety of models that provide various air flow patterns.

Also, the thermal system’s air flow outside of the enclosure must not be restricted.  Air flow restriction can impact the rated capacity of the cooling equipment. Again, repositioning or relocation of the system may be required.

In summary, many factors should be considered during the design stages of an enclosure application, including what the future may bring.  All of these considerations at the design stage will lead to an enclosure installation that will provide few if any surprises during the system integration and commissioning.  It will also provide an environment for the equipment inside of the enclosure that will provide many years of satisfactory and reliable operation without service interruption.