The Cabinet Cooling System Choice: How to Ensure Success

The best time to consider cabinet cooling system options is during the cabinet design stage. Crucial decisions that affect heat load and methods of cooling can then be made in conjunction with equipment selection and cabinet layout. It's also the correct time to evaluate the implications of the cabinet's location and to identify environmental constraints.

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The careful evaluation of all these factors helps ensure the successful choice of the cabinet cooling system.

Cabinet Size and Shape Affects Cooling Design

The type of cooling solution has a bearing on equipment layout. With fan cooling, equipment that generates heat should be placed near the bottom where the air is cooler. But if a side-mounted cooling solution is used, hot equipment should be placed next to the cool air inlet.

Consequently a wide, short cabinet may be better for fan cooling, whereas a tall and narrow cabinet is more suited to side-mounted cooling. Also, take into consideration that the greater surface area of a large enclosure will have better heat conduction properties than a small enclosure.

Assess the Heat Load

Calculate the cabinet heat load by adding together the heat generated by each piece of equipment. To calculate the heat generated by power conversion equipment like variable frequency drives, inverters, rectifiers and transformers, subtract the device efficiency from one (1-Efficiency) and multiply this by the input current times the input voltage. You can reduce the total heat load considerably by choosing efficient power conversion equipment.

If the cabinet is outdoors, solar radiation will contribute towards the total heat load. Try to minimize this by shielding the cabinet.

Identify Ambient Conditions

Establish the ambient conditions at the location where the cabinet is to be installed. Key considerations include:

• Maximum ambient temperature: Establish the highest ambient temperature recorded for that location by consulting local climate data, if outdoors. If indoors, identify heat sources and, if possible, measure the air temperature.
• Dust and dirt: Is the location excessively dusty?
• Risk of corrosion: Are corrosive vapors, including salt spray, or chemicals present?
• Water: Will the enclosure be sprayed, washed down or subject to excessive splashing?
• Humidity and dew point: In humid locations, condensation may occur inside the enclosure when the temperature falls below the dew point; check to see if this is a possibility.

These factors will largely determine what cooling options are viable and help establish the required NEMA rating of the cabinet.

Select the Enclosure Temperature

Choose a maximum enclosure temperature that's lower than that of the most sensitive equipment. It's wise to adopt a conservative approach and increase the margin of safety by selecting an enclosure temperature that's several degrees below the maximum temperature; 95 °F is a good choice for most electrical equipment.

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Enclosure Temperature Management Calculation

It's now possible to calculate the cabinet cooling system capacity using the information that's been collected. Preferably use an online Enclosure Temperature Management (ETM) calculator as this method of calculation has proven to be simpler and more accurate than manual calculations. If you are unsure which is the best cooling method, repeat the calculation for all three cooling options; Air Conditioners, Heat Exchangers and Filtered Fans.

Cabinet Cooling System Choices

After identifying the viable cooling solutions, the next step is to decide between open loop and closed loop cooling:

Open loop cooling:

Open loop cooling draws outside air into the enclosure using filtered fans  and is most suitable for NEMA Type 1 and Type 3 ventilated enclosures that are not subject to excessive dust, heavy splashing or water spray. An optional 3R Fan Shroud Kit can be specified for outdoor use. Always bear in mind that fans only remove heat and cannot cool the enclosure below the ambient air temperature.

Closed loop cooling:

With closed loop cooling, the enclosure cooling circuit is completely separate from the external air cooled circuit. The two options are air to air heat exchangers and enclosure air conditioners. An air to air heat exchanger is a good choice for moderate heat loads and its only limitation is that, like cooling fans, it cannot cool below the ambient air temperature. If the ambient temperature is higher than the required enclosure temperature or the heat load too large, an enclosure air conditioner is the best choice. Another benefit of an air conditioner is that it can be equipped with an optional heater package to prevent internal condensation in cool, damp weather. Air conditioners and air to air heat exchangers can be supplied for NEMA Type 12, 4 or 4X enclosures to suit all environmental requirements. Heavy-duty filters are also available to provide added protection to air conditioner condensers in extremely dusty conditions.

The Recipe for Success

Most enclosure cooling problems can be avoided by incorporating the cabinet cooling system design into the initial design phase. Benefits include the ability to select the best cooling option and the optimum internal layout; these can be drawn up before the detailed panel arrangement is completed to suit cooling airflow requirements and ensure a uniform internal temperature profile. 

When cooling an electrical enclosure, airflow is an important factor to consider. However, to add to the complexity, different types of thermal management products use air differently. This post describes a "must-know" concept that will ensure success for your cabinet cooling application. In order to make this distinction, it's important to first know the difference between open-loop and closed-loop cooling, and understand why one system may be the better option for a specific application.

1. Open-Loop Cooling

Open-loop cooling describes a process that replaces the air inside a chamber with 'fresh' cooler air from outside the chamber. Open-loop cooling is the most commonly used process when the available air supply is cool enough and clean enough to provide the required heat removal. Open-loop ventilation uses ambient air to remove the heat, and may consist of small muffin-type fans that exhaust or supply an electrical cabinet, with optional filters to prevent airborne aerosols and dust from entering the cabinet. Enclosure cooling fans have the advantage of utilizing a minimum amount of cabinet space and will move a substantial volume of air where flow is virtually unimpeded. Cost and complexity is minimized with this type of ventilation process. Where density of components impedes airflow, packaged blowers or motorized impellers may be arranged to operate against these higher static pressures. With a rack enclosure, supplemental fan trays may be used to spot cool or supplement other air-moving devices. See Figure 1 below:

2. Closed-Loop Cooling

Closed-loop cooling describes a process that reconditions (reuses) the air inside an enclosure. Air conditioners and heat exchangers are examples of cooling products that cool enclosures using a closed loop system. The purpose of this system is to prevent contaminated air from entering and causing damage to critical components, and when maximum internal cabinet temperatures cannot be maintained using open-loop ambient air cooling, closed-loop systems are used to satisfy this requirement. If the ambient air is harsh, containing dirt, oil, humidity or corrosives, this will adversely affect the performance or ultimate longevity of the components. Many applications using intricate electronic/electrical components require a closed-loop cooling system to dissipate heat buildup without introducing outside contaminated air. Air-to-air heat exchangers, water-to-air heat exchangers and air conditioning units are great options to cool a confined amount of air within an enclosure. Air conditioners have the added ability to maintain a temperature lower than ambient and reduce the humidity within the controlled space. See Figure 2:

In harsh environments involving high temperatures, heavy particulates, oil or chemicals capable of damaging components, ambient air must be kept out of the enclosure. Sealed enclosures are generally used, with closed-loop cooling consisting of two separate circulation systems in a single unit. One system, sealed against the ambient air, cools and recirculates the clean cool air throughout the enclosure. The second system uses ambient air or water to remove and discharge the heat.

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