Over the past few decades, the use of sophisticated, high density electronics within automation and process control panels has become commonplace. With electronic enclosures, effective thermal management and its related costs are important considerations in managing these valuable assets.
Understanding the panel equipment manufacturer's specifications regarding the maximum allowable operating temperature is the starting point in the selection process for enclosure cooling. Most modern electronics, including common devices such as variable-frequency drives (VFDs), programmable logic controllers (PLCs), transformers and relays, are designed for internal panel air temperatures between 104 and 122°F (40 and 50°C). Common sense dictates maintaining the panel temperature at or below the lowest required operating temperature.
But how do you maintain a tightly packed enclosure at the appropriate temperature in a relatively warm ambient environment? Some methods used cannot be recommended for some or even most applications. These include:
- Measures such as cutting holes in the panel and installing filtered fans. Filtered fans can work in clean environments, but few industrial settings are so contaminant-free that this is a practical approach for many. Moreover, it cannot be used for outdoor installations. If the filter is dense enough to prevent the entry of contaminating dirt and moisture, it will quickly clog and become a preventive maintenance headache.
- Leaving the panel open on unusually hot days to provide ambient air cooling. While this may temporarily resurrect an overheated device, this approach ensures that dirt, oil, corrosive moisture and other hazards will attack the electronics. Such attacks shorten the operating lives of the electronic components. In addition -- and no less important -- leaving the panel open creates OSHA safety issues.
It is a classic Catch 22: combat heat damage or battle dirt and moisture. Without some form of cooling, problems or even complete failures will occur due to heat buildup within these enclosures if the heat load is beyond the ability of the cabinet's natural convection cooling to dissipate.
Enclosure Cooling OptionsConsider some of the most popular enclosure cooling choices as well as some of the factors that make them more or less suited for a given application.
Thermoelectric Devices. These solid-state air-conditioners provide effective cooling but require 1 W of power to remove 1 W of heat. Typically, they are used only in small cabinets due to the energy costs.
Compressed Air Coolers. These devices use plant air to create a cyclone effect that cools the cabinet interior. The cost of a compressed air supply should be considered as well as whether the compressed air is dry and oil-free.
Air-Conditioners. When electronics first made their way into the plant, air-conditioners had to be used because the low thermal thresholds of early electronics required that the devices inside be "refrigerated" to below ambient conditions. As a result, air-conditioners are a frequently used panel cooling method. Today, however, components are made to withstand much more heat without harm or performance de-rating, and they do not require refrigeration. Because air-conditioning units cost more to install and maintain -- and use far more energy than most other options -- where possible, they are being replaced by alternative cooling solutions.
Heat Exchangers. Air-to-air and air-to-water heat exchangers also can be used for enclosure cooling. Benefits of heat exchangers include low power consumption and long life. They can be used in new panel installations and retrofits.
More on Heat ExchangersAir-to-water heat exchangers are a popular choice. The water industry has a built-in advantage in that it can easily employ available sources of water (typically under 2 gal/min) for use in air-to-water panel coolers. Unfortunately, relatively few other types of facilities have easy access to a water supply for this type of cooling.
These devices cost pennies on the dollar per BTU/hr compared to all other methods and are the most "green" and environmentally friendly solution as well. Air-to-water heat exchangers are closed designs, completely reliant on water temperature for their cooling effectiveness, and they do not have fans or fins that require filtering in dirty environments. They are also a natural fit for hazardous location installations: with a purged panel, standard muffin fans -- rather than explosionproof versions -- can be used.
Because air-to-water exchangers do not have active cooling, the cooling effectiveness relies on the water temperature, as previously noted. However, air-to-water units can address even large heat loads using "ground temperature" water in the 65°F (18°C) range. Also, the clean but heated outlet water can be reused somewhere else in the process, thereby taking advantage of the heat energy it carries.
An alternative is the heat-pipe-based, air-to-air heat exchanger. This design takes advantage of simple physics to offer high thermal transfer capabilities while consuming very low electrical energy. Air-to-air heat exchangers can perform at their full rated level for as much as 15 years, with the only failure point and replacement cost being the electric muffin fans they employ.
According to H.P. "Hank" Van Ormer of Air Power USA Inc, Pickerington, Ohio, an independent compressed air system consultant, "Open blow, refrigeration and compressed air cooling may be replaced with ‘heat pipe' heat exchangers with a potential energy savings of 3.5 to 4 kW each on an average cabinet. The initial [equipment investment] cost … offers a resultant power savings of $1,000 to 2,000 per year each."
The Achilles heel of air-to-air heat exchangers is their reliance upon ambient air temperature. In high ambient conditions where an insufficient panel-to-ambient temperature differential (∆T) exists for effective cooling, a below ambient solution (thermoelectric device, compressed air cooler or air conditioner) is a necessity.
Whatever your requirements, look at the long-term energy and maintenance costs as well as initial purchase price when evaluating your panel cooling alternatives. The initial cost is known, but you should probably assume that operating and energy costs will only continue to climb. Give them serious consideration when making your decision.