This article was updated May 3, 2023. It was originally published Oct. 11, 2001.
Engineers are always pushed to design products that are smaller, operate faster and do it all for less money. An unfortunate result of this miniaturization is an increase in heat density—more heat, less volume and often both. This creates a problem for enclosures: getting rid of the heat all those chips create. And any drives and transformers inside enclosures add to the problem due to their inevitable heat losses which create more heat.
In general, electronics begin failing at about 122°F and semiconductors’ lives are cut in half if operating temperatures go 20°F over the chip’s maximum operating temperature.
All of this adds up to one thing: the need to get the heat out.
Keeping it Cool
There are important criteria for heat removal from enclosures. The first is whether they are open to the air or closed and airtight. Heat naturally dissipates from inside open enclosures through air flow. But in closed enclosures, heat can only escape through the walls and roof. Another criterion is the enclosure’s exposed surface area. The more surface area, the more heat that can be dissipated. The final criterion is where the enclosure is located. An enclosure will stay cooler if it is in the middle of a well-ventilated room rather than tucked into a corner or up against a wall.
Many machine manufacturers offer several cooling options for enclosures. A common mistake is waiting until the design is almost complete before considering these options. It can force engineers to try to squeeze a cooling device into an already crowded space. And the cost of fixing an overheating enclosure that is already installed and being used is greater than that of putting in the extra design time on the front end of the project.
When sizing a cooling option for an enclosure, consider:
Maximum enclosure temperature at the warmest time of the year. What is the heat loss from electronics inside the enclosure? What are the maximum ambient temperature and the required internal temperature? And what is the size of the enclosure? To answer these questions, first evaluate the temperature at the site for the enclosure.
One common selection and sizing error is to underestimate the enclosure temperature during the warmest months of the year. And enclosures near heat sources such as paint ovens, furnaces and plastic molding machines can see ambient temperatures as high as 140°F during summer months.
Even industrial air conditioners do not provide enough cooling when ambient temperatures exceed 130°F. This is because refrigerant leaving the compressor is at around 150°F. As ambient temperatures go above 130°F, there is too small a temperature difference between outside and inside temperatures for the refrigerant to condense. For these situations, engineers should look at using air-to-air or air-to-water heat exchangers or vortex coolers.
Generally, internal enclosure temperatures should stay below 100°F. This is true for enclosures housing common electrical components such as drives and transformers which are usually rated up to 104°F operating temperature.
Cleanliness of the ambient air. Filter fans are inexpensive and small, but they need regular maintenance to be effective. In dirty environments, filters may require weekly maintenance.
Air conditioners, on the other hand, use two separate air cycles (closed-loop cooling) with a dust-tight seal between ambient and internal air cycles. However, air conditioners must also have regular filter maintenance or condenser coil cleaning. Air conditioners should always be wired to door switches to avoid excessive condensation from opening and closing doors.
Air-to-air heat exchangers also offer closed-loop cooling and need less maintenance than air conditioners due to the wide-channel spacing in the heat exchangers stopping them from trapping dust.
Vortex coolers use compressed air piped in from a clean part of the factory, along with an in-line filter that lowers the chances of dirt getting in the enclosure and reduces filter maintenance.
Air-to-water heat exchangers offer the lowest maintenance because no ambient air enters.
Finally, if the ambient air is corrosive, critical components must be protected using polyurethane or phenolic coatings. Air-to-water heat exchangers can be fitted with stainless-steel piping for applications with corrosive water, and vortex coolers are available in stainless-steel versions.
Maintenance requirements. Another question to consider is how difficult or costly is maintenance? In remote locations or where maintenance costs are high, a low-maintenance design can save a significant amount of money. One scheme common in the auto industry is to use an air-to-water heat exchanger to cool individual enclosures and have several heat exchangers use a common chiller. Air-to-water heat exchangers need little to no maintenance, and a single chiller requires less maintenance than several air conditioners.
It's also critical to use cooling devices with maintenance-friendly features such as easily accessible fans and filters, quick plug-style connectors and easily accessible coils or heat exchange cassettes. It’s also advisable to mount cooling devices in enclosures such that technicians need not kneel or use a ladder to service them.
Machine operator concerns. This area is often left unevaluated. But in some plants and factories, noise becomes a consideration, particularly if operators work near the cooling device. To avoid this problem, use lower noise cross-flow-style blowers and fan-speed controllers so that 100% rpm levels (and noise) are only used when needed and noise is minimized.
Avoid using vortex coolers in noise sensitive areas; they always lead to significant levels of noise. Of course, mount the cooling device so air discharge is not directed toward the operator.
Avoid airflow obstructions. They can hinder a cooling unit’s performance. Most cooling devices using fans require 6 to 8 in. of free space on the air inlet and outlet to work effectively. If space-is restricted, it may be more effective to use an air-to-water heat exchanger or vortex cooler. Airflow paths inside the enclosure should evenly cool the interior. And avoid air short cycles which arise when air does not circulate throughout the enclosure.
Hose-down requirements. Hose-down refers to any hose-directed spray near the cooling device. In food processing and other applications that require NEMA-4X enclosures, cooling devices must be protected from the spray and mounted to ensure a seal between the cooling device and enclosure.
NEMA-4 upgrades are available for air conditioners, heat exchangers, vortex coolers and thermoelectric coolers. An advantage of air-to-water heat exchangers is the ability to use them in NEMA-4 environments without special upgrades.
Outdoor conditions. Special steps must often be taken to protect cooling devices that may be exposed to freezing temperatures, rainfall, or blowing dust. Most cooling devices, even filter fans, can be used outdoors with appropriate upgrades. For air conditioners, upgrades include rain covers, polyurethane coatings on critical components and modifications should a compressor need to operate at ambient temperatures below 45°F.
Many heat exchangers can operate outdoors without upgrades. Rain covers are available for filter fans used outdoors. However, in dusty environments, filter maintenance for fans can be a drawback.
Monitoring cooling device performance. This means keeping an eye out for a failure and have strategies developed for responding to cooling device failures. One tactic uses thermal sensors in an uninterruptible power supply to notify the machine operator when temperatures reach critical levels. A more direct approach directly monitors the cooling device and provides a signal in the event of failure.
Some air conditioners can output diagnostic signals indicating fan, compressor, or sensor failure, or even a clogged filter. These signals can be added into the machine controls to send operators a message pinpointing the nature of the problem, thereby speeding up repairs. For instance, fan rpm can be monitored and an alarm signal generated when the speed drops below a critical value.
Air conditioners and filter fans account for 90% of all enclosure cooling devices. Consequently, a few basic calculations can help in the sizing process. But first a quick rundown of cooling variables:
Qe is the total power loss of electronic equipment (W).
Qs is the heat dissipated or absorbed through enclosure surface (W).
Qt is the required cooling capacity or amount of heat to be removed (W).
Ti is the maximum internal enclosure temperature (°C).
Ta is the ambient temperature (°C).
V is the air displacement of the filter fan (m3/hr).
A is the exposed surface rea of the enclosure surface area (m2).
k is the enclosure’s heat-transfer coefficient (W/m2•K); for sheet steel k is ± 5.5 W/m2•K; for plastic it is ± 3.5 W/m2•K.
The basic equation for an air conditioner’s cooling capacity is:
Qt = Qe –kADT
where DT = Ti– Ta.
Assume a single, freestanding enclosure 2.5 m high, 0.9 m wide, and 0.75 m deep. From an enclosure surface area factor table, the effective surface area is 8.4 m2.
An ambient temperature of 55°C and a required internal enclosure temperature of 40°C gives a DT of –15. Assume that heat loss, Qe, is 700 W, and k is 5.5 for a sheet-steel enclosure.
Plugging into the cooling capacity equation yields: Qt = 700 –(5.5)(8.4)(–15) Qt = 1,394 W.
To convert W to Btu/hr, multiply by 3.413. Therefore, choose an air conditioner with a minimum cooling capacity of 1,394 W or 4,760 Btu/hr.
For filter fans, the basic equation is:
V = f(Qt/DT)
where f is the filter-fan factor.
For example, suppose that the ambient temperature is 20 °C, the required enclosure temperature is 40°C, and the heat loss is 700 W. At 200 m above sea level, f is 3.2 m3K/Wh. Therefore, the required volume of air displacement is:
V = 3.2 (700/20) V = 112 m3/hr.
Felix Klebe was a product manager for climate controls at Rittal Corp. when this article was originally published.