When it comes to cooling enclosures, it's time to think inside the box.
Edited by Miles Budimir
Today's industry mantra is to design products that are smaller, faster, and cheaper. A by-product of this miniaturization is an increase in heat density — more heat, less volume, and often both.
High temperatures as well as temperature fluctuations can stress sensitive electronic equipment. The failure rate for such equipment increases with peak temperatures above 122°F (50°C), heat pockets, condensation, and conductive dust.
One rule of thumb says the average life span of semiconductors is cut in half with a 20°F or 10°C increase in operating temperature over the maximum operating temperature. Yet high temperatures in enclosures can hardly be avoided because electronic equipment such as transformers and drives generate heat in the form of power loss.
Get the heat out
All of this adds up to one thing: the need to get the heat out. In principle, heat can be dissipated through conduction, convection, and radiation. In conduction, heat transfers from object to object through direct contact. Heat transfer by convection uses air or a liquid as a medium. It absorbs and releases energy in the form of heat. Natural convection transfers heat by relying on density changes to cause fluid motion, whereas forced convection requires a pump or fan to move the fluid. Finally, radiation transfer of heat takes place via electromagnetic waves, the best example being the sun heating the earth. Conduction and convection play an important role in enclosures and electronic cabinets. Radiation is not a big factor.
An important criterion for heat removal from enclosures is whether the enclosure is an open (air can move freely through) or closed (airtight) system. While heat naturally dissipates from the inside of an open enclosure through a flow of air, heat can only be dissipated from a closed system through the walls or roof.
Another important consideration is the total exposed surface of the enclosure. Together with the location of the enclosure, these two factors determine the amount of heat dissipated. So for instance, an enclosure in the middle of a room can dissipate or absorb more heat than one placed next to a wall or in a corner.
Considering these factors early in the design process ensures proper selection of a cooling device best suited for the environment. Many machine manufacturers offer several cooling options depending on the machine's install location. A common mistake is waiting until the overall product design is almost complete before considering its cooling. This results in trying to squeeze a solution into an allocated area of the enclosure. The cost of field retrofitting a problem application later is greater than the extra time put into the design process on the front end of the project.
A basic calculation goes like this: The total heat load equals the heat loss from electronics minus convective heat transfer through the enclosure walls. This gives the total heat load to be removed. Knowing an internal required temperature and maximum ambient temperature then lets you determine which cooling options are possible.
When sizing a cooling option for an enclosure, consider:
• Maximum temperature at the enclosure at the warmest time of the year. Common questions asked during the sizing of enclosure cooling include: What is the heat loss from the electronics inside the enclosure? What are the maximum ambient temperature and the required internal enclosure temperature? What is the size of the enclosure?
To answer these questions, first evaluate the temperature at the site for the control enclosure. One of the most common selection and sizing errors is to underestimate the temperature at the enclosure site during the warmest months of the year. Enclosures located near local heat sources such as paint ovens, furnaces, and plastic molding machines can experience ambient temperatures as high as 140°F during summer months.
Even industrial air conditioners do not provide sufficient cooling when the ambient exceeds 130°F. This is because the refrigerant coming out of the compressor is around 150°F. As the ambient goes above 130°F, there is too small a temperature difference for the refrigerant to condense. These cases require considering air-to-air or air-to-water heat exchangers or vortex coolers.
Generally, an internal enclosure temperature of 90 to 100°F is targeted. This rule of thumb holds true for enclosures housing common electrical components such as drives and transformers which are usually rated up to a 104°F operating temperature.
• Cleanliness of the ambient air Although filter fans offer the advantage of low cost and small footprint, they rely on regular maintenance of the filter to maintain effectiveness. In dirty environments, the filter may require weekly maintenance.
Air conditioners offer the advantage of two separate air cycles (closed-loop cooling) with a dusttight seal between the ambient and internal air cycles. However, air conditioners must have either 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 the advantage of closed-loop cooling. Generally, they need less maintenance than air conditioners because the wide-channel spacing in the heat exchange cassette prevents trapping dust inside the heat exchanger.
Vortex coolers use compressed air piped in from a clean part of the factory, along with an in-line filter to reduce the chance of dirt entering the enclosure and help reduce filter maintenance.
Air-to-water heat exchangers offer the lowest maintenance of these systems because no ambient air enters.
Finally, in areas where 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-level requirements Another question to consider is how difficult or costly is the maintenance? In remote locations or areas where maintenance costs are high, a low-maintenance design can save a significant amount. A strategy becoming more common in the automotive industry is to use air-to-water heat exchangers to cool individual enclosures in conjunction with a central chiller. Air-to-water heat exchangers need essentially no maintenance, and one chiller requires less maintenance than multiple air conditioners. It's also critical to design products with maintenance-friendly features such as easily accessible fans and filters, quick plug style connectors, and easily accessible coils or heat exchange cassettes.
A practice that helps ensure regular maintenance mounts the cooling device in an enclosure location so those responsible can work on the unit without kneeling or using a ladder.
• Machine operator concerns is an area often left unevaluated. In some environments, noise becomes a consideration, particularly if machine operators work near the cooling device. Strategies include lower noise cross-flow-style blowers and fanspeed controllers so that 100% rpm (and noise) only occur when needed. Avoid the use of Vortex coolers in noise sensitive areas. Of course, design the location of the cooling device so an air discharge is not directed toward the operator.
• Airflow obstructions can hinder a cooling unit's performance. Most cooling devices using fans require 6 to 8 in. of free space at the fan air inlet and outlet to work effectively. In space-restricted areas, it may be more effective to use an air-to-water heat exchanger or a Vortex cooler. Airflow paths inside the enclosure also need careful consideration to ensure even cooling inside the enclosure and avoid air short cycles. Short cycles occur when the air does not fully circulate throughout the enclosure. In the case of an air conditioner, air is deflected from the exhaust of the unit into the return without fully circulating throughout the enclosure.
• Hose-down requirements call for special treatment. Hose-down refers to any hosedirected spray near the cooling device. In food-processing and other environments that require NEMA-4X enclosures, the cooling device must be protected from the spray and mounted to ensure an environmental seal between the cooling device and the 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 require special consideration because cooling devices may be exposed to freezing temperatures, rainfall, or blowing dust. Most cooling devices, even filter fans, can be used in outdoor environments with appropriate upgrades. For air conditioners, this includes rain covers, polyurethane coatings on critical components, and possibly modifications for low ambient operation should a compressor need to operate at ambient temperatures below 45°F. Many heat exchangers can operate outdoors without special upgrades. Rain covers are available for filter fans used outdoors. However, in dusty environments, filter maintenance for the fans can be a drawback.
• Monitoring cooling device performance means keeping an eye out for a failure. This area has increasingly grown over the last few years. As part of the overall system reliability, it is important to develop strategies for responding to cooling system failure. One tactic uses the thermal sensors in a UPS 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 provide diagnostic signals indicating fan, compressor, or sensor failure, or even a clogged filter. These signals can then be integrated into the machine controls to provide the operator with a message pinpointing the nature of the problem, thereby speeding up the repair. For instance, fan rpm can be monitored and an alarm signal produced when the speed drops below a critical value.
Qe = total power loss of electronic equipment (W) Qs = heat dissipated or absorbed through enclosure surface (W) Qt = required cooling capacity or amount of heat to be removed
(W) Ti = maximum internal enclosure temperature (°C) Ta = ambient temperature (°C) V = air displacement of filter fan (m3/hr)
A = exposed enclosure surface area (m2)
k = enclosure heat-transfer coefficient (W/m2•K)
sheet steel ± 5.5 W/m2•K
plastic ± 3.5 W/m2•K
Basic cooling calculations
Air conditioners and filter fans account for 90% of all enclosure cooling devices. Consequently, a few basic calculations can help in the sizing process.
The basic equation for cooling capacity required of an air conditioner 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 the 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 sheetsteel 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.