Some electrical and electronic devices are designed with housings and fittings so that they can be installed in a standalone manner almost anywhere, but such devices are the exception rather than the rule. Most electrical and electronic components must be installed in an enclosure of some type to protect them from temperature extremes, moisture, and other harmful conditions.
Certainly, damage due to mechanical reasons, vibration, contaminates, and more is always a possibility. But defending electrical and electronic systems from high (and low) temperatures and any amount of moisture are typically key concerns. Design engineers, therefore, need to select the right devices for controlling temperature and implement measures to address both temperature and moisture issues. This article examines the challenges and identifies a variety of thermal management solutions.
Protecting the interior of an enclosure from elemental extremes is the primary issue, but it is equally important to control the interior climate at an optimal and consistent level. Environmental controls should perform their duty and must be instrumented so that users receive an alarm if conditions are becoming problematic.
High temperature is an obvious problem for most electrical or electronic systems. Excessive heat can be generated by the electronics themselves, or from outside influences like sun exposure. Overheating may result in failed circuitry and overcurrent protection trips, and it will typically shorten the lifespan of components. Low temperature is usually much less of an issue, but it is associated with moisture accumulation, which can cause serious problems.
This is because low interior temperatures can result in moisture condensing within an enclosure, leading to droplets of moisture and the potential for dripping and accumulation. Too much moisture can lead to corrosion and arcing faults, both of which will cause failures.
Another possible situation is where an application may require both heating and cooling depending on the time of day. This can be a consideration in areas where there are temperature fluctuations, such as exposed areas subject to nighttime lows but daytime direct sunlight. In this case, both cooling and heating must be designed to work together to maintain the ideal conditions within the enclosure.
Applying devices and design strategies to control electrical and electronic enclosure interior temperature conditions is generally called “thermal management.” Superficially, this seems straightforward: Install a sufficiently sized heating/ventilation/air-conditioning (HVAC) system for each enclosure. Sometimes, such as for large data center installations, this is exactly the approach that is used.
But as a practical matter, adding HVAC to many types of electrical and electronic enclosures is impractical because:
- The HVAC hardware is expensive to install and maintain.
- HVAC equipment and ducting consumes significant working space.
- Energy operating costs are high.
- Many installations use dozens or hundreds of smaller enclosures, which can’t be effectively conditioned with HVAC systems.
Design engineers, therefore, really need compact, effective, energy-efficient thermal management solutions, which can be installed on a targeted basis as needed.
Most people are familiar with basic heaters, air conditioners, and associated thermostats like those used in cars and homes. Each of these devices, or industrialized versions, can be used for conditioning control panels. But there are other more specialized devices for the service (Figure 1).
Here is a rundown of thermal management devices for enclosure temperature and moisture control:
Air conditioner: Air conditioners for enclosure cooling function are like consumer versions, but they are packaged in compact housings rated for NEMA 12, 4, and/or 4X environments. They typically use on-board controls and maintain closed-loop air circulation within the enclosure, so no extra moisture or contaminants are introduced inside (Figure 2). However, these devices consume significant power, take up lots of space, and require periodic service.
Heater: Heaters for control panel usage are most often electric-resistance type and may or may not include controls on-board. Design engineers need to ensure these heaters are touch-safe and mounted in the lower area of the enclosure.
Circulating fan: Air conditioners and some heaters will include a circulating fan to move air within the enclosure and promote heat transfer. However, users can also install separate directional circulating fans to assist. Another option, if it is acceptable for outside air to enter the enclosure, is to install filter fans (or passive louver plates) that pull cooler outside air into the enclosure and exhaust hot inside air.
Thermostat: Some heating and cooling solutions include on-board digital or electro-mechanical thermostats for temperature control, but those that don’t will need a standalone thermostat wired to perform this function. Thermostat contacts can also be monitored by a supervisory control system for indication and alarming purposes.
Thermoelectric cooler: Thermoelectric coolers use the Peltier Effect to perform cooling, instead of a compressor like an air conditioner, so they have fewer moving parts and require virtually no maintenance.
Vortex cooler: Vortex coolers use pressurized air and separate the flow into hot and cold air streams (Figure 3). The cold stream is applied to the enclosure interior. Vortex coolers can be a compact cooling method where larger devices will not work, and they are exceptionally reliable and suitable for operation in harsh environments.
Heat exchanger: A heat exchanger is a closed-loop system that simply exchanges heat inside an enclosure with a cooler outside environment. Heat exchangers have a low operating cost and are useful when enclosures contain heat-generating devices, and when the enclosure is installed in a larger conditioned space.
Hygrostat and hygrotherm: Moisture can build up in any type of enclosure, even with proper environmental ratings, because it can be introduced when the cover is open, or it might intrude via conduits. Enclosure heaters can prevent the formation of condensation, but only if they are controlled to maintain the relative humidity below 60 percent, which is possible if the enclosure interior is maintained at a temperature as little as 9 °F higher than outside ambient air. A hygrostat (also known as a humidistat) senses the relative humidity and can keep it within prescribed limits. A hygro-therm performs this task, and it also performs a dual function by acting as a thermostat.
Selection and Sizing
Design engineers need to gather information about the intended installation to perform the necessary selection and sizing steps. This requires:
- Evaluating the enclosure contents, how much heat is internally generated (in BTU per hour or watts), and at what temperature ranges those components are rated to operate.
- Determining the outside ambient temperature range and if there are additional heating factors (especially a heat load from sun exposure).
- Understanding the availability of utilities like electricity and/or compressed air.
- Selecting the solution(s) and sizing them.
Some solutions, like air conditioners and thermoelectric roolers, can actively remove moisture from the enclosure, so there may need to be provisions for a condensate drain. Economics plays a role in terms of upfront installation cost, ongoing operational energy cost, and future maintenance needs. Technologies with more moving parts may perform well but will typically have a higher lifetime total cost because they may fail at some point. Simpler technologies like vortex coolers are very reliable but may consume a significant amount of compressed air.
Qualified manufacturers and distributors of thermal management products will have sizing tools and support staff available to help design engineers choose the right options for their applications. They can take advantage of these resources to find the best technical and most cost-effective solutions for providing thermal management that protects electrical and electronics installed within enclosures from the effects of temperature and moisture.
Jim Krebs is a Technical Marketing Engineer at AutomationDirect (Cumming, GA). For more information, visit here .