In the automation and process industries, the complications that arise from improper enclosure temperature control are significant. Aside from the actual expense to repair or replace electrical equipment that breaks down or fails due to enclosure overheating, the costs of unwanted downtime in production also must be factored in.
Fortunately, these issues can be prevented through the use of proper enclosure cooling. Effective enclosure cooling accounts for all temperature and environmental factors that impact the life and operation of electrical equipment. Here are six complications that are easily avoided with proper enclosure temperature control.
1. Short Component and Equipment Life
The service life of electrical equipment is directly related to its operating temperature. It has been shown using the Arrhenius equation that, in simple terms, the life of electrical equipment is reduced by half for every 18°F (10°C) temperature rise.
A good example of how this affects modern equipment is to consider the impact on a variable-frequency drive (VFD). The maximum allowable operating temperature of many VFDs is 104°F (40°C). At this maximum temperature, service life has most likely already been affected. Considering it is not uncommon for the temperature in a poorly ventilated enclosure to exceed 125°F (52°C), the lifespan of a VFD, in this case, would be less than a few years.
2. Breakdowns due to Internal Enclosure Heat Loads
Three factors determine an electrical enclosure’s temperature:
- Ambient temperature.
- The rate of heat removal capable by the temperature control system.
- Heat load.
Heat load is a representation of the total heat generated inside the enclosure.
Three primary sources of heat contribute to total load:
- The heat generated by the equipment.
- Heat that transfers between the enclosure and the air outside the enclosure.
- Heat from solar radiation if the enclosure is outdoors.
Internal heat is mostly generated from the heat loss of the electrical equipment in the enclosure. For equipment such as VFDs, the heat generated can be calculated from the efficiency of the drive. This varies between 95 and 98 percent, and a 10 kW drive with an efficiency of 95 percent will generate 500 W of heat.
The same philosophy applies to transformers and power supplies. For control equipment such as relays, contactors and PLCs, it is safe to assume that all of the electrical energy used is converted into heat. To figure the efficiency of the drive, check the manufacturers’ catalogs.
Assuming the ambient temperature is hotter than the enclosure, heat also flows into the enclosure from the environment. Naturally, if the enclosure is warmer, heat will flow out of the enclosure. The amount of total heat transferred internally from outside the enclosure is determined by the difference in temperature, the surface area of the enclosure, insulation and the finish, color and material of the enclosure. Also, if the enclosure is exposed to the sun, solar heat must be taken into account.
The total heat load is the summation of all these factors and can be manually calculated. However, it is equally effective and less time-consuming to use online tools available from temperature control manufacturers that calculate the enclosure’s cooling requirement based on all these elements.
It is essential that the enclosure’s temperature control system has the capacity to remove this heat, or the enclosure temperature will increase until a thermal equilibrium is reached. Without cooling, that equilibrium point is more than likely above the equipment’s safe operating temperature.
3. Electrical Equipment Failure due to Dusty Applications
The negative impact of dust on electrical equipment deserves attention due to the growing number of enclosures located in remote areas or near debris caused by processing and production. This is not a serious issue if the equipment is properly sealed, but for technical reasons, many pieces of equipment are not sealed. A good example is the moving contacts on electrical contactors. Although protected, the contacts are open to the environment.
Research shows that dust on electrical contacts causes an increase in contact resistance. At mains voltages, this inevitably leads to overheating of the contacts, causing them to burn out and fail. At lower DC control voltages, dirt on contacts prevents the contacts from closing, leading to an open circuit.
In a dusty environment, it is essential that the electrical enclosure is at least partially sealed and fitted with filtered fans. Highly sensitive or critical electronics can be installed in a sealed NEMA 12, 4 or 4X enclosure fitted with a closed-loop enclosure temperature control system.
4. Effect of Ambient Temperature on Cooling Capacity
In a location where the external ambient temperature is higher than safe operating levels, the only practical method of maintaining the enclosure’s internal temperature is to use an active cooling system such as an enclosure air-conditioner.
An often-overlooked component of temperature control is the relationship between cooling capacity of the air-conditioning unit and the ambient temperature. Ambient air is used to cool the refrigerant in the condenser coil of an air-conditioner, so the cooling capacity is dependent on the temperature of the ambient air. If an air-conditioner’s nominal capacity is calculated at an ambient temperature of 104°F (40°C), the capacity at another temperature will be different. If the ambient temperature is higher, the capacity will be lower. Therefore, it is necessary to consult the air-conditioner performance curves to establish the actual capacity at ambient temperature.
The effect of ambient temperature is significant. An air-conditioner’s capacity at 125°F (52°C) ambient temperature can be 70 percent of the capacity at 104°F (40°C). If not taken into account, lower available cooling capacity will result in excessively high electrical enclosure temperatures that lead to equipment failure.
5. Unplanned Maintenance Caused by High Electrical Enclosure Temperatures
The combined effects of high electrical enclosure temperatures and dust collecting inside the enclosure are such that electrical breakdown and failures are inevitable.
The first sign of such a problem will be unexplained tripping of equipment such as VFDs, power supplies and circuit breakers. Another possible sign is the incorrect operation of control circuits that produce unpredictable results. This is especially noticeable in high speed industrial communication circuits.
Eventually, more sensitive pieces of equipment will fail, leading to hard-to-detect problems and unplanned downtime.
6. A Halt in Production
Along with unplanned maintenance, there can be a cessation of production, and it is not uncommon for the impact of these halts to be severe. Additionally, the failure of critical equipment may have a cascade effect and cause subsequent damage of electrical and mechanical equipment.
Thermally induced failures are unpredictable. Often, by the time a specialist is called, they have frequently disappeared — only to reappear once the plant is back in production. In fact, when unexplained equipment failures occur, the possibility of electrical enclosure overheating should be one of the first things to be examined.
High-speed servo drives are particularly susceptible to this type of failure, which is often time consuming and difficult to resolve.
When specialized equipment fails, there may be long delays in obtaining replacement parts. They could vary from several days to as long as a month, which may mean extended plant downtime.
Don't Risk It, Install Enclosure Cooling
It is a given that modern electrical control equipment tends to have relatively low maximum operating temperatures. In fact, they are generally much lower than that of old-fashioned electromechanical control equipment. A good rule of thumb is to design an electrical enclosure for a maximum temperature of 95°F (35°C). Keep in mind that during summertime, the maximum outdoor temperature throughout most of the United States gets pretty close to 95°F (35°C). Some areas are even hotter.
In such situations, an effective solution is to install an enclosure air-conditioner. A properly sized air-conditioner ensures that a temperature of 95°F (35°C) is maintained within the enclosure under all conditions. As a further benefit and precaution, a closed-loop cooling solution will prevent the ingress of dirt, dust and debris into the enclosure and help ensure reliable operation.