Electrical equipment is almost always specified by manufacturers for operation within an optimal temperature range. That range is defined by the conditions that will ensure the equipment’s reliability, performance, efficiency and physical integrity are protected. At higher temperatures, the efficiency of equipment decreases, and component damage or failure may result. For instance, a modern variable-frequency drive (VFD) typically has an efficiency of 93 to 97 percent and is designed for operation below 104°F (40°C). For every degree increase above the maximum intended operating temperature, efficiency is de-rated by 2 percent.
In addition to an increased risk of equipment failure, high operating temperature can result in long-term costs due to decreased reliability and lifespan. For a business, this could mean unexpected costs associated with frequent maintenance, component failure, downtime for spare parts, and, in some cases, replacement of the entire system. Fortunately, this added cost, which can range from tens of thousands to hundreds of thousands of dollars, can be avoided with a little foresight that includes a temperature control system.
Temperature-Related Causes of Electrical Equipment Damage
The lifetime and failure-free operation of electrical equipment is strongly dictated by environmental conditions and the mode of operation. Each component or type of electrical equipment has its own maximum operating temperature as specified by the manufacturer.
In many process applications, high operating temperature can increase the potential hazards to equipment operation. Some of the common risks associated with high internal heat are:
- Unexpected equipment shutdown.
- Deteriorated equipment performance.
- Equipment damage.
- Shortened equipment life.
- Fire hazard.
Unexpected Equipment Shutdown. Some manufacturers program equipment components to toggle off at high temperatures.
Deteriorated Equipment Performance. High temperature can cause erratic equipment performance and lead to undesirable results.
Equipment Damage. Equipment operating beyond its manufacturer-specified operating temperature can become damaged, requiring repair or even replacement. PLCs and electrical loop controllers do not have high temperature tolerance.
An efficient temperature control system improves reliability and lifespan of the enclosure system by controlling the temperature within the enclosure to meet the operating temperature requirements of the most critical component.
Shortened Equipment Life. Constant exposure to high temperature accelerates equipment aging. Components work harder at high temperatures and, thus, experience more wear and tear, resulting in a shortened equipment life.
Fire Hazard. Rare, but not uncommon, excessive heat inside an enclosure could result in melting of the equipment parts and even fire in some cases.
Most manufacturers emphasize the effect of temperature on their equipment by including a performance de-rating factor for high temperature operation. As a result, identifying the most critical component and controlling the temperature to match the component’s operating temperature range are important.
Low temperature environments have a different effect on electrical equipment. The two main areas of concern for equipment operating at low temperature are safety and function. The common risks associated with low temperature are:
- Equipment startup issues.
- Material properties.
- Arcing due to condensation.
- Ice formation.
Equipment Startup Issues. Starting motors and variable-frequency drives at low temperatures is difficult because oil and grease lose their lubricating properties.
Material Properties. Low temperatures have a dramatic effect on material properties — particularly plastics and elastomers used in the construction of electrical equipment. Low temperatures can make the materials unsafe and unsuitable for use.
Arcing Due to Condensation. Water vapor in the air condenses at low temperature. Insulators lose their properties at low temperature. The combined effect of conductors in contact can cause arcing issues and increase the risk of fire.
Ice Formation. Formation of ice on the equipment casing increases the risk of short circuiting and breakdown of electrical circuitry.
Humidity: A Consequence of Fluctuating Temperatures
For the normal operation of electrical equipment, manufacturers typically specify a relative humidity of 60 percent (±30 percent) at a temperature of 86 to 104°F (30 to 40°C) without the presence of condensation Though outdoor equipment is more threatened by humidity and condensation, indoor equipment in highly humid conditions also is susceptible to failure without a proper condensate management system. Humidity in the enclosure can be detrimental to electrical equipment, causing corrosion, electro-migration, moisture absorption, short circuiting and premature aging.
Water in contact with the unprotected metallic surfaces of electrical equipment causes corrosion. Corroded joints in sensitive equipment may be irreparable; thus, joints require replacement, ramping up costs. The presence of an electric field and moisture can cause electro-migration that can be seen as the formation of pits and dendrites on metal surfaces in contact with an insulator.
In addition to an increased risk of equipment failure, a high operating temperature can result in long-term costs due to decreased reliability and lifespan.
Polymers used in electrical equipment for insulation purposes lose their properties when they absorb moisture in the air. This can expose the equipment to problems. Moisture in the air (usually mixed with particulate matter) is a good conductor of electricity. When water droplets come in contact with electrical circuits, they can cause short-circuiting or total breakdown of the circuit components. And, humidity increases aging of equipment by increasing the rate of wear and tear of components.
The Generation of Enclosure Heat and Humidity
Temperature control is critical in protecting electrical equipment, but what factors contribute to heat and humidity? What factors should be accounted for in specifications?
The primary source of heat comes from the electrical equipment itself within the enclosure. The second law of thermodynamics puts a limit on the amount of useful work that can be derived from a machine; it follows that no system can operate at 100 percent efficiency. Thus, the remaining energy is used to overcome friction or released into the environment as lost heat. This lost energy increases the temperature of the enclosure air and the temperature of the components of the equipment, creating a cycle of heat generation.
The secondary source of heat load in an enclosure comes from the environment. The common sources of external heat are heat transfer due to ambient temperature, solar heat gain and other equipment located in the vicinity of the enclosure.
When the external ambient air is hotter than the equipment’s operating temperature, heat transfer will cause an increase in the enclosure’s internal air temperature. The rate of heat flow depends on the magnitude of the temperature difference and the enclosure’s material and medium of insulation lining. To put it in perspective, a temperature difference of 15°F between the enclosure air and ambient air can contribute to 5.1 BTU/hr for every square foot of surface area of the enclosure. Assuming a medium-sized enclosure, this translates to almost 22 BTU/hr of additional heat that must be dissipated to the external air.
The incident radiation from the sun can cause an increase in the internal temperature of an outdoor enclosure. The extent to which solar heat gain can increase the enclosure temperature depends on factors such as the location of the enclosure, any insulating coating on the enclosure, the color and texture of the enclosure outer body, ambient temperature and humidity, air composition in the location, altitude and wind parameters. Assuming there is no internal heat from the equipment, to maintain a medium-sized black outdoor enclosure at ambient temperature, 1308 BTU/hr of heat would need to be eliminated. With a gray outer body, only 838 BTU/hr would need to be dissipated.
Indoor and outdoor enclosures commonly are located alongside other equipment. The thermal footprint of industrial equipment such as blast furnaces, industrial ovens and foundry equipment in the vicinity of the enclosure can increase its internal temperature.
Natural Convection vs. Active Cooling
A general misconception is assuming the heat dissipated by the equipment is not significant enough to warrant the installation of an active temperature control system. More often than not, engineers eliminate heat by increasing the size of the enclosure or adding a ventilation system that makes use of natural convection. Although this is the lower-cost option, it is an insufficient method for many applications as equipment evolves to require active temperature control.
There are several impediments to using natural convection as a solution to enclosure temperature controls, including:
- Hot spots.
- Absence of early warning.
- Environmental limitations.
- Mandatory closed-loop cooling.
Temperature control solutions range from simple filtered fans to heat exchangers and air conditioners. The choice of a solution depends on the application itself. Depending on the sophistication of the solution and application requirements, the costs vary.
Hot Spots. Heat inside an enclosure rises to the roof. Without a ventilation system that accounts for this, there will be a temperature differential between regions inside the enclosure. This phenomenon can result in thermal hot spots that can spiral into a cascading temperature increase in these regions. Hot spots can lead to temperature-related equipment failure.
Absence of Early Warning. The absence of an active cooling system makes it impossible to cool the enclosure to below ambient temperatures, which can result in poor equipment performance or failure if the ambient temperature is higher than the specified operating temperature of the equipment. High heat inside enclosures cannot be recognized immediately because natural convection does not use metrological equipment or digital controllers to send out high temperature alarms.
Environmental Limitations. Natural convection relies too much on wind patterns, indoors and outdoors. Consequently, any change in wind pattern can result in insufficient cooling of the enclosure. The reliance on air for natural convection extends to air composition as well. Enclosures located in harsh environments such as petrochemical plants cannot depend on natural convection for efficiency cooling because it can result in contamination of the enclosure. The use of filters with natural convection is limited because there is no fan to actively move air through the enclosure. There is an inherent pressure drop in the flow that is associated with all filters.
Mandatory Closed-Loop Cooling. Some applications require sealed enclosures. In these cases, a closed-loop cooling system is required. Natural convection requires larger enclosures and an effective ventilation system that allows cool, clean and dry air for heat dissipation. Space constraints and extreme ambient conditions may prove unfavorable for natural convection to cool the enclosure.
What a Temperature Control System Can Do
No matter the application, an efficient temperature control system improves reliability and lifespan of the enclosure system by:
- Controlling the temperature within the enclosure to meet the operating temperature requirements of the most critical component.
- Minimizing pollutants, dispersed oil and moisture from the air inside the enclosure.
- Reducing humidity and handling the condensate in an efficient manner that does not require water drains or energy consumption.
- Enabling monitoring and control of the enclosure temperature and air conditioner performance remotely.
- Efficiently protecting equipment from possibly hazardous ambient conditions.
- Reducing utility costs associated with low efficiency performance of equipment at high temperatures.
- Minimizing maintenance costs and the need for repair and replacement of components.
- Eliminating costs of failed equipment that result in downtime.
Temperature control solutions range from simple filtered fans to heat exchangers and air conditioners. The choice of a solution depends on the application itself. Depending on the sophistication of the solution and application requirements, the costs vary. PC