When planning an enclosure layout, cooling is sometimes an afterthought. However, the enclosure air conditioner can have a tremendous impact on the overall performance and efficiency of industrial operations. Proper and efficient cooling can prolong the life of installed equipment, save energy and utility costs, and protect against unscheduled downtime. In fact, using a properly selected enclosure air conditioner can lead to increased productivity and profitability for your plant.
Factors to ConsiderInternal Heat Load. The internal heat load is the amount of heat energy produced by the electronics inside the enclosure as a result of unused electricity running through the components. Knowing the amount of heat energy (in BTU/hr or watts) that will be created by the equipment housed in the enclosure is crucial to specifying the proper air conditioner capacity for an application.
One way to determine the heat load inside an enclosure is to add together the heat loads of all of the installed electronic components as specified by the component manufacturers (table 1).
Another approach is to calculate the electricity consumed by the electronics and then multiply that number by the system efficiency. The resulting number equals the need for cooling capacity. For example, if a 20-percent efficient electronics system consumes 500 W of power, the system is using only 100 W of electricity for its actual function. The remaining 400 W is dissipated in the form of heat energy.
Cooling Capacity. In the world of electronics, cooling capacity is the maximum amount of thermal energy that an environment-control product can remove. This parameter also is shown in BTU/hr or watts. The cooling capacity, or performance, of a specific enclosure air conditioner not only depends on its overall design but also on various application-specific factors such as the ambient temperature, the maximum allowable internal temperature and the operating frequency (in Hertz).
The ambient temperature (TA) can affect the cooling capacity of an air conditioner substantially. If an air conditioner operates in high ambient temperatures (for example, some air conditioners can operate in 131°F environments), it will provide less cooling capacity. This limitation occurs because air conditioners work by pulling the hot air from inside the electrical enclosure and transferring the thermal energy to the surrounding environment. The hotter the outside air, the lower the ability of the air conditioner to transfer the enclosure heat energy out through the condenser coil. The converse is also true: When air conditioners are placed in areas with lower ambient temperatures, the heat transfer through the condenser coil into the ambient air is greater, and the cooling capacity of the air conditioner is increased.
The maximum allowable internal temperature (TI) also is relevant to the cooling capacity of an air conditioner. This parameter determines how much thermal energy needs to be removed from an enclosure. Typically, air conditioners operate by maintaining temperatures that do not exceed a specified setpoint. A recommended setpoint for enclosure air conditioners lies between 86 and 104°F (30 and 40°C), depending on the electronics installed in the enclosure. Lower-temperature setpoints can lead to excessive condensation and should be avoided.
To illustrate this principle, consider an air conditioner with a setpoint of 95°F (35°C) and a differential (or switching hysteresis) setting of 9°F (5°C). The temperature inside the enclosure is allowed to increase to 95°F before the air conditioner starts to run to cool the temperature down to 86°F (30°C). Once the differential to the setpoint is reached, the air conditioner will shut off until the enclosure temperature again rises to the 95°F setpoint.
The third factor that influences the cooling capacity of an air conditioner is the operating frequency. In North America, 60 Hz is the norm, but 50 Hz is used throughout much of the world. Dual-rated air conditioners can operate at both frequencies. When an air conditioner is operating at 60 Hz, the fans and compressor actually rotate faster than at 50 Hz, resulting in higher performance for the air conditioner at 60 Hz.
When evaluating an air conditioner that is stated to have a certain cooling capacity, it is important to consider under what temperature conditions and at which operating frequency that cooling capacity is provided.
Helpful ToolsPerformance Diagrams. One way to determine the cooling capacity of an air conditioner under variable conditions is to use a performance diagram (figure 1). These charts show the cooling capacity of an air conditioner based on the requirements of DIN 3168 as well as under different temperature scenarios, including maximum operating conditions. Viewing performance in a diagram format can help users understand how a particular air conditioner will perform in a specific application.
Sizing Software. When selecting an air conditioner, the easiest way to figure out how an air conditioner will perform at given temperatures is to use sizing software. These convenient tools typically walk users through the various factors that impact an application and then determine the need for cooling. Some software products can calculate the internal enclosure temperature without any means of cooling before predicting how many BTUs or watts of cooling capacity the application requires. Air conditioner manufacturers’ software often will also suggest appropriate part numbers to simplify ordering.
Manual CalculationsEfficiency. Reducing power consumption and increasing efficiency are vitally important to protecting the environment and saving money during air conditioner operation. The formula to determine the efficiency of an air conditioner is the ratio between useful cooling capacity and power consumption. More efficient air conditioners have higher cooling efficiency factors (see web exclusive sidebar: Increasing Cooling Efficiency Ratio).
The Impact of Humidity. An unavoidable side effect of using air conditioners is the need to dehumidify the enclosure’s interior air. As the enclosure cools down, part of the humidity contained in the air condenses on the evaporator coil. Traditionally, this condensate has been discharged from the enclosure by using condensate hoses and collection bottles. More advanced cooling units reduce humidity through integrated condensate evaporators.
The amount of condensate that is created depends on relative humidity, the air temperature in the enclosure, the evaporator coil and the air volume present in the enclosure. The Mollier h-x diagram (figure 2) is used to show the water content of air depending on its temperature and relative air humidity. For example, consider an enclosure air conditioner with a temperature setpoint (internal enclosure temperature) of 95°F and a relative ambient air humidity of 70 percent. If 95°F air is exchanged over the evaporator coil, the surface temperature of the evaporator coil (evaporation temperature of the refrigerant) is approximately 64°F (18°C). At the outer layer, adhering to the surface of the evaporator coil, water (condensate) is deposited at the dewpoint. The difference, ΔX = X1 – X2, indicates the amount of condensation that occurs per 2.2 lb of air with complete dehumidification.
The air tightness of an enclosure plays an important role in the amount of condensation that will occur in an application. Because the quantity of ambient air - and, as a result, the amount of humidity - is limited in a properly sealed enclosure, the amount of condensation also will be limited. Conversely, an enclosure that is not properly sealed will see higher levels of condensation. Ambient (humid) air can enter through poorly sealed cable entries, damaged or open enclosure doors and damaged enclosure gaskets - resulting in increased condensation.
If, for example, ambient air is entering the enclosure at a rate of 5 m3/h, a permanent condensation amount of 2.7 oz/h (80ml/h) can occur. For this reason, it’s always recommended that control panels be operated with the enclosure doors closed and that all sides of the enclosure be properly sealed and gasketed. In addition, it is advisable to use a door switch that interrupts the operation of the air conditioner while the enclosure door is open and to set the internal temperature of the enclosure only as low as is actually needed.
In conclusion, selecting the right air conditioner for an industrial enclosure application is crucial to maximizing efficiency, performance and overall return on investment. Knowing what factors to consider and taking the time to evaluate the available products properly can reduce utility costs, drastically improve the life and reliability of installed equipment, and solidify operations as a whole through increased productivity and reduced downtime.