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Readers help a reader solve this control problem. Next month's problem: How can we improve combustion control?
A Reader Writes:
We specify coolers for enclosures that end up in a wide range of ambient temperatures and humidities. My boss says we should simplify operations by using the same refrigeration unit, but this means in some cases the unit will have much more than the calculated minimum capacity. What problems could this cause? How could we avoid them?
--from March 2003 CONTROL
Stop at the Worst-Case Scenario
When calculating cooling needs, it is essential that the worst-case scenario be used to determine the size of the cooling unit. Electrical and electronic equipment must be protected from severe high temperatures (even for brief periods of time) and dramatic temperature changes. This may only occur at certain times or when certain pieces of equipment are running. This inevitably leads to a cooling unit that appears to be oversized for the application.
...Standardizing the AC unit across various applications may result in a very large cooling unit on a low cooling-need cabinet. The result is a cooling unit that would run for very short periods of time. The short run time may not be enough for the unit to remove humidity. Humidity is as much a problem as high temperatures and can damage equipment.
...Cycling a cooling device more often (turning it off and on frequently) leads to premature failure and increased maintenance. Having an oversized or undersized cooling unit leads to wasted power (added power costs), decreased cooling performance, reduced equipment life, and additional maintenance needs.
...Each application has its challenges and must be researched to provide the most economical cooling solution. Sizing cooling equipment often requires trade-offs (cabinet size, position from external heat sources, etc.) Cooling equipment is based on worst-case scenarios to prevent elevated temperature from occurring.
...Too large a cooling unit leads to inadequate dehumidifying and decreased cooling equipment life. But too small a unit will stress the cooling equipment and possibly not be able to handle worst-case cooling needs. Reserve capacity is often added to the cooling needs as insurance and to provide for added future additions of heat-producing equipment. Though standardization may prove to be economical up-front, longterm the costs are much higher.
Brian Mordick, Product Manager, Thermal Products
Hoffman Enclosure Co., www.hoffmanonline.com
Beware Overtemperatures and Alarm Trips
For best results, be careful not to oversize the unit. Be certain that both the evaporator and condenser air flow paths cannot short circuit or are impeded. Be cautious of adding protective covers to the outside of the unit that may reduce air flow and unit thermal performance. Seal the electrical enclosure to prevent humidity and outside air from entering. Closed-loop enclosure cooling is the goal.
...This said, it is inevitable that one size (capacity) air conditioner unit may be called upon to handle all of the applications for a user's product line.
...The factory low-temperature cut-out setting (the temperature at which the compressor will shut off) is typically 75° F. Overcooling the electrical components can become a problem in some circumstances. The immediate danger is of cycling the compressor on too many times per hour. This will shorten the compressor life and result in a sudden failure.
...A compressor short cycling time-delay relay is an option that is available from enclosure air conditioner manufacturers. This will delay the restarting of the compressor in the cases where the enclosure air has been cooled down rapidly and then reheats quickly due to the internal and external heat loads.
...Unfortunately, large swings in the enclosure temperature can result as a unit cools down the air within the enclosure. Sunlight, ambient, and internal heat sources will quickly raise the temperature of the relatively small amount of recirculated air within the enclosure. Although the thermostat has a differential setting that will allow the enclosure temperature to rise 10-15° F before it restarts the compressor, this can occur rapidly. The time delay relay will prevent this but allow the internal temperature to rise even further. At times, systems with alarms can have nuisance trips as the temperature can spike several times an hour.
...Also, critical electronic components will be exposed to wide swings in temperature and relative humidity. In cases where outside air leaks into a cabinet, condensation could form when surfaces are cooled below the dewpoint of the outside air that is leaking inwards.
...Sizing should be limited to a reasonable factor of safety. This is recommended to be not more than 25% above the estimated worst-case loads taking solar, internal, and ambient temperature heat gain into account. The enclosure surface area and thermal insulation (if used) also should be a part of the load calculation. Calculation assistance can be found on most manufacturers web sites and in catalogs.
Bruce K. Kreeley, Director of Engineering
Many Aspects Can Be Standardized
If you only use one refrigeration unit, you will end up with too much capacity on some enclosures. Take air conditioners as an example. If you select one air conditioner for all of your enclosure cooling needs, you will have to size the unit for your worst case, which is the hottest ambient temperatures and the highest heat loads.
...For one application, this might be a great solution, for another application the unit may be significantly larger than required. From a performance point of view, an oversized unit will perform in the application. The unit will cool rapidly, but will be idle for long periods of time between cycles. Some potential issues for oversizing an air conditioner are:
* High purchase costs for the unit.
* Higher current draws, resulting in high peak operating costs.
* Inefficiency, resulting in unbalanced or low cycling rates.
* Physical mount limitations, sometimes resulting in using a larger enclosure than necessary.
* Added size and weight for handling.
...The best solution is to pick a minimum number of refrigeration units. Start by selecting one type of refrigeration unit, one operating voltage, one type of controller/thermostat, a minimum of physical sizes and capacities, and a preferred supplier to minimize costs.
...Most suppliers provide multiple capacity (BTU/hour) units in a single physical size. Let your cooling capacity requirements determine the number of units required. Reducing other variables to a single item will greatly reduce the average number of units required for projects or maintenance needs while still meeting the objective of standardized sizing.
...Another solution is to use a single size unit, but in multiple quantities. Instead of using one large unit, you can use several smaller units. Make sure that the units can operate together, cycling at the same time. This is typically accomplished with units that have a communication bus. Units are programmed to operate together, cycling on and off at the same time. With this option, a common size is maintained, but the cooling capacity is closer matched to the application.
Steve Pucciani, Product Manager, Thermal Management Products
One Size Does Not Fit All
It is imperative that a reasonable operating environment be maintained inside the electrical enclosure. After all, the goal is to keep the control system components comfortable and operating properly.
...Reasonable environment means the internal humidity and temperature will not vary beyond the component specifications listed by the individual manufacturers. Specification limits keep products from failing prematurely, limit corrosion effects on metals, and avoid weakening plastics with excessively cold temperatures. Electronic devices, integrated circuits, and printed circuit boards stress and fail when operating temperatures are overcooled or heated.
...As a worldwide supplier of electrical terminal blocks and signal conditioners, we have found that if the specifications for relative humidity do not go beyond a showering downpour condensing on the sides of the electrical enclosure (above 93% RH), all is well.
...Likewise, if the temperature does not exceed the melting point of most plastic and insulation materials, all will be well. We have found that if the normal operating temperatures within the enclosure are within the range of 5-40Â° C, which covers applications throughout the world, generally the products will continue to function normally. Should these temperatures be exceeded, a derating factor can be applied to compensate for the extremes.
...The environmental enclosure designer should take into account the components in the enclosure, the volume, and air flow paths around the components such as transformers, PLCs, power supplies, terminals, signal conditioners, and wiring. The environmental control system should match the internal wattage load and the external influence of geographic location of the final installation with room to spare.
...With regard to the cooling and heating system, one size does not fit every enclosure application. It is a calculation that takes a bit of research and time.
Ed Promin, Engineering Manager
How Can We Improve Combustion Control?
We are burning bark and wood byproducts in furnace cells and are having trouble controlling fuel level and combustion. We are able to measure fuel level with a laser instrument but the fuel quality varies from good bark to sawdust to dirt. Sometimes the bark feeder winds up but does not put in more bark, then overfeeds, which causes problems from poor combustion airflow to plugging. How can we improve control of this process?
Please send your comments, suggestions, or solution for this problem (or a problem of your own that you would like to pose to other colleagues) to CONTROL Problem Solving Ideas, email@example.com, 555 W. Pierce Rd., Suite 301, Itasca, IL 60143; Fax 630/467-1124. Responses will be published in our July issue.
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