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Readers help a reader solve this control problem. Next month's problem: How can we improve combustion control?
* 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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