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Control Systems Selection Guidelines : |
Process controls, over temperature controls, level controls, sensors, power controls, and panels. Now that you have selected the heater(s) for your process, it is time to choose control components, panels, and sensors, to provide the desired results. |
In order to assemble a complete control system, you will need the following information: |
- Voltage, wattage, current (calculated from voltage and wattage),
- Number of zones: (different sections controlled differently),
- Area location or classification: (indoor, outdoor, explosion hazard), and
- The desired process temperature range, as well as permitted deviations should be specified. Close control and/or control of one pass heating of gas or liquids will probably require electronic control.
- Process accuracy issues: For large mass processes (big tanks, large blocks of metal) where the temperature won’t or can’t move quickly, and the temperature requirement is not critical, mechanical bulb and capillary thermostats can usually be used, or if electronic control with indication is needed a simple On/Off controller with a contactor is necessary.
- Process speed: For processes, having low mass, fast, accurate control is important. A proportional or PID controller with an SCR power controller would be a good choice.
- Process upset: If the process is subject to upset, (oven door opened for new batch, for instance), a PID control will be required for good results. This is also the case if heating liquid or gas (air) in one pass. An SCR will be needed as well.
- Environmental (ambient conditions): Process controls, over temperature controls, and accessories must be selected with the surrounding area in mind. Wet, dry, and explosion hazard areas must be considered, as well as the ambient temperature range the equipment will operate in. Mechanical controls should not be exposed to temperatures above their stated range. Electronic controls are designed to operate in an ambient Temperature of above 32°F, and below a stated maximum, usually 120 or 140°F.
- Safety: An over temperature control should be included to protect process, area, heater(s), and/or product in the event of a primary control failure, or interruption of flow in moving systems. If the power control is an SCR, a contactor or shunt trip should be provided so the load can be shut down, even if the SCR’s are shorted. If heating confined liquid or gas, an approved mechanical temperature/pressure relief valve is also required. For some areas, ASME certification may be required on pressure vessels.
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