Shift Controls was approached to design and fabricate a heater control panel to deliver power to a 105 kW, 480 VAC, 3-phase immersion heater used for batch heating of a liquid in a large tank. The system also required monitoring and interfacing with a plant wide DCS system.
The control panel will be used to batch heat liquid, with a relatively small heater, in a large storage tank before further processing. This configuration will mean that the heater will be at 100% output while filling and then turn off when the set point is reached. The client also prioritized minimizing the panel's capital cost as these panels will be installed at multiple sites.
Shift Controls listened to the client's requirements and tailored a solution that met both the process and cost requirements.
Due to the batch process, the heater would be either 100% on during the heating phase or off after the heating and tank fill is complete, a simple hysteresis heating control strategy with contactors was implemented. This design was chosen to simplify the panel and reduce total cost. A design with SCRs would be unnecessary for the process and would require active cooling since the panel was to be installed outdoors where the temperature could reach as high as 120°F.
Electrical Requirements
The panel is protected by a main circuit breaker with an external disconnect handle. High speed fusing is implemented to protect each of the three individual power contactors and heater circuits.
The client requested staging the 3 heater circuits so they do not all energize at the same time, causing a large current spike when the contactors energize. Shift Controls designed a timed staged control scheme that uses timers to pull down the three contactors in timed succession to reduce the current inrush peak. This design also helps with maintenance temperature control when the tank is close to its set point and requires less power to maintain temperature, reducing temperature fluctuations.
Environmental Requirements
In discussing the project requirements with the client, it was determined the panel was going to be installed outside where the ambient temperature could be as high as 50°C (120°F). The high temperature requirement dictated the component selection and the panel design. Using contactors to control power to the heater circuits greatly reduced the cost, as they do not need active cooling to operate at higher temperatures.
Control Requirements
The control panel has a temperature controller which measures the process temperature and a safety limit controller which measures the heater element temperature. The process or the limit controller can shutdown the heaters by de-energizing the contactors if the process or the heater element exceeds a specified limit. A latching over-temperature alarm indicator and reset button is mounted on the panel door in addition to a SAFE/RUN switch, that allows the panel to be placed in standby when heating is not needed. The process temperature is re-transmitted by a 4-20mA output to the plant wide DCS system for remote monitoring.
Custom Designs
Shift Controls has extensive experience in designing and fabricating custom panels to meet the most difficult design and environmental requirements. Like this project, we listen to your specific project requirements and custom design a system that meets your process and cost requirements. Note that the product pages on our website are examples of a standard product line we have developed. A significant portion of our systems are custom built units, designed for all manner of industrial control, including heaters, motor control, PLCs and general process control. As always, feel free to contact us regarding your project or if you have any questions.
