Shift Controls was approached to design and fabricate a heater control panel to deliver power to four 50 kW, 480 VAC, 3-phase heaters for a total of 200 kW and 234 amps. The system also required control interfacing with a plant wide DCS system.
The panel is protected by a main circuit breaker with an external disconnect handle. High speed fusing is implemented to protect the SCRs and each individual heater. Each of the 4 heaters is divided into two circuits for a total of 8 circuits. Each circuit is protected by a 3-pole circuit breaker. Each device acts as power distribution, 2 contactor loads from the feed breaker, 4 SCR loads from the 2 safety contactors, and 8 heater loads from the 4 SCRs. This made the best use of the panel space without the need for dedicated distribution blocks.
With any panel, providing adequate cooling is one of the most important and overlooked design considerations. It is typical for an SCR to generate 1 - 1.5 Watts of heat per switched amp, per switched control leg. This can be a significant amount of heat to deal with in a confined enclosure.
In discussing the requirements with the client, it was determined the panel was going to be installed where the ambient temperature could be as high as 40°C (105°F). This high temperature requirement dictated the component selection and the panel design. A typical solution is to mount the SCR heat sinks external to the enclosure to reduce or eliminate the amount of internal cooling required. However, due to the high ambient temperatures and heater power, active SCR cooling was required for this application. The SCRs were specified to be Watlow Din-a-mite Style Cs with active SCR fan cooling to be able to deliver the current at the elevated ambient temperatures.
To be able to dissipate the large amount of heat produced by the SCRs and other internal components, heat calculations were performed and a large fan, vent, and wash down hoods were specified to ensure the panel internal temperature did not exceed 50°C.
The power to the heaters is controlled by a distributed control system (DCS) with an analog 4-20mA SCR power signal and a 24 VDC digital panel enable. It is important to our client to integrate the heater control with their DCS and control all four heaters with a single 4-20mA signal. To drive all four SCRs with a single signal, a high powered signal isolator was used to drive the SCRs.
The control panel also handled safety over-temperature thermocouples and limit controls. Each of the four integrated heater sheath thermocouples is read by the panel for over-temperature limit control. Due to the high channel count, a Watlow F4T controller was specified and configured to read the temperature of each heater with a limit card. The controller will shutdown the heaters by de-energizing safety contactors if any heater exceeds a specified limit. The F4T has the advantages of a touchscreen, which makes viewing the temperatures and limits much easier than having 4 separate limit controllers. The F4T controller also provides over-temperature alarm contacts to the DCS system to alert the control system an over temperature and shutdown event has occurred. In addition, the F4T provides Modbus TCP communication for the capability to interface all of the measurements and parameters with the DCS. An E-Stop button is provided on the panel to allow the field operators to de-energize the heaters if necessary.
Shift Controls has extensive experience in designing and fabricating custom panels to meet the most difficult design and environmental requirements. Like this project, we can integrate the panel into an existing DCS system or design as a full stand-alone system. Note that the products 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.