Shift Controls was approached to design and fabricate a heater control panel to deliver power to a large 800 kW, 480 VAC, 3-phase process heater with a full load current of 963 amps with UL508a certification. In addition, the panel also required control and monitoring interfacing with a plant wide DCS system.
The panel is protected by a 1200 amp main circuit breaker with an external disconnect handle. Each of the three phases is carried by four 350 MCM conductors and distributed from the main breaker to two safety contactors. Each safety contactor provided a SCR with a positive way of disconnecting power in case of an over-temperature condition of the heater elements or downstream process, an enclosure air conditioner failure, or a plant or local E-Stop button push. The power to the heater is controlled by two of Watlow’s new 700A Aspyre Series advanced SCR controllers. These controllers allow for advanced monitoring of load voltage, current, and power and various control firing strategies and communication options. The power output from the SCRs is distributed to the 12 heater circuits which are protected by individual 100A circuit breakers.
Safety is always the highest priority in our designs. While not required by code, we separated the low voltage control components and control connections inside a separate enclosure. Low voltage control power and signals are passed between the power and control enclosure with a small pass-through. This allowed the high voltage and power components to be isolated from the control components. The field maintenance staff does not need to open the large power enclosure to perform control commissioning or maintenance.
The panel is located outside at a location that could reach 105 °F in the summers and a freezing cold -20 °F in the winters. These requirements made active cooling and heating absolutely necessary. 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. To keep the SCRs and internal components within the required temperature limits, thermal calculations were performed and a large air conditioner was selected with an integral heater. In addition, the stainless steel enclosure provides great outdoor corrosion resistance and reflects much more solar radiative energy than a painted steel panel, lowering the cooling requirements.
The process setpoint is controlled by a distributed control system (DCS) with an analog 4-20mA SCR power signal and a 24 VDC digital panel enable. In addition, there is an alarm status contact and three temperature 4-20mA re-transmits so the DCS can monitor the heaters and have adjustable alarm setpoints in the control room.
Local to the panel, a Watlow F4T controller was specified and configured to read the temperature of the heater elements with limit cards. The controller will shutdown the heaters by de-energizing the safety contactors if any thermocouple exceeds a specified limit. The F4T has the advantages of a touchscreen, which makes viewing the temperatures and limits much easier than having 2 separate limit controllers and a process controller. In addition, the F4T provides Modbus TCP communication for the capability to interface all of the measurements and parameters with a DCS. An E-Stop button is provided on the panel and wired into the plant emergency shutdown system to allow the field operators to de-energize the heaters in an emergency. A local Run/Safe switch, alarm light and reset button, and heater power on indicator light were also provided.
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 provide a custom design for large heating applications in excess of 1000 amps with UL508a certification. 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.