Case study: Control system maintains temperature in industrial ovens

Controlling 32 independent heating zones from one PLC and HMI has its challenges.
By Control Engineering Staff June 4, 2008

Since 1979, Georgia Oven Co . has been developing custom thermal devices, ovens and furnaces for customers with plastics, textiles, medical, electronics and pharmaceutical applications. Today the company exclusively produces vacuum and atmospheric ovens for a variety of industries.

John Starnes, electrical designer for Georgia Oven, says the company is constantly exploring new technologies for customers that cannot use off the shelf designs or require heavily automated production type units.

One such application brought the company to Watlow , via Atlanta-based distributor Star Electric Inc. Georgia Oven’s customer was ordering a large multiple heating zone atmospheric oven to bake polyimide on stainless steel coils that make hard drive suspensions. The thermal system needed to control 32 zones of temperature with quartz glass panel heaters, each with its own limit and ability to communicate with a PLC and HMI.

The control scheme had to support changing failed system components easily without altering or shutting down the other operating zones.

Watlow’s Ez-zone ST achieved all of these operational objectives. Each package included the temperature controller, high limit, shutdown contactor, heat sink and solid state relay (SSR). The 32 units were bussed together into the PLC using Modbus, allowing the operator to set the temperature to all the zones by using a PLC.

By having the set-point retransmit via Modbus, Watlow could also individualize high limit problems and controller errors on the HMI from each of the Ez-zone units. The use of a multi-loop controller would not have solved the need for individuality in the event of a controller failure. The Ez-zone ST breaks out each zone, which allows the user to isolate one quickly or swap it out.

Starnes says that using Ez-zone ST on the initial two ovens has streamlined communications and reduced panel space for their customer, among many other benefits. “The units are simple to integrate, smaller and increase the ovens’ efficiency. These are meaningful benefits for our customer, who has come to expect innovation from us,” he says.

—Edited by Peter Welander, process industries editor, PWelander@cfemedia.com ,
Process & Advanced Control Monthly
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