A manufacturer of dog and cat food needed a control system for a food extruder that accepted frozen raw ingredients in a hopper at one end, cooked the food by pushing it through a steam-heated barrel with a rotating screw, and formed the profile of the final product with a die at the other end. Subsequently, the product was sliced into pellets and dried.

The problem the company faced was that the raw meat and other food had to be safely and uniformly cooked, although its initial temperature, density and composition could vary. An alarm had to report when product exited the extruder below a user-specified minimum temperature. The flow of steam had to be controlled based on pressure, and the system had to be easy to operate.

St. Louis-based Watlow provided RTD sensors and a PPC-2000 programmable process controller, which could be customized to provide a sufficient number of analog inputs and control loops to control the system.

Representatives of Conrad Kascik, a Midwest distributor of Watlow products, created a logic program that averages the temperatures measured by three RTDs at the extruder's output and provides that average as the feedback for PID control of the screw speed. Cooking time is automatically adjusted so when the final temperature of the product is slightly below setpoint, the screw slows, adding cooking time, and when the temperature is slightly above setpoint, the screw speeds up, shortening cooking time.

A second channel controls steam flow by adjusting a valve based on feedback from a pressure transducer. The PPC-2000 ensures product safety by monitoring each RTD individually for deviations from specified limits. An interface panel allows operators to monitor the process and change the temperature and pressure setpoints.

Production resulted in steam pressure controlled to within +/-0.5 psi and product temperature controlled to within +/-2oF deviation from setpoint.

For more information, contact Watlow at (800) 492-8569 or www.watlow.com.