Screen breakage problem solved by replacing nylon with profile wire screen

When Coffee Process Technology, Houston, TX, encountered screen breakage problems as it continuously dewatered a thick coffee slurry, the company experimented with a variety of screen mesh sizes and production changes. The nylon screens continued to fail. The sticky solids were difficult to discharge because they built up inside the screen and caused frequent failure. Coffee Process Technology is no novice when it comes to roasting and decaffeinating coffee the family has been doing it for three generations, and they have facilities in Houston, Mexico, and Spain but screen breakage in this installation was proving a costly headache.

The process involves initial dewatering in a Carter-Day spin dryer which reduces the waste stream coming from the decaffeinating operation, from 45 percent water to approximately 29 percent. The critical problem was how to remove the remaining water from the coffee slurry and yield a readily disposable dry cake.

The equipment selected for this application was a Kason Centri-Sifter^® centrifugal screen separator. This compact sifting unit utilizes a rotating paddle assembly inside a cylindrical, horizontal screen basket. The material to be screened is mechanically conveyed into the sifting chamber by a helical rotating paddle. No contact is made between the paddle edges and the screen, but as the centrifugal force propels the slurry against the screen openings, the water passes through and the "dry" cake is ejected through the discharge spout.

The Centri-Sifter model selected was direct driven by a 3HP, TEFC motor, operating at 950 RPM. It is designed with a wide inspection door and a removable "overs" discharge spout. Quick-release fittings facilitate screen changing and cleaning. An anti-blinding wiper assembly was incorporated to extend the screening cycle, but failure of the nylon screen at the overs end presented a high maintenance problem.

The solution was found with a change from nylon to a stainless steel 0.03 inch wedge wire screen basket, with 0.005 inch (longitudinal) slotted openings.

These rugged, durable wedge wire screens provide on-stream reliability and allow higher sifting rates because paddle tip speed can be increased without concern over screen breakage. According to plant manager Carlos de Aldecoa, Jr., the process for dewatering the coffee slurry is now running smoothly and the reclaimed water is being economically recirculated into the system. Switching from nylon screens to a stainless steel profile wire basket did the trick.