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Modified Screw Heat Exchanger

Screw Heat Exchanger Keeping Itself Clean

| Author / Editor: Matthias Ginster* / Wolfgang Ernhofer

Compared to a conventional screw heat exchanger the new design, a 2:1 screw pitch ratio — and a corresponding 1:2 speed ratio between the two shafts — provides self-cleaning operation, aided by narrow gaps between the screw flights.
Compared to a conventional screw heat exchanger the new design, a 2:1 screw pitch ratio — and a corresponding 1:2 speed ratio between the two shafts — provides self-cleaning operation, aided by narrow gaps between the screw flights. (Picture: Köllemann)

For many industries that produce, process, or handle bulk solids, the screw heat exchanger is a well-known piece of equipment for heating, drying and cooling. With the same working principle as a conventional screw conveyor, however, most screw heat exchangers rely on the flowability of the bulk solid for good mixing and to keep their heat transfer surfaces clean. As a result, these standard screw heat exchangers cannot be used for sticky, pasty or adhesive materials or slurries.

With today’s process industries producing an incredibly wide variety of products, engineers naturally sometimes require the ability to heat or cool bulk solids that are less easy to handle.

Over the years, bulk solids specialist Köllemann has taken note of the rising need for advanced processing techniques and the need to handle materials that fall outside the standard range of bulk solids. As the company began to carry out R&D in this specialized field of application, the research process led to many new ideas and techniques.

The starting point in the design of many kinds of machines for solids handling has been the multi-shaft principle. Versatile as this equipment family is, however, traditional designs are not suitable for all duties. To this day it is a common misconception that an ordinary twin-shaft screw heat exchanger can easily work with sticky or pasty materials.

In the worst case, in fact, an ordinary twin-shaft machine will run idle with completely clogged screw flights. As a result there will be no significant flow of either material or energy. This is because much of the material sticks to the flights, where it is only slightly deformed; it maintains its position on the shaft but is not conveyed to the outlet.

A major problem with these difficult materials is that they form relatively thick layers on the surface of the shafts. This significantly decreases heat exchange efficiency by insulating the heating or cooling surfaces.

Developing a New Concept

One of the company’s earlier designs featured two contra-rotating shafts, and each driven by its own motor and lacking a spur gear to synchronize them. The idea was to use electronic frequency converters to alternate the relative speeds of the shafts very slightly. Each variation in speed changed the positions at which the flights meshed, forcing them to scrub off adherent material from one other.

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