April 13, 2006

Centrifuges used in food processing usually do the same thing as a screw press: they separate liquids from solids. In this capacity, centrifuges can be extremely effective, especially with materials that cannot be separated in a screw press.

Centrifuges are precision high-speed machines, and they generally require high horsepower motors. In general, they are thought of as expensive machines, that are expensive to operate, and that require frequent expensive maintenance. Consequently, a rule of thumb is that, if a screw press can do the job, it will be preferred over a centrifuge.

This is not to say that centrifuges are not popular machines. Far more of them are sold than screw presses.

There are two basic categories of centrifuges: the sedimentation (decanting) type and the filtration (screen) type. The most common sedimentation type is the continuous horizontal solid bowl decanter centrifuge. A decanter centrifuge separates solids by taking advantage of the difference in specific gravity between the suspended solids in a flow and the specific gravity of the liquid that conveys the solids. Pumping this flow into a spinning bowl, with an internal screw spinning at a slightly different speed, results in a machine where high consistency solids can be augured out one end while clarified liquid flows over a ring dam at the other end. The ring dam is adjustable and the pool depth can be chosen to optimize either dryness of the cake or clarity of the liquid.

Another fairly common sedimentation type centrifuge is the vertical disc bowl centrifuge. It operates virtually continuously with periodic ejection of solids.

The second basic category is the filtration type centrifuge. Filtration type centrifuges have a screened surface on a portion of the spinning basket. As the solids pass over this screened surface, additional moisture is removed from the solids. Thus, a drier cake is produced. Of course, the centrate (filtrate) will have more suspended solids, since some of these may get through the screen.

Filtration type centrifuges can be either batch or continuous. As the name implies, the flow into a continuous filtration centrifuge separates with solids continuously flowing from one end and the centrate from the other. In contrast, the batch centrifuge allows solids to accumulate within the machine. These centrifuges are programmed so that feed is stopped and the centrifuge is slowed down to discharge speed, so that a plow will periodically discharge the cake.

Examples of the continuous filtration type centrifuges are the pusher and the constant angle bowl centrifuges. The former are common in the chemical industry for dewatering and washing crystalline products, and the latter are found mainly in wet starch refining processes. Neither type functions well outside of these applications.

Issue 172