Everyone knows that screw presses can be used to reduce the moisture content of a material. Less appreciated are the limits that exist. Each material has a natural limit below which the moisture content cannot be reduced in a screw press.

These limits vary widely. For example, lumber sawdust can be pressed down to almost 50% moisture; fresh green alfalfa will go to 72%, while tomatos can be reduced to only 86%. These limits arise from the chemistry of the water in the material.

A screw press will remove the free and interstitial water in an organic material. However there are two forms of water that cannot be separated by mechanical force: (1) the water that is electrically attached by hydrogen bonding and (2) the hydrated water that is chemically bound to the molecules of the solids.

To remove these two forms of water the chemistry must be changed. In the case of citrus peel, this is achieved by adding lime, which reacts with the peel.

Most commonly the chemistry is changed with heat. Heating press cake through a dryer will reduce the moisture content right down to zero. In some cases, the heat is added in the form of steam: a steam dryer will do the same thing.

As recently as 1970 steam was added in Vincent screw presses used for pressing both citrus peel and fish. This was done by injecting the steam through holes drilled in the resistor teeth. About once a year we sell a press with provision for steam or water injection through the resistor teeth.

Some competitors direct steam through a hollow screw shaft. FKC and Dupps offer the heated shaft feature for paper mill sludges. It is not an energy efficient way to dewater sludge; but, the few points of moisture content reduction achieved can be sufficient to meet regulatory demands.

Some screw presses are designed to apply a great deal of friction to a material. The heat generated is sufficient to cook the moisture out of the cake. Low cake moisture is obtained; however, the electrical energy requirement is great. Stord uses this technique in pressing citrus waste.