Screw Press
The press consists of a cylindrical perforated cage through which runs a pair of closely
fitting screws. The screw presses, due to the turbulence and kneading action exerted on the fruit
mass in the press cage can effectively break open the unopened oil cells and release more oil.
Digested fruit is continuously conveyed through the cage towards an outlet restricted by a cone,
which creates the pressure to expel the oil through the cage perforations (drilled holes).
The crude oil pressed, which consists of water, dirt and palm oil is passed to the
purification section. At the same time, the press cake, which discharged from the end of the
press, contains mainly the fiber and nuts. There is a relationship between nut breakage and the
percentage of fibre passing to the clarification section. Nut breakage should be properly
measured by the presence of broken shells as with Tenera material the variation in shell
thickness can at times be heuristic, as some of the kernels have little or no shells surrounding
them. The appearance in the press cake of smashed kernels normally indicates that the press
pressure requires immediate adjustment or that the worm screws are in need of rebuilding.
The optimum conditions for operation of the twin-screw press will depend upon
conditions of the fruit when it enters the screw press to achieve effective use of the press. To
establish the proper condition, an accurate and true record of press expelled material samples is
essential. Considerable efforts have been made in the past to improve the oil extraction
efficiency of screw presses. Most of them have focused on optimization of process variables
such as applied pressure, pressing temperature and moisture conditioning of the fed samples
(Ohlson, 1992). Attempts to improve upon existing screw presses by developing a suitable and
efficient oil expression device have remained isolated.
Type of Screw Press
There are two distinct methods of extracting oil from the digested material. One system
use mechanical presses and is called the ‘dry’ method. The other called the ‘wet’ method uses
hot water to leach out the oil.
In the ‘dry’ method the objective of the extraction stage is to squeeze the oil out of a
mixture of oil, moisture, fibre and nuts by applying mechanical pressure on the digested mash.
There are a large number of different types of presses but the principle of operation is similar for
each. The presses may be designed for batch (small amounts of material operated upon for a time
period) or continuous operations.
Batch presses
In batch operations, material is placed in a heavy metal ‘cage’ and a metal plunger is used to
press the material. The main differences in batch press designs are as follows: a) the method used
to move the plunger and apply the pressure; b) the amount of pressure in the press; and c) the
size of the cage.
Different designs use either a screw thread (spindle press) or a hydraulic system
(hydraulic press) to move the plunger. Higher pressures may be attained using the hydraulic
system but care should be taken to ensure that poisonous hydraulic fluid does not contact the oil
or raw material. Hydraulic fluid can absorb moisture from the air and lose its effectiveness and
the plungers wear out and need frequent replacement. Spindle press screw threads are made from
hard steel and held by softer steel nuts so that the nuts wear out faster than the screw Hydraulic
presses are faster than spindle screw types and powered presses are faster than manual types.
Some types of manual press require considerable effort to operate and do not alleviate drudgery.
Continuous systems
The early centrifuges and hydraulic presses have now given way to specially designed screw-
presses similar to those used for other oilseeds. These consist of a cylindrical perforated cage
through which runs a closely fitting screw. Digested fruit is continuously conveyed through the
cage towards an outlet restricted by a cone, which creates the pressure to expel the oil through
the cage perforations (drilled holes). Oil-bearing cells that are not ruptured in the digester will
remain unopened if a hydraulic or centrifugal extraction system is employed. These presses act
as an additional digester and are efficient in oil extraction. Moderate metal wear occurs during
the pressing operation, creating a source of iron contamination. The rate of wear depends on the
type of press, method of pressing, nut-to fibre ratio, etc. High pressing pressures are reported to
have an adverse effect on the bleach ability and oxidative conservation of the extracted oil.
