Pressing Station


The purpose of this station is to press the oil out from the palm fruit mesocarp


Equipment and Function

The equipment to do this is the digester and the screw press. The digester is like a big blender and has a central shaft with arms that rotate and mesh up the fruit. This action breaks up the oil cells and conditions the fibre for pressing.

The screw press is a machine with twin screws that rotate in opposite directions. The mesh of palm fruits with the nuts is fed into the screws from the digester. The mesh as it travels along the screw press gets squeezed and the oil is discharged through a press cage (a cylinder with holes). At the end of the screw press is a cone or a plate that exerts pressure on the mesh. The pressure is normally automatically controller by a hydraulic system. The higher the pressure of the cone the more pressure is applied to the mesh. The hydraulic system gets feed back from the power consumed by the screw press motor (Amperes). The correct pressure is when a small percentage of nut break and the oil to dry matter of the press fibre is about 8 %. If the pressure is set to high the nut breakage increases and kernel is lost. A simple test is to take a small amount of fibre and twist it in your hand, No oil should come out of the fibre, and it should not stain a cloth.

Start Up Procedure

To start the press station, the digester needs to be filled with sterilised fruit. The steam valves should be opened and the digester filled to 80%. When this has been achieved the press should be started with the cone fully pushed in.  The press hydraulic cone control should be set to manual control.  The digester feeding door can be half opened. The operator should observe the fibre coming out of the screw press, when the fibre gets to be dry the cone should be pulled out, little by little. The digester feeding door can now be fully opened and the press cone control put in auto control mode.

Normal Operation

Due the operation time of the screw press, the following must be monitored

  1. The digester level

Make sure that the digester is at least 75 % full. This is achieved by opening and closing of the digester feed door or normally by controlling the quantity of fruit coming from the thresher. The fruit going to the thresher is controlled by the tipper or the auto feeder at the top of the thresher. This in turn depends on the rate the fruit cages are tipped.

There fore the digester level during the mill operation depends on the rate of tipping of fruit cages. The rate of tipping of fruit cages must be made equal to the desired mill capacity.

In 30 tons per hour mill with fruit cages of 10 tons each, a cage must be tipped every 20 minutes. The cycle time for tipping a cage must be 20 minutes. By cycle time we mean the time taken to move the fruit cage into the tipper, tip the cage, and move the cage out and move a new sterilised fruit cage into position for tipping. The timing can be planned by observing the mill operations and timing each step. Once the time for the various operations has been determined, then this must be implemented by informing the mill operator. If during the mill operations, a breakdown occurs then the process must be stopped or adjusted to match the conditions. During the life of the screw press, wear takes place and towards the end of the life of the screw press, the throughput drops. If it drops more that 20 % then the screws need to be replaced.

If a palm oil mill has a capacity of 45 tons per hour and has 3 screw presses of 15 tons per hour each, the capacity will be 45 tons when all the screws are new. When they wear out the capacity of the mill will drop to 36 tons per hour if we follow the 20 % drop in throughput rule. To have a higher mill capacity, the screws in the screw press must not be changed at the same time. The screw must be changed at 2/3 life of the screws, then average mill through does not drop more than 10 % any one time.


Shut Down Procedure

When it is time to stop the process, there must be no more fruit coming from the thresher to the digester. Then the digester must be allowed to be empty. This can be determined by observing the ampere meter of the digester and by observing the fruit in the feed chute of the digester. When the digester is nearly empty the press cone control must be set to manual control and the cone pulled out to reduce the pressure on the press cage. The press must be allowed to run so that most of the fibre is discharged from the press. If this is not done it will be very difficult to start the pressed the next day.  Some mill completes the press by adding nuts with the press cone out, at the inlet so that the screws are completely empty. This should be done if the press is to be stopped for more than a day. For normal operations when the mill is stopped for up to 12 hours this is not necessary.

Maintenance Checks

During operations the press operators should be instructed to monitor the press and digester operation and immediately report any unusual noise or occurrences.

The gearbox oil level should be monitored weekly by the mill fitters.

Product Sampling

The press cake fibre should be sampled every 2 hours. A small quantity approximately 200 grams should be taken and put in a container. At the end of the shift this sample can be taken to the laboratory for analysis.

Process Efficiency

A press that is working well, the oil content in the fibre should be less than 8 % to dry matter and the nut breakage less than 15%. The performance of the screw press is also influenced by the efficiency of the digester, the steriliser and the crop ripeness. Very unripe crop is very difficult to digest and will result in a higher oil loss in the fibre. If the results of the laboratory sample are outside the acceptable limits, the cause needs to be investigate and corrected.

Design Parameters

The capacity of the screw press is one of the main factors that will determine the processing capacity of the mill. Therefore the press will be required to be maintained well and worn screws replaced as soon as they reach the end of their life. Most mills have a spare screw press so that when one unit is maintained the mill will continue to achieve the processing capacity.

For 45 tons per hour mill 3 screw presses of 15 tons per hour capacity are required. This most mill consultants will install 4 screw press, three in operation at any one time and one spare. The question is will a mill with 3units of  15 tons per hour screw presses achieve a capacity of 45 tons per hour our long period say average for a month. The majority of the mills do not. Why ? The reason is that 15 tons per hour is the maximum capacity of the screw press, not the average capacity. When the mill does not achieve the rated capacity, the owners of the mills who are mostly not technical people are unhappy and feel that the mill is not running well.

There are two possible solutions to this problem. One is to ask the screw press designers to design a press that have an average capacity of 15 tons per hour over the operation life of the screws or to rate the mill as 40 tons per hour mill.

Labour Requirement

One press operator and a helper can operate a press station with up to 6 screw presses.