Capacity Measurement

April 20, 2006

There are many ways to measure the throughput capacity of a screw press. As emphasized in brochures, press capacity can vary a great deal, depending on the nature of the material being fed to it.

Generally, it is hard to measure the pounds per hour (pph) of material being fed into a screw press. Instead, we focus on measuring the flow rates of press liquor and press cake, and then adding them to get the total.

Steady flow conditions must be established before measuring flow rates. Specifically, it is a poor practice to measure the time required for a given sample of material to feed through a press. Instead, establish steady flow operation and then, during this period, capture timed samples of press cake and press liquor.

Press liquor flow is best measured by diverting the flow into either a 5-gallon pail or, in larger presses, a 55-gallon drum. Timing the seconds to fill the container allows calculation of gallons per minute (gpm). Multiplying this figure by 500 gives the flow in pph in most cases.

At times the press liquor flow goes into a tank or pit. This makes it easy to calculate the flow. Simply measure the cross section dimensions of the pit or tank, and time the seconds or minutes required for a change in level of liquor in the tank.

Other times, the press liquor drain line is close to floor level, making it impossible to capture the flow in a 5-gallons pail. In this case, a heavy duty plastic bag can be used to collect a timed sample of press liquor.

The flow rate of press cake is frequently best measured by using a tarpaulin. This requires up to five people: four to hold the corners of the tarp, and one to run the stop watch. The process is to swing the tarp into the cake discharge, allowing it to fill until a sample in the range of 50 to 150 pounds is captured. With the elapsed seconds and sample weight known, the pph rate is easily calculated.

In some applications, screw press capacity is measured in tons per day of dry solids. Therefore, when catching and weighing press cake, it is important to save a sample of this cake and measure its solids content.

There are cases where only one flow rate can be timed and weighed. For example, maybe press liquor can be captured, but press cake is inaccessible. In this situation, samples of inbound material, press cake, and press liquor should be collected for moisture content analysis. Knowing the three moisture contents and one flow rate, the other two flow rates can be calculated. It involves five unknowns and five equations, so it is best if you liked algebra class.

When running tests like these, it is convenient to also measure the bulk density of the press cake. This is easily done by weighing a 5-gallon pail full of cake. Knowing that five gallons is two-thirds of a cubic foot makes the calculation easy. Alternatively, a cardboard box can be measured, filled, and weighed. Questions requiring bulk density data arise from time to time.

Issue 177