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July 12, 2024
Hardly a week goes by at Vincent where we do not recommend that someone do a fist test. That involves taking a clump of material and squeezing it in your fist to see if you can separate any water. It is a quick way to get a feel for if a screw press will do the same thing. Or, by squeezing the press cake from a screw press, it is a quick way to check the performance of that press.
What we look for from the fist test is:
a) A thick stream of water drains out;
b) A steady stream of a dozen or more drops gradually fall out of your fist;
c) Only one to three drops of water fall out;
d) The best you can do is to get a film of water to appear between a couple fingers;
e) Not even a film of water can be separated, no matter how tight or long you squeeze, indicating you’ve reached the practical limits of this test.
Of course, there are some things where the fist test is a failure. That includes most DAF sludges, some pomaces, and some fluids from grease traps. With these you cannot get any liquid to separate. We call this the “mashed potatoes” effect. Squeeze mashed potatoes in your fist, and no matter how slow and careful you are, what squirts between your fingers is identical to what is left in your fist. The odds are slim that a screw press will do any better.
We recently had a demonstration of the limits of the fist test. We collected samples of press cake from two customers on the same day. Both plants make instant (water-soluble) coffee. Both had nearly identical 16” presses, about the same age, running close to the same screw rpm and pressure on the discharge cone. They were both pressing spent coffee grounds to make fuel for their boilers. However, the plants had differences in the coffee beans they were processing; the grind, roast, and extraction processing steps had variations, and one spray-dried their product while the other freeze-dried it.
We ran the fist test on samples of press cake from both plants. The results were exactly what we wanted to see: my palm did not even get damp, let alone could I squeeze a film of water from either sample. The samples gave every appearance of being identical and ideal for burning.
However, the moisture results from the lab told the real story. One plant normally produces press cake with 55% moisture content, and my sample came out at the low end, with 49% moisture. The other plant normally sees their press cake with 62% to 66% moisture content, and my sample came out with 69%.
Since we could not squeeze any free water from either sample, those two tests did show us that both presses were operating near their peak performance. But it left us in a quandary to explain the big difference between the two facilities.
In closing this newsletter, we need to remind everyone of another simple test which is especially useful once you’ve reached the limits of the fist test. That is the sock test, which is capable of producing higher pressures. Put your clump of sample in a piece of cotton cloth. Twist that into a very tight ball. See what comes through fabric. Then un-twist the ball and look at the fiber left in the cloth. That will give you a good idea of what a screw press can do.