We occasionally are queried about the pressure exerted on material as it goes through our screw presses. This usually arises because people are familiar with hydraulic ram (piston type) presses. Also, we have one competitor who publishes a ranking of internal pressure figures for their screw presses.
We wish it were otherwise, but the bottom line is that we cannot measure pressure on material in our screw presses. The relationship is poor between the air pressure applied to the discharge cone and the pressure exerted on the material within the press.
When, during the design of a screw press, we select the diameter of the air cylinder for the discharge door, we do look at the annular area at the cake discharge. (That annular area is the difference in cross sectional area between the screen diameter and that of the screw shaft.) We select an air cylinder which will keep us within a range that is consistent with other similar screw presses. The math is straight forward.
The actual pressure, however, is heavily affected by three other factors:
- How slippery is the material being pressed?
- How hard does the screw push the material toward the cake discharge? and
- How fast is the screw turning?
It is like a pump: the discharge pressure depends not only on the restriction at the outlet, but also on the viscosity of the fluid, the configuration of the impeller, and the rotational speed of that impeller.
To illustrate the slippery factor: if we run algae through the press and put the air cylinder pressure at 30 psi, the cone will go dead shut, the algae will slip inside the press, and the pressure exerted on the algae will be close to zero. In contrast, if we run ground glass from a medical waste disposal facility into the same press, with the regulator set at 30 psi, the forces in the press may be great enough to rip the flights off the screw and burst the screen wide open.
The screw configuration also affects the pressure. Just as a bolt with fine threads can exert more force than one with coarse threads, a tight pitch screw exerts more axial thrust than a long pitch screw. Furthermore, if we step up the diameter of the screw shaft as it progresses toward the cake discharge, the pressure gets multiplied. Thus we see that the screw design affects the discharge pressure independently of the air pressure on the discharge cone.
Another factor is the pressure in the inlet hopper. Just like the suction head at the inlet to a pump, pressure at the inlet can affect the pressure at the discharge. Experimentally we have fed our presses in closed piped systems, using positive displacement pumps, so that we have 60 psi in the inlet hopper. And there are installations with 30' tall hoppers mounted over the inlet to the presses, giving about 15 psi static head at the inlet to the screw. As a rule, this does not work; most times material is plastered against the screen of the press, blinding it so that no press liquor gets through. But, indisputably, it does increase internal pressure independently of other design factors.
Because of the many factors and conditions which affect screw press operation, we avoid rating our presses with a single compression ratio. Instead we look separately at the change in pitch of the flights; increase in shaft diameter; and air pressure on the discharge cone.