Twin Screw Press Evolution
NOVEMBER 11, 2015
Issuance of US Patent 6,550,376 in 2003, covering design features of a twin screw press, marked Vincent's introduction of our Series TSP screw presses.
After a long incubation period, sales are now consistently being generated in several applications. These applications center on situations where the throughput of a single screw press must be severely de-rated because of slippage inside the screw press.
Dewatering shrimp waste (heads and tail shells) is our most notable success. It is very difficult to get shrimp waste to feed through a single screw press. Using a single screw press, it is necessary to use a VFD programmed for a continuous cycle of forward-reverse motion. This operating mode breaks up co-rotation inside the press.
With our twin screw press, one screw pushes the material surrounding the other screw, which keeps material moving forward. This effectively overcomes much of the slippage. In general we have found that a twin screw press has three times the throughput capacity of a single screw press with the same screw diameter.
One of our customers has a pending start-up of a spent coffee application with a twin screw press. Ten percent of the time this soluble coffee plant runs coffee which has been ground to a powder rather than flaked. This fine waste slipped severely during testing with a single screw press. A similar situation was observed pressing exhausted grounds from secondary extraction at a UK instant coffee factory. We expect the twin screw design to overcome the problem.
Other good candidates for twin screw presses include dewatering spent distillers and brewers grains, cream of coconut, and pineapple juicing facilities. A pineapple application ran successfully earlier this year in Mexico.
Design of twin screw presses has improved remarkably in recent years. The most effective change has been to switch to conical shaft screws. As material progresses through the press, the conical shafts push the material outward against the screen. This configuration was achieved by increasing the center-to-center distance between the overlapping screws.
Originally Vincent's twin screw presses had long L/D ratios (length of the screen divided by the diameter of the screw). This design required seven stages of compression. We found that a standard 4:1 L/D works fine when combined with conical shafts. This allows shortening the twin screw presses to five stages of compression. The screens of these presses are now interchangeable with those of single screw presses, allowing us to improve deliveries and pricing.
We observed that material pressed between two screws tends to push the screws apart. This caused flexing which could push the screws outward into the screens. We remedied this by making the screw shafts from 17-4PH alloy instead of 304 stainless. The 17-4PH is much stiffer and stronger. (Shortening the L/D and increasing the shaft diameters also helped.)
A major problem with early twin screw presses was that press cake jammed at the discharge cone. This firmly packed press cake would push the cone open, resulting in less squeezing despite using a high air pressure on the cone actuator. The problem was overcome by adding "wing feeders" to the tips of the last flights of the screws. These wing feeders strip the cake away from the pinch point at the discharge.
Originally all of Vincent's twin screw presses used expensive semi-custom gearboxes with dual output shafts. Today all our smaller twin screws use a standard single output shaft gearbox with spur gears mounted on the two shafts. In this configuration the screw driven by the gearbox in turn drives the second screw. This use of an off-the-shelf gearbox has proven a win-win for everyone.