Dewatering centrifugal dryer (Spinner)

Dewatering centrifugal dryer (spinner)

In a plastic recycling line, a dewatering centrifugal dryer is the machine that throws free water off washed flake by spinning it at high speed against a screen. It sits after washing and before any heated drying stage, and it exists for one reason worth being blunt about: mechanical water removal is cheap and thermal water removal is not.

The same machine is sold under several names — dehydrator, mechanical dryer, spinner, centrifuge machine. They all describe the same thing. What follows is how much water it actually takes off, why the throughput figure for film is a fraction of the figure for rigid material, and how to read a range where two models share a rotor diameter.

Why the cheap water comes off first

Water leaves washed flake in two very different ways, and the order they happen in is what decides your energy bill.

Spinning it off costs the electricity to turn a rotor. Boiling it off costs fuel, and it costs it for every kilogram of water still on the flake when it reaches the heated stage. A centrifugal dryer is therefore not competing with a hot-air dryer — it is protecting it. Every point of moisture the spinner removes mechanically is a point the heater does not have to pay for.

This machine takes a plastic flake stream down to a residual moisture content of <1–8%, with throughput reaching 2,000 kg/h. Where you land inside that range is a material question, not a machine question, which is the next thing worth understanding.

Used together with an electrical heating box, the same unit reaches higher drying efficiency — useful where the flake has to arrive genuinely dry rather than merely de-watered.

What is inside the housing

The rotor

A high-speed, open-flow rotor carrying paddles welded along the shaft. It runs at a stable 1,500 RPM, and stability matters more than peak speed on a machine that is throwing wet, unevenly distributed material against a screen all day.

The modular screen basket

Flake is flung outward, water passes through the screen, flake does not. The basket is modular, which is the difference between a screen change being a scheduled job and being a rebuild.

Access and corrosion resistance

An easy-access hatch simplifies cleaning and maintenance, and the construction is corrosion resistant — a genuine requirement rather than a specification flourish, because this machine spends its entire service life full of wet plastic and process water.

The residual moisture range, and why it is so wide

A stated figure of <1–8% looks imprecise until you consider what the machine is actually doing. Centrifugal force removes free surface water — water sitting on the outside of a piece of plastic. It cannot remove water that is trapped somewhere the force cannot reach it.

That is why the same machine gives very different answers on different materials. Dense rigid regrind has a low surface-area-to-mass ratio and sheds water readily, landing at the low end. Film is the opposite: thin, light, enormous surface area for its weight, and prone to folding water into itself. It lands at the high end.

So the honest way to read the specification is: the low end is what rigid flake achieves, the high end is what film achieves, and the number you get is set by what you feed it.

Why film throughput is a third of rigid throughput

This catches people out at the sizing stage, so it is worth stating before the table rather than after it. On the same machine, film capacity runs at roughly a third of rigid capacity.

On the WTS-650 the split is 500–1000 kg/h rigid against 300–400 kg/h film. On the WTS-850 it is 1500–2000 kg/h rigid against 500–600 kg/h film. The machine has not changed; the material has.

The reason follows directly from the point above. Film carries far more water per kilogram and gives it up more slowly, so it needs more residence time in the rotor to reach the same dryness. More residence time at a fixed rotor volume means less material per hour. If you are sizing a dryer for a film line off a rigid-material capacity figure, you will undersize it by a factor of about three.

Four models, and one pair that shares a rotor

Two things in this table repay a second look.

The WTS-850 and the WTS-850L have the same rotor diameter. Both are Φ850 mm, so the L is not a wider machine — it is a longer one, with a bigger motor to turn it. Look at what that buys: rigid capacity moves from 1500–2000 kg/h to 2000 kg/h, barely a change, while film capacity moves from 500–600 kg/h to 800–1000 kg/h, a jump of over half. The L variant’s gain is almost entirely on film. That is the residence-time argument showing up in the product range: length gives dwell, and dwell is exactly what film needs.

The VTS-450 has no film figure at all. The soft film row is blank for the smallest model. Read that as the range starting at the WTS-650 if film is your material.

Where it sits, and what it feeds

The dryer takes washed flake from the wash stage and hands it on, usually through a cyclone and a screw or pneumatic conveyor, to storage, to a hot-air drying stage, or directly to a pelletiser. In a PET bottle line the same machine type is used twice — once mid-line to strip liquid off the flake, and once at the end to take surface water off the finished product.

What comes after it depends on how dry the next process needs the material to be. Flake going to a washing line’s storage silo can leave the spinner as it is. Flake going into an extruder generally cannot, because water flashing to steam in a melt is a defect mechanism, not a moisture inconvenience.

What it runs on

The machine is effective on shredded film, rigid regrind and mixed plastic streams, and is widely applicable to PP, PET, HDPE, LDPE, LLDPE and ABS. It suits both soft and rigid plastics, including film, raffia and regrinds.

The polymer matters less than the physical form. Two materials of the same polymer, one as thick regrind and one as thin film, behave like different feedstocks in this machine — which is the whole point of the two separate capacity rows in the table.

Keeping it working

A machine that is permanently wet and permanently spinning has a short list of things that decide its service life.

The screen basket needs cleaning. Fines and fibre blind the perforations, and a blinded screen holds water in rather than letting it out, so residual moisture creeps up before anything sounds wrong. The modular basket and the access hatch exist to make this a routine job.

Wear parts are consumables. The structure is robust and the wear parts are wear-resistant, but plastic flake at 1,500 RPM is abrasive; the paddles and screen are the parts that go.

Corrosion resistance is what buys the years. On a machine in a demanding wet environment, the difference between a long service life and a short one is usually not the rotor — it is whether the structure around it survived the water.

See it running in a line

Frequently asked questions