KWS Manufacturing

Screw Conveyor Solutions for Four Problem Applications

Do you need a screw conveyor to handle a difficult material or meet tough application requirements? Read this article for practical advice on selecting a screw conveyor that’s up to the challenge.


One of the workhorses of dry bulk solids handling, the screw conveyor provides enclosed transfer while moving materials horizontally, vertically, or at an incline. The versatile conveyor’s wide range of configurations, components, and construction materials makes it suitable for handling the most problematic materials and application requirements.

The screw conveyor typically consists of a screw mounted in an enclosed U-shaped trough (or tubular housing). The screw can have one or several sections, each consisting of flighting mounted on a pipe. At one end, the screw is connected to a rotating drive shaft, and the opposite end is connected to an end shaft. The screw is supported by bearings at both ends. If the conveyor is long enough to require more support, hanger bearings can be suspended from the top of the trough to support the screw at points between the screw sections.

The following sections provide guidelines for handling four difficult conveying applications.


1. Conveying a Wet, Sticky Material

Screw Conveyor Solutions for Four Problem Applications

If you handle a wet, sticky material, such as filter cake from a centrifuge, a screw conveyor that has a standard screw with center pipe isn’t the answer: The wet, sticky material will adhere to the screw and pipe where the screw flight meets the center pipe, eventually clogging the conveyor and reducing its capacity. A good solution for preventing this problem is to use a shaftless screw conveyor.

This unit, as shown in Figure 1, can efficiently convey semifluid, sticky materials because the screw is simply a spiral with no center pipe. The shaftless screw conveyor is usually constructed with a high-strength carbon steel spiral and a stainless steel trough with a reduced-friction liner and high-torque drive unit. The conveyor is suitable for moving many wet, sticky materials in chemical, pulp and paper, Environmental, and other industries.

Certain industries, such as chemicals and foods, require all product-contact surfaces in the screw conveyor to be stainless steel to prevent corrosion or contamination. For these applications, a carbon steel shaftless screw isn’t suitable; instead, a stainless steel ribbon screw (with a shaft) is the best choice. To provide good release of sticky material and prevent buildup, the ribbon screw flight surfaces or the entire screw can be polished to a very fine finish or electropolished to achieve a mirror finish. The screw and conveyor surfaces can also be covered with a nonstick coating, such as Teflon, or another coating engineered for your application.

2. Resisting Wear from an Abrasive Material

When you use a screw conveyor constructed with standard carbon steel or stainless steel components to move an abrasive material such as iron ore, titanium dioxide, foundry sand, or cement clinker, the conveyor will usually wear rapidly. This will lead to frequent conveyor maintenance and downtime, higher overall operating costs, and the equipment’s premature failure. One common method for combating these problems is to use abrasion-resistant plate or hard surfacing on the trough and screw. Hard surfacing is a layer of abrasion-resistant material applied over softer standard construction materials to lengthen component life and reduce overall operating costs.

Hard surfacing can be applied to screw conveyor components constructed of carbon steel, stainless steel, or special metal alloy base materials. The hard surfacing itself can consist of various combinations of chromium, tungsten, molybdenum, carbon, cobalt, and other more exotic materials. The best combination for your application depends on the conveyor component’s base material as well as your conveyed material, application requirements, and operating environment.

Hard surfacing can be applied in different ways. Conventional welding methods using stick or wire electrodes are most common. In a few light-duty and precision applications that require screw flights made of a thin base metal, a plasma or oxyacetylene torch is used to apply a powdered abrasion-resistant material to the metal.

Working with an application engineer at KWS to select the right hard-surfacing material and application method for your screw conveyor will ensure that it handles your abrasive material and provides many years of reliable service.

3. Preventing Dust Leaks

When you’re moving a fine, dusty material in a screw conveyor, you need to ensure that the cover is tightly sealed to the trough to prevent leaks that can create worker health risks and combustible dust hazards. This requires selecting the right cover thickness, gasket material, and, in some cases, clamps.

The best way to get a tight seal on a screw conveyor is to use a flanged cover that’s bolted on 12-inch centers (or closer) with a high-quality compressible gasket between the cover and trough. The cover is made of carbon steel or stainless steel to match the trough construction materials, and the steel thickness and turned-down edges at both sides of the cover provide the cover’s rigidity, improving its ability to seal to the trough.

The typical cover’s steel thickness is from 12 to 14 gauge, but a thicker cover that provides additional rigidity can be required for a very dusty application. A gasket made of a nitrile rubber blend with closed-cell foam construction will provide a positive seal even if the sealing surfaces are irregular. This gasket material is available in black for standard applications and white for chemical, food, and other sanitary applications.

Bolting the cover to the trough rather than clamping it is generally better because bolting eliminates safety hazards and prevents worker injuries. But for some applications — such as those requiring regular hanger bearing maintenance or frequent washdowns — clamping is more practical. Two clamps that seal well and provide easy access to the conveyor are a toggle clamp with an adjustable clamping force and a pivoting C clamp with a knob that allows the operator to apply a large amount of force to seal the cover.1

4. Cooling an Extremely Hot Material

Screw Conveyor Solutions for Four Problem Applications

A very hot material discharged from a rotary kiln or similar equipment can damage your downstream process and handling equipment. Common examples are limestone exiting a kiln and fly ash discharging from a boiler. A screw conveyor can be designed to cool the material as it’s transferred — for instance, cooling a material that leaves a kiln at 1,400°F to less than 150°F — indefinitely extending your equipment life.

In a cooling screw conveyor (also called a heat transfer screw processor), a cooling medium — typically cool water — continuously circulates through a hollow trough jacket or the screw, or both, to transfer heat from the hot material. In the screw, the cooling medium circulates through the pipe and hollow flights. A cooling screw conveyor with twin screws is shown in Figure 2.

To select a cooling screw conveyor for your hot material, you’ll need to work with an application engineer at KWS to calculate the conveyor’s surface area and design the cooling medium flow to match your application’s heat load: that is, the amount of heat you must remove from the hot material. We will calculate this heat load based on your material’s inlet temperature and desired outlet temperature and the cooling medium’s temperature and flow rate, then will size the conveyor to handle the heat load — and your material’s volumetric flowrate — with a conservative safety factor.


Reference:
1Toggle clamp available from DE-STA-CO, Auburn Hills, Mich. (www.destaco.com); pivoting C clamp available from The Witte Co., Washington, N.J. (www.witte.com).