Vertical screw conveyors:

A vertical screw conveyors conveys material upward in a vertical path. It requires less space than some other types of elevating conveyors. Vertical screw conveyors are also referred as ‘Lifts’ or ‘Elevators’. Vertical screw conveyor can handle most of the bulk materials provided there is no large lump. The maximum height is usually limited to 30m.

The vertical screw conveyors system is a great solution to many material handling problems by providing a cost effective elevating action with minimum space requirements. The vertical is fed by a horizontal feed screw which is synchronized with by speed and feed rate. Both the feed screw and the vertical are driven by independent screw conveyor drives.

A vertical screw conveyors consists of a screw rotating in a vertical casing. The top bearing for the screw shaft must be designed to stand against radial and thrust loads. A suitable inlet port at the lower end and a discharge port at the upper end of the casing are provided. Feeding a vertical screw conveyors deserves careful consideration. Most materials are fed to the vertical conveyor by a straight or offset horizontal feeder conveyor. The ideal operation of a vertical screw conveyor is to have a controlled and uniform volume of material feeding.

Uneven feeding and start stop operation may adversely affect the performance of the vertical screw conveyors in terms of speed, capacity and horse power.

Discharge Arrangement in Vertical Screw Conveyors

Discharge of material in a vertical screw is achieved through an opening similar to that of a horizontal conveyor. The discharge spout may be connected to an elbow or other type of discharge to move material into a subsequent conveyor or process.

To ensure positive discharge of material, vertical screws are often affixed with “kicker paddles”, other flat paddle‐type or reverse flights to foster complete discharge of the material. Even so, there are times when it is advisable to provide a safety overflow. This is usually an opening diametrically opposite and above the discharge spout, arranged to spill the material if the discharge spout becomes clogged and unable to handle the normal material flow.

Hanger or Stabilizer Bearings in Vertical Screw Conveyors

Intermediate hanger or stabilizer bearings usually are necessary in vertical screw conveyors when extended heights of lift are required, to eliminate excessive screw deflection and “whip.” These hanger or stabilizer bearings are positioned between the sections of the screw and are supported between the housing flanges. The particular kind of hanger or stabilizer bearing to use is determined by the characteristics of the material being handled.

Some materials travel upward in a mass and would experience obstruction by an intermediate stabilizer or hanger bearing. One such material is cottonseed. With it, hanger bearings are not used. Certain other materials tend to center the screw within the housing, thus eliminating the need for stabilizer bearings. Some lighter duty materials can also allow for wear‐shoe made out of a material such as UHMW; this is simply a flight that extends slightly past the O.D. of the screw to assist in centering and eliminating “whip.”

Vertical Screw Conveyors Construction Materials

1. Carbon Steel
2. Abrasion resistant steel
3. Stainless steel

Average capacities and speeds of Vertical Screw Conveyors

What are the things you need to consider to put up a Vertical conveyor system?

1.) Lump Size – Material with large variance in particle size, or excessively large lumps tend not to properly convey in a vertical screw. In general, free‐flowing material that does not degrade will work suitably in a vertical screw. Friable material along with extremely fine powders that aerate will often have issue working properly in a vertical screw.
2.) Extremely Dense Material – Very dense material that has little to no compressive capability may not convey properly due to the fully loaded condition and speed at which a vertical screw operates.
3.) Abrasive Materials – Due to the high speeds and full contact of the material to the screw, abrasive materials may rapidly degrade components, along with prematurely wearing seals and drive components.

Vertical screw conveyors or some special design of vertical screw conveyor finds wide application in ship unloading.

Practical experience with these conveyors has shown that the resistance factor for vertical conveyors is higher than those of the horizontal conveyors. Resistance factor λ may be taken as 5.5 to 7.5 for grains. 6.5 to 8.3 for salt.

Design of Vertical Screw Conveyors

The driving power of the loaded screw conveyor is given by:

P = P_H + P_N + P_st

Where,

P_H = Power necessary for the progress of the material

P_N = Driving power of the screw conveyor at no load

P_st = Power requirement for the inclination of the conveyor

Power necessary for the progress of the material P_H:

For a length L of the screw conveyor (feeder), the power PH in kilo watts is the product of the mass flow rate of the material by the length L and an artificial friction coefficient λ, also called the progress resistance coefficient.

P_H = I_m.L. λ.g / 3600 (kilowatt)

= I_m.L. λ / 367 (kilowatt)

Where,

I_m = Mass flow rate in t/hr

λ = Progress resistance coefficient

Each material has its own coefficient λ. It is generally of the order of 2 to 4. For materials like rock salt etc, the mean value of λ is 2.5. For gypsum, lumpy or dry fine clay, foundry sand, cement, ash, lime, large grain ordinary sand, the mean value of λ is 4.0.

In this connection it should be noted that the sliding of the material particles against each other gives rise to internal friction. Other resistance due to grading or shape of the output discharge pattern contributes to the resistance factor. That is why the parameter λ is always higher than that due to pure friction.

Drive power of the Vertical screw conveyor at no load, P_N:

This power requirement is very low and is proportional to the nominal diameter and length of the screw.

P_N = D.L / 20 (Kilowatt)

Where,

D = Nominal diameter of screw in meter

L = Length of screw conveyor in meter

Power due to inclination: P_st

This power requirement will be the product of the mass flow rate by the height H and the acceleration due to gravity g.

P_st = I_m.H.g / 3600

= I_m.H / 367

H should be taken positive for ascending screws and will be negative for descending screws.

Total power requirement of Vertical Screw Conveyor:

The total power requirement is the sum total of the above items

P = (I_m (λ.L + H) / 367) + (D.L /20) (Kilowatt)

Advantages of Vertical Screw Conveyor

• Efficient design
• Easy of installation
• Requires less space
• More cost effective
• Can be dust tight
• Material discharge in any direction

Disadvantages of Vertical Screw Conveyor

• If the rotation of the vertical screw is stopped, the conveyor will be full of materials. It is also true that if the vertical screw is left turning but the feed of material ceases, the vertical screw conveyor will not empty itself, some of the material will be left in the housing in an amount depending on the material characteristics. It is important to realize, however, that material left over from previous operations will be the first to discharge when the vertical screw conveyor is restarted.
• The ideal operation of a vertical screw conveyor is to have a controlled and uniform volume of material fed to the unit. Uneven or surge loads and start, stop operations can affect specifications of speed, capacity and horsepower. Some granular or pelletized materials can roll to the bottom of the vertical section after stopping and will then create a starting problem.

Applications of Vertical Screw Conveyor

• Animal feed
• Pet food
• Food processing
• Ice
• Bulk powders
• Bio solids
• Pellets
• Bone and Blood meal