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Screw Conveyor design calculations

Design of screw feeders

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Page summary
1. Definition of a screw feeder
2. Position in the process
3. Important considerations for the design
4. Screw flight design
5. Dosing
6. Common problems with screw conveyors

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1. Design procedure

How to size a screw feeder ?

Different design procedures can be found in the industry. The one presented in this page is derived from the CEMA methodology, it allows to perform a basic design but is not aimed at a detail design and manufacturing - ALWAYS refer to a reputable company before manufacturing a screw conveyor, for examples companies part of the CEMA association.

The procedure presented in this page will allow to approximate the capacity of the screw conveyor considering different design parameters like... [                 ]. The methodology is aimed at designing quite large screws for coal, cement...etc... industries, for smaller screws please sense check the results given by the procedure.

A typical screw conveyor design is shown below :

Figure 1 : Screw conveyor principle drawing and key components

2. Design of a new screw conveyor

Known : capacity required of the screw conveyor, material
Unknown : screw size and characteristics

2.1 Simplify formula

The capacity of a screw conveyor with a standard screw flight can be estimated the following way :

With

Q = screw capacity in kg/h
D = screw diameter in m
S = screw pitch in m
N = screw speed in rpm
α = loading ratio
ρ = material loose density in kg/m3
C = inclination correction factor

Step 1 : define the requirement

Define the capacity required for the screw conveyor. The design of the screw must reach a capacity equal or greater than this value.

Example : the requirement for a screw conveying sugar is 3500 kg/h.

Step 2 : calculate the capacity of the screw conveyor

• Assume a diameter D

• Define the screw pitch (depends on the characteristics of the product to be conveyed, among other parameters) according to the diameter of the screw.

Pitch	Pitch length S
Standard	S=D
Short	S=2/3*D
Half	S=D/2
Long	D=1.5*D

• Estimate the loading ratio α of the screw according to the flow properties of the solid to be conveyed

Material	Min loading ratio	Max loading ratio
Not free flowing	0.12	0.15
Average flowability	0.25	0.30
Free flowing	0.4	0.45

To be noted that these are orders of magnitude only. Values can be more or less high, in some cases the loading ratio can even reach 95%.

• Define if the screw conveyor is flat (which is always preferable) or has to be inclined. Determine the correction factor corresponding.

Inclination in °	Correction factor C
0	1
5	0.9
10	0.8
15	0.7
20	0.65

• Adjust the screw speed so that the capacity of the screw is higher than the requirement.

Example :

• The diameter assumed is 0.1 m
• No specific duty for the screw, the pitch is chosen standard, S=0.1 m
  
• Sugar is free flowing, a loading of 0.45 is selected
• The screw is installed without inclination, C=1
• Sugar density is 800 kg/m3

The calculation gives 17 kg/h for 1 rpm. Adjusting the speed, 207 rpm are required to reach a capacity of 3500 kg/h.

Step 3 : compare the calculated capacity to the max screw speed

Some reference max screw speed are given in the table below :

Screw diameter in m	15%	30%A	30%	45%
0.1	69
0.15	66
0.23	62
0.25	60
0.30	58
0.36	56
0.41	53
0.46	50
0.51	47
0.61	42

If the calculated speed at step 2 is < than the max speed for the screw diameter selected, the design can be kept.

If the calculated speed at step 2 is > than the max speed for the screw diameter selected, the design is not suitable and the calculation must be run again by changing a parameter, typically the diameter.

Example :

• The diameter selected was 0.1 m for which the max screw speed at 45% is advised to be 190 rpm
• The calculated speed is too high, the calculation must be done by changing a parameter. We can select a diameter larger, D=0.15 m
• The calculation is run again and this time a speed of 62 rpm is calculated. It is less than the max recommended, speed, the screw design can be selected.

2.2 Alternative formula given by CEMA

The CEMA association gives the capacity of a screw conveyor as :

C = 0.7854*(Ds2-Dp2).P.K.60/1728

With :

C = capacity in ft3/h/rpm
Ds = Diameter of the screw flight in inches
Dp = Diameter of the pipe -shaft - supporting the screw flight in ches
P = pitch of the screw in inches
K = percent trough loading

It is the capacity over 1 rpm. If the dimensions are know, in the case of an existing screw, the screw capacity / h can then be found by multiplying by the rpm at which the screw is used.

The formula gives similar results as the one stated above.

Speed of screw conveyor

The speed of the screw conveyor can be found by dividing the required capacity Q by the calculated capacity C : N = Q/C

It can also be calculated the following way when standard screw design are at disposition : N = Q.CF0.CF1.CF2.CF3/C1

With :

N = screw speed in rpm
Q = required capacity in ft3/h
C1 : conveying capacity at 1 rpm, tabulated
CF0 = overload capacity = 1.1 to 1.2
CF1 = conveyor pitch factor
CF2 = type of flight factor
CF3 = mixing paddle factor

Checking the capacity of an existing screw conveyor

4. Power requirement of screw conveyor

5. Example of calculation

Let's say an engineer wants to size a screw conveyor to convey wheat flour at 5 t/h.

At a density of around 0.510 kg/m3, which gives a volumetric flow of 5000/0.510 = 9800 l/h = 9.8 m3/h. To convert in ft3/h, it is necessary to multiply by 35.3147 which gives 346 ft3/h.

STEP 1 : understand the service needed

The screw is to be operated flat, not inclined, there is no particular need for special flights, so a plain flight will be used (flight is not cut, it is not a ribbon, no paddles...).

Flour is flowing quite well and is fine, the filling rate is assumed to be min 45%.

STEP 2 : assume a screw size

The screw size is assumed at 150 mm to start with, it is around 6 in. A standard pitch is assumed.

STEP 3 : define the standard capacity of the screw

From the tables, a 6 in screw conveyor with a filling rate of 45% has 368 ft3/h at 165 rpm and 2.23 ft3/h at 1 rpm.

STEP 4 : calculate the required speed

N = 346 * 1.2 * 1 * 1 * 1 / 2.23 = 186 rpm

STEP 5 : control if N is < max rpm of the screw

186 rpm is > than 165 rpm, a bigger size must be chosen.

The same approach is repeated for a 9 in screw conveyor. From the tables, a 6 in screw conveyor with a filling rate of 45% has 1270 ft3/h at 165 rpm and 8.20 ft3/h at 1 rpm.

N = 346 * 1.2 * 1 * 1 * 1 / 8.20 = 50 rpm

This time the required speed is clearly on the safe side vs the tabulated value. The screw can be chosen.

Auger conveyors are usually placed below hoppers where they can convey horizontally the product to another process operation. When the conveying involves also a weighing, the screw feeder is actually used as a dosing equipment.

Screw conveyors can also introduce solids in a pneumatic conveying system. In most of the case, they are used in vacuum transport since there is no pressure problem, but some designs can also used to feed the product towards a pressure conveying line. For this particular purpose, the screw flight design must allow a compression of the solid so that the plug formed avoids the pressurized air of the conveying line to come back through the screw. This cannot be used for all products, thus, for pressure conveying, airlock rotary valves are usually preferred to screw conveyors for introducing the product.

Screw feeders are found in any industries handling bulk materials, to convey and / or to dose bulk solids :

- Food processing
- Pharmaceuticals
- Grains / Animal Feed
- Water treatment (sludge processing)
- ...

3. Important considerations

What is important when designing a screw conveyor ?

Industrial Auger conveyors can be very long reaching >5 m, however this kind of design can only be used with non sensitive product and when no cleaning is required. It is indeed not possible to remove the flight of such screws, and if some designs exist to still access the inside (trough design - show example), the efficiency of cleaning is reduced and the time to clean will be long. Besides, the length of the screw makes it prone to bending and to possible metal / metal contacts with the housing, risking to pollute the product with metal chips. As a reference, a screw flight of 4 m can bend up to 7-8 mm from the shaft straight axe, if no proper provision have been taken to allow for such bending, metal / metal contact can happen.

It appears better, for hygienic applications, to request the following characteristics :
- Short screws
- Extractible flights
- 2 bearings / although cantilevered designs are available for very short screws of 1-1.5 m
- Flat operation
- Tip speed 1 m/s and pressurized bearing seals - for ATEX prevention

Inclined screws are tempting because they allow to solve many process problems especially when retrofitting installations, one should however be very careful that these types of screws are usually difficult to access and extract for cleaning, may be also more sensitive to bending and metal metal contacts, and are not very efficient (efficiency is decreasing with the angle).

Table 1 : Screw conveyor capacity as a function of its inclination

Angle of screw (degrees)	Percent of max capacity (%)
<8	100
20	55
30	30
45	0

There are different types of screw feeders that can be found in process industries :

- Shafted screw conveyors
- Shaftless "Pig tail" screw feeders
- Flexible screw conveyors
- Trough screw conveyor or tube screw conveyors
- Single or double screw (particularly used in loss in weight feeders)

Screw conveyors are rotating equipment, which them a risk for operators who would try to reach the inlet of outlet of the screw. For this reason, a special attential must be paid to the inlet and down pipe. If they are equipped with flexible especially, those flexible should be dismantable only with a tool and the operator trained to stop the machine if such dismantling is necessary. In case it is not possible, a cross in the pipe preventing to reach the screw flight can be installed, provided that it does not prevent the flow of product. In any case, a risk analysis must be carried out by the plant operator to make sure the access is properly protected.

4. Screw conveyor design

What are the characteristics of a screw flight (pitch...) ?

The screw flight can take different shape and pitch according to the application in which it is used.

- Standard flight : constant pitch = 1 diameter ; works for most of the applications

- Short pitch : used for inclined screws, can also be used for materials that are easy to fluidize

- Enlarging pitch : smaller pitch at beginning of the screw, can be used for feeders below a hopper to provide a constant feed.

- Shortening pitch : large pitch at the beginning and shorter pitch at the end, this will create compression, it is not a common design and can lead to a lot of thrust on the screw, power requirement, and possibly mechanical damage. If the screw is properly designed, this screw flights can be used as feeding devices towards a pressure system, since the powder compressed at the end of the screw will act as a plug preventing the gas to leak towards the screw.

- Ribbon : used mainly for sticky materials

Other designs are available with paddles or cut flights when there is a need for mixing the material.

In applications requiring high hygiene degree of finition, the shaft must be plain, not hollow, and the flight must be fully welded and polished. This in order to avoid that material get trapped in non accessible areas and pollute later good products.

The design of the shaft and the motor must be well aligned so that the motor is able to run the screw and start it from 0 speed or run it overloaded, when it requires the most torque. Conversely, the shaft must be designed to sustain such a torque and not break when it is applied.

5. Dosing

When used in dosing applications, the screw must be equipped with a motor on VFD in order to run in coarse speed and fine speed (see dosing page). Screw conveyors are most often integrated in automatic Gain in Weight or Loss in Weight systems. When running Loss of Weight, the screw feeder can be operated either in volumetric mode or on automatic gravimetric mode.

6. Common problems with screw conveyors

Screw conveyors are usually reliable equipment, however a certain number of issues may require some corrections :

Table 2 : common problems with screw conveyors

Issue	Root cause and action
Metal - metal contact	Bearing damage - change the bearing Incorrect clearance screw flight / housing - review the design or change the flight Foreign body in the screw - review foreign body prevention prior to screw conveyor
Bearing damage	Powder has entered the bearing - check bearing seal and flushing
Screw performance below design	Screw flight mounted in the wrong direction Bad feed of powder to the screw - check the feed of product if no bridging or ratholing
Screw conveyor blocked	Check if outlet valve is opened Check pitch of the screw - the powder should not be compacted
Inaccurate dosing with screw	Size too big - check screw size or adapt screw flight profile at the end of the screw