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Screw Conveyor design calculations step by step

Design of screw feeders (capacity, size)

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Page summary
1. Screw conveyor design : methods
2. Design procedure of a screw conveyor
3. Screw conveyor size and capacity calculation tool

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1. Screw conveyor design : methods

How to size a screw feeder ?

Screw conveyors are among the most widespread equipment for transporting and dosing bulk solids. It is thus required in many projects to calculate the size of a screw conveyor in order to reach a required capacity. It can be also required to check the capacity of an existing screw in case the material handled change or the capacity required increases. Few formula exist in the literature and are presented in this short Engineering Guide.

Warning : It must be noted that those formula aim at giving an idea of the size of the screw conveyor (which means its diameter) and the speed at which it will operate, based on some assumptions and some design decisions (choice of screw pitch, inclination). The formula can also be used to roughly check the capacity of an existing screw feeder. They are not meant for a detail design and for manufacture of a new screw, a task that must always be given to a specialized company (for example adhering to CEMA association in US, or SHAPA in UK).

This page is considering that the material is not lumping. It is important, especially when using the screw in mining, concrete industries where special precautions, not detailed here, should be taken.

A typical screw conveyor design is shown below :

Figure 1 : Screw conveyor principle drawing and key components

2. Design procedure of a screw conveyor

How to calculate the size and capacity of a screw feeder ?

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

2.1 General 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	S=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%.

• For inclined screw conveyor design : 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%B	45%
0.1	69	139	69	190
0.15	66	132	66	182
0.23	62	122	62	170
0.25	60	118	60	165
0.30	58	111	58	157
0.36	56	104	56	148
0.41	53	97	53	140
0.46	50	90	50	131
0.51	47	82	47	122
0.61	42	68	42	105

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.

Note that when handling powders susceptible to lead to a dust explosion, the max speed should give a conveyor tip speed < 1 m/s.

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*(D_s²-D_p²).P.K.60/1728

With :

C = capacity in ft³/h/rpm
D_s = Diameter of the screw flight in inches
D_p = Diameter of the pipe -shaft - supporting the screw flight inches
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.

3. Screw conveyor size and capacity calculation tool

Warning : this calculator is provided to illustrate the concepts mentioned in this webpage, it is not intended for detail design. It is not a commercial product, no guarantee is given on the results. Please consult a reputable designer for all detail design you may need.

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