|1. Screw conveyor
design : methods
|2. Design procedure of a screw conveyor|
|3. Screw conveyor
size and capacity calculation tool
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
Known : capacity required of the screw conveyor, material
Unknown : screw size and characteristics
The capacity of a screw conveyor with a standard screw flight can be estimated the following way :
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
Example : the requirement for a screw conveying sugar is 3500 kg/h.
Step 2 : calculate the capacity of the screw conveyor
|Pitch||Pitch length S|
|Material||Min loading ratio||Max loading ratio|
|Not free flowing||0.12||0.15|
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
|Inclination in °||Correction factor C|
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 :
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|Screw diameter in m||15%||30%A||30%B||45%|
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
The CEMA association gives the capacity of a screw conveyor as :
C = 0.7854*(Ds2-Dp2).P.K.60/1728
C = capacity in ft3/h/rpm
Ds = Diameter of the screw flight in inches
Dp = 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.
You can find a calculation tool here applying the
formula given in this page. Please use only this tool to get a rough
idea of a screw size, do not perform detail design with this Excel