Screw conveyors are used in transporting goods relatively short distances and for angles from zero to 60 degrees. They are used in for example in agriculture to transfer crops. For longer distances belt conveyors are better solution for their ability to have adjustable belt length. However, they have relatively smaller inclination degrees. For upwards (higher than 60 degrees) it is better to use bucket conveyor due to their ability to transport goods to even 90-degree angles. Screw conveyors can also be used as dosing machine. There are also small movement resistances when moving screw without load, it can handle high temperature ma-terials and it has relatively high transportation capacity. []
Figure 2-1 Screw conveyor example. In figure we can see input head of the screw conveyor and screw.
(Wikipedia)
Screw conveyors transport goods with a screw. It pushes goods forward with each cycle.
Amount of goods which can be transported depends on screw properties such as length of screw, distance between screw pitches, diameter of the screw, and rotational speed of screw.
Screw conveyor needs motor, which is usually induction motor with straight fastening or belt coupling.
Usually screw conveyors are used in off-on setting. The electrical motor is started with start-ing resistor or with wye-delta style. Transfer capacity cannot be controlled in this kind of setting, but instead screw conveyor is run at max transfer limit.
With frequency converter it is possible to control motor with a torque or speed controller. It is also possible to detect some fault situations and run with a soft starter for screw conveyor.
Transportation capacity can be easily adjusted for production needs with frequency con-verter, as well as limiting torque, fast reaction to fault situations and optimal transportation speed for preserving material attributes. Also, earlier mentioned savings are one upside for using frequency converter.
Calculation of screw conveyor capability
In this chapter are presented essential equations for the screw conveyors. They are derived from the ISO 7119-1981 standard. Screw conveyor power consumption, mass flow, volume flow rate, and needed motor speed are presented in this chapter. These equations are then used later on modelling of the screw conveyor system. The following variables that are later on measured on actual setup or in simulation model: Mass flow rate and screw conveyor overall power consumption P. Motor speed n is taken from either frequency converter or from the simulation model variable. With these it is possible to calculate screw conveyor energy efficiency.
Screw conveyor volume flow rate in m3/h can be calculated with 2.1 (ISO 7119-1981, 1980)
= 60 , (2.1)
where is filling coefficient, D is diameter of the screw,S is pitch of the screw andn is the rotation speed of the screw in rpm.
When a bulk material density in kg/m3 is known, screw conveyor mass flow rate can be calculated with (ISO 7119-1981, 1980)
, (2.2)
Screw conveyor overall power consumption can be calculated (ISO 7119-1981, 1980)
, (2.3)
wherePh is the power necessary for the progress of the material, Pn is the drive power when no load is on the screw conveyor, and Pst is the power consumption due to inclination. All the powers are given in watts, W.
Power necessary for the progress of the material can be calculated with formula 2.4
= = , (2.4)
whereL is the length of the screw, is progress resistance coefficient, andg is the accelera-tion due to gravitaaccelera-tion.
Drive power of the screw conveyor at no load can be calculated.
= , (2.5)
whereD is diameter of the screw andL is the length of the screw.
Power consumption due to inclination can be calculated
= = , (2.6)
whereH is height where the material is transferred, andg is acceleration due to gravitation.
Combining all the previous formulas, overall power consumption can be calculated.
= ( )+ , (2.7)
Further, it is possible to calculate mass flow when power is known.
= , (2.8)
By contributing (2.1) and (2.2), and then reducing the formula, rotational speed of the screw in rpms can be calculated.
= , (2.9)
Calculation of screw conveyor system efficiency Screw conveyor efficiency can be calculated as
= , (2.10)
where Poutis mass of material that gets through the screw conveyor (tons/h) and it can be calculated from (2.2).Pinis consumed power (kW) and it can be calculated from (2.7). This leads that the efficiency is presented in tons/kWh. This tells how much material is trans-ported with specific amount of energy. For example, if mass flow is 10 tons per hour and used power is 0.8 kW, then screw efficiency is 12.5 tons/kWh. However, it is usually better to express energy efficiency in fuel consumption format rather than in fuel economy format.
In figure 2-2 is shown relationship between fuel consumption and fuel economy. As can be seen from the figure, fuel economy gives non-linear relationship for the fuel consumption.
When fuel economy increases from 0.01 tons per Wh to 0.02 tons per Wh, fuel consumption drops from 100 Wh to 50 Wh. But when fuel economy increases even more from 0.02 tons per Wh to 0.03 tons per Wh, fuel consumption drops from 50 Wh to 33.3 Wh. With the first case fuel consumption dropped 50 Wh, but the next step dropped fuel consumption only 16.7 Wh. Because of this, in this thesis fuel consumption Wh per tons is used instead of fuel economy tons per Wh [US Gov 2018].
Figure 2-2 When fuel economy increases from 0.01 tons per Wh to 0.02 tons per Wh, fuel consumption drops from 100 Wh to 50 Wh. But when fuel economy increases even more from 0.02 tons per Wh to 0.03 tons per Wh, fuel consumption drops from 50 Wh to 33.3 Wh. With the first case fuel consumption dropped 50 Wh, but the next step dropped fuel consumption only 16.7 Wh.
Screw conveyor related patents
There are lots of screw conveyor related patents even from 1955 onwards. However, they are usually mechanical patents and they aim to improve screw or screw intake capabilities.
Energy efficiency, frequency converter or energy efficient frequency converter screw con-veyor patents are rare [Wredfors 2013]. An example of the mechanical screw concon-veyor pa-tents is US patent 20130167740. Invented by Johann Doppstadt and Horst Berger in 2011, for the original assignee Doppstadt Familienholding Gmbh, this screw conveyor patent fo-cuses on how to dry wet materials with screw conveyor [Doppstadt et al. 2011].
Figure 2-3 Patent number 20130167740. Invented by Johann Doppstadt and Horst Berger in 2011, this screw conveyor related patent was designed to dry wet materials with screw conveyor systems.
Another mechanical screw conveyor patent is US2830695 A. Invented by Marion H. Fen-nimore and Ivan J. Stephenson in 1955, it was designed for situations when normal screw conveyor can’t use desired direct path due to obstacles. This patent focuses on how to make screw conveyors flexible, so that screw conveyors can be bent around, under, or over the obstacles. It has joints which will become curved when screw conveyor is bent. [Fennimore et al. 1955]
Figure 2-4 US2830695 A, screw conveyor with joints. Designed by Marion H. Fennimore and Ivan J.
Stephenson for situations when normal screw conveyor can’t use desired direct path due to obstacles.