Screw Feeder (Auger) Calculation for Variable Flow Rate with Single Screw

What's a BD?

https://www.bechtel-wuppertal.com/media/1859/calculations-screw-conveyor-bechtel.pdf

BD is bulk density mentioned in SI unit

Horizontal or sloped? The Bechtel reference applies only to horizontal.

What do you think the screw motor variation limit is? 15:1 is easily accomplished with some care sizing the motor, keeping in mind constant HP above base frequency. If you add cooling you can control to lower speeds.

Your 32.5:1 turn down requires control down to 2Hz, which can be done with cooling and encoder feedback, and perhaps with sensorless vector control, on a motor by motor basis, low slip design.

If you don’t need absolutely continuous feed for the process, intermittent feed at the low end is also quite feasible.

Other solutions would seem to be much more complicated, reducing your overall reliability and increasing your investment, extending delivery, complicating diagnosis and repair/maintenance costs...

hi rwilliams,

you are correct on your calculation - the difference on the Hz is 70 Hz to 2 Hz for the 0.2kg/min to 6.5 kg/min which results in the 12 kg/hr to 390 kg/hr respectively.

To achieve the output on a single fixed pitch screw feeder, we have to work on a large range of the Hz, which can eventually affect the motor efficiency.

the charging rate can be - 5 to 15 mins per batch

<...the charging rate can be - 5 to 15 mins per batch...>

Well, is it <...5...mins...> or <...15 mins...> or something in between?

If it happened faster or slower then would it matter?

Why is variable speed needed at all? Could one not just run the <...single fixed pitch screw feeder...> on a single speed for a fixed period of time?

Please zoom out a bit and describe the circumstances in which this equipment finds itself.

The effect on motor efficiency is a minor consideration, this sort of operation is practiced in many machines over a wide range of industry. A short charging time & suitable rest interval may allow you to dispense with the cooling needs at low speed high torque operation. A motor design engineer can advise you with the duty cycle & ambient temperature information. It doesn’t get much simpler.

Your high turn down ratio throws up another problem. Unless your ingredient is free flowing like plastic granules you will have trouble feeding the auger. To guarantee the auger is always full you need to agitate the feed hopper. Agitation suitable for 6.5kg/min is likely to compact material flowing at 0.2kg/min causing it to bridge. An intermittent vibrator set to a mark space ratio which mirrors your turn down might work but some trial and error (and a back up plan) will be needed during set up.

For the turn down ratio expressed in various posts rather than using a VFD why not use a three phase stepper motor of the appropriate size. From my experience with VFD drives on screw feeders on flocculant and lime the minimum speed is 10% of full motor speed. Below this value if the motor is a TEFC motor then the fan does not produce enough cooling and the motor will overheat unless an external cooling fan is used but that increases the complexity.

I have used a three phase stepper motor of 5KW to drive a batching worm. The step mark space can be adjusted to drive the motor at slow speeds but clogging can occur.

You do not say what the product is so its characteristics are unknown as its cohesion or particle size. I have used screw feeders on lime powder to floc which is a long chain polymer with a grain size similar to refined sugar.

First, I do not understand why are you designing a screw conveyor for the first time without any help. Have the MBAs and managers in your company fired all of the senior staff in an effort to "save costs" ?.... Are you the sole young engineer in your company recently hired to do all of the technical work ?

Second, I do not understand why you are designing a screw conveyor at all. Screw conveyors are designed for a specific service, specific conveyed materials and use minimally expensive MOC...... They are like light bulbs.... Specify the right one, use it up, discard and replace it when necessary.

Third, experienced screw conveyor vendors are more than happy to perform all of the necessary flow and hp calculations for you, if you provide geometry to them and award them the contract. This is the normal course of business ....

Fourth, KWS is an experienced and highly respected American vendor of conveyors. They have an intensive and detailed calculation methodology available that is far superior to most anything available on the internet .... the link is here:

https://www.kwsmfg.com/wp-content/themes/va/pdf/Screw-Conveyor-Engineering-Guide.pdf

Fifth, since CR4 is an second-string engineering forum that caters to freshers, students and people with zero experience, there are no normal niceties such as "please and thank you" often used ....

I can only assume that this information helps you, because you will probably ignore this thread forever in the future, not say thank you, and will never complete the thread

What is being propelled - sewage sludge, for example? Temperatures? Pressures?

Please describe the process being designed. If in doubt, consult a qualified local Process Engineer.