Mass Flow Rate on a Vibrating Screen

Posted in: , on 4. Oct. 2003 - 18:57

Determining Mass Flow Rate on a Vibrating Screen

Hello George,

Many times my company has been requested to determine the mass flow rate on both vibratory feeders and screeners. We are a control systems integrator, and commonly instrument screw and belt conveyors by weighing the live load and measuring the conveyor speed (mass x velocity). Typically the throughput on the vibratory applications are way too high to consider loss-in-weight technology, and there seems to never be enough headroom to install a mass flowmeter device.

I am considering installing tension loadcells into the feeder support cables and monitoring the eccentric motor speed. Will the RPM of the vibratory drive be “relative” to the material velocity? It is critical for the RPM vs velocity relationship to be repeatable, however with our PLC-based electronics it does not have to be linear because we can calibrate multiple points on a curve. Thank you in advance for your assistance.

Regards, Delmar Schmidt

Melfi Technologies Inc.

Phone: (281) 298-8398

Fascinating Question.........Love It

Posted on 8. Oct. 2003 - 11:02

Thank you for your question. I am a in the field MACARONI and cheese type guy and will need more help understanding your fine question.

I assume you are talking about ELECTROMAGNETIC volumetric type feed devices.........say on the underside or live bottom if you will of a discharge bin setup?

Lets establish thee application first then we shall proceed from there please.

I am interested in learning here also.

THANKS George ASSINCK Limited, Markham Ontario


Best Regards, George Baker Regional Sales Manager - Canada TELSMITH Inc Mequon, WI 1-519-242-6664 Cell E: (work) [email][/email] E: (home) [email][/email] website: [url][/url] Manufacturer of portable, modular and stationary mineral processing equipment for the aggregate and mining industries.

Re: Mass Flow Rate On A Vibrating Screen

Posted on 13. Oct. 2003 - 04:57

For the sake of this thread let us invent a typical application. To simplify this discussion I will eliminate the secondary variable of changing the vibratory rate (motive speed):

A rectangular electromechanical screener, with a fixed-speed motor, is 4-feet wide has an inlet-discharge length of 8-feet. It is starve-fed, receiving rice from a wild-flow stream at an average rate of 20T/hour, however the flow will unpredictably swing from 10T-30T/hour due to changes in the upstream process. The screener is removing both fines and trash, however assume the amount of reject material is negligible so we will disregard this loss.

My requirement is to meter an additive into the rice stream at a ratio of 0.1%. Somehow I must determine the instantaneous rice flowrate, but I don’t have enough space to install a weighbelt, flowmeter or other ancillary piece of equipment. I assume the screener should behave similar to a vibratory feeder, as it will convey the rice horizontally and in a controlled fashion from the inlet to the discharge.

The conveying time each grain takes to traverse the length of the screener should be relatively constant, because it is dependant on the motive force supplied by the vibrator. My hypothesis is that if the fixed-speed motor maintains a stable and repeatable material velocity, the live weight (bed depth) of the rice within the screener will change proportionally to the mass flowrate.

It is also my hypothesis the efficiency of the screener will decrease proportional to the increase in loading (rate). This should further increase the bed depth relative to the throughput, which will assist in my rate measurement. This efficiency relationship does not have to be linear, however it is critical to be repeatable.

My plan is to suspend the screener housing on loadcells and weigh the contained rice to derive changes in mass flowrate. I can catch timed samples at varying flowrates to calculate the rate; I will not need to attempt a theoretical mathematical computation.

Please confirm my hypothesis that the material velocity (traverse time) will be constant and repeatable with respect to material throughput. Thank you again for your assistance.


Posted on 16. Oct. 2003 - 03:06

You are a very sharp guy: I can confirm that that feed rate and volume for a given weight of material for rice will be constant. Constant, taking into consideration the material flowability every day is constant.

Flow rate will be constant if moisture content is constant. Assuming moisture content and bulk density or weight of the material per cu foot is constant - Bed depth will be constant at a given ANGLE of repose of the vibrating unit.

Efficiency (a key factor) is dependant on staying within the rules of BED DEPTH calculations. TOO DEEP equals inefficient and carryover when the fines should pass. TOO THIN on the bed depth has a tendency to allow the material to be TOO active and also creates inefficiency.

Your hypothesis seems quite correct to me. TRAVERSE time down the length of the screen will be constant as long as the angle of the vibrating device remains the same. It can actually be timed by flourescent painting some particles and timing the seconds from the start to the discharge end. This is your foot travel rate per minute. I do this with rocks....I do not know how you would do it with rice.

My thoughts........George

Best Regards, George Baker Regional Sales Manager - Canada TELSMITH Inc Mequon, WI 1-519-242-6664 Cell E: (work) [email][/email] E: (home) [email][/email] website: [url][/url] Manufacturer of portable, modular and stationary mineral processing equipment for the aggregate and mining industries.