Material Handling - Feeder Motor Speed Control VIA Blower Pressure

[jdbrandt]

I've done both dilute phase (which this IS, by the way, regardless of your boss's opinion) and dense phase transport systems, mostly of cement, but also of flour, mag stearate, guar gum, etc. Pressure switches are commonly used in those industry, just like Smoot uses. Now, assuming a Roots' blower type of PD blower, you can 'sense' the air moving with a pitot tube sensor. Granted, they're a bit tough in a material stream, but, they do make them for this. What I generally did was to monitor the pressure in the pipe...if it went too high, I'd keep the blower running, but pause the rotary feeder. Too low...you might be out of raw material, rat-holed or bridged, and just be blowing air into the hopper. You may also want to use an analog output from your blower drive to use blower horsepower as a measure. I don't have the curves for a PD blower, but I'll be the manufacturer can say how many amps equal how much air. Remember, a deadheaded impeller type fan moves no air, and hence, draws very little current. I don't know what the rules are for a PD.

 

[jdbrandt]

Dude: If you have load cell feedback on the silo, you know the feed rate of the material filling it....just do a 'transfer of custody' style interlocking and you're done.

 

[jdbrandt]

My biggest concern would be the abrasiveness of the mica. Abrasiveness, is that even a word. Anyway, even it a powdered form this mica is very abrasive. We have had to put wearbacks on piping turns and transitions. All the target type flowmeters that I have used have been in fluid or steam flow applications.

I am glad I found this place. For years I have talked about this stuff to people and they just look at me with a blank stare.

Robert

You've got good reason to worry. The pipes are actually the least of your problems. If you're using a RJPFF bag filer, the bag material selection and baffling of the house are critical to making all this work. MICA travels in the direction of the air, and, as a result, it hits the bags like little flying knives. Your bag supplier should have spec'd the right bags, else he will become you 'new bestest friend', and you will spend all your time changing bags.

 

[R_Stone]

It may not work for your application, but here is an interesting product:

http://www.auburnsys.com/homepage.htm

Tom, you read my mind. Actually what I have gone with on this and on or baghouses as well are the series 70 and 30 particulate monitors and filter monitors. These are induction units since typical opacity equipment would not last in this environment.

FilterSense


Thanks Tom, great minds think alike.

Robert

 

[Tom Jenkins]

You may also want to use an analog output from your blower drive to use blower horsepower as a measure. I don't have the curves for a PD blower, but I'll be[t] the manufacturer can say how many amps equal how much air. Remember, a deadheaded impeller type fan moves no air, and hence, draws very little current. I don't know what the rules are for a PD.

I can help there.

A PD blower (Positive Displacement) is essentially a constant volume flow rate device at a constant rpm. I never use a meter on PD blowers for air flow - I just calculate the flow based on the following formula:

CFM = CFR x (rpm - slip)

CFM = Cubic Feet per Minmute, of course, and is the inlet side volumetric flow rate

CFR = Cubic Feet per Revolution, also known as the displacement of the blower

rpm = actual blower drive shaft speed, which may not equal motor speed

slip = slip rpm, which is the extra turns the blower impellers need to make up for leakage between impellers and housing

So, if you know the ratio of blower speed to motor speed, and you get the slip and CFR from the manufacturer's data you can get the flow rate of the blower. Unlike a centrifugal fan or blower a PD cannot be dead-headed - it will continue to push air. If the discharge is plugged the pressure will rise until something blows. That's why you always need a pressure relief valve on PD blowers!

You are right that the pressure is related to amps, but flow isn't. This again is different from centrifugal fans and blowers, where amps is roughly proportional to flow rate. For a PD blower:

bhp = (FHP x rpm/1000) + rpm x CFR x dP x Fg

bhp = blower shaft horsepower

FHP = friction horsepower = losses for bearings gears etc. and is usually expressed in bhp per 1000 rpm

dP = pressure difference from inlet to outlet

Fg = gas horsepower factor, usually expressed as bhp/psi/CFM

FHP and Fg are also available from the blower manufacturer.

So, for a given blower at a constant rpm the motor current draw will increase roughly proportionally to discharge pressure. At a constant discharge pressure the blower will draw roughly the same amps regardless of speed and flow rate, because torque is related to amps and torque is roughly proportional to pressure.