how to measure vacuum suction
#11
  
I'm attempting to repair a commercial/industrial vacuum. The contractor uses these while grinding concrete. there are two of them; one 120v and one wired 249v but they're basically the same units. Eash has three motors and another small one that drive the "clicker" for the filter pleats. The 120v version will trip a 20a breaker frequently, so I've been asked to separate the load onto two 120v cords which is basically the way the 240v is wired.(so he can plug into different circuits on a residential site). The problem is that the 120v unit currently performs better than the 240v one even though it just came back from the "authorized repair shop. It was sent there because it may have been plugged into 3 phase power, and I think there were issues with the magic smoke, though I'm not sure. I would like an easy way to measure suction in a manner that they can be compared (to each other as well as to other units) more accurately than just holding my hand over the hose. It's concrete slurry that we're trying to suck up so we need to test at relatively high velocity as well as high suction.
Sign at N.E. Vocational School Cabinetmaking Shop 1976, "Free knowledge given daily... Bring your own container"
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#12
  Re: how to measure vacuum suction by MstrCarpenter (I'm attempting to re...)
Get a Magnehelic.  For DC systems 0 - 15" WC will work.  For vacuum systems you need one that goes up to 80 - 100".  If you'd rather measure velocity, get a hot wire anemometer or manometer and pitot tube.  If you have to do the measurements when dust/slurry is going through the system you'll need to be careful which type unit to use.  I'm confident the magnehelic will work in dirty environments, but I'm not sure about the others.  

John
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#13
  Re: how to measure vacuum suction by MstrCarpenter (I'm attempting to re...)
I'd make sure to have an ammeter in the rig -- that's the only way to tell how hard a motor is working.  Might also help to tell if one motor is weaker than the others. Laugh
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Wild Turkey
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#14
  Re: how to measure vacuum suction by MstrCarpenter (I'm attempting to re...)
(01-07-2019, 10:55 AM)MstrCarpenter Wrote: I'm attempting to repair a commercial/industrial vacuum. The contractor uses these while grinding concrete. there are two of them; one 120v and one wired 249v but they're basically the same units. Eash has three motors and another small one that drive the "clicker" for the filter pleats. The 120v version will trip a 20a breaker frequently, so I've been asked to separate the load onto two 120v cords which is basically the way the 240v is wired.(so he can plug into different circuits on a residential site). The problem is that the 120v unit currently performs better than the 240v one even though it just came back from the "authorized repair shop. It was sent there because it may have been plugged into 3 phase power, and I think there were issues with the magic smoke, though I'm not sure. I would like an easy way to measure suction in a manner that they can be compared (to each other as well as to other units) more accurately than just holding my hand over the hose. It's concrete slurry that we're trying to suck up so we need to test at relatively high velocity as well as high suction.

 Take clear plastic tubing secure it in a U shape to a board that is about ( I am guessing ) 7 or 8 ft long and fill half way up with water and connect one end of tube to a plug that has a barbed fitting in it , other end of tube open.  Mark a line on the tube of the water level when at rest, start vacuum and mark how high the water level goes on vac 1 then try it on vac 2 . This will measure suction but not cfm.  Roly
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#15
  Re: how to measure vacuum suction by MstrCarpenter (I'm attempting to re...)
(01-07-2019, 10:55 AM)MstrCarpenter Wrote: I'm attempting to repair a commercial/industrial vacuum. The contractor uses these while grinding concrete. there are two of them; one 120v and one wired 249v but they're basically the same units. Eash has three motors and another small one that drive the "clicker" for the filter pleats. The 120v version will trip a 20a breaker frequently, so I've been asked to separate the load onto two 120v cords which is basically the way the 240v is wired.(so he can plug into different circuits on a residential site). The problem is that the 120v unit currently performs better than the 240v one even though it just came back from the "authorized repair shop. It was sent there because it may have been plugged into 3 phase power, and I think there were issues with the magic smoke, though I'm not sure. I would like an easy way to measure suction in a manner that they can be compared (to each other as well as to other units) more accurately than just holding my hand over the hose. It's concrete slurry that we're trying to suck up so we need to test at relatively high velocity as well as high suction.

 Take clear plastic tubing secure it in a U shape to a board that is about ( I am guessing ) 7 or 8 ft long and fill half way up with water and connect one end of tube to a plug that has a barbed fitting in it , other end of tube open.  Mark a line on the tube of the water level when at rest, start vacuum and mark how high the water level goes on vac 1 then try it on vac 2 . This will measure suction but not cfm.  Roly    
( inches of water column will be twice the elevation rise )
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#16
  Re: RE: how to measure vacuum suction by Roly ([quote='MstrCarpente...)
(01-07-2019, 06:33 PM)Roly Wrote:  Take clear plastic tubing secure it in a U shape to a board that is about ( I am guessing ) 7 or 8 ft long and fill half way up with water and connect one end of tube to a plug that has a barbed fitting in it , other end of tube open.  Mark a line on the tube of the water level when at rest, start vacuum and mark how high the water level goes on vac 1 then try it on vac 2 . This will measure suction but not cfm.  Roly    
( inches of water column will be twice the elevation rise )

Yep, you just made a manometer.  That's exactly what I used to measure the SP at various locations in my central DC ductwork.  It works fine for typical DC's but you are going to need really tall ceilings to use one with a vacuum system which draws 80" of water, or more.  


John
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#17
  Re: RE: how to measure vacuum suction by jteneyck ([quote='Roly' pid='7...)
(01-07-2019, 08:06 PM)jteneyck Wrote: Yep, you just made a manometer.  That's exactly what I used to measure the SP at various locations in my central DC ductwork.  It works fine for typical DC's but you are going to need really tall ceilings to use one with a vacuum system which draws 80" of water, or more.  


John

John, I actually have a manometer but I purchased it for gas piping. I guess inspectors frown upon the tube stapled to a board version. Seems kind of ironic seeing that's what is used to check the accuracy of the ones with gauges. Roly wrote that doubling the amount of rise will be the "inches of water column; but that's the "new" way. We actually don't need to mark the board in half-scale or double the amount of rise from the water's starting level. (Which, by the way, can change over time due to evaporation.) The way I was taught (a long time ago Smile ) was to just measure the distance between the two water levels.

So John, what range of "water column were you getting on your D.C. system? If I am to measure real world applications, I think I need to have a consistent portion of the hose open. So I'm thinking if I increase the restriction until the motors develop a noticeable load, can I then measure the suction with a 1/4"  tube sealed to the vacuum hose further upstream? Do you think this data will be useful, as in how much slurry can this suck up from how far away. The tipping point is when suction is too low to keep slurry from spraying out from under/around the shroud? 
 
Wild Turkey, I will check the amps of each motor, but I'm not sure if I can check them under load, I think the unit has to be assembled to create a seal for suction. Also, as I mentioned this unit just came back from the "authorized" shop. I don't know what exactly was damaged or repaired, but it wasn't as good as the 120v unit since they got it (used). So another question; I've cleaned up armatures with 600-1000 grit sandpaper, Is this an acceptable practice?
Sign at N.E. Vocational School Cabinetmaking Shop 1976, "Free knowledge given daily... Bring your own container"
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#18
  Re: RE: how to measure vacuum suction by MstrCarpenter ([quote='jteneyck' pi...)
Before I re-engineered (and I am an engineer so I can say that with no disclaimers!) my DC system I measured a static pressure at the inlet to my cyclone of only 5 - 6" and as low as 2" at one machine upstream.  Obviously, it didn't work very well.  After all my modifications I measured 10 - 11" water 3' from the inlet to the cyclone.  And I did exactly as you described; I just measured the distance between the two water levels.  And the static pressure at the machine that was 2" before was now over 4" which turned out to be enough to capture the sawdust I was making.  

For a DC or vacuum system moving air motor load is maximum at maximum flow - i.e. with all the blast gates open.  The load decreases as you close off gates, so if you want to measure max. load open all the gates.  If your vacuum system is moving a liquid slurry, slurry should move through the vacuum hose (?) as long as there is flow, but I'm not sure which way motor load is going to go.  I'm sure that if you completely block off the inlet by putting the hose in a pit of slurry the slurry will rise up in the hose only to whatever height the vacuum is rated at and motor load will drop off, just like a sump pump that you ask to lift to a height higher than it's rated at.  What I'm not sure about is how the motor load will respond as air flow drops off as slurry flow increases.  It could be that it goes through some peak based on total mass flow; I just don't know.  But I'm sure that if you can keep your manometer hose from getting plugged with slurry that it will measure the static pressure in that line and you'll be able to figure it out by measuring it and the motor load.  All you need to measure the motor load is access to one of the conductors going to the motor, it doesn't have to be at the motor itself. 

I hope this helps. Good luck.

John
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#19
  Re: RE: how to measure vacuum suction by jteneyck (Before I re-engineer...)
(01-08-2019, 12:08 AM)jteneyck Wrote: .....What I'm not sure about is how the motor load will respond as air flow drops off as slurry flow increases.  It could be that it goes through some peak based on total mass flow;....
I hope this helps. Good luck.

John

Thanks, it does! There's three motors on each vacuum, and I'm not (easily) able to put a load on a single motor as there will obviously be leakage back thru the non-running motors, but if all six motors are all drawing about the same current unloaded, then I'll assume they're fine. Now if I compare the current and water column of the "good" vacuum with the "bad" one using the same filter, hose, and air restrictor (a controlled "dust shroud") I might find that sweet spot is missed simply because of a different hose, or shroud. You've also helped by reminding me that the more air it moves the more "work" it does. The 120v unit might be the one with leaks, which is causing it to trip 20a breakers.

I've got a new question on these vacuums. They each have a fourth motor that drives a "clicker" on the inside of the filter pleats with a separate switch. They have been running it with all four motors on. My thinking is that "clicking" the pleats while it is running will just serve to pack the debris more densely into the filter. What are the thoughts of those of you that have a similar function for your filters?
Sign at N.E. Vocational School Cabinetmaking Shop 1976, "Free knowledge given daily... Bring your own container"
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#20
  Re: RE: how to measure vacuum suction by MstrCarpenter ([quote='jteneyck' pi...)
I think it depends upon how it was designed.  With typical bag houses using mechanical shakers to clean the filter bags the bag house has to be offline before the shaking sequence is engaged.  But pulse jet baghouses typically operate online, one of their advantages.  

My guess is that with the system you have the cleaning was meant to be offline, as you suggested. That would always be a safe bet, even if it actually was designed such that it could be used online.  However, you may have to stop the process more frequently so that the filter cleaning process can be activated to clear the filters.  The best thing to do is to find the operating manual or contact the company who made it, if possible, to find out how it was designed to operate.  But if that is not possible, I would only run the filter cleaning in the offline mode until I had enough data to suggest it is safe to do while online.  

John
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