Vacuum readings from absolute to gauge or vice versa

[cityjack]

Good morning all,

I am working with a new scrubbing system to me. The previous system people are no longer available, so I am left to figure this out for myself. I'll try to make this as clear as possible. I am also attaching the oem pump curves as well.

I am sucking nasty hot (130F) exhaust gases out of the end of an adhesive extruder. Nasty crap. The gases are being pulled through a condensation/H2O trap. I look at the trap as a water filter. 2” pipe all the way from the extruder to the liquid ring vacuum pump. Nasty hot gas into the top of the trap, cooler cleaner gas/air out of the trap to the pump. Then out of the pump to a drain collection tote to be properly disposed of.
At the top of and right at the extruder where the gas is exhausting, before the H2O trap, I am reading 35CM/Hg on a vacuum gauge. There is no gauge on or at the pump. I am trying to see if what I am getting at this gauge correlates to the top pump curve. See attached curves. I am trying to see how efficient the pump is operating compared to what the curve says it should be doing.
Here is my math, please let me know where I am missing the "boat".

I need to find the absolute vacuum given what I’m seeing on the gauge(35CM/Hg). I think. The temp of the operating liquid for the pump is 15 deg C, so I am thinking I should see around 75 mbar absolute according to the top curve. 0 (zero) is a perfect vacuum. 14.7/psi or 76CM/Hg is atmosphere. 35CM/Hg on the gauge (negative pressure). Since absolute is atmosphere plus gauge, I take 76CM/Hg and add a negative 35CM/Hg. That gives me 41 CM/Hg absolute. I think. 41CM/Hg to mbar is 546ish mbar absolute. That puts me way the heck off the curve at 15C.
I do not see why I am so far off the curve.

Any help would be greatly appreciated.

Thank you and have a nice weekend.

Sid

 

[EdStainless]

Are you sure that your gauge is reading 35cm below atmospheric and not 35cm absolute pressure?
But in either case you are at a pressure well above the cavitation limit for this pump, but...
You should add a gauge at the pump intake.
It could be that the pressure drop through the system means that the pump is seeing a much lower intake pressure.

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P.E. Metallurgy, Plymouth Tube

 

[cityjack]

Thank you so much for your reply sir.

I do definitely need a gauge at the pump intake. That's what I'm used to. When the system is off, the gauge at the exhaust goes to zero. The scale is 0 - 76CM/Hg. That tells me at atmospheric(system off) there is no vacuum. In my mind, that's gauge pressure. The gauge really is negative. Hence my math of adding a -35CM(gauge)to 76CM(atmosphere) coming up with 41CM. Gauge is zero when off(atmosphere), 35CM/Hg when running. I'm thinking if it were absolute, when system is off, it would go to 76CM on the gauge.
Your last statement, you mean through the system because of the drop, Im getting a vacuum loss(Rising closer to atmosphere)? Do I understand you correctly sir?

Thanks a ton sir. I appreciate your ear.

Sid

 

[EdStainless]

It wounds like you have the pressure correct.
No, the pressure nearer the pump will be lower nearer the pump.



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P.E. Metallurgy, Plymouth Tube

 

[cityjack]

Closer to the pump would be a greater vacuum, further away from atmospheric pressure?

 

[georgeverghese]

If I read the curve correctly, the no go zone for this vac pump is below some 75mbar abs (5.7cmHg abs) at an operating temp of 15degC. So you can afford some (546-75) = 470mbar of pressure drop in the line from the extruder exit through the condensation trap and into the pump. In comparison, from steam tables, we have a sat vap pressure of 1cmHg for pure water at 15degC.

 

[cityjack]

Thank you sir. I appreciate your help, really. It looks like you're thinking that a loss of 470mbar would be the drop of vacuum through the pipes and water trap that I could withstand before it gets to the pump? Anything more than that, my pump becomes ineffective. These units drive me crazy. Just when I had a handle on Torr with my old place of employment for vacuum ovens. I do know Torr and mbar are almost a 1:1. It just seems like 470mbar is alot to lose. 13mbar for just water I can handle as you are seeing. But 470 is a lot.

I need to put a gauge on the pump. I spoke with the tech rep for the pump this morning. He said not only a gauge on the pump, but absolutely a flow meter going to the pump is critical.

Thank you again sir.

Sid

 

[LittleInch]

That was going to be my point - your picture and text don't mention flow anywhere.

It looks to me like your vacuum pump is barely able to keep pace with the vapours being produced and can only get the vacuum down to your 540 odd mbar.

Any chance you can totally block off the pipe with the guage on and see how long it takes the pump to get down to its 75 odd mbar?

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[cityjack]

Littleinch, thats a good idea. I am a new hire here, going on 10 days, so I am not exactly sure what production schedules look like in order to experiment. At least you are seeing my math is somewhat accurate. I'll keep you posted. I need to find a flow meter that isn't a 1000.00 dollar Keyence. 1 inch pipe, any suggestions?

Thank you for your help.

Sid