Pumping out of a vacuum chamber

[ports394]

I have a vacuum evaporator... the concentrated material falls into a sight glass/tube at the bottom, and we just have a hose barb connected to a peristaltic pump.

The evaporator is running at 20" Hg vacuum. the pump can't create enough suction to pull material out. (Without vacuum running, it can).

How do I go about pumping that material out? A diaphragm pump can create more suction... or should something like a rotary valve we used instead?

 

[KiwiMace]

You have around 30kpa to play with. I'd go with the rotary valve gravity solution.

 

[ports394]

KiwiMace, thank you. I don't understand what you mean with 30 kpa to play with. Do you mean suction pressure till I hit full vacuum?

 

[KiwiMace]

Yep. You are going for a net positive pressure on the feed side that exceeds your losses - like any other flow situation. You can draw a perfect vacuum, and even close is unreliable. At best there is about 10"HG, 30kPa, or 5psi to work with - not a lot.

So I think your idea of a rotary valve that maintains your evaporator pressure is the best solution, although I'm surprised the the peristaltic pump doesn't achieve the same objective. Maybe too much pressure drop in the hose barb, or some kind of surface tension effect of the concentrate in this area is preventing the pump from priming under gravity.

 

[ports394]

KiwiMace, the pump didn't work so well originally because it was about 6 feet away, and about 10 feet of tubing. I moved it closer, and the pump is struggling, but it can do it.

 

[77JQX]

Can you put the pump at a lower elevation, and have the suction tube filled with liquid? That should help.

 

[ports394]

77JQX, Good suggestion, I did actually set it up the way you said, when I moved it earlier. I wasn't even thinking about filling the suction tube.. It just worked out that the only place I could put the pump was lower.

Also, we had been using the tubing in the pump head for a while.. so I replaced that.. wow it was flat!

 

[KiwiMace]

Try a short, braided line then.

 

[ports394]

Kiwimace, I'm using norprene. it's pretty soft tubing. I just have to be more diligent about replacing it. I've got 1 pump, and a 50 foot case of it. No need to be cheap, right?

 

[Compositepro]

Place your vacuum chamber on the second floor, 30 ft. above your draw-off valve, and you will not need any pump at all. The point is that in a perfect vacuum the only thing that will cause fluid to move into a pump suction is gravity.

 

[ports394]

In a perfect vacuum yes... but I'm at 26" Hg.. not 29.5"

 

[Compositepro]

But do you understand the point? If you did you would realize how important the liquid head is at your suction as well as the flow resistance of your tubing. If your tubing is collapsing under vacuum, a peristaltic pump obviously will not work.

 

[ports394]

Composite, yes I understand your point. But I think you misunderstood my post about the tubing collapsing. I was talking about the tubing that is inside the pump head... the section that is constantly rolled on. The rest of it is fine. The tubing is rated for 26" of vacuum anyhow.