Draining under vacuum

[meandmyostrich]

I am looking to drain liquid carry over in the suction pipeline to a scrubber fan
Obviously the suction side of the fan is under (slight) vacuum - any ideas on how to drain?
As it is a scrubber the fan needs to be running 24/7
I would rather avoid a manual operation if possible

 

[bimr]

A drain on the bottom of the duct along with a loop seal should work. It is hard to say anything else since what will work depends somewhat on the ductwork layout.

 

[quark]

Bimr is right. That is a standard practice. Just make the difference in leg heights of loop seal equal to that of fan static(a worst case scenario). The water will be flowing out forever, without difficulty.

 

[handee]

Not quite sure what a loop seal is, but I would use what's called a "P-trap" (just the kind of trap you find under your sink). As quark said make the sure the liquid height difference exceeds the fan static, which I'm guessing can not be more than a few inches of water.

Fran McConville

 

[meandmyostrich]

Yes, as it is a fan and the ducting diameter is large, the vacuum is low
What I should have mentioned before is that I am concerned about liquid accumulating in the fan and destroying the bearings
I was concidering some kind of lute with a drain at the bottom, but was concerned the fan would pull through the drain itself
Obviously if there is a constant lquid level this would not happen - I am imagining that the p-trap works in a similar way to an inverted bucket steam trap?

 

[quark]

fxm,
Loop seal = p trap

meanDMO,
Not exactly. Inverted bucket trap is driven by steam pressure where as the P trap works by gravity.

Regards,

 

[Montemayor]

I think that what is applied here is not a "p" trap in the conventional sense. A conventional "p" trap is a "U" shaped liquid seal for positive pressure sealing - much like a kitchen sink drain. In the case of an industrial tower, vessel, or suction of a fan I have successfully used an inverted "U". Of course, the top of the inverted "U" must have a small breather pipe vent (3/4 to 1" diam) to avoid a syphon effect. The height and size of this type of drain is calculated just as quark has pointed out.

The inverted "U" also has the advantage of not requiring the pressure/vacuum source being located higher up in order to avoid having to dig a pit in the ground to accomodate the conventional "p" trap in the case of excessive static heights.

 

[quark]

One problem I see with inverted traps is the drain pan height limitation(loop height should be less than that of drain pan).

Secondly, there is always a constant level of water in the pan equal to the seal height in the loop. I always insist to keep the air handling units on a 6"(atleast) pedestal.

To avoid liquid carryover, air velocity should be less than 500fpm(or we have to have drift eliminators).

Regards,

 

[rmw]

Graham has a technical paper on their site about this topic at

http://www.graham-mfg.com.

They obviously have to deal with draining out of some serious vacuum pressures.

rmw