DH drain flow in negative pressure

[JoshNZ]

What sort of psi differential would you expect to see across a timber stack inside a kiln? I write this after I've written everything below thinking now it's probably straight forward - after looking up inches of water/psi conversion, but pretty interesting nevertheless, I'll leave it all there.

Ive come across an odd problem with my kiln with the DH drain apparently blocking.

The DH (which is on the floor on the negative pressure side) has evaporator coils one on each side, both have a drip tray below them which are piped to the centre of the unit to a T joint, where a hose connects and the hose exits through a hole in the floor, and into a 20L/5gal jug to monitor water removal. Several times lately I've gone to empty it, pulled the hose out and seen it flow constantly for about 20-30 seconds.

Last night I drove past the jug near full thinking I must come back and empty that tonight, forgot, and this morning the jug had not overflowed. I opened the door slowly crept into the kiln and sure enough the drip trays were flooded.

I closed kiln up, went back underneath to where the jug is and very gently lifted the hose out, no water drained out. I slowly lowered the hose outside of the jug and away it went draining the flooded trays.

The jug is definitely well below the trays. The only theory I can come up with is the drip trays being in the negative pressure side of the kiln are drawing that water level up the drain hose. But could there really be enough of a pressure differential to draw the water level up ~300mm?

And after googling.. 12 inches of water, could there be as much as .45psi across the timber stack?

I was playing around with an air flow calculator to try and grasp it in practical terms. Supposedly if I were to poke a 1" diameter tube 3ft long through a gap in the timber stack, nothing else changed, I should see air flowing through at about 13/gal min.

I think I've accidentally measured the pressure of the negative side of the timber stack, the differential is probably even more being that the positive side is above atmosphere.

[tawilson]

Put a loop in the hose so you've created a trap. Air handlers need traps because of the negative static pressure.

[doc henderson]

If the drain hose is sealed going into the jug, it could also be + pressure in the jug.  the weight of the water should allow it to overcome the neg. pressure of the kiln.  may need to vent the jug.  does the hose end up submersed in the water?  you can make a manometer with a piece of tubing that has a U-shaped low area with water in the loop.  under pressure, the difference of the levels of the water on each side of the U tell the pressure in whatever units you measure it in.  cm or inches of water pressure.  you should be able to observe it without opening doors or shutting down the system.  If the side of the water in the U that raises up is nearer the system, that is - pressure.  and vice versa.  you can cheaply measure the pressure on both sides of the stack, and in the drain tube and jug, simultaneously if you have enough clear tubing.  you can add food coloring to the water for ease of reading if you like.

[K-Guy]

@JoshNZ 

I made this for our kilns but it should solve your problem.

KilnTech Episode 9: L53/L200 Drain Line Installation - YouTube

[JoshNZ]

The loop will solve the problem of the kiln drawing in air through the drain line but to keep it draining and overflow the jug, it needs to go lower than the top of the water column the negative side holds right?

I just found the whole situation interesting, I didn't expect to see a measurable pressure difference across the timber, I knew there would be a slight one but surprised it's so much, enough to pull water. Next time I load it I'll put a hose under the stack like you suggest doc and see what I've got exactly. Good idea.

Thanks for the vid K-Guy I'll ziptie a loop up.

[doc henderson]

prob on the front and back side of the stack.  also, in the jug to see if it develops + pressure.

[JoshNZ]

Oh the jug is open at the top it's just a ~1/2" hose poked into the 2" opening for the lid, definitely atmospheric pressure in there.

Something setup permanent would be interesting to check each time you'd loaded your charge, would tell you relative to any other load how well it's stacked/air gaps blocked off etc.

[doc henderson]

could set up a manometer like the old gages used on 3-and 4-cylinder motorcycles to adjust carbs.  if the loop fixes it great.  If it was all form the neg. pressure from the fan blowing through a pile, then I think it should drain when the kiln is off.  there is a pressure difference to make air move, but I have a hard time thinking it is enough to lift water.

[JoshNZ]

I'll let it fill up again, pull the hose out, then shut the fans down and see what happens 

It didn't seem right to me either but then running the numbers through that airflow calculator, it says a 1" pipe poked through the stack would see a ~12gal/min air flow as a result of that pressure differential, that does seem realistic to me.

I'll drain the hose and splice a clear tube to it tomorrow and see if it draws any water up from a cup

[doc henderson]

you would have to change the flow rate to pressure.  and neg. pressure vs the pull from the weight of the water (siphon) pressure.  if the end sits in the bucket of water, may be a vapor lock and then the loop will not help.  the chamber has high and low spots, but a net 0 unless the vents are blowing out, with no return air.  If it is all local neg. pressure on the side with the DH, then turning off the fans should fix it, or opening a door.  in my opinion.  if it is drafting air from out of the bucket, it should equalize and then drain.  I'm sure it is something simple, but it may not be one of the above-mentioned simple issues.  fun puzzle.  

[JoshNZ]

Well you're quite right doc, that's not what is going on. Buggered if I know why it blocks.

The problem in my last theory was that the drip trays didn't empty when I opened the door to go in the kiln, couldn't be any negative pressure at that point.

I left it until today and the jug was overflowing when I arrived, but I went in before touching it and the drip trays were flooded. I took the hose out gently and just waited, and after a moment they started draining.
Don't know what causes it to start or stop =/.

[doc henderson]

I bet it is the hose submerged in an overflowing jug of water.  it must work till then, since the jug is full.  If it was something else, it might stop halfway.  get a bigger jug, or let it run on the ground for a day.

[JoshNZ]

The jug overflowed this time. I can't see how that would do anything what's the difference between the tip of the hose being submerged under a 4" of water vs 14" when it's full? If the top is open?

I could zip tie a bracket to the handle so the drips free fall into the jug above the lid

[doc henderson]

I assume the top of the jug is below the tray, as I think you said it is under the kiln.  the force to drain the tray once the water is in the tube, is the delta H.  the difference in the height of the column of water.  if you lifted the jug higher than the tray and the tube was full of water, you would siphon back into the tray.  as the jug fill, the delta H force gets smaller.  if there is resistance in the system, or air causing a lock, it may back up the system.  can you show a pic of the setup?  how far is the top of the jug, below the tube opening at the tray?  any chance the tube could be floating up and causing a kink?  can you run the tube out to the open into a low tub with an air gap from the tube to the tub above the max water level?  if this solves the problem, then there you have it.  good luck, and thanks for the feedback.

[doc henderson]

here is another idea.  I assume you cannot sit there all day watching.   get a jug of water, or a hose with nozzle, and slowly add water to the tray and watch it drain.  when it backs up, then look at the jug below.  maybe you can recreate the problem in 10 minutes and see.  I would start with what you mentioned, creating an air gap above the water level in the jug.  I assume the DH drips water in cycles of cold and warm.  (Compressor on and off).  so, the tube could empty completely (full of air after a cycle) then after the tip is submerged and it fills again, may get an air lock.

[doc henderson]

you could even put a smartphone to watch the jug and tube underneath, and face time yourself from the kiln!   8)  

[JoshNZ]

It's that height, ~8" below the floor plus the thickness of the floor (8"?) Plus the height of the trays 4" maybe.

I'll make something to hold the hose end pointing downward but above the jug and just park the jug under it each time



 

[doc henderson]

sorry for all the replies, but I am on a stent of 12-hour night shifts.  thanks for the distraction.