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Condensing fume falling out of local exhaust

MatthewL (Chemical) (OP)
30 Apr 12 8:13
Hi all,

File this one under "Law of Unintended Consequences" :P
I work at a nylon plant, where we extrude plasticized nylon thru dies at high (450-550 F) temperatures using an underwater pelletizer.  During the setup for the extrusion, the operators have to purge the barrel and die of the pelletization system into a drum, which releases a large amount of plasticizer fumes.  There is a local fume exhaust system, consisting of a fan w/knockout drum, exhaust header and individual drops to four extruders.  There is only one operator for the extruders.  I was tasked with reducing the amount of fumes that escape the local exhaust, as it is an irritant and possible sensitizer.  After looking at the system I noticed that there were manual blast gates at each station, and in conversations with the operators, found that most of them never used the manual gates.  I had them try using the gates, which did reduce the amount of fumes at the station they were working at, but they frequently would just leave them all open.  To force them to use the blast gates, I automated them with a simple PLC that only allows at most two blast gates to be open at any time.  The operator has to push a button on the control panel at each station to open the blast gate at their location, closing a blast gate at another station.  It works well for removing fumes BUT now the plasticizer that condenses in the lines (BP ~600F) is running back out of the line once the gate is closed to that section.

Does anyone have a way to reduce this backflow without ripping out the blast gates or adding another exhaust system?  I am considering drilling a 1 to 2" hole in each blast gate to allow some draft at each location while maintaining a higher draft at the open location (the drops are 6" lines with flexible strip wound metal trunks to position the exhaust pickups).  Another thing I noticed is that the lines have a lot of dust/nylon buildup that is acting like a sponge, so any ideas on cleaning the lines and preventing additional buildup once cleaned would probably also help.

Thanks,

Matt
chicopee (Mechanical)
30 Apr 12 13:38
I would explore drip legs. I am not sure that you could eliminate the dust so a periodic cleaning program seems warranted as you probably have a serious fire exposure.
MatthewL (Chemical) (OP)
30 Apr 12 14:36
Chicope,

There are drip legs on the process side of the blast gates, however, the material is condensing in the initial vertical section of the exhaust and dripping out of the intake of the vent lines.

Matt
clay87 (Mechanical)
1 May 12 22:45
Great work on the PLC.  Perhaps change the closed position of the gates to almost-closed.  This you can at least undo.  

Are the lines sized appropriately to keep dust from falling out?
MatthewL (Chemical) (OP)
2 May 12 7:04
Clay,

Thanks, it took the C&I engineer and me some work to get a functioning system.  Basically, when the operator pushes the start button at a station, the PLC sets that positioner's value to 2 and subtracts one from all other non-zero positioners.  If the value is greater than zero, it's open, if zero, closed, allowing them to keep two valves open at a time, or, by pushing a button twice, opens just the one they are at.  The valves are an open/closed design using a solenoid and air cylinder (http://www.kbduct.com/special-autoair.html) and I can't see a good way to alter the stroke.  The dust problem is not so much from the equipment, just what's normally in the air.  The plasticizer itself (I think) is causing the problem, by initially condensing on the duct, then giving a tacky surface for the dust to stick to, and then gets in a feedback loop (dust sticks to plasticizer, which allows more plasticizer to stick, which attracts more dust, etc).  FPM readings for the ducts when all were open (before I made the change) were in the 1000 to 2000 range, and with all other ducts closed I get readings in the 3000 to 4000 FPM range, and with two open, 2700 to 3500 FPM, all are averaged from readings taken at the face of the opening.

Matt

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