Re: Fume hood

MessageSo, the 6" pipe (let's call it the vacuum run) is drawing from inside or outside the house? If inside, do you need to provide make up air? I'm wondering if there is the potential for backdraft? The 4" joins the 6" how? using a reducer and a Y-connector or a T-connector? Thx.

~m
  ----- Original Message -----
  From: Best, Dianne
  To: alt-photo-process-l@sask.usask.ca
  Sent: Monday, October 31, 2005 7:27 AM
  Subject: RE: Fume hood

  Ether is VERY explosive! According to the MSDS, it has a lower explosive limit of 0.9% - that's scary!

  Ether compounds (collodion) was the only reason for doing the fume hood so the hood was designed for high air flow and a spark-proof exhaust.

  The fan is a "duct fan" from a local surplus/discount mechanical store (Princess Auto) that cost about $45.00 It is built with a shaded pole motor (no brushes or commutator) and runs a steel shaft in bronze bushings so it is absolutely spark-proof. The only hazard from the motor would be if the coil overheats (which it shouldn't) or if the plastic fan blades build up a static charge when the humidity gets really low (mid winter). I even soldered all the wire connections before putting Marettes over them - just extra insurance against sparks!

  The ductwork is all galvanized steel and is electrically grounded. From the hood to the fan chamber is a short length of 4" - the whole system was originally 4" but I had to change the fan to 6" to get enough air flow. From the fan to the outside is all 6" galvanized. The exterior portion is a simple snorkel - two 90 degree elbows so the exhaust is pointed to the ground and out of the rain - with a fiberglass window screen over the end to keep bugs out. The pipes and elbows are all painted flat black on the inside to stop light reflecting down into the darkroom.

  I designed the system so that the fan sits in the outer window plate and blows air up the snorkel. The rest of the system sees a vacuum created by the fan. This ensures that any air leaks in the system are drawing air from inside the house, not blowing explosive fumes into the darkroom.

  350 CFM is probably over-kill for this fume hood but I'd rather loose a little heat in the winter than risk reaching that 0.9% lower explosive limit with the ether!

  Dianne
Received on Mon Oct 31 09:52:46 2005