The Vespiary

The Hive => Chemicals & Equipment => Topic started by: lugh on July 13, 2002, 01:19:00 AM

Title: Electric Motor Explosion Proofing Modification
Post by: lugh on July 13, 2002, 01:19:00 AM
From Ind Eng Chem  675 (1926):

To prevent sparks from the motor igniting volatile solvent fumes, the principle of the Davy lamp was applied. The air holes at the brushes were covered with 50 mesh copper gauze, welded on. The motor was then run free and under load in explosive mixtures of benzene, gasoline vapor and natural gas. No explosions occured, although the brushes were sparking badly. Under similar conditions the same unprotected motor invariably detonated the gas mixtures. It should be noted that this modification gives no protection from the improbable event of a spark between the shaft and bearings, but this is a rare occurance.

SWIL would test this in a small test chamber before trying it with diethyl ether or hydrogen  ;D  SWIL sees no reason brazing couldn't be used to attach the mesh either  :)
Title: j3al7o82
Post by: MescalToad on July 13, 2002, 09:26:00 AM
Good luck!
Title: ?
Post by: Fully_Auto on July 19, 2002, 10:54:00 PM
Can someone please explain why adding copper mesh to the open spaces on a motor will prevent it from detonating explosive gasses even though sparks are still forming.

I looked up Davy lamp on google but I still don't understand. Is the copper just absorbing and displacing the heat?  Why won’t an electrical spark ignite the gasses present?
Title: heat absorption
Post by: goiterjoe on July 19, 2002, 11:00:00 PM
the copper mesh absorbs the heat out of the electrical sparks and prevents the gasses that are present from hitting their autoignition temperature.  A davy lamp won't work in the presence of vapors such as gasoline that are already at their autoignition temperature normally.  I doubt it would work for something like hydrogen or methane either.
All paths are the same: they lead nowhere
Title: Thanks goiterjoe
Post by: Fully_Auto on July 19, 2002, 11:23:00 PM
That's what I assumed.

Does the motor have to be grounded in order for this to work?
Title: corrections
Post by: goiterjoe on July 19, 2002, 11:33:00 PM
apparently this will work fine for hydrogen and methane, since they have ignition temperatures well above ambient temperatures.  why wouldn't you want to use a grounded motor?  It shouldn't make any difference unless you have a problem with voltage fluctuation on the utility lines that service your house, like from lightning strikes or conductor faults. 
All paths are the same: they lead nowhere
Title: Davy Lamp
Post by: lugh on July 19, 2002, 11:47:00 PM
The Davy lamp was invented for using in coal mines to protect the miners from the ingition of firedamp  :)  The articles states it was tested in natural gas, also known as methane or firedamp, so that's not an issue. It appears it would be okay to use around diethyl ether as well, see:

http://www.physics.purdue.edu/reports/demo/heat/Ht-12.html (http://www.physics.purdue.edu/reports/demo/heat/Ht-12.html)




http://tfk.factmonster.com/ce6/sci/A0842924.html (http://tfk.factmonster.com/ce6/sci/A0842924.html)



The Davy lamp is based upon the principle that to be ignited a substance must first be heated to its kindling temperature and that if such heating is prevented combustion will not occur. The flame in the lamp is surrounded by a metal-gauze screen that distributes the heat over a large area so that the maximum temperature of the screen is below the ignition temperature of the flammable gas mixture (e.g., firedamp).

(https://www.thevespiary.org/rhodium/Rhodium/hive/hiveboard/picproxie_docs/000331864-file_kn0s.gif)
Title: Very interesting
Post by: Fully_Auto on July 20, 2002, 12:13:00 AM
Who ever invented this spent way to much time hanging out with his lamp :)

Thanks for taking the time to respond to my ramblings.  :)
Title: davy lamp ?
Post by: platcat on July 21, 2002, 11:49:00 PM
Is swip to understand the exxxxpensive equipment he was pricing can be substituted with a 20" window fan
which has the motor housing blanketed in copper gauze?? Is the brazing/welding done for a reason or can swip use nylon tie straps?If nessessary swip will solder to keep heat at minimum. Swip  is dreaming of a visit to mart mart for some scrubbing pads. If he gets the right answer.lol. the copper kind that is!! that will solve another pricy problem. Good work Bee's :)
"The crux of the biscuit is the apostrophe"FZ
Are we there yet?
Title: 50 mesh
Post by: lugh on July 22, 2002, 01:37:00 AM
You need to use 50 mesh copper gauze, that's not going to be real cheap, but it would allow you to use such a fan more safely if you follow the directions exactly. Copper scrubbing pads are not a suitable substitute, forget about the zip ties also  :(