Hearthlist Digest #300 - Sunday, May 9, 1999
 
Re: ODS
  by "Skip Jackson" <[email protected]>
ODS
  by "Craig Issod" <[email protected]>
Re:ODS
  by <[email protected]>
 

(back)
Subject: Re: ODS
From: Skip Jackson <[email protected]>
Date: Sat, 8 May 1999 21:34:30 -0400
 
Dear Craig,
	The gas is directed from a jet across the ODS (a bimetal post) and
the thermocouple.  With a normal amount of oxygen the gas ignites at the
opening tip of the jet.  As the oxygen level decreases the gas ignites
farther away from the jet because the gas has to find more oxygen in
order to burn, ie the flame is lifting.  The ODS is placed at the exact
distance from the jet so that as the flame lifts past it the ODS cools
which shuts off the pilot which shuts off the main valve.
	I would fax you a diagram but I do not want to deprive the
Hearth Education Foundation the $125 you can pay them for a manual.
Skip Jackson
Fireside Engineering
 
 
 
 
 

(back)
Subject: ODS
From: Craig Issod <[email protected]>
Date: Sat, 8 May 1999 21:34:30 -0400
 
Thanks, Skip...that's the technical info I was looking for. I have an older
HEARTH manual and it only has a basic diagram and description, not the
compete one that you gave.
 
 
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Craig Issod             HearthNet at http://www.hearth.com
[email protected]    Everything your Hearth Desires
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(back)
Subject: Re:ODS
From: [email protected]
Date: Sun, 9 May 1999 17:51:45 -0400
 
In a message dated 99-05-08 19:05:23 EDT, you write:
 
<< BUT, someone in the industry told me it was not exactly like that..and that
 the bimetal thingy on the side had something to do with it...
  >>
 
Craig:
 
The fundamental reason an ODS works is what Skip Jackson said. The gas/air
mixture is flowing out of the pilot at a certain velocity. Under normal
conditions the flame is able to propagate back against this flow and remain
stable at the piliot opening. In other words the rate at which the flame is
able burn through the mixture (the "flame speed") is equal to or greater than
the speed of the gas/air mixture.
 
When the combustion air is depleted of oxygen, combustion cannot occur as
quickly: the flame speed decreases. When the flame speed becomes less than
the velocity of the gas/air mixture, the flame begins to lift off the pilot.
ODS pilots are designed, and their orifice carefully sized, so that before
the oxygen concentration falls below 18 percent the flame will have lifted
sufficiently to allow the thermocouple to cool so it isn't producing the
voltage to keep the pilot valve open.
 
And yes, that is also the reason you can blow a candle out. When you blow on
it you increase the velocity of the mixture against which the candle flame
must burn, and eventually you literally blow the flame away from the fuel and
heat source needed to keep combustion going. (The cooling effect of the
"wind" and the changing of the fuel/air mixture also contribute, but the main
thing going on is the excessive velocity relative to the flame propagation
speed.)
 
As Skip says, this explained pretty well in the HEARTH Gas Appliance
Technicain manual, and more discussion and the opportunity to ask questions
like this takes place at the HEARTH Gas Training Course.
 
I'm not aware of a bimetal thingy playing an essential role in this process
on the ODS pilots I've seen in use in this industry. If there is such a
thing, it is probably no more than a supporting role to the simple, elegant
and relatively foolproof effect described above.
 
Dave Johnston