Draft Checks

Steve,


Your mention of hopper got me curious about what the #6 would read with the mag opened.

So, I just switched my mano probe from the pipe back over to door frame screw hole, again. The reading was .015 (down from .03 in the pipe just before the thimble). Same difference of .015 as I found the first time.

Then I opened the loading door and an inch and, as to be expected, it dropped to .00.

So then I opened the magazine cover compleatly. The reading only dropped to .01. Closed the cover and it climbed back up to .015. Did that several times, each time letting the reading stabilize for 30 seconds. And each time getting the same results.

I was a bit surprised it didn't also go to zero with the mag cover open. The mag is mostly full of nut coal and the bottom is in contact with the firebed, so that all must offer more resistance to air flow into the barrel than a partially open door. That explains why the mano readings don't climb as fast in DD when I'm reading the mag as they climb with the load door open.

Just though you'd also like to know.

Paul

KingCoal wrote: ↑

Fri. Feb. 16, 2018 6:23 pm

OMG Joe please don't. there's no tellin what you might start seein and then we'll have to start all over again

Lets hope he wouldn't want to dance with the Blue Ladies.

Paul

Trust me Paul, them ladies wouldn't want to dance with me. I have 2 green feet. (Or is that "left" feet?)
What-ever, it would be a serious embarrassment.

In my mind these pressure readings you guys are all seeing when you open various doors and such all come back to the volume flow capacity of the chimney. For example, the drop to zero in the CC when you open the load door. The pipe being 6 inches and the load door being bigger can allow in more air than the chimney can take, the flow volume capacity of the chimney is overwhelmed and the CC is able to match ambient pressure of the room. The magazine is another great example. You take the lid off expecting the CC to drop to zero but it doesn't. Since not enough air can get thru it to satisfy the negative pressure in the barrel, the chimney can keep up with the volume allowed thru the mag opening.

Lee, i was sure you would be back with this explanation. i figured the answer was the over supply of flue volume but didn't want to mis- state it.

i have much more perimeter opening around the top of my hopper than Paul has with the mag in the #6 so not surprised mine goes all the way to .0 when open.

also not surprised my hopper has a rope gasket and cam lock to pressure seal it.

all good stuff, thanks
steve

Lightning wrote: ↑

Fri. Feb. 16, 2018 9:29 pm

In my mind these pressure readings you guys are all seeing when you open various doors and such all come back to the volume flow capacity of the chimney. For example, the drop to zero in the CC when you open the load door. The pipe being 6 inches and the load door being bigger can allow in more air than the chimney can take, the flow volume capacity of the chimney is overwhelmed and the CC is able to match ambient pressure of the room. The magazine is another great example. You take the lid off expecting the CC to drop to zero but it doesn't. Since not enough air can get thru it to satisfy the negative pressure in the barrel, the chimney can keep up with the volume allowed thru the mag opening.

Lee,

That might be with some instances, but the central chimney I'm using was built for three coal stoves, not just one. It's 12-1/2 by 12-1/2 inches square inside.

The cross section of the 6 inch stove pipe is about 28 square inches, the cross section of the chimney is 156 square inches - more than five times the pipe cross section.

Pressure drop in the barrel with the load door open does not mean exhaust has stopped moving (loss of draft). It just means it's moving slowly. When I open the loading door and the mano reading goes to zero, there is still pressure drop inside the chimney system that is allowing air movement into the stove through the open door. The stove does not back draft into the room. There is not the slightest whiff of exhaust when I stand close in front of the stove with my nose above the open loading door. Same as a fireplace will still draw with a mano reading of zero when taken in the fireplace opening.

That's one of the points of using a plenum, to slow the movement of gases down.

Paul

Right right, I'm with ya on all that. My term describing the chimney as being "overwhelmed" meant the 6 inch pipe.... and your right, the reading of zero doesn't mean that flow stopped, I'm sure it's moving quite well in fact. It's just that the negative pressure in the barrel is being satisfied by the open load door or you could say that the open load door is overwhelming the ability of the pipe to maintain negative pressure in the barrel.

The open load door didn't overwhelm the pipe either.

When I put the probe back in the pipe near the chimney thimble, and then re-opened the load door, the mano reading climbed as the top of the firebed mound up to the bottom of the magazine started burning hotter with the increase in feed air. The increasing mano reading showed that the pipe was handling the increased exhaust volume as an increase in velocity through the pipe to get to the far larger capacity handling chimney. A mano reading in the chimney would have been lower as the exhaust gases slowed down again on reaching such a large area. Just like a plenum effect again.

Paul

Right, I was just trying to get at the nuts and bolts of the pressure observation in the barrel.

So what do you suppose the pressure observations would be along the entire flue gas path if you intentionally stopped it's flow completely?

geez, i'm glad all i wanted to know was if the resistance of the base burner passages could be measured and what it might be.

this stuff make my thinker skip

And the ten thousand foot view is that draft applied to the stove is the only # needed to run any stove/range. Actually that’s not quite true, it’s fun to know. I have not used my draft gauge in a long time. Originally they were not used at all

What's interesting about air resistance or should I say the resistance of objects in its path is that there is an exponent involved. As velocity increases and drag thru turbulence is introduced, the resistance becomes squared relative to the object. And I think this is why there can be a reading in the pipe, but none in the CC although there still is a flow. lol

But it's probably more likely that the fuel bed itself is creating it's own little push that's just enough to equalize the pressure in the CC with all the resistance going on downstream. I think a reading under the grates thru the primary air opening would confirm it.

Exhaust flow through a stove and chimney system doesn't always do what people think.

With our mano's, one component we are not measuring is gas velocity, which also has an effect on gas pressure through an enclosed space.

As for what resistance of gases to flow does, as I said in another post, I've often heard that stoves should not have more than two 90 degree bends in the exhaust pipe, or the resistance to flow will cause draft problems. The fly in that ointment is that base heaters and base burners add many more tight bends and narrowed passages of "resistance" and they work fine. My range has five 90 degree and one 180 degree bends before the exhaust exits the stove, then there's two 90 degree elbows in 8 feet of 6 inch pipe. Four 90's and one 180 with the GW #6, plus two 90's in it's 8 foot x 6 inch stove pipe.

According to what I've read over the years about stove installation wisdom, neither stove should draft well, yet they do in both direct and indirect draft.

Paul