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Flue size?

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Posted by Tina & Shane on October 16, 2003, 3:49 am
 I am building a Corn fired boiler and I need to know what the min. flue
size should be. My current furnace is a 100,000 input/ 80,000 output L.P.
fired unit. On the coldest days of winter it will run 50% duty, 15 min. on -
15 min off, with the burner cycling on and off during the on times. Can I
assume that I would need 50,000 Btu's input for the corn boiler since it
will be using half the needed Btu's on a constant basis?

 My current furnace uses a 5" flue and the flue gases run in the 140-160
Deg. range. My thinking is that since the L.P. furnace uses a 5" pipe for
the 100,000 input Btu's I can use a pipe with half the volume, say a 4" pipe
for the 50,000 needed for the corn boiler. Also figure in that the flue
gases will be in the 70-90 Deg. range, hopefully.

 The heat exchanger is a 13" cylinder 36 " high. with both ends sealed.
Thirty .750" ID copper tubes will pass through both ends to be encompassed
in water that is in the 13" cylinder. The 30 copper tubes (13.2 sq in".)will
be tilted at 10 deg. and should allow for adequate air flow for a 4" (12.6
sq. in.") flue pipe. The burner will be underneath the lower part of the 13"
cyl. with flue gases going through the copper tubes. I expect that alot of
the heat will be directly transfer to the bottom of the heat exchanger with
the rest be transferred as it goes through the copper tubes, just like a
convential water heater. With the bottom of the cyl. and the 30 copper tubes
I come up with 2660 sq. in. of surface area, does that sound adequate for
50,000 Btu's? There is room for more copper tubes or the cyl. could be
taller if needed.

 I just checked my water heater, 32,000 Btu's input. It has 630 sq. in. of
surface for heat transfer. That seems pretty meager compared to my figures
for the corn burner. No wonder I burn 200 gal. of  L.P. in the summer for
heating water.

  Any input would be appreciated.



Posted by Loren Amelang on October 16, 2003, 6:53 pm

What you haven't told us is the water flow rate around the copper tubes
inside your heat exchanger, or whether there is any forced turbulence in
there. I don't know how you would calculate the performance of such a
device, but someone probably does.

My wood-fired boiler is inside-out from yours - the water is in the 14 32"
tubes, arrayed over the firebox to get radiant heat from the coals as well
as the combustion gases. It has no trouble absorbing 50 KBTU with a 3 GPM
circulator running.

On the output to storage side, I have 250' of 1/2" tubing in five parallel
runs in the atmospheric hot water storage tank. Since the water in the tank
is only slowly convecting, it takes a lot more area to accomplish the same
amount of heat transfer. If you use enough circulation and maybe add some
baffles to increase the flow past your tubes inside your exchanger, you
will probably increase its efficiency.


Posted by daestrom on October 16, 2003, 8:23 pm

Such a cool flue temperature for your LP furnace makes me wonder if it is a
condensing furnace (condenses the water out of the flue gases).  This type
is efficient and has a small water pump to drain off the condensate (or it
sometimes goes directly to the floor drains).  These can even be vented
sideways since it has a 'forced draft' blower on the fire box.

I've never seen a 'corn boiler', but I wonder if it can operate with such a
cool flue?  If it tries to condense the moisture, it might also 'soot up'.
Solid fuel might have some ash that gets carried up the flue and could
affect proper operation.

What does the manufacturer for the corn-boiler recommend?  Or is this DIY?

If the LP is forced-draft and the corn-boiler is natural draft, then flue
sizes can't be directly compared like this.  You don't say the type of draft
used by each, so take this into account.


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