Sneak Peak Video of the 
New Solar Hydrogen Home DVD
Coming SOON!

Download Over 100Meg of
FREE Hydrogen Video
Ride in the Famous H2 Geo
Click Here

chimney air-air heat exchangers
1 nov 1995
does anyone use a woodstove or fireplace chimney as an air-air heat exchanger?

seems like this might work with a bare fluepipe running up through a masonry
chimney, or a triple wall air-cooled "all-fuel" fluepipe with cold outside
air coming down the from the outside, to a point lower than the air intake 
for the fireplace, to supply combustion/ventilation air which is heated as
it travels down the outside of the fluepipe. 

i suppose this is a "counterflow air-air heat exchanger" as explained on
page 3-4 of the 1993 ashrae handbook of fundamentals. if the hot flue gas
enters the fluepipe at a temp thi, say 68 f (when there is no fire) and the 
cold outside air enters the top of the chimney at a temperature tci, say 32 f,
and the amount of airflow is, say, 50 cfm, and the fluepipe is 6" in diameter
and smooth and 16' long, what will the temperature tco of the cold air that
enters the house be, and what will the heat exchanger efficiency be?

the area of the fluepipe is a = 16' x pi x 6"/12" = 25 ft^2. the u value of
the smooth fluepipe surface is about 1.5 for each side, if the air and flue
gases are flowing slowly, so the overall u value is 1.5/2 = 0.75 btu/hr-f.
the number of exchanger heat transfer units is approximately

  ntu = au/cmin = 25 ft^2 x 1.5/2/50 = 0.375, 

  e = ntu/(1+ntu) = 0.375/1.375 = 0.27 

    = (tco-tci)/(thi-tci) = (tco-32)/(68-32), and the incoming air temp is
    
  tco = 32 + 0.27 (68-32) = 42 f.

we might do better with earth-coupled air for winter ventilation, but
if we need a chimney anyway, why not use it in this way? 

if we roughen the fluepipe somehow and make the chimney fit around the pipe
more closely, and increase the air velocity to v = 2 mph, using a small
blower for incoming air, the thermal conductance of the fluepipe airfilms
would increase to about 

  u = 1/(1/uinside + 1/uoutside) = 1/(1/2 + 1/(2+v/2) = 1.2, 

  and the ntu would increase to about 25 ft^2 x 1.2/50 = 0.6

  and the heat exchanger effectiveness increases to 

  e = 0.6/1.6 = .375, so

  tco = 32 + 0.375 (68-32) = 45 f.

we could turn on the blower when the fluepipe gets hot, with a thermostat,
or when the humidity rises, in an airtight house.

what will the incoming air temperature be when there is 600 f flue gas going
into the chimney, and how much of this "waste heat" can be recovered in btu/hr?
one could use the formula above, substituting 600 for 68. how much wood could
we save during an 8 hour fire every day over a 200 day heating season, with
one of these chimneys, assuming a cord of wood contains the heat equivalent
of 100 gallons of oil at 100k btu/gallon?

the outgoing flue gas temp, tho, would come from the formula

  e = (thi-tho)/(thi-tci) = (600-tho)/(600-32) = 0.375, so

  tho = 600 - 0.375 (600-32) = 387 f,

which is greater than 212 f, so the water in the flue gas would exit as
vapor, but the creosote may condense. so the fluepipe joints should be
installed "downhill," lapped so the creosote runs back into the woodstove. 
does all this conflict with building codes, eg using triple-wall all-fuel
fluepipe? i don't know. if it does, perhaps an exception should be made,
in the name of energy efficiency.

nick



I got ALL of these 85 Solar Panels for FREE and so can you.  Its in our Ebook
Ready for DOWNLOAD NOW.

Site Meter