From nick@vu-vlsi.ee.vill.eduThu Nov 2 21:13:09 1995 Date: 1 Nov 1995 08:53:31 -0500 From: Nick Pine To: london@sunsite.unc.edu Subject: Chimney air-air heat exchangers 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