Article 31567 of alt.solar.thermal: Path: news.misty.com!not-for-mail From: nicksanspam@ece.villanova.edu Newsgroups: alt.solar.thermal Subject: Re: cheap and easy space heater panel? Date: 24 Dec 2008 10:46:30 -0500 Organization: Villanova University Lines: 37 Message-ID: References: <5364b731-021a-480f-92a8-eea359c434a8@k18g2000yqj.googlegroups.com> <52a1ac57-c6de-4c25-a590-9d37f47bd4a3@a12g2000yqm.googlegroups.com> NNTP-Posting-Host: acadia.ece.villanova.edu X-Trace: max.inside.misty.com 1230133592 6847 153.104.44.130 (24 Dec 2008 15:46:32 GMT) X-Complaints-To: abuse@misty.com NNTP-Posting-Date: Wed, 24 Dec 2008 15:46:32 +0000 (UTC) Xref: news.misty.com alt.solar.thermal:31567 Father Haskell wrote: >> >No baffles or absorber panels? >> >> Right, altho a transpired mesh absorber (eg dark window screen) >> would improve efficiency. > >How does mesh compare to corrugated aluminum, painted black? Better and cheaper. A layer of black window screen would reduce reradiation loss to the glazing and might keep the air between the screen and the glazing close to 70 F, so a 1'x1' box with R1 glazing would lose (70-30)1ft^2/R1 = 40 Btu/h on a 30 F day. With 90% transmission, it would gain 225 Btu/h in full sun, for a 185 Btu/h net output. A panel with 10 cfm of airflow behind it producing H Btu/h of net heat with a max temp of 70+H/10 and average 70+H/20 and H = 225-glazingloss = 225-(70+H/20-30)1ft^2/R1 makes H = 185/1.05 = 176, no? Nick This seems like a good time to make a "heat trap" like an igloo for the (20 F) barn cats, with a sleeping platform that's above the top of an entrance below it, so cat-heated air (which rises) can't escape through the entrance. We might put a cardboard box upside down on top of 3 strawbales in a U-shape, with a 4" gap between 2 of them so the cats can enter via the gap/tunnel and climb up to lie on the strawbales under the box. We could pile strawbales all around and over the box or drape it all round with rags on the outside. A 2'x2'x1' tall box with 12 square feet of 1" R4 rag surface and 2 20 watt (68 Btu/h) cats could stay 70 F until the barn temp dropped to 70-2x68x4/12 = 24.7 F... 2" of rags would make this minus 21.7 F. Merry Christmas to all, including the kitties (who might prefer a few creatures stirring, eg some tasty mice.) Date: Sun, 28 Dec 2008 11:31:10 -0800 (PST) From: nick pine Subject: A condensing woodstove chimney? Newsgroups: alt.energy.homepower Eli wrote: >A few year ago, I fooled around with a forced draft of my wood stove >chimney by installing a duct fan blade near the top of 6" stainless >steel chimney... for the sake of discussion, assume we can perfect a >forced draft. The way to capture the heat going up the chimney is to >have the first section of pipe above the stove to have a water jacket >where the flow of water can capture the chimney heat. Air seems simpler. >... Cooling the chimney will dampen draft. I would like to see a >forced draft with a pressure sensor that would automatically turn on >when the sensor determines forced draft is needed to overcome reduced >draft from water jacket or any other reason. An airtight stove with a combustion air intake port and a manual adjustment might not need this. > It seems to me that we could capture a lot more heat from wood > combustion if we can cool the chimney without risking back draft by > using a forced draft. With a condensing chimney, a 20K Btu/h woodstove might produce 3K Btu/h (15%) more heat, even when burning damp wood. In the calc below, a counterflow air-air heat exchanger with a 10'x6" flue pipe inside a 10'x10" pipe with 500 cfm of 70 F room air flowing between them cool 3 cfm of combustion gas from 600 to 86 F with a 97% heat exchanger efficacy... http://www.engineeringtoolbox.com/fuels-air-flue-gas-d_170.html says it takes at least 70 cubic feet of air to combust a pound of wood with a heating value of about 7000 Btu, and bone-dry wood has 20% more (low) heating value than wood with 20% moisture. 20 PI=4*ATN(1) 30 HEAT=20000!'combustion heat (Btu/h) 40 HHV=7000'wood high heating value (Btu/lb) 50 PPH=HEAT/HHV'dry wood consumption (lb/h) 60 CAN=70'combustion air need (ft^3/lb) 70 CMIN=CAN*PPH/60'combustion airflow (cfm) 80 PRINT"Heat (Btu/h):";HEAT,"Wood (lb/h):";PPH,"Cair (cfm):";CMIN 90 LP=10'pipe length (feet) 100 DP=6/12'inner pipe diameter (feet) 110 A=LP*PI*DP'inner pipe area (ft^2) 120 U=3/4'pipe wall conductance (Btu/h-F-ft^2) 130 NTU=A*U/CMIN'Number of Heat Transfer Units 140 CMAX=500'room air fan cfm 150 PRINT"Pipe (ft):";LP,"Fan (cfm):";CMAX,"NTU:";NTU 160 Z=CMIN/CMAX'capacity rate ratio 170 ETERM=EXP(-(1-Z)*NTU) 180 E=(1-ETERM)/(1-Z*ETERM)'heat exchanger efficacy 190 THI=600'incoming flue gas temp (F) 200 TCI=70'incoming room air temp (F) 210 THO=THI-E*(THI-TCI)'outgoing flue gas temp (F) 220 PRINT"Hx Eff:";E,"Thi (F):";THI,,"Tho (F):";THO Heat (Btu/h): 20000 Wood (lb/h): 2.857143 Cair (cfm): 3.33 Pipe (ft): 10 Fan (cfm): 500 NTU: 3.534292 Hx Eff: .9703182 Thi (F): 600 Tho (F): 85.73138 With a 40K Btu/h fire, the efficacy drops to 83%, but the chimney still condenses, with Tho < 212... Heat (Btu/h): 40000 Wood (lb/h): 5.714286 Cair (cfm): 6.66 Pipe (ft): 10 Fan (cfm): 500 NTU: 1.767146 Hx Eff: .8270361 Thi (F): 600 Tho (F): 161.6708 The setup might look like this, in a fixed font like Courier: |<-- 10' -->| 10" pipe _______________________________ ______________________________c f room 6" pipe / -------------------------- --u <= a air || -----------------------||------ n flue|| || ||_________ cooled flue gas||^ || ||--------- gas exits--> ||| ||| || || ||v cl ------- | combustion air || | |_____________c__|| | stove |-------------d--- -------- intake port | drip | | | |bucket| ----------------------------------------------------------------- The 6" fluepipe could be inside a 10" pipe with 2 capped 6" Ts and a 10" T with a 10" to 6" reducer. Grainger's $70.85 4C847 550 cfm 10" fan (controlled by a flue thermostat in series with a room temp thermostat) could push room air into the 10" pipe and pressurize the combustion air intake port. Condensate could drip from the lower T with a liquid trap in a cap cl with a corrosion-resistant liner. A one-way motorized or passive plastic film damper cd could prevent reverse combustion airflow as the stove cools. The fresh air side of the damper box might contain a CO detector that makes an alarm and turns on the fan if CO appears. Nick Date: Tue, 30 Dec 2008 00:41:18 -0800 (PST) From: nick pine Subject: Re: A condensing woodstove chimney? Newsgroups: alt.energy.homepower harry wrote: > > The setup might look like this, in a fixed font like Courier: |<-- 10' -->| 10" pipe _______________________________ ______________________________c f room 6" pipe / -------------------------- --u <= a air || -----------------------||------ n flue|| || ||_________ cooled flue gas||^ || ||--------- gas exits--> ||| ||| || || ||v cl ------- | combustion air || | |_____________c__|| | stove |-------------d--- -------- intake port | drip | | | |bucket| ----------------------------------------------------------------- >The 6" fluepipe could be inside a 10" pipe with 2 capped 6" Ts and >a 10" T with a 10" to 6" reducer. Grainger's $70.85 4C847 550 cfm >10" fan (controlled by a flue thermostat in series with a room temp >thermostat) could push room air into the 10" pipe and pressurize >the combustion air intake port. Condensate could drip from the lower >T with a liquid trap in a cap cl with a corrosion-resistant liner. >A one-way motorized or passive plastic film damper cd could prevent >reverse combustion airflow as the stove cools. The fresh air side of >the damper box might contain a CO detector that makes an alarm and >turns on the fan if CO appears. > The system you describe is in fact "induced draught". Forced draught > would be blowing air into the stove (hence pressurising it). Look at the little down-arrow. The fan blows air into the stove. > Any such system in a domestic house is foolish because if the fan was to >fail, you and your family would stand a excellent chance of being gassed... Hence the combustion air check valve cd and the CO detector/fan controller. > There are numerous problems with condensing boilers even with gas > (which is the simplest to deal with) revolving round the acidity of > the condensate. You will not get a solid fuelled applianced to work > on condensing for this reason What's an "applianced"? The final low-temp part of the fluepipe needs to be corrosion-resistant, eg PVC. > also tars and creosotes condense out so insulatiing the heat exchange > surfaces especially with wood. "Insulatiing"? "Especially with wood"? Have you too long the German language speaking been? >The tars are difficult to remove and if a large amount catches fire Underwater? > the heat is so intense the chimney may well melt/burn through and > burn the building down in the worst case. Surely there are worser cases. > Tar deposition is bad enough on ordinary stoves a condensing stove > would be much worse. But tars and creosotes would condense out in water. > It's a good idea to make the top section of the chimney removeable > for this purpose. There is no chimney. > And the wood must be dry or vast amounts of energy are wasted > evaporatiing the moisture out of the wood. "Evaporatiing"? This should work fine with damp wood, recovering all that energy. > Also tar deposites are increased. What's a deposite? Tar is a non-flame problem. Maybe the fan should run regardless of the room temp if the flue gas temp near the stove ever drops to 300 F, indicating no flames. It might shut off if it stays below 300 for 5 minutes, when the stove runs out of fuel. > Woodburners are vastly labour intensive due to the neccesity of > cutting, stacking, storing and drying the wood. Good exercise. Warms you twice. You can also burn trash in a woodstove. My PhD friend Rich Komp says plastic bottles burn just like kerosine, chemically-speaking. > They are also filthy objects to have in the house So are toilets. >the, ash gets everywhere. "The, ash"? > Forget catalytic converters. OK. > They are expensive, do not increase efficiency, the precious metals > are "poisoned" by the combustion products. That is, if the tars > don't cover them up first. I'm planning to use this stove: http://www.homedepot.com/webapp/wcs/stores/servlet/ProductDisplay?storeId=10051&langId=-1&catalogId=10053&productId=100291302 which does not have a catalytic converter. > Harry, heating engineer Representing the state of the art? :-) Nick