Posted by nick pine on December 28, 2008, 2:31 pm
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 000!'combustion heat (Btu/h)
40 HHVp00'wood high heating value (Btu/lb)
50 PPH=HEAT/HHV'dry wood consumption (lb/h)
60 CANp'combustion air need (ft^3/lb)
70 CMINÊN*PPH/60'combustion airflow (cfm)
80 PRINT"Heat (Btu/h):";HEAT,"Wood (lb/h):";PPH,"Cair (cfm):";CMIN
90 LP'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 CMAXP0'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`0'incoming flue gas temp (F)
200 TCIp'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):
3.333333
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):
6.666667
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|| || ||_________
gas||^ || ||--------- cooled flue
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
Posted by daestrom on December 28, 2008, 3:26 pm
nick pine wrote:
> 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 000!'combustion heat (Btu/h)
> 40 HHVp00'wood high heating value (Btu/lb)
> 50 PPH=HEAT/HHV'dry wood consumption (lb/h)
> 60 CANp'combustion air need (ft^3/lb)
> 70 CMINÊN*PPH/60'combustion airflow (cfm)
> 80 PRINT"Heat (Btu/h):";HEAT,"Wood (lb/h):";PPH,"Cair (cfm):";CMIN
> 90 LP'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 CMAXP0'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`0'incoming flue gas temp (F)
> 200 TCIp'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):
> 3.333333
> 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):
> 6.666667
> 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|| || ||_________
> gas||^ || ||--------- cooled flue
> 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.
A CO detector is a good idea around any wood-burner. Especially if the
draft is small (tight house, cold flue, short vertical, etc...).
If the wood is about 20% moisture, then I make it out that for a 5.7 lbm/hr
fire (40,000 BTU/hr), you have about 2.4 lbm/hr of H2O in the flue gasses
(about 1.14 from the moisture in the wood and the rest from combustion
products). So a perfect condensing flue you might get an additional 2400
BTU/hr (about 6% increase over the 40,000 BTU/hr).
Not sure that it's really worth all of this, since you probably will only
recover some fraction of that. Unless all the stainless fittings and flue
are cheap to come by. Of course using a circulating fan helps distribute
the heating regardless of the flue setup, so I don't consider the cost of
operating the fan to be a problem.
But a cold flue temperature also will condense a lot of other compounds
besides just the water. Of course that depends on just how well secondary
air is used to burn them before going up the flue. Or maybe a catalytic
converter? Might have to clean the flue often or provide a drain that can
handle the tar/goo that you might get.
daestrom
P.S. Thanks for putting the units on many of your variable calculations.
Makes it a lot easier to follow along :-)
Posted by nick pine on December 28, 2008, 7:10 pm
> If the wood is about 20% moisture, then I make it out that for a 5.7 lbm/hr
> fire (40,000 BTU/hr), you have about 2.4 lbm/hr of H2O in the flue gasses
> (about 1.14 from the moisture in the wood and the rest from combustion
> products). So a perfect condensing flue you might get an additional 2400
> BTU/hr (about 6% increase over the 40,000 BTU/hr).
Norbert Senf says a condensing chimney can add 9% to the heating value
of bone-dry wood. It might add 29% to 20% wood.
>Not sure that it's really worth all of this, since you probably will only
> recover some fraction of that. Unless all the stainless fittings and flue
> are cheap to come by.
I'm thinking the parts exposed to low-temp condensation might be
corrosion
resistant vs stainless, and the rest can be galvanized and
periodically
replaced.
> But a cold flue temperature also will condense a lot of other compounds
> besides just the water. Of course that depends on just how well secondary
> air is used to burn them before going up the flue.
I'm planning to use an Englander airtight stove with secondary
combustion,
about $1K from Home Depot.
> Might have to clean the flue often or provide a drain that can
> handle the tar/goo that you might get.
I'm planning to use a drain, as shown in the diagram.
Nick
Posted by nick pine on December 30, 2008, 3:41 am
> > The setup might look like this, in a fixed font like Courier:
|<-- 10' =
-->|
10" pipe _______________________________
______________________________c =
f room
6" pipe / -------------------------- --u <= a ai=
r
|| -----------------------||------ n
flue|| || =
||_________
gas||^ || =
||--------- cooled flue
||| ||| || gas exits-->
||| ||| =
||
|| ||v =
cl
------- | combustion air ||
| |_____________c__||
| stove |-------------d---
-------- intake port | drip |
| | =
|bucket|
-----------------------------------------------------------------
> 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
Posted by nick pine on December 30, 2008, 7:42 pm
>... It takes energy to evaporate water. This energy is lost. Just goes up the chimney in the form pf steam.
Not if we condense the water vapor.
Nick
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>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.