Garn wood consumption.

[Gasifier]

Some things in this wood heating thing I still have a hard time getting my head around. You just brought up one. If the draft fan in a lambda boiler is slowed down - based not on burn conditions or fuel load but rather rising water temps & closing delta T's - wouldn't that lower the combustion efficiency? It is cutting down the air supply while the fuel supply might be remaining constant - which seems to me at that point to be setting up for a less efficient burn.

EDIT: That was replying to jebatty's post - gasifier got in ahead of me on the reply.

 

[ewdudley]

If the draft fan in a lambda boiler is slowed down - based not on burn conditions or fuel load but rather rising water temps & closing delta T's - wouldn't that lower the combustion efficiency? It is cutting down the air supply while the fuel supply might be remaining constant - which seems to me at that point to be setting up for a less efficient burn.

The rate of fuel supply is the rate of gasification, which is for the most part the rate of pyrolization sustained according to the rate of primary air supply. So if primary and secondary air supplies are decreased together then combustion efficiency can remain high. Lambda controlled gasifiers can take it a step further by fine tuning the ratio of primary to secondary air supply as total air supply is reduced.

Overall efficiency (for non-condensing solid fuel units) is determined by how complete the combustion process is, how little excess oxygen (and nitrogen) is introduced (and heated), and how low the (non-condensing) exhaust temperature is. So if combustion is complete, excess oxygen is minimized, and flue temperature is minimized; then efficiency is maximized.

This implies than optimum efficiency can be maintained by lowering combustion rate as water temperature increases, assuming constant heat exchange area.

 

[heaterman]

Again, this is only true under certain conditions for certain boilers that are not sophisticated enough to control flue temperature to the minimum non-condensing flue gas temperature throughout the burn cycle.

Any boiler is more effective when firing into low temperatures, but as you well know efficiency is measured according to the temperature and composition of the flue gases, not the temperature of the water at the heat exchange surfaces.

That is true.
Efficiency is measured by analyzing the flue gas of any fuel burning appliance at a given point in time and under prevailing conditions. Efficiency does however rise and fall in a boiler in tandem with the water temperature in the vessel. If you look at a chart or graph of efficiency vs water temperature for any boiler you can see that even the best gas boilers will be around 85-87% when working into water temps in the 180* range.
If you take the same boiler, under the same circumstances and fire it into 110* water the graph will show efficiencies over 94-95% because the vapor in the flue gas is now collapsing back into liquid and releasing its latent heat.

The same thing, albeit to a lesser extent, happens in a wood fired boiler that can tolerate low return water temps. But alas....we are splitting hairs here. A far larger factor is the moisture content of the fuel in terms of overall thermal efficiency.

 

[jebatty]

... based not on burn conditions or fuel load but rather rising water temps & closing delta T's

EW answered this better than I could, and I did not say that draft fan was regulated only by rising water temp and closing delta-T's. Those factors likely are involved, and consistent with EW's statement, the Froling also measures oxygen and varies the primary and secondary to maintain high efficiency.

 

[maple1]

Think I got it.

After a year with mine, I have no doubt a lambda can get more out of the same wood. There are times with mine that I have to crack open the fire door to let more air in, on startup. Just too much off-gasing all at once if I load too much in, for the conditions in the boiler (everything cold) - and the secondary burn comes & goes as the oxygen gets used up & then rushes in again. The so-called 'puffing', I believe.

 

[TCaldwell]

keep it going, this is a great thread

 

[Robert V]

So, in simple terms, the greater the temp difference, aka Delta T, the better heat transfer from the HX to the water being heated. As the two become closer in temp, the more difficult it is to transfer heat. So, in a real life application, what would be the ideal temps, both high and low, for an unpressurized system? At what point are you spinning your wheels by getting the water too hot or letting it go too cold? What is the harm in allowing the water to cool too much?

 

[wardk]

So, in simple terms, the greater the temp difference, aka Delta T, the better heat transfer from the HX to the water being heated. As the two become closer in temp, the more difficult it is to transfer heat. So, in a real life application, what would be the ideal temps, both high and low, for an unpressurized system? At what point are you spinning your wheels by getting the water too hot or letting it go too cold? What is the harm in allowing the water to cool too much?

Hi Robert; I don't have the answers ,I hope the experts will chime in . I have a couple questions about your system, how hot do you run it and what temp do you refire say after 24hrs? I haven't used panel rads but was considering them for some cold areas, do you find them efficient and how hot do you run them?Thanks

 

[heaterman]

You are correct Rob. It's a basic law of physics that the greater the temp difference is, the "faster" the higher temperature will transfer to the lower.
It's also why a person has to upsize a plate type heat exchanger by a much larger degree than you would think to get approach temperatures within 5* or less.

The chart I posted tells a couple stories if you look at it. It's easy to see how heat transfer accelerates as the differential between flame or heat exchanger surface temp and water become greater. The other factor you see in play there is modulation, which is not common in wood burners but interesting to note anyhow. I think, that efficiency goes up at lower burn rates because the ratio between flue gas volume and sq ft of heat exchanger surface increases.

 

[Robert V]

e="wardk, post: 1563237, member: 23023"]Hi Robert; I don't have the answers ,I hope the experts will chime in . I have a couple questions about your system, how hot do you run it and what temp do you refire say after 24hrs? I haven't used panel rads but was considering them for some cold areas, do you find them efficient and how hot do you run them?Thanks[/quote]

wardk,
I am not very experienced with my system yet. I have only been running it about 2 months. So far I have been heating the water to around 190 deg and allowing it to cool to 135 ish. I am running a water to air hx in the basement, 30 plate hx for dhw, and 10 panel radiators of various sizes for the main level. Heater man did the heat loss calculation and help me design the system. The panel radiators are working really great and I am glad I went with them. The temp to them varies based on whatever the water temp is. They are capable of putting off a lot of heat but most of the time they are just a touch warmer then the air in the room. They are silent, self governing, and seem to be very efficient. The house never felt this warm with our propane forced air furnace. How long have you had your garn? What kind of system do you have?

 

[Robert V]

You are correct Rob. It's a basic law of physics that the greater the temp difference is, the "faster" the higher temperature will transfer to the lower.
It's also why a person has to upsize a plate type heat exchanger by a much larger degree than you would think to get approach temperatures within 5* or less.

The chart I posted tells a couple stories if you look at it. It's easy to see how heat transfer accelerates as the differential between flame or heat exchanger surface temp and water become greater. The other factor you see in play there is modulation, which is not common in wood burners but interesting to note anyhow. I think, that efficiency goes up at lower burn rates because the ratio between flue gas volume and sq ft of heat exchanger surface increases.

I have a long way to go but I am catching on to this stuff. Never thought I would find home heating so interesting. Hard to explain my interest to other people. Anyone relate to that?

 

[heaterman]

e="wardk, post: 1563237, member: 23023"]Hi Robert; I don't have the answers ,I hope the experts will chime in . I have a couple questions about your system, how hot do you run it and what temp do you refire say after 24hrs? I haven't used panel rads but was considering them for some cold areas, do you find them efficient and how hot do you run them?Thanks

wardk,
I am not very experienced with my system yet. I have only been running it about 2 months. So far I have been heating the water to around 190 deg and allowing it to cool to 135 ish. I am running a water to air hx in the basement, 30 plate hx for dhw, and 10 panel radiators of various sizes for the main level. Heater man did the heat loss calculation and help me design the system. The panel radiators are working really great and I am glad I went with them. The temp to them varies based on whatever the water temp is. They are capable of putting off a lot of heat but most of the time they are just a touch warmer then the air in the room. They are silent, self governing, and seem to be very efficient. The house never felt this warm with our propane forced air furnace. How long have you had your garn? What kind of system do you have?[/quote]



Love to hear that. It's really difficult to describe the difference in the "feel" of heat from a rad vs the same room temp with forced air. It's as close as you can get to having a wood stove in each room.

 

[wardk]

wardk,
I am not very experienced with my system yet. I have only been running it about 2 months. So far I have been heating the water to around 190 deg and allowing it to cool to 135 ish. I am running a water to air hx in the basement, 30 plate hx for dhw, and 10 panel radiators of various sizes for the main level. Heater man did the heat loss calculation and help me design the system. The panel radiators are working really great and I am glad I went with them. The temp to them varies based on whatever the water temp is. They are capable of putting off a lot of heat but most of the time they are just a touch warmer then the air in the room. They are silent, self governing, and seem to be very efficient. The house never felt this warm with our propane forced air furnace. How long have you had your garn? What kind of system do you have?

Love to hear that. It's really difficult to describe the difference in the "feel" of heat from a rad vs the same room temp with forced air. It's as close as you can get to having a wood stove in each room.[/quote]
My system has the garn supplying a 70 plate hx to pressurized secondary, the loads are hydronic basement floor,water to air hx forced air, 2 cast iron rads, 2 baseboard and a sidearm for dhw. I have just put a Burnham lp boiler inline waiting on a gas fitter to check out my install and do a first fire. This system heats a 3900 sq ft house and warms a 1800sq ft shop. Still fine tuning with the help of the experts on this forum.