Measuring chimney draft
Some text that I found on the net that needs to be re-written and cleaned up. Mostly it applies to factory stacks but I think some of the same priciples apply:
Draft pressure is critical to the design of the particular heating system and generally falls into one of four categories:
- Atmospheric systems are very common and depend entirely on the slightly negative stack pressure (due to the heated flue gases being lighter than air and naturally rising) to safely exhaust flue gases to the outside, while at the same time pulling in sufficient combustion air. These systems have draft diverters or hoods located immediately downstream from the heat exchanger which allow room air to be pulled in and mixed with the products of combustion before entering the vent system.
- Power Burners have a mechanical blower, which delivers combustion air to the flame, but also rely on a precisely controlled overfire draft to maintain consistent combustion air intake. This generally requires the installation of a barometric control.
- Balanced Draft boilers, which are designed to operate under a positive pressure in the combustion chamber, generally have a breach damper (either manually or automatically controlled), which maintains the boiler combustion chamber and flue gas passageways under a positive pressure to maximize efficiency. Manufacturers’ positive pressure requirements vary widely. However, a precisely controlled negative draft in the stack is still required to remove the products of combustion at a controlled rate and to allow for the exact amount combustion air to be introduced to the flame.
- Forced Draft systems also have a mechanical combustion air blower but are designed for a positive over fire pressure created, in part, by resistance to flue gas flow in the stack, which also operates under a positive pressure.
You ask how to optomise flue velocity....may I suggest that this is the wrong question: What you need is the stack diameter and stack system design which will optimise the static pressure developed at the base of your carbonizing chamber, just below the grates. It is the static pressure developed below the grates that draws air down through the sawdust bed.
The first step is to install a static pressure gage which reads the negative static pressure developed below your grates.. This will give you insights on how to operate your present system in a manner to get maximum static pressure development.
This can be done with a piece of 1/2" steel tubing or pipe, inserted into the chamber below the grate and sealed with an appropriate packing. Clay packed in place with appropriate support is quite adequate. Run this pipe up vertically about 5 ' above ground level, and then add a piece of clear plastic tubing, formed into a "U" shape. Add water to the tube. Read the difference in water height in each leg, as the static pressure developed. I would guess you would develop in the range of 1" static pressure development.
I think you may be optimistic by a factor of 5, give or take. Check out the table at http://www.ikweb.com/enuff/public_html/Chimdr.jpg
To measure variations of less than 0.1"WC will require at least, some form of inclined manometer made of clear plastic tubing, at best something much more expensive.
Another issue is the static pressure loss from the air intake of the firebox at the base of the chimney. The movement of the additional air, required for complete combustion, into the firebox comes at the expense of at least some of the chimney's total draft.
This will be a powerful tool, to aid you in determining the importance of the many variables associated with the operation of your system.
1: You can determine the optimum positioning of the firebox cover, which lets air into the stack for ignition and on-going combustion of flare gas.
2: You can check for leakage in the system, once you have a general procedure and empirical data developed.
3: You can develop optimal standards for the rate of charging of new sawdust
4: You can determine the maximum desirable height of charcoal in your system, before you stop charging fresh sawdust. (For example, once the system is up to temperature, I would guess that you will find that you get excellent carbonization rates until you reach a height of say 2', and thereafter, the gas flow rates drop significantly because of flow resistance through the deeper bed. Expressed in different terms, you may very well be able to get 300 kG/day out of a given system, if you charge to only 2' height, but if you charge to 3' height, your daily production drops to only 150 kG/Day
The key thing, I would guess, is operation of the inlet air control at the base of the stack, for combustion of flare gases. Too much air, and you have excessive pressure loss because of greater gas flow, and you lower the temperature of teh products of combustion, which reduces the draft developed. Too little air, and you don't combust all the flare gas, and don't develop maximum stack temperature. Basically, restrict the air supply progressively, until smoke is visible at the stack outlet, then open it progressively, until the smoke is almost gone. Watch the Manometer, to determine conditions yielding maximum pressure differential.
This then tells you how to operate your air inlets to get maximum stack temperature.
Then, with flare gas combustion conditions optimal, you experiment with other conditions. My initial guess is that you should try to maintain maximum suction under the grate, and that when your bed height is too high, you will probably see a drop in the draft, because the reduced gas flow results in lowering stack temperatures, and consequently, lower draft.
The theory certainly suggests that lower temperatures reduce draft. However my interpretation of Jay Shelton's book on Solid Fuels is that in practice, with chimney diameters in this range 20cm +/-, average flue gas temperatures over 600F(333C) actually slightly reduce the chimney capacity, due the higher friction losses associated with the higher velocities of the hotter, less dense, flue gasses. A drop to 400F and you loose a few percent also.
Optimal design of the stack is not simple. You need to know the approximate stack gas composition, its flow rate, and temperature. The two resulting dimensions are stack diameter and stack height. Sometimes there are external considerations, which change things markedly. Perhaps the optimal stack was 1' diameter, and 40' tall, but you have cheap access to a 2' diameter culvert, 20' long. Perhaps there are height restriction constraints.. A good starting point would be a velocity of about 300 feet per minute.
Lets consider a minimum instead of an optimum. I figure that Elsen's goal of around 30kg/hr of sawdust converting to 7kg/hr of charcoal would yield a producer gas combustion rate that would require a safe minimum chimney diameter of 20cm when 6 m tall. A larger size might let you speed up the process. My hunch is that between 18cm and 30cm dia. there lies an optimum that will not be worth pursuing. It would result in only a slight improvement. Pick a size in that range that is available. What ever is cheapest. After all Elsen, your experience will soon be worth more than my words, and next month, you'll be wanting a bigger system.
Stacks should, in theory, be insulated to maximize average stack temperature. However, you have to be very careful here.... if you insulate at the base, the stack gets too hot, and the steel scales. Scaling gets bad above about 800 degrees F. If the steel stack does not glow in the dark, then you are below 800 F. Ideally, what you should have is insulation on the inside of the stack, up far enough to prevent scaling temperatures, and then insulation on the outside, to reduce heat loss from there on, to maximize average stack temperature.
Hopes this helps you get a bit further with your operation. Once you get some of the above implemented, perhaps there are some other things that can be done for increase performance even further.
I would certainly agree with Kevin, that having some way to measure the draft at various points in the system would be ideal for developing an understanding of the system, and what various changes actually mean.