Post by tumppiw on Dec 15, 2012 10:26:47 GMT -8
Hi fellow rocketeers!
I'm a farmer from Finland, and have been into RMHs for a few years now. One has been happily warming our goatshed and a few have done our cooking. This forum has been a great source for information, especially the p-plate has made a difference in all our rockets.
Now I've started to thing about building a bigger rocket burner for our grain dryer. Typically those things run with a 300 kW oil furnace. The furnace heats air, which is blown through a circulating mat of grain. I've added solar air heat and coupled our house wood chip boiler to the drier, but those amounted only for 60 kWh, so about 250 kW is still missing.
So the design parameters for the rocket heater are:
- 250 kW of power output for 4-5 hours at a time, used to supply 60-80 C degree air
- equals approximately 50-70 kg of wood/h, c.a. 250-350 kg wood/batch
- with sufficient burning air (excess air factor of 2) being around 7 m3/kg wood, the airflow is 350-500 m3/h
- preferably no stored heat, rapid cooldown is needed for grain before storage
For simplicity, I'm planning on skipping heat exchange and putting the exhaust gases directly into the air intake of the drier. (In the old days, all driers worked with smoke coming through the grain.) A concept plan is attached.
Since the amount of fuel is pretty big, I'm planning on putting a "f�rugn" for the fuel. (The first barrel.) I'm not sure what it's called in English, but they are used in Finland & Sweden. ( www.novator.se/bioenergy/wood/B6.pdf and www.warmax.fi/eng/product_info.asp) Basically they gasify the wood which is stored in an airtight container. They are famous for burning just about anything. Air is fed vertically below the gasifier and is preheated by the gasifier walls.
I tried to do some dimensioning, using an equation of Q = 0.65 * A (2*g*h*dT/T0)^0.5, for the airflow (m3/s). The equation is for sizing natural draft in chimneys, I'm not sure how well it applies to rocket chimneys. With dT = 750 K, and pipe diameter of 200 mm, a rocket chimney with a height of 1100 mm should draw enough burning air. The vertical barrel on top is used to dilute the exhaust gases with intake air to get lower temperatures for the drier.
So does anyone have experience on rocket burners for >200 kW output? Does the dimensioning make sense? I was planning on making the burn tube and riser from castable vermiculite clay and insulate with rock wool. I know that people have had bad experiences in increasing the size of the downdraft wood box, but maybe this air tight construction might work around it. The barrel in the plan is a first iteration, in reality the box would need to be big enough to hold all the wood for the batch.
Thanks & greetings from snowy Finland!
I'm a farmer from Finland, and have been into RMHs for a few years now. One has been happily warming our goatshed and a few have done our cooking. This forum has been a great source for information, especially the p-plate has made a difference in all our rockets.
Now I've started to thing about building a bigger rocket burner for our grain dryer. Typically those things run with a 300 kW oil furnace. The furnace heats air, which is blown through a circulating mat of grain. I've added solar air heat and coupled our house wood chip boiler to the drier, but those amounted only for 60 kWh, so about 250 kW is still missing.
So the design parameters for the rocket heater are:
- 250 kW of power output for 4-5 hours at a time, used to supply 60-80 C degree air
- equals approximately 50-70 kg of wood/h, c.a. 250-350 kg wood/batch
- with sufficient burning air (excess air factor of 2) being around 7 m3/kg wood, the airflow is 350-500 m3/h
- preferably no stored heat, rapid cooldown is needed for grain before storage
For simplicity, I'm planning on skipping heat exchange and putting the exhaust gases directly into the air intake of the drier. (In the old days, all driers worked with smoke coming through the grain.) A concept plan is attached.
Since the amount of fuel is pretty big, I'm planning on putting a "f�rugn" for the fuel. (The first barrel.) I'm not sure what it's called in English, but they are used in Finland & Sweden. ( www.novator.se/bioenergy/wood/B6.pdf and www.warmax.fi/eng/product_info.asp) Basically they gasify the wood which is stored in an airtight container. They are famous for burning just about anything. Air is fed vertically below the gasifier and is preheated by the gasifier walls.
I tried to do some dimensioning, using an equation of Q = 0.65 * A (2*g*h*dT/T0)^0.5, for the airflow (m3/s). The equation is for sizing natural draft in chimneys, I'm not sure how well it applies to rocket chimneys. With dT = 750 K, and pipe diameter of 200 mm, a rocket chimney with a height of 1100 mm should draw enough burning air. The vertical barrel on top is used to dilute the exhaust gases with intake air to get lower temperatures for the drier.
So does anyone have experience on rocket burners for >200 kW output? Does the dimensioning make sense? I was planning on making the burn tube and riser from castable vermiculite clay and insulate with rock wool. I know that people have had bad experiences in increasing the size of the downdraft wood box, but maybe this air tight construction might work around it. The barrel in the plan is a first iteration, in reality the box would need to be big enough to hold all the wood for the batch.
Thanks & greetings from snowy Finland!