Post by timbee on Jun 18, 2011 7:12:35 GMT -8
Hi All first post although i have been ghosting the forum for awhile. Ive been having great fun reading back through all the old threads so hopefully wont ask any questions that have already been dealt with.
Ive just finished a rocket powered water heater. Basically its a 55 gallon drum(44 gallon here in Aussie) laid on its side in a frame with an all steel rocket underneath.The drum has a 1/2 copper heat exchange coil spiraled through the center so that the drum itself is unpressurized and vents to the atmosphere. The rocket is 6" x 10" x 1/4" steel box section , heat riser is approx 44" , burn tunnel approx 20", feed tube approx 12" . The drum is shelled in old roofing iron with approx a one inch gap all round but with a bulge on the bottom where the riser feeds in of approx 6". This shell is covered in some kind of mineral insulant (not fiberglass) that i found in the shed and finally skinned over in more roofing tin to try and make it look pretty ,not very successfully i might add but at least it keeps the insulation dry!
I'm very happy with the results, one load of reasonable hardwood approx 12" long gets the water to showering temp in about an hour (its winter here and were just getting frosts) and somewhere between that and 2-3 loads gets the entire 200 liters boiling. Sorry i cant be more specific with wood usage but the unit has been in constant use by up to 15 people on a daily basis since and with the insulation is keeping the water hot enough for quite a few showers the next morning after its evening fire up. So i haven't had a chance to run any cold start tests.
I have a question though and its to do with something I've experienced and something that has been touched on a few times , namely steel vs brick or refractory risers. A number of times Donkey has mentioned a quicker start up for steel risers as his reason for leaning that way. The thing is i built my riser out of heavy steel by preference for durability and ease of construction, but i fully expected it to be a slower starter than if i had used brick. My reasoning is that steel is a good conductor compared to brick, it heats quickly and also cools quickly .Brick on the other hand is slower to heat up and slower to give up its heat. On cold start up the steel being a better conductor absorbs more heat from the hot gasses in the warm up phase thereby lowering combustion temp and gas volume which also has an adverse effect on gas velocity through the system, hence a sluggish start. Brick on the other hand having lower thermal conductivity absorbs less heat at a slower rate thereby leaving more heat in the combustion chamber hence higher gas temps higher gas volume and higher velocity.
Of course the point may be moot as once the two differing materials are heated through to the insulative layer the temperature will creep up and eventually stabilize, but until that happens i cant help but think that the steel riser will be a bit more sluggish. Its like if you had two identical rocket stoves ,one with a 1/4" steel riser and one with a 1/4" brick or refractory riser , i bet i could hold onto the brick riser a lot longer than the steel one on start up. Where's the heat that's not burning your hand on the brick riser that is on the steel riser? My guess is in the combustion gasses . Is my reasoning off ? I just cant seem to picture it any other way unless it has something to do with laminar flow being greater in the smoother steel riser and or the brick having a greater surface area due to surface texture or some other odd factor i hadn't thought of.
Cheers Tim
Ive just finished a rocket powered water heater. Basically its a 55 gallon drum(44 gallon here in Aussie) laid on its side in a frame with an all steel rocket underneath.The drum has a 1/2 copper heat exchange coil spiraled through the center so that the drum itself is unpressurized and vents to the atmosphere. The rocket is 6" x 10" x 1/4" steel box section , heat riser is approx 44" , burn tunnel approx 20", feed tube approx 12" . The drum is shelled in old roofing iron with approx a one inch gap all round but with a bulge on the bottom where the riser feeds in of approx 6". This shell is covered in some kind of mineral insulant (not fiberglass) that i found in the shed and finally skinned over in more roofing tin to try and make it look pretty ,not very successfully i might add but at least it keeps the insulation dry!
I'm very happy with the results, one load of reasonable hardwood approx 12" long gets the water to showering temp in about an hour (its winter here and were just getting frosts) and somewhere between that and 2-3 loads gets the entire 200 liters boiling. Sorry i cant be more specific with wood usage but the unit has been in constant use by up to 15 people on a daily basis since and with the insulation is keeping the water hot enough for quite a few showers the next morning after its evening fire up. So i haven't had a chance to run any cold start tests.
I have a question though and its to do with something I've experienced and something that has been touched on a few times , namely steel vs brick or refractory risers. A number of times Donkey has mentioned a quicker start up for steel risers as his reason for leaning that way. The thing is i built my riser out of heavy steel by preference for durability and ease of construction, but i fully expected it to be a slower starter than if i had used brick. My reasoning is that steel is a good conductor compared to brick, it heats quickly and also cools quickly .Brick on the other hand is slower to heat up and slower to give up its heat. On cold start up the steel being a better conductor absorbs more heat from the hot gasses in the warm up phase thereby lowering combustion temp and gas volume which also has an adverse effect on gas velocity through the system, hence a sluggish start. Brick on the other hand having lower thermal conductivity absorbs less heat at a slower rate thereby leaving more heat in the combustion chamber hence higher gas temps higher gas volume and higher velocity.
Of course the point may be moot as once the two differing materials are heated through to the insulative layer the temperature will creep up and eventually stabilize, but until that happens i cant help but think that the steel riser will be a bit more sluggish. Its like if you had two identical rocket stoves ,one with a 1/4" steel riser and one with a 1/4" brick or refractory riser , i bet i could hold onto the brick riser a lot longer than the steel one on start up. Where's the heat that's not burning your hand on the brick riser that is on the steel riser? My guess is in the combustion gasses . Is my reasoning off ? I just cant seem to picture it any other way unless it has something to do with laminar flow being greater in the smoother steel riser and or the brick having a greater surface area due to surface texture or some other odd factor i hadn't thought of.
Cheers Tim
