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Post by whazzatt on Aug 29, 2019 9:41:41 GMT -8
Hello fellow pyromaniacs ;-)
I have been contacted by someone who wants me to help build several rocket heaters to add to a school's pool heating setup.
I have not seen the pool or the warehouse-esque structure that houses it yet, but I have been told that the current 'regular' furnace is not doing much to heat the water despite the considerable expense to build it. Sounds like the furnace, which is located 50 metres away from the pool, has a grid-like pipe system above the flames that the water is pumped through. The water then travels the 50m in underground pipes before entering the pool via 8 inlet feeds.
The guy who contacted me wants me to build a test rocket heater at one of the inlets. He does not know how the rocket heater would actually heat the water, or how to build the unit, but the idea is for me to do that and then he will test the temps pre and post rocket heater, which will determine how many more units need to be built at other inlets.
He seems to think he needs another 25kw but I have a strong feeling that there are many variables to the situation that he is not taking into consideration - for example, he cannot tell me for how long the additional heaters would need to produce 25kw, and he has not considered the difference between the energy needed to get the pool to desired temperature and the energy needed to maintain the pool at the desired temp.
The current furnace is being fed manually, the fuel being long 'quarter rounds' from a local saw mill.
My thinking is along the following lines:
* J-tube test unit, because a) a batch box might heat the water too much too quickly and we don't want the swimmers to burn at the inlets inside the pool, and b) the J-tube can have a 'slide-feed' constructed above the mouth so that those long quarter rounds can slide down, thus keeping the rocket going for a few hours before needing to be tended to.
* On top of the regular upside down oil-drum radiator of the rocket heater will be a smaller barrel, with thick copper pipe coiled around the oil-drum radiator. Water first enters from ground level, runs up and through the coiled copper pipes, then into the barrel, before leaving the top of the barrel to the pool inlet. There is a pump moving the water through the system, so I hope I don't need to think too much about thermosyphon effects; the rate at which the water moves through the system will need to be kept to a minimum.
* Inside the pool venue, build a big batch box (perhaps 20cm) that heats a big brick bell so that heat can be stored in the thermal mass at night, which should prevent pool heat loss too.
Any thoughts?
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Post by gadget on Sept 9, 2019 18:32:41 GMT -8
I heated 385 gallons of water all last winter with a rocket mass heater. From what I have seen, it take a ton of heat to raise the temp just a little bit so I would not worry about output temps being to high.
With out knowing what size pool and assuming its not insulated, I would think you would need at least a dozen 8" rocket mass heaters running 24/7 to heat the pool. Maybe 6 people staffed to feed and chop wood - 3 shifts a day and a small forest near by for fuel. Maybe just go 2 dozen heaters and run for 12 hours a day.
Seriously though, I don't know if its possible in a practical way. I'm not good at math but I don't think its a good idea from what I have experienced heating water with wood.
A natural gas heater that is fully automated and designed for the task would be the best/cheapest option. Maybe some solar thermal panels so any greenies feel less guilty and can enjoy the warm water.
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Post by whazzatt on Dec 5, 2019 2:10:41 GMT -8
So I declined to help out with the project, so the guy went ahead with some other plans. Take a look: photos.app.goo.gl/mXPmikbT6kwNLVMG9Unsurprisingly, it's not working. Yesterday I was asked to come take a look at why things aren't working as they hoped - lots of reasons that I won't go into detail about here. I am basically being begged to try a big J-tube rocket retrofit (several reasons for J preference over Batch). It is an opportunity to be involved with the building of a massive system, so I am kinda tempted. But I need help from the gang here. Please check the album. 4 pics there: 1) the adapted wood-burning unit that simply does not work for this application 2) the 'heat-exchanger', which has many metres of pipes coiled around the inside 3) heat-exchanger (2 of the tripod legs are in clear view), with the port through which heat is supposed to enter (but it is currently not working very well) 4) A pic I found online that I will soon refer to (source: permies.com/t/27583/rocket-stoves/rocket-stove-water-floor-heater) The idea would be to slide the current wood-burning unit out (pic 1), and build a very large J-tube unit in it's place. The port into the 'heat-exchanger' (pics 2 and 3) would be sealed, with a new one (the size of the proposed riser) created in the middle of that bottom plate. The current chimney (not pictured, but it is currently constituted by the narrowing of the heat exchanger to a traditional square chimney at the top) would be sealed. The heat exchanger would then look like a traditional barrel or drum, slightly curved at the top (like a boiler/geyser drum). The riser would be super long - all the way to the top of the modified heat exhanger, with the usual top-gap between top of riser and heat exchanger's new surface. No water-carrying pipes would be in the path of the heat entering the top of the heat exchanger. An exit flue would be added to the bottom of the heat exchanger (middle-middle of pic 2), which would technically become a bell. Exit flue would lead to a chimney that is at least 3 metres higher than the apex of the structure's roof. Chimney CSA can be slightly oversized to encourage draw. Questions: 1) Pic 4, the borrowed pic, shows a situation similar to what is happening on the inside of the bell - pipes will be exposed to very high temps (Note that the water is not being passively moved through the system; pumps are involved!). Is this a 'metal is doomed' scenario or is that only when exposed to direct flame (e.g. burn chamber and riser). 2) The riser will be up to 4 metres long, maybe a bit longer. I have never seen such a long riser actually built. Is this a viable design feature? (My gut feeling is that it is). 3) Is this another waste of time, energy, and resources? I would never have encouraged a person to try and heat a large swimming pool with wood combustion, but clearly he has gone ahead and tried... and failed. This would be a last resort to salvage things... and I would be given the chance to be involved in a really big build! What do you think? (Gadget obviously thought it a dead end, but that was when the situation was slightly different...)
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Post by Deleted on Dec 5, 2019 7:31:28 GMT -8
Swimming pools at the Olympic Games are 50m long 25m wide, at least 2m deep with 10 lanes. It has about 3000 cubic meter. In order to train for competitions, a common condition for school swimming pools, a swimming pool must be 50m long, but may be smaller in width. It requires about 3500 KWh to change the temperature of 3000 cubic meter by 1°C.
Obviously, even with a very large Rocketstove one could warm up at most a paddling pool for toddlers.
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Post by whazzatt on Dec 12, 2019 7:31:22 GMT -8
I agree that this is a lost cause. But the guy is desperate. He and company have spent a small fortune on the set-up (I promise it is not me!). He wishes to go ahead and try a big rocket despite the 10+ points I have given in writing that state why it won't work! I have again not yet agreed! I'd like to run this by the community here first. At www.batchrocket.eu/en/9-english/7-building there is a graph stating that a 250mm batch box can get 8.6kW from 27.8 kgs of wood. This has given him a small bit of hope. Not necessarily getting my hopes up. But, I would have an alibi to build a massive batch box, oh yeah! And the guy does have constant access to a lot of wood, with a work-force to run the fires - that's what was happening with the failed set-ups anyway. So I'll throw this out to you all: I'm not asking if you think it will work to heat the water! I'm asking about the likelihood of the following retrofit pushing the hot air through the system successfully. Please take a look at the existing heat exchanger, which I will call the big bell: photos.app.goo.gl/mXPmikbT6kwNLVMG9I have added a fifth picture - my excellent and highly skilled drawing of the inside of the bell. The bottom plate (or floor) of the bell is clearly visible in the photos. The current (failing) furnace input square area is also visible. The square input area is cut into the bottom plate, i.e. the bottom/floor of the big bell. Slightly above that is another 'diffuser' plate (see drawing, pic 5), which has many round holes cut into it in order to dissipate heat more widely into the bell (for the original failed set-up). Lots of pipes are coiled above the inner 'holey' plate, mostly accumulating nearer the inner wall of the bell (Kind of like the 4th pic in the album, i.e. the illustration I found online). I am thinking of the following: - a 250mm open-door batch box replacing the failed furnace. Easy enough to remove the furnace. - the batch box would be like one of Peter's old work-shop units (pictured at the top of the batchrocket link). - two-and-a-half (ish)barrel system that reaches the bottom plate. Cut a round entrance (around the present square one to 'remove' it). - seal the present 'chimney' (see my drawing) at the top with a plate (thus creating an almost-proper bell), and do the usual exit flue from bottom barrel as per usual approach. Question 1: that diffiuser plate - is it going to impede flow, in which case should it simply be removed? The total surface area of those holes is quite large. Question 2: would it be better to seal the present square entry area, and rather cut a barrel-sized round hole in the centre of the bottom plate of what would become a proper bell? I'm thinking that heat entering the middle of the bell is better than an off-centre entry. Question 3: is the tapered chimney area a bad idea for the bell? The present chimney could be cut lower and sealed with a plate, creating more of a traditional bell shape.
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Post by Deleted on Dec 13, 2019 7:56:28 GMT -8
28Kg of wood release about 110KWh, that is just about 3% of the about 3500 KWh required to change the temperature of 3000 cubic meter by 1°C. It would require about a metric ton of wood for 3500 KWh. A pool with 4 useable lanes would have to be 15m wide, wich still would have about 1875 cubic meter requiring about 2200KWH to change the temperature by 1°C. You may consider to build a 30inch/750mm stove.
How big is the pool actually? How much volume?
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Post by whazzatt on Dec 18, 2019 5:57:19 GMT -8
How big is the pool actually?
How much volume?
I just received the answer to this question. 640,000 liters!!! Damn! In the most recent conversation I had with the guy, I really emphasized how much of a bad idea it is to try meet his kW requirements with wood combustion. He seemed to get it, probably because even the people who constructed the current failing furnace are apparently also starting to admit what a bad idea it was! So the guy asked if I'd help build a test unit for the purpose of experimentation, presumably with a smaller volume of water. Thanks Karl
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Post by Deleted on Dec 18, 2019 7:21:14 GMT -8
640 cubic meter require about 750KWh, corresponding to about 220 kg of wood, to change the temperature by 1°C. You may offer to build a 30inch/750mm stove without any kind of guaranty.
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Post by pigbuttons on Dec 30, 2019 19:33:53 GMT -8
At least you won't have to worry about it flashing to steam and blowing up. sic.
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