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Hot Water
Jan 31, 2014 16:01:32 GMT -8
via mobile
Post by ronyon on Jan 31, 2014 16:01:32 GMT -8
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Post by endrunner on Jan 31, 2014 20:28:36 GMT -8
I have actually built a couple of revisions of various boxes to heat water with an 8" using thermosiphon pumping in an unpressurized loop. I found coils introduce a lot of friction and really slow water movement. My last rev raised the temperature 100F at 1 gpm. My exit temperatures on the flue pipe were still 350F, so clearly I am not extracting enough heat. Been puzzling over next move. It seems like I should be able to get to 75k BTU/hr without a pump, but not there yet.
Donkey, I think you will need more head room above your heat riser to move into the bell.
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Deleted
Deleted Member
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Post by Deleted on Feb 1, 2014 5:09:23 GMT -8
Donkey's setup is safe for different reasons: 1. The water in the tank isn't under pressure. 2. Because of this the water cannot get any hotter than 212F. 3. Because the additional energy required for phase change cannot be transfered to the water inside the coil it cannot boil at 212F. |
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Post by wrekinwanderer on Feb 13, 2014 12:20:15 GMT -8
Hi Ga2500ev,
Thanks for your breakdown of spacing needed between tank and sides of the bell.
Apologies. I didn't make myself very clear about the lack of a coil in the setup I was describing. I shall try again!
In my set up the tank to be heated in the bell, is fed from a small header tank which receives domestic water under pressure. In my case the header tank is about 1m above the hot water cylinder. Therefore the water in the tank is not under mains pressure.
There are three tubes coming out of the top of the tank in the bell, cold-in, hot-out and the third tube is an expansion tube that rises up to the header tank. Therefore the system is open. This is the pressure release valve. As the hot water in the bell increases in temperature, the pressure increases, it expands and water goes safely up the expansion tube and back to the header tank. The water in the tank can boil without a problem. This is a standard open-vented hot water system in the UK. The circuit is open, nothing is under pressure, nothing to go boom. Cold water is fed from the header down a tube into the bottom of the hot water tank, which pushes the hot water out as required. Simple.....
Hmmmm, if I was going to use a coil, I wonder if silicon tube would work. Temperature rated to 250 degrees C. Cheaper than copper?
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Post by 2tranceform on Feb 13, 2014 13:05:02 GMT -8
.... Hmmmm, if I was going to use a coil, I wonder if silicon tube would work. Temperature rated to 250 degrees C. Cheaper than copper? Silicon tubing does have low thermal conductivity. |
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Post by ronyon on Feb 13, 2014 22:09:52 GMT -8
wrekinwanderer, does the system use a pump? If not and it is not pressurized, what propels the heated water out of the tank? Gravity?
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Post by Donkey on Feb 19, 2014 9:37:48 GMT -8
Hey y'all.. Due to conversations elsewhere, I've made an .XL sheet for calculating heat generated and transferred to the water.My math skills are pretty rudimentary (and my understanding of spread sheets) but I think that what I've done is correct.. There are links to my source material inside the .XL sheet. It should be self explanatory but if anyone needs explanation of an item or my assumptions, go ahead and ask.. This is really meant to start a conversation. Anyone interested in double-checking this, correcting my mistakes, adding stuff I've missed or pointing out a better way to this?? (please) |
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Post by Donkey on Feb 19, 2014 9:41:25 GMT -8
Oh.. It's based on a J-tube rocket stove, 8 inch model. The stove design at the top of this thread..  |
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Cramer
Junior Member
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Post by Cramer on Feb 19, 2014 18:40:12 GMT -8
Hey y'all.. Due to conversations elsewhere, I've made an .XL sheet for calculating heat generated and transferred to the water.My math skills are pretty rudimentary (and my understanding of spread sheets) but I think that what I've done is correct.. There are links to my source material inside the .XL sheet. It should be self explanatory but if anyone needs explanation of an item or my assumptions, go ahead and ask.. This is really meant to start a conversation. Anyone interested in double-checking this, correcting my mistakes, adding stuff I've missed or pointing out a better way to this?? (please) A few questions I am not quite understanding: Where did your assumption of % efficiency of heat transferred to water come from? Is this the water in the buffering tank or the water in the tubing you speak of? Is the coil in the buffering tank in a closed loop to a holding tank? |
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Post by Donkey on Feb 20, 2014 1:52:07 GMT -8
Where did your assumption of % efficiency of heat transferred to water come from? That is an editable field. I didn't know what to set it at, so made it variable. Good one.. As it is, It would be the water in the buffering tank, though one would assume that as long as the water in the coil is NOT moving, it would equalize with the buffer water. This could branch into a section that would determine the output temperature of the coil with variables for initial temperature, volume and velocity.. I suppose it could be, but no.. With this design, the coil is more like an on demand system, it runs to the tap. |
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Cramer
Junior Member
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Post by Cramer on Feb 20, 2014 5:43:34 GMT -8
Fluid Transmission Surface Fluid Overall Heat Transmission Coefficient
(Btu/ft2 hr oF) (W/m2 K)
Water Mild Steel Water 60 - 70 340 - 400
I found this data, I have no idea how to interpret it but someone else might.
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Post by Donkey on Feb 20, 2014 19:26:33 GMT -8
Where did your assumption of % efficiency of heat transferred to water come from? I don't think that I adequately answered this question. Sadly, the 20% figure seems to fit pretty well with what I'm seeing on my shower heater. I just fiddled with different numbers till it felt close to right. 20% feels close and it's a nicely round number. |
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Post by Donkey on Feb 20, 2014 19:27:29 GMT -8
Fluid Transmission Surface Fluid Overall Heat Transmission Coefficient (Btu/ft2 hr oF) (W/m2 K) Water Mild Steel Water 60 - 70 340 - 400 I found this data, I have no idea how to interpret it but someone else might. Got a link to the source material? |
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Cramer
Junior Member
Posts: 129
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Post by Cramer on Feb 21, 2014 3:01:41 GMT -8
Fluid Transmission Surface Fluid Overall Heat Transmission Coefficient (Btu/ft2 hr oF) (W/m2 K) Water Mild Steel Water 60 - 70 340 - 400 I found this data, I have no idea how to interpret it but someone else might. Got a link to the source material? If you mean the shower does not get hot enough then that closed loop idea into a holding tank with a mixing valve at the outlet might be a solution. It is easier to make hot water out of warm water than to make hot water out of cold water right? Just don't blister yourself man!! |
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Post by Donkey on Feb 21, 2014 22:15:33 GMT -8
Oh, no.. Don't get me wrong, the water gets PLENTY hot.
3 loads of manzanita (seriously heavy wood) is enough to pop the safety valve, 3 loads of redwood scrap (very light wood) are enough for 4 hot showers (if they're short).
I thought that the efficiency of my heater was much higher than 20% and was a bit surprised at how low it turns out to be. I don't really know, but I wouldn't be surprised AT ALL if the maximum possible were something like 40%..
Honestly, I've never really spent a lot of thought on the numbers, they've not mattered much to me. I'm far more interested in the ride itself, the bang-for-buck quotient and the numbers don't really describe that very well. The map is NOT the territory.
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