[Closed] Any thermal flow dynamics experts in da house?

No idea, but that's awesome.

I assume the heat exchanger is full of water rather than air?

K, so you're saying 3 pipes in parallel versus 1 pipe 3 times the length. With the same flow rate I reckon you're right in your last thought; the temperature will be the same at the outlet for the same flow rate. BUT I suspect for the same pressure you will get a lower flow rate in the long pipe at a higher temperature.

I am not the expert you asked for, not familiar with fluid flow and heat transfer equations, but I did spend far too long at uni studying engineering.

I'd second goldfish.

As an aside are you going to put a thermostatic valve in there to mix some cold water? Should stop the chance of drunk people scolded ING themselves

Losses in a pipe due to friction are given by 4flv^2 / 2d where:

f = coefficient of friction for the pipe material / condition
l = length of pipe
v = velocity of flowing fluid
d = diameter of pipe

Mass flow rate in a pipe is pAv where:

p (greek rho) is density of fluid
A = area of pipe
v = velocity of fluid

So to get the same mass flow rate through your single 15mm pipe (1.5x more diameter, 2.25x more area than a single 10mm pipe), the fluid will have to flow somewhat faster (3/2.25 = 1.33 x faster - assume the 10mm pipe has an area of 1, therefore 3 pipes have an area of 3).

The overall length of pipe is the same (3x10m vs 1x30m).

Frictional loss in the 1x30m 15mm bore system will therefore be higher as the square of the speed increase is 1.77 and the diameter increase is only 1.5, so you'll have about 1.77 / 1.5 = 18.5% more loss.

So probably you'll get less pressure with the 15mm bore tubing. But you have to factor in that you presumably have a lot more junctions, fittings, elbows, etc. with the 10mm system which will add a LOT of loss, so the larger diameter system could well still come out on top as it's "cleaner".

What they said [ he says feigning comprehension] but remember hotter water at a higher pressure also means more loss[ more hot water used] so you will need more heat anyway.

Essentially if you want it hotter use more fuel

The flow through the system is constant, negating the need for any expansion mechanism or storage facilities.

Hot things expand so either it needs to be pressure tight or have space for expansion of the water.

Wow, thanks for the replies guys. In reply to Footflaps, the heat-exchanger comprises a secondary chamber above the fire box which the heated air/smoke must pass through to get to the chimney. There is also a very large lump of heavy steel in there to act as a heat sink to provide some thermal mass. Around that limp of steel is wrapped 10m of 10mm micro-bore copper pipe with a continuous flow of water through it.
@ TheBrick, it is almost impossible to scald yourself with it as you have to reduce the flow to a dribble before it gets really really how. I am putting a stop on the feed tap so it is not possible to turn it down that low. If it starts to get uncomfortably warm it is a simple matter of increasing the flow (opening a tap on the feed pipe) and it quickly cools down a bit. Too cold and you turn the tap back down. The fire is pretty small and slow to respond and once going seems to happily give 30 minutes of pretty constant heat before needing making up again.
@steomcd, thanks for the maths. Unfortunately I may have not been too clear but I would replace the 3 x 10' of 10mm pipe with a single 30' length of 10mm microbore. I would simply loop 2 of the ends together each side as it is currently set up so the water goes in, loops out, back in, loops out, back in then out to the shower.

I think the easiest thing to do is try it 🙂 won't take 20 mins to make up a new inlet and replumb the microbore as it is all on compression fittings anyway.

You can coil round the chimney to get a secondary feed as well as I have done that before and it works well- worked well enough to be gravity fed.

OP- I'm tempted to go out and buy a hat just so I can doff it to you!

Hot things expand so either it needs to be pressure tight or have space for expansion of the water.

I assume it expands out of the shower head.

I don't think it will make a noticeable difference with either of your two scenarios, if you want the water hotter you need to either keep it in the heat exchanger longer (so need a longer tube for a given flow rate) or improve the heat transfer (increase the temperature differential between water and heat source, or increase the surface area of the tubes).

Pre-heating the inlet on the chimney will help.

You have the same amount of pipe in the heatexchanger in both scenarios - there will probably be very little difference in terms of heat transfer / temperature.

For the highest pressure at the shower head, you want the lowest pressure drop possible across the system.

Pressure drop across a section of pipe is proportional to the fluid velocity through that section [b]squared[/b] (as stevomcd states above).

Flow through many pipes in parallel = lower velocity through each pipe = lower pressure drop through the heat exchanger (compared to a single long higher velocity pipe coiled around). This gives you the lowest pressure drop, and greatest potential flow through the system.

However, more flow through (basically) the same heat exchanger with the same heat source = lower temperature.

Get more pipe in that heat exchanger!

So it is basically much as I thought then. I initially went with 3 parallel pipes over 1 long one as I assumed (rightly it seems) that this would result in a slower flow to give the water a bit more time to heat up. I have toyed with the idea of pre-warming the water on the chimney and may do that for a MK2 version (I have a cunning plan involving a CO2 pub gas bottle). Think I will try this one "as is" and see what the consensus is. No-one is expecting a power shower in the middle of a field to be fair. Anything better than ice cold (the water is piped direct from a mountain stream 🙂 ) has to be a bonus really.

Probably a good mind set to take in the design is 'how much can I cool the heat from the fire with the water'. That's the same as heating the water.

Heat transfer depends on many paramters including fluid velocity, thermal conductivity of all materials, surface roughness, turbulence, surface area and exposure time.

More tubes of thinner wall would be the best solution but that will bring higher pressure loss.

Higher the fluid velocity the higher the heat transfer, eg you blow on something to cool it faster but that would be the cooling effect on the pipe, ie higher flow rate will extract more heat from the boiler but due to low exposure time the actual water temp would be lower.

Using large diameter pipes means there will be a greater temperature gradient across the water but obviously the flow will best be assuned as turbulent.

Using a larger overall cross section by using smaller pipes in parallel will reduce the velocity for the same flow rate so this will help reduce losses but there will be a trade off due to increased akin friction.

Temperature differential (?T) is also another factor in heat transfer but in your case you want to hit a certain temp so you always have to face the same transfer gradient due to ?T.

I think you would be on the right track to work out the circumferential area per volume over a 1m length. Then work out the time the water is in that metre of pipe. Ignore the other stuff for now.

Preheating g is a good idea, extract every bit of heat you can and start with the cooler parts and finish with the hotter parts as if you get it the wrong way round you will reduce the ?T effect.

If you are relying on convective flow eg to using a dual circuit with the first a closed loop the. Make that a large bore system as small bore will kill the flow and you will end up with purely conductive transfer.

Hi
I am a thicko, but I spend many hours a week heating large volumes of liquid.
If I was trying to build a log powered hot water system based around the gas bottle chimenea I would change 2 things.
Your feed water should either be wrapped around the chimney and then round the upper section of the gas cylinder , or feed down the inside of the flue. Inside of the flue would have sooting issues so would need to have the ability to brush it out. Outside wrapping would benefit from insulation.

I would plumb it in 15mm , 22mm would be overkill and the distance between your heat source and the center of the pipe is greater , meaning low flow oor turbulance for dissapation.

I am a little unclear as to the other end , is it direct or tank fed . Tank fed I would go PHE , direct from stove top , thermostatic mixer set at 38'c with cold feed for safety .

Basically you want a large length of decent size tubing being heated up over a sensible timeframe to a good temperature. Tubular HE over fire pit might the best solution but its £ £ £

Good idea except the lack of expansion.

what happens when you turn off the tap? I assume there is water still in the coil, even a small amount?

old solid fuel boilers were most dangerous when they had been drained down, seal the ends and the remaining water boils and makes the water boil and cause an explosion. What happens if someone puts shower hose back with a kink in it?

I would put multiple PRV's in the system , and maybe an air seperator

Had another thought; You mention that the copper is coiled around a lumb of steel for thermal mass, I'd be tempted to remove the steel and use the volume for more tube. Also, I would try to maximise the amount of tube with a direct line of sight to the fire - radiant transfer is much more effective than convective (Radiant is proportional to ?T^4, convective is only proportional to ?T).

I have now studied the picture in more detail.

The shower head is like, 4ft away ? So on start up any amount of water in the coil will be very hot, then after 10 secs drop to ' just about ok for a 3min rinse down temp?'

I think a re think is in order. You need a cylinder ( IMO ) old school insulated copper one as a break/ buffer tank .Drain it down at end of day , then fire up the chimenea and set flow rate , use a ballcock to stop flow when tank is full?. You can make your heater more efficient for sure , but at the unacceptable end of the danger / reward scale.

@ Singletrackworld. I will be rigging the shower so that it is impossible to have the fire lit and the water turned off. There will be a constant throughput of water even when the shower is not being used or if the fire is not lit. The only regulation will be the ability to turn the flow up or down a bit to control temperature. With the fire flat out, you need to reduce the flow to a dribble (hardly a shower) before it becomes too hot to bare. I agree with all your points if it were to have an on/off function, and I even began with the intention of doing it that way. However, the fact that the water comes from a stream means there was no need to save water, thus going with a constant through-put model.

@irelanst cheers for that idea, I will see how this version works out and if it proves to be too cool for comfort I will try your suggestion.

here will be a constant throughput of water even when the shower is not being used or if the fire is not lit.

The advantage is that it is more instant than a storage system and a storage system will probably require you to mix cols water at some point

It will be running straight back into the same stream it came out of about 100 foot higher up the hill! In terms of wood usage, based on trial runs I reckon it will use about 1/2 of a pallet to run for a couple of hours each end of the days over a weekend.

The disadvantage of a storage system is that the first couple of peeps get a warm shower and the following 20 either have to wait for it to warm up again or have a lukewarm/cold shower. Plus you need a header tank, and expansion tank, a hot water tank and an indirect means of heating the water in said tank.