Thread: Heat Exchanger

Hi Christian,
Why plaster? I thought it was a pretty good insulator?
How do you feel about breaking into your heat exchanger with 1/2" pipe and setting up say 6 parallel loops?

This is going to run at tank pressure?

5C rise on 620ml in a minute with 100W. My maths says you'd be better off with a electric element on the tank.

Your pictures arent working for me

Stuart

G'Day Stuart,

I updated the pictures.

I used plaster to increase the mass and to even out the heat between the coils. Others seemed to have used it for their systems. Being "pour-able" its easy to encase the coils. Only problem is if you want to remove the coils later on to make it parallel it makes it pretty hard .

My math comes to the same conclusion as yours unless there's a way to increase the flow rate and keep the temperature differential.

Regards, Christian

Howdy,
Connecting a Wood Stove
Heat exchangers on a wood stove will heat water to boiling point and beyond(steam). Hot water expands when heated. If it were a closed mains pressure system you would need a pressure relief valve and about 20 mins after you started the fire water would start to run out the pressure relief valve and run and run and run.
the link above is just the first one from google. The systems available place a coil inside the mains hot water service that by convection heats the water.
payback period is the only concern, most of these types of systems are sold into areas without a reliable power supply or no power at all.
cheers

Hi Christian,
Thanks for the pictures.
While not fixing your current issue, one thing I've liked the look of*. Have you looked into waste water heat exchangers that are fitted to the outlet of the shower before sewer. They preheat both the cold water going to the shower and the cold water going to the hot water system. The ones I have seen claim some pretty good numbers for heat recovery.

I'll try and dig up some links if you like....... it was a while ago.

Stuart

*I can't fit one without a pit and a pump. Though the claimed savings are high enough to run a pump depending on one thing and other.

This one Stu? WaterFilm or Certified and approved drain water heat recovery (DWHR) | RenewABILITY Energy

Christian, I would have made that with at least 1/2" manifolds and then 1/4" risers. As you say the flow restriction on such a length of small pipe is high.

Cheers,
Ew

Hi Christian,


Congratulations for having a go. I'm really interested in this sort of stuff, although I myself are yet to try anything similar. One observation, your 5 deg C increase with 600ml of water is governed by the heat transfer of the exchanger. So, with your system as it is, if you were able to say double the flow and get 1.2L/min then you would enjoy a temp increase of only 2.5 deg C. I was under the impression that a small flow rate should not necessarily be an issue if the heat transfer is high and the temp change is accordingly high. 100 L at 50 deg C has the same amount of heat as 200L at 25 deg C. Using the specific heat of water, your flow rate of 620ml/min and the temp change of 5 deg C you can quite easily work out the rate of heat transfer in Kw for your system.

Specific heat water is approx. 4.2Kj/Kg/deg C
so a change of 5 deg C of 620ml is 4.2x5x0.62 = 13Kj heat. This happens over 1 minute (60 seconds) for rate of heat, or power transfer is

13000j/60 = 217W

So, based on your observations, your heater exchanger is extracting just over 200W of heating power from your stove. This is the limiting factor of the system, not the flow rate. You may find that you have used too much copper coil. I'm sure a bit of research would reveal the optimum heat transfer of a copper heat exchanger per unit length. you may well find that part of your heat exchanger are "giving up" some of the transferred heat.

You could experiment with shortening your copper tube, using a different heat transfer medium instead of gypsum and perhaps some insulation on the outside part of the exchanger (side not facing the stove)

Just spitballing. It's not all gunna be gold!

Cheers,

Simon

Originally Posted by Ueee

This one Stu?

Thanks Ewan, that be them.

Stuart

Originally Posted by Wrongwayfirst

Howdy,
Connecting a Wood Stove
Heat exchangers on a wood stove will heat water to boiling point and beyond(steam). Hot water expands when heated. If it were a closed mains pressure system you would need a pressure relief valve and about 20 mins after you started the fire water would start to run out the pressure relief valve and run and run and run.
the link above is just the first one from google. The systems available place a coil inside the mains hot water service that by convection heats the water.
payback period is the only concern, most of these types of systems are sold into areas without a reliable power supply or no power at all.
cheers

Hello Wrongway,

I'm familiar with that page. The point of this design is that the water doesn't boil as it's on the outside of the stove. 20min and the stove itself hasn't even warmed up. The flue pipe and wet back type options run the risk of boiling the water. It is a good practise though to have a pressure relief valve in the circuit.

Christian

G'Day Simon, Ewan and Stuart,

Yes some things take some experimentation. You don't know unless you try it out.

That 5degC figure is based on a flow rate of 2.5L/min. I got that with the garden hose connected. If I could heat the hot water system that much over a firing of the wood stove i'd be happy as I wouldn't have to turn on the electric element in the tank (btw before anyone mentions I know there is a regulation minimum tank temperature of 55deg to prevent nasties).

I thought that the length would have been OK. The design from the link I reproduced used 75ft (22.7m) of copper tube whereas I only used 16m. Teaches me to believe everything I read on the internet ). A manifold design is probably the way to go to increase the flow rate.

Waste water heat exchangers look interesting to pre-heat the cold water to the shower but it would be a bit of a nightmare to plumb it up at my place. Seems good if you have a basement and your hot water is always "hot".

Christian

Originally Posted by simonl

One observation, your 5 deg C increase with 600ml of water is governed by the heat transfer of the exchanger. So, with your system as it is, if you were able to say double the flow and get 1.2L/min then you would enjoy a temp increase of only 2.5 deg C.

Well not exactly. If you double the flow the temp rise wont be halved(though in this case it might get close to it). But you could keep the flow the same and use less power at the pump. So still 200W out but only 50W in.(in nice round numbers there is likely more to it, I'm pretty sure there is a velocity squared rule in there somewhere).

But yes, ditching the plaster on the heater side of the exchanger would be the first move I think.

Christian,
You gave 5C rise, but what are the temps? as your input temp goes up things are going to get worse than the bucket numbers.

Stuart

Originally Posted by Stustoys

Well not exactly. If you double the flow the temp rise wont be halved(though in this case it might get close to it).

Hi Stuart,

Is that because the efficiency of heat transfer is expected to change with different flow rates?

Mathematically, for a given system, you can either have 620ml at 5 deg increase or 1240ml at 2.5 deg increase for the same amount of heat input & same conditions?

Simon

Originally Posted by simonl

Is that because the efficiency of heat transfer is expected to change with different flow rates?

ummm that might depend on your definition of efficiency.
But, the greater the temp dif the more flow you get.
To put it another way, there comes a point where doubling the length of pipe will do nothing(well ok maybe "almost" nothing) to the temp output. So depending on one thing and another, cutting the pipe on half and doubling the flow might make no difference(well ok maybe "almost") to the output temp.

There are also the savings in pump power mentioned above.

Stuart

I can't add anything to the maths, but we've had a wet-back stove heat exchanger into a mains pressure tank for 30 years (until we took the stove out a couple of years ago for 'domestic' reasons).
Our hot water service was an off the catalogue model (we had to order it to made brecause even then they were only made to order): it has a 6m of 3/4" copper coil in the top 1/3rd of the height of the tank. That coil is/was connected to the we back bottom to top and top to bottom respectively. The water in this cuicuit is moved by thermosyphon only and heats up the top 1/3er of the tank directly and preheats the bottom 2/3rds by internal thermosyhon. The hot water tank also has connections in the bottom 2/3rds for solar panels (but we never installes any). The stove circuit has its own header tank with float valve and overflow to leave it open for expansion.
The mains pressure tank has a pressure relief valve of course. When we had the stove going all day in winter, the pressure relief valve would occasionally discharge a little when the tank got close to boiling temp and actually boiled a few odd times in those 30 years, but not often. When no-one is home to use up any hot water, the stove wouldn't be going either....
Also, wet backs for stoves are available or can be made in different sizes to ensure that not too much heat is extracted and prevent the temperature in a given tank to get too high. Our tank is 400 litres.
I have no idea if you can still get those kinds of hot water services, but they were commonly used in split level or multi story rural houses to enable hot water outlets higher than the hot water service - as in our case.
Might provide some food for thought.

When I worked at Beasley hot water we made all sorts of HW systems like the one that Joe described. Now they have been bought by Rinnai they may have been axed by the company. I remember one unit was a instantaneous gas heater screwed onto the side of a tank that could be connected to solar. Provided that there is a relief valve somewhere and a way of circulating the water the rest is just pipework.

Stuart, what you are describing with respect to heat exchangers is correct with parallel flow units but not necessarily true for the type of exchanger that Christian tried. Heat exchange relies on temperature difference (greater difference = greater transfer) If the side of the stove is assumed to be a constant temperature plate, the longer the tube the more opportunity for heat to transfer. When the water temperature approaches the plate temperature the transfer will taper off but if there was only a 5 degree gain I would suggest that point is a way off. (The relationship is actually logarithmic)

Christian, well done for giving it a go. Advice after the event doesn't help much but like most others I think the tube size is too small. On the Solar panels Beasley made the risers were 3/8" minimum (may have been 1/2") and the headers bigger again. Plaster is a pretty woeful conductor of heat. You may have been better off strapping blocks of Al to the pipes or even (as we did for the solar panels) soft soldering a sheet of copper onto the bent tube (incidentally I'm told although I've never tried it that using cold tea will slow down the setting of plaster*).

Michael

*I also briefly worked in a plasterboard factory. The setting speed of the plaster depends a lot on the way it has been ground. We had stuff ground in a ball mill that would wet set in less than 30 seconds. One of the uses was sealing leaks on the mixing equipment.