Thread: Heat Exchanger

G'Day Michael,

I think that if I get around to making a mark 2 of the heat exchanger i'd go for 3/8" coil and use a copper sheet to increase the heat transfer as you suggested. I know now to play around with the flow before putting it all together ). I also agree with your comment to Stuart about the heat transfer from the side of the stove. With the side of the stove anywhere between 65 and 80degC at the temperature I was wanting to heat at 37 - 42deg there was still plenty of temperature differential for the copper tube to pick up heat.

Christian

Originally Posted by Michael G

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)

But halving the length of tube gives you that greater difference(in fact by cutting the coil in half you would more than double the flow assuming the pump was up to it. Which at .6l a minute it would almost have to be). But yes as we've said the first problem would appear to be the plaster needs to go. But given Christians other numbers, even a 33C rise with the current flow rate will only warm his water tank 10C.
We are assuming of course that the water in the bucket wasn't 70C.
As long as the heat exchange is a fairly air tight fit to the back of the heater and the back was insulated, would radiation and convection do?

Originally Posted by Michael G

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.

Sounds fun, how hot did it get?

Originally Posted by simonl

Hi Stuart. Think we are on the same page here. By efficiency I meant the amount of heat actually extracted by the heat exchanger divided by the total heat available in the system (ie total amount available to the heat exchanger) x 100

In this case where the heat avalable is not fixed(we arent talking about a flame in an insulated box were some sort of counterflow heat exchanger could be used)
If all you are worried about is the heat exchanger efficiency, have as many parrallel loops
as you can and crank the pump pressure up to get minium temp rise. As flow goes up and temp rise comes down your heat exchanger efficiency goes up.......... but you have to run the stinking pump .
As you say, there would be a sweet spot for system efficincy.(or even building the dmn thing)

Great project.
I have 2 observations to make and add to the discussion.

Pump. These are designed for closed system and basically overcome the thermo-siphon so the hot water returns from the solar panels to the tank that is usually lower than panels. They are not very good lift height rated and low flow so maybe in a loop would perform better?

Coil. Being vertically mounted the hot water will want to stay inthe top half of the coil loops? Most solar panels and HX offer a bottom to top flow. As has been suggested larger tube top and bottom with many smaller vertical tubes may siphon better. And better flow with less resistance etc.

I did similar experiments about 30 years ago. Currently have solar on the roof and a month ago the safety valve failed and it rained hot water at 3am ! Fun.

My latest experiment is in monitoring the boost to see how much the solar saves.

James


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the coil should have been a serpentine arrangement for vertical install..pump will over come your circular exchange
Oval tubing in lieu round would give better heat transfer...annealed copper tubing could be run thru a roller skipping a small section where a bend is required
a bending spring or hand made wooden roller would give nice tight bends
heat conducting paste applied to pipe and plate will also give better heat transfer
If a copper plate had been used brown tip silver brazing tube to plate my have been the way to go

Hi Christian,
A few years ago i um "theoretically" made a pair of SHW panels. They are 1800 x 1200, 3/4" manifolds with 1/2" risers every 150mm. Fins are .6mm ally, fully wrapped on the risers and siliconed in place. Theoretically of course. The panels double as a window awning on our NE wall, they are tilted to 60 deg to get the best out of the winter sun, and are double glazed with clear laserlite, insulated with high temp insulation at the back.
They reach well into the 80's regularly and i have seen them at over 100 in summer. They heat a SS tank (ex gas heated tank) which then feeds out boosted tank via temp valve.
Pump is a 240v 3 speed job, controller is home made using an Arduino and DS18B20 digital temp sensors. Theoretically.
If i did it again i would have done some things differently, but overall i am very happy with the system.
The idea came from here: Solar Water Heating Projects and Plans
Here are some hi res rendered images of the panels....

Cheers,
Ew

Ewan, nice job on the solar panels. Its amazing what you can do with 3D rendering and photoshop to bring these "theoretical" projects to life ;-). BTW is that Felder green in the background?
On the todo list is a solar air heater but that will be some time off.



Eskimo I like the idea of flattening the tubing. I saw the oval section tubing on the link to the waste water heat exchanger that Stuart mentioned.

James, I would have thought the pump would have performed better. The lift height was rated to 5m maybe it does perform better when its in circuit but the performance wasn't convincing.

Again thanks for the suggestions. I think i'll get a coil of 3/8" and play around with length vs flow rates. In term of picking up heat I think a design with a squashed coil soldered to a sheet of copper would be the go.

Christian