Heat pipes

[Solar Mike]

I'm playing around designing a greywater heat recovery system and am thinking of using heat pipes as the double wall heat exchange mechanism.

I have built a test heat pipe using an evacuated copper tube with acetone as the phase change media. It works very well, but not at a low enough temperature to be efficient. Acetone normally boils at 55c, when the heat pipe is evacuated the liquid to vapor phase change starts around 16c but only at a low rate; at 40c or above you can hear the stuff boiling in the tube and it is very efficient at transferring heat.

I require an efficient rate of heat transfer around 10 to 30c as it is in this range that the waste water say from a shower occurs.


Next is to try a refrigerant gas like R410a or similar or even propane, Has anyone done this or experimented along these lines?


Cheers
Mike (NZ)

[Tesla]

Hi Mike
Welcome to the forum. I played with this technology a little but havn't got around to building one. Just some pointers here for where to find info. I should imagine you might be using a GFX gravity feed exchange type for the shower water - Perry's chemical encyclopedia excellent referrance. Kirkothemers book I hear has info on water mix pH charts but could not get a hold of one, usually found at the likes of CSIRO's libraries. Those black solar heat pipes use evacuated water, in Brisbane they teach this with renewable energy courses somewhere and I also found more info on a chinese website which manufacture them. Another book is Callum Coats book Viktor Schaubergers Living Energies. One other important area to look at is the geometry of your heat pipe. From measurement I found water from the rose is close to 42C entering drain at 36C and in NZ supply water around 11C. I considered using this heat as a preheat then into a scroll on R22 to heat to about 45 with mixer to achieve temp. This was an instantanious water heater to hopefully draw 10A or less. The Renew magazine has limmited data. Best of luck, and please keep us informed on your progress of this interesting subject.

[Solar Mike]

Heat pipe update:

Plan was to try several refrigerant gasses in the heat pipes; first problem was how to accurately measure 100cc of liquid gas into each pipe. This is just enough to fill the evaporator region of the pipe that is in contact with the grey-water. Solution was to make a small charge tank from 1" copper tube fitted with suitable fittings and a couple of ¼" ball valves. Fill the evacuated charge tank from the pot of gas; then empty this into the pipe.

First I tried butane, as its readily available and ozone friendly, didn’t work that well, at the low temperatures of approx 30C of the waste water there just aren’t enough vapor molecules to convey much heat.

Next tried R22 (borrowed from a friendly refrigeration engineer whom lives down the street), what a difference a much lower boiling point gas makes, the ½" dia pipes can move approx 500 watts of heat from the grey water to the shower cold water supply. However I require 4KW or so of heat transfer to be of any real use, so the next step is to try a bigger cross section pipe. This will be continued...

Comments:
Re GFX, looked at these but they take up too much vertical space under the house, I wanted to build something compact and easily retro-fitted.

Instantaneous heating with a heat pump scroll compressor moving heat from the waste water to the cold seems a good idea albeit bit more complex and expensive.

Cheers
Mike

[Tesla]

Hi Mike
For the accurate measurement of your refrigerant there are two options. 1, you can buy a charging cylender (mainly used for domestic purposes). 2, to use a copper tube with the capacity of 20% more than you require (safety) then look up a chart of that refrigerant and weigh in that amount as the volume will change with temperature.
Just a thought to possibly help improve your heat xfer rate - use an ultrasonic atomiser in the bottom of your heat pipe. They can be found at garden shops for making mist, don't cost much and use very little energy to boil water.

[Tesla]

Almost forgot about that GFX. The one available on the market is - well not of the best design and too long. To fix this problem I calculated to use three smaller tubes which spread out in an egg shape pipe which thins the waste water and increases the heat xfer. This method also halves the total length of heat exchanger. Don't forget to get the water spinning in the right (clockwise) direction for southern hemisphere. and don't use the internal tube as it reduces heat xfer surface.

[Peter_1]

How do you transfer the heat to the 'cold' water?
Disadvantage in my opinion: if your grey water has 40°C and your incoming is 12°C (like in our country), you only can preheat the cold to +/- 26°C as both will equalize to the same temperature. So you loose 14°C.
If you use a heatpump, you can recover a lot more.
I experimented once with a copper coil in a grey water storage tank and retracted the energy during the night at low electricity rates.

[Solar Mike]

Heat from the condenser end of the heat pipe is transferred to the cold water running to the shower mixer by direct conduction in a counter flow arrangement. Typically the shower waste water running down the plug hole is near 30c, the cold water gains approx 14c rising to approx 26c.

Not perfect but is very cost efficient recovery mechanism compared to a powered compressor arrangement. Our regs stipulate that any grey water recovery devices must be double walled. Currently I have a 6m coil of 1/2" copper spiral tube, passing the cold water, running inside the drain pipe to recover heat - not strictly legal. so I cannot build any for friends etc. The heat pipe presents a double wall so is allowed.

Ideally one would collect the grey water in an insulated tank and extract all usable heat via a heat pump back to the cylinder, will have to talk to the engineer, see if he has any old heat pumps that he is ripping out that I can scavenge some bits off to give this a go at home.

[tmm]

Mike, the major problem with grey water or for that matter sewage is corrosion. I enhanced a design for the aquatic sector for small chiller applications. The media flows around the immersed helical heat exchanger with the refrigerant circulating within. The superheated gas was cooled via a closed loop water circuit, with the internal conditions maintained by careful chemical dosing, the 'warm water' being circulated through a plate heat exchanger for 'pre-heating' the hot water requirements although with the lower temperatures which this produces it would be better to fine tune the system to underfloor heating applications.
If you look at Ground Source Heat Pumps, where the technology is very similar, you might find some insperation to progress your ideas.

tmm