Backyard Aquaponics
A place to discuss aquaponics

With a counter flow heat exchanger the temperatures are nearly the same all the way along the pipe. I'd put the heat pump booster at one end and put the cold end outside for simplicity (turn it into a higher efficiency heater).

Try the heat exchanger by itself first - the difference in heat may not be worth the effort of adding the heat pump.

In my mind you have two buildings, separated by a distance, one hot, one cold. Bury the heat exchanger tube in a channel between the two. An experiment I saw years ago for extracting heat from shower grey water to reheat incoming cold water got 2kW of heat using 6m of concentric 50mm PVC and half inch copper. I'd be guessing that you'd be able to out perform that using this thin wall low pressure (100kPa, not 10kPa - I checked yesterday) irrigation pipe. The majority of the resistance is in the boundary layer, as I said earlier.

Remember to insulate the pipe outside.

NJH, do you have a link to the thin wall stuff?

Nup, but you can buy it at B

We don't have a 'B', Nathan. We have HD or L, for example. As they say...a picture is worth a thousand words!

JP, i'd hazzard a guess that it is LDPE domestic irrigation tube,

I googled a picture of the thin wall irrigation tube, but it appears to have drip fittings built into it. I will try out the rubber tubing I have already.

Finallyh got around to an attempt to construct a spiral heat exchanger (like this: http://www.midcoast.com/~bo/HeatXchanger.html ) to do this temp differential thing.

The center is made of PVC pipe, and the spirals are formed by PVC flashing. Warm indoor water would be pumped into one of the center holes, and cold outdoor water would be pumped into one of the outer perimeter slots. The water would traverse around the spirals and leak out, the newly-heated outdoor water would go indoors and the newly-cooled indoor water would go outside.

I started by cutting one of my large PVC drain pipes down the middle with a hack saw and glueing it back together to form an "s". This didn't work as the glued seam was way too weak to support any force while the coil was wound up. Instead of making an S I glued two pipes together, and put slots in them to let the water get out. I slid one of the pipes along the miter saw to gouge a kind of groove in the side of the pipe so there was more contact where the pipes touch. This worked OK.

I bought some PVC weatherstripping tape that is used to form a seal between a truck bed and a truck cap. It had adhesive on one side. I ran two strips of tape along each spiral of flashing to keep the spirals a reliable distance apart.

Then I glued the end of the flashing to each of the center pipes and wound the spirals up tightly, and ended up wrapping the whole thing up with some duct tape.

...

...

I might not try hooking this up to water - the main thing I got from this exercise was in working out construction issues. There are several things I will be trying on the next go-round.

1. The spacing needs to be farther apart to allow for biofilm.
2. The flashing needs to be taller to allow for slowing of the flow as it gets dirty.
3. I do not trust the PVC foam material - it has a nasty smell when you open the package, and God knows what is in the adhesive.
4. The outer slots need to have a pipe-friendly shape rather than a narrow slot shape.
5. The center axle is acceptable this way.

I will be looking for rectangular rubber weather stripping, or maybe use rubber or silicone tubing for the spacers. I also thought about maybe trying something like a thick felt strip. It would be much better IMO to use bolts or something like that than to use the PVC glue to hold the flashing onto the center tubes. It would make it possible to take the thing apart and put it back together again, perhaps for cleaning or to change the spiral length; it would be nice to minimize the use of the PVC glue as well. I like that the thing can be held together by tape or some straps.

The plan is to get this worked out so that it can be inserted into (and removed from) a bucket, kind of like a removable filter thingo.

Hi Dave,
Nice project, 10 out of 10 for the neat job so far
Commercial spiral exchangers use a tubular manifold with lots of smaller tubes connected to it for one path and wind them into a spiral. The tubes form the walls of the other path. Your design is easier to make but still no walk in the park

Increasing the gap is good as it will be less restrictive to the flow, if you can find some flashing with an embossed pattern it will create more turbulance at the boundaries for better efficiency.. a MKII version at a later date perhaps.
Closed cell neoprene is another option for sealing.

You could use a spiral wire or small flat section sandwiched between the plastic side plates. Heating the wire/bar just enough to create the mirrored spiral housings for the flashing to fit into?

Maybe useful when you get into the mass production stage

Hey Hex, the spiral exchangers I've seen so far are made of welded curved plat5es - do you have a picture of one that uses tubes?

Are you talking about something like this:
http://www.usui.co.jp/eng/products/15_spiral_2.html

The third one in the picture looks like what you are talking about maybe?
http://www.southwestthermal.com/product ... dplate.htm

Hi Dave,
Searched high and low and as yet haven`t found a pic..i`m sure i had one somewhere.
They`re similar to the curved plate jobs except they use tubes in place of the plate.
Imagine a length of swimming pool collector rolled into spiral ..this takes the place of your solid spiral partition wall.
Put it a circular enclosure and fire one fluid thru the tubing and the other fluid down the spiral pathway created by the "wall" of tubing.
The tube wall is exposed to two flows (one on each side) giving reasonable efficiency and a large surface area in a compact package.

I`ll try to hunt down a pic


Here ya go, page 2 in this pdf:
http://www.sentry-equip.com/PDF%20files/Spiral.pdf

Thanks Hex!

Found this link:
http://www.graham-mfg.com/downloads/14.pdf

I'm thinking it would be a lot easier to just wind pairs of rubber tubing around a spool of some sort, and make sure the pumps have enough head or a valve to turn them up enough to blow out the biofilm. Trade the inefficiency of the tubes only touching on two sides for convenience. Maybe they could be pushed together to form a more rectangular cross section. I might start looking into getting long lengths of 1/2" or 3/4" tubing and a couple strong little pumps.