pool heater heat exchanger materials

By rlitman (***)

Date 03-21-2008 18:42

Can't speak with authority on the aluminum corrosion issue, but my gut tells me this isn't good.
You'll need to guarantee a high enough flow rate to ensure that the exiting water temperature is low enough to not damage your piping (most likely PVC, right).
That will also guarantee that your exchanger metal is below the condensing temperature, and the flue gases will almost certainly eat your tubing from the outside in, in short order, unless you can make this out of something like titanium, or short of that, 316Ti Stainless.

What about doing this with a closed loop, so you don't pump the pool water through your core?
Say something like this:
Take an old boiler, and raise it up so you can light your wood fire beneath it. Fill with clean water, and install an overpressure safety valve, and expansion tank.
You can use cast iron here, and even a cast iron pump (say a small Taco 007), to pump water through a loop.
Loop around 15 feet of PEX in your pool, and pump through this loop. Set up the aquastat to only pump when the core temperature is above 140F or so (to stay out of condensing mode). You just need to be sure to choose a pump large enough to make sure that the temperature can be kept below 180F (for the sake of the PEX.

As for having plenty of wood, here's something to consider:
18000 gallons of water weighs 144000 lbs. It takes 1 BTU to raise 1 pound of water 1 degree F.
There's a bunch of data on how much heat you can get from wood, and I found this on google really quickly:
http://www.daviddarling.info/encyclopedia/W/AE_wood_heat_value_BTU.html
So, using an average for Oak, you should ideally need around 33lbs of wood to heat your pool 1 degree. Kinda reasonable I guess.

By scotty68 Date 03-21-2008 22:36

Thanks for the info-I never thought of an independent closed heat loop in the pool itself.My thoughts are at the present time stainless is about $13.00 per linear foot. Copper I havent priced recently but am certain this is high as well. Not as much as titanium. Thinking aluminum might have been the way to go-but was concerned about corrosion. I might just have to bite the bullet. Most advertised pool heaters are made of stainless-316-cupro-nickel or copper. Probably for a reason. Thanks.

By rlitman (***)

Date 03-22-2008 06:08

Never heard of copper in a pool heater, but its not my thing. Seen many in 316, and cupro-nickel (I'm guessing some sort of monel). Titanium is the preferred choice, and is actually very common.

By 803056

Date 03-22-2008 17:55 Edited 03-22-2008 18:00

I would use copper tubing in the fire box. It has both excellent resistance to corrosion and high thermal conductivity.

I've made copper coils for heating homes with wood fired boilers without any problems. The system uses a safety valve to prevent over pressure or over temperature problems. I use two basic designs, one with a 15 gallon tank heated by convection and one that uses a circulator pump running continuously. I like the tank type system where the circulator only cycles when the water reaches 150 to 180 degrees for heating homes because it uses less electricity. You can purchase a brass or bronze pump so there is no "rust" introduced into the pool water.

I packed the copper tube with dry sand and capped both ends with caps (soldered). Then I bent the copper tubing (1 inch) around a 12 inch pipe (that's what the customer wanted). Once the tube was bent, the caps were removed and the sand poured out. Make sure you use dry sand so it will pour out easily. You can bend the copper "cold".

As for the temperature of the flue gas, I wouldn't be overly concerned. I doubt the boiler will be so efficient that you would cause a serious condensation problem. You might get some creosote build up, but that can be easily removed by opening your damper and burning the fire hot for a short while. I found that the creosote flakes and falls into the boiler if the stack is straight and above the boiler.

I heated my home with a wood fired boiler I built thirty years ago. I had to stop burning wood a couple of years ago because my wife got asthma when she hit the ripe old age of 45. This year I started to burn charcoal and that didn't seem to bother her. It isn't cheaper than oil, but at least the money stays here in the USA. I do hate sending my money overseas to the Middle East or China. It's a balance of trade thing that I have and a general distain for the corporate blood suckers (have you noticed how inexpensive Chinese made Nikes are at $100 + a pair). It drives my wife crazy because I make her buy American when ever possible. She right about it becoming more and more difficult, but I am getting off the subject.

You can also consider using black plastic pipe to build a solar heater. It is amazing how easy a solar collector is to construct and how effective the solar collector can be. Just leave your garden hose in the sun for an hour or so and then turn it on. The water is hot enough to be uncomfortable until the unheated tap water comes through. In your case, you can use convection heating to "pump" the water through your system. Cold water in the bottom of the collector and hot water out the top. It is simple and cost effective. I read somewhere that you can gather about 150 watts of energy from the sun for every square foot of collector surface area. A hollow deck consisting of a top plate of aluminum "diamond plate" with an insulated bottom plate could work very nicely. The deck can be painted black for better efficiency. There is no pressure involved, but you would have to use a circulator pump if the deck is above the water line of the pool. The aluminum alloy should be fine with the amount of chlorine in the pool. Many of the appliances used around the pool are made of aluminum alloy. Use an alloy that doesn't use magnesium as it's primary alloying constituent.

Best regards - Al

By gwg

Date 03-22-2008 20:26

For this type of application, you can use Type 439 ferritic stainless steel tubing. Excellent thermal conductivity, excellent resistance to corrosion in water containing chlorides and it is readily available, and easy to form. I would not use aluminum tubing because depending on your water quality and in the event you don't have water flowing through this heat exchanger, and you continue to fire you will have a problem. Why, because Aluminum looses considerable strength above 400 deg F!

Why don't you go with solar heating?

By 803056

Date 03-23-2008 03:27

Do you know how much pressure is required to heat the aluminum up to 400 degrees?

I agree that I wouldn't use aluminum or copper pipe if there is a chance high temperatures and pressures were involved, but this system will never see more than a 50 degree to 100 degree temperature rise if it is an open system. I can't see where this would be any thing other than an open system if it is used to heat pool water to extend the pool season.

The copper tubing is easy to work with, readily available, and inexpensive compared to stainless steel tubing. The copper is easily joined by soldering. No inert gas or welding machine required.

Best regards - Al

By rlitman (***)

Date 03-23-2008 04:21

I've done the packed sand technique. Worked very well, until the one day that it didn't come out.
That's when I switched to packed salt. Packs the same, but if it doesn't pour right out, it can always be dissolved.
Annealing the copper pipe may make bending easier. If you start out with 10 foot straight lengths of copper pipe, it really could use annealing first, if it came in a roll, then not so helpful.

Condensation has more to do with core temperature than just efficiency. With a very high flow rate, the heat exchanger will never get hot, and you will get condensation while it stays below the dew point. Get over 140F, and that's not so much of a problem.
Running your heat exchanger directly off of the pool pump will keep you below the dew point, but will also keep your temperatures in a safe range, provided that the pump doesn't fail, while the fire is burning, but then there's the condensation problem.
Using a separate (slower) circulator will allow you to get the heat exchanger hotter (say cycling between 140 and 180F), but requires an aquastat, and controls.
Let your outlet water get too hot, and you may damage the pool's piping (which is designed for cool water). If you set it up to mix with the main pump's circuit (or maybe even forego the circulator, and use something like a monoflo tee to divert a portion of the flow through the heat exchanger) . . . you may be able to get something rather ideal here. But some number crunching should be in order, and with the variable output nature of a wood fire, it can be a little complex here.

I don't know much about pool chemicals attacking copper, but I know that the pH needs to be buffered above 7.

BTW, wouldn't a pool solar cover work about as well as a solar collector over the pool?

By scotty68 Date 03-23-2008 15:15

Thanks for the great ideas and education.I have only seen these wood burning pool heaters on-line and it didnt look as though much thought was put into them.Ill explain what i have seen. The pool pump does all the pumping-a simple supply filtered water in and warmed up water out. This heater is on a sight called warm water solutions.They sell for around $2000-$2500. Obviously there are many variables with a wood heater and you also need to attend it because if you loose the pump and the fire is still going its possible to have melt down. My wife starts to freak everytime i get a "good Idea". But its been done before so its possible and for myself would be practical.Aluminum pipe is very affordable-I just didnt know if it would withstand corrosion inside by chlorine-or outside by flue gas.Again the water is pumped so fast through the exchanger it would probably never even get hot to the hand. Some of the things you guys discussed are beyond my knowledge at this point-why would it be a concern about condensation?dew point? If its practical to explain im always listening.Alloy pipe as we all know since at some time we may have worked with it is big bucks. Im going to call around for real prices on 316SS and cupro -nickel .Titanium is way out of the? and is a pain in the a** to weld properly in the garage not to mention i would need to invest in trailing cups and other apperatus for my tig torch. I was looking at doing this in tubing 1-1/2"X.065 wall. Keep the great ideas coming.No such thing as a wasted education.And youve definetly have me looking at things i didnt see before.Gotta go kids are ODing on peepsand crap.

By 803056

Date 03-23-2008 16:23

The solar pool cover is a great idea and less complex than building a solar collector, but you can't swim in the pool if it is covered.

I've never had a problem with condensation in my wood boiler during start up or operation. Any time you burn a hydrocarbon based fuel you will produce water as a by-product, but I'm not that sure I would be overly concerned with that issue. I suppose if it was found to be an overwhelming issue, you could add a catch trough to capture the condensate and drain it to the outside of the boiler. I suspect it would boil off from the heat of the fire and simple exit the stack as steam as long as the flue gas was above 212 at the top of the stack.

The total heat gain is a function of the BTU produced by the heating unit and how much of that heat is transferred to the water over time. The temperature of the water going out minus the temperature of the water going in times the volume will determine the volume of heat added to the pool water. It would be most effective when a thermal pool cover is over the pool surface to minimize evaporation and heat loss to the environment. Insulation on the walls of the pool would help as well.

I like some of rlitman's ideas. The salt trick sounds interesting. I agree that the rigid copper pipe would be difficult to bend without heating it. The coiled tubing is what I've used.

As long as the water is flowing, brazing or soldering would work with the copper tube or pipe. The operating temperature will not be more than 212 if the system is not pressurized and the system is full of water. Besides, the gurgling sound produced by the boiler will keep your wife's attention focused on it instead of the other things you are doing, like rough-housing with the kids in the water. She will be so busy making sure it doesn't explode, she'll pay no attention to you and the kids.

It used to keep my wife awake at night's until she got used to hearing the sounds the boiler makes. Then in a short while she could tell when the boiler needed wood by the absence of noise. On the cold nights she would give me a shake and say, "I don't hear anything, I think the boiler needs wood."

I would say, "You better go check it then." and roll over and go back to sleep. Ahh, the simple life.

Best regards - Al

By DaveBoyer (*****)

Date 03-24-2008 03:34

Rigid copper pipe: You can't bend it much before it fractures, it is pretty work hardened from drawing. This makes it easy to handle & work with for straight runs. I have used the "K" grade bendable copper pipe, it works easily. I think the larger sizes came as streight lengths, Mine was all salvaged.

By 803056

Date 03-24-2008 04:10

What did you build with it?

Al

By DaveBoyer (*****)

Date 03-25-2008 05:35

I used the flexable pipe in hooking up the solar panel tank I replaced last fall. By bending I could get around some obstructions easier than with fittings.
This was at least the second time the pipe was recycled. At least some of it was in the tear out of parts of the old system, and I KNOW my Dad didn't buy it new when He put the system together 25 years ago. I was working from a pile of used bits & pieces, had more when done than when I started, as I pulled out a bunch of now superflous runs.

By rlitman (***)

Date 03-24-2008 13:54

Exactly. But K is quite a bit more expensive than L because of its thicker walls (when you're not buying scrap). That, and while its kinda easy to bend, its still not "dead soft".
I just spend 15 minutes or so, with my air-mapp or air-acetylene torch, and work my way from one end of the 10' stick to the other.
Just one very large, but soft, flame; get one end to a dull-red, and slowly move that heated spot over the length of the pipe. You'll end up with something almost as bendy as a garden hose. Just remember that copper work hardens, so don't bend more than necessary, or you'll have to anneal it again.
The only down side to this technique, is that when you heat the copper to a dull red, it will get covered in a nasty fire scale, that makes soldering almost impossible, but brazing with a copper-phosphorus rod (skip the silver, it's not necessary, and only adds to the cost), works, WITHOUT ANY FLUX, just the same dull-red heat.

BTW, Al, I need to offer my apologies.
I'm still sure you'll get condensation on the pipes (and creosote), but in retrospect, I'm remembering that there's more to the story.
Burning natural gas, creates nitrates (because of the high flame temperature), which dissolved into the condensation, create nitric acid (corrosive).
Burning oil, creates sulphates (because of oil's sulfur content, but flame too cool to create nitrates), which dissolved, create sulphuric acid.
But, we're talking about burning wood, and it's just not nearly as corrosive here. A regular rinse with a soapy baking soda solution, should keep much of the exterior corrosion at bay.

By 803056

Date 03-24-2008 18:35

No apology is necessary.

That's what this forum is all about, an exchange of ideas and information.

You presented some interesting information. Thanks - Al