non-stainless steel batch water heater?

[dinges]

G'day,

The other day I've found an old gas-tank (LPG, not gasoline) of 100l (26 gallon). My immediate thought was 'batch heater!'. Actually, I've held the saw to start working on it. (no worries, it'll be flushed out twice with water to make sure no more gas is inside; I don't want this to be my last project)

However, I'm wondering: the tank is plain steel (it's quite rusted on the outside, but only superficial). Will rust (on the inside) be an issue? I initially assumed it wouldn't (since no air will be inside), but on the opposite side: there's water, it's hot (up to 80 deg.C?) and there's dissolved oxygen in the water...

I also have a 25 l tank (8gallon) from an old air compressor, but I think it's too small and it's plain steel too...

If I had other/better containers I'd use those, but alas.

So, has anyone else made batch-heaters out of these kind of tanks? Did you experience excessive rusting?

Other question: did you use special black paint (to withstand high temperatures), or plain paint; does this hold up well?

Looking forward to your opinions,

Peter,

The Netherlands.

[nothing to lose]

I use all kinds of tanks for various things.

 First be warned the tank could still be a bit dangerous. I don't really know if liquid propane is as bad as gasolene or not for getting into the pores of the metal and the seams. Take a gasolene tank, flush it out many times, start cutting with a torch and get a big BOOM from the fumes that never quit comming out. Seen people fill a gasolene tank full of water also and still go boom, probably a little air pocket in there that still has fumes. Of course you may not be planning to use a torch on it, but saws and drills can spark and set things off too. If your cutting it, are you welding it later??

 So I would double check the saftey of how you plan to work with it just in case.

As for using those tanks, they should work for awhile. Yes, I am sure they will rust out, but if in decent shape it should take years perhaps. Air compressor tanks rust out holes from the inside just being used as air tanks, from the moisture in the air turning to water, then people don't drain them half the time so they rust out.

I've cut open old well bladder tanks, yes the tops were even rusty inside. Most recent one when I cleaned it up inside after removing the bladder, I was getting a rusty mud as I scrubbed it a bit. This one will be a heat storage unit. I cut it open cleaned it, removed the bladder, put in a copper tubbing coil, welded it back together, sealed around the tubbing with JB Weld. Have not installed it in the Van yet. Purpose of this one is to hold 20gal hot water, the tubing connects to the heater hoses for the rear heater in the van already. So when I drive the hot coolent will flow through the copper tube inside the tank and heat the water in the tank. When I stop to take a nap in the back of the van I got 20gal hot water radiating heat to keep the rear of the van warm longer. It will already be warm because of the rear heater running while driving, tank is just to keep it warm after heater is turned off.

 I did not want to put 20Gal coolant into the tank though, that would have been easier I guess. Anyway that tank was made for water wells, and it was rusty inside.

I also made one nearly the same to use as part of a still when I get my alcohol fuel permit, it also was rusty and muddish like when I cleaned it out. The mud is the surface rust being washed and scrubbed loose.

Even galvanized tanks I get alot of are rusted from the inside out. Many are also from water wells or storage. I find a little pinhole leak in one near a small brownish area, poke around it a little with a screwdriver or such and I find a bigger hole comming out from inside. The other day I wanted a fitting from a rusty tank, metal looked fine on the tank where the female fitting was welded on, but when I tried to turn the fitting I wanted out  the side of the tank ripped out, rusted inside. I knew that one was bad, but lots of good fittings on it, so I brought it home anyway.

 If you have a good tank to begin with it should last for years, but eventually it will rust out when used for water. For anything like heating water then I normally would use an old hot water tank, if you can. They have a liner in them, some kind of coating, not sure exactly what it is, I knew once, but they don't rust out nearly as fast as any other steel type tanks. Most I have found if they rusted, it was from the outside which is not coated the same way. I get hot water tanks free or up to $3 often from scrap yards or people just throwing them away, wanting them hauled off anyway. Most the time I get about $5-$10 of used copper or brass fittings on them also, then I don't have to buy those later. If you can, I recomend you look for those.

 Normally the only thing wrong with such tanks I get are they are full of sediment, if your cutting them open anyway it's very easy to get out, if not cutting them then a little time flushing them, poking around with a stick, a bit of shaking, and the sediment eventaully washes out. I used alot of these and clean them that way. Before we got a well, I carried 4 30gal or larger water tanks in the back of a truck and filled up at various places to bring home. All scrap yard or throw away tanks, no leaks, just needed cleaned out.

What are you using the water for, if just heating the house not important, but if it is hot water for people use, beware if you paint the inside of the tanks what you use, and I would not like rusty tanks either myself for bathing or drinking.

 Also along the idea hot water tanks don't rust out very fast if at all. About 7 years ago or longer I had some animals I needed feeders and waterers for. I cut several hot water tanks in half, they are still in the yard and woods, no rust sitting outdoors cut in half many years now.

[John II]

This might be helpful: I take a small gas motor and connect a flex pipe from it's exhaust pipe to the tank that I'll be working on...... then start it and run it long enough that I'm more than certain that the tank is filled with carbon monoxide. Then idle the engine down leaving it running and do your drilling sawing or cutting on the tank.

John II

[dinges]

Thanks for the reply.

As to your warning on gastanks: yes, I will need to be very careful. I don't want to read in the newspaper that something happened to me... No cutting will be necessary, and I don't intend to weld, but a good brushing to get the rust of should be enough if there is any gas left in the tank. I intend to fill it several times with water, shake it about, drain it, fill it with water, etc. Finally, put in a hose from the compressor and dry it/flush it out for 1/2 hour. That should do the trick. If not, and you suddenly notice I no longer visit the board, you can guess what happened ;-)

YOU CANNOT BE TOO CAREFUL WITH SUCH TANKS. I'd never recommend to anyone else doing this kind of work on it, but that's only because I don't other people much. (don't get me started on that one).

The tank would be mainly used for hot water for the washing machine and perhaps also showers, so rust shouldn't be there; I don't like all my white clothes ending up brown :-) , let alone myself.

My guess though is that any rust will float to the bottom, while hot water is being drained from the top.

BTW, I do have a copper water container, but it's basically a big bucket, without a top on it; i.e., not a pressure vessel. I might try to close it by soldering but even if I would, the dimensions are not as nice as the other (gas) tank (LPG, not gasoline). Also, I'm still doubting whether I should first construct a small one with the 25l compressor tank; then again, it would take some material and a lot of time, with only limited results. (After building my first small genny 'to get a feel for it', I will no longer spend lots of effort on small trinkets just to see whether it works or not, when with just a little more effort & material you can get 'the real thing'. I had been warned about this by e.g. Dan but am stubborn.

Think I'll use the gastank for starters. If in the mean time I find a better vessel (stainless steel or copper) in the coming years, it'l be just a matter of removing the old steel vessel and inserting the better one, without too much hassle.

BTW, I intend to use PVC hardware on the tank, because of possible problems with electrolysis (galvanic corrosion, i.e. steel vs. copper). Or should just covering all the contact points (threading etc) with teflon tape be enough?

Funny, a hot water batch heater seems very simple, but once you start thinking about it, there are lots of little things that can go wrong. Like pressure: when the water heats up, it expands. Normally, I don't see expansion tanks on batch heaters; that's because they're connected to a 'normal' water heater (backup heater). But if you connect it straight to the warm-water line, I think you will need an expansion tank.

Or am I wrong on this one? I did a quick calculation on it (pV=nRT), until I realized that one is only valid for gas... Water being almost incompressible.

Again, thanks for the input,

Peter,

The Netherlands.

[nothing to lose]

I had a shop instructor once say the same thing basically. He was more saying warm up the tank hot that way with exhaust and evaporate the fumes out and as long as the tank stayed hot you could weld on it or whatever. However the guy was a bozo and I never took his word for it. He could not even braze a cracked  cast iron engine block. Thought he had done a great job each time till it started to cool then he looked shocked when the crack crawled out from under the brass

 He did not drill the ends of the crack nor preheat the block. No WAY I was going to take his word for welding on a gas tank !!

Glad to have your second opion verifing the exhaust fumes for a gas tank

 I think your more correct with the carbon monoxide than he was about the heat. Lack of oxygen when filled with carbon monoxide would be more likely to prevent igniting any fumes inside than heating the tank would.

[nothing to lose]

Well as for rust, my geuss is that if you have a clean tank to begin with you will be running enough water through it that rust will not turn your clothes brown or you

Just eventually  the tank will probably rust out like most tanks do. Kinda thought about that some after the first post. Not sure if the rust will settle to the bottom or flush out with the water realy, but in constant use there should not be that much loose rust floating around to mater much. They don't rust that fast normally. Still not sure I would drink the water.

I would probably skip the copper bucket and save it for something better latter. Wish I had one.

I don't see much problems with many hot water tanks I drag in that used brass or copper fittings on the tanks, even Galvanized pipes seem to be ok. At least at point of contact, threaded fittings. Tape is or putty is recomended to prevent leaks and perhaps helps with this also?

I really don't know about the expansion tank, pretty sure it is not needed though.

 How hot do you plan to heat this water? A normal hot water tank is full of water and connected direct to the water lines, no expansion tank used. I would use a presure relief valve of course. If water expands too much, presure builds, the valve pops open and dumps a bit of water, valve closes. That's all a normal hot water tank has and of course when everything is working correct the valves never open to dump water. It's if somethng goes wrong that they are needed, like a thermostat sticks and creates an electric boiler, then you got pressure you need to dump. But that rarely happens.

 Now if you were making a boiler???

Well I better go work on something, got awhile before I need to be at the computer again (hopefully). Sometimes I got to be around the computer or phone waiting or working, so I get nothing done here for myself. Now I think I have a chance to do something for awhile

[Old F]

I am with Nothing to lose

For get the propane  tank and look for old hot water heaters there free for the hauling and have all the plumbing ports all ready  installed .

Here is a pic of what I did to a old gas fired one I use as a heat exchanger for my out door wood stove .

I caped the flue and  circulate  the hot water from the stove thru it and it heats the water  in the tank for the house.  Going on four years now and no rust problems.

Old F

[ghurd]

Is the entire center flue filled with hot water,

or is the hot water in plumbing?

G-

[nothing to lose]

I like that idea too.

 I think I may try that one myself.

Ghurd,

 The entire center tube/pipe is the heat exchanger. The part the gas fumes normally rise up through to be vented, just cap both ends connect pipes and fill with hot water.

I think this also has the advantage of slower flow through the tank center compared to a normal heat exchanger connected to the side correct? Say if using 3/4" pipe to connect to it, you go into a much larger area in the center maybe 2-3" I geuss. So the hot water in the larger exchanger in the center has more time to heat the water in the tank surounding it before being pushed out the other end.

Compared to a normal store bought exchanger which is a pipe inside a pipe and hot water volume is little and flys by faster is what I meant, I think I like this one better.

[dinges]

Thanks for all the feedback so far, it's informative!

I will use the LPG-tank, but first I will use the 25l-tank from the air compressor. This will be a small trial unit to test construction and whether it works good enough. It should also provide some hot water. Did some calculations, and when I use 1000W/m^2 solar radiation (a bit optimistic, 800W/m^2 may be more realistic in the summer?), the water should heat up to 19deg.C/hour. Note: this is without energy loss due to convection and radiation (black body). I calculated max. temperature when including only black body radiation; it was 99deg.C, just short of boiling. Of course, real temperature would be lower due to convection & conduction, but still. What I found most impressive was the 19deg.C temp rise per hour. Even with half that value I'd be very happy!

Here are some pix:

Despite its bad look, the rust is only superficial; easy to wipe of.

The original compressor; found at a dumpster by the road, couldn't leave it there :-) The air-hose had been cut off, just as the 220-wire, but it was still working apart from that. However, we already have plenty of good compressors, while I needed the tank. Plus it had some nice wheels for another project (welding cart; the first thing I'll weld when I learn it). The motor will be saved too; perhaps a motor conversion one day? Though it's less than ideal, because the compressor is integrated in the casting.

These old air-compressors are a little goldmine for the renewable energy guys; tank for a batch water heater; motor for a conversion. 2 wheels that will always find a use....

Tsk, the things people throw away ;-)

Peter,

The Netherlands.

[Old F]

Peter

Seeing that you have  a number of  tanks   all ready here is some thing you mite find interesting

http://www.motherearthnews.com/library/1978_March_April/Mother_s_Hydrostatic_Pressure_Tank_Tester

Have fun play safe

Old F

[nothing to lose]

The compressor and motor case casting are all one piece, or just the end cap for the motor on that one side.

If just the endcap and compresser case, you could maybe make an end cap fairly easy for the motor itself. I did that for one I got from a scrap yard. I have a lathe, but with aluminum you could probably do the same with a good drill and some thought.

Looks like nice tanks, LPG looks rough but since it's just surface still nice.

[dinges]

Only the endcap & compressor-case (at the left side) are cast as one piece; so you can't take off the compressor and end up with a motor, because then also the end-case of the motor is missing. You're right, you could probably fix this (I do have a lathe), but then again, I have a feeling that this motor plus compressor will be used in the future (as one piece); don't know what for, but a little voice in the back of my head says 'save it!'. And you've got to listen to the voices in your head.

Like I said, the LPG tanks looks much worse than it is; a few light brushed with a steel brush and you end up with bare steel. In the mean time, I've sawed of some of the attachments of the compressor tank; filed it nice & flat again. To my amazement, under the paint there's a very thin layer of rust, just about everywhere! The paint still attaches strongly to the tank, it's not that the paint is 'bubbling' off, but still...

Peter,

The Netherlands.

[Bruce S]

Hello Peter;

    If the tanks you're about to use are first open to the air, then filled with plain faucet water then drain it; you will not have a problem with the tank catching fire if you want to weld/cut on it.

An LPG dealer taught me this some years ago. He used to make BBQ pits out of the little 20 pound tanks we have around here for gas grilling. Very similar to the ones I saw strapped on old Jeeps in Amsterdam that have been converted to dual-fuel.

They will rust, if you just leave normal water sitting in the tank.The +200F water is going to cause the rust to form even faster. They don't seem to rust through just that rust forms by water sitting in the tank. I was told they don't have any kind of coating on the inside like steel oil tanks, so the LPG won't react to coatings under high pressure.

I thinking you could pass the water through a sediment filter to make sure only the water get to the clothes.

If however these tanks are the newer aluminum ones then you're all set.

Hope this helps

Bruce S

 

[dinges]

Hi Bruce,

Yes, I intend to put a filter in it to filter bigger rust-clumps out; though I don't think it will be very necessary.

Have just flushed out the 25l tank a couple of times; even though this is just an air-compressor tank, it had quite some rust in it. But after the first 10-20l of flushing, the water came out clear. Besides, the rust-particles were pretty big & heavy, so they'd normally sink to the bottom of the tank, while you extract the hot water at the top.

The tank isn't entirely clean yet.

Nothing to lose: I noticed in another thread that you had trouble cleaning water-heater tanks from scale; poking around with a stick etc.

Have you considered putting some steel balls in them (old ball bearing balls?), putting the container in the back of the car for a couple of days, whilst driving around? Sort of a poor man's tumbler. That should loosen much of the scaling (rust in my case), which should be easy to flush out afterwards. I intend to do this to my 25l compressor tank, maybe the gastank too if it's rusted inside. Seems much easier & less work than poking around with a stick...

Bruce, I didn't know that aluminium LPG tanks existed too. Will have to keep an eye out for them. This would be just perfect for a solar batch heater application.

Finally, anyone of you that built a solar batch heater: what kind of paint did you use; special high-temperature paint or just run-of-the-mill 'plain black' paint?

Peter,

The Netherlands.

[windyknight]

Peter,

I found that here in the uk we can buy aerosol spray cans of high heat black paint (for stoves, wood burners etc) which is matt black so no reflection and obviously high temp. resistant. It's easy to use and 1 can does several sq.m.!

As to your question about rust, we have used an old cast iron kettle on our wood burner for years - it rusts terribly- but tea still tastes ok! Seriously, only time water looks bad is first use of season after the summer after that its fine.

BTW i've just made a batch tank by inserting a home made heat exchanger and you can see lots of ** in the bottom and this tank is copper



ps I'm putting together a diary of it's installation for posting next week hopefully

keith