rectangular pressure vessel

Practically speaking, you'll do better to build an insulated box that will hold multiple prefabricated round pressure vessels that add up to your desired volume - or do what most of the commercial installs do - have an un-pressurized insulated hot water tank with a pressurized heat exchange coil or coils.
Rectangular is a lousy shape, and results in a lot of extra work - and finding a weldor qualified to work on pressure vessels is often a bit of a search, and costly when completed. I have such a pressure vessel, which I unfortunately only intercepted when the outer shell (it's a wood fired water boiler) was partly cut off by the folks getting rid of it (since they had set it up with no heat storage, it never quite worked right, so they were getting rid of it, and wanted to lighten the lift out - I would have happily hired riggers to get it whole, but too late).
It has an array of straps tying the flat sides together every few inches (and is a mere 15 lb vessel, as you intend). Would be a lot of welding - I'm just thinking to reuse the firebox in some non-pressurized or non-water (sand, perhaps) setting.

Thanks, I was afraid that, i knew there had to be valid reason for not seeing anything like that commonly used.
Automotive radiators are a rectangular pressure vessel, and I know that on off shore rigs where space is at a premium they are used..
I have a 500 gal propane tank but would have to build something to hold it and then run lines ect. Where does the pressure tank have to be installed in relation to the storage tank? Tkns ED

What is the operating pressure, maybe 100 psi?
If you go rectangular, you could do something like 3/8 or 1/2 tie rods on about 12" centers to greatly reduce the required plate thickness. But a 500 gallon "box" that operates at normal water pressures is going to require the involvement of a good engineer to be safe.
Another possibility might be using something like a bundle of up to 6" pipe. Unless ASME has changed, since the last time I looked (about 30 years ago), vessels less than 6" in diameter, or I think less than 5 psi are exempt.

He said 10-12 PSI.

"Ignoramus5615" wrote in message > He sure did, sorry I missed it.

I have a similar application, for a solar water-heating setup. Used (home made) copper heat-exchanger coils inside a rectangular tank fabricated from mild steel to hold about 80 gallons of hot water-glycol mix at 160F, zero pressure. Figured the corrosion inhibitors in the anti-freeze would prevent problems. After a year or so the flaking was incredible. Rebuilt it with stainless, no more problems.
Wayne

How about two home oil tanks or ten 55-gallon drums?
I would NOT close the tanks if there is the slightest chance of water boiling anywhere in the system.

I suggest that the question is whether this tank is a pressure vessel or not?
I know that you mention 10 - 12 PSI but where does that number come from? It is hardly sufficient to use for a household pressure water system. Are you perhaps thinking of the pressure exerted by the head of water inside the tank?
If the latter then it is not a "pressure vessel", as the term is normally used, and there shouldn't be any problem in building a rectangular tank.
In fact I'm sitting in a shipyard looking at several fuel tanks, temporally removed from a trawler, of the size you mention, welded up from mild steel plate, that have probably been in use for 5 - 6 years with no problems.
Bruce-in-Bangkok (correct Address is bpaige125atgmaildotcom)

It is for a hydronic hot water heating system, average operating pressure of 10-12 psi but could go as high as 30 psi in some circumstances.
There are circulation pumps in the system along with heat loads and the general head of two stories.
Thats what I was thinking of when I posted this question How do you think they would handle the forces of 15 psi?

Sorry, never heard of a "hydronic" heating system. Is that a thermal syphon system?
I've seen hot water heating systems in New England, called Base Board Heating because the radiators were located close to the floor, and they did not use a pressurized reservoir.
I hesitate to comment as I am not sure how you have the system designed but what is the advantage in pressurizing the reservoir? It would seem simpler to heat the vented reservoir and pump the heated water through the system.
When you mention 15 PSI and then say that pressure might go to 30 PSI you are talking about a 100% increase in pressure.
Certainly you can design a rectangular vessel to hold 30 PSI but it will probably either be made out of thicker gauge then necessary or internally braced to prevent the flat sides from dish panning.
Bruce-in-Bangkok (correct Address is bpaige125atgmaildotcom)

Considering the insurance issues if it fails, this deserves a properly engineered design or a commercial tank of known properties. My homebrew hot water tank is in an insulated box outdoors, and not connected to the water mains.

When burning oil or gas (as is common) they only have a small reservoir, so small you might not notice it - like a 2-1/2 to 15 gallon captive air tank. There's also a "boiler" water feed valve, not that the water is ever supposed to boil, or even get to 212F/100C - but 15 PSI is a typical system pressure, with a relief valve that should go at 25 but given the vagaries of time and age and manufacturing tolerances allowing for at least 30 is good practice. If the house it tall, more pressure may be required in the basement.

Thanks to all who posted, the question has become academic at this point. Making pressure vessel has more down side than up side. I'm instead going to use a non pressurized tank aka a concrete hot tub with a copper pipe heat exchanger array. ED

The older hot water systems work like that, the ones with cast iron radiators in particular.. they have an expansion tank at the highest point. My system has one of their boilers--> And due to the high cost of fuel I'm adding one of these--> The whole thing is somewhat complicated, there's a group of pretty sharp folks over here ---> a lot of inovating..hence my look at a different sort of tank.. which I may do, just not in the house but out in the shop.. Tnks
ED
snippage

"Hydronic" just means that the system uses hot water (not steam or air) to carry the heat to the rooms. Baseboard radiators, in floor radiant tubing and ceiling mounted hot water fed forced air heaters are all "hydronic" heating system. The hot water is typically sourced from a misnamed "boiler" that doesn't actually boil. These systems have a closed loop with an air chamber or diaphragm expansion tank to accommodate the changes in water volume as it heats and cools, and typically operate at about 15 PSI.
The most cost effective setup for the OP is most likely to use one or more of the off the shelf heavily insulated "indirect fired DHW tanks", which are just a big storage tank good for 80+ PSI and containing a heat exchanger coil inside which in normal operation is plumbed as a zone on the hydronic heating system.