Unusual Design Questions

The Tank Unusual Design Questions

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  • #11085
    The_AV8R
    Participant

    I’m a student who has been asked to design a 3000 L (+/- 792 gallon) hot water tank for a friend. I’ll describe the project a bit, and then I’ll ask my questions. If anyone has the time to chip in their $0.02, it would be very much appreciated!

    The main purpose of the tank is to use the lower heat exchange coil of glycol, from solar (a max of 8000W), to heat the water so that the upper coil of water may go out to heating pipes under his floor. In the summer, with the higher energy supply and lower heating demand, he would also like to use water from this tank for personal use such as showers, etc.

    To top it all off, this tank is to be made of fibreglass, so I won’t need a sacrificial anode, among other design reasons.

    My questions are :

    – For a 7′ tall, 4.5′ diameter tank, would I need a short diptube for outgoing hot water? I know it’s a silly question, but I am still unsure of how the hot water actually leaves the tank. I’d love to take one apart to find out, but alas, I can’t.

    – What are the risks of corrosion and sediment buildup in such a tank? What do I need to incorporate to limit this?

    – Where can I find different heat exchanger coil manufacturers?

    – I know it is best to pre-form each hole needed in the 3/8″ fibreglass, but what is the best way to create them once it’s made? Drilling? Puncturing? Would any fibreglass experts out there recommend any special gaskets?

    Thanks for reading, guys. I hope you can help me out. I’ll be happy to clarify anything.

    #11089
    Ej
    Participant

    Your biggest problem will be obtaining a pressure tested approval rating. Do a search on fiberglass pressure vessels.

    #11091
    The_AV8R
    Participant

    Thanks for the reply, EJ.

    The tank won’t be under pressure, and on the rare occasion that the water within it will be used for personal use, it will be pumped into the smaller general use tank.

    I did some quick calculations on the max tensile strength of e-glass (433 MPa, according to a recent book on composite materials I picked up), and the 7′ of depth accounts for only 1.4% of max load for that type of material.

    90% of the time, the tank will be used to heat this guy’s floor – is it bad to leave the same water sitting in that huge tank? He wants it that big so that he can go for a week without sunlight, something that happens often enough in the coastal area where I live.

    #11092
    Ej
    Participant

    Any time you apply heat and water pressure to the tank it will be treated as a pressure vessel and must undergo a certain safety level to in sure the integrity of the tank.

    #11093
    Larry Weingarten
    Participant

    Hello: Suggest you put a separate coil of copper pipe in the top of the tank solely for domestic hot water. Don’t mix with heating water nor with possibly stagnant tank water. I’d use the DHW coil year round by simply having it preheat the water heater. If there is heat to be had, good. This separates pressure from the big tank and uses it simply for BTU storage. Experience tells us that fiberglass does not hold up well, long term to hot water. A polypropylene or cross linked polyethylene (PEX) tank will work well if kept out of the sun. As to your tank holes question, look up “bulkhead fitting” in McMaster Carr or other supply house. Heat exchangers (if single wall is OK) may be made simply from soft copper coiled tube. A coil would prevent any sediment build-up as no fresh water would enter the tank. You might want to look at the info on “Hummingbird Hill” from the site map. This house uses a solar storage tank like what you’re working on.

    Yours, Larry

    #11094
    The_AV8R
    Participant

    Ej wrote:

    Any time you apply heat and water pressure to the tank it will be treated as a pressure vessel and must undergo a certain safety level to in sure the integrity of the tank.

    I’ll look it up.

    eleent wrote:

    Suggest you put a separate coil of copper pipe in the top of the tank solely for domestic hot water. Don’t mix with heating water nor with possibly stagnant tank water. […]

    Larry, I really prefer your approach, save for one thing : space in the tank for coils. I already have the solar loop on the bottom and the floor loop on top. Given the sheer size of the tank, I can see how I could fit in a third coil; but I don’t think it would be practical.

    Also, thanks for the heads up on long-term fibreglass. Can you define “does not hold up?” Does it crack? Leak? Expand significantly? I’ll be using a non-porous resin.

    Thanks again, gents

    #11100
    Larry Weingarten
    Participant

    Hello: I’m unclear why another coil towards the top of the tank for DHW would not fit/work. Mine is a 1000 gallon tank and those three coils fit easily. The 4.5′ diameter would actually work nicely with a standard coil of solf copper. Is it the hatch size?

    Experience with insulated fiberglass tanks had them swelling, splitting and delaminating. Get a good warranty! The rubber gaskets on the bulkhead fittings fail also. Putting penetrations only in the top of the tank gets around that latter problem. Use a bilge pump to drain it if/when needed.

    Yours, Larry

    #11220
    energyexpert
    Participant

    Sorry I’m behind on posts.

    My dad had a solar water heating system installed in 1983. It has a 250 gallon vented storage tank. The tank water is treated to prevent stuff growing in the water.

    The system drains back when the pumps are off so there is no freeze concern.

    The tank water is pumped from the bottom of the tank to the panels. Return water enters the top of the tank. When heat is needed another pump takes suction near the top of the tank and pumps water through a coil in the ductwork if tank temperature is > 90F. Water returns to the bottom of the tank. If < 90F pump does not come on but gas does.

    A separate coil is placed in the water at the top of the tank. Domestic water flows through it on the way to the WH. This is the only loop under constant pressure.

    David

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