Heat transfer to water storage tank

[NEIOWA]

Problem. Fire water storage tank with capacity of 20000gal. Tank is a flexible bladder 22.5'x26.5'x5.5'(a blivet to those in the military) uninsulated. Located in NE Iowa (cold winters). Design intent is year round storage of water for fire protection. Performance of tank in freezing conditions is unknown. Presumable freezes from all sides and top uniformly. Will tanks burst??? Even if does not burst loose capacity and likely outlet ports freeze over. Tank has 2" drain on bottom and 3x 4" inlets/outlets/vent ports on top. Installation will consist of 6ea tanks (total of 120000gal). Tanks will connect manifold assembly, to fire pumps (2x 750gpm) and then to a "fire watermain" buried below frostline leading to a fire hydrant/tanker fill (1000' of main). Minimum main size 6" plastic, more likely 8" or 10".

Project likely requires addition of BTUs from Dec-March. In what volume? How to do cost effectively? Constructing an enclosure with roof is expensive. Side wall only enclosure, less expensive. Dug hole/pit for tank relatively cheap, perhaps roof over. In any of these cases insulate? Gas/Electric etc heat expensive.

Potential solution is to bury a return line parallel to the watermain and install a circulating pump. Return line size? (2" is cost effective), pump size? How many BTUs possible to pickup with this system?

May want cover tanks in some manner to prevent degredation from UV. Is "wind chill" relevant with an inanimate object?

 

[chicopee]

CHECK W/ AREA WASTE TREATMENT PLANTS AND NOTE IF AERATION OF FLOCCULATION TANKS KEEPS SLUDGE (VERY HIGH WATER CONTENT FROM FREEZING. I BELIEVE IF TANK IS UNIFORMLY AERATED AND WATER IN TANK IS CONSTANTLY MOVING, YOU MAY NOT HAVE TO WORRY ABOUT FREEZING, HOWEVER MUCH DEPEND ON PLACEMENT OF TANK IN RELATION TO COLD WINDS.
I LIVED IN MONTANA FOR FOUR YEARS IN A RANCHTHE BACK IN THE EARLY 60'S AND THE COLD WATER TAP CAN FROM A SURFACE SPRING SOME 400' AWAY. TEMPERATURE IN JANUARY WOULD DROP TO -40 DEG F, YET WATER LINE WOULD NOT FREEZE AS LONG AS WATER TAP WAS NOT SHUT OFF.
IF IT IS DEEMED THAT WATER MUST BE HEATED CHECK INTO A DIRECT FIRED STEAM GENERATOR WHEREBY YOU CAN MODULATE THE FIRING RATE BASED ON AMBIENT TEMP AND WIND CONDITIONS.
NOTE A DIRECT FIRED STEAM GENERATOR WILL CAUSE THE WATER TO BECOME SLIGHTLY ACIDIC (CARBONIC ACID) FROM THE CO2 IN THE PRODUCTS OF COMBUSTION.

 

[KeItSiSt]

You asked alot of questions. Wind does have an effect. According to my map from the US weather bureau the lowest one-day mean temperature for that area is around 25F below zero. Not sure if you have a elevated steel tank or a steel suction tank but I will assume a steel suction tank. Based on six tanks at 20,000 gallons and a minimum water temperature of 42F and a wind velocity of 12 mph you will lose about 320,000 btu's per hour per tank. This will need to be put back into the water by an indirect heater like a boiler with a heat exchanger that circulates the water and adds back the btu's based on your requirements. Any reputable engineering firm should be able to help you along with a boiler representative. My charts came from National Fire Protection Association Bulletin NFPA No. 22. HOpe this helps give you an idea. Consult a reputable engineering firm for your specific requirements and wind velocities in your area.