Ice Out

Active mixing technology offers reliable prevention for water tank freezing without high energy costs for heating.
Ice Out
A typical installation.

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It’s hard to think winter when summer is just starting to bloom. But the fact is, winter in northern climates brings the risk of ice damage to elevated water tanks and standpipes, and thinking about that now allows time to prepare remedies.

There are various preventions for ice, which can do a variety of costly damage to water tanks. They include heating, tank insulation and tank mixing (and let’s not forget being located somewhere south of St. Louis).

PAX Water Technologies offers another solution called active mixing. It uses a low-horsepower electric mixer at the tank bottom to keep warmer water circulating upward and so keep the water at the top from freezing. Company CEO Peter Fiske (pronounced “Fisk”) talked about the technology in an interview with Water System Operator.

WSO: Why is icing in water storage tanks a problem? How does it do damage?

Fiske: Tank icing problems can be spectacularly bad. Steel and big chunks of ice don’t go well together. Falling ice chunks can cause punctures or scrape away coatings. Massive collars of ice can form from the low water level to the high water level, and the weight of it causes abnormal stress.

In addition, when you have an ice layer over the water in the tank, it prevents the heat in the water from warming the headspace. This means the temperature at the top of the tank cycles over many degrees from sunlight to nighttime, and that repeated expansion and contraction can put a lot of stress on the joints.

WSO: Why does ice tend to form toward the top of the tank?

Fiske: First of all, the top of the tank is more exposed. It’s up in the wind, so there is a higher rate of heat transfer out the top of the tank. The other thing is that ice floats — ice is one of the few solids that is lighter than its corresponding liquid. In addition, warm water just a few degrees above the melting point is actually denser than ice and colder water. So this warm water will sit at the bottom of a tank. In order for that water to become buoyant, it needs to be above about 40 degrees F.

So if you’re going to heat the water in the tank to prevent freezing, you can’t just keep it above the freezing point. You need to keep it at about 8 degrees F above the freezing point in order to promote convection that will help prevent ice formation on the water surface. It takes a lot of extra energy to promote what we call free convention — the natural buoyant rise of warm water.

WSO: How much energy does it take to prevent freezing by way of heating?

Fiske: For a 100,000-gallon tank, you need to add 100 kW of energy just to get that water at the tank bottom warm enough so that it starts to rise. That can be a very expensive energy bill in the winter.

WSO: What is the alternative to tank heating for ice prevention?

Fiske: The answer is forced convention, or active mixing — a process of physically moving the water around inside the tank, rather than just relying on buoyancy. Active mixing pushes the warmer water to the top of the tank, even though the water may be slightly heavier. It keeps the entire tank at the same temperature with a lot less energy.

WSO: What kinds of tank mixing methods are available?

Fiske: The first tank mixing technologies were passive mixing systems. One approach puts nozzles at the inlet so that during the fill cycle, water squirts around the tank and creates a mixing action. Then came draft tube mixers, which were originally designed for lakes and ponds but were applied to water tanks. In general, passive mixing systems alone have difficulty keeping tanks ice-free in very cold climates.

The latest technology, which is what we offer, consists of submersible mixers that sit on the tank floor and continuously push that heavier water up to the top. We offer PAX Water Mixer (PWM) 200 and 400 models. Both use one-third horsepower. The PWM 400 can mix tanks as large as 9 million gallons. The units are made of all 316 stainless steel. They are food safe and NSF 61 certified. They’re designed to be durable so that with proper maintenance they will last as long as the tank.

WSO: How much mixing power is required to keep tanks ice-free?

Fiske: That depends on the size and shape of the tank, the temperature of the water going into the tank, the ambient temperature outside, how long cold winter temperatures persist, the water turnover rate, and the tank material’s insulation capability — or R-value. For a 500,000-gallon tank that needs to turn over every two hours, you would want to specify a mixer rated for at least 4,000 gpm.

WSO: Is active mixing alone always enough to prevent ice formation?

Fiske: Mixing is very valuable in keeping tanks ice-free, but it will not melt ice if there isn’t enough heat in the water itself. So if you have very low or no turnover, as in fire tanks, or if you have extremely cold temperatures, or if you have a very tall, slender tank with a high surface area to volume ratio, mixing alone may not be enough.

For that reason, we offer integrated mixer-heating systems that combine a PAX mixer with a resistive heater. We customize the amount of heating to the specific demands of the tank. This prevents ice formation while greatly reducing the energy cost compared to just heating alone.

WSO: Is mixing of value only in cold weather for ice prevention?

Fiske: Ice is the concern in cold weather, but in warm weather there are concerns from the standpoint of water quality. Warm weather can create water-quality problems galore. In a tank that doesn’t have a mixer, you can see warm water temperatures in the top of the tank rise by almost 1 degree F per day, whereas the water at the bottom stays cold.

Because the inlet and outlet are at the bottom of the tank, that slug of warm water may just rise and float within the tank day after day. It will lose its chlorine residual and it may grow biofilms, potentially even bacteria. When you are sampling your system, you sample water from the bottom of the tank, when the water you should be most concerned about is at the top if the tank isn’t well mixed.

An active mixer takes each influx of water and circulates it with its fresh chlorine residual through all parts of the tank, making sure that fresh residual contacts all the wall surfaces, the appurtenances and the cathodic protection lines. It makes sure the tank is uniform in temperature and uniformly chlorinated. When the tank is uniform, you have a lot of advantages, including better, more stable water quality.

WSO: How should a mixing system be installed in an existing tank?

Fiske: We recommend doing the installation at a time when a tank is being taken down for maintenance. You can have it chemically cleaned and washed out and install the mixer at the same time. Then you have the benefit of a fresh, clean tank with coatings as close as possible to their original as-designed condition, along with well-mixed water inside the tank.


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