[APD] RE: Aquatic-Plants Digest, Vol 10, Issue 27

["Douglas Guynn" <d_guynn at sbcglobal.net>]

Cheryl Trine wrote:

This is a question I have had for a while, also, because I get calcium
deposits on my heaters which I have to remove periodically to keep the
heaters working properly.

OOO! OOO! OOO! Ask me! Ask me! (with hand raised and waving in the air).

I believe the answer to your question lies in a property know as "scaling
tendencies". It is a well known fact, especially among people using cooling
towers, that the tendency of materials in solution to deposit on a surface
is dependant on many things, including the total dissolved solids (TDS) of
the water, the concentration of the individual components of the dissolved
materials, and the temperature and pH of the water. High TDS, high
temperatures, and high pH tend to promote solid deposits, known as scale.

In a cooling water system, water is pumped into the top of the cooling
tower. This sides of this structure are open at the top and bottom, allowing
air to be pulled in around the bottom, flow up through the tower, and
exiting the top. The water falls through the cooling tower "fill" which may
consist of hundreds or thousands of horizontal wooden slats, or a structured
series of corrugated plastic or fiberglass sheets. This fill is designed to
spread the water out in thin layers to maximize the contact area and contact
time with the upward flowing air. The resultant evaporation cools the water,
and the heat is carried out with the air. The cooled water is then pumped to
heat exchangers in other parts of the facility. While cooling the material
in the heat exchangers, the water gets warm and is returned to the cooling
tower to be cooled again.

The evaporation also concentrates the solids in the water, which leads to
the same unwanted deposits (known as scale) you have seen on your aquarium
equipment. This scaling occurs first in the heat exchangers, because that is
the warmest place in the system. The same thing happens on your heater. When
you heater is on, the increased temperature of the surface of the tube
causes the water to deposit scale, probably in the form of carbonates.
Unfortunately, they do not "re-dissolve" once the temperature returns to
normal.

The cooling tower operator avoids this scaling using a number of methods.
One is chemical treatment. Anti-scaling chemicals are added, and the pH is
maintained just above neutral in most cases, because while even lower ph
discourages scaling even more, it also dissolves the metals of the
exchangers. A second method is "blow-down", where a small portion of the
circulating water is drained off and replaced with fresh water to keep the
TDS from getting too high. A third method is to limit the temperature in the
heat exchanger from getting high enough to trigger scaling. If the
temperature of the water exiting the heat exchanger exceeds a certain point,
the water flow through the exchanger is increased, which usually lowers the
outlet temperature.

While the first method is not a good fit with your problem, the second and
third methods may prove useful. The second method, TDS limitation, can be
accomplished by regular water changes. The usefulness of this depends on
what your change water is like. Also, "topping off", or replacing the
evaporated water, with RO or distilled water will help.

The third method can also be used. It may be that the heater you are using
is too powerful. If you are using a 300 watt heater, consider going to a 200
or even a 100 watt heater, depending on your tank size. This limits the
temperature on the heater tube, possibly eliminating the scale.

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