WATER CORROSION
Two concerns will immediately come to light when considering a water source heat pump, whether for ground water or for a closed loop application: Will there be enough water? And, how will the water quality affect the system?
Water quantity is an important consideration and one which is easily determined. The well driller must perform a pump down test on the well according to methods described by the Nation Well Water Association. This test, if performed correctly, will provide information on the rate of low and on the capacity of the well. It is important to consider the overall capacity of the well when thinking about a water source heat pump because the heat pump may be required to run for extended periods of time.
The second concern, about water quality, is equally important. Generally speaking, if the water is not offensive for drinking purposes, it should pose no problem for the heat pump. The well driller or local water softening company can perform tests which will determine the chemical properties of the well water.
Water quality problems will show up in the heat pump in one of more of the following ways:
1.Decrease in water flow through the unit.
2.Decreased heat transfer of the water coil (entering to leaving water temperature difference is less).
There are four main water quality problems associated with ground water. These are:
1.Biological Growth. This is the growth of microscopic organisms in the water and will show up as a slimy deposit throughout the water system. Shock treatment of the well is usually required and this is best left up to the well driller. The treatment consists of injecting chlorine into the well casing and flushing the system until all growth is removed.
2.Suspended Particles in the Water. Filtering will usually remove most suspended particles (fine sand, small gravel) from the water. The problem with suspended particles in the water is that it will erode metal parts, pumps, heat transfer coils, etc. So long as the filter is cleaned and periodically maintained, suspended particles should pose no serious problem. Consult with your well driller.
3.Corrosion of Metal. Corrosion of metal parts results from either highly corrosive water (acid water, generally not the case with ground water) of galvanic reaction between dissimilar metals in the presence of water. By using plastic plumbing or dielectric unions galvanic reaction is eliminated. The use of corrosion resistant materials (such as the Cupronickel coil) through the water system will reduce corrosion problems significantly.
4.Scale Formation. Of all the water problems, the formation of scale by ground water is by far the most common. Usually this scale is due to the formation of calcium carbonate, but magnesium carbonate or calcium sulfate may also be present. Carbon dioxide
gas (CO2), the carbonate of calcium and magnesium carbonate, is very soluble in water. It will remain dissolved in the water until some outside factor upsets the balance. This outside influence may be a large change in water temperature or pressure. When this happens, enough carbon dioxide gas combines with dissolved calcium or magnesium in the water and falls out of solution until a new balance is reached. The change in temperature that this heat pump produces is usually not high enough to cause the dissolved gas to fall out of solution. Likewise, if pressure drops are kept to a reasonable level, no precipitation of carbon dioxide should occur.
REMEDIES OF WATER PROBLEMS
Water Treatment. Water treatment can usually be economically justified for close loop systems. However, because of the large amounts of water involved with a ground water heat pump, water treatment is generally too expensive.
Acid Cleaning the Water Coil or Heat Pump Recovery Unit. If scaling of the coil is strongly suspected, the coil can be cleaned up with a solution of Phosphoric Acid (food grade acid). Follow the manufacturer’s directions for mixing, use, etc. Refer to the “Cleaning Water Coil”, Figure 24. The acid solution can be introduced into the heat pump coil through the hose bib A. Be sure the isolation valves are closed to prevent contamination of the rest of the system by the coil. The acid should be pumped from a bucket into the hose bib and returned to the bucket through the other hose bib B. Follow the manufacturer’s directions for the product used as to how long the solution is to be circulated, but it is usually circulated for a period of several hours.
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