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Working With It - Designing Open Loop Systems

Reinjection & Disposal

Water Quality Operation In a groundwater-source heat pump system, the water flows through the system once. A recharge well, or a surface lake, pond or stream, may be used for water disposal.

Since large quantities of water are used, it is not usually economical to treat the return water. If water treatment is required, it would be most cost-effective to use a ground-coupled closed loop approach.

Before using this system, a water sample should be analyzed by a reputable water quality laboratory, and the results interpreted for pH, high undissolved solids, iron, calcium and other minerals. The lab can also calculate the Langelier Saturation Index, which is a measure of corrosiveness.

The quality of the source water, at installation and in the future, impacts the performance and life of the heat pump. Keep in mind that a well, lake or pond the provides a good source of water one year may become a poor source the next year.

The principal concerns with a groundwater-source system are corrosion, scaling, encrustation and erosion.

Corrosion

Galvanic corrosion occurs when dissimilar metals are used together. For example, do not use iron or galvanized pipe together with the copper pipe used in the heat pump. Acceptable materials are copper, PVC, polyethylene, polybutylene and rubber.

Scaling

Scaling is the process where minerals precipitate out of the water and build up or scale on the inside surfaces of the pipes and water-to-refrigerant heat exchanger in the heat pump.

Scaling reduces heat transfer and increases pumping costs. The minerals that combine to form scale are normally present to some degree in well water. If the water temperature rises suddenly and/or the water pressure drops suddenly, the suspended minerals will be released and form a carbonate scale.

The first step to prevent scaling is to keep all water lines under pressure. The second step is to limit the water temperature rise from 20 degrees in the cooling mode. Most heat pumps don't raise the water more than 10 to 12 degrees when cooling. If a larger rise is noted, adjust the water flow rate. Scaling typically does not occur during the heating mode.

Encrustation

Occuring primarily in return wells, this is a build-up of a slimy orange-brown deposit caused by iron bacteria. This can clog the system as easily as scales.

Keeping the water lines pressurized and free of contact with air inhibits the growth of this bacteria. If it is found, it can be removed with periodic cleaning using a chorine bleach solution.

Erosion

It is also important to ensure that the water is free of sand or other particulates, since these could prematurely erode the heat pump. Proper screening or filtering can reduce the number of particles that pass through the heat pump, and thus minimize erosion.

Use a well screen with a fine enough mesh to filter out known sand-sized particles found will drilling. A filter should also be placed between the pressure tank and the heat pump intake to trap any possible particles. A replaceable cartridge, with the cartridge replaced by a screen, is suggested.

Some manufacturers offer a cupro-nickel heat exchanger as an option. Cupro-nickel has a higher resistance to abrasion than copper.

 
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