|
|
|
Water as it passes over the ground, through caves and springs picks up some of the elements from the limestone and other elements of nature which dissolved and remain. These elements collectively are called hardness. In a heavy use industrial steam boiler the water is could be completely replaced as often as once each hour. Obviously at higher turnover, temperatures and pressures the boiler would quickly have scale from this hardness that would reduce and ultimately prevent water circulation and heat transfer which will destroy the boiler. The higher the operating pressure of the boiler the more critical the removal of foreign items from the feedwater becomes. Large utility boilers operating at 3,000 psig + may actually use distilled water for ultimate purity.
The purpose of a water softener is primarily for the removal of hardness from the boiler makeup water. Makeup water is the water supplied from the municipal water system, well water, or other source for the addition of new water to the boiler system necessary to replace the water evaporated. Some filtering of the water may occur in the water softener but that is not the purpose of its design and too much of other pollutants in the water could actually foul the water softener affecting its operation. Hardness is composed primarily of calcium (Ca) and magnesium (Mg) but also to lesser amounts sodium (Na), potassium (P), and several other metals. Hardness is measured in grains with one grain of hardness in the water being 17.1 ppm of these elements. The purpose of using hardness as the unit of measure is that tests to measure in parts per million (ppm) are much more difficult and expensive to use. Hardness varies from area to area. Usually near salt water the hardness is very low as the limestone is virtually non existent and in mountainous areas where limestone is everywhere hardness is usually very high. All softeners soften or remove the hardness from the water. The primary minerals in the water that make "hard" water are Calcium (Ca++) and Magnesium (Mg++). They form a curd with soap and scale in piping, water heaters and whatever the hard water contacts. Hardness is removed from the water by a process known as positive ion exchange. This process could also be known as "ion substitution", for substitution is what occurs. Sodium (Na+) ions, which are "soft" are substituted or exchanged for the Calcium and Magnesium as the water passes through the softener tank. The softening media is commonly called resin or Zeolite. The proper name for it is polystyrene resin. The resin has the ability to attract positive charges to itself. The reason it does so is because in its manufacture it inherits a negative charge. It is a law of nature that opposite charges attract, i.e., a negative will attract a positive and vice versa. A softener tank contains hundreds of thousands of Zeolite beads. Each bead is a negative in nature and can be charged or regenerated with positive ions. In a softener, the Zeolite is charged with positive, "soft" sodium ions. As "hard" water passes through the Zeolite, the Calcium and Magnesium ions are strongly attracted to the beads. As the "hard" ions attach to the Zeolite bead, they displace the "soft" Sodium ions that are already attached to the bead. In effect, the Sodium is "exchanged" for the Calcium and Magnesium in the water supply with the Calcium and Magnesium remaining on the Zeolite beads and the Sodium ions taking their place in the water flowing through the softener tank. The result of this "exchange" process is soft water flowing out of the tank. It can now be readily understood that a softener will continue to produce "soft" water only as long as there are Sodium ions remaining on the Zeolite beads to "exchange" with the Calcium and Magnesium ions in the "hard" water. When the supply of Sodium ions has been depleted, the Zeolite beads must be "regenerated" with a new supply of Sodium ions. The regeneration of the Zeolite beads is accomplished by a three step process.
Water softeners come as single mineral tank units (simplex), double mineral tank units (duplex) and multiple mineral tank units. Since regeneration cycles can take approximately one hour simplex units are used only when this interruption can be tolerated. To avoid interruption duplex units are used so that the regeneration of one unit can be accomplished while the second unit is on line. Triplex or other multiplex units usually are the result of need for increased capacity and units can be added to keep soft water available. The reliability of new electronic metering/controls for regeneration have allowed users to depend on smaller units with more frequent regeneration.
BACKWASH: The flow of water through the mineral bed is reversed. The mineral bed is loosened and accumulated sediment is washed to the drain by the upward flow of the water. An automatic backwash flow controller maintains the proper flow rate to prevent the loss of resin.
BRINE DRAW AND SLOW RINSE: Ordinary salt has the capability to restore the exchange capacity of the mineral. A given amount of salt-brine is rinsed slowly through the mineral bed. After the salt-brine is drawn, the unit will continue to rinse slowly with water to remove all of the salt-brine from the media bed.
FAST RINSE: A high down flow of water repacks the mineral bed. Any trace of brine not removed in slow rinse is flushed to the drain. The unit is then returned to SERVICE the brine maker is refilled with fresh water to form salt brine for the next regeneration. The total regeneration time is approximately 60-90 minutes.
Demineralization / deionization.
Typically used on water that has already been prefiltered, DI uses a minimum two-stage process to remove virtually all ionic material remaining in water. Two types of synthetic resins are used, one to remove positively charged ions (cations) and another to remove negatively charged ions (anions). Resins have limited capacities and must be regenerated upon exhaustion.
|
|
Ion exchange deionizers (Dl) use synthetic resins similar to
those used in water softeners. 
