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Technologies - Heating and Cooling

Cooling towers are used for large-scale air-conditioning systems and to cool the circulating water used in oil refineries, chemical plants, and power stations. They are one of the largest non-residential water consumers. Water that falls through the tower is cooled due to evaporation and then returned to the top of the tower. The water that drops into the basin is on average 10 degrees F cooler than the water returned to the top. The towers vary in size from small roof-top units to very large structures; the taller the tower the greater the temperature drop.

Water is lost from a cooling tower by evaporation, drift, and bleed-off (water used to carry away tower-damaging mineral buildup; also known as blow-down). Water that must be added to the system to make up for these losses is called makeup water. Evaporative losses from a cooling tower can be estimated at 3 gallons per minute for 100 tons of cooling. That means that a tower or condenser which provides 500 tons of cooling evaporates almost 21,600 gallons over a 24-hour operating period.

Bleed-off Reduction
One of the major ways to improve cooling tower water efficiency is to reduce the amount of bleed-off (water used to carry away minerals). Water use can be reduced by 20% or more by circulating water through the tower more times before bleed-off occurs. This will result in a higher concentration ratio (or cycles of concentration). A cooler's concentration ratio is its measure of water use efficiency. The higher the ratio, the more water-efficient.
Bleed-off Reuse
Cooling tower bleed-off could be used to irrigate plants that are not sensitive to its high salt content. Another option is to convey it to water treatment plants. Not only would these practices save water, they may save money through reduced water and sewer costs.
Use of Sulfuric Acid
The use of sulfuric acid in the chemical treatment program of a cooling tower prevents scale buildup, so water can circulate more times before it is discarded or make-up water is added.  If sulfuric acid treatment is not feasible or practical, carboxylated polymers (carboxylates) can be used instead to control the system’s pH and prevent scale. The solubility of carboxylates is less than sulfuric acid, but still more than double than without treatment.
Conductivity Controller

This device automatically controls blowdown based on the maximum cycles of concentration that can be safely achieved. The resulting conductivity (typically measured as microSiemens per centimeter, uS/cm). A conductivity controller can continuously measure the conductivity of the cooling tower water and discharge water only when the conductivity set point is exceeded.


Evaporative coolers (also referred to as swamp coolers) increase the humidity of incoming air being drawn into a building and decrease its temperature. After a short period of operation, re-circulating water in an evaporative cooler assumes the temperature of the entering air. This temperature is theoretically the lowest temperature to which the entering air may be cooled.

The principal opportunity for conservation of the water used by evaporative coolers is to reduce the flow of bleed-off water discharged from the coolers. Typically, only a very small stream of bleed-off water is necessary for proper operation; but unfortunately, excessive amounts of water are often bled off. Not only does this waste water, it impairs the cooling efficiency of the cooler because the water does not become as cold as possible.

The three common types of evaporative coolers: recirculating, non-recirculating (once-through) and a design sometimes referred to as a “dump pump” are described below. The bleed off from all types evaporative coolers can be used to irrigate plants that are not sensitive to the water’s high salt content.

Recirculating Evaporative Cooler In this type of cooler, water that runs off the pads is captured and recirculated several times before it is discarded. A small amount of bleed-off is necessary to control concentrations of contaminants in the re-circulating water and prevent damage to the cooler's pads, thereby allowing it to operate effectively. Bleed-off valves can be adjusted to discharge the minimum amount of water necessary. Recirculating evaporative coolers use about 3 gallons of water per hour of operation.
"Dump pump” Evaporative Cooler The “dump pump” variety automatically empties and refills the water-collection pan every few hours of operation.
Once-through Cooling for Equipment

In dry climates, equipment may be cooled by a “once-through” or single-pass of water. This is the most water-intensive cooling process because after passing through and cooling the equipment, the water is often discarded. Equipment which might be cooled by once-through water include: degreasers, rectifiers, hydraulic equipment, x-ray machines, condensers, viscosity baths, air conditioners, air compressors, hydraulic presses, welders and vacuum pumps. Many types of water-cooled equipment can be replaced with energy-efficient, air-cooled models. Water use can also be reduced through re-circulation, by using water from non-potable sources and by reusing the once-through water for other purposes such as landscape irrigation or a cooling tower. These practices may not only save water, they may save money through reduced water and sewer costs
In Arizona’s active management areas, ADWR prohibits the use of once-through cooling water by industries that have their own wells unless the water is reused.

Boiler & Steam Generators Boilers are used in large heating systems or in industries where large amounts of process steam are used. Water is added to a boiler system to make up for the water losses and to replace water lost when the boiler is blown down to expel any solids that may have built up. Wherever practical, steam condensate should be captured and returned to the boiler for reuse as make-up water. A condensate return system saves water, reduces the costs of pretreating the boiler feed-water and reduces energy consumption. Boiler operating costs can be reduced up to 70 percent by installing a condensate return system. Water consumption by boiler systems vary depending on the size of the system, the amount of steam used, and the amount of condensate return.
Expansion Tanks An expansion tank provides a cushion of air for the expansion of water as it heats. This saves water in two ways: 1) it prevents the pressure relief valve from having to open and discharge water to relieve the pressure and 2) it eliminates the need to use cold water in mixing valves to cool the blow-down because the blow-down condenses and cools in the expansion tank. In a steel expansion tank the air and the water touch each other. In a bladder-type expansion tank the air and the water are separated by a diaphragm.
Humidifiers are used to add moisture to the air through evaporation, thereby increasing relative humidity levels. There are two major types of humidifiers: room humidifiers and central humidifiers. Room humidifiers are self-contained units that are not usually connected to water supply lines. Their reservoirs are manually filled. Central humidifiers are usually connected to the central heating system and water supply lines. Their reservoirs are automatically filled.
Water-Conserving Humidifiers Humidifiers that have a continuous bleed-off system, waste water. In these humidifiers, a constant stream of water leaves the reservoir and enters the sewer system at the same time a constant stream of potable water fills the reservoir. Recirculating humidifiers do not continuously drain and replace water. There are concerns about the possibility that recirculating humidifiers may contribute to “sick building syndrome”. An alternative would be to reuse the discarded water for another purpose, such as irrigating landscapes, rather than sending it to the sewer. Another option is to adjust the humidifiers to discharge the minimum amount of water necessary and avoid excessive bleed-off. Timers can be used to turn humidifiers on or off based on seasonal needs, or to control when water is pumped out of the reservoir to remove mineral build up.
Mist Cooling Systems Mist cooling systems have become available to businesses and homeowners as an outdoor cooling option in regions with high temperatures and low humidity. Water is pumped through the system and released in a fine spray which evaporates, forming cool barriers against hot, dry air. Each misting nozzle uses about half a gallon of water per hour. To conserve water, the systems should be operated only when people are using the area. Some systems have sensors to turn misters off when no one is present, or on/off switches that can be activated by customers as needed. It is also desirable to use misters with nozzles than can be independently controlled, to direct nozzles properly so that the system only cools the area intended, and to turn off misters when it is too windy or humid for them to work well.