Conservation of electricity and water are becoming more important for businesses to reduce operating costs. A painless method to conserve power and water is: automatic control of the water treatment aspect of the cooling tower.

Background
Cooling towers are pieces of equipment used to evaporate water, thereby reducing the temperature of the remaining water. The remaining cool water can then be used to remove heat from cooling equipment and the cycle repeats itself.

The water enters at the top of the tower and cascades over PVC fill or a tube bundle. This causes the water droplets to decrease in size and increase the surface area for rapid heat transfer. The evaporation rate is increased when the fans energize and blow air across the droplets, this occurs on demand when the temperature drop cannot be achieved by water flow alone.

When water is evaporated minerals are left behind and concentrate in the remaining water.  All minerals have a saturation point, normally based on the concentration, water pH, and temperatures.  If the concentration exceeds the saturation point, the minerals come out of solution and deposit on internal surfaces of the cooling system. This is referred to as “scaling”. The most prevalent scale is calcium carbonate, which precipitates (scales) in the hottest area of the cooling system. Scale has an insulating effect, which lowers the amount of heat that can be effectively removed by the cooling tower. Scaling causes pumps and fans to work longer to dissipate heat from the cooling equipment resulting in higher electrical costs and temperatures. For example, 1/10 of an inch of scale (calcium carbonate) increases cooling costs by approximately 40 percent.

Cooling Tower Conductivity Control
The most common method to prevent scale in cooling tower systems is to bind the scaling minerals and control their concentrations. Acid, typically sulfuric, can also be used to control the pH and reduce the alkalinity so that greater cycles of concentration can be utilized.

The most economical method is the use of a conductivity controller. These units operate on the principal of ions (dissolved solids) conducting electricity.  The more ions present the more conductance the water exhibits. Conductance is measured in uSeimens. After analysis of the water, an acceptable conductivity range can be calculated.  This method allows concentration of minerals in the tower water. Bleed-off only occurs as the saturation point is approached. The particular scale/corrosion inhibitor used by a company has a RSI/LSI number associated with its ability to inhibit scale. The parameters used to determine this number are conductivity, pH, temperature, alkalinity and calcium hardness.

A program that utilizes acid has the distinction of running higher cycles of concentration, because the pH is kept in range and the alkalinity is lowered and kept in a form that is more difficult to precipitate. By running higher cycles of concentration water usage is diminished. However, OSHA regulations have reduced most of the use of acid to larger towers or ones that are used in critical situations. A pH controller should always have a chemical pump alarm limit.

The controller activates a chemical pump that adds chemicals for scaling and corrosion proportional to the amount of bleed-off or makeup. This allows the system to maintain a constant chemical residual because for every gallon of makeup a specific amount of chemical is pumped. The most common method used for chemical feed is to inject the chemical after the bleed-off has taken place. This keeps a larger proportion of the chemical in the system instead of exiting through the bleed-off.

Biological growth (bacteria, fungi and algae) must also be controlled in cooling towers.  Bacteria are the worst offenders, but mold and fungus also cause problems. They can cause disease, corrosion and reduce heat transfer. Legionnaire’s disease is one disease that has been proven to inhabit cooling towers and may be transmitted from tower water in the drift of the tower. Heat transfer loss is caused by a slime mass exuded by bacteria. This mass tends to collect debris that acts as an insulator. Microbial induced corrosion (MIC) is caused by chemicals exuded by bacteria; many are acidic and pit the metal.

Reducing the risk of bio-fouling or infection is a fairly simple matter. Biocides must be used on a regular basis, preferably added by a controller or a timer-based pump. Manual addition in many cases is imprecise and not performed according to schedule. These biocides will not kill all the microorganisms in the system because systems are continually re-contaminated, but biocides will keep their levels under control where infection becomes unlikely. Biological growth is typically tested with a nutrient–based growth strip. Aerobic bacteria are normally the target of these tests and are measured in CFU (colony forming units).

Biocides must have a specific concentration over a specified contact time to get the necessary kill rate. The biocides need to be added as a slug dose 2-3 times per week. This is where biocide timers are used. The ideal biocide addition works like this:
Prebleed to lower the conductivity to prevent scaling when the bleed-off is closed.
Add dispersant to penetrate a slime mass or attached bacteria
Add biocide in proper dose in less than 1 hour.
Close bleed-off to maintain concentration of biocide in the system
After the allotted contact time return to normal functions
Other methods of biological control used are:
Sodium hypochlorite
hydrogen peroxide
Bromine based compounds
Peracetic acidThese biocides are referred to as oxidizing biocides and can be added as a slug dose or a continuous low dosage. An ORP controller can control these chemicals, maintaining a continuous level or an ORP spike.

Also, towers must be drained and cleaned at least once per year and preferably super-chlorinated.

Showing an ROI on Control Equipment
Chemical Pump Alarm Timers – De-energizes pumps when a pre-set amount time passes. This feature keeps the pump from emptying a barrel when a bleed valve becomes inoperative

Evaporation Credit- This is a method of measuring the evaporation and seeking credit on your sewerage bill. (See application note)

Flow switch- This device stops all pumps from running when a no-flow situation exists. Without this option chemicals can be pumped into a stagnant line causing unwanted reactions and possible damage.

Glossary
Biocide- a chemical that inhibits or kills biological growth, typically bacteria, mold and fungus
Bleed-off- water with a high concentration of minerals that is purposely drained from the tower so that less concentrated make-up water can dilute the mineral content
Conductance- a measurement of the ability of water to conduct electricity, typically caused by inorganic dissolved solids
Contact time- a time, determined by biocide manufacturers, that is required for the biocide concentration to be in solution with the bio-growth to achieve the desired kill rate
Chemical residual- the amount of a particular chemical in the system that is available to perform the desired reactions. This may differ from the theoretical amount added as precipitation and other reactions may occur to deplete the chemical.
Dispersant- a wetting agent used to penetrate biological slime masses and release them into the water to be contacted by a biocide.
Drift- the droplets of water that are entrained in the exiting air of the cooling tower fans.
Makeup- the water source used to replenish the level of water in the cooling tower as evaporation and other losses occur.
ORP- a measurement of the oxidation-reduction potential (ORP). This is a non-selective measurement used in towers to get a basic idea of the killing power of an oxidizer. Measured in millivolts, the zero point is 250mV. Typical ranges are 400-600 mV.
ORP spike- the raising of the ORP to a certain level and maintaining it for a specific time
pH- the relative acidity or basicity of substance. Typically measured in biological processes as 0-14, it is the negative log (p) of the hydrogen ion concentration (H). All inorganic acids contain hydrogen (H): HCL, H2SO4, HF, etc.
Precipitate- the act of a dissolved substance coming out of solution
PVC fill- the lattice work of PVC contained in many towers that is used to break the spray into smaller water droplets
RSI/LSI- Ryznar/Langlier Indices- These are equations that predict the scaling or corrosive potential of water without regard to the chemicals added. They use the following parameters in the equation: pH, conductivity, temperature, alkalinity and calcium hardness. Note that these parameters are very important in water treatment.
Saturation point- the point at which no more of a particular substance can be dissolved in a fluid. In cooling towers, this refers most often to calcium carbonate precipitation.
Scale and corrosion inhibitor- a blend of chemicals used to extend the solubility of particular compounds (i.e. Calcium carbonate) by distortion of chemical structure or controlled precipitation. Polyphosphonate is the most common of scale inhibitors. The corrosion additive is based on the type of metal in the system. Many times referred to as inhibitor.
Slug dose- a concentrated addition of chemical added to a system, usually biocides