1. Cooling Tower Workshop Water Efficiency for Cooling Towers Brent M. White “ Going Green Can Keep You Out of then Red” June 7, 2007 Hyatt Regency Tampa
26. Thank You! Any Questions? For more info: [email_address]
Editor's Notes
Introduce Self
Cooling Towers Use Significant amounts of water depending on their applications. (as much as 60% of a facilities total water use) Used as refrigeration systems such as at the St. Pete Times Forum to produce ice for skating Used in Air Conditioning source in large facilities Also used in process cooling as in circuit board manufacturing.
Cooling Towers use 95 percent less water than a single-pass cooling systems that discards water after a single use. Towers are one of the largest users in hospitals, hotels, industrial plants, office buildings and schools.
Cooling towers use evaporation to lower the temperature of incoming water and delivers it to part of a buildings operation, a specific piece of equipment or a specific process. This practice is most efficient when the maximum about of water surface area is exposed to the maximum flow of air.
Basically, there are two types of tower designs depending on the direction of air flow in relation to the direction the incoming water falls. Counterflow: Air moves vertically upwards towards the fall of water. Crossflow: Air moves perpendicularly across the fall of water.
Each type of tower has advantages and disadvantages to its design. These should be considered when choosing what type of tower to use in its operations.
Basic design specifics will determine which type of tower to use in a facility. Crossflow: When pump head, piping and operating costs needs to be minimized and ease of maintenance is a concern. Counterflow: When footprint, icing and increased pumping to mitigate pressure changes (like in a multi-story building using booster pumps) are of concern.
Minimizing water loss is possible by understanding the basic operating principles of cooling towers. Water is lost three ways: Evaporation, Bleed off Drift caused by wind
Evaporation in a cooling tower is caused when air comes in contact with the falling water releasing the latent heat in the form of water vapor. The amount of evaporation depends on the: - length of time the cooling water is in contact with the air; - the beginning temperature of the air and water; and the surrounding wind and humidity. This occurs at about the rate of 1 percent for every 10 degrees of temperature drop.
A tower will evaporate a total of 1 to 3 percent of the total volume of the circulating water. This works out to about 2.4 gallons per minute for every 100 tons of cooling. For a 1,000 ton tower this can work out to be as much as 34,500 gallons per day.
Bleed off, or blowdown is triggered by dissolved and/or suspended solids left in the cooling tower after evaporation reaching a pre-determined limit. These materials are left concentrated in the basin, or recirculating water inside the tower. High concentrations can cause scale buildup, corrosion or biofouling. This is measures in conductivity (microsiemans) or Total Dissolved Solids (ppm TDS)
Bleed off, or blowdown involves releasing a small amount of the basin water which contains a high concentration of TDS through a bleed off valve into the sanitary sewer. This is usually controlled in an automated process by a conductivity meter. A “batch method” releases a constant fixed amount of basin water to reduce TDS. This is the primary area for saving water.
Drift occurs when water drops are carried off by airflow during the initial stage when air and water meet inside the tower. Rates of drift are low, usually ranging between 0.05 and 0.2 percent of the airflow rate. Drift really isn’t critical to the efficiency of the cooling tower thus isn’t controlled. Other types of losses include valve leaks and drawdown.
As mentioned, efficiency of tower operations is related to blowdown, which is determined by the water quality inside the tower. Determines how much water is blowndown and how much is used to makeup that lost water. This can be defined by the concentration ration of makeup tower water conductivity vs basin tower water conductivity
To control these rates of bleed-off and make-up water use, you must apply the right amounts of treatment chemicals and involving other treatment methods as needed. Monitoring levels of 4 contaminants is needed: -Scale -Corrosion -Biological fouling -Foreign matter
Explain cooling tower schematic including bleed-off, drift, evaporation, make-up and the concentration ratio
To determine the concentration ratio for sub metered towers (metered at makeup and blowdown valves): Divide makeup water volume by blowdown water volume. This gives you a concentration ratio, otherwise known as cycles of concentration or how many cooling cycles a tower will use water before discharging it to the wastewater stream.
To determine this on an unmetered tower, divide the conductivity of the basin water by the conductivity of the makeup water. It’s a good idea to submeter all towers for accurate measurement of water use. Some utilities will even offer wastewater credits for water lost to evaporation, or water going through the main domestic supply meter, but not being introduced to the wastewater stream.
To calculate evaporation: again, for every 100 tons of cooling, multiply by 2.4 gpm. Extend that out to 24 hrs for a per day figure. To calculate bleed-off volume, divide the evaporation figure by the concentration ration minus 1. To calculate the makeup volume, add the evaporation volume to the bleed off volume.
Explain how the chart works in determining how much water can be saved. I.e.: Going from 2 cycles of concentration to 4 will save 33% of the water used. Technologies now exist that enable a tower to be ran at 90+ cycles without scaling or damage to the tower.
This graph shows the relationship between cycles of concentration and the volume of water used by the cooling tower.
To manually calculate the volume of water saved by changing the concentration ratio, use the following equation.
In summary, reducing bleed-off is the primary opportunity to saving water in a cooling tower. This can be done 3 ways: Improving system monitoring Upgrading cooling water treatment Using alternative sources of makeup water. Measures to do so include: Installing submeters to monitor water use Increasing cycles of concentration Operating bleed-off continuously Installing conductivity controls Make efficiency a priority with service providers.
Secondary measures include: Ozonation – reduces chemical use Sulfuric/Ascorbic Acid – may require a corrosion inhibitor. Calcium sulfate is more soluble than calcium carbonate Sidestream filtration – using filters to lower make-up water conductivity Installing tower covers – blocks sunlight and slows down biological growth inside the tower.
Other efficiency measures include: Using recycling and reuse from other sources that may have a better makeup conductivity level Magnets and electrostatic field generators that are reported to remove scale Install a water softener to the make-up feed to reduce conductivity level. (Better for smaller towers)
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