Cooling towers use significant amounts of water for processes like air conditioning and refrigeration. They are one of the largest water users in places like hospitals, hotels, and office buildings. Cooling towers work by evaporating a small portion of water to lower the temperature of the remaining water in an efficient closed-loop system. There are two main types of cooling tower designs, and water is lost primarily through evaporation, bleed-off of concentrated water, and minor drift. Increasing the concentration ratio by reducing bleed-off through improved monitoring, treatment, and alternative water sources can significantly conserve water use.

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

2. Use significant amounts of water: Refrigeration Systems Air Conditioning Process Cooling Cooling Towers

3. Use 95 percent less water than single pass cooling systems One of the largest water users in: Hospitals Hotels Industrial Plants Office Buildings Schools Cooling Towers

4. Uses evaporation Lowers water temperature of heated water from: A building’s operation A piece of equipment A specific process Evaporation is most efficient when: Maximum water surface area is exposed to the maximum flow of air. Cooling Tower Function

5. There are two types of tower designs: Cooling Tower Types Counterflow tower Air moves vertically upwards to the downward fall of water. Crossflow tower Air moves perpendicularly across the direction of the water fall

6. Design Specifics

7. Design Specifics Crossflow use criteria To minimize pump head To minimize pumping and piping first costs To minimize operating costs When ease of maintenance is a concern Counterflow use criteria When footprint is restricted When icing is of concern When increased pumping is designed for any pressure drop.

8. Cooling Tower Water Loss Minimizing loss if possible Understanding basic operating principles Water is lost three ways: Evaporation Bleed off Drift and other minor loses

9. Evaporation Warmed water goes from liquid to a vapor Removes the latent heat Cools the water left behind Evaporation depends on: Length of time water is in contact with the air Temperature of the air and the water Surrounding wind and humidity Occurs at a rate of about 1 percent for every 10 ° temperature drop

10. Evaporation Evaporation = 1 to 3 percent of the circulating water. (2.4 gpm/100 tons of cooling) Example: 1,000 tons of cooling loses about 24 gpm to evaporation (2.4gpm/100 tons x 1,000 tons) At 24 hours per day, loses would be 34,500 gpd

11. Bleed-Off Dissolved and suspended solids are left in the cooling tower after evaporation Concentrated in the recirculating water Scale buildup, corrosion or biofouling can occur Measured in Conductivity (  S) Total Dissolved Solids (TDS)

12. Bleed-Off “ Bleed-Off” or “Blowdown” involves: Releasing a small amount of recirculating water Water contains high concentrations of TDS Released through bleed-off valve Bleed-off water is sent to sanitary sewer Bleed-Off control Conductivity meter automates at a present value (High TDS -  S) “ Batch method” does this in large volumes until present TDS is reached Primary area for saving water

13. Drift and Other Losses Water drops carried off by airflow In the form of mist, or drift Drift release is not controlled Drift Rates are low Between 0.05 and 0.2 percent of the airflow rate Not critical to the efficiency of the cooling tower Other types of losses: Valve leaks Drawdown/draw-off

14. Cooling Tower Efficiency Related to the water quality inside the tower Conductivity (TDS/  S) concentration How much water is bled-off How much water is used to make-up Concentration ratio Make-up water -vs- Tower basin water Determines how much water the tower uses

15. Water Quality Requirements Includes Controlling the rates of bleed-off/make-up Adding the right amounts of chemicals Applying other treatment methods Monitoring levels of 4 contaminants Scale Corrosion Biological fouling Foreign matter

16. Cooling Tower Schematic Legend B: Bleed Off D: Drift E: Evaporation M: Make-up CR: Concentration ratio Water Balance M = E+B+D Concentration Ratio CR = M Quality / B Quality

17. Concentration Ratios For metered towers (gallons) CR = M/B, or CR = (B+E)/B Example: 250 ton tower – 24 hours M = 14,400 gallons B = 5,760 gallons CR = 14,400 gal / 5,760 gal CR = 2.5 Some utilities may provide a credit to the wastewater charges for evaporative losses with tower submetering.

18. Concentration Ratios For unmetered towers (gallons) Calculation based on the conductivity concentration ratio (TDS/  S) CR = [B] / [M] Example: Bleed-off conductivity = 1,400  S Make-up conductivity = 550  S CR = 1,400 / 550 CR = 2.5

19. Calculations Evaporation: Evaporating rate = 2.4 gpm/100 tons of cooling E = (2.4 gpm/100 tons) x (250 tons) x 24 hours x (60 min/hr) E = 8,640 gallons Bleed-Off: B = E/ (CR-1) B = 8,640 gallons / (2.5 – 1) B = 5,760 gallons Make-Up: M = E + B M = 8,640 + 5,760 M = 14,400 gallons

20. Savings from Increasing Ratios Most towers run at 2 to 3 cycles of concentration Cycle of Concentration – How many times the water is used inside the tower before being bled-off

21. Water Use –vs Concentration Ratio

22. Calculating Savings Increasing the concentration ratio saves water CR 1 = Ratio before increasing cycles CR 2 = Ratio after increasing cycles Percent conserved = CR 2 – CR 1 CR 1 (CR 2 -1) X 100 Example: CR 1 = 2, CR 2 = 6 Percent Conserved = 6 - 2 2(6-1) X 100 = 4/10 = 40%

23. Water Efficiency Measures Reducing bleed-off is the opportunity Bleed-off can be reduced 3 ways: Improving system monitoring and operation Upgrading cooling water treatment Use alternative sources for make-up water Efficiency measures include: Installing submeters and monitoring use Increasing concentration ratios (reduce bleed-off) Operating bleed-off continuously (no batch) Installing conductivity controls Make efficiency a priority with service providers

24. Secondary Efficiency Measures Ozonation Could reduce or eliminate chemical use High initial capital investment Requires careful management Not all towers are compatible Sulfuric / Ascorbic Acid May reduce water-use by 25 percent Can be hazardous without proper training May need a corrosion inhibitor Sidestream filtration Rapid sand filters or high-efficiency cartridge filters Install tower covers Blocks sunlight / limits biological growth (algae)

25. Secondary Efficiency Measures Recycling and reuse* * (All may require pretreatment) Nonpotable/Reclaimed water Reject water from reverse osmosis systems Wastewater from single-pass cooling systems Well water Magnets and electrostatic field generators Reported to remove scale Increases cycles of concentration Not well substantiated Increased energy costs and biofouling Water softeners Success in raising cycles of concentration Used if make-up water has a high conductivity

26. Thank You! Any Questions? For more info: [email_address]