Cooling Tower Cooling Tower WorkshopWorkshop
Water Efficiency for Cooling Towers
Brent M. White
“Going Green Can Keep You Out of then Red”
June 7, 2007Hyatt Regency Tampa
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Use significant amounts of water:
Refrigeration Systems
Air Conditioning
Process Cooling
Cooling TowersCooling Towers
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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 TowersCooling Towers
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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 Cooling Tower FunctionFunction
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There are two types of tower designs:
Cooling Tower Cooling Tower TypesTypes
Counterflow tower Air moves vertically upwards to the downward fall of water.
Crossflow tower Air moves perpendicularly across the direction of the water fall
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Design SpecificsDesign Specifics
Crossflow Counterflow
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Lower energy consumption Easy maintenance/de-scale More variations of water flow Reduced drift Lower operating costs
Efficient use of air Better tower performance Longer ranges Finer droplet size from spray heads
Dis
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Accelerated algae growth Possible orifice clogging Larger footprint Possible icing on louvers in colder climates
Higher pump head needs Increased operating costs Difficult to clean/de-scale More piping needed High inlet velocities may suck in trash and dirt
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Design SpecificsDesign 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.
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Cooling Tower Water Cooling Tower Water LossLoss Minimizing loss if possible
Understanding basic operating principles
Water is lost three ways: Evaporation
Bleed off
Drift and other minor loses
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EvaporationEvaporation
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
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EvaporationEvaporation
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
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Bleed-OffBleed-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)
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Bleed-OffBleed-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
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Drift and Other Drift and Other LossesLosses 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
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Cooling Tower Cooling Tower EfficiencyEfficiency
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
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Water Quality Water Quality RequirementsRequirements
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
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Cooling Tower Cooling Tower SchematicSchematic
LegendB: Bleed OffD: DriftE: EvaporationM: Make-upCR: Concentration ratio
Water BalanceM = E+B+D
Concentration RatioCR = MQuality / BQuality
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Concentration RatiosConcentration 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.
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Concentration RatiosConcentration 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
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CalculationsCalculations
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
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Savings from Increasing Savings from Increasing RatiosRatios
Concentration Ratios
Before Adjusting
Cycles 2 3 4 5 6 7 8 9 10 12 15 201.5 33% 50% 56% 58% 60% 61% 62% 63% 63% 64% 64% 65%
2 25% 33% 38% 40% 42% 43% 44% 44% 45% 46% 47%3 11% 17% 20% 22% 24% 25% 25% 27% 29% 30%4 6% 10% 13% 14% 16% 17% 18% 20% 21%5 4% 7% 9% 10% 11% 13% 14% 16%6 3% 5% 6% 7% 9% 11% 12%7 2% 4% 5% 6% 8% 10%8 2% 3% 5% 6% 8%9 . 3% 5% 6%
10 2% 4% 5%12 2% 4%15 2%
Concentration Ratios After Adjusting Cycles
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
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Water Use –vs Water Use –vs Concentration RatioConcentration Ratio
Water Consumption -vs- Concentration Ratio
3000
3500
4000
4500
5000
5500
6000
6500
7000
7500
8000
1 2 3 4 5 6 7 8 9
Concentration Ratio
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Water Use Per 100 tons of Cooling (GPD)Log. (Water Use Per 100 tons of Cooling (GPD))
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Calculating SavingsCalculating Savings
Increasing the concentration ratio saves water CR1 = Ratio before increasing cycles CR2 = Ratio after increasing cycles
Percent conserved = CR2 – CR1
CR1 (CR2-1)X 100
Example: CR1 = 2, CR2 = 6
Percent Conserved = 6 - 22(6-1)
X 100 = 4/10 = 40%
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Water Efficiency MeasuresWater 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
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Secondary Efficiency Secondary Efficiency MeasuresMeasures 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)
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Secondary Efficiency Secondary Efficiency MeasuresMeasures 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
Thank You! Any Questions?Thank You! Any Questions?
For more info: For more info: [email protected]@swfwmd.state.fl.us
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