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

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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

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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: Brent.White@swfwmd.state.fl.usBrent.White@swfwmd.state.fl.us