CTI STD-201 & ASHRAE 90.1 Mass Water Balance, Cycles of Concentration & Water Savings Optimization
Evaporation Loss (E)
192.0 GPM
43.6 m³/h (1.60% of Circ)
Blowdown Rate (B)
63.9 GPM
14.5 m³/h (at 4.0 COC)
Fresh Makeup Demand (M)
256.0 GPM
58.1 m³/h (122.9M Gal/yr)
Annual Savings @ Target COC
$70,400 / yr
9.58M Gal/yr Conserved
Makeup & Blowdown Water Demand vs Cycles of Concentration (COC)Blue: Total Makeup (GPM) | Amber: Blowdown Discharge (GPM)
Thermodynamics & Drift
Cooling Range (ΔT):20.0 °F (11.1 °C)
Heat Rejection Duty:120.0 MMBtu/h (35.2 MW)
Tons of Refrigeration:10,000 Tons
Drift Loss (D):0.06 GPM (86 gal/day)
Water Chemistry & Scaling
Basin Water TDS:1,000 ppm
Basin Silica Level (SiO2):72 ppm (<150 ppm Limit)
Silica Scaling Risk:SAFE (Below 150 ppm)
Target COC Silica Limit:8.3 Max COC
Operating Costs & Conservation
Annual Water Utility Cost:$799,000 / year
Annual Chemical Cost:$104,500 / year
Total Annual Operating Cost:$903,500 / year
Makeup Reduction %:-7.8% Water Demand
5 Fatal Traps & Engineering Pitfalls in Cooling Tower Operation
1. The Silica Precipitation Glass Hard Scaling Trap
Attempting to aggressively save water by pushing cycles of concentration above 8 to 10 in regions with high raw-water silica (e.g. western US or volcanic aquifers with (>25 ext{ ppm } SiO_2)) is catastrophic. When basin silica exceeds 150 to 180 ppm, it polymerizes into amorphous colloidal silica, forming a glassy, porcelain-hard silicate scale on condenser tube surfaces. Unlike calcium carbonate, silica scale cannot be dissolved by acid washing; it requires mechanical reaming or complete condenser re-tubing costing hundreds of thousands of dollars.
2. Over-Concentration Diminishing Returns Fallacy
Water savings follow an asymptotic curve ((M = E cdot [COC / (COC - 1)])). Increasing COC from 2.0 to 4.0 cuts blowdown and makeup demand in half, saving millions of gallons. However, pushing from 5.0 to 9.0 saves less than 2% to 3% additional water while quadrupling chemical inhibitor dosages, tripling biological biofilm growth risks, and pushing calcium hardness to precipitation thresholds.
Operating cooling towers with missing, warped, or damaged drift eliminator modules allows liquid water droplets (10 to 50 microns) to escape into the atmosphere. Warm cooling water (85°F to 105°F) is an ideal breeding incubator for Legionella pneumophila bacteria. Escaping drift travels up to 2 miles downwind, infiltrating building air handling unit (AHU) outdoor air intakes and causing fatal Legionnaires' disease outbreaks and massive corporate liability.
4. The Blowdown Valve Air-Binding & Siphon Failure Trap
Relying on manual intermittent blowdown or blowdown solenoid valves installed with improper pipe grade causes air-binding or silt blockage. When the blowdown line clogs, evaporation continues while blowdown drops to zero. Cycles of concentration quietly surge from 4.0 to over 15.0 in 48 hours. By the time operators notice high conductivity, the chiller condenser tubes are completely choked with calcium carbonate crust, driving chiller head pressure to high-pressure trip limits.
5. Under-Concentration High Blowdown Sewer Bill Shock
Operating a cooling tower with uncalibrated conductivity controllers stuck at 1.8 to 2.2 COC dumps massive volumes of lightly used water down the drain. Because municipal sewer charges are billed as a direct percentage of incoming metered makeup water, the facility pays double for water (supply fee plus sewer discharge fee). Elevating COC from 2.0 to 5.0 slashes water and sewer bills by 40% to 55%, commonly saving medium-sized facilities over $100,000 annually.
What are Cycles of Concentration (COC) in an industrial cooling tower?+
Cycles of Concentration (COC) represents the ratio of dissolved dissolved mineral ions (such as chlorides, silica, or total dissolved solids TDS) in the recirculating tower basin water compared to the incoming fresh makeup water: COC = TDS_basin / TDS_makeup. Because pure water evaporates into the atmosphere during evaporative cooling while dissolved minerals remain trapped in the basin, minerals concentrate over time. Operating at higher COC (e.g. 5.0 to 7.0 vs 2.5) saves millions of gallons of fresh water annually by reducing blowdown waste.
How is cooling tower evaporation loss calculated?+
Evaporation rate is directly tied to the latent heat of vaporization of water (approximately 1,000 to 1,050 Btu/lb). Under CTI STD-201 and ASHRAE standards, evaporation loss E is calculated from circulating water flow rate Q_circ (GPM) and cooling temperature range Delta T (deg F) via: E = 0.0008 * Q_circ * Delta T. For every 10 deg F of water cooling across the tower fill, approximately 0.8% of the total circulating water volume evaporates into the air stream.
What is blowdown and why is it necessary in open recirculating cooling systems?+
Blowdown (or bleed-off) is the deliberate, continuous discharge of a portion of mineral-concentrated basin water to the sewer or wastewater treatment plant. It is replaced by fresh makeup water to dilute mineral concentrations and maintain COC below the saturation precipitation limits of calcium carbonate, calcium sulfate, and silica. Without adequate blowdown, minerals precipitate onto heat exchanger tubes as hard insulating scale, severely degrading heat transfer and clogging water passages.
What is the modern standard for cooling tower drift loss?+
Drift is the unwanted loss of liquid water droplets entrained in the discharging exhaust air plume. Modern cellular polyvinyl chloride (PVC) drift eliminators reduce drift loss to between 0.0005% and 0.001% of the circulating water flow rate (per EPA Clean Water Act Section 316(b) and CTI standards). Uncontrolled drift not only wastes water and treatment chemicals, but also deposits corrosive salts on surrounding cars/switchgear and disperses aerosolized Legionella bacteria into the ambient air.
Why do returns diminish when increasing COC above 6.0 to 8.0 cycles?+
Water conservation follows an asymptotic hyperbolic curve: increasing COC from 2.0 to 4.0 cuts blowdown and makeup demand by over 50%. However, increasing COC from 6.0 to 10.0 provides negligible additional water savings (less than 3% to 5%) while drastically compounding chemical treatment costs. At high COC, silica exceeds its 150-180 ppm solubility limit and Langelier Saturation Index (LSI) spikes, requiring expensive polymer dispersants and hazardous acid feeds that outweigh the marginal water savings.