Tower Evaporation & Blowdown Engine

What is Cooling Tower Hydrology & Scaling Physics?

Cooling towers reject heat by evaporating water. Because pure H2O evaporates while heavy minerals (calcium, magnesium) do not, the remaining basin water rapidly concentrates into a highly saturated chemical slurry. To prevent this slurry from hardening into cement-like scale inside your chiller, the tower must continuously drain (blowdown) this dirty water and replace it with fresh municipal makeup water.

Mathematical Foundation

Stage 1: Evaporation Rate

E_{gpm} = \frac{GPM_{total} \times \Delta T}{1000}

E_{gpm}

= Volume of chemically pure H2O vapor physically evaporating into the atmosphere (GPM).

GPM_{total}

= The total condenser water flow rate pumping through the tower.

\Delta T

= Tower Range — the temperature drop between the hot return and the cold supply (e.g. 10°F).

1000

= Baseline latent heat approximation. It takes roughly 1,000 BTUs to evaporate one pound of water.

Stage 2: Continuous Blowdown Priority

B_{gpm} = \frac{E_{gpm}}{C - 1}

B_{gpm}

= The critical volume of concentrated water that must be drained into the sewer to prevent scaling (GPM).

E_{gpm}

= Calculated pure evaporation loss.

C

= Cycles of Concentration (Ratio of dissolved solids in the basin vs dissolved solids in the pure city water).

Stage 3: Total Makeup Obligation

M_{gpm} = E_{gpm} + B_{gpm} + D_{gpm}

M_{gpm}

= Total fresh water the city must supply to keep the tower alive.

D_{gpm}

= Drift and Windage loss — small droplets physically blown out the top by the fan. Usually negligible (0.01%) on modern towers.

Laws & Principles

THE LAW OF CONCENTRATION CYCLES: A tower operating at 4 Cycles of Concentration means the water in the basin has precisely 4 times the amount of dissolved minerals as the pure city water coming out of the hose. Running at 1 Cycle means you are dumping 100% of the water straight down the drain without concentrating it.
THE CALCIUM CARBONATE LIMIT: Most standard municipal water supplies can safely sustain between 3.0 to 5.0 Cycles of Concentration before the dissolved calcium carbonate covalently bonds to the copper condenser tubes inside the chiller.
THE BLOWDOWN PARABOLA: Moving from 1.5 Cycles to 3.0 Cycles saves millions of gallons of blowdown water per year. However, due to the hyperbolic nature of the formula, pushing a tower from 6.0 Cycles to 8.0 Cycles saves almost zero visible water while exponentially increasing the risk of catastrophic mineral scaling.

Step-by-Step Example Walkthrough

" An engineer oversees a 1,500 GPM condenser loop dropping 10°F across the cooling tower. Chemical treatment analysis limits safe municipal water operation to 4.5 cycles of concentration. "

1. Calculate vapor Evaporation: E = (1,500 GPM × 10°F) / 1000 = 15.0 GPM of pure vapor lost.
2. Calculate forced Blowdown: B = 15.0 GPM / (4.5 - 1) = 15.0 / 3.5 = 4.29 GPM of dirty drain water.
3. Calculate Total grid Makeup obligation: 15.0 GPM + 4.29 GPM = 19.29 GPM total.

Final Result: To safely reject the heat, the tower destroys 19.29 Gallons Per Minute of city water continuously. Over the course of a humid 24-hour peak summer day, this tower will consume 27,777 gallons of municipal water and dump 6,177 gallons into the sanitary sewer.

Quick Answer: How do you calculate Cooling Tower Blowdown?

To calculate cooling tower Blowdown, you must first calculate the pure Evaporation Rate by multiplying the condenser water GPM by the tower's temperature Drop (°F) and dividing by 1,000. Next, you determine your safety limit for Cycles of Concentration (C). Finally, you take the Evaporation Rate and divide it by (C - 1). This yields the exact Gallons Per Minute (GPM) of contaminated water the tower must actively drain into the sewer to prevent catastrophic mineral scaling.

The Hydrological Engine

Total Makeup = [ (GPM × ΔT) / 1000 ] + [ Evap / (Cycles - 1) ]

Scaling Variables:

Evaporation (The First Bracket): Determines how much pure Phase-Change vapor was destroyed to reject the chiller heat.
Blowdown (The Second Bracket): Determines how much dirty fluid must be drained to keep the mineral slurry suspended without petrifying on the pipes.

Scaling Risk vs Cycles of Concentration (C)

Cycles Limit	Diagnostic Assessment	Likely Physical Causes
1.0 to 1.5 Cycles	Massive Water Waste	Treating the tower like a once-through system. Extremely high municipal utility bills.
3.0 to 5.0 Cycles	Standard Safe Zone	Proper standard chemical dosing for hard municipal water.
6.0 to 8.0 Cycles	High Efficiency	Soft municipal water or aggressive acid-feed chemical treatments.
9.0+ Cycles	Extreme Scaling Risk	Almost certainly requires Reverse Osmosis (RO) makeup water or advanced ozone to survive without cementing the chiller.

Catastrophic Failures & Design Mistakes

The Closed Valve

To save money on utility bills, a facility owner completely shuts the blowdown drain valve on a 500-ton tower. The Evaporation continues perfectly, but the Blowdown drops to zero. Because there is no drain, the mineral concentration climbs to 25.0+ Cycles in a matter of days. The calcium in the water physically crystallizes, filling the chiller condenser tubes with solid white rock. The chiller must be rebuilt for $90,000.

The Chase for Perfection

An engineer spends $40,000 on advanced chemical skids to push a tower from 6.0 Cycles up to 10.0 Cycles of concentration. Because the blowdown equation is hyperbolic, pushing the tower to 6.0 cycles already captured 95% of the total water savings. The jump from 6.0 to 10.0 cycles barely saves 1 GPM of total water, meaning the ROI on the chemical skid will take roughly 80 years.

Field Design Best Practices & Pro Tips

Do This

✓Install dual water meters immediately. Municipalities will physically charge you for every drop of water that goes down the sewer drain. If you do not have dedicated sub-meters analyzing exactly how much water the cooling tower evaporated into the sky versus drained away via blowdown, you are illegally paying astronomical sewer fees for vapor that evaporated.

Avoid This

✗Never assume pure Evaporation is proportional to the heat load alone. If the fan VFD fails and the tower basin water heats up excessively, the laws of psychrometrics dictate that a larger percentage of the cooling will be accomplished by dry sensible heat transfer, reducing the actual evaporation rate below this calculator's nominal load profile.

Frequently Asked Questions

What are Cycles of Concentration?

It is perfectly defined as the ratio of dissolved total solids in your cooling tower basin divided by the dissolved total solids in your city makeup water line. If your tower is operating at 5.0 Cycles, it means the water in your tower holds exactly 5 times more minerals per gallon than the water entering it.

Why must I blowdown the cooling tower?

Because when pure water evaporates from the tower, all the heavy minerals (calcium, magnesium) are left behind in the basin. If you do not actively drain (blowdown) this highly concentrated nasty water, it will quickly reach extreme saturation levels and solidify into cement-like scale inside your expensive chiller pipes.

Does increasing Cycles of Concentration save water?

Yes, but with massive diminishing returns. Pushing a tower from 2.0 cycles to 4.0 cycles saves millions of gallons a year and is a huge financial win. However, pushing a tower from 7.0 cycles to 10.0 cycles saves almost nothing due to the hyperbolic nature of the math, while massively increasing your risk of destroying the condenser.

How does the tower know when to blowdown?

Modern towers use a chemical controller with a conductivity array sensor. The sensor constantly measures the electrical conductivity of the water (which directly correlates to how many minerals are suspended in it). When the conductivity hits your setpoint (e.g., 4.0 cycles), it electronically opens a motorized drain valve to dump water until the concentration drops via fresh makeup.

Cooling Tower Hydrology