Enter Operating Data
Only fields required by the selected method affect the evaporation result.
Example Data Table
| Method | Key inputs | Estimated evaporation | Use case |
|---|---|---|---|
| Engineering heat balance | 500 m³/h, 35°C hot, 29°C cold | About 5.23 m³/h before margin | Measured plant operation |
| Known heat rejection | 12,000 kW, 95% latent fraction | About 17.15 m³/h before margin | Equipment duty review |
| Practical rule of thumb | 500 m³/h, 6°C range, 0.00153 factor | About 4.59 m³/h before margin | Early feasibility check |
Formula Used
The calculator supports three useful evaporation approaches. Use consistent SI units for dependable results.
Symbols: Q is heat rejection in kW. V is circulation in m³/h. ρ is density. cp is specific heat. ΔT is water range. hfg is latent heat. E is evaporation in m³/h.
How to Use This Calculator
- Choose the method that matches your available plant data.
- Enter circulation and measured hot and cold temperatures for the heat balance method.
- Enter thermal duty and a realistic latent fraction for the known-load method.
- Use a locally verified practical factor for the quick estimate method.
- Set cycles, drift, and safety margin to estimate total makeup water.
- Select calculate. Review evaporation, blowdown, drift, and daily water demand.
- Download the result as CSV or PDF for reporting.
Understanding Cooling Tower Evaporation
Cooling towers remove heat from process water. Rejected heat leaves through evaporation. A small volume of water changes into vapor. That phase change carries energy. The remaining water returns to the system at a lower temperature. Accurate evaporation estimates support makeup water planning, treatment control, and cost reviews.
Why the Rate Changes
Evaporation depends first on heat rejection. A larger thermal load requires more water to evaporate. Circulating water flow matters. Flow and temperature range define the sensible heat removed from the loop. A wide range means the tower handles energy for the same flow. Ambient wet bulb conditions affect tower performance. They do not replace the heat balance calculation. However, they influence the approach temperature and achievable cooling.
Using the Heat Balance
The engineering method calculates heat from water flow, density, specific heat, and temperature range. That heat is divided by the latent heat of vaporization. The result is the theoretical evaporation mass. Converting mass to volume uses water density. This method is useful when hot water and cold water temperatures are measured. It provides an estimate. It adapts to different water properties or operating temperatures.
Practical Plant Inputs
Some facilities know heat rejection directly from equipment data. The heat load method is useful in that case. It applies a latent heat fraction because a small part of tower heat rejection is sensible. The rule of thumb method is helpful for quick checks. It uses a site factor multiplied by circulation and range. The default factor should be reviewed against plant records. Local climate, fan operation, and tower design can shift performance.
Water Losses Beyond Evaporation
Evaporation is not the only makeup water demand. Drift carries small droplets out with exhaust air. Eliminators keep drift low. Blowdown removes concentrated dissolved solids. Its rate rises when cycles of concentration are low. The calculator estimates blowdown as evaporation divided by cycles minus one. Total makeup equals adjusted evaporation, drift, and blowdown. A safety margin can cover uncertainty during early design or incomplete measurement.
Interpreting Results Carefully
The result is an operating estimate, not a guaranteed meter reading. Compare it with makeup meter data over matching periods. Check whether periods include startup, basin overflow, leaks, filter backwash, or chemical feed water. Use average values for planning. Use logged values for troubleshooting. Sudden changes may indicate sensor errors, a valve issue, altered cycles, or unexpected heat load. Trend evaporation, blowdown, conductivity, and makeup together.
Good Operating Practice
Keep flow meters and temperature sensors calibrated. Record hot and cold water temperatures at stable load. Confirm the circulation basis before entering a value. Use consistent units. Review latent heat when water temperature differs greatly from the default. Set realistic drift and cycles values. Increase the safety margin only when justified. Conservative margins can hide losses that deserve investigation. Regular water accounting helps protect equipment, budgets, and supply resilience each operating day.
Frequently Asked Questions
What does the evaporation rate represent?
It represents water converted to vapor while rejecting heat. The value is normally expressed in cubic meters per hour, liters per hour, or gallons per minute.
Which method should I select?
Use the heat balance method when measured flow and temperatures are reliable. Use known heat rejection when equipment thermal duty is available. Use the rule of thumb method for fast preliminary checks.
Why are hot and cold water temperatures needed?
Their difference is the tower range. Range indicates how much heat the circulating water loses. A larger range generally produces a larger evaporation estimate at the same flow.
Does humidity change the calculated evaporation rate?
Humidity affects tower capability and approach temperature. The heat balance result still follows the heat rejected. Compare calculated values with meters because weather and operation can change actual losses.
Which latent heat value is appropriate?
Use a value near 2,400 kJ/kg for typical warm tower water. Adjust it when operating temperatures differ substantially. Higher water temperatures slightly reduce latent heat.
Is drift included in evaporation?
No. Drift is liquid droplets leaving with exhaust air. Evaporation is vapor. Both require makeup water, but they should remain separate in water accounting.
Why does the calculator estimate blowdown?
Blowdown controls dissolved-solid concentration. It removes a portion of circulating water. The required amount depends mainly on evaporation and cycles of concentration.
What are cycles of concentration?
They compare dissolved solids in recirculating water with makeup water. Higher cycles reduce blowdown. Excessive cycles can increase scale, corrosion, or biological control challenges.
Is the practical factor exact?
No. It is a screening estimate. Validate the factor with site measurements, actual water chemistry, tower design, and seasonal operating records.
When should the estimate be checked?
Check it after load changes, water-treatment adjustments, meter repairs, or seasonal shifts. Review monthly trends even when the system appears stable.
Why compare the estimate with makeup meter data?
Comparisons reveal leaks, overflow, inaccurate instruments, or incorrect assumptions. Reliable evaporation estimates improve water control, costs, and resilience.