Evaporation Rate Calculator

The calculation is rooted in mass transfer principles driven by aerodynamic and thermodynamic forces. The fundamental mass transfer equation for evaporation is expressed as:

Evaporation Rate = (25 + 19 × v) × A × (ωmax - ωcurr)

Where v represents the air velocity passing over the surface in meters per second, A is the exposed surface area in square meters, and (ωmax - ωcurr) signifies the difference between the maximum saturation humidity ratio at the water temperature and the current ambient humidity ratio. When direct relative humidity and temperature are utilized instead of absolute humidity ratios, the saturation humidity is approximated using a temperature polynomial:

ωmax(T) = 0.003733 + 0.00032T + 0.000003T2 + 4×10-7T3

Multiplying these terms together accounts for both forced convection caused by wind speed and the vapor pressure deficit that pulls moisture molecules out of the liquid phase.

Let us walk through a practical scenario to see how the formula operates. Imagine an indoor industrial basin with a water surface area of 10 square meters. The ambient air is moving across the surface at a velocity of 2 meters per second. Suppose the maximum saturation humidity ratio at the water's temperature is 0.025 kg/kg, and the current ambient humidity ratio is 0.015 kg/kg.

Step 1: Calculate the aerodynamic wind factor.
Wind Factor = 25 + (19 × 2) = 25 + 38 = 63

Step 2: Determine the humidity ratio difference (vapor pressure deficit).y
Humidity Difference = 0.025 - 0.015 = 0.010 kg/kg

Step 3: Multiply by the surface area and combine all parameters.
Evaporation Rate = 63 × 10 × 0.010 = 6.3 kg/hour

Thus, under these specific conditions, water will evaporate from the basin at a rate of 6.3 kilograms per hour.

To ensure your evaporation calculations remain accurate and to manage moisture effectively in real-world applications, keep these guidelines in mind:

1. Account for Air Turbulence: Even minor increases in wind speed or indoor HVAC airflow dramatically accelerate evaporation because it strips away the microscopic layer of saturated air hovering just above the liquid surface.

2. Measure Surface Area Accurately: Always use the true exposed two-dimensional surface area of the liquid, rather than the total volume or container footprint, as boundary geometries heavily influence mass transfer.

3. Monitor Humidity Fluctuations: Ambient humidity changes throughout the day. Recalculate your rates during peak humidity (typically early morning) and lowest humidity (typically mid-afternoon) to find realistic daily averages.