Water Evaporation Rate Calculator
Evaporation Results
How to Calculate Water Evaporation Rate
Calculating the rate at which water evaporates from a surface is a critical task in various fields, from maintaining swimming pools to managing industrial cooling towers and reservoirs. Evaporation is driven by the energy difference between the water molecules and the surrounding air, heavily influenced by humidity and airflow.
This calculator utilizes a standard engineering approach derived from the Dalton-type mass transfer equations, often used in HVAC and environmental physics. It considers the vapor pressure differential and the mass transfer coefficient driven by air velocity.
The Physics Behind the Calculation
Evaporation occurs when the partial pressure of water vapor at the water's surface is higher than the partial pressure of water vapor in the surrounding air. The rate is governed by three main factors:
- Vapor Pressure Differential: The difference between saturated air at the water surface and the actual moisture content of the ambient air.
- Air Velocity: Moving air removes the boundary layer of saturated air sitting just above the water, accelerating evaporation.
- Surface Area: The larger the exposed area, the greater the total volume of water lost.
The Formula
While there are several empirical formulas (like Stower's or Carrier's), a robust method involves calculating the difference in Humidity Ratio (kg of water / kg of dry air). The simplified logic used here is:
Where:
- V = Air velocity over the surface (m/s)
- A = Surface Area (m²)
- W_s = Humidity ratio of saturated air at water temperature
- W_a = Humidity ratio of the ambient air
Understanding the Inputs
Water Temperature: Warmer water has higher kinetic energy, leading to a higher saturation vapor pressure and faster evaporation.
Relative Humidity: Higher humidity in the air reduces the capacity of the air to absorb more water, slowing down the evaporation rate.
Air Velocity: Even a slight breeze can significantly increase evaporation by stripping away the moist air layer directly above the water.
Practical Applications
Swimming Pools: Uncovered pools can lose significant amounts of water and heat energy. A pool cover eliminates air velocity at the surface and traps humidity, stopping evaporation.
Aquariums: Understanding evaporation helps in planning auto-top-off (ATO) system requirements to maintain salinity stability in saltwater tanks.