How to Calculate Evaporation Rate of a Liquid

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Evaporation Rate Calculator

Results:

Evaporation Rate: 0.00 kg/hour

Equivalent to: 0.00 Liters/hour

Daily Evaporation: 0.00 Liters/day

function calculateEvaporation() { var A = parseFloat(document.getElementById('surfaceArea').value); var Tw = parseFloat(document.getElementById('waterTemp').value); var Ta = parseFloat(document.getElementById('airTemp').value); var RH = parseFloat(document.getElementById('humidity').value); var v = parseFloat(document.getElementById('windSpeed').value); if (isNaN(A) || isNaN(Tw) || isNaN(Ta) || isNaN(RH) || isNaN(v)) { alert("Please enter valid numerical values."); return; } // Magnus Formula for Saturation Vapor Pressure (hPa) function getVaporPressure(temp) { return 6.112 * Math.exp((17.67 * temp) / (temp + 243.5)); } var Ps = getVaporPressure(Tw); // Saturation vapor pressure at water temp var P_sat_air = getVaporPressure(Ta); // Saturation vapor pressure at air temp var Pa = (RH / 100) * P_sat_air; // Actual vapor pressure of air // Using the Penman-style mass transfer equation for open water // E = (a + b * v) * A * (Ps – Pa) // Constants a and b for kg/(h*m^2*hPa) var a = 0.0175; var b = 0.024; var evaporationRate = (a + (b * v)) * A * (Ps – Pa); // If Pa > Ps, condensation occurs technically, but we treat it as 0 for simple evaporation if (evaporationRate < 0) evaporationRate = 0; document.getElementById('rateKgH').innerText = evaporationRate.toFixed(4); document.getElementById('rateLH').innerText = evaporationRate.toFixed(4); // 1kg water ~ 1L document.getElementById('rateDay').innerText = (evaporationRate * 24).toFixed(2); document.getElementById('evapResult').style.display = 'block'; }

Understanding Evaporation Rate Calculation

Evaporation is the process by which a liquid turns into a gas at its surface before reaching its boiling point. Calculating the evaporation rate is critical in fields such as civil engineering (reservoir management), chemical processing, and HVAC design.

The Physics Behind the Formula

The rate of evaporation depends on the difference in vapor pressure between the liquid surface and the surrounding air. This is often modeled using a variation of Dalton's Law. The primary drivers are:

  • Vapor Pressure Gradient: The higher the liquid temperature, the higher the saturation vapor pressure at the surface. If the air is dry (low humidity), the gradient is steeper, leading to faster evaporation.
  • Surface Area: Evaporation is a surface phenomenon. Doubling the surface area effectively doubles the evaporation rate.
  • Wind Speed: Air movement replaces the saturated air layer immediately above the liquid with drier air, maintaining a high concentration gradient.

How to Use This Calculator

To get an accurate estimate, you need five specific data points:

  1. Surface Area: The total area of the liquid exposed to air in square meters.
  2. Water Temperature: The temperature of the liquid at its surface.
  3. Air Temperature: The ambient temperature of the surrounding air.
  4. Relative Humidity: The current moisture content of the air as a percentage.
  5. Wind Speed: The velocity of the air moving across the surface in meters per second.

Practical Example

Imagine a swimming pool with a surface area of 32 m². The water temperature is 28°C, the air temperature is 30°C, humidity is 40%, and there is a light breeze of 2 m/s.

  • Ps (at 28°C): ~37.8 hPa
  • Pa (at 30°C, 40% RH): ~17.0 hPa
  • Evaporation Rate: Using the formula, this would result in approximately 2.15 kg/hour, or roughly 51.6 liters of water lost per day.

Factors That Increase Evaporation

If you need to reduce evaporation (e.g., in a pool or industrial tank), you should focus on covering the surface or reducing the temperature. Conversely, to increase evaporation (e.g., in a salt pond or drying process), you should maximize surface area and increase air circulation.

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