function calculateEvaporation() {
// 1. Get Input Values
var tLiq = parseFloat(document.getElementById("liquidTemp").value);
var tAir = parseFloat(document.getElementById("airTemp").value);
var rh = parseFloat(document.getElementById("relHumidity").value);
var velocity = parseFloat(document.getElementById("airVelocity").value);
var area = parseFloat(document.getElementById("surfaceArea").value);
// 2. Validation
if (isNaN(tLiq) || isNaN(tAir) || isNaN(rh) || isNaN(velocity) || isNaN(area)) {
alert("Please enter valid numbers for all fields.");
return;
}
if (area <= 0) {
alert("Surface area must be greater than 0.");
return;
}
// 3. Physics Constants
// Standard atmospheric pressure (kPa)
var P_atm = 101.325;
// 4. Calculate Saturation Vapor Pressure (Tetens Equation)
// Formula: P = 0.61078 * exp((17.27 * T) / (T + 237.3)) (Result in kPa)
// Saturation vapor pressure at liquid temperature
var Psat_liq = 0.61078 * Math.exp((17.27 * tLiq) / (tLiq + 237.3));
// Saturation vapor pressure at air temperature
var Psat_air = 0.61078 * Math.exp((17.27 * tAir) / (tAir + 237.3));
// Actual vapor pressure in air based on Relative Humidity
var P_air_actual = Psat_air * (rh / 100);
// 5. Calculate Humidity Ratios (Mixing Ratio) – kg water / kg dry air
// x = 0.622 * P_vapor / (P_atm – P_vapor)
var x_s = 0.622 * Psat_liq / (P_atm – Psat_liq); // Saturation humidity ratio at water surface
var x_a = 0.622 * P_air_actual / (P_atm – P_air_actual); // Actual humidity ratio of air
// 6. Calculate Evaporation Rate
// Using standard engineering mass transfer formula:
// m (kg/h) = (25 + 19 * v) * A * (x_s – x_a)
// This is a common empirical relation for water surfaces.
var evaporationFlux = (25 + 19 * velocity) * (x_s – x_a);
var totalEvaporationKgPerHour = evaporationFlux * area;
// Handle Condensation (Negative Evaporation)
var label = "Evaporation";
if (totalEvaporationKgPerHour < 0) {
totalEvaporationKgPerHour = Math.abs(totalEvaporationKgPerHour);
label = "Condensation";
}
// Convert to Liters (assuming water density approx 1 kg/L)
var totalLitersPerHour = totalEvaporationKgPerHour;
var totalLitersPerDay = totalLitersPerHour * 24;
// 7. Update UI
var displayMass = totalEvaporationKgPerHour.toFixed(3) + " kg/hr";
var displayVol = totalLitersPerDay.toFixed(2) + " Liters/day";
if (label === "Condensation") {
displayMass += " (Gain)";
displayVol += " (Gain)";
document.getElementById("resMassRate").style.color = "#dc3545"; // Red for condensation warning
} else {
document.getElementById("resMassRate").style.color = "#28a745"; // Green for evaporation
}
document.getElementById("resMassRate").innerHTML = displayMass;
document.getElementById("resVolRate").innerHTML = displayVol;
document.getElementById("resVpLiq").innerHTML = Psat_liq.toFixed(3) + " kPa";
document.getElementById("resVpAir").innerHTML = P_air_actual.toFixed(3) + " kPa";
document.getElementById("result-area").style.display = "block";
}
How to Calculate Rate of Evaporation of Liquid
Calculating the rate of evaporation is essential for various industries, from HVAC engineering and swimming pool maintenance to chemical processing and environmental science. Evaporation occurs when liquid molecules gain enough kinetic energy to escape the surface and become vapor. This calculator uses standard mass transfer equations to estimate how quickly a liquid (specifically water) will evaporate under given conditions.
Key Factors Influencing Evaporation
The rate at which a liquid evaporates is not static; it fluctuates based on the dynamic relationship between the liquid and the surrounding environment. The primary variables include:
Liquid Temperature: Higher temperatures increase the kinetic energy of molecules, raising the saturation vapor pressure at the surface and accelerating evaporation.
Air Temperature & Humidity: These factors determine the vapor pressure of the surrounding air. Dry air (low humidity) can accept more moisture than saturated air, promoting faster evaporation.
Air Velocity (Wind Speed): Air moving across the liquid surface removes the boundary layer of saturated vapor, replacing it with drier air and maintaining a high evaporation gradient.
Surface Area: Evaporation is a surface phenomenon. Doubling the surface area generally doubles the total evaporation rate.
The Calculation Logic
While there are complex thermodynamic formulas for precise laboratory conditions, engineering applications often use empirical mass transfer formulas. This calculator utilizes a variation of the Dalton equation methodology, often expressed as:
Evaporation Rate = Θ × A × (xs – x)
Where:
Θ (Theta): An evaporation coefficient that changes based on air velocity (e.g., 25 + 19v).
A: The surface area of the liquid pool.
xs: The humidity ratio (kg water/kg dry air) of saturated air at the liquid's surface temperature.
x: The actual humidity ratio of the ambient air.
Interpreting the Results
Positive Rate (Evaporation): Indicates the liquid is losing mass to the air. This is the standard operational mode for cooling towers or drying pools. Negative Rate (Condensation): If the ambient air is warmer and more humid than the liquid surface (i.e., the dew point of the air is higher than the liquid temperature), moisture will condense into the liquid rather than evaporate from it. This calculator flags this occurrence as "Condensation."
Practical Applications
Understanding these rates helps in determining makeup water requirements for swimming pools, estimating solvent loss in open tanks, and designing humidification or dehumidification systems for indoor environments.