* Warning: Gasoline vapors are highly flammable. This estimation uses the Mackay & Matsugu mass transfer model for a non-boiling pool. Actual rates vary by gasoline blend.
function calculateEvaporation() {
var area = parseFloat(document.getElementById("surfaceArea").value);
var wind = parseFloat(document.getElementById("windSpeed").value);
var tempC = parseFloat(document.getElementById("temperature").value);
if (isNaN(area) || isNaN(wind) || isNaN(tempC) || area <= 0 || wind < 0) {
alert("Please enter valid positive numbers for Area and Wind Speed.");
return;
}
// Constants and Physics
var tempK = tempC + 273.15;
var R = 8.314; // Ideal gas constant J/(mol·K)
var MW = 0.100; // Molecular Weight of Gasoline approx 100 g/mol (0.1 kg/mol)
var density = 0.74; // Average density of gasoline kg/L
// 1. Calculate Vapor Pressure (P_vap) in Pascals
// Approximation: At 20°C (293K), Pvap is approx 55,000 Pa (55 kPa).
// Using a simplified exponential correlation for Gasoline range:
// P(Pa) = 55000 * exp(0.035 * (T_C – 20))
var pVapPa = 55000 * Math.exp(0.035 * (tempC – 20));
// 2. Calculate Mass Transfer Coefficient (K) in m/s
// Using Mackay & Matsugu (1973) correlation for pool evaporation:
// K = 0.00482 * U^0.78 (where U is wind speed at 10m height)
// Adjusted slightly for surface level winds often used in spill modeling:
// K ~ 0.002 * U^0.78
var effectiveWind = Math.max(wind, 0.1); // Prevent 0 division/stagnation issues
var K = 0.002 * Math.pow(effectiveWind, 0.78);
// 3. Calculate Evaporation Flux (E) in kg/s
// Formula: E = (K * A * MW * P_vap) / (R * T)
var rateKgPerSecond = (K * area * MW * pVapPa) / (R * tempK);
// 4. Convert Units
var rateKgPerHour = rateKgPerSecond * 3600;
var rateLitersPerHour = rateKgPerHour / density;
var pVapKPa = pVapPa / 1000;
// Display Results
document.getElementById("massRate").innerText = rateKgPerHour.toFixed(2) + " kg/hr";
document.getElementById("volRate").innerText = rateLitersPerHour.toFixed(2) + " L/hr";
document.getElementById("vapPress").innerText = pVapKPa.toFixed(1) + " kPa";
document.getElementById("resultOutput").style.display = "block";
}
Understanding Gasoline Evaporation Physics
The rate at which gasoline evaporates from a spill is a critical factor in environmental safety and fire hazard assessment. Unlike simple water evaporation, gasoline is a complex mixture of hydrocarbons, making its volatility highly sensitive to temperature and airflow. This calculator estimates the evaporation rate using mass transfer principles derived from environmental engineering models.
Key Safety Note: Gasoline vapors are heavier than air and can travel along the ground to ignition sources. A rapid evaporation rate increases the concentration of flammable vapors in the immediate vicinity.
Variables Affecting Evaporation
Surface Area (A): The most significant factor. If a gallon of gas spills into a thin puddle covering 5 square meters, it will evaporate much faster than if it is contained in a bucket with a 0.1 square meter opening. The calculator uses a linear relationship between area and rate.
Wind Speed (U): Air movement strips away the saturated vapor layer directly above the liquid, allowing more liquid to vaporize. This is modeled using the mass transfer coefficient ($K$), where higher wind speeds exponentially increase evaporation.
Temperature (T): Heat increases the internal energy of the liquid molecules, raising the Vapor Pressure. As shown in the calculator results, even a 10°C increase in temperature can significantly boost the evaporation rate due to the exponential nature of vapor pressure.
The Calculation Model
This tool utilizes a simplified version of the Stiver and Mackay evaporation model, often used in hazardous chemical spill modeling (like the EPA's ALOHA software). The core logic determines the mass flux based on:
$$ E = \frac{K \cdot A \cdot M \cdot P}{R \cdot T} $$
Where $P$ is the vapor pressure of the gasoline, $M$ is the molecular weight, and $R$ is the gas constant. The calculator assumes an average molecular weight of 100 g/mol and a vapor pressure curve typical for summer-grade gasoline.
Why This Matters
Professionals use these calculations to determine the "source term" for dispersion modeling. Knowing the rate of evaporation helps in:
Estimating the duration of a flammable atmosphere.
Calculating the necessary evacuation distance downwind.
Determining the urgency of applying foam suppressants to a spill.