The Water Condensation Rate Calculator is an essential tool for HVAC engineers, facility managers, and technicians. It estimates the amount of water extracted from the air when it passes through a cooling coil or dehumidifier. Calculating this rate is crucial for sizing condensate drain lines, pumps, and understanding the latent cooling capacity of a system.
How It Works
Condensation occurs when air is cooled below its dew point temperature. As the air cools, its capacity to hold water vapor decreases. The excess moisture turns from a vapor into liquid water (condensate).
This calculator determines the humidity ratio (specific humidity) of the air entering the system and the air leaving the system. The difference between these two values represents the amount of water removed per pound of dry air.
The Calculation Formula
The standard HVAC industry formula for calculating the condensation rate in Imperial units is:
Condensate (lbs/hr) = 4.5 × CFM × (W_in – W_out)
Where:
CFM: Airflow rate in Cubic Feet per Minute.
W_in: Humidity ratio of entering air (lb of water / lb of dry air).
W_out: Humidity ratio of leaving air (lb of water / lb of dry air).
4.5: A constant derived from the density of air (approx. 0.075 lb/ft³) and time conversion (60 min/hr).
Why CFM Matters
The volume of air moving through the system acts as the multiplier. Even a small drop in humidity can generate significant water volume if the airflow (CFM) is high enough.
Temperature Drop
The "Outlet Temperature" must be lower than the entering air's dew point for condensation to occur. If the coil isn't cold enough, sensible cooling happens without dehumidification.
Drainage Sizing
Knowing the gallons per hour generated helps in sizing condensate pumps and drain traps correctly to prevent overflow and water damage.
Input Parameters Explained
Inlet Temperature & RH: The condition of the air before it hits the cooling coil (e.g., return air from a room or hot outdoor air).
Outlet Temperature: The temperature of the air immediately after leaving the coil. This is typically 50°F – 55°F for standard air conditioning.
Outlet RH: The relative humidity of the air leaving the coil. In most cooling applications involving condensation, the air leaves near saturation (90% – 100% RH).
function calculateCondensation() {
// 1. Get Inputs
var cfm = parseFloat(document.getElementById('airflow').value);
var tInF = parseFloat(document.getElementById('tempIn').value);
var rhIn = parseFloat(document.getElementById('rhIn').value);
var tOutF = parseFloat(document.getElementById('tempOut').value);
var rhOut = parseFloat(document.getElementById('rhOut').value);
// 2. Validation
if (isNaN(cfm) || isNaN(tInF) || isNaN(rhIn) || isNaN(tOutF) || isNaN(rhOut)) {
alert("Please enter valid numerical values for all fields.");
return;
}
if (rhIn 100 || rhOut 100) {
alert("Relative Humidity must be between 0 and 100.");
return;
}
// 3. Helper Functions for Psychrometrics
// Convert F to C
function fToC(f) {
return (f – 32) * 5 / 9;
}
// Calculate Saturation Vapor Pressure (Tetens Equation) in hPa (mbar)
function getSatVaporPressure(tempC) {
return 6.1078 * Math.pow(10, (7.5 * tempC) / (tempC + 237.3));
}
// Calculate Humidity Ratio (W) in lb_water / lb_dry_air
// P_atm is assumed standard sea level pressure 1013.25 hPa
function getHumidityRatio(tempF, rhPercent) {
var tempC = fToC(tempF);
var Ps = getSatVaporPressure(tempC); // Saturation Pressure
var Pv = Ps * (rhPercent / 100); // Partial Vapor Pressure
var Patm = 1013.25; // Standard atmospheric pressure in hPa
// Formula: W = 0.62198 * Pv / (Patm – Pv)
// Result is kg/kg which is equivalent to lb/lb
var W = 0.62198 * Pv / (Patm – Pv);
return W;
}
// 4. Calculate Humidity Ratios (W)
var W_in = getHumidityRatio(tInF, rhIn);
var W_out = getHumidityRatio(tOutF, rhOut);
// 5. Calculate Delta W
// If W_out > W_in, physically condensation is not happening (it would be humidification),
// so we clamp to 0 for condensation rate.
var deltaW = W_in – W_out;
if (deltaW < 0) deltaW = 0;
// 6. Calculate Condensation Rate
// Formula: lbs/hr = 4.5 * CFM * deltaW
var lbsPerHour = 4.5 * cfm * deltaW;
// Conversions
// 1 gallon of water approx 8.34 lbs
var galPerHour = lbsPerHour / 8.34;
var galPerDay = galPerHour * 24;
// Grains per pound conversion (1 lb = 7000 grains)
var grainsRemoved = deltaW * 7000;
// 7. Update UI
document.getElementById('resLbs').innerText = lbsPerHour.toFixed(2) + " lbs/hr";
document.getElementById('resGal').innerText = galPerHour.toFixed(2) + " gal/hr";
document.getElementById('resGalDay').innerText = galPerDay.toFixed(1) + " gal/day";
document.getElementById('resGrains').innerText = grainsRemoved.toFixed(1) + " gr/lb";
document.getElementById('results').style.display = "block";
}