function updateLabels() {
var radios = document.getElementsByName('ct_units');
var unit = 'imperial';
for (var i = 0; i < radios.length; i++) {
if (radios[i].checked) {
unit = radios[i].value;
break;
}
}
if (unit === 'imperial') {
document.getElementById('label_flow').innerText = "Recirculation Flow Rate (GPM)";
document.getElementById('label_temp_in').innerText = "Hot Water Inlet Temperature (°F)";
document.getElementById('label_temp_out').innerText = "Cold Water Outlet Temperature (°F)";
document.getElementById('ct_flow').placeholder = "e.g., 1000";
document.getElementById('ct_temp_in').placeholder = "e.g., 95";
document.getElementById('ct_temp_out').placeholder = "e.g., 85";
} else {
document.getElementById('label_flow').innerText = "Recirculation Flow Rate (m³/h)";
document.getElementById('label_temp_in').innerText = "Hot Water Inlet Temperature (°C)";
document.getElementById('label_temp_out').innerText = "Cold Water Outlet Temperature (°C)";
document.getElementById('ct_flow').placeholder = "e.g., 200";
document.getElementById('ct_temp_in').placeholder = "e.g., 35";
document.getElementById('ct_temp_out').placeholder = "e.g., 30";
}
// Hide results when unit changes to avoid confusion
document.getElementById('ct_result').style.display = 'none';
}
function calculateEvaporation() {
// Get Inputs
var flow = parseFloat(document.getElementById('ct_flow').value);
var tIn = parseFloat(document.getElementById('ct_temp_in').value);
var tOut = parseFloat(document.getElementById('ct_temp_out').value);
// Determine Unit
var radios = document.getElementsByName('ct_units');
var unit = 'imperial';
for (var i = 0; i < radios.length; i++) {
if (radios[i].checked) {
unit = radios[i].value;
break;
}
}
// Validation
if (isNaN(flow) || isNaN(tIn) || isNaN(tOut)) {
alert("Please enter valid numerical values for all fields.");
return;
}
if (flow <= 0) {
alert("Flow rate must be greater than zero.");
return;
}
// Logic
var range = tIn – tOut;
if (range <= 0) {
alert("Inlet temperature must be higher than outlet temperature (positive range required).");
return;
}
var evaporation = 0;
var unitText = "";
var unitTemp = "";
// Constants
// Imperial: E (GPM) = Flow (GPM) * Range (F) * 0.0008
// Metric: E (m3/h) = Flow (m3/h) * Range (C) * 0.00144
// Note: 0.0008 factor assumes ~1000 BTU/lb latent heat.
// Metric derivation: 0.0008 * 1.8 (F to C conv) = 0.00144.
if (unit === 'imperial') {
evaporation = flow * range * 0.0008;
unitText = " GPM";
unitTemp = "°F";
} else {
evaporation = flow * range * 0.00144;
unitText = " m³/h";
unitTemp = "°C";
}
// Calculate Daily Loss (24 hours operation)
var dailyLoss = evaporation * 24;
var dailyUnit = (unit === 'imperial') ? " Gallons/Day" : " m³/Day";
// For Gallons/Day, we need to convert GPM * 60 * 24
if (unit === 'imperial') {
dailyLoss = evaporation * 60 * 24;
}
// Display Results
document.getElementById('ct_result').style.display = 'block';
document.getElementById('unit_range_res').innerText = unitTemp;
document.getElementById('res_range').innerText = range.toFixed(1);
document.getElementById('res_evap').innerText = evaporation.toFixed(2) + unitText;
document.getElementById('res_daily').innerText = Math.round(dailyLoss).toLocaleString() + dailyUnit;
}
Understanding Cooling Tower Evaporation
In any open recirculating cooling tower system, evaporation is the primary method of heat rejection. As water evaporates, it removes the latent heat of vaporization from the remaining water, thereby cooling it down. Accurately calculating the evaporation rate is critical for determining make-up water requirements, sizing pumps, and managing chemical treatment programs.
The Evaporation Rate Formula
The industry-standard rule of thumb for estimating cooling tower evaporation is that for every 10°F of cooling (Range) achieved, approximately 1% of the recirculation flow rate is evaporated. However, for more precise calculations, we use specific thermodynamic constants.
Imperial Formula (US): E = R × ΔT × 0.0008
Metric Formula (SI): E = R × ΔT × 0.00144
Where:
E = Evaporation Rate (GPM or m³/h)
R = Recirculation Flow Rate (GPM or m³/h)
ΔT (Range) = Difference between Hot Inlet and Cold Outlet temperatures
0.0008 = Evaporation Factor (Imperial). Derived from 1 / (Latent Heat of Vaporization ≈ 1000 Btu/lb) × Specific Heat of Water (1 Btu/lb·°F). Roughly 0.08% loss per 1°F.
0.00144 = Evaporation Factor (Metric). Derived from converting the Imperial factor for Celsius units (0.0008 × 1.8).
Why is this calculation important?
1. Water Budgeting: Knowing your evaporation rate helps facility managers predict water costs. Since evaporation represents pure water loss, it must be continuously replaced by fresh "make-up" water.
2. Chemical Treatment: As pure water evaporates, dissolved solids (minerals) remain behind and concentrate in the basin. This increases the "Cycles of Concentration." Chemical inhibitors are dosed based on the volume of fresh water added, which is directly driven by evaporation and bleed-off rates.
3. Bleed-off Sizing: To prevent mineral scaling, a portion of the concentrated water must be drained (bleed-off). The amount of bleed-off required is a function of the evaporation rate and the target cycles of concentration.
Example Calculation
Consider a cooling tower with a recirculation rate of 1,000 GPM. The water enters the tower at 95°F and leaves at 85°F.
Flow (R): 1,000 GPM
Range (ΔT): 95°F – 85°F = 10°F
Calculation: 1,000 × 10 × 0.0008 = 8 GPM
In this scenario, the cooling tower loses 8 gallons of water every minute strictly due to evaporation. This totals 11,520 gallons of water per day.