Enter total heat to be removed (Note: 1 Tower Ton ≈ 15,000 BTU/hr)
°F
°F
Please enter valid inputs. Inlet Temp must be higher than Outlet Temp.
Temperature Range (ΔT):–
Fluid Factor:500
Required Flow Rate:–
function calculateRecirculationRate() {
// Get input values using var
var heatLoadInput = document.getElementById('heatLoad').value;
var inletTempInput = document.getElementById('inletTemp').value;
var outletTempInput = document.getElementById('outletTemp').value;
var errorMsg = document.getElementById('errorMessage');
var resultBox = document.getElementById('resultBox');
// Reset display
errorMsg.style.display = 'none';
resultBox.style.display = 'none';
// Parse floats
var heatLoad = parseFloat(heatLoadInput);
var inletTemp = parseFloat(inletTempInput);
var outletTemp = parseFloat(outletTempInput);
// Validation
if (isNaN(heatLoad) || isNaN(inletTemp) || isNaN(outletTemp)) {
errorMsg.innerText = "Please fill in all fields with valid numbers.";
errorMsg.style.display = 'block';
return;
}
if (inletTemp <= outletTemp) {
errorMsg.innerText = "Inlet temperature must be greater than outlet temperature to calculate cooling range.";
errorMsg.style.display = 'block';
return;
}
if (heatLoad <= 0) {
errorMsg.innerText = "Heat load must be greater than zero.";
errorMsg.style.display = 'block';
return;
}
// Calculation Logic
// Formula: GPM = Heat Load / (500 * Delta T)
var range = inletTemp – outletTemp;
var constant = 500; // 8.33 lbs/gal * 60 min/hr * 1.0 specific heat
var gpm = heatLoad / (constant * range);
// Display Results
document.getElementById('resRange').innerText = range.toFixed(1) + " °F";
document.getElementById('resGPM').innerText = gpm.toFixed(2) + " GPM";
resultBox.style.display = 'block';
}
Calculating Cooling Tower Recirculation Rates
The recirculation rate, or water flow rate, is a critical parameter in the design and operation of cooling tower systems. It represents the volume of water circulating through the system per minute required to reject a specific amount of heat. Understanding this rate ensures that pumps are sized correctly and that the cooling tower operates at peak thermal efficiency.
The Recirculation Rate Formula
To determine the required flow rate in Gallons Per Minute (GPM), engineers use a fundamental thermodynamic equation derived from the specific heat capacity of water. The standard formula used in the HVAC and industrial cooling industry is:
GPM = Q / (500 × ΔT)
Where:
GPM: The recirculation rate in Gallons Per Minute.
Q: The Total Heat Load in BTU/hr (British Thermal Units per hour).
ΔT (Range): The difference between the water temperature entering the tower (Hot Water) and leaving the tower (Cold Water) in °F.
500: A "fluid factor" constant derived from: 8.33 lbs/gal (weight of water) × 60 min/hr × 1.0 (specific heat of water).
Understanding the Inputs
1. Heat Load (BTU/hr)
This is the amount of thermal energy the system needs to reject to the atmosphere. If you are working with tons of cooling, you can convert it. While a standard chiller ton is 12,000 BTU/hr, cooling towers are typically sized at 15,000 BTU/hr per ton to account for the heat of compression added by the chiller.
2. Cooling Range (ΔT)
The Range is simply the temperature drop across the tower. Standard HVAC design conditions often utilize a 10°F range (e.g., entering at 95°F and leaving at 85°F), though industrial processes may require significantly larger ranges.
Calculation Example
Consider a facility that needs to reject 3,000,000 BTU/hr of heat. The system is designed to take water in at 100°F and cool it down to 85°F.
Heat Load (Q): 3,000,000 BTU/hr
Range (ΔT): 100°F – 85°F = 15°F
Calculation: 3,000,000 / (500 × 15)
Denominator: 7,500
Result: 400 GPM
In this scenario, you would need a pump capable of moving 400 gallons of water per minute through the cooling loop to maintain the desired temperatures.
Why Accurate Calculation Matters
If the recirculation rate is underestimated, the water will not carry enough heat away from the source, leading to overheating equipment and system trips. Conversely, overestimating the flow rate can lead to oversized pumps, increased energy consumption, and water velocity issues that may damage piping or cooling tower fill media.