The chilled water flow rate is a critical parameter in HVAC (Heating, Ventilation, and Air Conditioning) systems. It determines how much chilled water needs to circulate through a building's cooling coils to meet the cooling demand.
The Formula:
The flow rate (Q) of chilled water is calculated using the following formula:
Q = P / (ρ * c_p * ΔT)
Where:
Q is the flow rate (typically in m³/s or L/s).
P is the cooling load (in Watts).
ρ (rho) is the density of water (approximately 1000 kg/m³).
c_p is the specific heat capacity of water (approximately 4186 J/kg·°C).
ΔT (delta T) is the temperature difference between the supply and return chilled water (in °C).
A simplified version, commonly used in engineering when dealing with Watts and °C, is:
This simplified formula assumes standard water properties and a common temperature difference. The factor 4.186 is derived from 1000 (kg/m³) * 4186 (J/kg·°C) / 3600 (s/hr) * (1000 L/m³), then simplified to 4.186 for kW and L/s.
Why is it Important?
An accurately calculated chilled water flow rate ensures that your cooling system operates efficiently. Too little flow can lead to insufficient cooling and potential system strain, while too much flow can result in wasted energy and reduced dehumidification effectiveness.
Example Calculation:
Let's say a building zone requires a cooling load of 50 kW, and the chilled water system is designed for a temperature difference (ΔT) of 5.5 °C. Using the simplified formula:
Flow Rate (L/s) = 50 kW / (4.186 * 5.5 °C)
Flow Rate (L/s) = 50 / 23.023
Flow Rate (L/s) ≈ 2.17 L/s
This means approximately 2.17 liters of chilled water per second must circulate through the system to meet the 50 kW cooling demand with a 5.5 °C temperature difference.
function calculateFlowRate() {
var coolingLoad = parseFloat(document.getElementById("coolingLoad").value);
var deltaT = parseFloat(document.getElementById("deltaT").value);
var resultDiv = document.getElementById("result");
resultDiv.innerHTML = ""; // Clear previous result
if (isNaN(coolingLoad) || isNaN(deltaT)) {
resultDiv.innerHTML = "Please enter valid numbers for all fields.";
return;
}
if (deltaT <= 0) {
resultDiv.innerHTML = "Temperature difference must be greater than zero.";
return;
}
// Simplified formula: Flow Rate (L/s) = Cooling Load (kW) / (4.186 * Temperature Difference (°C))
var flowRateLps = coolingLoad / (4.186 * deltaT);
// Convert L/s to GPM (Gallons Per Minute) for common reference
// 1 L/s = 15.8503 GPM
var flowRateGpm = flowRateLps * 15.8503;
resultDiv.innerHTML = "