How to Calculate Air Flow Rate in Cooling Tower

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Cooling Tower Air Flow Rate Calculator

(Avg. 60-80 kJ/kg for 20-25°C WB)
(Typically 100-140 kJ/kg)

Calculation Results

Heat Load (Q): kW

Mass Air Flow Rate (G): kg/s

Volume Air Flow Rate: m³/hr

Air Flow in CFM: ft³/min

function calculateAirFlow() { var waterFlow = parseFloat(document.getElementById('waterFlow').value); var tempIn = parseFloat(document.getElementById('tempIn').value); var tempOut = parseFloat(document.getElementById('tempOut').value); var hIn = parseFloat(document.getElementById('hIn').value); var hOut = parseFloat(document.getElementById('hOut').value); var density = parseFloat(document.getElementById('airDensity').value); if (isNaN(waterFlow) || isNaN(tempIn) || isNaN(tempOut) || isNaN(hIn) || isNaN(hOut) || isNaN(density)) { alert("Please enter valid numeric values."); return; } if (hOut <= hIn) { alert("Outlet enthalpy must be greater than inlet enthalpy."); return; } // Specific heat of water in kJ/kg·°C (approx 4.187) var Cp_water = 4.187; // Density of water (approx 1000 kg/m³) var rho_water = 1000; // 1. Calculate Heat Load (Q) // Q = (m_dot_water * Cp * delta_T) // m_dot_water in kg/s = (m³/hr * 1000) / 3600 var massFlowWater = (waterFlow * rho_water) / 3600; var range = tempIn – tempOut; var heatLoad = massFlowWater * Cp_water * range; // in kW // 2. Calculate Air Mass Flow Rate (G) // G = Q / (h_outlet – h_inlet) var airMassFlow = heatLoad / (hOut – hIn); // in kg/s // 3. Convert to Volume Flow Rate (m³/hr) // Volume = (Mass / Density) * 3600 var airVolFlow = (airMassFlow / density) * 3600; // 4. Convert to CFM (1 m³/hr = 0.588578 cfm) var airCFM = airVolFlow * 0.588578; document.getElementById('resHeatLoad').innerText = heatLoad.toFixed(2); document.getElementById('resMassFlow').innerText = airMassFlow.toFixed(2); document.getElementById('resVolFlow').innerText = airVolFlow.toLocaleString(undefined, {maximumFractionDigits: 0}); document.getElementById('resCFM').innerText = airCFM.toLocaleString(undefined, {maximumFractionDigits: 0}); document.getElementById('results-area').style.display = 'block'; }

How to Calculate Air Flow Rate in a Cooling Tower

Calculating the air flow rate is critical for the design and optimization of a cooling tower. It determines the capacity of the fans and the efficiency of the heat exchange process between hot water and ambient air.

The Core Formula

The calculation is based on the energy balance principle, where the heat lost by the water must equal the heat gained by the air. The fundamental formula used is:

G = Q / (h₂ – h₁)

  • G: Air mass flow rate (kg/s)
  • Q: Heat load or heat rejected by the water (kW)
  • h₂: Enthalpy of air at outlet (kJ/kg)
  • h₁: Enthalpy of air at inlet (kJ/kg)

Step-by-Step Calculation Guide

Step 1: Determine Heat Load (Q)

Before finding the air flow, you must know how much heat the water is losing. This is calculated as:

Q = Water Flow Rate × Specific Heat × (Water Inlet Temp – Water Outlet Temp)

Step 2: Determine Air Enthalpy

Enthalpy represents the total heat content of the air (sensible + latent). You usually determine the inlet enthalpy (h₁) using the ambient Wet Bulb Temperature and the outlet enthalpy (h₂) based on the target saturation at the exit.

Step 3: Calculate Air Mass Flow Rate

Divide the total heat load by the difference in enthalpy. This gives you the mass of air needed per second.

Step 4: Convert to Volume Flow (CFM or m³/hr)

Since fans move volume, not mass, you divide the mass flow by the air density (typically 1.2 kg/m³ at sea level) to get the volumetric flow rate.

Practical Example

Suppose you have a cooling tower with the following parameters:

  • Water Flow Rate: 100 m³/hr
  • Water Inlet: 37°C | Water Outlet: 32°C
  • Inlet Air Wet Bulb: 24°C (Enthalpy ≈ 72 kJ/kg)
  • Outlet Air Enthalpy: 112 kJ/kg

Result: Using the logic above, the Heat Load is approx 581 kW. The mass air flow rate would be 14.5 kg/s. Converting this at a standard density of 1.2 kg/m³ results in approximately 43,500 m³/hr or 25,600 CFM.

Why Accuracy Matters

If the air flow rate is too low, the cooling tower will fail to achieve the desired "approach" (the difference between outlet water temp and inlet wet bulb temp). If it is too high, you waste energy on fan power and increase the risk of "drift" (water droplets being carried out of the tower).

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