Mixing Air Calculator

Mixing Air Calculator (HVAC Psychrometrics)

Air Stream A (e.g., Outdoor Air)

Air Stream B (e.g., Return Air)

Mixed Air Results:

Mixed Air Temperature

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Mixed Relative Humidity

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Total Airflow

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OA Percentage

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function calculateMixedAir() { var flowA = parseFloat(document.getElementById('flowA').value); var tempA = parseFloat(document.getElementById('tempA').value); var rhA = parseFloat(document.getElementById('rhA').value); var flowB = parseFloat(document.getElementById('flowB').value); var tempB = parseFloat(document.getElementById('tempB').value); var rhB = parseFloat(document.getElementById('rhB').value); if (isNaN(flowA) || isNaN(tempA) || isNaN(flowB) || isNaN(tempB)) { alert('Please enter valid numeric values for flow and temperature.'); return; } var totalFlow = flowA + flowB; if (totalFlow <= 0) { alert('Total airflow must be greater than zero.'); return; } // Formula for Mixed Temperature: Tmix = (V1*T1 + V2*T2) / (V1 + V2) var mixedTemp = ((flowA * tempA) + (flowB * tempB)) / totalFlow; // Approximation for Relative Humidity mixing (Linear approximation for engineering estimates) // In precise psychrometrics, one should use Humidity Ratio (W), but for most HVAC field estimates, // linear RH mixing at standard pressure is the common rule of thumb. var mixedRH = ((flowA * rhA) + (flowB * rhB)) / totalFlow; var oaPercent = (flowA / totalFlow) * 100; document.getElementById('resTemp').innerText = mixedTemp.toFixed(1) + ' °F'; document.getElementById('resRH').innerText = mixedRH.toFixed(1) + ' %'; document.getElementById('resFlow').innerText = totalFlow.toLocaleString() + ' CFM'; document.getElementById('resPercent').innerText = oaPercent.toFixed(1) + ' %'; document.getElementById('airResult').style.display = 'block'; }

Understanding Air Mixing in HVAC Systems

In the field of HVAC (Heating, Ventilation, and Air Conditioning), determining the properties of mixed air streams is a fundamental task. Most commercial air handling units (AHUs) mix outside air (OA) with return air (RA) to provide ventilation while maintaining energy efficiency. This resulting mixture is known as Mixed Air (MA).

The Mixed Air Temperature Formula

The temperature of the air mixture is a weighted average based on the volume of each air stream. The standard formula used by engineers is:

T_mix = (V₁ × T₁ + V₂ × T₂) / (V₁ + V₂)

Where:

• V: Airflow Volume (typically measured in Cubic Feet per Minute – CFM)
• T: Dry Bulb Temperature of the air stream

Why Calculate Mixed Air?

Knowing the mixed air condition is critical for several reasons:

1. Coil Load Calculation: To size cooling and heating coils, you must know the temperature of the air entering the coil.
2. Economizer Verification: Engineers use these calculations to ensure that air dampers are modulating correctly to bring in the right amount of outdoor air.
3. Freezing Prevention: In cold climates, mixing too much freezing outdoor air with return air can freeze the hydronic coils in an AHU.
4. ASHRAE Compliance: Ensuring the minimum required outdoor air (per ASHRAE 62.1) is being delivered to the occupants.

Real-World Example

Imagine an office building during a hot summer day:

• Outside Air (Stream A): 2,000 CFM at 90°F
• Return Air (Stream B): 8,000 CFM at 74°F

Using the calculator, the total airflow is 10,000 CFM. The mixed air temperature would be:

(2,000 × 90 + 8,000 × 74) / 10,000 = 77.2°F

This means your cooling coil "sees" 77.2°F air at its inlet, which it must then cool down to the supply air setpoint (usually around 55°F).

A Note on Humidity

While this calculator provides a linear approximation for Relative Humidity (RH), precise psychrometric calculations require converting RH to "Humidity Ratio" (grains of moisture per pound of dry air). However, for quick field estimates and standard comfort cooling ranges, the linear volume-weighted average used here provides a reliable estimate for HVAC technicians and facilities managers.