Mixed Air
Calculate the mixed air temperature and flow rate when combining two air streams in HVAC systems. Free online thermodynamics calculator with interactive charts.
About This Calculator
The Mixed Air Calculator helps HVAC engineers, technicians, and students determine the resulting temperature and flow rate when two air streams are combined. In heating, ventilation, and air conditioning (HVAC) systems, mixed air typically refers to the combination of return air from inside a building with fresh outdoor air intake. Understanding the mixed air properties is essential for proper system sizing, energy efficiency analysis, and indoor air quality management.
The calculator uses the weighted average formula: Tmixed = (T1 × V1 + T2 × V2) / (V1 + V2), where T is dry bulb temperature and V is volumetric flow rate. This formula assumes adiabatic mixing with negligible heat transfer to surroundings, which is the standard approach in HVAC engineering. The total volumetric flow rate is simply the sum of the two stream flow rates: Vtotal = V1 + V2.
Regional Notes
Worldwide: The calculator uses metric units (degrees Celsius for temperature, cubic meters per second for flow rate) which are the international standard for HVAC and thermodynamics calculations. Engineers worldwide can convert from imperial units (CFM, °F) before entering values if needed.
Applications: Mixed air calculations are used in commercial building HVAC design, industrial ventilation systems, hospital air handling units, cleanroom environmental control, and residential HVAC system optimization. Proper mixed air temperature calculation helps reduce energy consumption by optimizing the ratio of return to outdoor air.
Frequently Asked Questions
What is mixed air in HVAC?
Mixed air in HVAC refers to the combination of two separate air streams — typically return air from inside a building and fresh outdoor air — into a single mixed stream. The resulting mixed air has properties (temperature, flow rate) that depend on the properties and proportions of the original streams.
How do you calculate mixed air temperature?
Mixed air temperature is calculated using the weighted average formula: Tmixed = (T1 × V1 + T2 × V2) / (V1 + V2), where T1 and T2 are the temperatures of the two air streams in degrees Celsius, and V1 and V2 are their volumetric flow rates in cubic meters per second.
Why is mixed air important in HVAC design?
Mixed air is crucial in HVAC design because mixing return air with fresh outdoor air improves energy efficiency by pre-conditioning the air before it enters the heating or cooling system. This reduces the load on HVAC equipment, lowers energy costs, and maintains comfortable indoor air quality.
Is air mixing an adiabatic process?
Yes, air mixing is considered an adiabatic process because heat transfer to the surroundings is negligible. No work is involved in the mixing, so the energy balance simplifies to m1h1 + m2h2 = m3h3, where m represents mass flow and h represents enthalpy of each stream.
What units are used for mixed air calculations?
Temperature is measured in degrees Celsius (°C) and flow rate in cubic meters per second (m³/s). The calculator automatically converts between metric units and displays results with proper unit labels for HVAC and thermodynamics applications.
Can I use this calculator for any two gas streams?
Yes, the mixed air formula applies to any two gas streams being combined. While it is primarily designed for HVAC air mixing (return air and outdoor air), the weighted average temperature formula works for any two gas or fluid streams at constant pressure.
How accurate is the mixed air temperature calculation?
The calculation is mathematically exact for the weighted average formula. Results are displayed to two decimal places for temperature. For real HVAC systems, accuracy also depends on proper measurement of air stream temperatures and flow rates.
What is the difference between mixed air and supply air?
Mixed air is the combination of return air and outdoor air before conditioning. Supply air is the air that has been conditioned (heated, cooled, or dehumidified) and is delivered to the occupied space. The mixed air calculator computes properties before conditioning occurs.