Mixed Air Calculator - Mixed Air Temperature, Humidity & Enthalpy

A mixed air calculator blends outdoor and return airstreams at different temperatures and humidities into a single mixed air state. It applies mass and energy conservation to deliver the mixed dry-bulb temperature, humidity ratio, specific enthalpy, and the back-calculated relative humidity of the supply air. Engineers use it for cooling coil sizing, economiser setup, indoor air-quality audits, and energy modeling.

• • Cooling coil selection: Confirm the mixed air temperature and humidity ratio entering a chilled-water or DX coil before consulting manufacturer selection software.
• • Economiser control setup: Estimate how much outdoor air to admit at each outdoor temperature while keeping the mixed temperature above the cooling-coil dew point.
• • Indoor air-quality audits: Track outdoor air fraction during minimum outside air measurements and verify compliance with ASHRAE 62.1.
• • Energy modeling: Generate the mixed-air state used by hourly simulation tools as the boundary condition for downstream plant equipment.

The calculation follows the two-stream psychrometric mixing equations in the ASHRAE Handbook (Fundamentals), Chapter 1. Both airstreams mix adiabatically at constant atmospheric pressure, so energy and water mass are conserved.

When the inlet temperatures differ by more than about twenty degrees Celsius, converting volumetric flow to mass flow using the air density calculator gives a noticeably different mixed temperature.

When the inlet temperatures differ by more than about twenty degrees Celsius, converting volumetric flow to mass flow using the Air Density Calculator gives a noticeably different mixed temperature.

The calculator applies mass and energy conservation to two adiabatic airstreams. Each inlet state is converted into a humidity ratio using the Magnus-Tetens saturation pressure, then the two states are averaged using their flow rates.

• m_oa, m_ra: Outdoor-air and return-air flow rates in cubic feet per minute or kilograms per second.
• T_oa, T_ra: Dry-bulb temperature of each airstream in degrees Celsius.
• w_oa, w_ra: Humidity ratio of each airstream in kg water per kg dry air.
• h_mix: Moist-air specific enthalpy of the mixed stream in kJ/kg dry air.

The Magnus-Tetens equation P_sat = 0.61078 exp(17.27 T / (T + 237.3)) returns the saturation vapour pressure in kilopascals for a Celsius dry-bulb temperature. The humidity ratio w = 0.622 P_v / (P - P_v) then converts vapour pressure P_v into kilograms of water per kilogram of dry air at atmospheric pressure P.

Specific enthalpy h = 1.005 T + w (2501 + 1.88 T) is the standard ASHRAE moist-air form using 1.005 kJ/(kg K) and 1.88 kJ/(kg K) for the specific heats of dry air and water vapour, and 2501 kJ/kg for the latent heat of vaporisation of water at zero degrees Celsius. Because the equation is linear in w and nearly linear in T over the HVAC range, the mixed enthalpy is also a flow-weighted average of the two inlet enthalpies to within about one percent.

According to ASHRAE Handbook (Fundamentals), mixed-air temperature and humidity ratio follow from flow-weighted averaging of the inlet states

The humidity ratio calculation here uses the same Magnus-Tetens saturation pressure and ideal-gas conversion that the Absolute Humidity Calculator uses to report absolute humidity in grams per cubic metre.

Four psychrometric ideas drive the mixed air calculator: saturation pressure, humidity ratio, specific enthalpy, and conservation of mass and energy across the mixing box.

Each concept feeds the formula chain. Saturation pressure determines the inlet humidity ratios, humidity ratio determines the back-calculated mixed relative humidity, and specific enthalpy is the result that downstream cooling coils remove.

The adiabatic-mixing assumption is strong enough for nearly every commercial air-handling unit but breaks down when condensate forms inside the mixing section.

The Vapor Pressure Deficit Calculator uses the same saturation pressure to express the drying power of the air, which helps explain why mixing two streams does not always produce a simple humidity average.

Use the mixed air calculator by entering the volumetric flow, dry-bulb temperature, and relative humidity of each airstream. Results update as you type.

1. 1 Enter the outdoor-air flow rate: Type the design or measured outdoor-air volumetric flow in cubic feet per minute.
2. 2 Set the outdoor dry-bulb temperature and relative humidity: Use the local weather data, design day values, or measured sensor readings.
3. 3 Enter the return-air flow rate: Enter the return-air volumetric flow in cubic feet per minute. For a constant-volume system, return flow equals supply flow minus any exhaust.
4. 4 Set the return dry-bulb temperature and relative humidity: Use the room or return-air plenum conditions.
5. 5 Read the mixed state and outdoor air fraction: The result panel lists mixed temperature, humidity ratio, enthalpy, and the back-calculated mixed relative humidity. The outdoor air fraction equals 100 x m_oa / (m_oa + m_ra).
6. 6 Adjust the flows to meet ventilation or coil targets: Increase outdoor CFM to lift the outdoor air fraction, or shift temperatures to estimate mixed states at different operating modes.

A classroom unit handling 5,000 CFM total with 1,000 CFM of outdoor air at 35 C and 30 percent RH and 4,000 CFM of return air at 24 C and 50 percent RH produces 26.2 C mixed air at 45 percent RH and 20 percent outdoor air, which is the boundary condition a downstream chilled-water coil would see at part-load.

Once the mixed air state is known, the Heat Transfer Conduction Calculator can size the downstream heating or cooling coil that brings the supply air to room conditions.

Using this calculator before coil selection, economiser setup, or energy modeling removes several manual steps and surfaces the property engineers actually need.

• • Single mixed state from two inlet states: Returns dry-bulb temperature, humidity ratio, enthalpy, relative humidity, and the outdoor air fraction at once.
• • ASHRAE-aligned equations: Uses the same Magnus-Tetens saturation pressure and ASHRAE moist-air enthalpy constants.
• • Real-time recompute: Results update on every keystroke so designers can compare design days and damper positions quickly.
• • Outdoor air fraction built in: Reports the percentage of the mixed stream from outside, which feeds ASHRAE 62.1 ventilation compliance.
• • Catches supersaturated mixes early: Caps the mixed relative humidity at 100 percent when warm saturated air meets cool saturated air.
• • Inputs that match field instrumentation: Accepts CFM, degrees Celsius, and percent, the same units carried by most building automation trend logs.

The calculator is most useful at the boundary between the airside and the plant side of an HVAC system. A mixed air temperature that is two degrees Celsius warmer than expected can push a chilled-water coil into dehumidification mode and force the chiller to run longer.

When the barometric pressure is well below 101.325 kilopascals, the gas laws calculator helps justify the pressure correction that the mixed air calculator assumes by default.

When the supply or return fan sits at altitude and the barometric pressure is well below 101.325 kilopascals, the Gas Laws Calculator helps justify the pressure correction that the mixed air calculator assumes by default.

Four factors dominate the accuracy of any such calculator: the inlet temperatures, the inlet relative humidities, the flow split, and the atmospheric pressure used to convert relative humidity to humidity ratio.

• • The model assumes adiabatic, constant-pressure mixing. If condensate forms inside the mixing section or a heat-recovery wheel leaks, the actual mixed state will differ and should be measured.
• • Volumetric flow is used as a proxy for mass flow. When the inlet temperatures differ by more than about twenty degrees Celsius, convert both flows to kilograms per second using the air-density calculator before averaging.
• • Atmospheric pressure defaults to 101.325 kilopascals. For sites above 1500 metres, set the local barometric pressure explicitly.
• • The Magnus-Tetens saturation pressure approximation is accurate to within about one percent for the 0 to 60 C range used here.

Two practical cautions are worth highlighting. First, mixing two saturated airstreams at very different temperatures can imply a humidity ratio that exceeds saturation at the mixed temperature, so the actual mixed stream sits at 100 percent RH with the excess moisture condensing out. The calculator caps the result at 100 percent to flag this case.

Second, the result panel reports mixed relative humidity back-calculated from the mixed humidity ratio. That value matches the formula chain, but a real mixed-air sensor will only agree closely when the mixing box truly behaves as an adiabatic, well-mixed plenum.

According to Wikipedia: Psychrometrics, psychrometric mixing conserves both energy and water mass when the process is adiabatic

Third, projects sitting above about 1500 metres of elevation should enter the local barometric pressure explicitly; the Boiling Point Altitude Calculator converts a measured station pressure into the equivalent altitude for that check.