KEVOS® MAKEOS · Engineering
Adiabatic Mixing of Two Air Streams (Mixed Temperature, Humidity, Dew Point, Fog Risk)
When outdoor and return air (or any two moist-air streams) mix without heat transfer, what temperature, humidity ratio, relative humidity and dew point does the mixed air have, and does the mixture stay clear of fog?
Adiabatic mixing of two moist-air streams, such as outdoor and return air in an air-handling unit: each stream’s humidity ratio, enthalpy and dry-air flow, the mixed temperature, humidity ratio, relative humidity and dew point, the outdoor share by mass, and a check that the mixture stays clear of fog.
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How this tool works
Inputs, units and where to find them
| Input | Unit | Typical source |
|---|---|---|
| Total (barometric) pressure | kPa | Barometric pressure at the site: 101.325 kPa at sea level. |
| Stream 1 (for example outdoor air) | ||
| Stream 1 dry-bulb temperature | °C | Dry-bulb temperature of the first stream, often outdoor air at the design condition. |
| Stream 1 relative humidity | % | Relative humidity of the first stream. |
| Stream 1 volume flow | m³/s | Volume flow of the first stream at its own temperature, from the damper or fan schedule. |
| Stream 2 (for example return air) | ||
| Stream 2 dry-bulb temperature | °C | Dry-bulb temperature of the second stream, often return air from the space. |
| Stream 2 relative humidity | % | Relative humidity of the second stream. |
| Stream 2 volume flow | m³/s | Volume flow of the second stream at its own temperature. |
| Limit | ||
| Highest acceptable mixed relative humidity | % | Highest mixed relative humidity you accept before fogging, wet filters or condensation become a risk. |
What each input means
Open an input to see what it is, its units, the worked example and the common mistakes, with a small diagram.
What this means: Total (barometric) pressure
- What it is
- The total pressure of the moist air, the barometric pressure at the site, used in the humidity ratios and specific volumes.
- Units and format
- Enter a number in kPa: kilopascals. The form accepts 50 kPa to 110 kPa. Required: the tool does not calculate until every input is valid.
- Example
- The form opens with 101.325 kPa as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
- Effect on the result
- Changes the humidity ratios and mass flows slightly.
- Get it right
- Use about 101.325 kPa at sea level and the local value at altitude.
Stream 1 (for example outdoor air)
What this means: Stream 1 dry-bulb temperature
- What it is
- The dry-bulb temperature of the first air stream, often the outdoor air entering through the outdoor-air damper.
- Units and format
- Enter a number in °C: degrees Celsius. The form accepts 0 °C to 60 °C. Required: the tool does not calculate until every input is valid.
- Example
- The form opens with 5 °C as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
- Effect on the result
- A colder first stream lowers the mixed temperature and raises the fog risk.
- Get it right
- Use the design condition you are checking; the tool accepts 0 to 60 °C.
What this means: Stream 1 relative humidity
- What it is
- The relative humidity of the first air stream, which with its temperature sets the moisture it brings to the mix.
- Units and format
- Enter a number in %: percent: the number is read as a percentage, so type 8 for 8 %. The form accepts 0.1% to 100%. Required: the tool does not calculate until every input is valid.
- Example
- The form opens with 80% as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
- Effect on the result
- Higher humidity raises the mixed humidity ratio and dew point.
- Get it right
- Cold outdoor air often has a high relative humidity but little moisture.
What this means: Stream 1 volume flow
- What it is
- The volume of the first stream flowing each second at its own temperature, which with its specific volume gives its dry-air mass flow.
- Units and format
- Enter a number in m³/s: m³/s. The form accepts 0 m³/s to 1000 m³/s. Required: the tool does not calculate until every input is valid.
- Example
- The form opens with 0.5 m³/s as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
- Effect on the result
- A larger flow moves the mix towards the first stream’s condition.
- Get it right
- Enter 0 to see the second stream alone.
Stream 2 (for example return air)
What this means: Stream 2 dry-bulb temperature
- What it is
- The dry-bulb temperature of the second air stream, often the return air from the conditioned space.
- Units and format
- Enter a number in °C: degrees Celsius. The form accepts 0 °C to 60 °C. Required: the tool does not calculate until every input is valid.
- Example
- The form opens with 22 °C as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
- Effect on the result
- A warmer second stream raises the mixed temperature.
- Get it right
- Measure in the return duct before the mixing box.
What this means: Stream 2 relative humidity
- What it is
- The relative humidity of the second air stream, which with its temperature sets the moisture it brings to the mix.
- Units and format
- Enter a number in %: percent: the number is read as a percentage, so type 8 for 8 %. The form accepts 0.1% to 100%. Required: the tool does not calculate until every input is valid.
- Example
- The form opens with 50% as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
- Effect on the result
- Higher humidity raises the mixed humidity ratio and dew point.
- Get it right
- Occupied spaces add moisture, so return air is often wetter than supply air.
What this means: Stream 2 volume flow
- What it is
- The volume of the second stream flowing each second at its own temperature, which with its specific volume gives its dry-air mass flow.
- Units and format
- Enter a number in m³/s: m³/s. The form accepts 0 m³/s to 1000 m³/s. Required: the tool does not calculate until every input is valid.
- Example
- The form opens with 1.5 m³/s as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
- Effect on the result
- A larger flow moves the mix towards the second stream’s condition.
- Get it right
- Supply flow minus outdoor flow gives the return flow when the system balances.
Limit
What this means: Highest acceptable mixed relative humidity
- What it is
- The highest relative humidity of the mixed air you accept before filters, coils or ducts risk getting wet.
- Units and format
- Enter a number in %: percent: the number is read as a percentage, so type 8 for 8 %. The form accepts 1% to 100%. Required: the tool does not calculate until every input is valid.
- Example
- The form opens with 90% as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
- Effect on the result
- Only moves the line between clear and near-saturation results.
- Get it right
- Allow a margin below 100 % for incomplete mixing and sensor error.
Method and formulas
- Each stream: p_v = φ p_sat(T) (Hyland–Wexler), ω = 0.622 p_v ÷ (P − p_v), h = 1.005 T + ω (2,500.9 + 1.82 T), v = 0.287 (T + 273.15) ÷ (P − p_v) and ṁ_a = V̇ ÷ v.
- Mixed state: ω₃ and h₃ are averages weighted by dry-air mass flow (mass and energy balances).
- Mixed temperature T₃ = (h₃ − 2,500.9 ω₃) ÷ (1.005 + 1.82 ω₃); vapour pressure p_v3 = ω₃ P ÷ (0.622 + ω₃); relative humidity = p_v3 ÷ p_sat(T₃).
- Example: 0.5 m³/s at 5 °C and 80 % mixed with 1.5 m³/s at 22 °C and 50 % gives 17.56 °C at 57.7 % (dew point 9.14 °C), clear of fog.
Limitations
- Perfect, adiabatic mixing; real mixing boxes stratify, so coils and filters can see colder or wetter air than the average.
- Air temperatures from 0 to 60 °C; relative humidity is taken over liquid water.
- When the result shows fog, the true mixed state is saturated with some water condensed; the reported state is the unrealisable straight-line mix.
- Example values are illustrations. Replace them with values for your actual system.
Typical uses
- Find the mixed-air condition entering a coil from outdoor and return air
- Check for fog or frosting risk when cold, humid outdoor air meets warm return air
- Set minimum outdoor air or economiser limits
Who it is for
- Mechanical services designers
- HVAC technicians and commissioning engineers
- Facility engineers
What you get
- Mixed temperature, relative humidity, dew point, humidity ratio and enthalpy
- Dry-air mass flows and the first stream’s share
- Fog and near-saturation check
Interpreting the result
- On a psychrometric chart the mixed state lies on the straight line between the two streams, closer to the larger one.
- Fog appears when that line crosses the saturation curve, most often with very cold, humid outdoor air.
Before relying on the result
- Measure the mixed-air temperature at several points across the coil face to check for stratification.
References
- Çengel Y. A. and Boles M. A., Thermodynamics: An Engineering Approach, 5th edition (McGraw-Hill, 2006), chapter 14 (sections 14-2 and 14-7: humidity and adiabatic mixing of airstreams). Method reference; no tables, figures or examples are reproduced.
- ASHRAE Handbook—Fundamentals (SI), chapter 1 Psychrometrics: Hyland–Wexler saturation pressure.
Learn the Concepts Behind This Tool
These KEVOS® Library readings explain the method this tool uses, with worked examples.
Method explained here · Library collection
Psychrometrics for HVAC and process air: humidity, dew point and the loads you cannot feel
In Fluids, HVAC and Building Services
What it covers: A practical guide to moist air: dry-bulb, wet-bulb, dew point, humidity ratio and enthalpy, the main processes on the psychrometric chart, sensible and latent loads, and condensation.
Method explained here · KEVOS® original book
Mixing outdoor and return air
In Humidity Control by the Numbers
What it covers: The mass, water and energy balances for mixing outdoor and return air, outdoor-air share by mass, the straight-line rule, fog when cold and humid streams meet, minimum outdoor air and economy cycles.
Worked example · KEVOS® original book
Moist air and the wet-bulb temperature
In Cooling Towers by the Numbers
What it covers: Work out the air a tower uses: humidity ratio, enthalpy, wet-bulb and dew point from temperature and humidity, why enthalpy drives a tower, and catch a 3 pm temperature paired with 9 am humidity.
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