Advanced Mixed Air Calculator
Example Data Table
| Stream | Flow | Temperature | Relative Humidity | Pressure | Use Case |
|---|---|---|---|---|---|
| Outdoor Air | 500 cfm | 35 °C | 45% | 101.325 kPa | Ventilation intake |
| Return Air | 1500 cfm | 24 °C | 50% | 101.325 kPa | Room return stream |
| Bypass Air | 100 cfm | 18 °C | 60% | 101.325 kPa | Optional third stream |
Formula Used
The calculator uses a moist air energy balance. Relative humidity is first converted into humidity ratio.
Humidity ratio: W = 0.621945 × Pv / (P − Pv)
Moist air enthalpy: h = 1.006T + W(2501 + 1.86T)
Mixed humidity ratio: Wm = Σ(ṁda × W) / Σṁda
Mixed enthalpy: hm = Σ(ṁda × h) / Σṁda
Mixed dry bulb temperature: Tm = (hm − 2501Wm) / (1.006 + 1.86Wm)
Here, T is in °C, P is pressure, Pv is vapor pressure, W is humidity ratio, h is kJ/kg dry air, and ṁda is dry air mass flow.
How to Use This Calculator
- Enter the flow, temperature, humidity, and pressure for each air stream.
- Choose the correct units for flow and temperature.
- Enable the third stream when a bypass or make up stream exists.
- Add a target supply temperature when heating or cooling duty is needed.
- Press the calculate button to view the result above the form.
- Download the CSV or PDF report for records and design notes.
Mixed Air Temperature Guide
Why Mixed Air Matters
Mixed air temperature is the dry bulb temperature after two or more air streams blend inside a duct, plenum, or air handling unit. The value matters because coils, fans, dampers, and controls respond to this combined condition. A simple average can be wrong when each stream has a different flow rate, humidity level, or pressure.
How the Calculation Works
This calculator treats each stream as moist air. It first converts temperature, pressure, relative humidity, and flow into a consistent set of properties. Volumetric flow is converted to dry air mass flow by using specific volume. Then the tool balances enthalpy and humidity ratio. The final dry bulb temperature comes from the mixed enthalpy equation, not from a guessed average.
Where It Is Used
The method is useful for HVAC design, psychrometric checks, economizer analysis, ventilation studies, and troubleshooting. It helps compare outdoor air, return air, bypass air, and make up air. It also shows whether a target supply temperature needs heating or cooling after the streams combine.
Mass Flow and Humidity
Moist air mixing depends on mass. A small hot stream may not shift the final temperature much if its mass flow is low. A large cool return stream can dominate the mixture. Humidity also matters because water vapor carries latent energy. That is why the enthalpy based result is often more realistic than a dry temperature average.
Input Quality
Use accurate field readings when possible. Measure air temperature away from coils and walls. Enter pressure near the same location as the flow reading. Choose the correct flow unit. Check that relative humidity is within a realistic range. For best results, compare the output with a psychrometric chart or trusted building automation data.
Reports and Notes
The final values can support reports and design notes. Download the CSV for spreadsheets. Download the PDF for quick sharing. Keep the example table nearby when learning the inputs. The calculator is a planning aid, so important projects should still be verified by qualified engineers and calibrated instruments. Avoid mixing units within the same study. Save one copy for design assumptions and another for measured readings. This makes changes easier to trace. The extra notes field can record damper position, season, sensor name, or test condition for review before final approval.
FAQs
What is mixed air temperature?
Mixed air temperature is the final dry bulb temperature after two or more air streams combine. It is commonly used in HVAC systems, ducts, plenums, economizers, and air handling units.
Why is flow rate important?
Flow rate controls the mass contribution of each stream. A high flow stream has more influence on the final mixture than a low flow stream at the same temperature.
Does humidity affect the result?
Yes. Humidity changes moist air enthalpy. Water vapor carries latent energy, so humid air can shift the final mixed temperature and heat balance.
Can I mix Celsius and Fahrenheit inputs?
Yes. Select the correct unit for each stream. The calculator converts all temperatures internally before applying the energy balance.
What pressure should I enter?
Use the local air pressure near the measurement point. Standard sea level pressure is 101.325 kPa, but actual site pressure may differ.
What is target sensible duty?
Target sensible duty estimates heating or cooling power needed to move the mixed air to a selected dry bulb temperature. Positive means heating. Negative means cooling.
Can this replace a psychrometric chart?
It can support quick analysis and reports. Critical engineering work should still be checked with trusted psychrometric tools, calibrated instruments, and professional judgment.
Why is dry weighted temperature also shown?
Dry weighted temperature is a simple mass weighted dry bulb average. It is shown for comparison with the enthalpy based moist air result.