Mixed Air Temperature Calculator
Calculate Mixed Air Temperature
Enter the temperature and airflow rate (CFM) for two air streams to find the temperature after they mix.
Total Airflow: — CFM
Weighted T1*CFM1: —
Weighted T2*CFM2: —
Mixing Analysis
| % Air Stream 1 | CFM1 | CFM2 | Mixed Temp (°F) |
|---|---|---|---|
| – | – | – | – |
| – | – | – | – |
| – | – | – | – |
| – | – | – | – |
| – | – | – | – |
| – | – | – | – |
What is Mixed Air Temperature?
Mixed air temperature is the resulting temperature when two or more air streams with different temperatures and flow rates are combined. This is a very common scenario in Heating, Ventilation, and Air Conditioning (HVAC) systems. For example, when outside air is mixed with return air from inside a building before being conditioned, the temperature of the combined air stream is the mixed air temperature. Our Mixed Air Temperature Calculator helps you find this value quickly.
Understanding mixed air temperature is crucial for HVAC system design, energy analysis, and ensuring occupant comfort. Engineers and technicians use this calculation to determine the load on heating or cooling coils and to size equipment properly. The Mixed Air Temperature Calculator is a valuable tool for anyone working with air handling systems.
Common misconceptions include thinking it’s a simple average of temperatures (it’s weighted by airflow) or that humidity doesn’t play a role (it does for enthalpy, but this calculator focuses on dry-bulb temperature based on sensible heat balance).
Mixed Air Temperature Formula and Mathematical Explanation
The mixed air temperature (Tm) is calculated based on the conservation of energy (specifically sensible heat, assuming no phase change or significant moisture addition/removal during mixing). The formula is a weighted average of the temperatures of the individual air streams, weighted by their respective airflow rates:
Tm = (T1 * CFM1 + T2 * CFM2) / (CFM1 + CFM2)
Where:
- Tm = Mixed Air Temperature
- T1 = Temperature of Air Stream 1
- CFM1 = Airflow rate of Air Stream 1 (in Cubic Feet per Minute)
- T2 = Temperature of Air Stream 2
- CFM2 = Airflow rate of Air Stream 2 (in Cubic Feet per Minute)
This formula assumes the density and specific heat of the air are constant for both streams and the mixed stream, which is a reasonable assumption for typical HVAC conditions.
| Variable | Meaning | Unit | Typical Range |
|---|---|---|---|
| Tm | Mixed Air Temperature | °F or °C | 0 – 120 °F |
| T1, T2 | Temperature of individual air streams | °F or °C | 0 – 120 °F |
| CFM1, CFM2 | Airflow rate of individual streams | CFM (or m³/s) | 50 – 100,000+ CFM |
Practical Examples (Real-World Use Cases)
Example 1: Mixing Return Air with Outside Air
An air handling unit (AHU) is mixing return air from the building with fresh outside air. The return air is 800 CFM at 75°F, and the outside air is 200 CFM at 95°F.
- T1 = 75°F, CFM1 = 800 CFM
- T2 = 95°F, CFM2 = 200 CFM
- Total CFM = 800 + 200 = 1000 CFM
- Tm = (75 * 800 + 95 * 200) / (800 + 200) = (60000 + 19000) / 1000 = 79000 / 1000 = 79°F
The mixed air entering the cooling coil will be 79°F. This is higher than the return air, increasing the cooling load due to the hot outside air.
Example 2: Bypass Air in an AHU
Some air (1500 CFM at 55°F) goes through a cooling coil, while another stream of air (500 CFM at 78°F) bypasses the coil and mixes with the cooled air downstream.
- T1 = 55°F, CFM1 = 1500 CFM
- T2 = 78°F, CFM2 = 500 CFM
- Total CFM = 1500 + 500 = 2000 CFM
- Tm = (55 * 1500 + 78 * 500) / (1500 + 500) = (82500 + 39000) / 2000 = 121500 / 2000 = 60.75°F
The supply air temperature after mixing will be 60.75°F. Using the Mixed Air Temperature Calculator helps determine the final supply temperature.
How to Use This Mixed Air Temperature Calculator
- Enter Air Stream 1 Temperature (T1): Input the temperature of the first air stream in degrees Fahrenheit (°F).
- Enter Air Stream 1 Airflow (CFM1): Input the volumetric flow rate of the first air stream in Cubic Feet per Minute (CFM).
- Enter Air Stream 2 Temperature (T2): Input the temperature of the second air stream in °F.
- Enter Air Stream 2 Airflow (CFM2): Input the volumetric flow rate of the second air stream in CFM.
- Calculate: The calculator will automatically update the results as you type. You can also click the “Calculate” button.
- Read Results: The “Mixed Air Temperature” is the primary result. You can also see the total airflow and the weighted temperature contributions from each stream.
- Reset: Click “Reset” to clear the fields to their default values.
- Copy Results: Click “Copy Results” to copy the main result and intermediate values to your clipboard.
The table and chart provide further insight into how the mixed temperature changes with different mixing ratios.
Key Factors That Affect Mixed Air Temperature Results
- Inlet Temperatures (T1, T2): The initial temperatures of the air streams are the most direct factors. Higher or lower initial temperatures directly influence the mixed temperature.
- Airflow Rates (CFM1, CFM2): The proportion of each airflow rate determines its “weight” in the average. The stream with the higher CFM will have more influence on the final temperature.
- Proper Mixing: The formula assumes thorough mixing. If the air streams don’t mix completely before the point of temperature measurement, the actual temperature might vary.
- Heat Gains or Losses: The calculation assumes no heat is gained from or lost to the surroundings during mixing (adiabatic mixing). In reality, there might be slight heat transfer through duct walls.
- Air Density and Specific Heat: While assumed constant, significant variations in temperature, pressure, or humidity between the streams could slightly affect these properties, though it’s usually negligible in standard HVAC.
- Measurement Accuracy: The accuracy of the input temperatures and airflow measurements directly impacts the accuracy of the calculated mixed air temperature.
Our Mixed Air Temperature Calculator provides a reliable estimate based on the provided inputs and the standard formula.
Frequently Asked Questions (FAQ)
- What units are used in the Mixed Air Temperature Calculator?
- The calculator uses degrees Fahrenheit (°F) for temperature and Cubic Feet per Minute (CFM) for airflow. Ensure your inputs match these units.
- Can I use Celsius with this calculator?
- This specific calculator is designed for Fahrenheit. You would need to convert Celsius temperatures to Fahrenheit before using it: F = (C * 9/5) + 32.
- What if I have more than two air streams?
- You can adapt the principle. First, mix two streams, then mix the result with the third stream, and so on. Or, extend the formula: Tm = (T1*CFM1 + T2*CFM2 + T3*CFM3 + …)/(CFM1+CFM2+CFM3+…).
- Does this calculator account for humidity?
- No, this Mixed Air Temperature Calculator calculates the dry-bulb temperature based on sensible heat balance. For calculations involving humidity and latent heat, you’d need a psychrometric calculator or enthalpy-based calculations.
- Why is airflow rate important?
- Airflow rate (CFM) determines how much heat energy each air stream contributes to the mix. A stream with higher CFM has a greater impact on the final temperature.
- What is CFM?
- CFM stands for Cubic Feet per Minute, a common unit for measuring the volume of air flowing through a duct or space per minute.
- Where is the mixed air temperature calculation used?
- It’s used in HVAC design for air handling units, economizer cycles, ventilation systems, and energy recovery ventilators to determine temperatures at various points in the system.
- Is the mixing process always perfect?
- No, perfect mixing is an ideal assumption. In real systems, some stratification might occur, especially if the mixing chamber is short or poorly designed. The calculated value is the average temperature assuming complete mixing.
Related Tools and Internal Resources
- HVAC Design Guide: Learn more about the principles behind HVAC system design and air mixing.
- Psychrometric Chart Tool: For more advanced calculations involving humidity and air properties.
- Duct Sizing Calculator: Calculate duct sizes based on airflow and velocity.
- Energy Efficiency in HVAC: Understand how air mixing and temperatures impact energy consumption.
- Air Change Rate Calculator: Calculate how quickly air is replaced in a space.
- Ventilation Basics: An introduction to ventilation principles and standards.
Using our Mixed Air Temperature Calculator alongside these resources can provide a comprehensive understanding of air management in buildings.