How to Calculate Mixed Air Temperature

Mixed Air Temperature Formula:

\[ T_{mixed} = \frac{(m_{oa} \times T_{oa} + m_{ra} \times T_{ra})}{(m_{oa} + m_{ra})} \]

Mass Flow Outside Air (m_oa):

kg/s

Outside Air Temperature (T_oa):

°C

Mass Flow Return Air (m_ra):

kg/s

Return Air Temperature (T_ra):

°C

Mixed Air Temperature (T_mixed):

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1. What is Mixed Air Temperature?

Mixed air temperature is the weighted average temperature resulting from mixing outside air and return air in HVAC systems. It represents the temperature of air after combining fresh outdoor air with recirculated indoor air.

2. How Does the Calculator Work?

The calculator uses the mixed air temperature formula:

\[ T_{mixed} = \frac{(m_{oa} \times T_{oa} + m_{ra} \times T_{ra})}{(m_{oa} + m_{ra})} \]

Where:

\( T_{mixed} \) — Mixed air temperature (°C)
\( m_{oa} \) — Mass flow rate of outside air (kg/s)
\( T_{oa} \) — Outside air temperature (°C)
\( m_{ra} \) — Mass flow rate of return air (kg/s)
\( T_{ra} \) — Return air temperature (°C)

Explanation: The formula calculates a mass-weighted average temperature, where each air stream's contribution is proportional to its mass flow rate.

3. Importance of Mixed Air Temperature Calculation

Details: Accurate mixed air temperature calculation is essential for HVAC system design, energy efficiency optimization, thermal comfort analysis, and proper sizing of heating/cooling equipment.

4. Using the Calculator

Tips: Enter mass flow rates in kg/s and temperatures in °C. All mass flow values must be non-negative, and the sum of mass flows must be greater than zero for valid calculation.

5. Frequently Asked Questions (FAQ)

Q1: Why use mass flow instead of volume flow?
A: Mass flow provides more accurate energy calculations since air density changes with temperature and pressure, while volume flow varies with these conditions.

Q2: What are typical values for HVAC systems?
A: Typical outside air percentages range from 10-30% of total airflow, depending on building occupancy and ventilation requirements.

Q3: How does this affect energy consumption?
A: Proper mixed air calculation helps optimize economizer operation, reducing mechanical heating/cooling loads and saving energy.

Q4: What if the total mass flow is zero?
A: The calculation is undefined when total mass flow is zero, as there is no air to measure temperature from.

Q5: Can this be used for other fluid mixtures?
A: Yes, the same principle applies to any fluid mixture where you need to calculate the weighted average temperature based on mass flows.