Mixed Air Calculator
Calculate mixed air properties for HVAC systems combining outside and return air streams
Air Mixing Calculations
Mixing Pressure
Standard atmospheric pressure is 101.325 kPa (1 atm)
Outside Air (Stream 1) - Inputs
Use either relative humidity or humidity ratio
Outside Air Properties
Return Air (Stream 2) - Inputs
Use either relative humidity or humidity ratio
Return Air Properties
Mixed Air Properties (Stream 3)
Mixed Air Temperature Conversions
Energy Balance Equations
Mass balance: ṁ₁ + ṁ₂ = ṁ₃
Energy balance: ṁ₁h₁ + ṁ₂h₂ = ṁ₃h₃
Vapor balance: ṁ₁ω₁ + ṁ₂ω₂ = ṁ₃ω₃
Enthalpy: h = 1.006T + ω(2501 + 1.86T)
Example Calculation
HVAC Air Mixing Example
Given: Outside air stream and return air stream
Outside air: 32°C, 60% RH, 20 m³/min
Return air: 14°C, 100% RH (saturated), 50 m³/min
Pressure: 1 atm (101.325 kPa)
Solution:
Mixed air temperature ≈ 18.93°C
Mixed air RH ≈ 88.79%
Total flow rate = 70 m³/min
Energy Conservation
Process: Adiabatic mixing (no heat transfer)
Mass conservation: Total mass flow is preserved
Energy conservation: Total enthalpy is preserved
Result: Mixed air properties determined by weighted average
HVAC Applications
Building HVAC
Air conditioning system design and optimization
Hospital Systems
Critical air quality and ventilation control
Industrial Processes
Process air conditioning and humidity control
Energy Efficiency
Optimizing energy consumption in air handling
Key Concepts
Adiabatic mixing process with no heat transfer
Conservation of mass and energy principles
Psychrometric properties and relationships
Enthalpy-based energy balance calculations
Return air and outside air mixing ratios
Understanding Mixed Air Calculations
What is Mixed Air?
Mixed air refers to the combined air stream that results from mixing two separate air streams with different properties. In HVAC systems, this typically involves combining outdoor fresh air with return air from the building to achieve desired temperature and humidity conditions.
Why Mix Air?
- •Energy efficiency by pre-conditioning outdoor air
- •Meeting ventilation requirements while controlling costs
- •Achieving optimal indoor air quality
- •Reducing heating and cooling loads
Calculation Principles
Energy Balance:
ṁ₁h₁ + ṁ₂h₂ = ṁ₃h₃
- ṁ: Mass flow rate (kg/s)
- h: Specific enthalpy (kJ/kg)
- ω: Humidity ratio (kg/kg)
- 1,2,3: Outside, return, and mixed air streams
Note: Mixing is considered adiabatic (no heat transfer to surroundings) and occurs at constant pressure.
Psychrometric Properties
Dry Bulb Temperature
The temperature of air measured by a standard thermometer. It represents the sensible heat content of the air.
Humidity Ratio
The mass of water vapor per unit mass of dry air (g/kg). It directly relates to the latent heat content of the air.
Enthalpy
The total heat content of moist air (kJ/kg), including both sensible and latent heat components.