With this mixed air calculator, you can easily determine the mixed air stream properties without the need for a psychrometric chart. Efficient HVAC system design requires accurately calculating the mixed air stream's properties, such as enthalpy, humidity ratio, dry bulb temperature, and more. If you're wondering how to calculate mixed air dry bulb temperature, look no further!

Keep on reading to learn:

  • What mixed air is, and its importance in HVAC systems;
  • What HVAC return air is and the difference with HVAC outside intake air;
  • How to calculate mixed air temperature; and
  • How to use the psychrometric chart to calculate mixed air properties.

We've also included an example of calculating mixed air properties using this calculator.

What's mixed air?

Mixed air refers to the stream of air that results from combining two separate streams of air with different properties, such as temperature, humidity, and flow rate. The resulting mixture of air will have unique properties that depend on the properties of the original streams and the proportion in which they were mixed.

Air mixing chamber that displays incoming streams, outside and return air, and the outgoing stream of mixed air.

Mixed air is a crucial aspect of HVAC systems (heating, ventilation, and air conditioning) used in various applications, including buildings, hospitals, and industrial processes. In these applications, it's necessary to mix a fraction of the conditioned air (return air stream) with the fresh outdoor air (outdoor air stream) to achieve the desired indoor air temperature and humidity. By mixing the return and outdoor air, the system can pre-condition the air before sending it back to the occupied space.

How to calculate mixed air properties: Mixed air calculations and formulas

Now that we know more about mixed air and its importance in air conditioning, we can explore how to determine its properties.

Psychrometric charts are handy for determining the state and properties of moist air. By using these charts, we can obtain information about parameters such as dry bulb temperature TdbT_{\rm db}, humidity ratio ω\omega, vapor pressure PvP_v, dew point temperature TdpT_{\rm dp}, enthalpy hh, relative humidity % RH\% \rm{\ RH}, and specific volume vv.

Take a moment to learn more about this topic with our psychrometric calculator

In addition to the properties we can read from the psychrometric charts, we'll need to use some formulas. When we mix the two air streams, the heat transfer to the surroundings is so small that we can consider it to be an adiabatic process. This means that the energy balance does not include the heat term. Plus, no work interactions are involved in the mixing process, and any changes in kinetic and potential energies can be ignored. As a result, we can express the energy and mass balances using the following formulas:

Energy balance:

m˙1h1+m˙2h2=m˙3h3\dot{m}_1\,h_1 + \dot{m}_2\,h_2 = \dot{m}_3\,h_3

Dry air mass balance:

m˙1+m˙2=m˙3\dot{m}_1 + \dot{m}_2 = \dot{m}_3

Vapor mass balance:

m˙1ω1+m˙2ω2=m˙3ω3\dot{m}_1\,\omega_1 + \dot{m}_2\,\omega_2 = \dot{m}_3\,\omega_3

By combining these expressions, we can obtain the following relationship:

m˙1m˙2=ω2ω3ω3ω1=h2h3h3h1\cfrac{\dot{m}_1}{\dot{m}_2} = \cfrac{\omega_2 - \omega_3}{\omega_3 - \omega_1} = \cfrac{h_2 - h_3}{h_3 - h_1}

where:

  • m˙\dot{m} — Mass flow rate;
  • hh — Enthalpy;
  • ω\omega — Humidity ratio; and
  • 1,2, and 31, 2, \rm{\ and \ } 3 — Subscripts for outside, return and mixed air respectively.

The steps to determine the properties of the mixed air stream using the psychrometric chart are as follows:

  1. With the temperature and relative humidity of the outside air (stream 1) and the return air (stream 2), enter the chart and read their enthalpy hh, humidity ratio ω\omega, and specific volume vv.

  2. With these values, calculate the mass flow m˙3\dot{m}_3, enthalpy h3h_3, and humidity ratio ω3\omega_3 of the mixed air stream (stream 3) using the mixed air calculation formulas shown above. These last two define the state of the mixture.

  3. Enter to the psychrometric chart with the enthalpy h3h_3 and humidity ratio ω3\omega_3 and read other properties of the mixture, such as dry bulb temperature Tdb 3T_{\rm db \ 3}, relative humidity % RH3\% \rm{\ RH}_3, and the specific volume v3v_3.

  4. Calculate the volumetric flow rate V3=m˙3v3V_3 = \dot{m}_3 \cdot v_3 using the specific volume v3v_3 and mass flow rate m˙3\dot{m}_3.

🙋 Learn more about relative humidity and dew point temperature with Omni's relative humidity and dew point calculators.

An example using the mixed air calculator — How to calculate mixed air dry bulb temperature

Now that we understand what mixed air is and how to calculate its properties, let's see how we can make these calculations quite simple with this mixed air calculator. To show how to use this tool, we'll use an example.

Suppose we have an air conditioning process where the stream of HVAC return air is saturated at 14 °C and has a volumetric flow rate of 50 m3/min. This stream is mixed adiabatically with a stream of HVAC outside air intake of 32 °C and 60% relative humidity, with a volumetric flow rate of 20 m3/min. Our task is to determine the properties of the resulting mixed air stream. We'll consider that the mixing occurs at a standard atmospheric pressure of 1 atm.

To solve this example with the mixed air calculator:

  1. Begin by specifying the mixing pressure, which in this case is 1 atm or 101.35 kPa. The calculator is preset to this value by default.

  2. Input the known properties of the outside air into the Outside air section of the calculator. In our example, this includes dry bulb temperature (Tdb1) of 32 °C, relative humidity (% HR1) of 60, and volumetric flow rate (V1) of 20 m3/min.

  3. With these inputs, the calculator will determine the remaining properties of the outside air:

Property name

Symbol

Value

Humidity ratio

ω1

18.025 g/kg

Dew point temperature

Tdp1

23.27 °C

Specific volume

v1

0.8895 m3/kg

Mass flow rate

m1

0.37474 kg/s

Enthalpy

h1

78.33 kJ/kg

Vapour pressure

Pv1

2.854 kPa

  1. Repeat the same process for the return air stream, inputting the known values of temperature (Tdb2) of 14 °C, 100% saturation (% RH2), and volumetric flow rate (V2) of 50 m3/min, this time in the corresponding Return air section.

  2. The calculator will determine the remaining properties of the return air:

Property name

Symbol

Value

Humidity ratio

ω2

9.965 g/kg

Dew point temperature

Tdp2

14 °C

Specific volume

v2

0.8265 m3/kg

Mass flow rate

m2

1.0083 kg/s

Enthalpy

h2

39.255 kJ/kg

Vapour pressure

Pv2

1.598 kPa

  1. Once both streams are defined, the tool will calculate the mixed air temperatures along with the other properties associated with the resulting mixed air stream (stream 3). In our example, we get the following results:

Property name

Symbol

Value

Volumetric flow rate

V3

70 m3/min

Dry bulb temperature

Tdb3

18.93 °C

Relative humidity

% RH3

88.79 %

Humidity ratio

ω3

12.15 g/kg

Dew point temperature

Tdp3

17.04 °C

Specific volume

v3

0.8436 m3/kg

Mass flow rate

m3

1.383 kg/s

Enthalpy

h3

49.84 kJ/kg

Vapour pressure

Pv3

1.9414 kPa

With the mixed air calculator, we can quickly and easily determine the properties of a mixed air stream without the need for a psychrometric chart.

🙋 Note that you can also use this tool to calculate any of the streams as long as you provide the properties of the other two.

FAQ

Is air mixing an adiabatic process?

Yes, air mixing is considered an adiabatic process, as there is very little heat transfer to the surroundings. Also, as there's no work involved in the mixing process, the energy balance equation simplifies to:

m1h1 + m2h2 = m3h3

Here, m represents mass flow, and h is the enthalpy of each stream. The subscripts 1, 2, and 3 refer to the outside, return, and mixed air, respectively.

What's mixed air dry bulb temperature if h=38kJ/kg and ω=8g/kg?

The dry bulb temperature is Tdb = 17.63 °C. You can find this value using a psychrometric chart by locating the intersection of these two values and moving vertically to the horizontal axis to read the dry bulb temperature.

Alternatively, you can use this formula:

h = 1.006 × Tdb + ω × (2499.86 + 1.86 × Tdb)

Substitute the known values of enthalpy h and humidity ratio ω, and solve for Tdb.

How do I calculate mixed air properties?

To calculate mixed air properties, use a psychrometric chart or mixed air calculation formulas. The general procedure is as follows:

  1. Determine the enthalpy, humidity ratio, and specific volume of the HVAC return air and outside air streams.

  2. Calculate the mass flow, enthalpy, and humidity ratio of the mixed air stream using these expressions:

    m1h1 + m2h2 = m3h3
    m1 + m2 = m3
    m1ω1 + m2ω2 = m3ω3

  3. With h3, and ω3, determine the dry bulb temperature, relative humidity, and specific volume.

  4. Calculate the volumetric flow rate with:

    V3 = m3v3

Gabriela Diaz
Air mixing chamber that displays incoming streams, outside and return air, and the outgoing stream of mixed air
Pressure
Mixing pressure (Pa)
inHg
Outside air (stream 1) - Inputs
Volumetric flow rate (V1)
ft³/min
Dry bulb temperature (Tdb1)
°F
Relative humidity (% RH1)
%
Humidity ratio (ω1)
g
/kg
Outside air (stream 1) - Properties
Dew point temperature (Tdp1)
°F
Vapour pressure (Pv1)
inHg
Specific volume (v1)
ft³/lb
Mass flow rate (m1)
lb/s
Enthalphy (h1)
kJ
/kg
Return air (stream 2) - Inputs
Volumetric flow rate (V2)
ft³/min
Dry bulb temperature (Tdb2)
°F
Relative humidity (% RH2)
%
Humidity ratio (ω2)
g
/kg
Return air (stream 2) - Properties
Dew point temperature (Tdp2)
°F
Vapour pressure (Pv2)
inHg
Specific volume (v2)
ft³/lb
Mass flow rate (m2)
lb/s
Enthalphy (h2)
kJ
/kg
Mixed air properties (stream 3)
Volumetric flow rate (V3)
ft³/min
Dry bulb temperature (Tdb3)
°F
Relative humidity (% RH3)
%
Humidity ratio (ω3)
g
/kg
Dew point temperature (Tdp3)
°F
Vapour pressure (Pv3)
inHg
Specific volume (v3)
ft³/lb
Mass flow rate (m3)
lb/s
Enthalphy (h3)
kJ
/kg
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