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Two-air-stream mixing

Balance two known dry-air mass streams and check that the resulting mixture stays unsaturated.

Balance two known dry-air mass streams and check that the resulting mixture stays unsaturated.

How this calculation works

Enter the dry-air mass flow, humidity ratio and specific enthalpy for each incoming stream. These quantities must use a consistent dry-air basis; a volume flow cannot replace a mass flow without conversion. The calculator adds the flows and takes mass-weighted averages of moisture content and enthalpy. It then rearranges the moist-air enthalpy relation to obtain the mixed dry-bulb temperature. Common absolute pressure is used to check the physical state of both inlets and the mixture. A larger stream therefore has more influence on the mixed properties than a smaller stream. This is an adiabatic, single-pressure balance with no fan heat or added water. If mixing would produce supersaturation, the calculator rejects that condition because condensation changes the simple vapor-only energy balance.

Inputs and units

  • Stream 1 dry-air flow (kg/s)
  • Stream 1 humidity ratio (g/kg dry air)
  • Stream 1 enthalpy (kJ/kg dry air)
  • Stream 2 dry-air flow (kg/s)
  • Stream 2 humidity ratio (g/kg dry air)
  • Stream 2 enthalpy (kJ/kg dry air)
  • Common absolute pressure (kPa)

Method and formula

m = m1+m2; Wm = (m1W1+m2W2)/m; hm = (m1h1+m2h2)/m; Tm = (hm−2501Wm)/(1.006+1.86Wm), with Wm in kg/kg for the temperature equation.

Worked example

Example inputs

  • Stream 1 dry-air flow: 1 kg/s
  • Stream 1 humidity ratio: 8 g/kg dry air
  • Stream 1 enthalpy: 40 kJ/kg dry air
  • Stream 2 dry-air flow: 3 kg/s
  • Stream 2 humidity ratio: 10 g/kg dry air
  • Stream 2 enthalpy: 50 kJ/kg dry air
  • Common absolute pressure: 101.325 kPa

Calculation steps

  1. Total dry-air flow = 1 + 3 = 4 kg/s.
  2. Mixed humidity ratio = (1 × 8 + 3 × 10) / 4 = 9.5 g/kg dry air; mixed enthalpy = (1 × 40 + 3 × 50) / 4 = 47.5 kJ/kg dry air.
  3. Convert 9.5 g/kg to 0.0095 kg/kg. Mixed temperature = (47.5 − 2501 × 0.0095) / (1.006 + 1.86 × 0.0095) ≈ 23.191556 °C. All three states are checked at 101.325 kPa.

Example results

  • Total dry-air flow: 4 kg/s
  • Mixed humidity ratio: 9.5 g/kg dry air
  • Mixed specific enthalpy: 47.5 kJ/kg dry air
  • Mixed dry bulb: 23.19155587 °C

Assumptions

  • Adiabatic mixing at one pressure, with no fan heat, water injection or condensation.
  • Flows are dry-air mass flows, not moist-air mass or volume flows.

Limitations

  • Each stream and mixture must be an unsaturated state between −20 and 50 °C.
  • A rejected supersaturated mixture requires a two-phase energy balance.
  • Preliminary educational check; verify inputs and equipment data before design or operation.

Sources

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