# MoistAirTab™ Example 5 - Humidification

#### Example 5

Steam (or liquid water) injection is frequently used in air conditioning for humidification. Moist air at 20°C db and 8°C wb is being humidified by injecting saturated steam at 110°C to a final stream with a dew point of 13°C. The rate of dry air flow is 2 kg/s. What is the final db of the moist air and what is the steam injection rate?

#### Solution

The solution is presented below as a table generated from Excel. The gray shaded areas represent the input data.

Key concepts:. | | | | | |

1. Adiabatic mixing assumes that no net gain or loss of total energy in the mixing process |

2. The dew point temperature specifies the moisture content in the final mixture. |

Overall Enthalpy balance | m_{air }h_{1} + m_{w} h_{w} = m_{air }h_{2} (1) |

Water balance | m_{air} W_{1} + m_{w} = m_{air} W_{2} |

or | m_{w} = m_{air} (W_{2} - W_{1}) (2) |

From (1) and (2) | (h_{2}-h_{1})/(W_{2}-W_{1}) = h_{w} |

or | h_{2 }= h_{w} (W_{2}-W_{1}) + h_{1} (2a) |

| | | | | |

| **Units** | **Moist Air** | **Steam** | **Outlet Mixture** | |

Pressure | bar | 1.01325 | 1.01325 | 1.01325 | |

Dry air flowrate | kg_{dry air}/s | 2 | | 2 | |

Dry bulb temperature | ^{o}C | 20 | 110 | 21.14 | |

Wet bulb temperature | ^{o}C | 8 | | 16.08 | |

Dew Point temperature | ^{o}C | -8.72 | | 13 | |

Humidity ratio | kg_{w}/kg_{dry air} | 0.0018 | | 0.0094 | |

Enthalpy of moist air | kJ/kg_{dry air} | 24.69 | 2690.95 | 45.06 | *solved by equation (2a)* |

Relative humidity | % | 12% | | 60% | |

Steam flowrate | kg/s | | 0.0151 | | *solved by equation (2)* |