Steam Humidifying Air

Overview

Engineering reference data for Steam Humidifying Air in air psychrometrics.

Key Formulas

Humidity Ratio

$\omega = 0.622 \frac{P_v}{P_a}$

Mass of water vapor per mass of dry air.

Relative Humidity

$\phi = \frac{P_v}{P_{vs}} \times 100\%$

Ratio of actual to saturation vapor pressure.

Wet Bulb Temperature

$T_{wb} = T_{db} - \frac{P_{vs} - P_v}{\gamma}$

Temperature measured by wet-bulb thermometer.

Enthalpy of Moist Air

$h = c_p T + \omega h_g$

Sensible + latent heat per unit mass of dry air.

Variables

Methods of Humidification

The extracted text describes two primary methods for humidifying air, each following a distinct thermodynamic process.

Humidifying with Added Water

When liquid water is added to an airstream without an external heat supply, the process is adiabatic. The air's state changes along a constant enthalpy line on the Mollier or psychrometric chart. This results in a decrease in dry-bulb temperature as the humidity ratio increases.

Humidifying with Added Steam

When steam is added to the airstream, the process follows a line of constant dh/dx ratio. For saturated steam at atmospheric pressure, this ratio equals the latent heat of vaporization of water (2502 kJ/kg), causing a near-constant dry-bulb temperature (increase typically less than 1°C). This process approximates a horizontal constant temperature line.

Additional Key Formulas

The following formulas are extracted from the examples and are critical for calculating humidification loads.

The mass flow rate of water or steam required for humidification:

$$\dot{m}_w = \frac{\dot{v} \rho (x_{out} - x_{in})}{1}$$

Where:

• $\dot{m}_w$ is the mass flow rate of water/steam (kg/s).
• $\dot{v}$ is the volume flow rate of air (m³/s).
• $\rho$ is the density of air (kg/m³).
• $x_{in}$ and $x_{out}$ are the humidity ratios at inlet and outlet (kg_w/kg_da).

The total sensible heat added to the air stream by steam (or the change in enthalpy) is:

$$q = \dot{v} \rho (h_{out} - h_{in})$$

Where:

• $q$ is the heat transfer rate (kW or kJ/s).
• $h_{in}$ and $h_{out}$ are the specific enthalpies of moist air at inlet and outlet (kJ/kg).

Steam Requirements Data

The following table provides an example of steam required for humidification based on outdoor conditions, derived from the provided text.

Interactive Charts

References

• Original Source