Humidifiers

Overview

Engineering reference data for Humidifiers 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

Safety Considerations

A critical safety warning must be included for any humidification system using water:

Warning: Water-borne bacteria causing Legionnaires' disease (Legionella) may develop in humidifying water systems and be transferred to humans via circulating air. Proper water treatment with chlorine dioxide or similar biocides is vital to mitigate this risk.

Humidification Processes

The thermodynamic process of adding moisture to air follows specific paths on psychrometric charts. The text describes two main scenarios:

1. Evaporative Humidification: When water evaporates into the air using its own heat, the process follows a constant enthalpy line, approaching the saturation curve.
2. Steam Humidification: When steam is injected, the enthalpy of the air increases. For practical purposes with saturated steam at atmospheric pressure, the process approximates a nearly horizontal line on a Mollier diagram and a vertical line on a psychrometric chart, with a negligible dry-bulb temperature increase (typically less than 1°C).

Key Process Formulas

The key relationships governing humidification can be expressed as follows:

• Evaporative Process (Constant Enthalpy): $h_1 = h_2$ Where $h$ is the specific enthalpy of the air.

• Steam Injection Process (Humidity Ratio Increase): $\omega_2 = \omega_1 + \frac{\dot{m}_{steam}}{\dot{m}_{dry\_air}}$ Where $\omega$ is the humidity ratio (kg/kg dry air), and $\dot{m}$ is mass flow rate.

• Approximate Temperature Change for Steam Addition: $\Delta T_{db} \approx 0 \quad \text{(for saturated steam at 1 atm)}$ This is a simplification; actual temperature change depends on steam temperature and pressure.

Humidifier Types

The extracted text lists three primary mechanical humidifier types:

• Sprayed Coils: Water is sprayed onto a finned coil within an air stream (2-3.5 m/s velocity). The coil provides evaporation surface and can be heated. Unevaporated droplets are collected and recirculated.
• Spinning Discs: Water flung from a rotating disc into a toothed ring, atomizing into fine droplets that evaporate in the air stream.
• Steam Humidifiers: Low-pressure steam is injected into the air via a distribution tube. Considered lower risk for microbial transfer as water is boiled.

References

• Original Source