Dew Point Flue Gases

Overview

Engineering reference data for Dew Point Flue Gases 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

Acid Dew Point Considerations

In combustion systems, the presence of sulfur in the fuel leads to the formation of sulfur trioxide (SO₃). When combined with water vapor, this forms sulfuric acid (H₂SO₄), which condenses at a much higher temperature than water alone. This is known as the acid dew point.

The acid dew point temperature is critical for material selection and corrosion prevention in boilers, flue gas ducts, and air preheaters. Operating above the acid dew point is essential to prevent severe corrosion and fouling.

Key Dew Point Formula

A widely used empirical formula for estimating the water dew point temperature ($T_{dp}$) of a flue gas is based on the partial pressure of water vapor ($P_{H_2O}$):

$$T_{dp} = \frac{4030 \cdot (B + \ln P_{H_2O})}{20 - B - \ln P_{H_2O}}$$

Where:

• $T_{dp}$ is the dew point temperature in Kelvin (K).
• $B$ is a constant related to the fuel type (e.g., ~1.84 for natural gas).
• $P_{H_2O}$ is the partial pressure of water vapor in the flue gas, typically in atmospheres (atm).

Factors Affecting Dew Point

The dew point of flue gases is primarily influenced by:

• Moisture Content: Higher moisture content increases the dew point.
• Fuel Composition: Fuels with higher hydrogen or sulfur content increase the water and acid dew points, respectively.
• Combustion Air Humidity: Humid air introduces additional moisture, raising the dew point.
• Excess Air: While necessary for combustion, excessive air dilutes the flue gas, slightly lowering the partial pressure of water vapor and thus the dew point.

References

• Original Source