Thermodynamic Properties of Moist Air
Definition
Moist air is a binary mixture of dry air and water vapor. In the study of thermodynamics and psychrometrics, it is treated as a mixture of two ideal gases, where dry air follows the ideal gas law and water vapor is treated as a superheated vapor at low partial pressures. The thermodynamic properties of this mixture are essential for analyzing air conditioning, drying processes, and cooling towers.
Main Content
1. Humidity and Absolute Humidity (Humidity Ratio)
- Humidity ratio ($\omega$) is the mass of water vapor present in a unit mass of dry air. It is expressed in kg of water vapor per kg of dry air.
- It is calculated using the ratio of partial pressures: $\omega = 0.622 \frac{P_v}{P_b - P_v}$, where $P_v$ is the partial pressure of vapor and $P_b$ is the barometric pressure.
2. Relative Humidity and Degree of Saturation
- Relative humidity ($\phi$) is the ratio of the partial pressure of water vapor in the mixture to the saturation pressure of pure water vapor at the same temperature.
- Degree of saturation ($\mu$) is the ratio of the actual humidity ratio to the humidity ratio at saturation for the same temperature.
3. Enthalpy and Dew Point
- The enthalpy of moist air ($h$) is the sum of the enthalpy of the dry air and the enthalpy of the water vapor associated with it ($h = c_p t + \omega h_g$).
- The Dew Point Temperature ($T_{dp}$) is the temperature at which water vapor in the air begins to condense into liquid droplets when the air is cooled at constant pressure.
Working / Process
1. Determining Partial Pressures
- Identify the total atmospheric pressure of the environment.
- Measure the dry-bulb temperature of the air sample to look up the saturation pressure from steam tables.
2. Calculating the Humidity Ratio
- Using the measured dry-bulb and wet-bulb temperatures, use the psychrometric equation: $P_v = P_{vsw} - \frac{P_b(T_{db} - T_{wb})}{1544 - 1.44 T_{wb}}$.
- Substitute $P_v$ into the humidity ratio formula to determine the moisture content in the air.
3. Plotting on the Psychrometric Chart
- Locate the dry-bulb temperature on the horizontal axis and move vertically to the calculated humidity ratio.
- Identify the intersection point, which defines the state of the moist air, allowing for the determination of enthalpy, relative humidity, and specific volume.
Psychrometric Chart Representation:
| / (Enthalpy)
| /
|/ (Constant Humidity Ratio)
+----------------------------> Dry Bulb Temp (C)
|
| / (Relative Humidity Curves)
| /
| /
|/
Advantages / Applications
- HVAC Design: Used to determine heating, cooling, humidification, and dehumidification requirements for buildings.
- Industrial Drying: Crucial for controlling moisture removal in food processing and timber industries.
- Cooling Towers: Essential for evaluating the efficiency of heat rejection by evaporating water into an air stream.
Summary
Moist air thermodynamics involves the analysis of dry air mixed with water vapor to predict behaviors in atmospheric conditions. By calculating properties like humidity ratio, relative humidity, enthalpy, and dew point, engineers can effectively control air quality and thermal comfort.
Key terms: Dry-bulb temperature, Wet-bulb temperature, Psychrometry, Saturation pressure, Condensation.