Reading the humidity ratio on a psychrometric chart is a straightforward process primarily involving the chart's vertical axis. The humidity ratio, often denoted as 'W' or 'ω', represents the mass of water vapor per unit mass of dry air and is a crucial parameter for understanding air conditioning and drying processes.
Understanding Humidity Ratio on the Chart
To accurately read the humidity ratio:
- Location of Scale: The humidity ratio has its scale on the Y-axis (the vertical axis) of the psychrometric chart. This axis is typically labeled with units such as pounds of water per pound of dry air (lbw/lbda) or grams of water per kilogram of dry air (g/kg).
- Lines of Constant Humidity Ratio: Lines of constant moisture ratios are horizontal across the chart. These parallel lines allow you to determine the humidity ratio for any given state point of air on the chart by tracing horizontally to the Y-axis.
- Relationship to Saturation Lines: Interestingly, saturation temperature lines are parallel to the humidity ratio lines. These lines meet the wet bulb temperature scale at the corresponding wet bulb temperature, providing another point of reference on the chart's intricate network of lines.
Steps to Read Humidity Ratio
Follow these steps to determine the humidity ratio for a specific air condition:
- Locate the Air State Point: First, identify the specific state point of the air on the psychrometric chart. This point is usually determined by knowing at least two independent properties, such as dry-bulb temperature and relative humidity, or dry-bulb temperature and wet-bulb temperature.
- Trace Horizontally: From the located air state point, draw or mentally trace a straight horizontal line (parallel to the lines of constant humidity ratio) directly to the left, towards the Y-axis of the chart.
- Read the Value: The point where your horizontal line intersects the Y-axis is the humidity ratio of the air at that state. Read the value from the scale on the Y-axis.
Example:
Imagine you have an air sample at 75°F (dry-bulb) and 50% relative humidity.
- Find 75°F on the X-axis.
- Follow that vertical line up until it intersects with the 50% relative humidity curve. This is your state point.
- From this intersection point, trace a horizontal line to the left, meeting the Y-axis.
- The value on the Y-axis (e.g., approximately 0.009 lbw/lbda or 9 g/kg) is your humidity ratio.
Key Aspects of Humidity Ratio Measurement
Understanding the properties and layout of the psychrometric chart is essential for accurate readings.
| Feature | Description |
|---|---|
| Axis | Y-axis (vertical axis) |
| Units | Typically lbw/lbda (pounds of water per pound of dry air) in IP units, or g/kg (grams of water per kilogram of dry air) in SI units. |
| Lines | Lines of constant humidity ratio are horizontal and parallel to each other. This parallelism also extends to saturation temperature lines, which align with wet-bulb temperature scales. |
| Significance | Critical for calculations involving moisture removal (dehumidification) or addition (humidification), as well as determining latent heat loads in HVAC systems. |
| Relationship | Humidity ratio is directly related to the partial pressure of water vapor in the air mixture and is independent of temperature, making it a stable measure of moisture content. |
The ability to accurately read humidity ratio from a psychrometric chart is fundamental for engineers, technicians, and anyone involved in air system design, analysis, or troubleshooting.
