The psychrometric chart is a graphical tool that displays the physical and thermal properties of moist air at a constant pressure. It serves as a visual map for understanding the complex relationships between temperature, moisture, and energy within an air-water vapor mixture. Professionals in heating, ventilation, and air conditioning (HVAC) rely on this chart to analyze air patterns and predict how changes in heat or moisture will affect the air’s condition.
Decoding the Key Air Properties
The foundation of using the chart lies in understanding the multiple properties of air represented by various lines and axes. The horizontal axis at the bottom of the chart is dedicated to the Dry-Bulb Temperature (DBT), which is the standard air temperature measured by a regular thermometer. Lines of constant DBT run vertically up the chart.
The vertical axis, typically on the right side, represents the Humidity Ratio (or moisture content), which is the mass of water vapor contained per unit mass of dry air. Horizontal lines across the chart correspond to a constant humidity ratio. Moving higher on the chart along this axis means the air contains more moisture.
Relative Humidity (RH) is shown by curved lines that sweep diagonally from the bottom left toward the top right of the chart. These lines indicate the percentage of water vapor in the air compared to the maximum amount the air can hold at that same temperature. The curve along the top boundary of the chart represents 100% RH, known as the saturation line.
The Wet-Bulb Temperature (WBT) is indicated by diagonal lines that slope gently downward from left to right, intersecting the saturation curve. WBT is the temperature air reaches when cooled by evaporating water into it until it is saturated. The difference between the dry-bulb and wet-bulb temperatures, known as the wet-bulb depression, relates directly to the relative humidity of the air.
The total energy content of the moist air, known as Enthalpy, is represented by a set of diagonal lines that are nearly parallel to the wet-bulb temperature lines. Enthalpy is a measure of the total heat energy in the air and water vapor mixture. These lines allow for calculations of heat added or removed during air conditioning processes.
Plotting Points and Finding Unknown Values
To use the psychrometric chart effectively, a specific condition of the air must first be defined as a single point, known as a state point. A state point can be fixed by knowing any two independent properties. For instance, knowing the dry-bulb temperature and the relative humidity is sufficient to locate the point.
The process begins by locating the first known property on its corresponding scale. Next, the second known property, such as the relative humidity curve, is located on the chart. The precise intersection of the lines or curves representing these two known values establishes the unique state point for that air sample.
Once the state point is plotted, all other properties of the air can be read directly from the chart by tracing along the relevant lines that pass through that point. For example, by following the diagonal wet-bulb line or the horizontal humidity ratio line, the corresponding WBT and moisture content values can be determined.
Analyzing Common Air Conditioning Processes
The psychrometric chart is a dynamic tool that shows how air properties change as it undergoes various air conditioning processes, which are represented by lines connecting an initial state point to a final state point. Sensible Heating or Cooling involves only changing the temperature of the air without adding or removing any moisture. This process is shown as a horizontal line moving right for heating or left for cooling, since the dry-bulb temperature changes but the humidity ratio remains constant.
When air is only Humidified, moisture is added to the air while the dry-bulb temperature is held constant. This process is represented by a vertical line moving straight upward on the chart, resulting in a higher humidity ratio and often a higher relative humidity. Conversely, ideal Dehumidification without cooling would be shown as a vertical line moving downward, removing moisture while maintaining the same dry-bulb temperature.
A common process, Cooling and Dehumidification, occurs when air passes over a coil whose surface temperature is below the air’s dew-point temperature, causing water vapor to condense. On the chart, this is typically shown as a line that slopes diagonally downward and to the left, indicating a decrease in both dry-bulb temperature and humidity ratio. This combined process is used in most air conditioning systems.
The process of Mixing Two Air Streams, such as outdoor air and return air, is visualized by drawing a straight line between the two individual state points. The resulting state point of the mixed air will fall somewhere on this line, with its exact location determined by the mass flow rate ratio of the two streams. This geometric representation simplifies the calculation of the resulting temperature and moisture content of the combined air.