Psychrometric Chart Basics

An introduction to understanding and using psychrometric charts in HVAC.

Understanding the Psychrometric Chart

A psychrometric chart is a graphical representation of the thermodynamic properties of moist air. It's an indispensable tool for HVAC engineers and technicians to analyze air conditioning processes, determine air properties, and design systems for optimal comfort and efficiency.

Sample Psychrometric Chart

A visual representation of a typical psychrometric chart.

Key Properties on a Psychrometric Chart:

  • Dry-Bulb Temperature (DBT): The actual air temperature, typically shown on the horizontal axis.
  • Wet-Bulb Temperature (WBT): Indicates the temperature air would cool to if moisture were evaporated into it until saturation. Lines of constant WBT slope diagonally downward from left to right.
  • Dew Point Temperature (DPT): The temperature at which air becomes saturated and condensation begins. Horizontal lines often represent constant DPT.
  • Relative Humidity (RH): The ratio of moisture in the air to the maximum amount it could hold at that temperature, shown as curved lines (typically from 10% to 100% saturation).
  • Humidity Ratio (or Specific Humidity): The mass of water vapor per unit mass of dry air, typically shown on the vertical axis to the right.
  • Enthalpy: The total heat content of the air (sensible + latent), shown on diagonal lines, often extending from the saturation curve.
  • Specific Volume: The volume per unit mass of dry air, shown as less steeply sloped diagonal lines.

Common HVAC Processes Visualized on the Chart:

  • Sensible Heating/Cooling: Horizontal movement left (cooling) or right (heating).
  • Humidification/Dehumidification: Vertical movement up (humidifying) or down (dehumidifying).
  • Cooling and Dehumidification: Movement down and to the left (typical AC coil process).
  • Evaporative Cooling: Movement along a constant wet-bulb line, up and to the left.
  • Mixing of Airstreams: A point on a straight line connecting the two initial air states.

Note: Psychrometric charts are typically constructed for a specific barometric pressure (e.g., sea level). Corrections may be needed for significantly different altitudes. This is a basic overview; mastering the chart requires practice and deeper study.