Psychrometric Chart
The psychrometric chart is an interactive tool for visualizing and analyzing air conditions in HVAC systems.
Understanding the Chart
Section titled “Understanding the Chart”Axes and Lines
Section titled “Axes and Lines”Dry-Bulb Temperature
Section titled “Dry-Bulb Temperature”The horizontal axis shows dry-bulb temperature (°F or °C). This is the temperature measured by a standard thermometer.
Humidity Ratio
Section titled “Humidity Ratio”The vertical axis shows humidity ratio (lb/lb or g/kg). This is the mass of water vapor per mass of dry air.
Relative Humidity Lines
Section titled “Relative Humidity Lines”Curved lines showing constant relative humidity from 10% to 100% (saturation line).
Wet-Bulb Lines
Section titled “Wet-Bulb Lines”Diagonal lines showing constant wet-bulb temperature, used for evaporative cooling analysis.
Enthalpy Lines
Section titled “Enthalpy Lines”Lines of constant enthalpy (BTU/lb or kJ/kg), representing total heat content.
State Points
Section titled “State Points”A state point on the chart represents a specific air condition:
- Dry-bulb temperature
- Wet-bulb temperature
- Humidity ratio
- Relative humidity
- Enthalpy
- Specific volume
Using the Chart
Section titled “Using the Chart”Placing Points
Section titled “Placing Points”Add a state point in either of two ways:
- Click anywhere on the chart to drop a point at that condition, or
- Click Create State Point in the State Points panel to add one you can type exact values into
Set the dry-bulb temperature plus either the wet-bulb temperature or relative humidity — the remaining properties (dew point, humidity ratio, enthalpy, and specific volume) are calculated and shown in the point’s details. Toggle Draggable to control whether the point can be repositioned by dragging it on the chart.
Moving Points
Section titled “Moving Points”Drag points to see how properties change:
- Move horizontally for temperature change
- Move vertically for humidity change
- All properties update in real-time
Connecting Points
Section titled “Connecting Points”Draw a process line between two state points:
- Click Create Process Line in the Process Lines panel
- Choose the From and To points
- Click Create — the process line is drawn between them
Each process line takes an optional Flow Rate (CFM or L/s) — the volume of the stream that drives the loads — and a Path (Straight or Saturated Cooling). It reports its Sensible Heat, Latent Heat, and Total Heat from those inputs. You need at least two state points before a process line can be created.
HVAC Processes
Section titled “HVAC Processes”Sensible Cooling
Section titled “Sensible Cooling”Horizontal line to the left:
- Temperature decreases
- Humidity ratio stays constant
- Common for cooling coils above dew point
Sensible Heating
Section titled “Sensible Heating”Horizontal line to the right:
- Temperature increases
- Humidity ratio stays constant
- Common for heating coils
Cooling with Dehumidification
Section titled “Cooling with Dehumidification”Line toward saturation then left:
- Air cools to dew point
- Moisture condenses
- Continues along saturation line
Humidification
Section titled “Humidification”Line upward (higher humidity ratio):
- Steam injection (isothermal)
- Evaporative (adiabatic)
Mixing
Section titled “Mixing”Straight line between two air streams:
- Mixed condition on the line
- Position based on airflow ratios
Elevation Adjustment
Section titled “Elevation Adjustment”Psychrometric properties vary with altitude:
- Set site elevation
- Chart adjusts automatically
- Sea-level vs. high altitude
Practical Applications
Section titled “Practical Applications”Coil Analysis
Section titled “Coil Analysis”Visualize cooling coil performance:
- Entering air condition
- Leaving air condition
- Sensible Heat Ratio
Economizer
Section titled “Economizer”Analyze mixed air conditions:
- Outdoor air point
- Return air point
- Mixed air result
Dehumidification
Section titled “Dehumidification”Evaluate moisture removal:
- Coil leaving condition
- Reheat requirements
- Latent capacity
Saving & Exporting
Section titled “Saving & Exporting”Download as PNG
Section titled “Download as PNG”The Download as PNG toolbar button captures the current chart as a PNG image, including the state points and process lines.
Save the chart
Section titled “Save the chart”Use Save to store the chart — its points, process lines, elevation, and view settings — under a name. Saved charts are listed in the History sidebar, where you can reopen, rename, or delete them. New Chart starts a fresh, empty chart.