Psychrometrics is the study of moist air, and the psychrometric chart plots six linked properties so you can read how air changes during heating, cooling, mixing and humidification. At a typical 25 °C and 50 percent relative humidity the air holds about 10.5 g of moisture per kg of dry air and carries roughly 50 kJ/kg of enthalpy; the chart lets you find that point and any resulting state without a single formula.
The Six Properties on the Chart
Every point on the chart is defined by dry-bulb temperature on the x-axis, wet-bulb temperature, dew point, relative humidity, humidity ratio in g/kg, and enthalpy in kJ/kg. They are not independent; fix two and the rest follow. At 25 °C and 50 percent RH the humidity ratio is about 10.5 g/kg and enthalpy about 50 kJ/kg, the pair you use for cooling-load and duct-sizing math. Learning to locate this point is the first skill in reading the chart.
How the Lines Are Drawn
Dry-bulb temperature runs horizontally; the curved relative-humidity lines bow from lower left to upper right and meet at 100 percent saturation, the saturation curve. Diagonal lines sloping down-right are wet-bulb and constant-enthalpy; near-vertical lines are constant dew point; and the nearly horizontal lines are constant humidity ratio. The table summarises what each line family means when you move along it.
| Line family | What is constant | Direction | Process example |
|---|---|---|---|
| Vertical | Humidity ratio | Up or down | Sensible heating or cooling |
| Curved RH | Relative humidity | Bowing | Saturation at 100 percent |
| Diagonal | Enthalpy | Down-right | Adiabatic humidify |
| Horizontal | Dew point | Left to right | Dehumidify on cooling |
Reading the Common Processes
A vertical move up at fixed dry-bulb is sensible heating: no moisture leaves, so the humidity ratio stays put while enthalpy rises. A cooling coil pulls the state down and left along a curve to the saturation line, removing sensible heat first then condensing moisture as it crosses the dew point, about 14 °C at our example point. Adiabatic humidification, such as a wetted pad, moves down-right at near-constant enthalpy, trading temperature for moisture. Mixing two streams is a straight line whose position is weighted by the mass flow of each.
Using the Chart for Real Design
Suppose you mix 20 percent outdoor air at 35 °C and 80 percent RH with 80 percent return at 24 °C and 50 percent RH. The mixed point lies 20 percent of the way from return toward outdoor, landing near 26 °C and 56 percent RH. The cooling coil must then carry that air down to a 12 °C supply, crossing the dew point and condensing roughly 4 to 6 g/kg of moisture. The chart shows both the sensible drop of about 14 °C and the latent load in one picture, so you size the coil in kW not guesswork.
When to Use the Chart vs Software
- Use the chart for quick checks, commissions, and teaching where a visual answer beats a number.
- Use it to verify software output when a coil size looks wrong by a factor of two.
- Use psychrometric software when you process 100 zones or run hourly energy simulation.
- Use the chart for mixing and bypass calculations where the geometry is intuitive.
Avoiding Reading Errors
The most common mistake is confusing wet-bulb and dry-bulb: the wet-bulb line is the diagonal, not the horizontal axis, and using the wrong one doubles the enthalpy error. The second is ignoring altitude; charts are drawn for sea level, and at 1,500 m the saturation pressure shifts, so use an altitude-corrected chart. Finally, read enthalpy at the point, not at the dry-bulb, because latent load hides in the humidity ratio that the simple temperature never shows.
Keruite manufactures this equipment at an 80,000 m² facility in Dezhou, China, with an in-house AMCA-210-style test laboratory; every unit is balanced to G2.5 and performance-tested before shipment, and coil enthalpy performance is verified against psychrometric points on the test rig.
Mixing Two Airstreams on the Chart
Mixing is pure geometry: plot the outdoor and return states, join them with a line, and place the mixed point a fraction of the way from return toward outdoor equal to the outdoor mass fraction. With 20 percent outdoor at 35 °C and 80 percent RH mixed with 80 percent return at 24 °C and 50 percent RH, the point lands near 26 °C and 56 percent RH, and the enthalpy there sets the coil duty. Getting this wrong by using volume instead of mass fraction shifts the point 1 to 2 °C and oversizes the coil by 5 to 10 percent, so always weight by mass of dry air, not by the simple percentage of supply volume.
Reading Sensible and Latent Split
The cooling coil process moves the state down and left to the saturation curve then along it, and the horizontal distance is the sensible drop while the vertical drop to the dew-point line is the latent removal. A coil that must take 26 °C mixed air to a 12 °C supply crosses the dew point near 14 °C and condenses roughly 4 to 6 g/kg, so the total enthalpy drop of about 30 kJ/kg is the real coil load, not the temperature alone. Designers who size on temperature only short the coil on moisture and get a clammy space in humid weather.
Case Study: A 30,000 m³/h AHU
A 30,000 m³/h air-handling unit was sized on a 14 °C temperature drop and left the space humid. Plotting the mix on the chart showed the true load was 32 kJ/kg of enthalpy, not the 22 kJ/kg the temperature-only calc assumed, because the 30 percent outdoor air at 80 percent RH carried heavy latent load. The coil was reselected for the full enthalpy and a reheater added for control, after which the supply met both temperature and humidity and the complaint calls dropped from daily to zero across one summer.
FAQ
What are the six psychrometric properties on the chart?
The chart shows dry-bulb temperature, wet-bulb temperature, dew point, relative humidity, humidity ratio in g/kg, and enthalpy in kJ/kg. A point at 25 °C and 50 percent RH sits near 10.5 g/kg moisture and 50 kJ/kg enthalpy, the values used for every HVAC load calculation.
How do I read relative humidity from the chart?
Find the dry-bulb temperature on the horizontal axis, move up to the point, and read relative humidity from the curved lines crossing it; at 25 °C and 50 percent RH the dew point is about 14 °C. The curved RH lines run from the lower left to the upper right and meet at 100 percent saturation.
What does a straight line up mean on the chart?
A vertical line up at fixed dry-bulb is sensible heating or cooling with no moisture change; humidity ratio stays constant. A line to the upper left at constant enthalpy is adiabatic humidification. Cooling coil process is a curve down to the saturation line, removing both sensible and latent heat at once.
Why does the chart matter for HVAC design?
It turns air-mixing and cooling into geometry you can read, so you size coils and reheats correctly. A mixed airstream of 20 percent outdoor at 35 °C and 80 percent RH with 80 percent return at 24 °C lands near 26 °C, and the coil must remove both the sensible and the latent load shown.