Definition
An engineering tool that displays the thermodynamic properties of air (temperature, humidity, enthalpy, volume) on a single chart. Used in the design of dehumidification and air conditioning processes, determining system operating points, and energy calculations.
Detailed Explanation
The psychrometric chart is an engineering tool that displays the eight fundamental thermodynamic properties of moist air on a single 2-dimensional graph. These properties are: dry bulb temperature, wet bulb temperature, dew point, relative humidity, specific humidity, enthalpy, specific volume, and vapor pressure. If any two independent properties are known, the others can be read directly from the chart.
The chart is drawn for standard atmospheric pressure (101.325 kPa). Values shift with altitude or pressure changes; ASHRAE charts are therefore published at sea level reference. For high-altitude facilities (Erzurum at 1850 m, Konya at 1016 m), pressure-corrected charts or calculation software must be used.
HVAC processes are represented as lines on the chart: heating is horizontal-right, cooling is horizontal-left, dehumidification is downward, humidification is upward, and cooling+drying is the lower-left diagonal. This directional analysis is the visual foundation of system design.
Axis Conventions
ASHRAE (American) convention: x-axis = dry bulb temperature (°C) y-axis = specific humidity W (g/kg dry air) Curved lines = relative humidity (%) Diagonal lines = enthalpy (kJ/kg)
Mollier (European) convention: x-axis = specific humidity W (g/kg dry air) y-axis = enthalpy h (kJ/kg) or temperature (°C) Vertical lines = relative humidity (%)
The axes differ, but the content is identical between conventions.
Practical Example
Consider an AHU (air handling unit) design problem. The mixed air point is 28°C, 60% RH. The supply air target is 14°C, 95% RH (condensation limit). The work the cooling coil must perform is visualized as a line between the two points on the chart.
Values read from the chart: • Mixed air: W₁ = 14.3 g/kg, h₁ = 64.8 kJ/kg • Supply air: W₂ = 9.3 g/kg, h₂ = 37.5 kJ/kg • Total load (enthalpy difference): Δh = 27.3 kJ/kg • Latent load (W difference): ΔW = 5.0 g/kg → ≈ 12.5 kJ/kg • Sensible load: 14.8 kJ/kg
Without the chart, this analysis takes minutes; with the chart or psychrometric calculator, it completes in seconds. The NKT psychrometric calculator on the tools page performs this calculation using ASHRAE 2017 Fundamentals correlations.
Engineering Note
Modern HVAC design uses software calculations instead of printed charts, but chart-reading skill is still critical:
• Quick design verification — visually confirms whether an AHU component is operating correctly within seconds. • Training and proposal presentations — the fastest way to explain system behavior to clients. • Field troubleshooting — visually diagnosing the cause of abnormal condensation, mold, or freezing problems.
What to check when selecting a chart: atmospheric pressure (sea level or altitude?), temperature range (low-temperature dry rooms require charts extending to −40°C), unit system (SI vs IP).
The NKT psychrometric calculator tool supports six operations: dehumidification (cooling+drying), heating coil, cooling coil, air mixing, humidification, and evaporative cooling. All calculations follow the ASHRAE Fundamentals 2017 standard and results can be downloaded as PDF reports.