The psychrometric chart is the fundamental analytical tool of mechanical HVAC engineering. It graphically represents the thermodynamic properties of moist air and every process that changes them — from cooling coil sizing and AHU design to cleanroom humidity control and energy recovery.
Every point on the psychrometric chart represents a unique, fully defined thermodynamic state of moist air. The horizontal axis is dry bulb temperature (Tdb). The vertical axis is humidity ratio (W) — the mass of water vapour per kilogram of dry air. The curved boundary on the left is the saturation curve (100% RH), representing the maximum moisture the air can hold at each temperature. All real air states exist to the right and below this curve.
Overlaid on the chart are families of lines for constant relative humidity (curved), constant wet bulb temperature (diagonal), and constant specific enthalpy (nearly parallel to wet bulb lines). These lines allow any two known properties to uniquely locate the full air state.
No HVAC design decision stands alone. Cooling coil selection, AHU sizing, chiller plant capacity, humidifier specification, heat recovery efficiency, duct insulation thresholds, and indoor air quality compliance — all are grounded in psychrometric analysis. The chart translates between the invisible physical properties of air and the engineering quantities that determine system performance and energy consumption.
In practice, psychrometric analysis determines the sensible and latent load split of a cooling or heating process, which directly drives equipment selection and energy modelling. A 10°C drop in dew point target can halve the latent load on a cooling coil — this is not visible without plotting the process.
Any two independent properties fully define the moist air state. PsychroFlow calculates all eight from any valid pair using ASHRAE Handbook of Fundamentals equations.
The standard air temperature measured by a thermometer. Forms the horizontal axis of the chart. Used for sensible heat calculations: Qs = ṁ × cp × ΔTdb.
Temperature at adiabatic saturation. Diagonal lines slope down-right across the chart. Critical for cooling tower performance, evaporative cooler effectiveness, and coil selection.
Temperature at which air becomes saturated when cooled at constant pressure. Determines condensation risk on cold surfaces. Key for window glazing selection and cold pipe insulation design.
Ratio of actual water vapour pressure to saturation pressure at the same temperature: RH = Pw/Pws × 100%. Shown as curved lines. Critical comfort and IAQ parameter — ASHRAE 55 targets 30–60%.
Mass of water vapour per kg of dry air. Forms the vertical axis. Constant W means no moisture addition or removal (pure sensible process). W = 0.621945 × Pw/(P − Pw).
Total heat content per kg of dry air: h = 1.006T + W(2501 + 1.86T). Used for all coil duty calculations. Qtotal = ṁda × Δh. Does not equal sensible heat unless W is constant.
Volume of moist air per kg of dry air. Used to convert between volumetric flow (m³/s from fans) and mass flow (kg/s for energy calculations): ṁda = Qvol/v.
Partial pressure of water vapour in the moist air mixture. Foundation of all humidity calculations. Saturation vapour pressure Pws(T) is computed via the Magnus formula. RH = Pv/Pws × 100%.
When moist air is cooled below its dew point over a cooling coil, the process line extended to the saturation curve defines the Apparatus Dew Point (ADP) — the effective mean surface temperature of the coil. It is not the refrigerant temperature; it is the thermodynamic fingerprint of the coil performance.
The Bypass Factor (BF) quantifies the fraction of air that passes through the coil without making effective thermal contact with the coil surface:
A BF of 0.05–0.15 indicates good coil design (multi-row, adequate fin density, low face velocity). A BF above 0.25 suggests poor coil contact — often caused by excessive face velocity (>2.5 m/s), too few rows, or wide fin spacing. The Contact Factor (CF = 1 − BF) represents the fraction of air fully conditioned to the ADP.
The Sensible Heat Ratio describes the proportion of total cooling or heating that changes the air temperature (sensible) versus the proportion that changes the moisture content (latent):
SHR is graphically the slope of the process line on the psychrometric chart. A horizontal line (SHR = 1.0) is pure sensible heating or cooling. A steeper downward slope indicates significant dehumidification.
Design SHR targets vary by application: offices 0.85–0.95, hospitals 0.75–0.85, pharmaceutical cleanrooms 0.65–0.80, data centres near 1.0. Matching the supply air SHR to the space load SHR is fundamental to avoiding over- or under-dehumidification.
Air crosses a cooling coil below its dew point. The process line moves toward the lower-left, ending on or near the saturation curve at the ADP. Dominant process in air conditioning design. Determines cooling coil UA, refrigerant capacity, and chiller plant sizing. Both sensible and latent loads must be met simultaneously.
Temperature rises at constant humidity ratio (W). Process line is horizontal. Used for heating coils, electric strip heaters, reheat coils in VAV terminal units, and heat pumps in heating mode. Relative humidity drops significantly during sensible heating — this drives winter humidification requirements.
Steam (isothermal): W increases at constant Tdb — vertical line on the chart. Adiabatic (evaporative): follows the wet bulb line, T drops while W rises. Steam humidification is used in critical environments (hospitals, laboratories, cleanrooms) where temperature control precision is essential. Sizing: ṁsteam = ṁda × ΔW.
Near-constant enthalpy process along the wet bulb line. Temperature falls as humidity rises. Effectiveness = (Tdb,in − Tdb,out) / (Tdb,in − Twb,in). Most effective in hot, dry climates (effectiveness 70–90%). Widely used in data centres, industrial applications, and as pre-cooling for DX systems.
A heat exchanger transfers enthalpy from exhaust air to supply air (or vice versa). HRV (sensible only) moves state along a line toward the exhaust condition. ERV (enthalpy exchanger) transfers both sensible and latent energy. Efficiency = (hout − hin) / (hexhaust − hin). Critical for energy-efficient ventilation in ASHRAE 90.1 compliance.
Outside air (OA) and return air (RA) are mixed upstream of the conditioning coil. The mixed air state lies on the straight line between OA and RA states, at a position set by the mass flow ratio. Reducing OA fraction saves cooling energy but reduces IAQ. Minimum OA is governed by ASHRAE 62.1 or local equivalent. PsychroFlow calculates the mixed air state for any OA fraction.
Atmospheric pressure decreases with elevation following the ICAO standard atmosphere model. This is not a minor correction — at 1750 m elevation, atmospheric pressure is approximately 82 kPa versus 101.3 kPa at sea level, an 18.5% reduction. Since humidity ratio W is directly proportional to vapour pressure and inversely proportional to atmospheric pressure, a given relative humidity corresponds to a significantly lower W at altitude.
PsychroFlow applies the ICAO standard atmosphere formula for all calculations:
where z is elevation in metres. This matches the formula used in ASHRAE Handbook of Fundamentals and major simulation tools (HAP, IDA ICE, EnergyPlus).
The practical consequences of altitude for HVAC design are significant:
Defines the acceptable temperature and humidity ranges for human comfort. The comfort zone on a psychrometric chart typically falls between 20–26°C dry bulb and 30–60% RH. Operative temperature, clothing insulation (clo), and metabolic rate are additional variables. PsychroFlow can overlay the ASHRAE 55 comfort zone on the chart.
Sets minimum outdoor air ventilation rates and indoor air quality requirements for non-residential buildings. Directly impacts the OA/RA mixing ratio, which is a core PsychroFlow process type. Moisture-related IAQ requirements (prevention of mould at surfaces <70% RH) are determined through dew point analysis.
Specifies minimum energy efficiency requirements including heat recovery, economiser cycles, and humidity control. The psychrometric chart is the primary tool for demonstrating compliance with economiser operation limits and energy recovery calculations.
PsychroFlow implements the equations in ASHRAE Handbook of Fundamentals Chapter 1 for all property calculations. Saturation pressure uses the Antoine/Magnus equation. Humidity ratio, enthalpy, specific volume, wet bulb, and dew point are all computed per ASHRAE 2017 HoF for metric SI units.