Psychrometric chart. Calculation pressure 101.33 kPa. Blue curves — φ (relative humidity), %; almost horizontal lines — isotherms; sloping ones — isenthalps i=const; the bold lower one — the saturation line φ=100%. Hover over the chart to magnify it.
Reference
Units
Temperatures (dry-bulb, wet-bulb, dew point) are in °C. Relative humidity φ is in per cent. Humidity ratio d is in grams of moisture per kilogram of dry air (g/kg). Enthalpy i is in kJ per kilogram of dry air. Pressure is in kilopascals (kPa); partial vapour pressure pv is in pascals.
Pressure and altitude
A standard printed chart is drawn for a single barometric pressure — 101.325 kPa (sea level). At another altitude it lies: at lower pressure the same relative saturation corresponds to a higher humidity ratio. Enter the altitude above sea level and the pressure is recalculated automatically (ISA), or type the barometric pressure by hand. All φ and saturation lines are redrawn for the given pressure.
Why the result differs from a printed chart
Reading sloping lines off a printed chart by eye gives an error of 1–3 %. Here the properties are computed from exact ASHRAE correlations (saturation pressure — Hyland–Wexler) instead of being read visually. Small differences from a particular printed chart are normal: they fall within its own error and depend on the set of formulas (ASHRAE against a textbook).
Air-handling processes
From the computed point you can draw a process line: heating (d=const), cooling (dry or with dehumidification down to saturation), adiabatic and steam humidification, mixing of two streams. The tool shows the outlet point and the loads: total, sensible and latent (kW), condensate (l/h) or water/steam flow (kg/h) and the process slope ε = Δi/Δd. Flow is set in m³/h (at the inlet point) — the load is computed in kilowatts.
Process chain (air-handling unit)
Several processes form a chain: each next one starts from the outlet of the previous — this models an air-handling unit (heat recovery → heater → cooler → humidifier). The total load of the chain is shown under the table.
Room calculation (supply air)
From the indoor conditions (the current point) and the heat and moisture gains (sensible gains in kW, moisture gains in kW, supply temperature difference) the tool finds the supply air parameters, the required flow (m³/h) and the process slope ε. Zero moisture gains give a d=const line (ε is undefined); negative ones mean moisture absorption (ε is negative).
Outdoor design conditions by city
The «City» field fills in the barometric pressure of the location per SP 131.13330.2020 and offers to insert the outdoor design points: warm season parameter A (ventilation, t at 0.95 frequency), warm B (air conditioning, by specific enthalpy at 0.98 — the rigorous point from table 10.2 of the SP) and cold B (heating, the coldest five-day temperature at 0.92). The reference covers major Russian cities (including regional capitals); the list is growing. After insertion the pressure and the point can be corrected by hand — that does not change the reference.
Saving and Excel
The «Save and get a link» button creates a permanent link to the calculation (point + processes + supply air) — you can share it or paste it into a design report. The Excel export (.xlsx) contains live formulas: enthalpy, vapour pressure, specific volume and density are recalculated inside the workbook from the same constants as the engine (the values match the calculation).
Limits
Calculation range: temperature −70…+80 °C, pressure 30…110 kPa, φ from 0 to 100 %. Supply temperature difference — (0, 30] °C. Points above the saturation line (φ > 100 %, fog) are not computed — the tool says so. Equipment selection from the computed load will come in future versions.