Psychrometric Chart Calculator: Air States and HVAC Processes

Moist-air properties follow the ASHRAE Fundamentals 2017 equations openly implemented in PsychroLib 2.5.0: attached source s3, pp. 5 (wet bulb), 8 (Hyland–Wexler saturation pressure and enthalpy), and 9 (specific volume and standard atmosphere). The ice/water saturation-pressure transition is at 0.01 °C; the wet-bulb energy balance changes branch at 0 °C.

With pressure in Pa, $W=0.621945p_v/(p-p_v)$ and $p_v=RH\,p_{ws}$. Enthalpy in kJ per kg dry air is $h=1.006T+W(2501+1.86T)$. Specific volume is $v=R_{da}T_K(1+1.607858W)/p$, with $R_{da}=287.042$ J/(kg·K). Inverse saturation uses bounded bisection. Wet-bulb inversions bracket the ice (below 0 °C) and liquid-water (at or above 0 °C) branches separately; a known wet bulb selects its phase and an unchanged outlet retains it. Without a known phase, two-root states show both candidates and a warning; the numeric inverse follows the liquid-water branch. No bisection crosses the branch discontinuity. The independent property pairs use algebra or a one-dimensional root.

Dry-air flow is $\dot m_{da}=\dot V/v$. A wet-coil outlet conserves humidity ratio and enthalpy in a bypass/contact mixture: $W_o=BF\,W_A+(1-BF)W_{ADP}$ and $h_o=BF\,h_A+(1-BF)h_{ADP}$. A dry coil retains $W_A$ and uses $T_o=T_{ADP}+BF(T_A-T_{ADP})$. Mixing conserves dry-air mass, vapor mass and enthalpy. Its temperature is recovered from mixed $h,W$, rather than temperature-averaged. These balances are derived here; the ADP/BF approach is described in the EnergyPlus coil reference.

Evaporative effectiveness measures approach to the saturated point at inlet enthalpy. Feed-water enthalpy is neglected. Steam adds $\Delta W=\dot m_s/\dot m_A$ and $\Delta h=h_s\Delta W$ (EnergyPlus humidifier balances).

Signed air-side rates are $Q_t=\dot m_A(h_o-h_A)$, $Q_s=\dot m_A(1.006+1.86W_o)(T_o-T_A)$ and $Q_l=Q_t-Q_s$. Positive means energy added to air. This defines the split using the outlet humidity ratio; other conventions differ slightly. SHR is $Q_s/Q_t$ and is displayed as undefined at zero net duty. Drainage is $\dot m_A\max(W_A-W_o,0)$ for a conditioning process and zero for fog-free mixing. The chart is an original drawing from these equations; no published chart is reproduced.

• Ideal-gas dry air and water vapor at a common, constant total pressure. Humidity ratios, enthalpy and specific volume are per kg dry air, not per kg moist air.
• Standard-atmosphere altitude pressure is an estimate. The measured-pressure option accepts local absolute station pressure.
• Negligible pressure drop, fan heat, kinetic and potential energy changes. Reported coil duty is the air-side enthalpy change; drain-liquid enthalpy and refrigeration performance are excluded.
• Coil bypass factor is a dry-air fraction. The wet branch mixes saturated contact air at ADP with bypass air; this exactly conserves enthalpy and moisture and is slightly curved in dry bulb versus W coordinates.
• Steam specific enthalpy uses the same reference near liquid water at 0 °C. Its default 2676 kJ/kg is an illustrative rounded supply value; enter actual steam data.
• For mixing, total/sensible/latent rates compare stream A with the mixed state at A's dry-air flow. External mixer duty and added-water/condensate rates are zero.
• The green comfort band is an input visualization, not an ASHRAE 55 assessment. Scene labels stay in SI. The metric/US switch converts supported simple units and temperature offsets. Compound HVAC quantities retain their declared units; companion US tables provide BTU/h, BTU/lb, cfm and lb/h. Numeric fields accept compatible units typed with the value.
• US enthalpy is a unit conversion of SI enthalpy and retains its 0 °C datum. It differs from an IP chart using a 0 °F dry-air enthalpy datum; energy differences and process duties are unaffected by a constant datum shift.
• Flow is separately entered in every mode. Air state only exposes the same volume-flow / dry-air-mass-flow inputs; reported flows are based on that specified flow, not inferred from thermodynamic properties.

Dry bulb −40 to +60 °C; pressure 50–110 kPa (altitude −500 to 5000 m when estimated). Dew/frost-point inversion extends to −100 °C. Relative humidity must be positive and at most 100%; exactly dry air has no finite dew point and is excluded. Negative-moisture and supersaturated input states are rejected instead of silently clamping humidity to PsychroLib's numerical floor.

The two-property solver supports all 14 independent pairs. Dew point plus humidity ratio is dependent. Wet bulb plus enthalpy is rejected within 0.1 K of 0 °C because the inverse problem is singular/ill-conditioned there. Rounded inputs in that pairing may be sensitive even farther from freezing. The ice and liquid-water wet-bulb energy balances overlap near freezing, so a unique inverse wet bulb is not always determined by the air state alone. Entered negative wet bulbs use the ice-wick branch; entered nonnegative values use the liquid-water branch. Automatic inversions with two roots show a warning and both SI candidate temperatures; the numeric result uses water. Supercooled liquid-water wet bulbs below 0 °C are outside this implementation.

Cooling-coil effective surface temperature is restricted to 0–40 °C. Frost, freezing condensate, detailed part-wet coil behavior, fog-producing mixing, liquid carryover and nonideal high-pressure air are excluded. Liquid-water evaporative cooling requires a saturation endpoint at or above 0 °C. Constant-enthalpy evaporation neglects water and pump heat. Loads are not an equipment electrical rating or a substitute for manufacturer selection data.