Psychrometric Chart of an AHU

What is modelled

Moist air is described by its dry-bulb temperature and humidity ratio. Mixing two air streams lands on the straight line between them (in enthalpy and humidity ratio), a cooling coil pulls the air toward the cold, saturated apparatus dew point and stops short by the bypass factor, and reheat then moves the state sideways. The share of the coil load that removes water is the latent part, so the sensible heat ratio says how dry the process is. The states and processes come from the 3D page module; the chart and checks are added here.

Equations

• Humidity ratio w = 0.62198 pv / (p - pv), enthalpy h = 1.006 T + w (2501 + 1.86 T) kJ/kg dry air.
• Saturation pressure from the Buck formula, checked against Magnus.
• Mixing w = x wo + (1 - x) wr, h likewise.
• Coil outlet (w, h) = BF (w, h)in + (1 - BF)(w, h)ADP; load m (hin - hout) = sensible m (1.006 + 1.86 w) ΔT + latent m Δw (2501 + 1.86 T).
• Wet-bulb: h(Twb, ws) - (ws - w) 4.186 Twb = h(T, w).

Checks run in the page

• Mixing balance against the module state, and collinearity of five other mixtures.
• Buck against Magnus saturation pressure, and the humidity ratio round trip.
• Coil load split into sensible and latent, and the bypass-factor line to the dew point.
• Wet-bulb by bisection with an enthalpy balance.

Honest simplifications

Sea-level pressure, a steady process, a coil modelled by a single bypass factor and an apparatus dew point, no fan heat apart from a trivial amount, no humidifier here, and ideal-gas mixtures. Real coils depend on water temperature, face velocity and fouling.

• Press Monsoon and see most of the coil load go into the latent part: a low sensible heat ratio and a lot of condensate.
• Press Mild, economiser: the coil barely works because the mixed air is already near the supply state.
• Raise the bypass factor and watch the coil outlet slide back toward the mixed point.