Tools · Psychrometric Chart

Psychrometric Chart HVAC

Walk an air handler through its psychrometric processes.
Mix outdoor and return air, then cool, heat, and humidify. Pick a stage with the pills, edit its parameters in the panel, or drag OA and RA on the chart. The supply-air state and all intermediate points follow. New to psychrometrics? The Psychrometrics Basics lesson walks the seven properties on the chart and the dew-point gotcha first.

Inputs are seeded with an example — edit them to your numbers.

The chart is built for a wider screen — on a phone the three-column workspace collapses and the canvas compresses. The fastest path on a phone is typing each state into the Input column above; dragging OA / RA on the chart still works, but a laptop or tablet gives it room to read.

Air handler

Chart range

Outdoor air entering the air handler. Drag this dot on the chart, or edit values here.

Air returning from the conditioned space. Drag this dot on the chart, or edit values here.

Mass-weighted mix of OA and RA. The mixed point falls on the straight line between OA and RA on the chart, at the OA% fraction of the way from RA to OA.

MA dry-bulb (°F)—
MA humidity ratio (gr/lb)—
MA rel. humidity (%)—
Cooling coil

Coil leaving condition. Must be at or below entering DB; humidity ratio can't increase across a cooling coil.

Heating coil

Sensible heating only — humidity ratio is unchanged across the coil. Leaving DB must be at or above entering DB.

Humidifier

Adiabatic (constant wet-bulb) humidification — an evaporative pad or wetted media. Adds moisture; dry-bulb drops a little. Leaving RH must be at or above entering RH.

Supply air is the leaving state of the last enabled stage in the chain. Full property table at right.

Chart

Heavy green = saturation. Light curves rising right = constant RH; light dashed = constant wet-bulb. Process segments are colored: gray = mixing, blue = cooling / dehumidification, orange = heating, green dashed = adiabatic humidification.

Stages

Stage DB °F gr/lb RH % h Btu/lb

OA — Outdoor air

Dry-bulb (°F)—
Wet-bulb (°F)—
Dew point (°F)—
Rel. humidity (%)—
Humidity ratio (gr/lb)—
Enthalpy (Btu/lb)—
Specific volume (ft³/lb)—
Vapor pressure (psia)—
Barometric press. (psia)—

Properties are per pound of dry air, computed internally from the ASHRAE Fundamentals (Chapter 1) IP equations and converted to your selected display units — the formula forms quoted here (0.240, 0.444, CFM·60/v) are that IP formulation; metric readouts are exact conversions of the same math. RH is taken as the partial-pressure ratio (the usual approximation). Mixing is a mass-weighted enthalpy / humidity-ratio combination. Adiabatic humidification holds wet-bulb constant. Coil capacities use AHU airflow and inlet specific volume: Q = ṁ·Δh with ṁ = CFM·60/vin; Qsens = ṁ·(0.240 + 0.444·Win)·ΔT. For building feel and quick design-day state-point checks — not a substitute for a calibrated load-study tool.

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