The Psychrometric Chart: A Complete Guide for HVAC Engineers

The psychrometric chart is a graphical tool for calculating and visualizing the states of moist air. It lets HVAC engineers see the temperature, humidity ratio, enthalpy and relative humidity of air at a single glance — and draw any air-treatment process (heating, cooling, humidification and their combinations).

What the psychrometric chart is

The chart used across North America follows the Carrier layout and shows the thermodynamic states of moist air in a rectangular coordinate system:

• The horizontal axis — dry-bulb temperature $t$ [°F]
• The vertical axis — the humidity ratio $W$ [gr/lb dry air] — the amount of water vapor per 1 lb of dry air
• Oblique lines — enthalpy $h$ [Btu/lb dry air] and the wet-bulb temperature, read against the scale outside the saturation curve

The European Mollier layout carries the same physics with the axes transposed and enthalpy on an oblique axis of its own — which is why it is called the h-x diagram there.

The main constant-property lines

Line	Symbol	Physical quantity
Temperature	$t$	Dry-bulb temperature [°F]
Enthalpy	$h$	Enthalpy [Btu/lb]
Humidity ratio	$W$	Humidity ratio [gr/lb dry air]
Relative humidity	$\varphi$	Relative humidity [%]
Saturation curve	$\varphi$ = 100%	Air saturation limit (the dew point lies on it)

How to read the chart

Every state of the air is represented by a single point. From that point you can read off:

1. The temperature — drop straight down to the horizontal axis
2. The humidity ratio — project horizontally to the right-hand axis
3. The enthalpy — follow the oblique constant-enthalpy line up and to the left, out to the scale
4. The relative humidity — from the position between the φ = 0% line and the saturation curve

The dew point ($t_d$) always lies on the saturation curve (φ = 100%) at the same value of $W$ as the starting state — read it by going straight left.

The basic air-treatment processes

Sensible heating

A horizontal line to the right — the humidity ratio $W$ does not change, temperature and enthalpy rise, relative humidity falls.

Typical use: a heating coil (electric or hot-water) in the supply air.

Cooling and dehumidification

A line descending to the left — the air is cooled below its dew point and water vapor condenses out. Temperature, enthalpy and humidity ratio all fall.

Typical use: a finned cooling coil (chilled-water coil or DX coil).

Adiabatic humidification

A line following the constant-enthalpy line up and to the left. Temperature falls and the humidity ratio rises — this holds for humidification with atomized water or for an evaporative cooler.

Isothermal humidification

A vertical line upward at constant temperature — the humidity ratio rises. This holds for a steam humidifier (saturated or superheated steam).

Mixing of two streams

The state of the mixed air lies on the straight segment connecting the two inlet states — at the position corresponding to the mass ratio of the two streams.

Energy recovery (HRV/ERV)

The supply air moves from point 1 (outdoor air) to point 2 (air after the recovery exchanger). The air exhausted from the room simultaneously cools or heats the supply stream. The effectiveness of the exchanger determines how close point 2 lies to the state of the exhaust air.

The chart in practice: a cascade example

A typical air handling unit in winter operation:

1. Outdoor air (t = 5°F, φ = 80%) → enters the recovery exchanger
2. After energy recovery (t = 41°F) → sensible heating
3. After heating (t = 68°F) → steam humidification to φ = 50%
4. Supply to the room (t = 70°F, φ = 50%) → the room’s thermal load
5. Exhaust (t = 79°F, φ = 55%) → back to the recovery exchanger

This whole chain can be drawn as a series of connected segments on the chart — and that is exactly what PsychroView does.

Standards and calculation accuracy

PsychroView computes psychrometric quantities per ASHRAE Fundamentals 2021, Chapter 1, using the Hyland–Wexler equations for saturation vapor pressure — within this range it is a highly accurate method (deviations from reference data on the order of hundredths to a few percent). The engine works internally in SI and converts for display, which is why the numbers agree with the ASHRAE tables regardless of the unit system you have selected.

Design parameters for the indoor environment and ventilation follow ASHRAE Standard 55 (thermal comfort) and ASHRAE Standard 62.1 (ventilation for acceptable indoor air quality).

Carrier vs. Mollier

In North America the Carrier chart with rectangular coordinates is the standard. In Central Europe you meet the Mollier h-x diagram in SI units, with oblique coordinates and enthalpy on the oblique axis. Same physics, transposed axes — a European colleague reading your state points will find them all, just in a different place on the sheet.

PsychroView supports both chart types, which makes it practical when a project crosses the Atlantic.

Getting started

A web-based tool such as PsychroView takes the same route: enter the outdoor air state (temperature and relative humidity), add the treatment processes (heating, cooling, energy recovery), and export the result to PDF or DXF for the submittal.

Frequently asked questions

What is the difference between the Carrier and the Mollier chart? Both show the same physical quantities but differ in the orientation of their axes. The Carrier (psychrometric) chart has rectangular coordinates with dry-bulb on the horizontal axis — typical of the USA. The Mollier (h-x) chart has oblique coordinates with enthalpy on the oblique axis — typical of Central Europe.

Do I need to know the partial pressure of water vapor? No — it is enough to enter the temperature and relative humidity. PsychroView calculates all the remaining quantities automatically.

Can the chart be used for pressures other than standard atmospheric? Yes — PsychroView lets you enter a custom barometric pressure, which is necessary anywhere above sea level. At Denver’s elevation the difference is not cosmetic.

How many processes can be drawn at once? Without registration you can draw two processes. After registering you can chain more processes and model a whole air circuit in a single project.