Psychrometrics is the study of moist air: a mixture of dry air and water vapour. Almost every air-side decision in HVAC, from sizing a cooling coil to diagnosing a damp classroom, comes back to how temperature and moisture interact. Engineers who are comfortable with the psychrometric chart make better selections and spot problems faster.
This guide covers the properties that matter, how to read the chart, the common processes you will draw on it, and the equations that turn a chart reading into kilowatts. It is written for practising engineers, but building owners who want to understand why their system struggles with humidity will find it useful too.
The properties of moist air
Each point on a psychrometric chart represents one state of moist air. Once you know any two independent properties (at a given barometric pressure), you know them all.
| Property | Symbol | Typical SI unit | What it tells you |
|---|---|---|---|
| Dry-bulb temperature | t | °C | Ordinary air temperature, what a thermometer reads |
| Wet-bulb temperature | twb | °C | Temperature of a wetted sensor; reflects evaporative potential |
| Dew point temperature | tdp | °C | Temperature at which moisture starts to condense |
| Relative humidity | RH | % | Vapour present compared with the maximum the air could hold at that temperature |
| Humidity ratio | W | g/kg dry air | Actual mass of water vapour per kilogram of dry air |
| Specific enthalpy | h | kJ/kg dry air | Total heat content, sensible plus latent |
| Specific volume | v | m³/kg dry air | Volume occupied by one kilogram of dry air in the mixture |
Two of these deserve special attention. Humidity ratio (also called moisture content, often given in grains per pound in imperial units; 7,000 grains equal one pound) is the true measure of how much water is in the air. Relative humidity changes whenever the air is heated or cooled, even if no moisture is added or removed. Dew point is directly tied to humidity ratio, so it is the most useful single number for judging condensation risk on cold surfaces.
Reading the chart
The standard ASHRAE chart plots dry-bulb temperature along the horizontal axis and humidity ratio up the vertical axis. The curved left boundary is the saturation line (100% RH). Other features:
- Relative humidity lines curve upward from left to right, parallel to the saturation line.
- Wet-bulb lines slope down to the right and are nearly parallel to enthalpy lines.
- Enthalpy is read on a diagonal scale outside the saturation curve.
- Specific volume lines are steep diagonals.
Charts are drawn for a specific barometric pressure. Sea-level charts (101.325 kPa) are fine for Vancouver and Toronto, but in Calgary or Banff the error becomes noticeable. Use an altitude-corrected chart or software set to local pressure; see our guide on air density and altitude.
Common psychrometric processes
Most HVAC processes appear as simple lines on the chart.
Sensible heating and cooling
Heating or cooling without adding or removing moisture is a horizontal line. Humidity ratio and dew point stay constant, while relative humidity falls during heating and rises during cooling. This is why outdoor air at minus 20 °C and high relative humidity becomes very dry indoor air once heated to 21 °C.
Cooling and dehumidifying
When a coil surface is colder than the entering air's dew point, the air is cooled and moisture condenses. The process line runs down and to the left toward the coil's apparent dew point. The slope of this line is set by the room's sensible heat ratio, explained in detail in sensible and latent heat.
Humidification
Steam humidification is nearly a vertical line (moisture added with very little temperature change). Adiabatic humidification, such as wetted media or atomizing nozzles, follows a constant wet-bulb line: the air cools as water evaporates.
Mixing
When two airstreams mix, such as return air and outdoor air in a mixing box, the result lies on a straight line between the two states. Its position is set by the mass flow ratio. A 30% outdoor air fraction places the mixed point 30% of the way from the return state toward the outdoor state.
Chemical dehumidification
Desiccant wheels remove moisture and release heat, moving the state point down and to the right along roughly constant enthalpy. They are used where very low dew points are needed, such as ice arenas and some cleanrooms.
The everyday air-side equations
Charts show the process; equations size the equipment. At standard air conditions (density about 1.2 kg/m³), the following forms are widely used. Imperial equivalents are in parentheses.
| Load | SI form (W, L/s) | Imperial form (Btu/h, cfm) |
|---|---|---|
| Sensible | Qs = 1.23 x L/s x delta t (K) | Qs = 1.08 x cfm x delta t (F) |
| Latent | Ql = 3010 x L/s x delta W (kg/kg) | Ql = 0.68 x cfm x delta W (grains/lb) |
| Total | Qt = 1.2 x L/s x delta h (kJ/kg) | Qt = 4.5 x cfm x delta h (Btu/lb) |
The constants assume standard density. At altitude, or for very hot or cold air, correct them in proportion to the actual density, or use mass flow directly.
A worked procedure for a cooling coil
- Establish the room design condition, for example 24 °C and 50% RH.
- Calculate the room sensible and latent loads, and the sensible heat ratio.
- Draw the room ratio line from the room state at the slope given by the sensible heat ratio.
- Pick a supply air temperature on that line, typically 12 to 14 °C for comfort systems, and calculate the supply airflow from the sensible equation.
- Locate the mixed air state from the outdoor and return conditions at design outdoor air fraction.
- Draw the coil line from the mixed air state to the supply state and read the enthalpy difference.
- Calculate total coil capacity from the total equation, then confirm the leaving dew point is low enough to meet the room's latent load.
Software performs these steps automatically, but sketching the process by hand is the quickest way to check whether a coil selection makes sense.
Where psychrometrics solves real problems
Psychrometric thinking explains many field complaints:
- High humidity in a cool space: often an oversized constant-volume unit that cycles off before removing moisture, or a coil running too warm for the latent load.
- Condensation on windows or ducts: surface temperature is below the room's dew point. Lowering indoor humidity or warming the surface fixes it.
- Dry winter air: heated outdoor air has a very low humidity ratio; see humidification in cold climates.
- Wet economizer sections or frozen coils: poor mixing creates air streams near or below freezing.
ASHRAE Handbook, Fundamentals provides the governing property data and formulations. For design, use the edition referenced by your project and confirm local barometric pressure.
Frequently asked questions
Why is humidity ratio more useful than relative humidity?
Relative humidity depends on temperature, so the same air reads different RH values at different points in a system. Humidity ratio and dew point stay constant until moisture is actually added or removed, which makes them reliable for tracking moisture through an air handler.
Do I need an altitude-corrected chart?
Below a few hundred metres the error is small. In Calgary (around 1,000 to 1,100 m) and higher mountain towns, sea-level charts misstate humidity ratio and enthalpy enough to affect coil and humidifier sizing, so use local pressure.
What is the apparent dew point of a coil?
It is the point where the coil process line, extended, meets the saturation curve. It approximates the effective coil surface temperature and governs how much moisture the coil can remove.
Key takeaways
- Two independent properties fix the state of moist air at a given pressure.
- Humidity ratio and dew point measure moisture directly; relative humidity changes with temperature.
- Sensible processes are horizontal lines; cooling with dehumidification moves down and left; mixing falls on a straight line between states.
- The standard air equations convert chart readings into loads but must be corrected for altitude.
- Sketching the process on a chart is a fast sanity check on any coil or humidifier selection.
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