Psychrometric Chart
This page provides a comprehensive psychrometric chart reference for engineers and HVAC professionals. It explains the key properties of moist air, visualizes common air-conditioning processes, and includes interactive tools for precise air property calculations. Use this reference to optimize HVAC system design, energy efficiency, and indoor thermal comfort.
Overview
A psychrometric chart is a graphical representation of the thermodynamic properties of moist air. It combines multiple parameters, including temperature, humidity, enthalpy, and specific volume, in a single diagram. Engineers and HVAC professionals use this chart as a reference tool for analyzing air-conditioning processes, energy efficiency, and indoor thermal comfort.
The chart is standardized in ASHRAE Handbook – Fundamentals and ISO 7730, ensuring uniform property representation for design and analysis purposes.
Chart Properties
The following properties are typically shown on a psychrometric chart:
| Property | Symbol / Unit | Description |
|---|---|---|
| Dry-Bulb Temperature | DBT, °C / °F | Measured air temperature using a regular thermometer. Plotted along the x-axis. |
| Wet-Bulb Temperature | WBT, °C / °F | Temperature indicated by a thermometer with a wetted wick, showing cooling effect of evaporation. |
| Relative Humidity | RH, % | Ratio of current water vapor pressure to saturation vapor pressure at the same temperature. Plotted as curved lines. |
| Humidity Ratio | W, kg/kg | Mass of water vapor per unit mass of dry air. Plotted along y-axis. |
| Enthalpy | h, kJ/kg | Total energy content of air, including sensible and latent heat. Represented as diagonal lines. |
| Dew Point Temperature | °C / °F | Temperature at which air becomes saturated and water vapor begins to condense. |
| Specific Volume | m³/kg | Volume occupied by a unit mass of air. |
| Vapor Pressure | kPa | Partial pressure of water vapor in the air. |
Typical HVAC Processes
Psychrometric charts are used to visualize common air-conditioning processes:
- Cooling & Dehumidification: Movement left and downward on the chart, reducing DBT and humidity ratio.
- Heating & Humidification: Movement right and upward, increasing DBT and moisture content.
- Evaporative Cooling: Horizontal movement along constant WBT lines, common in cooling towers and indirect evaporative cooling.
- Air Mixing: Combining two air streams produces a point along the straight line connecting their respective states.
Interactive Psychrometric Chart
Use the interactive chart below to view air properties at any point. Hover over or click a point to display:
The dry bulb temperature is the regular air temperature measured with a standard thermometer. It indicates how hot or cold the air is. For example, a dry bulb temperature of 30°C means the air feels warm.
Relative humidity is the ratio of the current amount of water vapor in the air to the maximum amount the air can hold at that temperature. For example, 60% relative humidity at 25°C means the air is holding 60% of the moisture it could hold at 25°C.
The wet-bulb temperature is the lowest temperature that can be reached by evaporative cooling. For example, if a fan blows air over a wet cloth, the temperature it reaches is the wet-bulb temperature.
The dew point is the temperature at which air becomes saturated and water starts to condense. For example, if the dew point is 18°C, cooling air below this temperature will cause dew to form.
The humidity ratio (or moisture content) is the mass of water vapor per kilogram of dry air. For example, a humidity ratio of 10 g/kg means there are 10 grams of water vapor in 1 kg of dry air.
Enthalpy is the total heat content of the air per kilogram of dry air. For example, an enthalpy of 50 kJ/kg indicates the amount of energy in the air including both sensible and latent heat.
Specific volume is the volume occupied by a unit mass of dry air. For example, a specific volume of 0.85 m³/kg means 1 kg of dry air occupies 0.85 cubic meters.
Vapor pressure is the pressure exerted by water vapor in the air. For example, a vapor pressure of 2 kPa indicates the partial pressure of the water vapor component in the air.
Interactive Psychrometric Chart (SI Units)
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