A psychrometric chart is a map of the thermodynamic properties of moist air. Engineers use it to design HVAC systems, drying processes, greenhouses, laboratories, and comfortable buildings. Instead of calculating every air property separately, the chart lets you locate one air state and read several related values at once.
It matters because air temperature, moisture, and comfort are tightly connected in real systems.
Key Facts
- Dry-bulb temperature is read along the horizontal axis of most psychrometric charts.
- Humidity ratio is the mass of water vapor per mass of dry air, often in kg water/kg dry air.
- Relative humidity is RH = actual water vapor pressure/saturation water vapor pressure x 100%.
- Dew point is the temperature at which moist air becomes saturated when cooled at constant humidity ratio.
- Sensible heating or cooling moves horizontally on the chart when no moisture is added or removed.
- Enthalpy of moist air can be estimated by h = 1.006T + w(2501 + 1.86T), with T in degrees Celsius and w in kg/kg.
Vocabulary
- Dry-bulb temperature
- The ordinary air temperature measured by a thermometer shielded from radiation and moisture effects.
- Wet-bulb temperature
- The temperature measured by a thermometer with a wet wick, showing the cooling effect of evaporation.
- Relative humidity
- The percentage ratio of the actual water vapor in air to the maximum water vapor the air can hold at the same temperature.
- Dew point
- The temperature at which air reaches saturation and water vapor begins to condense if cooling continues.
- Humidity ratio
- The mass of water vapor mixed with each unit mass of dry air.
Common Mistakes to Avoid
- Reading relative humidity as a straight vertical or horizontal line is wrong because relative humidity curves sweep upward across the chart.
- Confusing dry-bulb temperature with wet-bulb temperature is wrong because wet-bulb temperature includes evaporative cooling and is usually lower than dry-bulb temperature for unsaturated air.
- Assuming cooling always removes moisture is wrong because cooling above the dew point changes temperature but does not cause condensation.
- Using a psychrometric chart made for the wrong pressure or elevation is wrong because air pressure changes saturation behavior and shifts the property relationships.
Practice Questions
- 1 Air is at 30 degrees Celsius dry-bulb temperature and 50% relative humidity. Using a psychrometric chart, estimate the humidity ratio and dew point temperature.
- 2 An HVAC coil cools air from 28 degrees Celsius to 18 degrees Celsius at constant humidity ratio. If the initial humidity ratio is 0.010 kg/kg, what is the final humidity ratio and what type of process path is this on the chart?
- 3 Explain why humidifying cool winter indoor air can increase comfort even if the dry-bulb temperature stays the same.