Psychrometry, also known as hygrometry, is a field of engineering that focuses on the physical and thermodynamic properties of gas-vapor mixtures. It involves the study of atmospheric air, which is a mixture of pure air and water vapor at atmospheric pressure. Psychrometry plays a crucial role in various industries, including engineering, HVAC (heating, ventilation, and air conditioning), and building materials.

The Basics of Psychrometry

Air Composition and Dalton’s Law

Air is predominantly composed of nitrogen (78% by volume) and oxygen (21%), with small amounts of carbon dioxide and other gases making up the remaining 1%. The composition of air remains consistent across different locations. However, the amount of water vapor in the air can vary significantly.

According to Dalton’s Law, the total pressure of a gas-vapor mixture is equal to the sum of the partial pressures exerted by each component gas. This means that the total pressure of the air is the combination of the pressures exerted by the dry gases and the water vapor.

Dry Bulb and Wet Bulb Temperature

Dry bulb temperature refers to the temperature of air as measured by a standard thermometer with a dry sensing bulb. On the other hand, wet bulb temperature is the temperature of air measured by a thermometer with a sensing bulb covered by a wet wick. The evaporation of water from the wick cools the thermometer, resulting in a lower wet bulb reading compared to the dry bulb temperature. The difference between these temperatures provides insights into the relative humidity of the air.

Relative Humidity

Relative humidity (RH) is a commonly used psychrometric unit that represents the amount of water vapor present in the air compared to the maximum amount it can hold at a given temperature. It is expressed as a percentage and calculated by dividing the partial pressure of water vapor by the saturation vapor pressure at the same temperature.

Psychrometric Properties and Calculations

Humidity Ratio

Humidity ratio, also known as mixing ratio or moisture content, refers to the mass of water in the volume occupied by 1 kg of dry air. It represents the amount of water required to be evaporated into 1 kg of dry air to achieve a specific condition. HVAC engineers often use this term as it remains constant unless cooled below the dew point temperature. Humidity ratio is typically expressed in kilograms per kilogram (kg/kg) or grams per kilogram (g/kg).

Psychrometric Chart

A psychrometric chart is a graphical representation of the thermodynamic properties of moist air. It shows the relationships between dry bulb temperature, wet bulb temperature, relative humidity, humidity ratio, and enthalpy. By using a psychrometric chart, engineers can analyze air-conditioning processes, perform energy and exergy assessments, and make informed decisions about HVAC system design and operation.

Applications of Psychrometry

Psychrometry finds applications in various industries and environments. Some notable examples include:

  • Lithium Battery Dry Rooms: Maintaining low humidity and dew point levels in rooms storing lithium batteries to prevent moisture-related damage.
  • Pharmaceutical Humidity Control: Achieving precise humidity levels in cleanrooms and pharmaceutical environments to ensure product stability and quality.
  • Dehumidification in Cold Stores: Preventing ice formation in food distribution centers and cold stores through effective dehumidification.
  • Food Production Humidity Control: Controlling humidity in food production facilities to maintain product quality and extend shelf life.
  • Warehouse and Storage Dehumidification: Protecting stored goods from moisture damage by maintaining optimal humidity levels.
  • Car Storage Humidity Control: Preserving classic cars and valuable vehicles by controlling humidity levels in storage facilities.
  • Ice Rink Dehumidification: Ensuring optimal ice conditions and preventing condensation in ice rinks and arenas.
  • Sports Hall Humidity Control: Controlling humidity and condensation in PVC structures and sports halls to maintain a comfortable and safe environment.
  • Confectionary Humidity Control: Maintaining precise humidity levels to prevent moisture-related issues in confectionary production.
  • Humidity Control within Temporary Structures: Preventing condensation and controlling humidity in temporary structures such as tents and event spaces.
  • PVB Glass Lamination Humidity Control: Achieving optimal humidity conditions for the lamination process of PVB (polyvinyl butyral) glass.
  • Power Station Preservation and Accelerated Cooling: Controlling humidity to prevent corrosion and ensure efficient cooling in power stations.
  • Museum and Archive Humidity Control: Maintaining stable humidity levels to preserve artifacts and prevent deterioration in museums and archives.
  • Flood Damage Building Drying: Facilitating the drying process and preventing further damage after water or flood incidents in buildings.
  • Silo Head Space Conditioning: Controlling humidity in silos to minimize moisture-related issues in stored grains and other commodities.
  • Temporary Desiccant Dehumidification Systems: Providing temporary humidity control solutions in various industrial and commercial applications.
  • Preservation of Military Equipment with Humidity Control: Protecting military equipment from moisture damage through effective humidity control.

The Benefits of Psychrometry in Engineering and Design

Psychrometry plays a crucial role in engineering and design processes, particularly in the HVAC industry. Here are some of the key benefits:

Comfort and Indoor Air Quality

By understanding the psychrometric properties of air, engineers can design HVAC systems that provide optimal comfort and indoor air quality. Controlling temperature, humidity, and air movement helps create a pleasant and healthy environment for occupants.

Energy Efficiency

Psychrometry enables engineers to analyze and optimize the energy efficiency of HVAC systems. By considering factors such as heat transfer, moisture load, and air distribution, engineers can design systems that minimize energy consumption and operating costs.

Moisture Control

Effective moisture control is essential in various industries, including food production, pharmaceuticals, and storage facilities. Psychrometry allows engineers to design dehumidification systems that prevent moisture-related issues, such as mold growth, corrosion, and product degradation.

Building Performance and Sustainability

Psychrometry contributes to the overall performance and sustainability of buildings. By considering factors like insulation, roofing, and ventilation, engineers can design energy-efficient buildings that promote occupant comfort, minimize environmental impact, and comply with sustainability standards.

System Design and Optimization

Psychrometry provides valuable insights for system design and optimization. By using psychrometric charts and calculations, engineers can select appropriate equipment, determine system capacities, and ensure efficient operation.


Psychrometry is a fundamental science that plays a vital role in various engineering disciplines, particularly in the field of HVAC. By understanding the properties of air and water vapor, engineers can design systems that provide optimal comfort, energy efficiency, and moisture control. The use of psychrometric charts and calculations allows for precise analysis and optimization of HVAC systems, ensuring the delivery of high-quality indoor environments. By harnessing the principles of psychrometry, engineers can shape the built environment of tomorrow, creating sustainable, comfortable, and efficient spaces for all.