Psychrometry

Psychrometry is the study of air-water vapor mixtures. It is also sometimes referred to as hygrometry. Many mechanical engineering devices exploit psychrometric processes, such as air conditioning systems and cooling towers. Two important principles upon which psychrometric relationships are based are the perfect gas equation and Dalton’s law regarding the mixture. The perfect gas equation, PV = RT, is a fundamental tool used to manipulate the characteristics as one sets out to analyze both definitions and various conditions. Since d = 1/v, another useful form of the equation is p = dRT.

Dalton’s Law of Partial Pressure: Dalton’s states that in a mixture of perfect gases, each constituent of the mixture behaves individually as the others are not present. This statement can be expressed as a series of mathematical relationships based on the following logic statements:

  • The total pressure of the mixture is the sum of the individual or partial pressures of the constituents.
  • The mass of the mixture is the sum of the masses of the individual constituents.
  • The temperature of each constituent is equal to the temperature of the mixture.
  • The volume of each of the constituents is equal to the volume of the mixture.
  • The enthalpy of the mixture is the sum of the enthalpies of the individual constituents.

Total pressure of the air-water vapor mixture (pB) can be expressed by Dalton’s law equation as the sum of the partial pressures of the constituents; p = pa + pv and if the water vapor is saturated p = pa + pvs.

Relative Humidity (ϕ): It is defined as the ratio of partial pressure of water vapor in a mixture and saturated pressure of pure water at the same temperature T.

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Humidity Ratio or Specific Humidity: The humidity (w) ratio is defined as the mass of water vapor per unit mass of dry air in the mixture of air and water vapor. This term can be img and if mixture is saturated ωS = 0.622 [pvs/( p − pvs )] as

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Degree of Saturation (μ): It is the ratio of actual specific humidity and the saturated specific humidity, both at the same temperature T.

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Dry Bulb Temperature (DBT): It is the temperature of the mixture as measured by a standard thermometer. The word ‘dry’ is used to imply that the sensor is exposed to the vapor mixture without any liquid present.

Wet Bulb Temperature (or Saturation Temperature, (WBT): It is the temperature at which water evaporating into moist air at a given dry-bulb temperature and humidity ratio can bring air to saturation adiabatically at the same pressure.

Specific Volume (v): It as commonly used, but with units of mixture volume per kilogram of the dry air.

Dew Point Temperature (DPT): It is the temperature of moist air saturated at the same pressure and humidity ratio as a given specimen of humid air. If we cool it further, water will start condensing and separates out as fog/dew.

FREQUENTLY ASKED QUESTIONS

What is the significance of the perfect gas equation in psychrometry?
The perfect gas equation, PV = nRT, is a fundamental tool in psychrometry as it allows us to manipulate the characteristics of air-water vapor mixtures. By rearranging the equation to p = dRT, we can analyze various conditions and definitions. The perfect gas equation is a simplification of real-world behavior, but it provides a useful approximation for many psychrometric applications, such as calculating the properties of air in air conditioning systems and cooling towers.
How does Dalton’s Law of Partial Pressure apply to psychrometry?

Dalton’s Law states that in a mixture of perfect gases, each constituent behaves individually as if the others were not present. In psychrometry, this means that the total pressure of the air-water vapor mixture is the sum of the individual partial pressures of the constituents. This can be expressed mathematically as p_total = p_air + p_water_vapor. Dalton’s Law is essential in psychrometry as it allows us to calculate the properties of the mixture based on the properties of its individual components.

What is the difference between psychrometry and hygrometry?

Psychrometry and hygrometry are often used interchangeably, but technically, hygrometry refers specifically to the measurement of the humidity of the air, whereas psychrometry is the broader study of the properties of air-water vapor mixtures. Psychrometry encompasses not only humidity but also other properties such as temperature, enthalpy, and density. While hygrometry is a subset of psychrometry, the terms are often used synonymously in practice.

How does the composition of atmospheric air affect psychrometric calculations?

Atmospheric air is a mixture of many gases, including nitrogen, oxygen, carbon dioxide, and water vapor. The composition of atmospheric air can affect psychrometric calculations, particularly when dealing with trace gases. However, for most psychrometric applications, the air can be approximated as a binary mixture of dry air and water vapor. This simplification allows us to use psychrometric charts and tables that are based on this binary mixture assumption.

What are some common applications of psychrometry in mechanical engineering?

Psychrometry has numerous applications in mechanical engineering, including air conditioning systems, cooling towers, and humidification/dehumidification systems. By understanding the properties of air-water vapor mixtures, engineers can design and optimize these systems to achieve efficient heat transfer, humidity control, and energy savings. Psychrometry is also essential in the design of HVAC systems, where it is used to calculate heating and cooling loads, and to optimize system performance.

How can psychrometric charts and tables be used in engineering design?

Psychrometric charts and tables are graphical and tabular representations of the properties of air-water vapor mixtures. They provide a convenient way to determine the properties of the mixture, such as humidity, enthalpy, and density, based on temperature and humidity ratios. Engineers can use these charts and tables to quickly and accurately calculate the properties of the air in various engineering applications, such as designing air conditioning systems, cooling towers, and humidification/dehumidification systems.

How does Dalton’s Law of Partial Pressure apply to psychrometry?

Dalton’s Law states that in a mixture of perfect gases, each constituent behaves individually as if the others were not present. In psychrometry, this means that the total pressure of the air-water vapor mixture is the sum of the individual partial pressures of the constituents. This can be expressed mathematically as p_total = p_air + p_water_vapor. Dalton’s Law is essential in psychrometry as it allows us to calculate the properties of the mixture based on the properties of its individual components.