PSYCHROMETRY

Psychrometry is the study of air–water vapour 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 law 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 vapour mixture (p_B) can be expressed by the Dalton’s law equation as the sum of the partial pressures of the constituents; p = p_a + p_v and if the water vapour is saturated p = p_a + p_vs

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

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

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

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 vapour 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 DBT and humidity ratio can bring air to saturation adiabatically at the same pressure.

Specific Volume (v): It is 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.