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Chapter: Mechanical - Engineering Thermodynamics - Gas Mixtures and Psychrometry

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Psychrometry

psychrometry is a term used to describe the field of engineering concerned with the determination of physical and thermodynamic properties of gas-vapor mixtures.

PSYCHROMETRY

 

psychrometry is a term used to describe the field of engineering concerned with the determination of physical and thermodynamic properties of gas-vapor mixtures. The term derives from the Greek psuchron meaning "cold" and metron meaning "means of measurement"

 

Common applications

 

The principles of psychrometry apply to any physical system consisting of gasvapor mixtures. The most common system of interest, however, are mixtures of water vapor and air because of its application in heating, ventilating, and air-conditioning and meteorology.

 

Psychrometric ratio

 

The psychrometric ratio is the ratio of the heat transfer coefficient to the product of mass transfer coefficient and humid heat at a wetted surface. It may be evaluated with the following equation


where:

 

r = Psychrometric ratio, dimensionless

 

hc = convective heat transfer coefficient, W m-2 K-1

 

ky = convective mass transfer coefficient, kg m-2 s-1

 

cs = humid heat, J kg-1 K-1

 

Humid heat is the constant-pressure specific heat of moist air, per unit mass of dry air.

 

The psychrometric ratio is an important property in the area of psychrometrics as it relates the absolute humidity and saturation humidity to the difference between the dry bulb temperature and the adiabatic saturation temperature. Mixtures of air and water vapor are the most common systems encountered in psychrometry. The psychrometric ratio of air-water vapor mixtures is approximately unity which implies that the difference between the adiabatic saturation temperature and wet bulb temperature of air-water vapor mixtures is small. This property of air-water vapor systems simplifies drying and cooling calculations often performed using psychrometic relationships.

 

PSYCHROMETRIC CHART


A psychrometric chart is a graph of the physical properties of moist air at a constant pressure (often equated to an elevation relative to sea level). The chart graphically expresses how various properties relate to each other, and is thus a graphical equation of state. The thermophysical properties found on most psychrometric charts are:

 

Dry-bulb temperature (DBT) is that of an air sample, as determined by an ordinary thermometer, the thermometer's bulb being dry. It is typically the x-axis, the horizontal axis, of the graph. The SI units for temperature are Kelvin; other units are Fahrenheit.

 

Wet-bulb temperature (WBT) is that of an air sample after it has passed through a constant-pressure, ideal, adiabatic saturation process, that is, after the air has passed over a large surface of liquid water in an insulated channel. In practice, this is the reading of a thermometer whose sensing bulb is covered with a wet sock evaporating into a rapid stream of the sample air. The WBT is the same as the DBT when the air sample is saturated with water. The slope of the line of constant WBT reflects the heat of vaporization of the water required to saturate the air of a given relative humidity.

 

Dew point temperature (DPT) is that temperature at which a moist air sample at the same pressure would reach water vapor saturation. At this saturation point, water vapor would begin to condense into liquid water fog or (if below freezing) solid hoarfrost, as heat is removed. The dewpoint temperature is measured easily and provides useful information, but is normally not considered an independent property. It duplicates information available via other humidity properties and the saturation curve.

 

Relative humidity (RH) is the ratio of the mole fraction of water vapor to the mole fraction of saturated moist air at the same temperature and pressure. RH is dimensionless, and is usually expressed as a percentage. Lines of constant RH reflect the physics of air and water: they are determined via experimental measurement. Note: the notion that air "holds" moisture, or that moisture dissolves in dry air and saturates the solution at some proportion, is an erroneous (albeit widespread) concept (see relative humidity for further details).

 

Humidity ratio (also known as moisture content, mixing ratio, or specific humidity) is the proportion of mass of water vapor per unit mass of dry air at the given conditions (DBT, WBT, DPT, RH, etc.). It is typically the y-axis, the vertical axis, of the graph. For a given DBT there will be a particular humidity ratio for which the air sample is at 100% relative humidity: the relationship reflects the physics of water and air and must be measured. Humidity ratio is dimensionless, but is sometimes expressed as grams of water per kilogram of dry air or grains of water per pound of air.

 

Specific enthalpy symbolized by h, also called heat content per unit mass, is the sum of the internal (heat) energy of the moist air in question, including the heat of the air and water vapor within. In the approximation of ideal gases, lines of constant enthalpy are parallel to lines of constant WBT. Enthalpy is given in (SI) joules per kilogram of air or BTU per pound of dry air.

 

Specific volume, also called inverse density, is the volume per unit mass of the air sample. The SI units are cubic meters per kilogram of dry air; other units are cubic feet per pound of dry air.

 

Specific humidity

 

Specific humidity is the ratio of water vapor to air (including water vapor and dry air) in a particular mass. Specific humidity ratio is expressed as a ratio of kilograms of water vapor, mw, per kilogram of air (including water vapor), mt .


Relative Humidity

The relative humidity of an air-water mixture is defined as the ratio of the partial pressure of water vapor in the mixture to the saturated vapor pressure of water at a prescribed temperature. Relative humidity is normally expressed as a percentage and is defined in the following manner


RH BH is the relative humidity of the mixture being considered;

 P(H2O) is the partial pressure of water vapor in the mixture; and

 P(H2O) is the saturated vapor pressure of water at the temperature of the mixture. The international symbols U and Uw, expressed in per cent, are gaining recognition.


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