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Psychrometric The field of engineering concerned with the determination of physical and thermodynamic properties of gas-vapor mixtures. Psychrometric History Willis Haviland Carrier (November 26, 1876 – October 7, 1950) was an American engineer and inventor, and is known as the man who invented modern air conditioning. For his contributions to science and industry, Willis Carrier was awarded an honorary doctor of letters from Alfred (N.Y.) University in 1942, was awarded the Frank P. Brown Medal in 1942, and was inducted posthumously in the National Inventors Hall of Fame (1985) and the Buffalo Science Museum Hall of Fame (2008). Common applications Although the principles of psychrometry apply to any physical system consisting of gas-vapor mixtures, the most common system of interest is the mixture of water vapor and air, because of its application in heating, ventilating, and air-conditioning and meteorology. In human terms, our comfort is in large part a consequence of, not just the temperature of the surrounding air, but (because we cool ourselves via perspiration) the extent to which that air is saturated with water vapor. 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: 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 Psychrometric ratio cont. 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. Heat Humid heat - Humid heat is the constant- pressure specific heat of moist air, per unit mass of dry air. Dry-bulb temperature – Common thermometers measure what is known as the dry-bulb temperature. Electronic temperature measurement, via thermocouples, thermistors, and resistance temperature devices (RTDs), for example, have been widely used too since they became available Dry-bulb temperature The dry-bulb temperature is the temperature of air measured by a thermometer freely exposed to the air but shielded from radiation and moisture. Dry bulb temperature is the temperature that is usually thought of as air temperature, and it is the true thermodynamic temperature. It is the temperature measured by a regular thermometer exposed to the airstream. Unlike wet bulb temperature, dry bulb temperature does not indicate the amount of moisture in the air. In construction, it is an important consideration when designing a building for a certain climate. Dry-bulb temperature cont. (DBT) is that of an air sample, as determined by an ordinary thermometer. It is typically plotted as the abscissa (horizontal axis) of the graph. The SI units for temperature are kelvins or degrees Celsius; other units are degrees Fahrenheit and degrees Rankine. Wet-bulb temperature The thermodynamic wet-bulb temperature is a thermodynamic property of a mixture of air and water vapor. The value indicated by a simple wet-bulb thermometer often provides an adequate approximation of the thermodynamic wet-bulb temperature. Wet-bulb temperature cont. A wet-bulb thermometer is an instrument which may be used to infer the amount of moisture in the air. If a moist cloth wick is placed over a thermometer bulb, the evaporation of moisture from the wick will lower the thermometer reading (temperature). If the air surrounding a wet-bulb thermometer is dry, evaporation from the moist wick will be more rapid than if the air is moist. When the air is saturated, no water will evaporate from the wick and the temperature of the wet-bulb thermometer will be the same as the reading on the dry-bulb thermometer. However, if the air is not Saturated saturated, water will evaporate from the wick causing the Sock temperature reading to be lower. Wet-bulb temperature cont. The accuracy of a simple wet-bulb thermometer depends on how fast air passes over the bulb and how well the thermometer is shielded from the radiant temperature of its surroundings. Speeds up to 5,000 ft/min (60 mph) are best but it may be dangerous to move a thermometer at that speed. Errors up to 15% can occur if the air movement is too slow or if there is too much radiant heat present (from sunlight, for example). A wet bulb temperature taken with air moving at about 1–2 m/s is referred to as a screen temperature, whereas a temperature taken with air moving about 3.5 m/s or more is referred to as sling temperature. Wet-bulb temperature cont. (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 (see Hygrometer). When the air sample is saturated with water, the WBT will read the same as the DBT. 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 The dew point is the temperature at which a given parcel of humid air must be cooled, at constant barometric pressure, for water vapor to condense into water. The condensed water is called dew. The dew point is a saturation temperature. The dew point is associated with relative humidity. A high relative humidity indicates that the dew point is closer to the current air temperature. Relative humidity of 100% indicates the dew point is equal to the current temperature and the air is maximally saturated with water. When the dew point remains constant and temperature increases, relative humidity will decrease. Dew point cont. temperature (DPT) is the temperature at which a moist air sample at the same pressure would reach water vapor “saturation.” At this point further removal of heat would result in water vapor condensing into liquid water fog or, if below freezing point, solid hoarfrost. The dew point temperature is measured easily and provides useful information, but is normally not considered an independent property of the air sample as it duplicates information available via other humidity properties and the saturation curve. Relative humidity Relative humidity is a term used to describe the amount of water vapor that exists in a gaseous mixture of air and water vapor. Relative humidity cont. Definition: 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 calculated by using the following equation: Relative humidity cont. (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. The concept that air "holds" moisture, or that moisture “dissolves” in dry air and saturates the solution at some proportion, is erroneous. Humidity ratio (also known as moisture content or mixing ratio) 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 plotted as the ordinate (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 determined by measurement. The dimensionless humidity ratio is typically expressed as grams of water per kilogram of dry air, or grains of water per pound of air (7000 grains equal 1 pound). Specific humidity is the proportion of the mass of water vapor per unit mass of the air sample (dry air plus the water vapor); it is closely related to humidity ratio and always lower in value. Specific Enthalpy Enthalpy is a measure of the total energy of a thermodynamic system. It includes the internal energy, which is the energy required to create a system, and the amount of energy required to make room for it by displacing its environment and establishing its volume and pressure. Enthalpy is a thermodynamic potential. It is a state function and an extensive quantity. The unit of measurement in the International System of Units (SI) for enthalpy is the joule, but other historical, conventional units are still in use, such as the small and the large calorie. Specific Enthalpy cont. 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 Enthalpy cont. Specific Volume In thermodynamics, the volume of a system is an important extensive parameter for describing its thermodynamic state. The specific volume, an intensive property, is the system's volume per unit of mass.Volume is a function of state and is inter-dependant with other thermodynamic properties such as pressure and temperature. For example, volume is related to the pressure and temperature of an ideal gas by the ideal gas law. Specific Volume cont. often confused as the inverse density of the mixture, rather it indicates the space occupied by the "dry" air with no consideration given for any potential water vapor. The SI units are cubic meters per kilogram of dry air; other units are cubic feet per pound of dry air. Psychrometric chart Psychrometric chart cont. The psychrometric chart allows all the parameters of some moist air to be determined from any three independent parameters, one of which one must be the pressure. Changes in state, such as when two air streams mix, can be modeled easily and somewhat graphically using the correct psychrometric chart for the location's air pressure or elevation relative to sea level. For locations at not more than 2000 ft (600 m) of altitude it is common practice to use the sea-level psychrometric chart. Psychrometric chart cont. Psychrometric chart cont. 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