Properties of Air and Other Gases
The thermodynamic and transport properties of gases and vapors are important in fan engineering. This chapter deals with the thermodynamic properties, especially pressure, temperature, humidity, density, and enthalpy Transport properties, such as viscosity, thermal conductivity, and diffusivity, are dealt with in subsequent chapters. The gaseous materials most frequently encountered in fan engineering are air and water vapor; accordingly, most of the data are for these substances. Some formulae have been written specifically
for these materials, but most are generalized to accommodate any gas.
Atmospheric Air
Atmospheric air is a mixture of dry air, water vapor, and impurities. Dry air is a mechanical mixture of gases, whose principal constituents are listed in Table 1.1. (The table values may be considered representative of the composition of normal outdoor air throughout the troposphere.) The amount of water vapor in atmospheric air will depend on weather conditions. The nature and amount of impurities in the atmosphere depend on the forces at work in producing and dispersing contaminants. Industrial, urban, rural, seaside, and
other areas have characteristic atmospheres due to differences in impurities.
Adapted from the data of J. A. Goff: "Standardization of Thermodynamic Properties of Moist
Air," Trans. ASHVE, vol. 55, 1949, pp. 462-464,
The reference for Table 1.1 lists neon, helium, krypton, hydrogen, xenon, ozone, and radon, totaling less than 0.0025 percent by volume, as the residual part of atmospheric air. ASHRAE1 also lists methane, nitrous oxide, sulfur-dioxide, nitrogen dioxide, ammonia, carbon monoxide, and iodine, totaling 0.0003 percent by volume, as constituents of normal, clean, dry atmospheric air. ASHRAE considers all these gases in the calculation of the apparent molecular weight of clean, dry atmospheric air and obtains a value of 28.9645. Rounding off and lumping the residuals with the nitrogen, as has been done in Table 1.1, yields an apparent molecular weight of 28.964.
Standard Atmosphere
In 1952, the National Advisory Committee for Aeronautics adopted the International Civil Aviation Organization's Standard Atmosphere. Portions of this Standard are given in Table 1.2. (The reference contains much more extensive data in both U.S. and metric units.) Temperatures t are based on 15°C at sea level and a lapse rate of 0.0065°C/m throughout the troposphere, and they are assumed to be constant throughout the stratosphere. The tropopause is considered to be at the level where the temperature becomes -56.50°C. Pressures p are based on 101.325 kPa at sea level, a gas constant of 287.04 J/kg-K, and the perfect gas laws. Densities r are based on the temperature and pressure at the altitude Z and the perfect gas laws. Absolute viscosities m , kinematic viscosities n , and speeds of sound c are based on relationships that will be explained in later sections dealing with these subjects.
Standard Air
In fan engineering, standard air is considered to be air with a density of 1.2 kg /m3 when SI units are employed, or 0.075 lbm/ft3 when U.S. customary units are used. These two values are not exact equivalents, but they are close enough for most fan engineering purposes. Neither do these values exactly
correspond to the sea level value given for the Aeronautical Standard in Table 1.2. Atmospheric air of the composition shown in Table 1.1 will have standard density at various combinations of pressure, temperature, and humidity. Two convenient combinations are shown in Table 1.3, one for dry air and another for moist air. Note that all the combinations listed in Table 1.3 utilize the standard barometric pressure at sea level.
The concept of standard air is useful in rating fans, ducts, and other air handling equipment. Often both duct losses and fan capabilities can be determined from standard air data and used without correction. Even when
the actual density is considerably different from standard air density, it is frequently more convenient to apply corrections to standard air data than it would be to publish separate data for each condition. A slightly different concept, that of standard, or normal, temperature and pressure (STP or NTP), is sometimes employed in specifications. The values for standard temperature and pressure may differ from those in Table 1.3. One should always verify the exact values of standard conditions before selecting a fan based on STP or NTP. See the discussion of conversion from standard, or normal, conditions to actual conditions
Molecular Weight
The molecular weight of a pure substance is the sum of the atomic weights of the atoms in a molecule of that substance. Water, for instance, has a molecular weight of 18.015 based on two atoms of hydrogen at 1.008 each and one atom of oxygen at 15.999, all on the carbon-12 scale. Because air is a mechanical mixture of gases, it does not have a true molecular weight. Dry air of the composition shown in Table 1.1 has an apparent molecular weight of 28.964. The apparent molecular weight M of any mixture of gases can be
calculated either from a volumetric analysis using
where f x is the volume or mass fraction of constituent x ; Mx . is the molecularweight of constituent x ; and n is the total number of constituents.
Examples 1.1 and 1.2 illustrate the use of these equations in calculating the apparent molecular weight of dry outdoor air of the composition shown in Table 1.1.
Differences in molecular weights for the same substance usually can be traced to either rounding off or to differences between the carbon-12 and the oxygen-16 scales. ASHRAE1 lists molecular- weights on the carbon-12 scale as 28.9645 for dry air and 18.015 34 for water. The previously used value of 28.966 for dry air was based on the oxygen-16 scale. Throughout the remainder of this handbook, a value of 28.965 will be used as the apparent molecular weight of dry air. A mole, abbreviated mol, is the base unit of substance in SI. As such, it is further defined as the amount of substance that contains as many elementary
entities as there are atoms in 12 grams of carbon-12. In fan engineering, the elementary entities of interest are molecules, and the usual units of mass are the kg or the lbm. One kg-mol of air will have a mass of 28.965 kg. One lbm-mol of air will have a mass of 28.965 lbm.
The unit of molecular weight is the kg/kg-mol in metric units, and the lbm/lbm-mol in U.S. units. The number of molecules in a kg-mol of any gas is 6.02252 ´1026 . There are 2.73177 ´1026 molecules in a lbm-mol of gas. The volume occupied by a mole of gas will depend on the unit of the mole and on the temperature and pressure. For a gas constituent, the mole fraction, volume fraction, and pressure fraction are equal.
The thermodynamic and transport properties of gases and vapors are important in fan engineering. This chapter deals with the thermodynamic properties, especially pressure, temperature, humidity, density, and enthalpy Transport properties, such as viscosity, thermal conductivity, and diffusivity, are dealt with in subsequent chapters. The gaseous materials most frequently encountered in fan engineering are air and water vapor; accordingly, most of the data are for these substances. Some formulae have been written specifically
for these materials, but most are generalized to accommodate any gas.
Atmospheric Air
Atmospheric air is a mixture of dry air, water vapor, and impurities. Dry air is a mechanical mixture of gases, whose principal constituents are listed in Table 1.1. (The table values may be considered representative of the composition of normal outdoor air throughout the troposphere.) The amount of water vapor in atmospheric air will depend on weather conditions. The nature and amount of impurities in the atmosphere depend on the forces at work in producing and dispersing contaminants. Industrial, urban, rural, seaside, and
other areas have characteristic atmospheres due to differences in impurities.
Adapted from the data of J. A. Goff: "Standardization of Thermodynamic Properties of Moist
Air," Trans. ASHVE, vol. 55, 1949, pp. 462-464,
The reference for Table 1.1 lists neon, helium, krypton, hydrogen, xenon, ozone, and radon, totaling less than 0.0025 percent by volume, as the residual part of atmospheric air. ASHRAE1 also lists methane, nitrous oxide, sulfur-dioxide, nitrogen dioxide, ammonia, carbon monoxide, and iodine, totaling 0.0003 percent by volume, as constituents of normal, clean, dry atmospheric air. ASHRAE considers all these gases in the calculation of the apparent molecular weight of clean, dry atmospheric air and obtains a value of 28.9645. Rounding off and lumping the residuals with the nitrogen, as has been done in Table 1.1, yields an apparent molecular weight of 28.964.
Standard Atmosphere
In 1952, the National Advisory Committee for Aeronautics adopted the International Civil Aviation Organization's Standard Atmosphere. Portions of this Standard are given in Table 1.2. (The reference contains much more extensive data in both U.S. and metric units.) Temperatures t are based on 15°C at sea level and a lapse rate of 0.0065°C/m throughout the troposphere, and they are assumed to be constant throughout the stratosphere. The tropopause is considered to be at the level where the temperature becomes -56.50°C. Pressures p are based on 101.325 kPa at sea level, a gas constant of 287.04 J/kg-K, and the perfect gas laws. Densities r are based on the temperature and pressure at the altitude Z and the perfect gas laws. Absolute viscosities m , kinematic viscosities n , and speeds of sound c are based on relationships that will be explained in later sections dealing with these subjects.
Standard Air
In fan engineering, standard air is considered to be air with a density of 1.2 kg /m3 when SI units are employed, or 0.075 lbm/ft3 when U.S. customary units are used. These two values are not exact equivalents, but they are close enough for most fan engineering purposes. Neither do these values exactly
correspond to the sea level value given for the Aeronautical Standard in Table 1.2. Atmospheric air of the composition shown in Table 1.1 will have standard density at various combinations of pressure, temperature, and humidity. Two convenient combinations are shown in Table 1.3, one for dry air and another for moist air. Note that all the combinations listed in Table 1.3 utilize the standard barometric pressure at sea level.
The concept of standard air is useful in rating fans, ducts, and other air handling equipment. Often both duct losses and fan capabilities can be determined from standard air data and used without correction. Even when
the actual density is considerably different from standard air density, it is frequently more convenient to apply corrections to standard air data than it would be to publish separate data for each condition. A slightly different concept, that of standard, or normal, temperature and pressure (STP or NTP), is sometimes employed in specifications. The values for standard temperature and pressure may differ from those in Table 1.3. One should always verify the exact values of standard conditions before selecting a fan based on STP or NTP. See the discussion of conversion from standard, or normal, conditions to actual conditions
Molecular Weight
The molecular weight of a pure substance is the sum of the atomic weights of the atoms in a molecule of that substance. Water, for instance, has a molecular weight of 18.015 based on two atoms of hydrogen at 1.008 each and one atom of oxygen at 15.999, all on the carbon-12 scale. Because air is a mechanical mixture of gases, it does not have a true molecular weight. Dry air of the composition shown in Table 1.1 has an apparent molecular weight of 28.964. The apparent molecular weight M of any mixture of gases can be
calculated either from a volumetric analysis using
where f x is the volume or mass fraction of constituent x ; Mx . is the molecularweight of constituent x ; and n is the total number of constituents.
Examples 1.1 and 1.2 illustrate the use of these equations in calculating the apparent molecular weight of dry outdoor air of the composition shown in Table 1.1.
Differences in molecular weights for the same substance usually can be traced to either rounding off or to differences between the carbon-12 and the oxygen-16 scales. ASHRAE1 lists molecular- weights on the carbon-12 scale as 28.9645 for dry air and 18.015 34 for water. The previously used value of 28.966 for dry air was based on the oxygen-16 scale. Throughout the remainder of this handbook, a value of 28.965 will be used as the apparent molecular weight of dry air. A mole, abbreviated mol, is the base unit of substance in SI. As such, it is further defined as the amount of substance that contains as many elementary
entities as there are atoms in 12 grams of carbon-12. In fan engineering, the elementary entities of interest are molecules, and the usual units of mass are the kg or the lbm. One kg-mol of air will have a mass of 28.965 kg. One lbm-mol of air will have a mass of 28.965 lbm.
The unit of molecular weight is the kg/kg-mol in metric units, and the lbm/lbm-mol in U.S. units. The number of molecules in a kg-mol of any gas is 6.02252 ´1026 . There are 2.73177 ´1026 molecules in a lbm-mol of gas. The volume occupied by a mole of gas will depend on the unit of the mole and on the temperature and pressure. For a gas constituent, the mole fraction, volume fraction, and pressure fraction are equal.
No comments:
Post a Comment