Pressure, conversion factors

Pressure conversion factors 166 Energy conversion factors inside back cover)... 

Pressure is defined as force per unit of area. The International System of Units (SI) pressure unit is the pascal (Pa), defined as 1.0 N /m. Conversion factors from non-SI units to pascal are given in Table 1 (see also Units and conversion factors front matter). An asterisk after the sixth decimal place indicates that the conversion factor is exact and all subsequent digits are 2ero. Relationships that are not followed by an asterisk are either the results of physical measurements or are only approximate. The factors are written as numbers greater than 1 and less than 10, with 6 or fewer decimal places (1). 

The dimensions of permeabiUty become clear after rearranging equation 1 to solve for P. The permeabiUty must have dimensions of quantity of permeant (either mass or molar) times thickness ia the numerator with area times a time iaterval times pressure ia the denomiaator. Table 1 contains conversion factors for several common unit sets with the permeant quantity ia molar units. The unit nmol/(m-s-GPa) is used hereia for the permeabiUty of small molecules because this unit is SI, which is preferred ia current technical encyclopedias, and it is only a factor of 2, different from the commercial permeabihty unit, (cc(STP)-mil)/(100 in. datm). The molar character is useful for oxygen permeation, which could ultimately involve a chemical reaction, or carbon dioxide permeation, which is often related to the pressure in a beverage botde. 

The water-vapor transmission rate (WVTR) is another descriptor of barrier polymers. Strictly, it is not a permeabihty coefficient. The dimensions are quantity times thickness in the numerator and area times a time interval in the denominator. These dimensions do not have a pressure dimension in the denominator as does the permeabihty. Common commercial units for WVTR are (gmil)/(100 in. d). Table 2 contains conversion factors for several common units for WVTR. This text uses the preferred nmol/(m-s). The WVTR describes the rate that water molecules move through a film when one side has a humid environment and the other side is dry. The WVTR is a strong function of temperature because both the water content of the air and the permeabihty are direcdy related to temperature. Eor the WVTR to be useful, the water-vapor pressure difference for the value must be reported. Both these facts are recognized by specifying the relative humidity and temperature for the WVTR value. This enables the user to calculate the water-vapor pressure difference. Eor example, the common conditions are 90% relative humidity (rh) at 37.8°C, which means the pressure difference is 5.89 kPa (44 mm Hg). 

Chemical Flowsheet Basic unit operation selection with flow rates, conversion factors, temperatures, pressures, solvents and catalyst selection Process synthesis route Laboratory and pilot scale trials Knowledge of existing processes... 

Process Safety and Pressure-Relieving Devices Appendix of Conversion Factors... 

A-l Alphabetical Conversion Factors, 416 A-2 Physical Property Conversion Factors, 423 A-3 Synchronous Speeds, 426 A-4 Conversion Factors, 427 A-5 Temperature Conversion, 429 A-6 Altitude and Atmospheric Pressures, 430 A-7 Vapor Pressure Curves, 431 A-8 Pressure Conversion Chart, 432 A-9 Vacuum Conversion, 433 A-10 Decimal and Millimeter Equivalents of Fractions,... 

Due to a fortuitous cancellation of units, volume can be expressed in cm3 and pressure in MPa to give the same results as volume in m3 with pressure in Pa, without invoking any conversion factors. Thus pV in Pa m3 gives Joules as does pV in MPa cm3. 

Pressure units are summarized in Table 4.1. It is important to be familiar with them and to be able to make conversions between them. In Example 4.2, for instance, the pressure in pascals could have been obtained by using a conversion factor derived from Table 4.1 ... 

Therefore, if we work in pascals and cubic meters, the work is obtained in joules. However, we might have expressed the pressure in atmospheres and the volume in liters. In this case, we may need to convert the answer (in liter-atmospheres) into joules. The conversion factor is obtained by noting that 1 L = 10 3 m3 and 1 atm = 101 325 Pa exactly therefore... 

We must combine the reading from the manometer with the barometric pressure to find the pressure of the gas sample. The manometer displays the pressure difference in millimeters of mercury, so conversion factors are needed to express the pressure in atmospheres and bars. 

Use the appropriate conversion factors to give the pressure in atm and bars. 

For relatively low pressure drops, the effect of compressibility is negligible, and the general flow equation [Eq. (10-29)] applies. Introducing the conversion factors to give the flow rate in standard cubic feet per hour (scfh) and the density of air at standard conditions (1 atm, 520°R), this equation becomes... 

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