Page 27 - Separation process principles 2

Dimensions and Units

Chemical engineers must be proficient in the use of three systems of units: (1) the Interna-
tional System of Units, SI System (Systeme Internationale d'unites), which was estab-
lished in 1960 by the 11th General Conference on Weights and Measures and has been
widely adopted; (2) the AE (American Engineering) System, which is based largely upon
an English system of units adopted when the Magna Carta was signed in 1215 and is the
preferred system in the United States; and (3) the CGS (centimeter-gram-second) System,
which was devised in 1790 by the National Assembly of France, and served as the basis for
the development of the SI System. Auseful index to units and systems of units is given on
the website at http://www.sizes.conz/units/index.htm
Engineers must deal with dimensions and units to express the dimensions in terms of
numerical values. Thus, for 10 gallons of gasoline, the dimension is volume, the unit is gal-
lons, and the value is 10. As detailed in NIST (National Institute of Standards and Tech-
nology) Special Publication 811 (1995 edition), which is available at the website
http://physics.nist.gov/cuu/pdf/sp8 11 .pdf, units are base or derived.

BASE UNITS
The base units are those that cannot be subdivided, are independent, and are accurately de-
fined. The base units are for dimensions of length, mass, time, temperature, molar amount,
electrical current, and luminous intensity, all of which can be measured independently. De-
rived units are expressed in terms of base units or other derived units and include dimen-
sions of volume, velocity, density, force, and energy. In this book we deal with the first five
of the base dimensions. For these, the base units are:

Base Dimension SI Unit AE Unit CGS Unit
Length meter, m foot, ft centimeter, cm
Mass kilogram, kg pound, lb, gram, g
Time second, s hour, h second, s
Temperature kelvin, K Fahrenheit, F Celsius, C
Molar amount gram-mole, mol pound-mole, lbmol gram-mole, mol

DERIVED UNITS

Many derived dimensions and units are used in chemical engineering.dSeveral are listed in
the following table:

Derived Dimension SI Unit AE Unit CGS Unit

Area = ~en~th~ m2 ft2 cm2
Volume = ~en~th~ m3 ft3 cm3
Mass flow rate = Mass/Time kgls lb,/h g/s
Molar flow rate = molls lbmoVh molls
Molar amount/Time
Velocity = LengthlTime m/s ft/h cm/s
Acceleration = Velocity/Time m/s2 ft/h2 cm/s2
Force = Mass Acceleration newton, N = Ibf dyne =
1 kg m/s2 1 g cm/s2

(Continued)

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