Page 28 - Separation process principles 2

xxxii Dimensions and Units

Derived Dimension SI Unit AE Unit CGS Unit
Pressure = ForceIArea pascal, Pa = lbf/in2 atm
1 ~/rn~ =
1 kg/m s2
Energy = Force Length joule, J = ft lbf, Btu erg = 1 dyne cm =
1N m= 1 g cm2/s2, ca1
1 kg m2/s2
Power = EnergyITime = Watt, W = hp ergis
WorkITime 1 J/s = 1 N mls =
1 kg m2/s3
Density = Mass/Volume kg/m3 lb,,,/ft3 g/cm3

OTHER UNITS ACCEPTABLE FOR USE WITH THE SI SYSTEM

A major advantage of the SI System is the consistency of the derived units with the base
units. However, some acceptable deviations from this consistency and some other accept-
able base units are given in the following table:

Dimension Base or Derived SI Unit Acceptable SI Unit
Time minute (min), hour (h), day (d), year (y)
Volume liter (L) = m3
Mass metric ton or tonne (t) = lo3 kg
Pressure bar = lo5 Pa

PREFIXES

Also acceptable for use with the SI System are decimal multiples 'md submultiples of SI
units formed by prefixes. The following table lists the more commonly used prefixes:

Prefix Factor Symbol

gigs 1 o9 G
mega 1 o6 M
kilo 1 03 k
deci lo-' d
centi 1 o-~ c
milli 1 O-) m
micro 1 o4 P
nano 1 0-9 n
pic0 10-l2 P 3

USING THE AE SYSTEM OF UNITS
The AE System is more difficult to use than the SI System because of the units used with 1
i
force, energy, and power. In the AE System, the force unit is the pound-force, lbf, which is 4
defined to be numerically equal to the pound-mass, lb,, at sea-level of the Earth. Accord- i
ingly, Newton's second law of motion is written,
i
1

where F = force in lbf, m = mass in lb,, g = acceleration due to gravity in ft/s2, and to com-
plete the definition, g, = 32.174 lb, ft/lbf s2, where 32.174 ft/s2 is the acceleration due to 1
gravity at sea-level of the Earth. The constant, g,, is not used with the SI System or the CGS 4 1
System because the former does not define a kgf and the CGS System does not use a gf. 1