Page 29 - Separation process principles 2

Dimensions and Units xxxiii

Thus, when using AE units in an equation that includes force and mass, incorporate g, to
adjust the units.

Example

A 5.000-pound-mass weight, m, is held at a height, h, of 4.000 feet above sea-level. Calcu-
late its potential energy above sea-level, P.E. = mgh, using eachof the three systems of units.
Factors for converting units are given on the inside back cover of this book.
SI System:

CGS System.

AE System:

However, the accepted unit of energy for the AE System is ft lbf, which is obtained by
dividing by g,. Therefore, P.E. = 643.5/32.174 = 20.00 ft lbf
Another difficulty with the AE System is the differentiation between energy as work and
energy as heat. As seen in the preceding table, the work unit is ft lbf, while the heat unit is
Btu. A similar situation exists in the CGS System with corresponding units of erg and calo-
rie (cal). In older textbooks, the conversion factor between work and heat is often incorpo-
rated into an equation with the symbol J, called Joule's constant or the mechanical equiva-
lent of heat, where,

Thus, in the previous example, the heat equivalents are

AE System:

CGS System:

In the SI System, the prefix M, mega, stands for million. However, in the natural gas and
petroleum industries of the United States, when using the AE System, M stands for thou-
sand and MM stands for million. Thus, MBtu stands for thousands of Btu, while MM Btu
stands for millions of Btu.
It should be noted that the common pressure and power units in use for the AE System
are not consistent with the base units. Thus, for pressure, pounds per square inch, psi or
is
~b*/in.~, used rather than lbf/ft2. For power, hp is used instead of ft lbf/h, where, the con-
version factor is