Page 31 - Coulson Richardson's Chemical Engineering Vol.6 Chemical Engineering Design 4th Edition

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CHEMICAL ENGINEERING
1.8. SYSTEMS OF UNITS
To be consistent with the other volumes in this series, SI units have been used in this
book. However, in practice the design methods, data and standards which the designer will
use are often only available in the traditional scientific and engineering units. Chemical
engineering has always used a diversity of units; embracing the scientific CGS and MKS
systems, and both the American and British engineering systems. Those engineers in the
older industries will also have had to deal with some bizarre traditional units; such as
degrees Twaddle (density) and barrels for quantity. Desirable as it may be for industry
world-wide to adopt one consistent set of units, such as SI, this is unlikely to come about
for many years, and the designer must contend with whatever system, or combination of
systems, his organisation uses. For those in the contracting industry this will also mean
working with whatever system of units the client requires.
It is usually the best practice to work through design calculations in the units in which
the result is to be presented; but, if working in SI units is preferred, data can be converted
to SI units, the calculation made, and the result converted to whatever units are required.
Conversion factors to the SI system from most of the scientific and engineering units used
in chemical engineering design are given in Appendix D.
Some license has been taken in the use of the SI system in this volume. Temperatures are
Ž
given in degrees Celsius ( C); degrees Kelvin are only used when absolute temperature
is required in the calculation. Pressures are often given in bar (or atmospheres) rather
2
than in the Pascals (N/m ), as this gives a better feel for the magnitude of the pressures.
In technical calculations the bar can be taken as equivalent to an atmosphere, whatever
definition is used for atmosphere. The abbreviations bara and barg are often used to denote
bar absolute and bar gauge; analogous to psia and psig when the pressure is expressed
in pound force per square inch. When bar is used on its own, without qualification, it is
normally taken as absolute.
2
For stress, N/mm have been used, as these units are now generally accepted by
engineers, and the use of a small unit of area helps to indicate that stress is the intensity of
force at a point (as is also pressure). For quantity, kmol are generally used in preference
to mol, and for flow, kmol/h instead of mol/s, as this gives more sensibly sized figures,
which are also closer to the more familiar lb/h.
3
3
3
For volume and volumetric flow, m and m /h are used in preference to m /s, which
gives ridiculously small values in engineering calculations. Litres per second are used for
small flow-rates, as this is the preferred unit for pump specifications.
Where, for convenience, other than SI units have been used on figures or diagrams, the
scales are also given in SI units, or the appropriate conversion factors are given in the
text. The answers to some examples are given in British engineering units as well as SI,
to help illustrate the significance of the values.
Some approximate conversion factors to SI units are given in Table 1.1. These are
worth committing to memory, to give some feel for the units for those more familiar with
the traditional engineering units. The exact conversion factors are also shown in the table.
A more comprehensive table of conversion factors is given in Appendix D.
Engineers need to be aware of the difference between US gallons and imperial gallons
(UK) when using American literature and equipment catalogues. Equipment quoted in an