Page 54 - APPLIED PROCESS DESIGN FOR CHEMICAL AND PETROCHEMICAL PLANTS, Volume 1, 3rd Edition

Process Planning, Scheduling and Flowsheet Design 39

Computers are quite adaptable to the following calcu-
Table 1-10
lations: distillation tray-by-tray and short-cut methods; tray Calculation Time Using Medium Size Digital Computer
hydraulics for bubble cap, sieve or perforated and "dual-
Hours
flow"; absorption, heat exchange including condensation, Total Elapsed Time
partial condensation, cooier-condensing, reboiling; dry- Calculation Preparation + Calculation
ing; compression; equilibrium flash; fluid flow including Preliminary Dis till a tion
two phase and many others. It is important to remember Number of trays, reflux ratio . 1-3
that good results cannot be obtained from a poor or inad- Tray-by-Tray Distillation
equate computer program. Thus, it is wise to invest the To 40 trays . 2-3
effort into the development of basically sound general To 100 trays . 3-5
purpose programs. With these many variations can be Tray Hydraulics . 1-3
Bubble cap, sieve, perforated
arranged to suit the special case. In order to have confi- Heat Exchangers
dence in the results of any computer program (whether Condensers, exchangers . 0.5-1.0
self-developed or purchased) it must be tested against Separators . 0.5
extreme conditions or limits. To purchase and use a pro- Flash Vaporization . 0.5
gram without testing is inviting errors. Oil Absorbers . 1-3
Safety Valves . 0.25
Some programs require only a few days to completely
program for general purpose use, while some others tions. It may not apply well to projects of less than 200
require several months of continuous effort. Whenever process man hours.
more than one individual is expected to use the comput-
er program, it is good practice to obtain the several views When a limited time is available to complete a project,
on attacking the problem, i.e., type of input data, solution this may be used to determine the estimates of manpower:
approach, range of variables, fixed conditions and type
and form of output or results. Estimated man-hours (process)
Average number of men= -------------
Table 1-10 illustrates some reasonable time require- (Total elapsed weeks) (30 Lo 33)
ments for solution of problems or designs when using a
medium-sized digital computer, using existing programs. Where: 30 to 33 represents the actual usable job-relat-
A very high speed machine might reduce the pure calcu- ed man-hours per 40-hour week per man, allowing for
lation time to a matter of minutes; however, the time average sickness, vacation, jury duty, etc.
required for (1) data collection (specific problem condi-
tions as well as physical data, (2) data input to the com- Approximate maximum number required = (l.67) (Avg.
puter, and (3) evaluation of results and preparation of number of engineers)
design specification sheets all remain essentially fixed. In
some situations the complexity of the calculations Example 1-1: Man-Hour Evaluation
requires the capabilities of the large machines, and in
these cases the time advantage can be the difference From an examination of the process flowsheet the
between a good result and none at all. Total plant mater- man-hours Lota! 685* for the significant equipment. Items
ial and heat balances are a good example. such as steam traps and miscellaneous small time-items
can he omitted from the total. *Includes 75 man-hours for
Estimated Total Process Man-Hours pipeline sizing.

After the man-hours have been estimated for all of the Total Estimated job Man - Hours = 685 = 1525
individual items of equipment on the project, a guide to 0.45
total man-hours is:
If the work must be complete, including flowsheet
supervision, etc., in three weeks:
Estimated Equipment man-
hours (including checking)
Total Estimatedjob Process = Average no. engineers required = 1525 15.9
Engineering Man- Hours 0.45 (3) (32)

This applies to work where at least 50 percent of the time This is impractical since a job of this magnitude cannot
is by electronic desk calculator for the numerical calcula- be planned and decisions reached in this time. Therefore,