Page 16 - Chemical Process Equipment - Selection and Design

RULES OF THUMB: SUMMARY

Although experienced engineers know where to find information 9. Compression ratio should be about the same in each stage of a
and how to make accurate computations, they also keep a minimum multistage unit, ratio = (P,JPl)l'n, with n stages.
body of information in mind on the ready, made up largely of 10. Efficiencies of reciprocating compressors: 65% at compression
shortcuts and rules of thumb. The present compilation may fit into ratio of 1.5, 75% at 2.0, and 8045% at 3-6.
such a minimum body of information, as a boost to the memory or 11. Efficiencies of large centrifugal compressors, 6000-100,000
extension in some instances into less often encountered areas. It is ACFM at suction, are 76-78%.
derived from the material in this book and is, in a sense, a digest of U. Rotary compressors have efficiencies of 70%, except liquid liner
the book. type which have 50%.
An Engineering Rule of Thumb is an outright statement
regarding suitable sizes or performance of equipment that obviates CONVEYORS FOR PARTICULATE SOLIDS
all need for extended calculations. Because any brief statements are
subject to varying degrees or' qualification, they are most safely 1. Screw conveyors are suited to transport of even sticky and
applied by engineers who are substantially familiar with the topics. abrasive solids up inclines of 20" or so. They are limited to
Nevertheless, such rules should be of value for approximate design distances of 150ft or so because of shaft torque strength. A
and cost estimation, and should provide even the inexperienced 12 in. dia conveyor can handle 1000-3000 cuft/hr, at speeds
engineer with perspective and a foundation whereby the reason- ranging from 40 to 60 rpm.
ableness of detailed and computer-aided results can be appraised 2. Belt conveyors are for high capacity and long distances (a mile or
quickly, partitcularly on short notice such as in conference. more, but only several hundred feet in a plant), up inclines of
Everyday activities also are governed to a large extent by rules 30" maximum. A 24in. wide belt can carry 3000cuft/hr at a
of thumb. They serve us when we wish to take a course of action speed of 100 ft/min, but speeds up to 600 ft/min are suited to
but are not in a position to find the best course of action. Of interest some materials. Power consumption is relatively low.
along this line is an ainusing and often useful list of some 900 such 3. Bucket elevators are suited to vertical transport of sticky and
digests of everyday experience that has been compiled by Parker abrasive materials. With buckets 20 X 20 in. capacity can reach
(Rules of Thumb Houghton Mifflin, Boston, 1983). lOOOcuft/hr at a speed of 100ft/min, but speeds to 300ft/min
Much more can be stated in adequate summary fashion about are used.
some topics than about others, which accounts in part for the 4. Drug-type conveyors (Redler) are suited to short distances in any
spottiness of the present coverage, but the spottiness also is due to direction and are completely enclosed. Units range in size from
ignorance and oversights on the part of the author. Accordingly, 3 in. square to 19 in. square and may travel from 30 ft/min (fly
every engineer undoubtedly will supplement or modify this material ash) to 250 ft/min (grains). Power requirements are high.
in his own way. 5. Pneumatic conveyors are for high capacity, short distance (400 ft)
transport simultaneously from several sources to several
destinations. Either vacuum or low pressure (6-12 psig) is
COMPRESSORS AND VACUUM PUMPS employed with a range of air velocities from 35 to 120ft/sec
depending on the material and pressure, air requirements from 1
1. Funs are used to raise the pressure about 3% (12in. water), to 7 cuft/cuft of solid transferred.
blowers raise to Uess than 40psig, and compressors to higher
pressures, although the blower range commonly is included in
the compressor range. COOLING TOWERS
2. Vacuum pumps: reciprocating piston type decrease the pressure 1. Water in contact with air under adiabatic conditions eventually
to 1Torr; rotary piston down to 0.001Torr, two-lobe rotary cools to the wet bulb temperature.
down to 0.0001Torr; steam jet ejectors, one stage down to 2. In commercial units, 90% of saturation of the air is feasible.
100Torr. three stage down to ITorr, five stage down to 3. Relative cooling tower size is sensitive to the dilference between
0.05 Torr. the exit and wet bulb temperatures:
3. A three-stage ejector needs lOOlb steam/lb air to maintain a
pressure of 1 Torr. AT ("F) 5 75 25
4. In-leakage of air to evacuated equipment depends on the Relative volume 2.4 1.0 0.55
absolute pressure, Torr, and the volume of the equipment, V 4. Tower fill is of a highly open structure so as to minimize pressure
cuft, according to w = kVZ3 lb/hr, with k = 0.2 when P is more drop, which is in standard practice a maximum of 2in. of water.
than 90 Torr, 0.08; between 3 and 20 Torr, and 0.025 at less than 5. Water circulation rate is 1-4gpm/sqft and air rates are
1 Tors. 1300-1800 lb/(hr)(sqft) or 300-400 ftlmin.
I .\
5. Theoretical adiabatic horseDower (THP) = T(SCFM)T, /8130al 6. Chimney-assisted natural draft towers are of hyperboloidal
~ I A.
[(F''/P1)' - 11, where TI is inlet temperature in "F + 460 and shapes because they have greater strength for a given thickness;
a = (k - I)/k? k = C,/C,. a tower 250 ft high has concrete walls 5-6 in. thick. The enlarged
6. Outlet temperature T, = G(P,/PJ. cross section at the top aids in dispersion of exit humid air into
7. To compress air from 100"F, k = 1.4, compression ratio = 3, the atmosphere.
theoretical power required = 62 HP/million cuft/day, outlet 7. Countercurrent induced draft towers are the most common in
temperature 306°F. process industries. They are able to cool water within 2°F of the
8. Exit temperature should not exceed 350-400°F; for diatomic wet bulb.
gases (C,,/C, = 1.4) this corresponds to a compression ratio of 8. Evaporation losses are 1% of the circulation for every 10°F of
about 4. cooling range. Windage or drift losses of mechanical draft towers