MATERIALS HANDLING Robert
E. Wright,
MONSANTO CHEMICAL COMPANY, ST. LOUIS 4, MO.
the barrel is below the ground and is of sufficient length to obtain , t h e desired net positive suction head. This eliminates the necessity of lowering the pump into a pit or elevating the suction that equipment has been developed during the war to cover tank above the pump in order to get sufficient head. wider ranges of temperature, pressure, and flow, to meet severely The use of special materials of construction should be mencorrosive or abrasive conditions, and to handle liquids and gases tioned. For example, a pump for the handling of corrosive more efficiently. Solids handling equipment, such as conveyers fluids has been developed which is made of impregnated electricand trucks, has been developed to save or eliminate labor and to furnace graphite. This material gives a pump which is mechanimove materials more efficiently. The sum of these developcally strong, highly resistant to thermal shock, and resistant to ments gives the engineer new tools for designing better and practically all acids, alkalies, and solvents. more efficient process plants. Sometimes the The pump is provided with a specially denew equipment makes it possible to perform signed rotary seal which eliminates the use entirely new operations. Because of the of a stuffing box. Another manufacturer has breadth of the field it is possible to mention developed a new solids handling centrifugal only a few typical advances made during the pump constructed of a tough abrasivewar years in materials handling equipment. resistant alloy, which increases pump life two LIQUIDAND GAS HANDLINQ. Pumps. to four times that of pumps constructed of Wartime expansion of chemical plants and ordinary materials. These are but two exthe huge programs of manufacturing high amples of the many special materials deoctane gasoline, synthetic rubber, and the veloped and used during the war. atomic bomb required entirely new designs During the past few years much use has for and radical improvements in pumps. been made of process pumps. Such pumps Many of the pump developments were dicwere used for years by the petroleum intated by special requirements. For example, dustry, but their use is now being extended the catalytic cracking process for production t o chemical plants where costly process liquids of high octane gasoline requires a pump which must not be wasted or contaminated. Process will handle molten salt at a temperature of pumps have been developed by several manu850" F. and with a specific gravity of 1.75. facturers to meet the requirements of difficult Not only is this an unusual type of material pumping problems involving hot liquids well to be pumped, but the quantity being above their flash points. Pumps are now handled is very large, and, under some conavailable to handle fluids at 1000' F. ditions, the salt is pumped under vacuum. To meet the need for a high efficiency pump The pump discharges 17,000 gallons of for small capacities a t high heads, several molten salt per minute against a head of 45 companies have developed such equipment feet. For that purpose a propeller-type as vertical multistage pumps, usually of pump is used with the pumping element imdouble case construction. A 54-stage pump mersed in the molten salt. Special materials of this type delivers 18 gallons per minute of construction, particularly for the bearings against 1100 pounds per square inch, operates operating in the molten salt, and attention a t 3500 revolutions per minute and has an to proper submergence to avoid cavitation efficiency above 50%. make it possible for pumps such as this to One of the unusual pumps developed duroperate satisfactorily in continuous operation. ing the past few years is an axial-flow pump I n recent years most pump users have apwith variable-pitch blades (8). The pitch of preciated the necessity for providing sufficient the blades can be changed while the pump net positive suction head on pumps. Where is in operation; this adjusts head and capacity liquids are handled at or near their boiling without throttling or speed reduction, yet points, failure to provide sufficient net posimaintains high efficiency over the whole range tive suction head may result in vapor locking of operation. Pumps of this type have been because of flashing of the liquid a t the built with 1400-horsepower motors to deliver impeller, and, in addition, serious cavitation 0 to 112,000 gallons per minute at heads from may be caused. Many of the pump manufacturers now state the net positive suction 0 t o 50 feet. On the opposite extreme an interesting new head requirements of their pumps, which is a pump of very small capacity has recently been great help toward ensuring a successful inannouriced which should prove useful in stallation. Several manufacturers have dechemical laboratories. This new device veloped a type of pump specially suited to handling liquids near their boiling point. (Figure 2) is not in itself a pump, but, when a resilient piece of tubing is connected through This new type of pump is a vertical multiit, the presser bars moving in sequence create stage unit mounted in a barrel, from which Figure I. Pump for Handling a positive unidirectional flow in the low flow the pump takes suction (Figure 1). The Liquids at Temperatures near range of 1.5 to 34 gallons per hour. Since it baae of the pump is a t ground level, whereas Their Boiling Points
A
GENERAL review of liquid and gas handling equipment, such as pumps, compressors, and vacuum pumps, reveals
8
January 1947
I N D U S T R I A L A N D E N G I N E E R I N G CHEMISTRY
is a positive displacement device, it can produce and maintain a vacuum of 27 inches of mercury. It will develop discharge pressure up to 15 or 25 pounds per square inch. One manufacturer has developed a small motordriven pump which is flanged at both ends. It is installed in the line just like a pipe fitting and looks like a piece of pipe, since its diameter is about the same as that of the pipe line. Such a pump saves foundations and piping. Much of the recent pump progress centers around improvements to the stuffing box. This may seem of minor importance, but the successful performance of a pump under difficult conditions depends on the stuffing box. To minimize pacliing difficulties most manufacturers have developed special design details and have tried to keep the stufing box pressures as low as possible. More radical departures in pump designs have completely eliminated the stuffing box. One manufacturer has developed a very advanced pump without a stuffing box, which makes use of an oil-filled motor built integrally with the vertical pump. The design can be adapted to high pressures and corrosive liquids. Another manufacturer has announced a new vertical pump which not only eliminates the stuffing box but, in addition, is self-priming. The principle of this pump seal is ingenious and effective without close tolerances. Still another method of eliminating stuffing boxes is the use of mechanical seals. In general, all mechanical seals close the space between the pump case and the pump shaft by a spring-loaded, hardened wearing surface. An example of such a seal is one in which the stationary portion is carbon and the rotating portion is stainless steel faced with stellite. Such seals make it possible to handle costly, difficult, or dangerous liquids without a stuffing box and without leakage. The increased use of the mechanical seal is
9
COURTESY, E. R. GOllNElL
Figure
2. Sigmamotor, a Small Capacity Pump
perhaps one of the most significant trends to record in wartime pump progress. For security reasons it is not yet possible to describe some of the special pumps developed on the atomic bomb project. Some idea of the nature of these developments can be learned from the Smyth report (J), which states that thousands of pumps were needed; also, “It must be remembered that these pumps are to be operated under reduced pressure, must not leak, must not corrode, and must have as small a volume as possible. Many different types of centrifugal blower pumps and reciprocating pumps were tried. In one of the pumps for the larger stages, the impeller is driven through a coupling containing a very novel and ingenious type of seal. Another type of pump is completely enclosed, its centrifugal impeller and rotor being run from outside, by induction.” Possibly these developments can be reported at a later date. Compressors. One of the wartime developments in compressors was the wide application of the gas turbineaxial flow compressor in Houdry units to produce large quantities of compressed air used in the process. The gas turbine is driven by the hot high-pressure flue gases from the process. In this application the gas turbine not only produces all the power needed to drive the compressor, but in many instances could even generate additional electric power. The gas turbine compressor set will probably be applied to other processes, because such units require no cooling water, they have a higher thermal efficiency than most present equipment, they cost less to install, and they are exceptionally useful in cases where there is surplus heat available from procFigure 3. Elliott-Lysholm Compressor ess.
I N D U S T R I A L A N D E N G I N E E R I N G CHEMISTRY
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Vol. 39, No. 1
minute at 100 pounds per square inch has been built which is smaller than its motor drive. A notable development in materials handling is the method used to move catalyst in the high octane gasoline plants. This is a new conceptiun in materials handling. It amounts to injecting suitwble amounts of a gas, usually air or oil vapor, into
Figure
4. Sectional View of
d
Three-Stage Oil Diffusion Pump (1)
A new form of compressor, the Elliott-Lysholm double helical compressor (Figure 3), was announced during the war. This is a positive displacement compressor with a volumetric efficiency approaching 100% and an adiabatic efficiency of 80 to 85%. This compressor is currently applied only to gas turbine power units, but its application will eventually be extended to other compressor problems. It delivers cleaii oil-free air and operates independently of changes in flow or pressure. An experimectal steeple compound unit to deliver 300 cubic feet per
Figure 5.
Shakeout for Unloading Hopper Cars
Figure 6.
Material Accelerator for Speeding Discharge from Cars
(Continued on page 40)
40
INDUSTRIAL AND ENGINEERING CHEMISTRY
MATERIALS HANDLING CONTINUED FROM PAQHI
10
Figure 7.
Cartcoop
powdered catalyst, which thus becomes aerated and takes on the properties of a fluid. Within limits the fluid density is controlled by varying the amount of gas added and by changing the rate of flow of the fluid mixture. The process operations of oil reaction, catalyst transfer, and catalyst regeneration are all accomplished by moving the catalyst in a fluid state. The compressed air or oil vapor used for aeration is obtained from a centrifugal blower which is notable because of its size (2300 horsepower and delivering 30,000 cubic feet per minute at 26 pounds per square inch). Vacuum Pumps. In connection with the atomic bomb project, in both thc electromagnetic and gaseous diffusion methods of isotope separation, problems of vacuum technique arose on a scale previously unheard of. The world’s largest diflusion pumps were used 011 the atomic bomb project. All-metal vacuum systems were devised which operate at pressures as low as 0.000001 mm. of mercury; this has never been accomplished previously except on a laboratory scale (1). The principle of diffusion as applied to vacuum pumps is not new, but the glass construction of the earlier pumps limited their size and capacity. The all-metal diffusion pump is simple and continuous, and requires no moving parts (Figure 4). The operation of these pumps depends upon the rapid evaporation of oil from an electrically
Vol. 39, No. 1
heated boiler in the base of the pump. The oil vapor from the boiler travels up through central chimneys to the various jets. The oil vapor molecules, directed downward by the jets, collide with the gas molecules that have diffused into the region. The gas molecules in the oil-vapor stream are compressed by additional jets to permit their removal by mechanical pumps. The oil vapors are condensed and return to the boilers for reevaporation. In the opinion of Restinghouse engineers, no limit has been reached as to size and capacity of vacuum pumps and systems, and the prospective further improvements in vacuum pumps and high vacuum technique promise an ever-inereasing use of this tool in industry. SOLIDSHANDLINQ.Palletized Materials. There is a marked trend toward the handling of materials on pallets from the point of origin, to boxcars, and finally to the point of use. This trend was greatly accelerated during the war by the request of the Army and Navy that suppliers make shipments to the armed forces on standardized pallets. The pallets are handled throughout by industrial fork-lift trucks. Fork-lift trucks and pallets greatly reduce manual labor; for example, in one case the time required to unload freight cars has been reduced from 56 to 2 man-hours, though typical savings are usually less. In addition to facilitating the handling of loaded pallets, fork-lift trucks make it possible to store materials in tiers in warehouses; this leads to better utilization of warehouse space. There were no specially notable developments in fork-lift trucks during the war, although there were numerous refinements in design. Within a warehouse pallets are often handled with hand trucks equipped with jacks, which lift the pallets enough so that they can be moved. During the past few years warehouse lift trucks have been developed that arc equipped
COURTESY. STEPHENS-ADAMSON
Figure 8.
Swiveloader for Boxcars
MANUFACTURINO COMPANV
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INDUSTRIAL A N D ENGINEERING CHEMISTRY
Bulk Handlitiy. -4iiew car unloader (Figure 5) has been developed which can empty hopper-bottom railroad cars containing coal, ore, minerals, and other free-flowing materials in as little as 90 seconds. The device is pliced on top of the hopper-bottom car by a hoist and produces a rhythmical seismic action throughout the car with a frequency of 1000 cycles per minute. Under the influence of this vibration even tightly packed granular materials flow out of the hopper doors in a steady stream. The action is so fast that entire trainloads can be emptied without uncoupling the locomotive. Figure 6 shows another newly developed device to assist in rapid unloading of hopper-bottom cars. This device loosens and accelerates the discharge of frozen coal and other materials. The power diggers first screw into the packed or frozen cars, then the direction of rotation is reversed, and the action of the screws pushes the loosened material through the bottom gates. A new device has been developed to unload bulk materials from boxcars. Known as the Carscoop (Figure 7),it is a small, highly maneuverable scoop on wheels. I t is so fast that an average operator can unload materials from a 56-ton car in as little as an hour and a quarter. Not only can the Carscoop easily move around inside the car, but can, if desired, leave the car and drive into the warehouse before dumping. The machine is available with either electric motor or gasoline drive. Power for the electric motor is supplied through a cable which automatically reels or unreels from a drum; tension is maintained on the latter by a hydraulic coupling driven from the main motor. Another recent development is the Swiveloader, which will fill the largest boxcars without hand trimming at the rate of 40 to 80 tons of material per hour (Figure 8). The bulk handling of materials has many natural advantages. The new devices described here, and other machines, greatly reduce the labor required and simplify the problems of bulk shipments. WAREHOUSE DRUM-HANDLING SYSTEM. A plant is currently being designed which requires the use of thousands of 55-gallon steel drums each day. A method has been devised to minimiae the handling of these drums, the general arrangement of which is shown in Figure 9. The principal feature of the system is a gravity roller conveyer in the form of an elongated spiral, which permits the “live” storage of hundreds of drums. The live storage section is filled by an automatic hoist, and the system is devised so that the drums pass through all storage and filling operations without being handled manually. This system shows the trend toward highly mechanized handling of materials. WEIGHINGDEVICES. Most of the weighing devices now used were available just before the war, but some are relatively new and were widely applied during the war. For example, two scale manufacturers have applied photoelectric eyes on dial scales to control weighing and batching operations and thus reduce the operations to an automatic basis. Another company has perfected a machine which will automatically eject from a package conveyer line any underweight or overweight packages. The machine has a green light to indicate weights within tolerance, a red light for underweight, and an amber light for overweights. The accuracy is extremely close, and, depending upon the speed desired, the packages can be culled to an accuracy of */*z ounce. An electronic weigher has been devised to weigh penicillin in the amount of 40 mg. to an accuracy of *2%. LITERATURE CITED
(1) Oolaiaco. A. P., and Hopper, D. L., Westinghouse E n g ~ . ,6, .. 103-7 (1946). (2) Reeve, L. N., and Scoville, J. D., Combustion, 14, 32-8 (1943). (3) Smyth, H. D., “Atomic Energy for Military Purposes”, 1st ed.. ChaD. X. . .D. 182. Princeton, N. J., Princeton Univ. Press, 1945.
MIXING CONTINUED FROM PAQB
30
the physical changes necessary to the processes”. The author states that a satisfactory basis of liquid agitation design can be obtained from data now existing on hydraulics and mechanics in chemical handbooks and equipment manufacturers’ publications, but no illustrations or data are presented, and no specific information is given as to how the formulas of hydraulics and mechanics can be applied. A general discussion of problems encountered in mixing of liquids in shallow tanks is the subject of an article by Bissell, Everett, and Rushton (3). An analysis is made of the design factors necessary to achieve satisfactory agitation under conditions of shallow liquid depth and when liquid depth is diminishing during the runoff in some mixing operations. Methods of avoiding difficulties are enumerated, and data are given on recommended limit ratios of tank diameter to minimum liquid depth. APPLICATIONS TO .PARTICULAR PROCESSES. Two articles appeared recently which deal specifically with mixer applications to the handling of pulp stock; the one by Keon (7) has been reviewed above. The article by Couture (6)discusses the uee of propeller and paddle agitators for wood pulp circulating and blending operations. Several setups are shown for the use of outside pumps to produce circulation and blending. Propeller design is said to be subject to computation by theoretical equations given in the article. No efficienciesare given, however, and no reference is made to experimental verification of the several mathematical examples worked out. Kugel (8) discusses mixing and mixing equipment, as applied especially in the pulp and paper industry. Sterrett (18) gives several methods suitable for agitation of pickling solutions. Lo Presti and Bandos (10) describe a method for shaking cathodes in small plating tanks and thereby causing sufficient agitation and mixing. HEATTRANSFER. An article (and later discussion) by Chaddock and Sanders (6) about the unsteady state heat transfer to liquids in tanks was inadvertently omitted from last year’s review. The authors developed equations for batch heating and cooling of liquids in mixing tanks for use of both internal coils and external heat exchangers. EQUIPMENT. Porter (18)and Lee (9)give information on how to design and construct a spiral ribbon suitable for attachment to a shaft for use as an impeller in mixing operations. Stein (17) describes an agitator made of Saran sized for use in a 65-gallon vessel. Three of the articles (11, 14,16) are reviews of recent developments up to 1946 in the field of mixing. LITERATURE CITED
(1) Asquith, J. P.,Ind. Chemist., 21, 203 (1945); 21, 262 (1945). (2) Atwood, E.H., PetroleumReJiner, 25, 125 (March 1946). (3) Bissell, E. S., Everett, H. J., and Rushton, J. H., Chem. d Met. Eng., 53,118 (Jan. 1946). (4) Brumagin, I. S.,Ibid., 53,110 (April 1946). (5) Chaddock, R. E.,and Sanders, M. T.,Trans. Am. Inst. Chem. Engrs., 40, 203 (1944);40,505 (1944). (6) Couture, J. W., Pulp Paper Mag. Can., 46,765 (1945). (7) Keon, J. J.,Ibid., 47, 188 (1946). (8) Kugel, F.,Papier Fabr. Wochbl. Papierfabr., 1944, 340. (9) Lee, C. A.,Chem. & Met. Eng., 53,166 (Feb. 1946). (10) Lo Presti, P.J., and Bandos, H., Metal Finishing, 43,406 (1946). (11) Lyons, E. J., Chem. Industries, 57, 856 (1945). (12) Porter, A. B.,Chem. & Met. Eng., 52, 123 (Sept. 1945). (129) Riley, D. F.,Manufacturing Chemists, 17, 27A (July 1946). (13) Rushton, J. H.,Can. Chem. Process Inds., 30, 55 (May 1946). (14) Rushton, J. H., IND.ENG.CHEM.,38,12 (1946). (16) Rushton, J. H.,Mack, D. E., and Everett, H. J., Trans. A m . Inst. Chem. Engrs., 42,441(1946). (16) Staff Review, Chem. & Met. Eng., 53,144 (Feb. 1946). (17) Stein, A,, Ibid., 53,147 (May 1946). (18) Sterrett, E., Ind. Finishing, 21, 110 (Sept. 1945).
