July, 1929
INDUSTRIAL A N D ENGINEERING CHEMISTRY
649
Mechanical Handling of Materials in and about the Chemical Plant 11-Belt Conveyors A. K. Bnrditt and W. F. Schaphorst 45 ACADEMY Sr
HE first part of this article ( I ) * briefly compared some of
T
the typical methods for handling bulk and solid materials in and about the chemical plant, arbitrarily classifying them under the headings of belt, gravity, flight., screw, and pneumatic conveyors and elevators. The discussion was, ofnecessity, element.ary arid brief, to serve as a background for a further analysis of each classification. The purpose of tlre present article is to point out some of tlre advantages and some of the principles in the design of belt coiivcyors particulnrly as applied in the chemical industries.
,Xeianr, N J
belt,. Carrier idler pulleys spaced at short intervals support the load surface aod prevent sagging between the end pulleys. The lower or return section of the belt is supported on return idlers spaced at less frequent intervals. The conveyor is usually driven from the discharge end of the belt so that the carrying surface will he under tension. The length of the conveyor is measured by the center distance betrveen the end pulleys. The belt is subject to abrasion from the material carried, to friction both oE the material and of tlre drive and idlcr pulleys, arid to deterioration from the conditions of operation such as moisture, heat., etc.-all of which in the chemical industries are usually more severe than in the cam of power transmission belt,ing. The choke of bclting for conveyors is therefore important and must he corit.rolled 1)y the particular conditions under which the belt is t,o operate. Manner of Loading
In loading belt conveyors it is important to avoid cxcessive \scar of t.lie materials, especially those of ail abrasive nature, on the I&. It is advisable that the material be travcliug nt t,he same rate of speed as that at wliich the belt is moving at the point of contact with the belt. Control of the speed of loading can he secured by means of the loading chute. In some cases it is necessary to use an automatic feeder or itnother type of conveyor, such as tile apron conveyor, for bnding the
Figure I-Belt
Conveyor Operating at a Speed of 200-300 Feet per Minute
As an economical method for lrnndling materials over long distances at it liigh rate of speed the belt conveyor and elevator is difficultto equal. Rulesfonnulated byHetzel(8) state that if the capacity of the conveyor is to be less than 50 t,ons pier hour the belt conveyor is less ecorromical than the flight, conveyor. If tlie capacity is to be over 100 tons per hour, belt conveyors will cost less tlisii flight conveyors and consume less power. If the conveyor is to Be shortcr than 75 to 100 feet, a flight conveyor may save space and maintenance costs since short conveyor belts \iear faster than do long ones. If the length of portage is to !x more t.han from 200 to 300 feet, the belt conveyor will save power and will cost less than the flight conveyor. It must be remembered, however, that there are exceptions to nearly every rule. The great capacity of belt coni'eyors is clearly shown on the long syst,em illustrated in Figure 1. Short portable belt conveyors are frequently more economical than other types. A large installation of belt conveyors i i i a salt plant, which will be described later in this article, inchides several short h i t systems. The belt conveyor consists essent.ially of a belt suspended betveen two end pulleys, one of which is used t o drive tlie belt. The material is carried on the upper surface of the * Received Anril 1, 1929.
* Italic number3 in parenthesis refer lo liferatare cited rf end of article.
Courtesy Sandvik Conacyor Mfz. Co. Figure Z--lfandlin$ Tartaric Sludge on s Metal R e l t C0"YeYOr
I N D UXTRIAL A,IrD ESGINEERISG CHEMISTRY
650
belt. Loading chutes should be designed and arranged to place the material in the center of the belt. The chute should be so inclined that the material passes onto the belt approximately in the direction of the belt travel. I n the case of small portable types of conveyors it is frequently impossible to observe all these precautions closely, since it is usually more economical to replace a small belt of this character than to try to use elaborate loading systems even where they are possible of application.
Courtesy Portable dlachrnery Co. Figure 3-Magnetic
Separator in End Pulley of Belt Conveyor
Types of Belting
Two types of belting are particularly common in conveyor installations, rubber and textile. A third type, the metal belt, is being increasingly recognized as worthy of consideration. For special conditions of operation various compositions of belting can be had. Textile belting varies considerably according to the service for which it is intended. It may be of cotton or canvas folded over itself a number of times and stitched or it may be of a solid thick weave. Textile belting can also be coated with rubber. Rubber belting is resistant to abrasion, as shown by the remarkable life of the automobile tire under severe road conditions. It is fairly resistant to corrosion and to the action of certain acids. Materials hotter than 150' F., however, rapidly deteriorate rubber belting and should be carried on specially designed belts. Some chemicals have a damaging effect on rubber and therefore should not be carried on rubber or rubberized belt. Organic solvents such as chloroform, toluene, and benzene; oils, greases, and oil emulsions; and strong oxidizing agents such as chromic acid concentrate, sulfuric and nitric acids-all rapidly deteriorate rubber. Rubber and textile belts are both highly flexible, which is important especially in the troughed type of conveyor. The number of plies of belting depends upon the width of the belt used and the grade of material handled and varies in thickness from 3 ply u p to 10 or 12 ply for coarse heavy materials and rery wide belts. A fair average for the number of plies according to the width of the belt is shown in Table I.
Conveyor belts should not have fewer plies than the number given in the minimum column for heavy service, nor more than the number given in the maximum column to avoid unnecessary stiffness in troughed conveyors. The belting requirements can be checked according to Formula 1:
X=
h. p. X 33,000 S X B
where X is the stress in belt in pounds per inch of width, S is the belt speed in feet per minute, and B is the width of the belt in inches. Dividing the value X by 20 gives about the number of plies necessary for strength. The stress for each inch of width should never exceed 20 pounds. The thickness of the rubber reenforcement of rubber belting varies according to the material handled in general practice. The &fanhattan Rubber Company recommends coverings as follows: I/;?- or 1/16-inch cover for light unabrasive materials such as wood chips, earth, or powdered coal; 3/12- or '/*-inch cover for medium weight or semi-abrasive materials such as sand, gravel, crushed rock: 6/m2- or 3/le-inch cover for heavy and abrasive material such as large rock, heavy ore, coke, cement clinker; 3/16- or l/rinch cover for extreme service such as ore or stone in very large lumps. Where materials have a high moisture content it is advisable to have rubber coating on the belt rather heavy and see that it completely surrounds the belt on the edges as well as on the upper and lower surfacen. If this covering is not sufficient to isolate the duck body of the belt, the life of the belt will be considerably shortened. In the case of hygroscopic crystals, there is a tendency for the water to ceep through any breaks in the covering and then for materials dissolved in the water to crystallize. This crystallization stiffens the belt, making it brittle and particularly liable to fracture. The life of rubber as well as textile belting can be considerably lengthened by careful inspection and study of its operating conditions. The splicing of the belting is a point rather frequently neglected but which deserves particular attention. It is safer to replace the splice before it fails and tears the ends
Figure 4-Weightometer Table I-Number WIDTH O F
BELT
I
Max.
Inches 12
14 16 1s 20 22
24 26 28
3 3 4. 4 4 5 5 5 5
Installed i n Belt Conveyor
of Plies according t o Belt Width
PLIES Min.
Vol. 21. No. 7
4 4 5 5 6 6 7 7 8
1
W I D T H OF
BELT
Inches 30 32 34 36 38 42
44 48
1
PLIES
Min.
6 6 6 6 6 7 7 8
Max.
8 S 9 9 10 10 10 11
of the belting than to allow it to close down the conveyor a t an inconvenient time or to damage the belt itself. Belting that is allowed to run off center will wear prematurely. The end pulleys supporting the belt should be properly leveled and squared with the center line of the conveyor. I n cases where an incline follows a horizontal run it is important that the idler pulleys merely support the belt. If the belting is allowed to hang suspended, it creates too much tension on the belt itself and results in stretching. Inspection should be frequent to see that the tension is great enough to prevent
1 N D USTRIAL AND EATDIIVEERIN(: CHEMlISTR Y
July, 1929
slippage, hut not so great as to place an undue strain on the belt. When carrying material tirat is hot enough to harden the ruhher covering of the heit, it is frequently the practice to
651
materials the carrying capacity of the flat heit conveyor is considerably less than that of the mme width t.roughed conveyor, but the metal belt has certain advantages over the rubher or textile belt. It is not affected by moisture, is easily cleaned, and will not contaminate the mat,erials carried. Considerably liigher temperatures r,an be carried on metal helt,s ttiaii OIL otlier types, and tile metal belt will operate in low temperatures that are hard on rubber and textile belting. hlonel metal helting possesses the well-known properties of moncl metal incliiding high resistance t,o corrosion. Figure 2 shows a metal conveyor carrxing tartaric sludge with about 60 per cent moisture content, nt an incline of 16 to 18 degrees. Pulleys
Courtesy Rorbcr-Creme Co.
FlBure 5-Unloading
Dump Cars Handling Peat
spray the belt with water, usually a t the tail pulley. The tail pulley sometimes rotates in a trough of water. This precaution should always be taken in carrying warm materials. Belts that operate a t high speeds frequently develop high poteiitials of stat.ic electricity. This is particularly true of transmission, where the charge is built up by the friction of the belt on the pulley as well as the separation of the belt from the pulley, but i t also exists in the case of conveyor belting. The Department of Agriculture has recently investigated this problem. Experimental tests of static electricity showed that belting sometimes builds up a potential as high as 40,M)O volts. The Department of Agriculture recommends the use of a conducting powder such as aluminum, bronze, or copper, sdded to a g o d Fade of spar varnish used to hold the powder on the belt. This compound makes the conduction of ruhher belting adequate to remove or neutralize static charges as rapidly as they are formed. Lampblack proved to he the most practical conducting dust and wibh spar varnish as the thinner gave a preparation which rvould not flame and cause a fire risk. In some chemical inrhintrii~st,!ie me of such a n antistatic compound would 113 advantageous, hut under in dinary conditions where there is no explosion hazard, when
C o w t a s y Portable Morhinery Co.
FiUure b-sforlns
Sacks in Warehouse uifh Portable Conveyom
belting operates at moderate or slow meeds. t.he nrohlem of static kleitricity is of little importance.' Metal conveyor belting is not SO flexible as rubber and textile belting and is used only for flat belt conveyors. For bulk
Tho end pulleys of belt conveyors sIi(~uldhe large in diameter, not. only to avoid sharp bending of the belt as far as possible but, in the case of the drive pulley, tu secure better traction. The pulleys shoiild be crmvnod s!iglrtly to assiire propor centering of the helt. Srmntq gives a nlininium diameter in inches for the end pirllcys OF five t i l i l l 3 the niimlar of plies in t,he belt. Present praciiw i n w>,>w?ym dosign scems to call for even larger i>nllcgs. Table I ! S'LIOWS ihc minirmim diameters generally ueed.
Flaurr 7-Losdin8
Table 11-Mlnimum
Inafailaflon fur Box Cess
Diameter of End Pulleys accwdle8 t o Width of ".It
Carrier idler pulleys are of two principal types to conform to tho two t,ypes of belt conveyors, flat belt and troughed. FlatAielt idler pulleys are usually mount,ed three on a single cross shaft, end to end. Troiigtied belt carriers are usually three t u fire in number and the end pulleys of each carrier unit are iiielinnd 2 5 to 30 degrees. The intermediate pulleys of the five-pulley iype are sloped lightly less to conform to the curvature of the carrier miit and trough of the belt. Plain, sleeve, or antifriction heairngs are supplied with carrier units, although the antifriction h a l i n g is preferable because of its saving in power and maintenance costs. The carrier units are spaced a t intervals, as shown in Table 111,according to tile width oi the belt and the load supported. Return idlers are similar in design to flat belt carriers. but the unita are spaced much fartherapart than are the carrier idlers.
INDUSTRIAL A X D EiVGIhrEERING CHEMISTRY
652
Table 111-Spscina of Carrier Idler Pulleys According to Width of Belt W=oml BELTOB
SPACINO
wmre BELT0s
Inikcr 12
Per, 4%-5
Inches 30 34 36 38 42 44 4s
14 16 18 20 22 24
4.g-5 4%-5
44% 4-496 4-456
3!4-4
SPACIRU
Peel
3%-4 3%-4 3%-4
5-3% 3-35 3-335 3-3%
The greater distances can be taken for loads of 50 to 100 uounds Der cubic foot and the lesser distances for loads of 100 or more pounds per cubic foot. Guide wllevs . . can be placed at intervals along the belt to assure centralization and in some e'ases to increase the height of the trough and consequemtly the capacity of the helt. It is sometimes necess:iry to make all pulleys of some special material or otherwise to protect them from deterioration from the material rarried in the convcyor. The rnetxl parts are sometimes ganitized to prevent incrustation and corrosion from salt, and monel met,al is sometimes used t,o prevent acid corrosion. Lubrication of modern belt conveyor pulleys is fregueiitly by means of pressure systems such as Alemite or Dot fittings. Tripper e belt tripper for distributing materials over bins, etc., ts essentially of vertically placed piilleys that form the belth,o an S and discharge the matefial into a straddle h o p per that chutes,it over tlie sides of the conveyor. The tripper is usually moved by hand or power, or automatically dong the conveyor so that t.he material can be discharged a t any point desired. Belt Conveyor Instdllation in Salt Plant
,As an example of a belt conveyor installation, the systern described below, installed in a large salt plant, is typical. A Link-Belt skip hoist elevates the rock from tlie mine to t,lic feeding hopper or a roller grizzly in the rear breakcr house, whicli delivers oversize to a 48-inch cooveyor serving a jaw crusher and fines to a 36-inch conveyor leading to a roll crusher. The product of the jaw crusher drops to either of two 3Ginch conveyors, one leacing to the roll crusher and the ot.her to a chute merging with that from the crusher. The crushed material then passes to the front hreaker house over a 30-inch main conveyor, iuhich was shown in Figure 3 of the previous article, and a cross conveyor of the same size, these feeding various further crusher and screening operations. These two conveyors have five-roll t.roughing idlers. The seventh conveyor is used in loading raw stock to cars at the first. fioor.
VOl. 21, No. 7
The first four conveyors are used as picking tables where blasting wire and other foreign matter are removed, and are operated at speeds of from 73 to 80 feet per minube. The other conveyors operate at from 212 t.o 250 feet per minut,e. About 150 tons per how pass through the system. The Alemite pressure system of lubrication is used and all bearings, which are of the taper roller type, are greased every 2 to 3 weeks. This requires tlic time of one man for somew11a.t less than 9 hours and about 15 pounds of grease. A11 surfaces of the idler and guide pulleys are granitized. Tahle IV the data Of the units.
Fi$.ure 8-Filline
C o u r f r g Porlabie Marhinery co. Boa Car w i t h Loose Material by Means of C O ~ Y E Y O T L ~
Special attention should be called to the slow speed at which the first four conveyors opentie. Their use as picking tables makes the reason obvious and also shom the versat,ility of belt conveyors. The cross conveyor is short, hut it is somet i m a cheaper in the end to install a less economical short belt conveyor for the sake of uriiforinidy in equipment. Power Requirements The power required to operabe belt conveyors under average conditions is shown in Formula 2. Where there is an incline in the conveyor, Formula 3 should he used. For conveyors under 50 feet in length 50 per cent should be added to the horsepower required arid for conveyors between 50 and 100 feet in length 25 per cent should he added. For every change in the angle of inclination aliout. 10 per cent can be added t o the number of liorsepower required to operate a belt coiiveyor. (2) (3)
Table IV-Specificatione of Belt Conveeyors in a Salt Plant (Belt
C U ~ V F Y U ~eSo u i ~ a e d with
Link-Belt idlers hnvine rianitized rolls nod Timken benrinrsi
c*aa,en
BBLT FUXCTiON
IDLBRS
SERVINO
Width Ply
-
Speed
Slope
Type
No. Diamete
Inihrr
Cmrher feed and
picking No. 1 picking table
Grirrly t o jaw crusher Jaw criishri t o roil crusher
No. 2 picking table
C.r?rrly to roll
No. 3 picking
Juwcrusherio main conveyor Rear io iiont brerkeicr buiiilding Rear to imnt
fnble
&%.in Eonveyni CrDm conveyor
crusher
breaker building Rock loadinz c o n v e ~ o r IIonnrr t o c r r
* Center dirfancr between end pulleys.
27
8
48
6
20
80
Fiat
1
4%
16
0
36
6
0
19 6
....
0
...
17
3
36
0
0
79.5
....
0
...
24
4
36
0
9
73
....
0
...
183
6
30
6
UI
250
Flat
11
4%
69
8
30
6 4
260 212
Flat
4 0
4%
18
23
3
11
26
18
variable
....
...
July, 1929
INDUSTRIAL AND ENGINEERING CHEMISTRY
where Cis a Dower constant as shown in Table V. T i s the load in tons per hour, L is the center-to-center length of the conveyor, and 11 is the vertical height of lift.
-
Table VI-Soeed
653
and C a n ~ c i of t ~Belt Convevors
W,D% 0s
nBLT Inrha 12 14
16 18
20 22 24 26 2s 80 32 34 36 3s 42
44 4s
Inches 2 2%
3 4 5
6
S 9
Inches 1% 2 2%
3 3% 4
% :
12
14
6 7
15
% ;
IS
9 10 11 13
16
19 20
a2 24
14
Table VII-Economic
Speeds of Operation
Lks. par cu. fl.
Cuke coarse stone Lime and cement Sand and gravel
33.5 185
65
110
Fl. DE, win 250 275
300 376
Other Applications Courleiy Link-Rcli Co.
Figure 9-Belt
Conveyor Which Serves Also a8 a Piekine Table
Speeds and Capacities Speeds of operation and capacities of belt conveyors are shown in Table VI. These data are for average performance conditions, but will serve as a guide for estimating tho performance of belt. conveyors under special conditions in the various clieniical industries. It will be noted that maximum capacity figures are given both in cubic foot per hour and in tons per hour. 'IXe labter figure represents the maximum weight that a conveyor should bear under nonnal conditions. The figures are given for a belt speed of 100 feet per minute and can readily be corrected for different belt speeds. Tahte V-Power
Consfants for Bel
The belt conveyor is the most universal of all kinds of conveyors, wliet.her in the permanent or fixed type of installation, the unit or somi-portable type, or the fully portable class of conveyor. Belt conveyors are used for unloading cars, for storage and reclamation of materials, for transporting not only loose materials but finished articles, cartons, boxes, or other containers, and for loading and shipping. Figure 3 shows an unusual combination of a magnetic separator wit,h the end pulley of a belt conveyor. Another combination showing the economy of mechanical handling of materials is the use of an automatic scale in connection with the conveyor as illustrated
0""e""rS
Inches 12
14 16
1s 20
22
24 26 28 30 32 34 36
2
x
2% 2N 3
3M
The belt conveyor cannot operate at the maximum speed shown in Table VI when carrying very light materials. Heavy bulk material can usually he moved Easter than light material. Typical economical speeds of operation for a few snbstances are shown in Table VII. The speed of operation of the conveyor will depend not only on the rate at which it is convenient to feed the material to the next process, hut also, in the case of fine materials, on the ability of the material to remain settled on the conveyor while in motion.
Courlrry .Sandah Conveyor
.W#.Co.
Figure lO--Modern Large-Capaciiy Metal-Belt lnstsllafion
in Figure 4. The weight of all material passing over the conveyor is recorded hv the Weiilitometer, so tliat a t any time the amount of mat,erial that has passed this particular point can be readily ascertained. Gondola cars are frequently nnloadcd over a hopper. Tho material can then he discharged onto a belt conveyor to carry it to the point of storage. A typical example of this kind of service is shown in Figure 5. The material is dumped from the cars into a hopper. A unit type of belt conveyor raises the material to a tower in t,be plant, where it is discharged onto another similar unit discharging into the storago yard.
wyors an effort is always made to feed the material onto the belt at a speed as nenr to that of the belt as possible. Figitre 9 shows a rotary grizzly under a breaker feeding one of the picking &Its mentioiied in the description of the installation of belt, conveyors in a salt plant. A model large-capacity installation of metal-belt conveyors i s shown in Figure 10. These conveyors receive cans of hot gresse from the filling machines and carry them across to t,iie lahcling uiscl~ines,the cans cooling enroute. Another view showing t,lie notable economy of mechanical handling is tlie large flat-type textile belt conveyor illustrated in Figure 11. This design moves large drums from one building to another a t the same time elevxt,ing them to a considerable height at a ratlwr sharp angle. Economy of Handling
large belt conveyor units show a material-liandling cents per ton. One such installation for Iinn(lling 60,000 tons of fine coal a year over a distmce of 400 fret, arid rlevating it a distance of 24 feet is cited as doing the iwrk a t W.0154 per ton. Smaller installations operate a t soii?rxliat less efliciency, but they also move material a t a fr:tction of tire cost of hand labor. It requires only 5 horseponer to operat,c x 12-inch belt conveyor handling 69 tons per l i o u r it distance of 300 feet. Such installations can frequently he rlrivcri from a linesltaft l'lie first cost of m a l l portable units in small, while the oost of operation of a small conveyor is negligible compared with thc wages t,liat, must he paid to men t o do the same work. Many plant,s fail to realize the time that is being lost and the amount. of wages t.liat are being paid for moving materials from place to place. Manufacturing plants, such as in the automotive iudustries, operating on a piece-work plaii of wage payment hsve been forced to take into account this tremendous waste of t,inie. The piece worker refuses to stir out of his tracks t o obtain his supplies, where on an hourly wage system he was willing to spend 10 minutes or so out of every hour t,o obtain what he needed for his job. These plants first hired errand boys to keep the mechanics supplied, but soon came t,he more economical conveyor systems that are now found in every automotive plant. By proper arrangement of equipment there are few plants so very small that mcchaiiieill handling will not save worthwhile amounts in the reduction of labor costs o its IOTV as
Vse of the hclt crirrveyor for the loading of freight cars is well ill
