Process of Flour Manufacture, by Percy A. Amos
In placing this little text book before the milling public, a brief statement as to its scope and aims will be advisable. To those in touch with school and institutions throughout the country, the now surprising thing is that so few attempt to start classes or lectures on the subject of flour milling. In view of the fact that the industry is a vast and important one, and that it has a distinct bearing upon the health and well being of the British public, this state of affairs is to be developed. No doubt one reason for this is that the number of men employed, directly or indirectly, in the trade is quite out or proportion to the enormous output of flour and offals, but at the same time, in large centers, a series of lectures by competent men would be appreciated by those actively employed on the mills, and by those whose connection is quite distant, but who have an interest in the matter. The author has had experience, during a series of lectures, delivered at the Manchester School of Technology, upon the subject, and included in the class have been screen men, milling operatives on the various machines, warehouse men, mill clerks, foreman, milling apprentices, flour salesmen, and travelers, and even provender millers. Those only casually acquainted with the industry have invariably expressed surprise and admiration for the elaborate storage, cleaning, conditioning and milling systems of modern mills, and have admitted that many prejudices which they shared with the general public have received severe shocks in the light of the facts. The English miller has many severe and oft times unjust criticisms passed upon him by the public generally, who in most cases are quite ignorant of the elaborate means employed to supply pure flour from a wide range of wheats. contamination and degrading in the wheat has now been reduced to the vanishing point, thus making for purity and the full nutriment of the valuable food stuff contained within the wheat berry. What is wanted for a sound health food is good strong white flour, not too fine or too white. The recent swing of the pendulum to the "gray" 80 per cent, "standard" flour has been really brought about by the public themselves. For many years they have been demanding very white bread and very white flour. The bakers has to comply with this, or lose their custom. They in turn, therefore, had to demand from the millers the fine white flour. The high cover numbers on the dressing machines resulted in the most valuable particles on the flour being partly eliminated, i. e. those contain gluten, which naturally remain with the coarser flour. The public therefore got starchy, weak flour, robbed of a lot of nourishment, and presently began to cry out for whole, of partly whole, meal, whereas by being content with creamy, medium flour, the best results would be obtained. All these points are dealt with and emphasized in the following pages. In a subject like Flour Milling, where every detail and theory of the series of processes must be pointed by reference to, and in many cases sketches of, some machine employed on this particular work, it is very necessary to choose some standard make amongst the best types; but, on the other hand, it is quite impossible to mention every make upon the market. The number of excellent and high class milling catalogues not issued by the leading milling engineers throughout the country can be obtained upon application, to fill the blanks unavoidably left in the present work.
Amongst the most advanced civilizations of the world, wheaten flour forms such an important part of the staple foods consumed that attention is being concentrated upon every step of the long process. The direct outcome of this has been that the last thirty years have seen more advancement than the previous thirty centuries. The old familiar millstones have practically ceased to exist as a wheat grinding medium, and have been replaced by the grooved chilled iron rollers which form the backbone of the roller system. The oldest history to hand relating to the grinding of wheat, or corn, dates back over 6,000 years, in the early days of the Egyptian nation. The stones in use were named "hand stones" or querns, and were simply two rough stones, the lower, which held the grains to be pulverized, has a slight depression in the top, the other one was simply a boulder, grasped to smash and crush the grains. It was simply a crude pestle and mortar. The quern was in use for 4,000 years, as the sole means of grinding or reducing wheat. Even to this day it is used among the North American Indians, and in most Eastern countries. Over 2,000 years ago the millstones, a crude form of the ones we have supplanted by rolls, came into use. They were driven by slaves or cattle. For 4,00 years the millstone was the only system in use. The only real progress made was in the sifting methods and the driving power. The Greeks first introduced a mechanically driven mill about 450 - 400 B. C. It was a water mill, and had a horizontal wheel, but no gearing. A hundred years later the Romans invented a vertical water wheel with gearing to stones. About four hundred years later still they made the "ship mill" which floated, and was worked by a flowing stream or tide. About 600 A. D. the wind mill was invented. At first the arms revolved upon a tripod stand. Later a turret or tower took its place, and from it evolved the old familiar wind mill. For hundred of years the flour milling industry has progressed very slowly, principally owing to the very severe laws and restrictions placed upon it. It is during the last thirty years that the real progress has been made. As the English nation grew, in size, the quantity of wheat growing steadily diminished. Wheat had to be brought from abroad in enormous quantities, and their diverse natures and many impurities demand a variety of treatment in cleaning, conditioning, and grinding. Steam was first used for driving mills about 1784, in London. Stones and bolting reels continued to be used right up to 1880's, and even later. A new type of purifier was patented by a Mr. Westrup in 1854. In 1870 Mr. Childs, an American, introduced wheat cleaners into England, and Messrs. Wegman's porcelain rolls also appeared. Whitmore's purifier and centrifugal appeared between 1870 to 1880. G. Buholz patented an iron roller system in 1874, and fitted up a Liverpool mill. But the great revolution in Flour Milling came in 1881. During that year a great exhibition of roller milling was held in London, and millers attended in large numbers, anxious to see the system that was at work on the Continent and in America, and which enabled the Hungarians, the Germans, and the Americans to flood the English market with flour of such fineness and whiteneness that the stone made flour at home had no chance of good sales. Parties of millers also paid visits abroad under the guidance of Mr. Carter, and being convinced of the hopelessness of fighting the roller system, wisely put the prejudice of years aside and set to work to install the new system, into their mills. To sum up: The history of Flour Milling can be expressed in periods:- 1. The hand stone or quern..................................4,000 B.C. to... 2. The slave and cattle driven millstones...........2,000 B.C. to... 3. The Greeks single water wheel millstones.......450 B. C. to... 4. The Romans geared water wheel with several stones....................................................350 B.C. to... 5. The wind mills..........................................................600 A.D. to... 6. The steam driven mills............................................1784 A.D. to... 7. The roller mill system and cleaning systems......1881 A.D. to... 8. The present day perfection of the roller mill system and development of grain handling, storage, cleaning, and conditioning, reduction and the artificial bleaching and conditioning of flour........................................................1912 A.D.
A comparison with the old system of reduction by stones in one operation. It is now widely recognized that wheat cannot profitably be reduced upon stones. The method is too crude and haphazard for a scientific age. The material, even in such excellent stone as French Burr, is too friable, uneven and coarse in texture to obtain the requisite fine grooving for a gradual reduction of the wheat berry. On stone, the wheat was reduced in one operation, and the resulting pulverized mass was then dispatched for grading and dressing. The method used for dressing or furrowing the face of the millstones. The grooving did not radiate from the center of the stone, but was set out at a tangent, from the "drift" circle. The main grooves, or "master furrows," varied in number with the number of "quarters" into which the stone surface was divided. The smaller furrows intersecting the remainder of the quarters were termed "secondary furrows."The top surface if the stone left between the furrows was termed the "land," and was usually "snecked" or "cracked," to give the maximum grinding power. the grinding action was obtained by fixing one stone and revolving the other against it. The general practice favored fixing the bottom stone, termed the "bed stone," and rotating the top stone, termed the "runner." The feed of wheat was shoveled into the "eye" of the stone by a shaker or joggle feed, and getting into the furrows, was dragged over between the surfaces of the land, in widening circles, until it finally fell out round the circumference of stones in the form of meal. The centrifugal action of the revolving stones helped this to travel outwards. The duties of the furrows was threefold: (1) To distribute the wheat and meal over the grinding surface; (2) to partly reduce it; (3) to supply air for ventilation ( or to act as ventilating "flyers") tending to keep the grinding surfaces cool. Two distances were recognized in millstones, to be maintained whilst in operation. High grinding (well apart for light treatment). Low grinding (set close for heavy searching grinding). A further look to the millstones will enable the working of the stones to be more clearly followed. The scheme of "cracking"the land, or grinding surface, will be noticed, this being the first real grooving introduced in grinding wheat, and followed up in modern break rolls. The stones were supported either on a floor, or a special iron or timer hursting. When the stone milling began to be seriously threatened, many exhaustive experiments were carried out to determine wheat the millstone was really capable of . Stones with ten, twelve, and sixteen quarters were tested, with one, two, three furrows each respectively, but while a marked improvement was effected over the older style of dressing, the best work was far behind the roller system. It was found, in most cases, that the greater the number of furrows, the more severe the treatment of the wheat became.Stones 4 feet diameter, and at a speed of 120 revolutions, gave most satisfactory results. Stones were usually run against the sun (clockwise). One of the great disadvantages of stones, especially felt as the mills became of larger capacity, was the large fluctuation of power required. As may be understood, the increase of pressure over a 4 foot pair of stones soon doubled the friction, and power required to rotate them. test made upon a pair of 48 inch stones at 120 revolutions, and producing 300 pounds of fine meal per hour, showed that they took nearly 10 horse power. Unless carefully dressed, and run truly with a good even feed, the stock, or grist, from stones will be found heated as it discharges. This method of reducing the wheat berry favors the manufacture of a large percentage of fine pulverized bran, a most undesirable thing. All attempts to harness stone grinding to a gradual breaking down and an efficient system of grading, purification, and dressing were found of but little real use, and the roller mill of metal doomed the millstone to practical extinction. The modern idea of cracking or shearing open the wheat berry, in preference to crushing it, was faintly shadowed out, with the most efficient methods of stone dressing. There the land and furrows of the top stone runner, and the action was somewhat like that of the groovings on the latest break roll. In the early days of roller milling, several mediums were tried, porcelain amongst other substances being given quite a long trial as a smooth roll, reducing flour stock from the purifiers. But for general reliability and even wearing and freedom from cracking, under the heavy pressures used, cast iron rolls deeply casehardened by chilling in the mould, were found best, and have now become practical universal. Many strange ideas and theories were advanced, before the different phases and limitations of the new system were realized. What, then, does modern milling aim at, as differing materially from the old methods on stones? Instead of pulverizing the grains into one muddled mass of torn bran skins, burst flour bag, and vegetable tissue, it operates upon them by a series of steps, "breaking down" the berry slowly, and at each operation, sifting away the freed particles of the endosperm, or flour bag, before dispatching the open grains to the next roll for further treatment. This goes on until nothing but the bran skins remain, the whole of the interior having been removed. This section of roller milling, termed the break system, now consists of four distinct operations, or of breaking down on four sets of rolls. At first, the highest possible grinding was performed on the 1 break roll, the remaining ones being called upon to do the real work. This led to the use of five and even six break systems, in order to deliver nothing but perfectly cleaned wheat skins form the last break. These systems because unwieldy, and, as aspiration of all rolls came into practice, the trouble of "blue flour," which necessitated the gentle first break operation, was greatly diminished, and rather, more searching treatment made possible on this roll. A reaction took place, and three break systems came into vogue. Gradually the reasonable middle course has been found, and present day systems of four breaks are in the majority, and will probably be so, as long as milling by rollers obtains. The grooving of break rolls is a matter of first importance. The shape, size, length, angle, and speed of grooves and cutting edges have a distinct bearing upon each other, and these must be maintained in the correct proportions. The standard, "saw tooth" or "German" type of grooving was evolved from many varieties of shapes, and standardized as in present day use. This shape, with the chisel formation of each cutting edge. It gives best results after a few weeks' work and wear, when the first keen edge had been smoothed away by the wheat. Then comes the matter of angle at which these grooves are cut. First a number of rolls had them cut parallel with the axis, and a distinct looking tendency was the result, besides failure to open out the wheat berry efficiently. Marks of the corrugations were left upon the bran coats. Grooves were then cut diagonally with the roll axis, and were also more or less a failure when run as to interlock. The correct system was found when the spiral grooves were run across one another scissors fashion. A clipping action was obtained which opened the grain cleanly, with a minimum of friction and damage to stock. Special machines have been designed for cutting the spiral grooves on the rolls, and the travel of the cutting tool can be so adjusted that the amount of spiral is increased or diminished at will. A good definition of spiral grooving is "The course a groove takes, which, while encircling a cylinder, advances along it at an angle with the axis." The angle has now been fixed at 12 degrees, and can be considered the standard. Yet another matter had to receive careful attention before the roll became efficient and economical in breaking wheat. Whilst both rolls had, of course, to revolve towards each other, to carry stock through, one had to travel much faster than the other, no shearing being possible otherwise. The proportion is now fixed at 2 1/2 to 1. The slow roll grooves "hold" the grain whilst the fast roll grooves shear it across. It will be noticed that the fast roll has its corrugations with the short edges downwards, whilst the slow roll has its long sides downwards. This further assists the shearing action. The size of grooving for each roll in the break system, is governed by the size of feed particles. In the case of the first break, the whole wheat to be treated, must be allowed sufficient space in the grooves, whilst passing through the "nip" of the rolls, to prevent any crushing action, and this must be followed on every roll in the break system. The correct position of the pair of grooved rolls. It may appear at first sight that these groovings will tend to cog in with each of the horizontal grooves, but a consideration of the fact that the rolls rotate inwards at the point or edge of contact will prove the scissors action to be effected. The average diameter of rolls had been fixed at 10 inches. This gives the best period of contact when running; the angle at which the surfaces and grooves of a pair of rolls approach each other varies with diameter. Thus an 8 inch or 9 inch roll gives a much sharper angle than the flat circumference of a 13 inch or 14 inch diameter, and the happy medium has been struck in the 10 inch. Having now dealt with the main principles involved in the first section of the roller system, we will proceed to enumerate the complete list of operations, order to point the way to the treatise to follow. They consist, in order of rotation of: (1) Breaking down, or opening the wheat berry, in a series of grindings, on grooved rolls; (2) Scalping, or the gradual removal of the particles of interior flour bag, in as large and uncontaminated a condition as possible, on sifting or pneumatic machines; (3) Cleaning, or scraping, the bran, on finely grooved rolls; (4) Bran dusting and meal grading or dressing on sifting reels; (5) Chop system, or preliminary grading and dusting of scalper stock on sieves or reels; (6) Purification, or the final dusting, sizing, and grading of same stock, on a combined sifting and aspirating machine, which weighs the stock to a fine degree by upward currents of air, and dispatches the groups to the different flour making reduction rolls; (7) Reduction of these particles, by heavy pressure between smooth rolls, to the fineness of flour; (8) Scratch system, or the retreatment of semolina (the largest flour particles), to which bran snips are still adhering, upon finely grooved rolls, and special grading and dusting machines, of the finest of the flour particles from the reduction rolls; (10) Further grinding and dressing of residue, until the safe working limit has been reached, when the hopeless muddles of flour and bran particles are tailed away and dispatched as offals; (11) Bran rolling, or the flattening of bran particles into large flakes. To help impress the method of linking up the separate process upon the reader's mind. A list of the various machines used throughout the flouring section of the mill will now be useful (under the headings already referred to): (1) Four roller mills, with grooved rolls, either horizontal, vertical, or diagonal; (2) reels, sifters (rotary or reciprocating) plansifters, and pneumatic scalpers - with covers of wire or grit gauze; (3) and (4) reels, centrifugal, or plansifters with wire or grit gauze covers; (5) reels, centrifugal, or plansifters with wire, grit gauze and silk covers; (6) purifiers, sieve or air circuit types; (7) four roller mills with smooth or very finely grooved rolls, either horizontal, vertical, or diagonal; (8) rolls as above, but finely grooved, and reels, centrifugal, or plansifters, with wire or grit gauze covers; (9) centrifugal, plansifters, rotary sieves, and sometimes reciprocating sieves for tailings, silk covers; (10) more reduction rolls and dressing machines; (11) large smooth rolls as for reduction. There are of course conveying tackle and spouts, but these do not really enter into the manufacture of the flour. In order to avoid confusion and to enable the student to understand the terms employed in following chapters, it will be well now to give a table, explaining each in turn. Scalping. - Sifting away these loosened contents. Dressing. -Sifting away flour particles. Throughs. - Stock dressed through cover. Over tails. - Stock dressed over cover. Tail sheets. - End section of dressing or grading cover clothed coarsely to dress through all but the largest particles on centrifugal and reels. Cut offs. - Similar end cover on purifiers. Tins or trays. - Stock, lifted by air currents from purifier sieve cover and deposited in trays of purifier. Semolina. - the largest granular particles of endosperm released by the break rolls, dressing through from 18 to 40 mesh per inch. Middlings or "Midds." - The medium granular particles of endosperm, released by the break rolls, dressing through 40 to 84 mesh per inch (includes two or three grades). Dunst. - Finest granular particles of endosperm released by the break rolls and dressing numbers (106) meshes per inch. Flour. - Finely reduced semolina middlings and dunst dressing through from 94 to 156 meshes per inch. Bran meal. - Soft branny stock, through 36 mesh. Fine sharps. - Small hopeless mixture of flour, soft fibrous stock, and bran shreds, through 50 mesh. Coarse sharps. - A similar mixture through 30 to 40 mesh. Fine bran. - Small particles of wheat skins passing through 14 mesh. broad bran. - Coarse or large particles of wheat skins passing over 12 mesh. Greys. - Small particles of bran in flour or stock, which appear grey from beneath flour. Blue flour. - flour from breaks containing large percentage of cellulose matter (cell walls of endosperm) of a very light and filmy nature. Beeswing. - Fine, filmy flakes of cellulose matter and single bran coats (badly adjusted scour beaters often cause this).
Three types of flour roller mills are employed in breaking down the wheat berry. The horizontal, the vertical, and the diagonal. Each has special advantages, but modern practice favors the diagonal type as being most suitable for good all round work. roller mills are made in which the rolls are placed two and three high in one frame. Whilst these certainly economize floor space, the distance of top rolls from the base renders the casing susceptible to vibration; the feeds and deliveries are somewhat complicated, some of the drives and gears are out of reach, and pressures cannot be so finely adjusted as in the usual type or four roller mill. All these points tend to keep the tall framed mills out of the break and reduction systems. The three types of roller mills are- (1) The horizontal rolls, which can be easily adjusted as the weight of rolls does not come into play. Large floor space is taken up, vibration is liable to affect the pressures and render them uneven. They are now used mostly upon bran for bran cleaning, no heavy pressures are required in such work. (2) The vertical roll, which has the advantage of more reliability on heavier classes of work, less floor space, and more uniform work. The delivery from the rolls can be easily inspected. It shows to least advantage upon light fine stock. (3) The diagonal rolls combine the advantages of both the former types. The frame takes up an average floor space and height. The position of the rolls in relation one to the other, is so arranged that they approach each other at an angle when pressure is applied for grinding. This is a distinct advantage, obviating any tendency to injure roll surface or stock by sudden "gripping." The angle of roll axis, approximately 45 degrees, is an ideal one at which to receive the feed into the "nip," and facilitates the arrangement of feed rolls and shelves. Grist, issuing from the underside of rolls, can be readily examined, and the scrapers, in the case of the reduction rolls, set accurately. The principle upon which these roller mills are constructed and fixed, in order that they work imposed upon them, while maintaining accuracy and easy running with the minimum of power per sack, is- (1) Rigidity.- The frame and the mechanism contained in it must be perfectly rigid and solid, so that all stresses and pressures can be easily taken up. The roller mill must be firmly fixed, avoiding all vibration. (2) Driving.- Must be ample strength, smooth running, free from noise and jar, and reliable in uniform speed. Belts being apt to slip in this particularly severe form of driving, gear wheels of the chevron type are in general use. Each pair of rolls being bound together, the exact differential speed in maintained, whatever the speed of the main spindle. The gears run in an oil tight case, the oil rendering them almost noiseless. The main drive from the mill shaft is by belt. (3) Bearings.- taking the great pressures put upon rolls and spindles during grinding, they must be of ample diameter and length. The minimum of friction and heating is obtained by maintaining an unbroken film of oil between the spindle and the bearing surface. All makers design each bearing with a self contained oil well and an automatic circulation of oil to and from it. Be renewing this oil when it becomes too dirty or too thick, almost perfect lubrication of the bearing is ensured - danger of heating and seizing is greatly reduced, and the power required is kept at the normal. Special roller and ball bearings have been tried on rolls, but have not been adopted generally, as they are apt to wear unevenly. (4) Rolls.- must have a very hard tough surface to resist the wearing action of the grains. The silica contained in the wheat causes this. It has been found that iron rolls cast in an iron mould, meet the case best, and for this reason. Immediately the metal is poured into the mould, the surface of molten metal coming in contact with it, are "chilled," that is, the metal suddenly cools and sets, thereby forming a crystalline structure of material, extremely hard. The interior of the hollow rolls, cooling at a normal rate, is not so hard, but remains tough and less liable to break or crack with a sudden shock or blow. Thus the body of the roll is strong to resist a bending or breaking action, and the surface to resist a frictional or wearing action. The rolls spindles are of steel, and are driven into the hollow rolls by hydraulic pressure. The rolls must be absolutely true throughout, to the 1/1000 part of an inch, and the depth of grooves (on breaks and scratch) not greater than half the diameter of particles ground, in order that they may not escape treatment. (5) The adjustment of rolls should be under perfect control. It must be extremely sensitive and accurate, for upon this depends successful breaking and grinding. A slight change in the atmosphere or the condition of stock, will often necessitate a correspondingly slight adjustment of the rolls - to either increase or decrease the pressure. The latest improvement in this section, is the micrometer adjustment, a delicate and ingenious gear which will now be described. The micrometer adjustment has been designed in order to obtain the minute and extremely finely graduated settings of rolls, with a comparatively large movement of the hand wheel. The mechanism operates thus- the tension rod, working pivoted lever carrying the roll, screws into the gunmetal nut, which is supported by a hand wheel on a ball seating and connected to it by a pin. The hand wheel sleeve is treaded coarser than the tension rod, and a fine adjustment is obtained by the difference in pitches of threads on these two. The rolls can be brought together by moving the hand wheel. One complete revolution will lift the hand wheel 1/8 inch up, but at the same time the lower tension rod 1/11 inch. Thus the tension rod has actually mover up 1/8 - 1/11 inch or 3/88 inch. The leverage of roll lever reduces this movement on the rolls to approximately 1/100 inch. (6) Throwing - out, or moving the rolls apart when some obstruction requires removing, when the feed has stopped, or the mill is shut down for the week end, must be easy and rapid. In some roller mills where the bottom roll is adjustable, this is accomplished by simply depressing a short lever, which operates an eccentric cam at each end of the rolls by means of a rod running parallel to rolls. This lever engages, at its lower end, with an eccentric pin and boss, attached to the tension rod carrying the pressure spring. This tension rod is also lowered slightly, the pressure between the fixed and the adjustable rolls is removed, and a slight gap left between them. A clearance of 1/8 - 3/16 inch being sufficient. The chevron gears must always be well in gear with each other. Throwing rolls wide apart would not admit of this, and would serve no useful purpose. The throw out lever also operates the feed rolls, stopping them and thus preventing any feed passing unground. The compression spring referred to, on which the adjustment lever and roll rest, is to allow the roll to "give" sufficiently to pass hard substances such as nails, which would otherwise damage the rolls, and to absorb vibration. The throw out lever in some makes is depressed to open rolls, and raised for the same purpose in other makes. The hand wheel sleeve, and with it the whole of the micrometer adjustment, is held firmly in place by the throw out cam. By depressing the lever, the cam is lowered by the eccentric pin, and the whole adjustment gear follows down with it. (7) Stops. - Rolls are provided with stop pin, designed to prevent the rolls actually touching. For a fine finish, however, it is practically impossible to keep them apart, should the feed suddenly cease. The rolls are riding on the stream of material, and it thus serves to hold them apart. stop the feed suddenly, and the great pressure behind the rolls will tend to bang them together. (8) The feed gear usually consists of a feed hopper with hinged valve gates and feed rolls. By means of adjustable pressure spring gear the value gates should be set to work automatically. The main duty of the feed gear is to widen the heap of stock in the hopper into a broad thin sheet of feed, which covers the whole length of rolls, in a consistently uniform spread of unvarying thickness. It is well to feed the rolls to the extreme ends to prevent the possibility of uneven wearing. Glass doors are provided for a good view of working and feed in hopper. Roller feed gears. - The most troublesome roll feed to deal with is that composed mainly of soft and branny stocks. If this type of feed can be efficiently regulated and spread it will be comparatively easy to manipulate other stocks. Some have devised a feed gear to overcome these difficulties (A first essential is that the gears shall be readily adjustable, and that the gate shall be lifted clear in the event of binding feeds.) The regulating and adjusting lever can be worked without opening the roller mill door. It had three positions - of angular movement - rocking spindle. In the first position it is thrust inwards sufficiently to "hold" the regulating link, the bell crank lever, and prevent the tension spring from coming into play against the feed plate. This is really the "locking" position. In the second position the lever is pulled forward to depress sufficiently to free the bell crank lever, and allow the spring full play, thus putting into action the automatic feed. In the third position the lever is pulled forward to its limits, and, lifting finger up against stud extension, raises the feed plate vertically. This latter operation can be done quickly, and allow any temporary clogging or obstruction of the feed to clear itself. The edge of feed plate and feed roll contact is exposed, and thus can be cleared by a spatula if desired. The makers claim this an an extremely sensitive gear. (9) Exhaust. - The main reason for roll exhaust are the removal of humidity and heat and the lifting away of light soft flour dust and cellulose matter released at each operation. By means of the internal trunks, incorporated in the machine, the aspiration is applied to the grist as it leaves the rolls. Here it has the greatest effect. The heat generated by grinding, combined with a certain amount of humidity, is drawn away, thus keeping the rolls cool and free from the tendency to sweat. On the breaks a small percentage of light cellulose particles released from the berry, and termed "blue flour," is also removed, and contamination of the break flour avoided. Two trunks should be applied to each roller mill, one at each end, and should be run into the side of the main exhaust trunks, to allow them to be periodically cleaned without driving the deposit back to the rolls. Automatic traveling scrapers or monkeys keep these clear. To be exactly effective the main exhaust trunks should be proportioned in sectional area to the number of rolls from which they are taking exhaust, at any given point en route. The average area of branch trunks is one square inch per 4 inches of roll surface, thus each pair of rolls on a 60 X 10 inch roller mill will require 60/4 = 15 square inches inside area trunks = say 3 1/2 X 4 inches. A four roller mill will thus require two such trunks, one operating each pair of rolls. Exhausting is especially necessary on last break rolls and on some patent reduction rolls, where the heavy pressures necessary aggravate the tendency to sweat. The fans at work upon these rolls should be large and steady running, and the tendency now is to exhaust from these into a tubular textile dust collector situated in the mill. All branches and connections should have slides for the regulation of the suction, and the economy of power. (10) Grooving. -The standard German saw tooth pattern of grooving is the best. For mixed wheats, with a four break plant, a good working standard is 1 break, 8 to 10 per inch; 2 break 16; 3 break 20, and 4 break 26 per inch. When the mill is large enough to have two or more pairs of rolls in each break, it is often advisable, with a wide range of wheats, to size into two grades for 1 break, and use eight and ten grooves for the large and small grades of mixture respectively. Break rolls should not be run too long before regrooving. The amount of break flour will be increased, as extra pressure will be required to force the rolls to their full capacity. Extra power will be required. The percentage of pure semolina and middlings will be diminished, and therefore, also, the patents flour. The bran will be deeply marked and crushed. The percentage of broad bran will be reduced. All the sharps will be too white and too plentiful at the expense of the flour output. Bran flour will be stained on damp days and mixtures. With grooving regular and fairly sharp to the touch, the percentage of semolina and middlings up to standard, the break flour small in quantity, the bran clean and "finished," and the work performed without undue pressures, the break rolls do not require regrooving. The average life of grooving on 1 and 2 break rolls is approximately eight to nine months. On 3 and 4 breaks, where the stock has lost its first hardness, from twelve to sixteen months. It is advisable to regroove rolls alternately, so that only one pair of over sharp rolls is at work at any time. (11) General construction of roller mills should be such that all parts are easily accessible. Feeds entering the "nip" of the rolls, or grist leaving the rolls, should be within easy sight and reach of operative of examination. Rolls as simply and strongly constructed as possible, self contained and practically air tight except at point of ingress of air supply to exhaust. Rolls must not work out of line after adjustment. They should be kept scrupulously clean and free from accumulation of moth or stock. Some makers of rolls are adopting a roll scalper system. That is, are embodying sieves or scalpers within the roll frames, so that the grist falls directly on to these as it leaves the rolls. The released stock is removed, and the reminder of the berry passes onto the following break direct. Break roll sizes and surfaces. - The 9 inch diameter rolls are usually made from 20 to 32 inches long, fast roll, 375 revolutions, gears 2 1/2 to 1. The 10 inch diameter rolls, the standard size, are made 24, 32, 40, 50, and 60 inches long respectively, fast roll 350 revolutions, gears 2 1/ to 1. Although most makes of roller mills have equal sized pairs of rolls, one well known firm of milling engineers advocate the use of differential sizes, 10 inch and 13 inch diameter rolls pairing. The actual standard travel of roll peripheries is approximately 900 feet per minute for fast or "cutting" roll, and 360 feet per minute for slow, or "holding" roll. The stock travels through the rolls at approximately 450 feet per minute. The size and number of rolls in a break plant is determined by the standard set, in "inches per sack." This term meaning the number of inches of roll length, or contact, employed for every sack of the mill's capacity per hour. From 30 to 40 inches per sack is the range of practice, spread over a four break system in certain proportions. It will be noticed that there is a method in this uneven division of surfaces, and that the 2 and 3 breaks always receive the most. The reason for this lies in the fact that the heaviest work, the releasing of the greater part of the semolina, middlings, and dunst, must be done on these two rolls, and that therefore extra surface must be granted, to keep the feeds as thin as consistent with good work. The 1 break has only to crack open the berries, and on the last, the 4 break, the bulk of the stock has been removed, so that, although the inches of grinding surface are less than on the previous rolls, the proportion of the surface to actual feed is really greater. this should be so, because the bran coats must be finally and completely "finished" and "cleaned" without further abrasion and powdering. Break systems, have ranged from three to six separate operations. For sometime the three break was in favor, but experience condemned it finally upon the following counts: (1) The sudden change in size of grooves from coarse to fine destroys the larger flakes of bran and makes too much fine midds and dust in place of semolina; (20 the greater pressure being required, there is the tendency to crush the grain and make too much break flour; (3) the increased pressure develops heat, and rolls will require more attention in working, lubrication, and aspiration; (4) to thoroughly clean bran in three breaks the sudden breaking down will mean bran fiber shredded on the 1 and 2 breaks, to the detriment of all break and purifier stock. The five and six breaks systems, although still practiced in some mills, have not much to recommend them. It must be clearly borne in mind that the four break system does not employ less actual roll surface per sack, than in the five or six break systems. The balance is kept by giving each break more surface. The net gain is, that the stock gets the minimum of friction and handling, consistent with proper breaking down, and that each break feed has ample surface on which to pass the maximum stream without over crowding. Rules for rollermen - Break system. -In starting up the rolls, they must be carefully timed and in easy running, ready for final adjustment as the feed enters. The feed to 1 break must be examined closely to determine that it is even in quantity and condition. The pressure upon the roll must be adjusted until the maximum feed is passing in a broad even stream right across the rolls and into the "nip" with precision. Constant touch must be kept with the screenman to guard against any likely variation in state of mixture. The work of succeeding 2,3 and 4 breaks must be quickly followed up in succession and the pressure set for each quality of feed. The bran skins should leave the last break in well open and well cleaned flakes, free from grooving impress. A final adjustment will probably be found necessary, to either ease off or put on pressure, to meet the slight variation caused by the rolls warming to their work.Periodic handling of all break stocks, to and from rolls, is very essential in order to check the work of the rolls. Care should be taken that the work of the scalpers ensures finished stock grading away and not feeding to following roll. Large percentages of break flour must be avoided. The reduction rolls will next require attention, one after another being carefully adjusted and set to a nicety until the whole range is full employed., A special set of notes will be given in the reduction section. To deal with matters applicable to either break or reduction rolls. Troughs of scalpers and dressers should be obtained so that it can be ascertained that the rolls are providing the correct feeds. A closely worked report of the first hour's run must be taken by the packers. This will enable the roller men and foreman to check the flour percentage in the aggregate and in the various grades, enabling them to gauge the work of the rolls. When all is in good running order, a constant watch must be kept upon the various drives, bearings, and mechanisms of the rolls. The sound of the running is a criterion of any defect or otherwise. Slipping belts, heated bearings, uneven feeds, or rolls out of truth, are the greatest troubles. Heated bearings often cause the "out of line" rolls, the first remedy for which is to get them up again, by relieving the rolls at the binding ends, and thus allowing the other ends to come together and equalize matters. A read justment must then be made, and a thorough lubrication and cooling of the bearings effected. This trouble will be rare if the oil wells are well supplied with clean oil, free from grit. Choking spouts are to be guarded against if the wheat or weather is specially damp. Exhausting must be maintained at the highest efficiency. Breaking belts are a feature of roll driving requiring immediate and rapid attention and repair. They usually "go" at the joint, as the great power transmitted means heavy pulls upon them. It often happens that leather laces are constantly pulling out, whereas a common Harris fastener, if properly hammered or in exact line with the stress, will last a long time. In order to cope with sudden chokes, the feed spout should be fitted with hand hole near floor level above. The feed can then be allowed to pile up until this is reached, when an operative can "fetch" the stock away on to the floor until rolls are in full running. Break System - Break Flour. - Much difference of opinion exists as to the making of break flour. Considering the matter impartially, the best policy is to make as little as possible whilst the break stock is in the presence of impure particles. The chances are against such stock or flour retaining its good qualities in color, bloom, and strength. The first break flours, and the last, or bran flours, are the two poorest grades on the system, whilst the second and third rank tenth and fifth respectively. In the great majority of cases where the break flour is excessive, it comes from the breaking down of middlings from the second and third breaks. It will be found to contain fair all round qualities, and it might be argued that, therefore, it need not occasion any comment or require any remedy to reduce its percentage. But it is really robbing the high grade flours of some of their roller feeds and thus reducing the flour output of the patents grades., This high percentage of break flours will probably be of good strength, fair color, and all round baking qualities, but only because it contains so much good stock that the contaminating particles are reduced to a relatively small proportion and thus do not make themselves so obvious. It is against all the principle of modern gradual reduction systems, with their elaborate grading and purification schemes, as it changes the graders and dusters into flour dressers; they, in turn, delivering dusty and insufficient feeds to the purifiers. Scratch Rolls and their Duties. - Scratch rolls are probably so called because their grooving is so fine that it comes under the heading of "scratches" rather than grooves. It is a cross between a break and a reduction roll; it is grooved like a fine break roll. It has differential speeds, like break rolls, and it runs at reduction roll speeds. It must be understood, in passing, that all roller mills, whether break, scratch, or reduction are similar in frame and general mechanical construction. The scratch stock is reduction stock with bran snips attached, and the duty of these rolls is to remove the snips from what would otherwise be pure semolina. If the break rolls are kept from a severe treatment of break stock, a certain amount of "scratch" material is bound to be passed by the scalpers. This consists of large particles of semolina with the outer portions of the wheat berry adhering. These are the outer layers of the endosperm, which will include the gluten and strength of the grain. This must be preserved for reduction into strong flour, by must be made to part with all other matter before being thus treated. The purifier tails this stock away, as impure because it cannot purify and render it fit for the final grinding into flour. The scratch rolls, with their fine groovings, are designed for the delicate work of treating this material. Being like an extremely fine break roll, they can, if the pressure is very carefully and exactly adjusted, do this work efficiently and with a minimum of powdering, sending the semolina back for repurification, but slightly reduced in size of particles.
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