NOVEMBER, 1933
Frr;cAE
8.
\\‘EIOHIVG MILL FOR
IKDUSTHI kL A l D ENGINEERING CHEIIISTRY
FEEDERDELWERIUG COiL FIRINGA FURSACE
TO .I
T h e weight fed is recorded and t h e temperature of t h e f u r n a c e held w i t h i n olose limits since the feeding is b y weight and not by volume.
feeder, particularly for coarze materials, is the reciprocat,ing plate feeder ~ ~ h i cisl i a plate located under the hopper with tlie open part of the hopper toward the discharge end of t,he feeder. A s the plate reciprocates back and forth, the back of the hopper keeps tlie material from flowing backward, and the feed is moved forward and off the end of the plate by displacement. The reciprocating movement is also effected by rapid vibration from ail electromagnet with certain forms of this type of feeder. The roll type of feeder operates on the same principle as the belt feeder, but, instead of a n endless belt, a large roll is placed underneath the hopper. The types of feeders described are used inairily for noiiflooding materials. For fine or bulky substances which tend to flood, the rotating pocket, feeder is used. It is simply a star pocket rotated in the hopper bottom; around part of the pocket are located curved plates t,o prevent the material from by-passing or flooding the pockets. The standard screw type of feeder consists of a screw enclosed in a stationary pipe located below the hopper, which moves the material from the hopper by displacement. It is nonflooding so long as the screw is kept full of material a t all times. Since all of these feeders are essentially volumetric, any change in the weight per cubic foot of the material, either by bridging, segrega,tion, or change in specific gravity, will cause a variation in the weight actually delivered by the feeder, even though the volume may be constant a t all times. This change in weight has a far greater effect on the subsequent apparatus or process than is often appreciated. As a matter of fact, the importance of feeding by weight was never realized until volumetric feeders were replaced b y constant-weight feeders in a number of processes and the effect on the subsequent apparatus and process was noted. I n the case of feeding pulverizing mills, the capacity wa.: considerably increased, and in feeding kilns and driers a more uniform product was secured with even less watching of the controls and considerably less fuel consumption. In the case of proportioning and mixing two or more materials, the advantage of delivering the product a t a uniform rate by weight is self-evident when it is realized that only by this means can the mix be uniform and do away with the necessity of making numerous tests and corrections before tJheblend or mix meets the specifications. A number of methods have been eniployed by those who recognize t h e importance of feeding by weight. One method is to deliver the feed into a counterweighted hopper; when a predetermined amount, has, been deposited in tJhe hopper,
1341
a contact is then closed which shuts the feed off. \Then :t definite time period has elapsed, the charge is dumped onto a conveyor or into the apparatus that; it feeds. Xodificat’ions of this principle have been employed by using periodically operating belt conveyors as well. Both methods are essentially a batch method of weighing arid feeding. Another system, which is continuous in operation and 1-ery simple, uses the principle of a balanced belt and counterweight. The belt operates continuously and delivers the material from the bin a t a continuous rate and a constant weight. Figure 5 illustrates the basic principle of this method of feeding: The conveyor belt is mounted on a franie and pivoted a t F . The counterbalance, A , is set for the desired feed rate. When the material, J , changes iii weight per cubic foot, the frame tilts up or down. Assuming that it first increases in weight per cubic foot, the actual weight of the material on the belt increases or tends to increase, and the pivoted frame tilts down and partially rotates about the pivot F . The gate, H , is partially closed through the linkage, C, and the volume moving out is thi.is decreased on the belt so that the weight is a t once corrected and held constant. Hence, the actual weight of the material delivered from tlie end of the belt always remains the same, regardless of what it weighs per cubic Foot or whether it is coarse or fine, so long as the counterweight is not moved. Thisdeviceis simple and inexpensire. It opens u p possibilities of feeding by weight without much capital outlay to obtain the desired result, with the consequent advantage of improving the operation throughout the entire process. Tests have been conducted on feeding such equipment as grinding mills. The increased capacity of the grinding mill, over and above that which was the maximum when fed by a voluinetric feeder, has in a number of cases exceeded 10 per cent’. Until the reason for this increase is analyzed, it mould seem to be an exaggeration. The grinding mill is a rather sensitive machine a t best when near its maximum capacity. I t s operation is greatly improved when it is not subjected to appreciable variations in rate of feed over short periods of time. -4s mentioned previously, the variation in volumetric feeding usually ranges between 20 and 30 per cent in weight. It is readily apparent, then, that for the normal operation of the mill, it must be operated at considerably les? than maximum capacity in order to keep it from overloading when fed by a volumetric feeder. When fed by a constantweight feeder, the mill may be fed at a considerably higher rate than previously; it is not now subjected to wide fluctuations in feed rates, with the result that the average feed-rate capacity can be increased considerably. The grinding cost per ton and wear on grinding parts are lowered in the same proportion. RECEIVEDSeptember 9, 1935.
Correction In the paper on “Consumers’ Crude Rubber Requirements” [IsD. ENQ.CHEM.,27, 1201-4 (1935)], the expression “low plasticity” is used to designate a soft rubber and not a stiff rubber. While the meaning of the expression is probably clear from the text of the article, attention is called to this variation from the usual method of expression in order to avoid misunderstanding. In measuring plasticity by any of the more common methods, the softer rubbers are usually designated by a lower figure than that used for the stiffer rubbers. ;Is a result the expression “low plasticity” in many rubber laboratories has become synonymous with “soft rubber.” I t is in this sense that it was used in t,lie article. GEORQE A. SACKETT
