SOME ASPECTS OF HANDLING POWDERS IN MECHANICAL EQUIPMENT 701 ditions of product, temperature, humidity, condensation, local vibration, periods of standing in settled condition. (c) Attrition may be related to the work input in excess of that required to overcome gravity or the direct action of a superimposed load Shearing under a high normal load or excessive dispersal of potential energy are obvious sources of breakage. (d) Segregation is minimised by maintenance of a high order of packing and minimisation of free surface activity and particle movement. (Received: $rd January 1969) REFERENCES (1) B.S. 3810 Pt. 2. (2) Jenike, A. W. Bull. Utah Engng. Exp. Stn. No. 108 (1961). (8) Jenike, A. W. Bull. Utah Engng. Exp. Stn. Bull. No. 128 (1964). (4) Reiner, M. Scientific American (December 1959). (5) Hvorslev, M. J. Ingenuiriudenskals skritter A. No. 45, 155 (1987). (6) Roscoe, K. H. Schofield, A. N. and Wroth, C. P. Geotechnique 8. 22 (1958). (7) Scarlett, B. and Todd, A. C. A.S.M.E. Paper No. 68-MH-6. (8) Reynolds, O. Phil. Mag. Set. 5, 9.0, 469 (1885). (9) Williams, J. C. and Birks, A. H. Powder Technol. 1, 199 (1967). (10) Ashton, M.D. et al, Rheological Acta 4, 206 (1965). (11) Ashton, M.D. et al, J. Sci. Instrum., 41, 768 (1964). (12) Farley, R. and Valentine, F. H. H. Powder Technol. 1, 844 (1967). (18) Jenike, A. W., Elsey, P. J. and Wooley, R. H., Proc. Am. Sec. Test Motor., 60, 1168 (1960). (14) Walker, •). M. and Cart, J. F. G.]{.G.B. S W. Region Publication Ref. SSD/SW[N.I$7 (December 1967). (15) Scarlett, B. and Todd, A. C. J. $ci. Instr., 1, 655 (1968). (16) Birks, A. Bradford University School of Powder Technology. (17) Bruff, W. and ]enike, A. W. Powder Technol., 1, 252 (1967). (18) Jenike, A. W. Powder Technol., 1, 287 (1967). (19) Walker, D. M. Powder Technol., 1, 228 (1967). (20) ]oranson, ]. R. Trans. ASME, 224 (May 1966). (21) Lee Y. Combustion Int. Combustion ]{ngng. Co•p. 31, 20 (1960). (22) Richmond, O. Mechanical Engng. 8õ, 46 (1968). (28) Gardner, G. C. C.E.R.L. Report RD/L/M/$0 (1968). (24) Gardner, G. C. Chem. Engng. Sci., 19, 288 (1964). (25) Bates, L. Bunker Design Symposium 1969. Instn. Mech. Engnrs. London. (26) I. Chem. E. Working party report 1966 "The Storage and recovery of particulate solids". (27) Bates, L. Chem. Process. Sup. 16 (August 1967). (28) Bagster, D. F. and Bridgewater, J. Powder Technol. 1, 189 (1967). (29) Carleton, A. J., Miles, J. E. P. and Valentin, F. H. H. A SME Paper GS-MHoeoe. (80) Peschl 1.A.S.2. ASME paper No. 68-MH.4. (81) Bates, L. ASME Paper No. 68-MH.oe. (82) Bruff, W. ASME Paper No. 68-MH.oe5. (88) Williams, J. C. Chem. Process. Supp. 6 (April 1965). (84) Mattee, H. Powder Technol., 265 (1967/68). (85) Roberts, A. W. ASME Paper No. 68-MH.5.

702 JOURNAL OF THE SOCIETY OF COSMETIC CHEMISTS (36) Williams, ]. C. Process. Engng. 173 (April 1965). (37) Wolf, E. E. and Hohenleilen, H. L. Trans. A.S.M.E. 67, 585 (1945). (38) Richards, J. C. Brit. Coal Util. Res. Assoc. Monthly Bull.,57, Nos. 27 (February 1968) Review No. 219. Introduction by the lecturer Films just sho•vn illustrated examples of the complex regimes of motion that may take place •vithin mechanical equipment. The feature to be noted is that motion in the bulk is rarely completely even and although slip planes may form on boundary layers of static material or bet•veen masses moving at different speeds, bulk shear allo•vs local differences in voidage (or compaction) according to the local stresses. Oper- ationally, the vie•vs •vere taken through sectionalized glass-sided hoppers fitted •vith discharge scre•vs and sho•ved the •videly ranging patterns of flo•v given by various scre•v designs and different test materials, thus confirming the inadequacy of design predictions based on mechanical features, dimensions or speeds of operation only. The change of state of po•vder in its path through various regions of the equipment also illustrated ho•v the elemental approach to po•vder route can indicate overall performance characteristics and expose detailed potential hazards to smooth oper- ation. DISCUSSION A MEMBER OF T}•E Am)•F•C•: Mention is made of "free surface activity" in relation to the unconfined moving boundary of a bulk mass. This brings to mind the concept of surface films and the like. Perhaps the definitions could be rephrased as "free surface motion" to distinguish the effect from particle surface effect. T}• L•CTURER: This is fair comment. At present there does not appear to be a firm terminology on such points and the basic difficulty is to tie do•vn the subject so that reference to phenomena is made •vith a common meaning. "Free surface activity" or "motion" is not a simple mechanism and one •vould •vish to define the depth of the active surface, size scan of particles and other aspects of the situation to have a reasonable appreciation of likely events. A dictionary of terms is certainly needed to introduce consistency and explain j argon essential to understanding and discussion. British Standard 3810: Part 2 defines many points for mechanical handling equip- ment and there is good uniformity of specialised terms and symbols amongst the academic teams now working in the field. When one descends to simple practical aspects and considers the many styles of motion which occur, simple group terms are inadequate to concisely express all particular styles and detail is frequently required on a fragmented basis. A M•.MB•.R OF T•. AuI)•c•: In the film a considerable length of uniform screw was exposed to the product and this common practice frequently leads to trouble. Should not one fayour a screw geometry which changes in section every 1• pitches or so? T• L•cTu•R: The most suitable form of screw to select depends on a number of economic and process factors but is most usually dictated by the flow pattern that