700 JOURNAL OF THE SOCIETY OF COSMETIC CHEMISTS economy. The main practical aspects of such a situation is to relate the savings given by incurring the risk of operational failure, taking account of the cost and certainty of various means of redress. 5. Expressions for flow formulae are limited to simple channels of conical or plane symetrical flow. 6. No account is usually taken of accelerations, isotropy, local failure planes, the effect of discrete particles as opposed to a continuum, and certain simplifying assumptions are necessarily included in theoretical analysis. 7. Indeterminate operational conditions, e.g. filling hopper or mixers do not permit starting conditions of a system to be predictable. CONCLUSION The subject contains a great many variables of which some may be trivial or irrelevant in some cases and dominant in others. It is hoped that in context it is permissible to outline a few simple notes and rules. (a) The sheer number of locations in a system where powder conditions are likely to change in state preclude any comprehensive analysis or tests to give absolute determination of powder condition. The object therefore should be to understand the cause and effect of various powder conditions and review each detailed point with respect to potential hazards. Major dangers such as arching, blocking, flooding, and compacting should receive detailed consideration where appropriate. The basic concept of a powder existing in a unique state for a specific voids ratio and state of stress enables:- (i) The consistency of the handling properties of various samples of a powder to be measured and compared. (ii) Specific design parameters for hoppers to be derived. (iii) Classification by comparison of flowability of powders. (iv) Prediction of bulk behaviour to be made in specified conditions. (v) The breakdown of powder/mechanical activities to be considered on the elemental scale of a system. (vi) Attention to be directed to the conditioning of powder state by the mechanisms and transfer points of a continuous handling system for a stream of powder. (b) The entire flow path of a powder should be reviewed in the light of powder condition and local behaviour under the separate features of flow failure or flooding, segregation, attrition, dust, wear, contamination, residue, dead pockets, cleaning, etc., taking account of all variable con-

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.