Powder Technology-Elsevicr Sequoia SA, Lausannc-Printed in LK Netherlands

Short Communication

117

A laboratory scale feeder for noncohesive powders

D. M. C. HORSLEY and E. ROTHWELL

Department of Clxmical _‘Gzgineerirtg and Fuel Technology-. LhirersiI_r of She$‘Xd. hfc_Dpin Sweet. SltcjieId Sl 3JD Is:. Brirain)

(Received February 21. 1972)

In the field of powder technolo_q there is a re- curring need for a simple device which is able to dispense powders at a controlled and reproducible rate For laboratory scale investigations there are few commercially available feeders which will perform adequately at rates as low as say, 25 kg h- ‘_ While previous workers have reported that screw and star feeders will operate satisfactoriiy’ with a delivery accurate to some + 3%‘, it was found that there existed a need for a simple versatile powder feeder that would reliably deliver non- cohesive granular solids with an accuracy of better than & lo%_

This communication outlines the design and gives typical pe~~orrnancc data of a feeder which, while being relatively cheap to construct and simple to operate, is capable of dispensing a wide range of non-cohesive granular solids with an accuracy and repeatability of better than +O.S”?.

The powder feeder and ancillary equipment

The feeder consists of a rotating wheel which continuously removes powder from a storage hopper mounted above it_ Material so removed is carried round with the wheel until, under the action of gravity, it detaches itself and falls into a delivery chute. This system, which may be considered as a development of the well established moving belt devices, relies upon an invariant powder bulk density in order to achieve constancy of mass delivery rate, since it produces a nominally fied volumetric rate, defined by the speed of the wheel and the height of the hopper discharge gate above the wheel. The volumetric discharge rate of the feeder is rendered constant for a given wheel speed by arranging that the hopper gate feeds powder into a trough 3 mm deep x 6 mm wide machined in the

Powder TedmoL, 6 (1972)

periphery of a l SO-mm-diam. wheel, eliminating any possibility of sideways flow of material from the gate and resultant imprecision in powder discharge rate The wheel rotates in sealed ball races and is driv-en via a 200 I 1 reduction gearbos by a “Velodyne” motor with integral tachogenerator. A schematic drawing of the complete assembly is shown in Fig. 1.

The Velodyne speed, and hence, for a given gate height and powder granulometry- the powder feed rate, is controlled by a thyristor unit operating as a closed loop controller_ The desired motor speed is selected by adjustment of a potentiometer fed from a reference voltage. This potentiometer is adjustable either in pre-set steps or continuously, as dictated by requirements. The voltage from this potentiometer is compared with both the Velodyne tachogenerator voltage and a derived voltage proportional to the Velodyne motor armature current The “error-‘ voltage from this comparator junction determines the phase angle at which the thyristor drive unit fires on each halfcycle of the ac mains input. and hence the output to the motor armature. This feed- back control system provides automatic compensa- tion for the fluctuations in motor speed which result from changes in applied load. Zener diode voltage regulators control the voltages feeding the separate motor and tachogenerator field coils, thus helping to stabilize the motor speed against mains voltage variations. These thyristor control techniques are more fully discussed by Merrett3--5.

Feeder calibration and operation

In order to eliminate errors introduced by manual weighing of the mass delivered in fixed time periods over the operating range of the feeder, all relevant information was converted into digital pulse. which were counted using conventional technique; Stan- dard Harwell 2ooO series equipment, being readily available, was used for pulse counting Three

_ separate pulse trains were generated viz I (i) 1-kHz pulses from a l-MHz quartz crystal

oscillator and frequency divider to give a time scale accurate to better than i part in 10s.

(ii) Pulses directly related to the motor speed, derived by coupling an ac tachogenerator to the motor drive shaft The ax. waveform produced was

SHORT COMMUIWXTlON.5

Fig I_ General la>out of the feeder assembly.

electrically squared and differentiated to give a train of 3 pulses per revolution of the motor.

(iii) Pulses related to the mass delivered. This pulse train was generated by a photoswitch that viewed the visual output of an Oertling TP41 top pan balance which received the feeder discharge. This balance displays on a screen a series of alter- nate light and dark bars every 100 mg, visual inter- polation to within 10 mg being made in normal operation by means of an inclined wedge. The photoswitch 7-x placed so that it did not interfere with the I_ ,-..~al bahmcc operation, but was able to sense the light-dark change on the scale, giving 2 pulse of the correct polarity for the addition of each 200 mg to the pan of the balance.

To determine the performance of the feeder, three 2axs scalers were arranged to count the three independent pulse trains, 2 30-minute warming up period allowing the motor and control unit to attain operating temperature An electronic gate inhibited the scalers until the arrival of the first output pulse from the photo- switch, thereby removing any dubiety in de- fining the origin of time at the start of 2 run_ The pre-set count limit facility of the scaIer measuring the mass delivered was used to inhibit the other two

scalers after a predetermined mass of powder had been fad to the balance. The two inhibited scalers were then read out automatically onto an Addo-X printer, thus registering the elapsed time and the number of motor revolutions for the delivery of the specified powder mass

For each powder selected to test the feeder, 12 sets of observations of elapsed time and motor revolutions were recorded for each selected motor speed setting; from these data, feed rates (g min-r) and motor speeds (in rpm) were obtained, and hence the mean feed rates and motor speeds together with their standard deviations. This procedure was,re- peated for several different non-cohesive powders, typical results being given in Tables 1 and 2 for C&100 mesh calcite and 120-150 mesh silica sand respectively; in each case the total delivered weight of powder was 80 g.

The data given in Tables 1 and 2 show that this simple feeder is capable of delivering, with accuracy, a range of noncohesive powders, providing a turn- down of approximately 7 I 1 in rate Also the stan- dard deviation of the delivery was less than 0.5% of the mean rate; indeed, at ah but the lowest rate, this standard deviation was less than 0.27% of the mean. It appears from these results that a large percentage

Powder Techno.L6(1972)

SHORT COMMUNICATIONS 119

TABLE I

CaJcite 60-100 nesh

Mean J&d rate Srd der. of mean B/A @ min-‘) fPearPre@min-‘) (“4)

(A) (B)

23%. J 055 026 1013 0.49 024 178.9 027 0.15 154.5 0.11 0.07 1303 0.19 0.15 106.0 021 020 84.0 0.16 0.19 613 0.15 0.14 37.9 0.16 042

____- ________ _.

Ar. moror speed Srd_ dec_ of moror D,‘C

(TJm) speed (vm) (“A) (C) (D)

‘S64 5.7 0.20 25553 25 0.10 2240 1.6 0.07 1915 1.5 00s 1601 13 0.0s 1289 0.7 0.05 1014 0.7 0.07

734 1.0 0.14 450 06 0.13

TABLE 2

Silica sand 12&150 mesh

Alean feed rare Std_ dec. of mean B;A AI*_ motor speed Sfd. der. of mofor D/C (g min-‘) feed rate (9 min- I) ( “‘) 0 (vm) speed (rpm) ( “3 (A) (B) (C) (D)

237.5 037 0.16 xv6 32 0.11 211.6 020 0.09 2520 33 0.13 186.5 031 0.17 “II 13 0.06 1625 0.11 0.07 1915 1.1 0.06 138.7 0.25 O.JS 162s 2.4 0.15 JlL7 030 027 1315 ‘0 O-15 90.0 024 0.16 JO45 1-s 0.17 64.5 0.09 0.14 74-s 1.8 0.24 39.6 O.JS 0.45 455 0.7 0.16

of the variation in feed rate may be attributed to motor speed drift, this being most significant at the highest and lowest speeds. Whereas, in most opera- tions involving the feeding of powders, the quoted precision of feed would be adequate, further slight improvements may be realised by the use of more sophisticated analogue or digital motor control systems to eliminate motor speed fluctuations.

The stated feed range of from - 40 to 220 g min - ’ was attained by the variation of one system para- meter alone, namely motor speed. However. it should be borne in mind that the range quoted does not represent the limit of this feeder_ Scaled versions of the same general design would allow considerable variation in the range of the feed rate; furthermore, higher or lower rates at any given motor speed can readily be achieved on the unit described by merely adjusting the height of the discharge gate. However, some loss of accuracy would be expected when the

--

gate-wheel clearance approaches the particle size of the feed The range of the feed rates available from the unit, but obviously not the turndown, can easiIy be varied by employing a different ratio gearbox in the wheel drive system

Apart from the unit3 obvious appkation to the delivery of powders at constant rate, additional applications resulting from the nature of the motor speed control system may be considered of value. The Velodyne motor and control system has an acceleration’ of some 8000-10,~ rpm s-l and consequently is able to follow faithfully any step change in the desired motor speed value This feature renders the system of U-W in determining the response of any dynamic system which is influenced by the feed rate of a particular powder_ Dynamic characteristics may also be derived from frequency response analysis, and to this end the sinusoidal modulation of feed rate about a mean level has been

Powder Techol, 6 (1972)

120 SHORT COMMUNICATIONS

real&d in practice by wiring a motor-driven variable resistance in series with the motor speed potentiometer, sinusoidal variations in the motor wed and powder flow rate being confirmed by pulse countkg techniques

REFERENCES

1 E. J. Davies, R. M. Jervis and G. Thurstield, J. P+ E, Sci InsrK. 3 (1970) 666.

2 W. k Perkins et nl Proc 2nd NatL Air Pollution Sjmp., USA, 195% quoted in H. I_ G- and \V_ R Lane. Pani- culare Ciouds. Dum, Smokes and Mist.% Span. Londoa 1961.

3 J. Msrett. Mullad Tech Cow, 9 (1966) 74. 4 J. Merrett. hlullnrd Tech Commzm. 11 (1970) 75. 5 F. C Williims and A M. Uttley. J_ 1-r. Eler Engrr. 93 (1%)

1256.