INDUSTRIAL AND ENGINEERING CHEMISTRY
664
LITERATURE CITED
Capriati, E., Ital. Patent 422,665 (June 20, 1947). Carson, B. B., and Ipatieff, V. N., J . PhzJs. Chem., 45, 431-40 (1941). “Denver SubA Flotation,” Bull. F10.B29,Denver Equipment Co., 1400-17th St., Denver, Colo. Faucounau, L., BUZZ.soc. china., [SI, 4 , 55-67 (1937). Cuyer, A., and Perren. R., Helc. Chi7n. Acta, 25, 1179-97 (1942). Ipatev, V., and Ipatev, V., .Jr., He?.., GZB, 856-90 (1929). Ipatev, V.. and T’erhorukii, V.,Ibid., 44, 1755-5 (1911). Perry, J. H., ed., “Chemical Engineers’ Handbook,” 3rd ed., 1). 1508, Kew York. McGraw-Hill Book Co.. 1950. P o i , L. 8 . ,“Froth Flotation, Industrial and Chemical Applica-
Vol. 44, No. 3
tion,” BUZZ. FIO.B46, Denver Equipment Co., Denver 17, Colo. (10) Bamans, C. H., “Engincering Metals and Their Alloys,” p. 152, New York, Maemillan Co., 1949. (11) Seidell, “Solubilities of Inorganic and Metal Organic Chemical Compounds,” I, 3rd ed., New York, D. Van Nostrand Co..
1940. (12) Shengle and Smith, J . A m . C h m . Soc., 21, No. 2 , 932-3 (1899). (13) Sneed, M. C., and Maynard, 3’. L., “General Inorganic Chemistry,” 4th ed., p. 790, New York, D. Von Nostrand Co., 1943. (14) Wise, W. S., Dodge, B. F., and Bliss, H., IND.
[email protected] 39, No. 5 , 632-6 (May 1947). RECEIVED for review April 28, 1951.
ACCEPTED September 28, 1951.
EngFnTring Bocess development I
RALPH E. PECK, RUSSELL T. GRIF~ITH’,AND K. NAGARAJA RA02 ILLINOIS INSTITUTE OF TECHNOLOGY, CHICAGO, ILL.
M
-4NY equations have been developed to calculate the time of drying to reach any given average moisture concentration for various drying conditions, but most of those equations are not applicable to materials of varying diffusivity or capillarity. The aim of this work is to develop equations relating the effective diffusivity t o the average total moisture content of material and the drying conditions. These relations can be applied in design calculations. The term effective diffusivity denotes the value of the proportionality factor not only for diffusion but for capillarity or any other force which may cause migration of moisture through a solid. This factor is a function of moisture concentration and thus varies throughout the slab. As all the data were obtained on a total veight basis, only average values of the effective diffusivity were obtained. Sherwood (10-15) was one of the first to approach drying calculations by utilizing the analogy of unsteady heat conduction in a slab. He considers that drying of fibrous materials takes place by diffusion of the moisture to the surface. Newman ( 7 , 8) applied the diffusion equation to the drying of solids of various shapes and various surface conditions. The latter equations take the surface film resistance into account, but are not explicit in concentration as a function of time and require a trial and error solution. Kamei and Siomi (5)found the diffusivity of moisture through paper, clay, and soap to change somewhat with temperature, humidity, and air velocity. Bateman, Hohf, and Stainm ( 1 ) reported drying curves for wood cylinders. Van -4rsdel ( 1 6 ) studied the effect of varying diffusivity in solids and outlined an approvimate numerical procedure to calculate drying rates. THEORETICAL ANALYSIS
The general differential equation for the unidirectional diffusion of the moisture through the sides of a rectangular slab is given by 1
Present addreaa, General American Transportation Corp., East Chicago,
Ind. 2 Present address, Consultant Djakarta, Indonesia.
(ECA)
Government
of
Indonesia,
Assuming D as constant, the solution of Equation 1 (5,4 ) can be written in the form (21
The constant represented by A, can be evaluated by the initial condition e = 0, C = COfor all values of 2. A,, then, is given ( 4 ) by
1
.
-4 sin 2ct, R
+ -21 a,R
The constants represented by a,, may be evaluated by applying the boundary condition a t the surface of the slab. At x = T
Applying Equation 4 to Equation 2 it follows that (5)
For large values of 0, i t has been shown (IS)that the series represented by Equation 2 is a rapidly converging series and for most practical purposes only the f i r s t term is important. Then AI and a1 need be evaluated. Equation 5 has to be solved by trial and error or by the help of trigonometric charts. However, approximations can be made t o evaluate a, for certain general cases. Where the solid offers the major resistance cot alR will approach H
2
- alR.
?r
This is seen to be true for small values of ,j - a,R by
reference to trigonometric tables. Therefore
March 1952
INDUSTRIAL AND ENGINEERING CHEMISTRY
Hence a, =
2(R
+ $)
(7)
D/k' is a length and accounts for the resistance of the air film a t the surface. Let D R+,=I, 12
(8)
Therefore (1,
=
7r
2L
1sin 2anR + 2 a&
Where alR approaches x / 2 , sin 2a1R approaches x - 2a1R. This substitution is good for thick slabs where the major resistance is not in the gas film'. Then from Equation 9 =
4Co sin alR
(10)
Substituting the value of A , in the first term of Equation 2 gives
a,R c = 4Co sin ex
Dall@cos
4%
4
4
A1
dZ
-because of this variation in k .
Cosin a,R 1
=
tion 15A is only good for thick specimens the data can be used this way where the major resistance is in the solid phase-i.e., where the slabs are fairly thick. D/k' has the dimensions of thickness and accounts for the sur1 face resistance and is seen t o vary as - where Vis the gas velocity. V" 1 1 If -- is plotted against -for one definite thickness the intercept V" should be the solid resistance. In the preeent investigation IC was found to vary with concentration and the data were correlated by Equation 19 so that I M = k when C = 1. In the present paper -is used in place of 1
A simplification of Equation 3 gives
A,,
665
alz
The average value of the concentration of moisture in the slab is given by
(12)
While Equation 15 applies to the ideal situation, it will have t o be modified in view of the following considerations: 1. The temperature of the specimen changes from the time it is placed in the dryer until the time of completion of the drying process. Similar temperature changes were observed in a series of exploratory runs on Balsa wood. The temperature of the slab was measured by placing thermocouples in three different locations. The substance first assumes the wet bulb temperature in a short time and from then on approaches the dry bulb temperature a t infinite time. The.change of temperature of the specimen and the consequent change in the vapor pressure relationships a t the surface should be taken into account in any rigorous treatment of the diffusion process in drying. P, and C, would always differ somewhat from wet bulb conditions. 2. During the drying process, the area through which diffusion takes place constantly changes. This may occur in more ways than one. The surface near the incoming stream is usually dried earlier than the center of the slab. The liquid interface may move into the slab and thus bring about a change in the area through which diffusion takes place. At the surface of the slab-i.e , when z equals R, the following relations hold good for heat and mass transfer:
This average is the concentration that is measured in all drying experiments and used in all the calculations. Using only the first term of the Equation 2
c
The change in with time is obtained by differentiating Equation 13 t o give Equation 14
For a particular thickness and constant diffusivity, Equation 14 can be written as dC' _ = -
kc
The area over which P, is effective will vary with the moisture concentration in most cases, particularly with lyophilic materials. The effective area for heat transfer will similarly depend on whether or not heat can be transferred through the edges. In the present investigation, the edges were coated with cellophane and diffusion was prevented from the edges. If the evaporative area, P, - Pa,is proportional to C,, then
Equation 17 is the usual boundary condition used by Newman and others in their derivations (7').
d6
where k =
Dd -.4LZ
TABLE I. DATAFOR RUN17 R
Substituting Equation 8 in Equation 15 there is obtained
1 =
&$(R
+E)
If 1 4 is plotted against R for constant boundary conditions the intercept where k = 0 gives the gas film resistance. .4s Equa-
(Weight of slab before drying 225.2 grams weight of the slab immediately after drying 45 grams dimensions of the Slab 6.9 X 4.0 X 0.85 inch, dry hulh temperature 140'' F., wct hulh temperature 83' F., humidity 10%) Time, Weight Dr ing Rate, Hours Lost. &am,/ (e) Grams c Mean 8 Hr./Sq. In. 0.0 4.0 R 70 n nfi
INDUSTRIAL AND ENGINEERING CHEMISTRY
666 I O
The third and impoitarit consideration is the change in the difiusivity that takes place during a particular run. This change has been ob0 seived by a number of author5 and attempts have heeii in a d v h y in a n v - - - - .autlioi 5 to derive .inalytical eupiei_--- r__4ioiiq aysuming ~ i ~ r y i ndiffusivity g 01 The analvtical pro02 - l o 29 30 40 veduieq lead t o Figure 1. Sample Plots o f Itate us. c for Runs at 10% liumidit? lather u n w i e l d y results. Besides, it is not alwavs possible to predict how the diffusivlty variep v4th the drying conditions The above considerations lead t o the conclusion that the changing boundary conditions and the changing diffusivity can be taken into account bv writing the rate Equation I 5 as 3
Iather
n hich
on simplification givr.
where p
+1=n
In Equation 18 the term C" is thus a correction term ivhich takes into account the considerations M e d . The data for balsa n-ood behave according to Equation 19. Stout et nl. ( 1 4 ) and Besser and Piret ( 2 ) observe similar behavior of rate data in their work on drying different materials. Equation 19 leads to the problem of integrating the rate equation to get the time necessary to dry a particular slab of material from a higher concentration to a lo~verconcentration. By plott'ing the data on a log-log paper with rate as t,he ordinate and the average, C, as the abscima, one obtains a straight line whose slope is n' and rat,e is W when C = 1. When the values of AI and n are known, Equation 19 can be easily int,cgratcd a s follows:
leas groups and for the oasc nlirrc )L equals 1 this equation rcduces t o the ordinary semilog plot encountered i n drying thcory. EQUIPMENT
The rect'angular slabs of balsa wood used in this stJudy were cut from the same stock. The thickness of the slabs ranged from about 0.2 to 0.85 inch and were all cut in such a manner that. when d pai,allclto tho dire(.placed in the dryer, the grain of the ~ ~ o ovas tion of the air stream. Previous to the test, Ihc samples were
Vol. 44, No. 3
soaked in water unt.il they attained constant weight m t l \ver(: dried in the dryer until they attained a low concentration. The initial free moisture concentration, designated as Co in t h e theoretical discussion, had a great influence on the rate 01 drying. The values of Co chosen for this study lay in a range between 2.6 to about 4. Table I summarizes the ranges of drying variables used in this study. Sample plots for runs a t 10% humidity are presented in Figure 1. The data for run 17R are presented in their entirety in Table I to demonstrate what data 'ivere taken during t.he experiment. The shell of the dryer, vhich is diagranimed in Figure 2, was fabricated from No. 20 gage, 18-8 stainless steel, of welded oondruct,ion. The dryer body wa mote fairly uniform air veloci ( m i s t w e floor space, The bod :isberlos insulation t o prevent loss through walls. l_)anip~rs placccl i n thc inlet : ~ n t iexit piper wcrc adjusted for control of quant,it,iesof fresh and recirculatcti air. The air velocity in the test section \vas evenly distributed by of a baffle installed ahead of t.he test section. This baffle ed of borosilicat,e glass tubc. \?-itti axe? p:mdIel t o the dirertion of flow.
Figure 2.
Diagram of Experimental Dryer
Screens i-iere placed betn-een the wet bulb and t,he test section to help give a better velocity distribution. Velocities were measured with a Hastings hot wire instrument. The velocity was determined across the duct and was relatively constant across the test section before inserting the specimen. A small observation n-indovl- was placed in the operating door so that the material could be inspected a t all times during the drying process. Both the operating door in the up er section and the maintenance door in the lower section were ruiber gasketed to prevent leaks. The air was circulated by a No. 1-H Aut,ovent volume blo\\-c:r. The fan was coated with Bakelite lacquer to prevent corrosion. A l/?-h.p., single phase, alternating current motor mounted on B. F. Goodrich Vibro-Insulators was used to drive the fan. T h c insulat'ors prevented transmission of vibrations that might impair accurate weighing of the sample. The air velocity in thc unit was regulated by means of a variable diameter pully on t h t . fan. Thorough mixing of air and water vapor was maintaii Iiy location in the lower sect,ion of two fanlike baffles with versed blades. HEATING SYSTEM.The air was %-armedby heating units constructed of Nichronie wire wound in open construction on frames located immediately after the blower. Four of the units, of about) 700 v a t t s capacity each, mere connected to s\vitohes and pilot lights on the instrument board. These were used to warm the dryer quickly and enough of them were used during a run to niaintain a temperature slightly below the desired operating t.emperature. A larger heating unit of 1300 watts capacity connected t o a Varinc t'ransformer, and a Leeds and Northrup temperature control unit was used to maintain the desired temperature. The temperature could be held a t any desired point in the rangc: of 80" to 200" F. without any perceptible variation. Twenty-four continuous hour recordings of thc dry bulb temperature ~ ~ r r ( 1 taken on a circular-type Leeds and Xorthrup recorder. t h i I r m Y I w SYGTEX.Tt'atcr vapor was introdurrd in t,hc lower section betmcn thr hcaters and the reversed fanlike bafflca by steam, which was generated in a sinall boiler located directly below t,he dryer. An 1800-watt heater was used only in warming up the unit. A second 600-watt heat,er was controlled (on and off) by a Leeds and Northrup controller. The water level in thc boiler was maintained by means of a float valve liquid-level control in the su ply tank. Build-up of pressure within the boiler was preventef by an overflow line leading from the boiler to the sewer. The overflow line was siphoning of liquid from t,he b The wct bulb temperalurc
March 1952
INDUSTRIAL AND ENGINEERING CHEMISTRY
Leeds and Northru recording unit. The distilled water supply for the wet bulb tRermometer wick was inside the dryer and, therefore, subject to the temperature within the system. To eliminate the possibility that the water drawn up into the wick would be warmer then the true wet bulb temperature, cooling coil and thermocouple were installed in the wet bulb t o cool the water to a point just above the true wet bulb temperature. Both the wet and dry bulb thermometers were located in the center of the air stream just before it entered the drying section. The wet bulb was shielded from the warmer surrounding bodies by being installed in a highly polished tube. The fluctuation of the wet
667
3. Have a uniform distribution of air velocity and temperature across the test section. 4. Afford a wide range of constant velocities a t a predetermined value. 5. Afford various ratios of recycled air. 6. Weigh the test sample automatically and continuously without interruption of the drying process. 1. Be corrosion resistant, particularly at high humidities. 8. Provide a means of visual observation of the test sample without interruption of the drying process. CORRELATION OF DATA
LO
28
k 24
22
20
0
01
02
03
04
06
OB
1 Figure 3. Plot of s ; ~us. V,,M
1
bulb temperature from the control point was =!=lo F. The wct tiulb recorder was the same type as the dry bulb recorder. Humidities were checked a-ith a Sargent Ventilated Hygrometer. RECORDING BALANCE (6). The samples to be dried were suspended from one end of a balance arm that extended into thc dryer through a slot out in the back of the dryer shell. A chain was suspended between the other end of the balance arm and a thin cord which was wound on a reel. The chain hung in a smooth catenary curve between the reel and the arm, and the balance was maintained by reeling in or letting out the chain as the weight of the sample varied. Velocities up to 150 feet per minute 10 did not hinder the proper operation of the balance. 40 This balancing device was controlled by means + J of an electrical circuit 30 with a contact plate on either side of the pointer 20 on the balance arm. A slight change in weight caused the pointer to I O touch one of the contact plates, which t h e n operated a relay, energizing a doubly wound THICKNESS-INCHES 110-volt reversible motor that was geared t o th(a Figure 4. Plot of Thickness vs. chain reel. The balance1 for Runs at 10% Humidity arm pointer circuit was T M energized b r a 45-volt dry -cell. A condenser was placed in parallel with the pointer t o reduce sparking of the contacts. A recording drum was located on the same axle as the reel. A recording pen was drawn transversely across the drum a t a uniform rate by a thread which passed over a pulley attached to the hour hand of an ordinary alarm clock. This pen gave a record of weight versus time. The initial and final weights were obtained by placing weights in a pan hung on the outer balance arm. Under the operating conditions used in these tests, the balance gave a sensitivity of better than 0.1 gram. The relationship between the length of chain taken in and weight lost was linear and equal to 4.25 inches on the recording drum per gram. A switch and pilot light for the reversible motor were placed on the instrument panel. The dryer described above was designed so that it would: 1. Maintain and record a constant dry bulb temperature a t a predetermined value. 2. Mgintain and record a constant wet bulb temperature a t a predetermined value. ~
All experimental data were taken on balsa wood. The plots for rate versus were all straight lines on a log-log scale similar to the ones shown in Figure 1 for 10% humidity. The plots show that the drying rate behaves as predicted by Equation 19. The rate used is based on the 70 area of the specimen and is expressed as grams of BO water lost per hour per square inch of the area of 50 the slab. M was calculated by reading the nu40 merical value of rate when 7k E equals 1 on the log-log 30 plot. The value of n is calculated by taking the 20 slope of the straight line on the rate versus plots. , I O ,’ 1 Values of M , n,and -
4 7
0
Figure 5. Plot of Thickness us.
1 for Runs a t 30% Hu-
4%
are tabulated for some runs in Table 11. 1 The value of - for
da
the various runs is utilized
midity
1 I
in two ways. plotted against
1 pr8 to
T~ is
obtain a fairly good straight line for the
data of Cheng and Griffith ( 4 A ) taken with 0.24-inch thick balsa wood slabs a t different velocities. Various exponents were tried for the velocity in the Figure 3 and 0.8 gave the best straight
TABLE 11. S U ~ M A OF R YRESULTS 1
Run No.
M
1R 2R 3R 4R 5R 6R 7R 8R 9R 10R 11R 12R 13R 14R 15R 16R 17R 18R
0.22 0.10 0.120 0 035 0.048 0,022 0.268 0.130 0.160 0.130 0 106 0.301 0.184 0.203 0,074 0 110 0.047 0.220 0.123 0.144 0.135 0.124 0.108 0.148 0.129 0.170
1ca
4c 8C
1oc
13C 7Gb
11G I9G a b
Cheng ( 4 A ) . Griffith ( 4 A ) .
a 0.55 0 817 0 815 1 950 1 850 2 140 0 836 1 890 1 550 1 660 2 090 0.930 1 060 1 140 1 540 1 850 2 280 1 320 0 566 0 517 0 542 0 565 0 565 0 666 0 588 0 635
2.13 3.16 2.87 5.35 4.54 6.73 1.92 2.77 2.50 2.77 3.06 1.82 2.33 2.22 3.68 3.02 4.61 2.13 2.55 2.63 2.72 2.85 3.04 2.60 2.78 2 42
Thiok-
H,
Bulb, Wet
Bulb, Dry
%%%
%
O F .
OF.
90 90 90 90 90 90 90 85 96 96 85 84 84 87 87 87 83 88 100
120 120 120 120 120 122 129 122 133 135 122 140 140 141 141 141 140 138 120 120 120 120 120 120 120 120
0.15 0.36 0.35 0.69 0.70 0.87 0.36 0.60 0.80 0.81 0.85 0,275 0.38 0.35 0.687 0.75 0.85 0.375 0.24 0.24 0.24 0.24 0.24 0.24 0.24 0 24
30 30 30 30 30 30 20 20 23 22 20 10 10
10 10 10
10 12 48 48 48 48 48 48 48 30
100
100
100
100 100 100 90
Vol. 44, No. 3
INDUSTRIAL AND ENGINEERING CHEMISTRY
668
line. This straight line is extrapolated to zero value of 1 axis. intercept of 2.2 is obtained on the
1 w8 and an
~
In Figures 4 and 5,
1
is plotted against thickness in inches
for 10 and 30y0humidity runs, respectively. The velocity of air
The change in the diffusivity and the boundary conditions of thc slab during the progrees of the run is taken into account by the introduction of ? in the theoretical Equation 15. The term M in Equation 19 is a function of the diffusivity and also of the total resistance to diffusion offered by the slab and the air film a t the surface of the slab. From Figure 5 the value of 1 .- dATi for a slab of 0.24-inch t,hicknessis equal to 2.75. This length is proportional to the total resistance of the slab) R and the air film, L-R. 1 From Figure 3 the value of __ a t infinite velocity or zero
dZ
thickness of the air film is 2.22. This length is proportional to the resistance offered by the solid only, R. Referring back to Figure 5 the intercept a t zero thickness is about 0.7 and this length, L -R, is proportional to the resistance offered by the air film. The total resistance in the case of 0.24-inch thick slabs is of the order of (2.22 plus 0.07) 2.92 or L. Figure 5 predicts a value of 2.75. The agreement is thus quite close. The air film resistance is thus
0'7$i00
or 24% of the total resistance, The correlations, t,here.~
fore, provide a method of evaluating the air film resistance and its contribut,ion to the total resist'ance. The correlations are limited to the case where the solid resistance is still the main resistance or where the thickness of the slabs is more than 0.2 inch.
("/.'"-,
Figure 6. Plot Showing the Scattering
of Data
CONCLUSIONS
used in these runs was 80 feet per minute and was kept constant a t this value.
For rectangular slabs of balsa wood, the plot of rate against average concentration (Figure 8) gives a straight line on log-log paper in the ranges of moisture concentration, humidity, air velocity, and thickness studied. The relation
Figure 6 is a plot of Equation 20 in which ordinate and -
(;)'I
-
?')
- 1 is the
(1 - n)M'O
- n) is the abscissa. In this relation
equals M A / W and converts the relation from a basis of square inches of area to a basis of unit weight of dry solid. DISCUSSION OF RESULTS
30
I
-
de-
M'
-,I{%"
theoretically derived is shown to be borne out by the experiments. 1 1 The plot of versus ~ 0 . 8is a straight line, which when extrapolated to the zero value of
1 vlo> gives an
intercept propor-
tional to the resistance in the solid phase.
The theoretical prediction of the relation between drying rate and the PO average concentiation iq n borne out fairly closelj by the expeiimental data. The agreement is very 10 close considering the fact that more than fifteen specimens were used in these investigations 0 0 02 04 06 08 IO Though the slabs were cut TUICKNESS- NCRES from the same stock they Figure 7 . Plot of Thickness differed in the the us. n fibers M-ere oriented in thcm The data taken by Cheng and Griffith ( 4 A ) on entirely different slabs closely follow the correlations presented in this paper. I n fact, the runs 1 or the correlation of velocity \I ith -- - resented in Figure 3 FI a
I. J , and Frank, A. C., Trans Am. SOC.Mech. Engrs., 68, 1135 (1946). (10) Sherwood, T.K., IND. ENG.CHEM.,21, 12-113 (1929). (11) Ibid., pp. 976-80. (12) Sherwood, T. K., Trans din. Inst. Chem. Enors., 23, 28-44 (1929). (13) Ibid., 27, 190-202 (1931). (14) Stout, L. E., Caplan, K. J., Raird, W. G., Ihid.. 41, 283-314 (1945). (15) Van Arsdel, W. B., Ibid., 43, 13-23 (1947).
RECEIVED for review December 9 , 1950
ACCEPTEDSovernber 15, 1981.
Operation of Pilot Plant Vinegar Generators EFFECT OF VARIOUS TYPES OF DILUTION WATER
RUDOLPH J. ALLGEIER, REUBEN T. WISTHOFF,
AND
FRANK M. HILDEBRANDT
U. S. INDUSTRIAL CHEMICALS CO., DIVISION OF NATIONAL DISTILLERS PRODUCTS CORP., BALTIMORE, MD.
T
H E quality of water used for industrial fermentations has become a problem of major interest in the past few decades. This factor of water quality is obviously an important one, if only for the reason that water constitutes such a large proportion of the solutions used for growth. Furthermore, in certain types of fermentations the water in the growing solutions becomes a part of the product. This is the case with beer, ale, and similar beverages and is also true in the case of distilled vinegar production with which this paper deals. I n early times river, pond, well, and spring waters were preferred by the fermentation industries, They yielded in most cases a soft, more or less organically pure water containing some salts necessary to the growth of organisms. However, as population increased, springs were not available and pollution problems forced the employment of treated water from municipal systems. Wells are still used by some plants. Even here, the water may be affected in unpredictable ways by changes in level of the water table which may in turn carry with it chemical alterations in the water itself.
Many studies have been made of the type of water suited for the production of beers and ales. Indeed, the quality of such beverages is, in some cases, determined largely by the type of water used, For manufacturers of vinegar, especially the socalled white or distilled vinegars, such studies have not been made or they have been inconclusive and not readily applicable to generator operation. One attempt to evaluate water was made by Wiistenfeld (3) who describes the type he considers best suited to vinegar manufacture. It has, therefore, seemed worth while to do some preliminary experimentation with the object of obtaining helpful information for the users of the modern Frings-type generators. GENERAL PLAN OF EXPERIMENTAL WORK
The first requirement in such a study is an experimental technique which will give information applicable t o large scale generators. A type of small scale generator has been described by Hildebrandt ( I ) with operation characteristics sufficiently close to those of the Frings generator to meet this requirement. Given
