C H E M I C A L
& M E T A L L U R G I C A L
E N G I N E E R I N G
v o l u m e 40 E s t a b l i s h e d 1902 n u m b e r 3
M C G R A W - H I L L P U B L IS H IN G C O M P A N Y , INC.
S. D. KIRKPATRICK, Editor
M ARCH , 1933
NOW, UNITED, WE G O AHEAD!
C V E N T S of the past fo rtn ig h t have clone
*“ m uch to subordinate com petitive and p a r
tisan interests in o rd er to accomplish a com
mon purpose— to c a rry on in m eeting the u rg en t dem ands of a national em ergency. W e have only started to fight but the very fact th at we have started on a united fro n t is en
couraging. F o r alm ost the first tim e since the fa te fu l crash of 1929, there is a hopeful change in public psychology. I t is as if we recognized a t last th a t all the processes o f tearin g down m ust be stopped and the drastic and disastrous industrial deflation b rought to an end. T he time has come w hen we m ust begin to rebuild and th a t process is one th a t calls fo r consolidation of energy and purpose, fo r cooperation in every line o f industry.
F o r m ore than th ree years we have concen
trated our attention on individual, company, o r industry problem s and, it m ust be adm itted, th a t th e record of achievem ent is not a source o f pride. W e have talked a lot about the basic problem s o f th e country b u t our actions, which speak louder th an w ords, have all been directed tow ard only one end— self preservation. W age an d salary cuts and long layoffs have curtailed purchasing power. P rice cutting and cut
th ro a t com petition have robbed us of profits.
All of this has been a p a rt o f th e vicious de
scending spiral in w hich we lost sight of our fundam ental interdependence— the fact th a t as a nation w e cannot continue to think and act in term s o f any group o r any given industry.
Tt took the banking crisis to m ake us realize th a t a fte r all we have only one interest to serve, th a t of the w elfare of the country as a whole.
W e have full assurance th a t this is the motive
th at will guide those men in W ashington who are charged w ith the responsibility of our n a tional governm ent. T hey m ust have the con
fidence and support o f all of us as individual citizens. B ut there is also a responsibility to the public interest on the p a rt of industry. It, too, is som ething we m ust recognize and su p port. T h e public dem ands th a t A m erican in
du stry retu rn s to the highest ethical plane of business practice. Com petition can no longer be ruthlessly destructive o f the gains th a t were built up th rough constructive cooperation over a period of years. F a ir w ages to labor, fair prices fo r raw m aterials, fa ir profits to the m an u factu rer and fa ir re tu rn s to the investor can only be fa ir and beneficial to the consum er.
All of these desirable ends will be attained when industry tu rn s once m ore to the ideals of co
operation and builds fo r the public interest.
* * *
In the chemical engineering profession and in m ost of the industries it serves th ere has been no serious breakdow n in ethical and busi
ness standards and practices. F o r th e m ost part, prices have been m aintained a t fa ir levels and w ith any re tu rn to w ard norm al produc
tion, profits to capital and wages to employees should be advanced proportionately. B u t it m ust be apparent to all of us th a t w e should not only continue on such a basis b u t by p re cept and example, we m ust aid in the process of rebuilding public confidence in A m erican industries and institutions. W e have reached the end of deflation and now can tu rn all of our energies in the direction of reconstruction—
once industry is really convinced of th a t f a c t
E D I T O R I A L S
Can’t W e Elim inate This Cause for Chiseling?
G
O V E R N M E N T A L extravagance lies a t the root of m ore of our present-day difficulties than are imm ediately apparent on the surface. I t can be shown, fo r example, th at one of the basic causes of th at evil w e know as “chiseling” is the fact th a t m uch of the bank credit norm ally available fo r the use of industry has been diverted to governm ental financing. In the present situation the federal, state and municipal authorities can borrow a t absurdly low interest rates.T h e result is a tem ptation to sell short-term paper to finance rapidly m ounting deficits. B ankers wishing to keep th eir institutions as liquid as possible rush in to purchase these governm ent obligations and thereby actually encourage extravagance in new expenditures o r in the m aintenance of existing services th a t are no longer needed.
In d u stry through its tax es m ust pay considerable of the bill fo r local, state, and federal governm ent.
W h en m ore people come to a realization th a t inflation o f governm ent is balanced by a corresponding deflation in industry, th ere will be a m uch g reater insistence on economy. P ro g ress in th at direction would help to re tu rn the flow of bank credit to industrial channels and thus elim inate one of the underlying causes fo r price cutting.
W ho W ill Speak For Process Industry?
P
R O C E S S IN D U S T R Y should have a com petent spokesm an to set fo rth its needs and interests w ith respect to pow er utilization and the developm ent of natural resources in the Tennessee R iver V alley. L ast m onth we explained w hy th at problem is im portant, and how partially advised public officials may go astray fo r the lack o f adequate technical advice. T he situation in W ashington, is, o f course, now largely overshadowed by the problem s of banking, budget balancing, and tax readjustm ent by the F ederal authorities. B u t this m erely postpones ; it does not elim inate the problem.M any process in d u stry groups could well join in having a study made o f th e Tennessee V alley situation.
A com m ittee o f outstanding individuals, like the T ra n s portation Com m ittee originally headed by Calvin Coolidge, would be highly desirable. Such a com m it
tee could form ulate a disinterested set o f findings re g ard in g the extent to which reforestation, utilization of
forest products and w ater pow er, and developm ent of m ineral resources is both practical technically and in the public interest.
M ay we suggest th a t such a com m ittee could very profitably be organized and financed fo r a simple straig h t-fo rw ard study? I t could be supported, fo r exam ple, by the fertilizer industry, electrochemical
and electrom etallurgical enterprises, by the principal groups using forest products such as the hardw ood dis
tillation industry, and by others. I f each such group would contribute m odestly to the cost o f such an in
quiry it would undoubtedly be possible to enlist a group of eminent, public-spirited individuals to form the committee and receive the advice of the supporting groups and others who m ight find any in terest in the m atter.
A long study or a very costly one w ould n o t be essen
tial. A few thousand dollars spent w ithin a few m onths might bring out convincingly to the public those real opportunities for advance w hich m ay be fo u n d in the Tennessee Valley, and show the im practicability of others th at have had popular su p p o rt because of the partisan propaganda. W h o will take the lead in such an enterprise?
Should W e P ut
Alcohol in Motor Fuel?
T
h o s e in our profession w ho m ay have talked rath er loosely about a chemical solution fo r the farm er’s problem may shortly have an opportunity to see the practical w orking out on a national scale of ju st such an experim ent. Technically th ere is no valid reason why alcohol cannot be m ade fro m excess grain and th ereafter m ixed w ith some p ro p o rtio n o f gasoline and used as a m otor fuel. B u t fro m an econom ic and practical industrial point of view such a p rogram as originated in the M iddle W e st and has now m oved on to W ashington is fra u g h t w ith m ost serious difficulties, both in policy and execution. I f th e fo rm er is to be decided fo r us by legislative o r executive fiat, the problem s confronting the chemical engineer are greatly simplified. W e can go ahead and produce the desired products in existing plants o r erect new ones even though we m ay have grave m isgivings about th eir immediate benefit to the fa rm e r and th e ir ultim ate effect on chemical industry.
Some who argue in fav o r o f the plan say th a t it is not to be judged solely on an economic basis. Eco
nomic soundness, they say, is relative fo r we m ust compare it w ith domestic allotm ent and o th er plans fo r farm relief m any of which are adm ittedly unsound.
T hat, in our opinion, is not a valid excuse fo r run n in g counter to the dictates of reason. I f w hat w e do to help the farm er is going to rise and plague him in years to come, the tim e to stop is b efo re w e sta rt. C er
tainly, it would be short-sighted fo r chemical industry to build, even w ith R .F .C . m oney, a strin g o f state o r privately operated alcohol plants th a t w ould continue to plague th at business long a fte r th e p resen t em er
gency has passed.
O u r considered opinion having in m ind th e best in terests of both the alcohol and petroleum industries, is th at chemical engineers should discourage and
114 Chemical & M etallurgical E n g in eerin g— Vol.40,No.3
oppose any drastic p ro g ram involving the erection o new plants to supply a m a jo r portion of the m otor fuel requirem ents of th e country. If a fa ir interpretation of the public in terest dem ands th a t the experim ent be tried, then it should be sta rte d in existing plants w hich are now operating a t less th a n a q u arter of capac
ity and could readily be changed over to the use o t g rain at a total expense of n o t m ore than $j,UUU,UW.
T h at it seems to us, is a fa ir wage considering the long odds th a t are against th e ultim ate success of any p rogram th a t opposes w h at seem to be sound economic
principles.
Proper V enting May
P reven t Secondary Explosions
S
E C O N D A R Y explosions are m ore serious than prim ary. O fte n it is the secondary w hich is called“ the explosion.” I f we stop the p rim ary puff there will be no big a fte rm a th .
T h ese fundam ental principles long vaguely recog nized lend special im portance to the chem ical-engineer
ing study presented in this issue by t h e engineers of the B u reau of C hem istry and Soils._ T h e ir w ork gives us a rational basis on w hich to forecast construction requirem ents if one is to control the d u st exp 0S' 0'1^
th a t inevitably m ay arise even in the m ost carefully planned and m anaged facto ry w here com bustible dusts m ust regularly be handled. I f buildings are constructed according to pro p er principles fo r venting, the bad con
sequences of these inevitable small “ pops” are m ini
mized and explosions of the p roportion of a disaster
need n o t follow. , , ,
O nly last m onth the w orld w as horrified by w hat occurred a t N eunkirchen, G erm any, w here an entire com m unity w as w recked by the explosion of equipm ent in the byproduct-coke dep artm en t of a small steel w orks. F u ll technical rep o rts are still lacking, b u t the m ore reliable new s services seem to i n d i c a t e clearly th at here again a m inor fire and explosion m the ta r-h a n dling facilities w as the initial cause o f the s e c o n d a r y
blast w hich did such terrib le dam age. T h is would seem to d em onstrate th a t the principles o f explosion venting, studied p rim arily fo r dusts by these B u r e a u engineers, could well be considered in any w orks h an d m g flam
m able gases o r vapors w hich m ay occasionally become m ixed w ith a ir in explosive proportions. T oo rnuc care and fo reth o u g h t cannot be given to this vital con
sideration of sa fe p lan t operation.
Secondary M aterials in Chemical Com petition
O
F L A T E it has become evident th a t secondary chemicals are exerting m ore th an th eir traditional effect on p rim ary m arkets. Recovery, h a s b e e n v a s t l im proved in m any instances and is being b ro u g h t evenM arch, 1933 — C hem ical & M etallurgical Engineering
closer to theoretical perfection. Ju s t w itness the case of a new sludge-acid process w hich is said to take any unseparated sludge acid, regardless of strength, and convert it into a clean S 0 2. T h e gas is then p u t tb ro u g a contact plant fo r the production of close to 100 per
cent of w ater-w hite acid of any strength fro m 98 per cent up. M akeup is readily supplied in the fo rm ot sulphur fo r the production of acid equal to o r larg er in am ount than the original acid. A lthough the process is still too new to hazard predictions, it m ay conceivably p u t refiners in the acid business, a t least to the ex ten t of supplying all of th eir own requirem ents.
T h ere are other cases w here recovered chemicals have clearly become secondary m aterials, w ith all th at secondary status implies. A n enterprising oil refiner has been successfully m arketing a special pickling acid for which the user pays a prem ium because of the content of certain inhibitors th a t naturally find th e ir way into the acid during its original use in the tre a t
m ent of distillates. M ost viscose rayon m akers are sell
ing a dilute contam inated caustic soda w hich is recov
ered from th eir steeping presses. Investigation of dialysis processes fo r p u rify in g such caustic is being actively pursued. A n d there are o ther heavy chemicals and certain solvents th a t m ay soon require considera
tion for th eir secondary status. A lthough sweeping generalizations cannot be m ade because each exam p e is likely to be peculiar to itself, nevertheless a generali
zation th at is both timely and safe is th a t such develop
m ents are fo be watched. Ju s t as m etal producers give heed to secondary supplies, chemical m anagem ent m us watch the signs and p ortents w here they bid fa ir to influence prim ary production.
Potash Independence Guaranteed for 1934
'n O T A S I I M IN IN G is to be undertaken by a second P r e s p o n s M e and well-financed c o g p a n , ^ t a c h ,s now startin g to sink a sh a ft near Carlsbad, N . M. E arly this sum m er operations are expected to be w e l l under w ay R eliable estim ates indicate full-scale production before the end of the calendar year. I t is im portant, therefore, to take new stock of our potash situation.
Bv the beginning of the active shipping season m 1934 the U nited States capacity for production of potash will apparently be about equal to dom estic r quirem ents. T h e m ajo r producing um ts will be S^ _ ceotible of considerable fu rth e r expansion. I t i s j v dent therefore, th a t national independence of foreign i u i U s of potash is an assured thing, even though im proved agricultural conditions, m c r e a s c d fertih zat on and a rejuvenated fertilizer business should re-establish as g reat a dem and as in the peak of the pas decade Am erican industry can, w ithout an u n w arran ted spirit of chauvinism , take g reat pride in th is achievem ent o the potash producers.
115
V e n tin g Dust Explosions
By HYLTON R. BROWN A N D RICHARD L. H A N S O N
C hem ical E n g in eers B u rea u o f C h em istry and S o ils
W a sh in g to n , D. C.
S
O M E du st explosions are certain to occur in industries which necessarily produce com bustible dusts.
T h e serious consequences o f such accidents are m inimized by the pro p er use of light construction, hinged panels, o r large glass and sash areas th a t give quick vent
ing at low pressures. B uilt up pressures produce undue spread o f flame and secondary explosions, which are responsible fo r the m ajo r p a rt o f the life loss and dam age to stru ctu re and equipment.
S afety codes prepared by the dust-explosion hazards com m ittee o f the N ational F ire P rotection Association u nder the leadership o f the chemical engineering division of the B ureau of C hem istry and Soils recom m end for venting a t low er pressures several types of sw inging vents. Some are held shut by w eight o r inertia alone;
others use a light friction catch. A friction device also is used to resist the opening of the vent th ro u g h o u t its sw ing and to hold it open. Still o ther vents, particularly sash, m ay be pushed from the fram e in such m anner that the explosion m ay pass out on all sides. T his opening is lim ited by folding o r sliding brackets
a t fo u r points. A nother type of o u t
let is form ed by the application of louvres.
I t is not the purpose of this article to com pare the m erits of the various types of sw inging vents, but ra th e r to show th eir desirability, as com pared w ith the use of fixed glass.
All o f the types have th eir peculiar advantages, depending upon the type of structure, the cost, and the fre quency and severity of the explo
sions to be anticipated.
In o rd er to obtain accurate data on the venting area required for various explosive dusts w ith the d if
feren t types of venting, special equipm ent has been installed a t the A rlington, Va.. experim ental station of the U . S. D epartm ent of A g ri
culture. T h is experim ental stru c
ture, in which the v enting o f du st ex
plosions can be studied, consists of a room , a gallery, and a tow er. The fixed glass window s in the structure provide a facto r of safety by b reak ing w hen the venting area is not sufficient to prevent the developm ent
o f excessive pressures. D ust m ay be sprayed into sus
pension in the air w ithin the stru ctu re from hem ispherical cups by jets of com pressed air. Ignition is produced by a glow ing electric heater coil. P ressu res produced by the explosions are recorded by rubber diaphragm m anom eters, installed a t appropriate points in the stru ctu re, to record pressures up to 800 lb. p er sq.ft.
T he m ethod of m aking the tests is as follow s: T he charge of du st to be used is placed in the cups, the vents to be used are unlatched or set to operate, the m anom eters are placed and the ignition coil is heated to its ap proxim ately constant tem perature. T h e com pressed a ir line leading to the je ts above the d u st cups is then opened, and as the d u st cloud form s around the coil, the explo
sion occurs.
Too m uch dependence m ay not be placed on fixed glass as a m eans of venting explosions in in d u strial plants.
S tan d ard factory sizes, 12x18 in. and 14x20 in., of double strength A -quality glass set in steel sash and wooden fram es has in a num ber of cases w ithstood pres
sures of m ore than 300 lb. p er s q .f t.;
and in several cases p ressu res of 400 lb. p er sq.ft. o r m ore w ere re
corded w ithout breaking the glass.
A ssum ing 300 lb. p er sq.ft. as the pressure such glass could be e x pected to w ithstand, the to tal p res
sure exerted on a 4 x 8 ft. w indow sash would be alm ost five tons. Such pressures are capable of causing serious dam age to both stru ctu re and equipm ent. T h is fact explains w hy in a num ber o f facto ry ex plosions, the entire steel sash has been blown out of its anchorage in the building w all and indicates the necessity o f providing som e form of explosion vents capable o f op erat
ing a t m uch low er pressure.
A ssum ing th a t the b reaking of glass indicated the pressu re a t which stru ctu ral dam age w ould occur, sev
eral series of tests w ere planned to d e te rm in e : ( 1 ) v enting area per u n it of volum e necessary to prevent stru ctu ral d am age; ( 2 ) effect of location and d istribution of v e n ts ; (3 ) effect o f differences in size and type of ig n itio n ; ( 4 ) reduction of D ust cups a n d e le ctric ig n ito r. E ig h t
6 in. cups in each o f th e th re e sec
tio n s of th e stru c tu re p e rm it dust- c lo u d fo rm a tio n in d ep e n d en tly o r
to g eth e r
116 Chemical & M etallurgical E ngineering — Vol.40,No.3
E x p erim en tal ex p lo sio n s tru c tu re . T h e ro o m 4x5x5 i t . ; th e gallery 2.5x2.5x20 f t.; th e tow er 3x3x12; to ta l v o lu m e 333 cu.ft. V ents o n room 0.1 to 1.55 sq.ft. each are h in g ed m eta l do o rs a n d p iv o ted sash of a p p ro x im ate ly 7 sq.ft. of glass. G allery has 8 h in g ed d o o r ro o f vents 1 sq.ft. each, an d 8 fixed glass w indow s 1.3 sq.ft. each. T o w er in clu d es o n e sw inging p an el 7 sqJEt., th re e h in g e d d o o rs 1.55 sq.ft.
each, a d ju s ta b le v e n t on top an d fo u r glazed w indow s 1.7 sq.ft. each
T y p ical ex plosion w ith o u t b reak ag e of glass
A lthough w as a .
had .a bearing on the results obtained, the general con-
elusion reached w as th a t to provide satisfacto ry venting before these pressures could be released th ro u g h open- fo r an explosion of starch d u st in a cube-shaped or ap- ings at the end of the gallery of the size recommendet proxim ately square room of small size, 3 sq.ft. of vent for cube-shaped room s. <•
fo r each 100 cu.ft. of volum e w ould be necessary. A few shots in the tow er indicated th at the same condi-
• W ith g rain d u st slightly different results w ere ob- tions m et in long narrow galleries also existed in deep, tained. S ix ty -six explosions w ere produced, w ith d if- sm all-diam eter bins, and venting a t th e top only, in ic feren t arran g em en ts of the vent openings. T h e am ount ratio recom m ended for.room s, .would not be satisfactory, o f vent opening in the room w as g radually reduced to In the room tests w ith the ignition at the center and the point a t which breakage of glass occurred. Based on the vents centrally located in each panel little coulc be th e resu lts obtained, it is recom m ended th a t in cube- done in the w ay of studying the effect of location and dis- shaped o r approxim ately square room s of small size, tribution of vents. T h e re w as an indication, how ever, 14- sq .ft. of vent be provided fo r each 100 cu.ft. th at one o r tw o large vents w ere m ore effective than sev-
A lthough no extensive series o f tests w ere m ade w ith eral sm aller ones totaling the same vent area is pro - o th er dusts, th ere w ere enough to indicate the pressures ably being due to the resistance offered to flow th ro u g 1 produced in com parison w ith starch and g rain dust. T he small openings. ,
results obtained w ith sugar, wood flour, sulphur, cork, T h e effect of location and distribution o f vents w as pow dered m ilk, soap pow der, and sim ilar products indi- m ost successfully studied in the gal ery tests In t es cated th a t the venting necessary fo r these dusts ranges tests the gallery w as used as a closed unit. he ignition betw een the requirem ents fo r starch and g rain dust. in each case w as located as shown in the center of
W h en the tests w ere extended to include the gallery gallery section below vents G and H . I h e m anom ete and tow er, the results indicated very definitely th a t the w as located as shown. G rain d u st w as used throughout, shape of the room is an im p o rtan t facto r to consider in Table |_ G au ery Tests (Grain Dust)
d eterm ining th e am ount an d the location of the vents re- No Total Vent GlaM Averse ? vent per q u ired to release explosion pressures w ithout stru ctu ral Vent* u.ed tmu Are**, Sq.Ft Broken Pr* ^ re 100 dam age. I t w as found, fo r instance, th a t an explosion ..."••_••••• l\1 sone 2M
prop ag atin g th ro u g h the gallery, which is 2^x2£x20 ft., 2 i.'o 312 *■'*.
p r o d u c e d a g u n b a r r e l e f f e c t w i t h r a p i d l y i n c r e a s i n g p r e s - h k m p (part opening). 3 i.7 None 153
sures. Glass along the side of the gallery would be broken 2 i.s None m 1.2
P u b lic a tio n o f t h i s r e p o r t is a u th o r iz e d t>y th e S e c r e ta r y o f A g ri- F ro m this table it m ay be seen how vents N and I a t t0 the opposite end from th e ignition are inadequate pro -
M arch, 1933 — C hem ical & M etallurgical Engineering
tection, although they m ore than conform to the total vent required fo r a cubical stru ctu re. Conversely, \ G provided all the vent necessary to prevent the breakage of glass. H e re the vent was m uch less than th a t required in a cubical stru ctu re of sim ilar volume. T h is vent, being so close to the ignition source, dissipated the pressu re rapidly and perm itted a very slow propagation tow ard the closed end of the gallery. T h is type of protection can not be advised because the exact point o f ignition in an indus
trial plant explosion is not predeterm ined, but it does show how much m ore effective the vent becomes when it is located close to the ignition source.
W h en the venting was distributed along the gallery and either four vents or all eight were used, the am ount of safe opening corresponded closely to th a t fo r the room tests. T h is is shown in the low er p art of T able I. A s the venting of the gallery a t the end opposite the ignition was extended to include N P and sufficient protection was furnished to low er the pressures and prevent glass damage. I t m ay be seen th a t this is not entirely due to the increased vent, but also to the increased distribution.
T h is advantage o f using distributed vents is still fu rth er em phasized in a com parison of several explosions of starch dust. W ith 3} sq.ft. of vent evenly distributed throughout the length of the gallery an explosion of starch d u st ignited by a flash from the room produced a pressure of 155 lb. p e r sq.ft. W ith 4 sq.ft. o f vent dis
tributed in the half of the gallery rem ote from the source of ignition an average pressure of 470 lb. p er sq.ft. was recorded. W h en all vents along the gallery w ere closed and the end le ft entirely open to provide 6$ sq .ft. of vent, large am ounts of glass w ere broken and p ressures w ere produced, exceeding the m anom eter scale but estim ated to be as high as 1,500 lb. per sq.ft.
I f any elongated stru ctu re is to be protected against dust explosions, w hether it be tow er, gallery, or L-shaped building, the building should be sectioned or zoned into cubical units. T h e vents m ay then be applied fo r each of these units a t the lim its of th e u nit itself. T h a t is, vents should be in the am ount recom m ended fo r the p a r
ticular dust, and fo r the volum e of th e cubical u n it and should be located in the roof o r o uter wall enclosing the cubical unit or zone being protected. F ro m this it may be seen th a t w hen a dust hazard is located in the center of a building between floors w here the distance to an outer wall exceeds the height of the ceiling, venting is difficult and complicated.
U ndoubtedly the size and type of ignition is responsi
ble fo r m any of the differences noted in du st explosion tests. (L o w er L im its o f C oncentration fo r E xplosion of D usts in A ir, by L . J . T rostel, and H . W . F rev ert, Cliem.
& M et. Vol. 30, Jan . 2S, 1924.) N o attem pt will be made in this article to draw a fine distinction betw een different types of ignition. A m arked difference should be noted, however, between w h at m ay be term ed p rim ary and sec
ondary ignitions. F o r instance, explosions in the gallerv and tow er occurred much' m ore quickly, and the pres
sures rose m uch m ore rapidly, when du st clouds in those parts o f the stru ctu re w ere ignited by a flash from a prim ary explosion in the adjoining room than w hen the coil alone w as used as the source of ignition. T his difference is believed to be due to the fact th at the flash
from the p rim ary ignition in the room , w hen p artly vented th rough a sm all opening into the gallery, took the form of a flame 8 ft. o r m ore in length. T h e flame ignited the du st cloud at about the sam e tim e th ro u g h o u t its length w hile the coil in the gallery itself produced an ignition a t only one point, and it w as necessary fo r this ignition a t the coil to propagate th ro u g h the d u st cloud, relatively a m uch slow er process w hich produced a slow er p ressu re rise.
E xplosions o f g rain du st ignited by a coil in the gallery produced an average pressu re of about 180 lb. p er sq.ft., w hen 3 sq.ft. of vent w as provided. U n d e r th e same con
ditions except th a t the ignition w as produced by a flash from a p rim ary explosion in the adjoining room, the average pressu re w as about 280 lb., and in one case glass w as broken. W ith g rain d u st ignited by the coil in the gallery a pressu re of 235 lb. p er sq.ft. w as recorded, w ith 2-f sq.ft. of vent. W ith the same vents, w hen the du st cloud w as ignited by a flash from the room, the pressures averaged m ore than 400 lb.
W h en the vent in the gallery was reduced to 2 \ sq.ft., an explosion of g rain du st ignited by a coil in the gallery produced a pressu re of 400 lb. p er sq.ft. W ith the same am ount o f vent and a sim ilar d u st cloud ignited by a flash from an explosion in the room , the pressu re w as 475 lb., and tw o w indow s w ere broken.
R esults indicate clearly the added difficulty encountered in try in g to vent a secondary explosion, and em phasize the advantage of providing sufficient openings a t all points in the stru ctu re to p roperly vent any p rim ary ex plosion a t the origin. B y such m eans the secondary e x plosion m ay be w holly prevented.
T h e venting area p er u nit of volum e previously recom mended in this article is believed to be the m inim um th a t should be used to provide protection against stru ctu ral dam age. W h ere additional protection is desired o r neces
sary to prevent dam age to equipm ent, e x tra vents should of course be provided. In fact any additional venting th at can be provided will pay good dividends should an e x plosion occur, because as show n in a study of the tests, the explosion p ressures drop rapidly as the venting area is increased beyond the m inim um am ount recom m ended.
T o show this reduction the p ressures recorded w ith d if
ferent vent openings have been plotted and curves draw n.
1 he data given in the diagram are fo r cube-shaped units or small square room s, and pending the securing of additional test data application to long n arro w room s and galleries or high tow ers and bins can be based only on data from a few shots in the gallery and tow er. T hese tests indicate th at the provision of vents in approxim ately the sam e ratio of area to volum e as used in the room will be satisfactory, provided the venting area is distributed in such a way th a t each cubical u nit of the stru ctu re has its share of vent. I t is presum ed th a t the increasing of this evenly distributed venting area will result in co rre
spondingly reduced pressures, as w as the case in the room.
D iagram of gallery
Room \
N I M
yianomehr r ? 1 r-7
L
__ r-7 K
1T7 J
C7
H I
....1 T o w er
118 Chemical & M etallurgical E n g in eerin g— Y'ol.40,No.3
R e d u c tio n of p re ssu re d u e to in crease of v e n t a rea in cu b ical s tru c tu re
D u rin g the m aking of tests to determ ine m ethods of venting o r otherw ise reducing the pressures th a t would be built u p in a room o r building w here dependence had been placed on fixed glass fo r venting, it w as suggested th a t outside glazing m ight p erm it the glass to be blown out of the fram es a t a pressu re below th a t required fo r breaking. A ccordingly, th e steel sash w as reversed, and a series of tests arran g ed to determ ine the pressu re at w hich glass w ould be blow n out w hen placed in the fram es from the outside w ith clips only, w ith plastic putty, an d w ith commercial putty.
T h ese tests w ith clips w ere to determ ine th eir relative resistance ra th e r th a n to determ ine th eir practicability.
T h e average pressu re a t w hich glass w as blow n out w hen clips alone w ere used w as about SO lb. p er sq .ft. P lastic p u tty of a stan d ard type w ith fo u r au x iliary clips was sim ilarly tested. T h is glaze required b u t 100 lb. per sq.ft. to blow o u t th e glass. In like m anner, com m ercial p u tty w ith fo u r clips p e r pane w as tested a fte r a few days aging. T h is required about 200 lb. p er sq.ft. to blow out the glass. C onsiderable added resistance is anticipated when glazing w ith com m ercial p u tty is tested fro m six m onths to a y ear a fte r installation.
T h ese results indicate th a t outside glazing w ith plastic p u tty has certain advantages w o rth considering in provid
ing venting area fo r the release of explosion pressures.
O utside glazing, how ever, probably w ould be m ore diffi
cult and m ore expensive, and replacem ent of broken panes would be m ore hazardous.
A n o th er m ethod suggested fo r reducing th e pressu re a t w hich fixed glass would break and th u s provide ad d i
tional ven tin g fo r an explosion before destructive pres
sures w ere reached w as to score o r p a rtly cut th e pane in such a way th a t com paratively low pressures w ould fra c tu re the glass. F ro m the results o f the few tests it w as concluded th a t this m ethod h ad some m erit. U ndoubtedly
the depth and the design of th e cut will affect to some ex ten t the breaking stren g th of scored glass. I t is be
lieved, however, th a t such cuts will p erm it glass to be blown out at from one-half to one-third the pressure n o r
m ally required to break it.
I t should be understood th a t such scoring of glass is not recom m ended as a substitute fo r m ovable vents but ra th e r as additional protection w here large areas of fixed glass are used in addition to the recom m ended am ount of vent. In fo rm atio n is not available concerning th e pos
sibility of fractu res occurring in scored glass due to ex
trem e o r sudden changes in tem perature o r to building vibration, but no breakage from this cause occurred d u r
ing the tests.
T h e series o f du st explosion venting studies originally planned have not been com pleted and only tw o dusts have been used to any extent, b u t it is felt th a t the results ob
tained and rep o rted in this article are sufficiently definite to indicate t h a t :
1. It is possible to vent dust explosions without structural damage.
2. Fixed glass offers too much resistance to permit de
pendence upon it alone for the release of explosion pressures without structural damage.
3. Many types of venting equipment, hinged doors, win
dows, and panels may be satisfactorily used to release explo
sion pressures provided sufficient venting area is used.
// / t /
\\
\
\\
\X
\
\V //
// /
a b c d
88 and 125 90 a n d 100 120 and 145 110 and 110 ( P o u n d s per Sq.Ft.)
No+e: S o lid lin e s in d ic a te cuts on outside s u rfa c e ; d o t t e d fin e s on in sid e su rfa ce
B re a k in g stre n g th of sco red glass
4. Vents near the source of ignition are more effective than those located some distance away.
5. The venting area required varies for different dusts.
6. Secondary explosions are more difficult to vent than primary explosions.
7. A definite reduction in pressure results as the venting area is increased.
8. Pressures may be released by lowering the resistance of fixed glass by mean* of outside glazing or by scoring.
B IB L IO G R A PH Y ON D U ST EX PLO SIO N S
D u st Explosions, P ric e an d Brown. N a tio n a l F ir e P r o t « t l ^ A l s o c l « t l M . B ostan, 1922.
A Review of Some R ecent D u st E xp losions, D av id J . P ric e and H y lto n H. Brown.
Safety C odes"ror^the P rev en tio n of D u st Explosions, U. S. D ept, or L ab o r B ui. No. 5 6 2 , New D a ta o r^O x y g en Concentration fo r Explosion P re v en tio n , P a u l W . E dw ards and
R . W . H a rris o n . Chcm . <t 1l e t . . A u g .. 1828- , „ ,
D u st M enaces I n d u s tr ia l P la n ts . D avid J . P ric e , Chem. <f M e t., M a r., 1 9 2 8 . Some Chem ical an d E n g in e erin g A spects of D u st Explosions, D av id J . I ric e . Chem.
S a fetjM n C r l n i i n ^ l u l p h u r an d H a rd R u b b e r, H . W . F re v ert, Chcm . & M e t., Dec. 8.
A P e c u lia r D u s t E xplosion, W. A. Noel. Chem. J M e t. ; A ug. 1 1 , 1 9 2 4 .
An In v estig a tio n of th e D ev a sta tin g Starch D u s t E xplosion a t I ekin !111, D ai Id J . P ric e , H y lton I t. Brown and P a u l W . E d w ard s, Chem. 4 M e t ., A p r. 1 4 , 1 9 2 4 . Low er L im its of C oncentration for E xplosion of D u sts In A ir , L . J . T ro ste l an d H . \ \ . H a z a f d ^ o f 'F la 'm m ib lf D u st' Shown 2ln ’ i R ecent E xplosion of A lum inum Pow der, D avid
J . P ric e and R o b ert M . B ak er. Chcm. i M e t . Nov. 1 2 . 19 2 3 .
P re v en tin g D u st E xplosions an d F ire s , D avid J . P r ic e . Chem . £ M et . M ay 21 19 2 3 . D ry L im e -S u lp h u r an d S u lp h u r D u st Explosions, F a u l W . E dw ard*, Chem . & M e t., P ro g re ss' M ade by2 F lo u r M illin g In d u stry In D u st E xplosion P re v en tio n , D a r Id J . P ric e , An Explosion o ^ H a r d ^ u b b e r D u st, D avid J . P ric e and H y lto n B . Brown, Chem . &
D u s t '^Explosion's— Some M ethods of P re re n tio n an d E n g in e erin g P ro b lem s D em an d in g A tten tio n . D a rid J . P ric e , Chem . * M e t.. M ar. 1 6 . 1 9 2 1 .
E n g in e e rin g P roblem s in DuBt E xplosion P re v en tio n , D av id J . P r ic e , Chem . d M e t., A D isastro u s E xplosion of A lum inum DU3t, D avid J . P r ic e , Chem . <f M e t., Nov. 1 0 ,
1 9 2 0 .
March, 1933 — C hem ical & M etallurgical E ngineering 119
M arch of Electrochemistry
By C. L. M AN TELL
C onsulting E n g in eer P r a lt In stitu te B ro o k ly n , N . Y .
T
H E C O M M E R C IA L developm ents of the electrochemical industries in th eir present day ex ten t is less than 50 years old, but the entire basic science and a rt go back little m ore than a century.
Electrochem ical processes require electrical energy, the existence o f w hich w as to all intents and purposes unknow n before 1800 w hen V olta discovered his voltaic pile. A : num ber of these couples together w as the first source of electrical energy used to break up w ater into its elements and provide fo r D avy’s arc in 1807. D avy’s assistant, F arad ay , discovered electrom agnetic induction as well as the underlying laws of the action of electrical energy on chemical substances and solutions in 1834.
D aniell’s chemical battery, invented in 1836, furnished a new and m ore pow erful source of electrical energy.
T h ro u g h its use came electroplating, developed by the Elkingtons in E ngland fro m 1836 o n ; and electrotyping by Jacobi in 1839. W rig h t in 1840 w orked out the basis
fo r plating solutions. In 1849 we find cadm ium plating discovered and only rediscovered some 70 years later w hen comm ercial quantities of the m etal became avail
able. B unsen m ade m agnesium in 1852 and lithium in 1855.
In 1859 P lan te learned how to store electricity in his lead storage cell. A lthough W o h ler m ade calcium car
bide in 1862, it w as 30 years later w hen try in g to make alum inum th a t W illson, an A m erican, stum bled on it again, to s ta rt th a t p resent day gigantic industry. In 1868 Leclanche m ade his cell o r battery, w hich in m odi
fied fo rm is the present day dry cell. A dam s com m er
cialized nickel plating in 1869 to m ake possible the nickel plated handle-bar of the bicycle age.
T h en began a new era of larg er quantities of pow er.
T he electrical generating m achine w as built. In 1875 A nthony and M oler made the first A m erican dynam o.
W eston in 1874 had built a low voltage plating g en era
120 Chem ical & M etallurgical E ngineering — Vol.40,No.3
tor, and five years later E dison produced the carbon fila
m ent incandescent lam p and B ru sh the carbon arc lamp.
In 1876 B ell’s telephone could operate w ith b a tte rie s ; and C arre, a F renchm an, had learned how to m ake carbon electrodes. E lkington learned how to refine copper elec- trolytically. Som e fo u r years later the Balbachs and the T h u m s w ere producing refined copper in the U nited S tates an d w orking out m echanism s to refine silver.
T h e year 1882 b rought fo rth the first electric m otor.
Electrical m achinery developm ent m oved apace. P a rso n s steam tu rb in e in 1884 and B rad ley ’s ro tary converter in 1887 w ere im p o rtan t developm ents.
H a ll in 1885 w orked out his alum inum process.
G assner in 1888 had converted L eclanche’s cell into its p resent day d ry form . C astner needed sodium to m ake alum inum , an d in 1890 had invented a cheap m ethod to m ake the m etal. H is m ercury type alkali-chlorine cell is operating today. A cheson h it upon silicon carbide, the first synthetic abrasive. Tesla, another of E d iso n ’s co-w orkers, m ade a m o to r driven by altern atin g cu rren t.
E lectric furnaces and electrolytic cells w ere still in th eir in fa n t stages but ready to grow up a t a trem endous rate. U n d e r the leadership of A dam s, the pow er plants a t N iag ara F alls w ere begun and blossomed in 1894.
H e re w as pow er in large quantities.
B ut new industries did not cease to be born. In 1895 M oorehead m ade 50 per cent ferrosilicon, so needed by th e steel m aker. A n overheated silicon carbide furnace in 1896 gave A cheson graphite, a lubricant superior to
the n atu ral m ineral. T h e tw entieth century w as born b u t not before Jacobs had learned to m elt alum inum oxide to b rin g fo rth a new , better, and stro n g er “ em ery,” an
other abrasive.
A t the daw n of the century, E dison b rought fo rth his nickel storage cell. H oopes in 1900 outlined the steps fo r the m an u factu re of p u re alum inum from H a ll’s prod
uct ; b u t he did not live to see it commercially produced alm ost a q u a rte r of a century later.
Conscious of th e grow ing im portance of the field, a g ro u p of leaders came together in 1902 to organize the A m erican Electrochem ical Society, which was to ^grow so international in scope th a t the term “A m erican w as later dropped. Becket, tw o years later, developed new an d p u re r ferro-alloys th rough the use of silicon instead of carbon. W h itn ey proposed the electrochemical theory' o f corrosion. B etts refined lead, B urgess iron, and C ot
trell b rought fo rth his m ethod of electrostatic precipita
tion of d u st and fum e.
T h e year 1905 b rought the cyanam ide process of F ra n k and Caro, whose w ork w as the basis of the trem endous nitrogen fixation plants of the U . S. G overnm ent in the W o rld W a r, w hen nitrogen com pounds w ere sadly needed fo r explosives; as well as the basis of the peace tim e uses of cyanam ide, fertilizers, am m onia, cyanides, and urea. D eF o rest in 1906 w ith his radio tube detector opened up the new field of electronics to th e w orld.
Iiy b in e tte in 1906 had w orked out the kinks of com m er
cial nickel refining.
March, 1933— Chem ical & M etallurgical Engineering ♦ 121
T hen came a lull. Chile had trem endous am ounts of low grade copper deposits. M illions w ere spent in w o rk ing out processes to win copper from ores o f 1.5 per cent copper, culm inating in 1912 in a trem endous de
velopm ent b ringing fo rth the lowest production cost copper in the w orld. T h e realm of electrow inning opened. A naconda and others in 1915 applied related m ethods to zinc to produce a p u re r form , and tw o years later byproduct cadm ium . M agnesium w as of g reat in
terest. In 1915 it sold fo r over $1 per lb. T h e annual production from byproducts of salt brines has grow n to over 1,000,000 lb., to sell a t one-third the original price.
A gain in d u stry had caught up w ith itself. N ew tools w ere needed. Electric furnaces o f increased utility w ere designed to find g reater application— the A ja x W y a tt in 1916; N o rth ru p ’s ingenious high-frequency furnace the same y e a r; and before the W o rld W a r ended the D etroit rocking furnace now widely used in th e non- ferrous m etal foundries. T he year 1919 b rought electro
lytic w hite lead. T h e follow ing year the D ow ns cell allowed the production o f sodium fro m cheap salt in
stead of caustic. E lectric furnace phosphoric acid was
Chemical Industry and the Five-Day W e e k
By FRED D. HARTFORD
D e n ve r, Colo.
S
U P E R V I S I O N o f the factory over th e w eek-end has always been a som ew hat troublesom e problem in sm elters, chemical plants and o ther continuous process industries. W ith the advent o f the five-day w eek fo r the office force and the m aintenance and y ard crew s, it is becoming still m ore difficult. W h en the la tte r d ep artm ents w ork on S atu rd ay m ornings, they can usually give their respective p arts of the burden sufficient m o
m entum to carry them th ro u g h until M onday m orning.
B ut w hen the week-end is increased to th ree nights and two days, 63 hours in all, supervisory strateg y and tac
tics, especially in m aintenance, are p u t to a severe test.
Back in the days w hen the w eek-end w as only 44 hours long, a certain large process plant, operating 24 hours a day and 7 days a week, .found it advisable to have present on S atu rd ay aftern o o n s and the Sunday day shifts a suitable m em ber of th e supervisory p e r
sonnel. T his tem porary supervisor shouldered th e re
sponsibility of keeping the plant up to th e p roper pitch and of taking charge in a general em ergency.
A t the beginning o f each year in this plant a schedule was arranged so th a t each m an eligible to be called knew in advance w hen he was supposed to serve. A holiday falling on a week day w as treated in the sam e m anner as a w eek-end; thus, over a period of tim e each m an would serve his pro p er proportion of such days. T h e schedule also provided th at no m an’s w eek-end or holi
day service should in terfere w ith his sum m er vacation.
In this plant all d epartm ent heads on salary— the assistant superintendent, the process forem an, th e d ep art
m ent process forem en, the research supervisor, th e chief
successful in the sam e year. In 1921 S heppard and his co-w orkers learned how to plate rubber. T h e n ex t year, F in k a t Colum bia converted the laboratory m ethod of chrom ium plating into a com m ercial process. Some years later, in 1925, F in k learned the tric k of reversing corrosion. A ntiques thousands of years old w ere re stored electrochem ically and m useum s rejoiced. B erry in 1922 used the electric furnace fo r clear fused qu artz fo r optical uses. L an g m u ir an d A lex an d er split the hydrogen molecule ap art, to let it recom bine and th u s m ake the basis of the atom ic hydrogen w elding torch.
In the last few years light m etals again, this tim e beryllium and lithium , are dem anding attention. P latinum is now p lated ; alum inum is given m any coatings and colors; the copper producer m akes his articles directly from the refining tank. Pow dered m etals, m ade electro- lytically, had trem endous developm ent in bearings, au to m otive and m achinery parts.
T h e electrochem ical industries are today m ore than 10 per cent o f all our chemical industries, th e annual value of products being $350,000,000,— tru ly g ian t steps over a century of achievement.
chem ist, the m aintenance engineer, and th re e assistant m aintenance engineers— w ere eligible fo r this w eek-end com m and. F o r the w eek-end routine m aintenance of plant m otors and pum ps, three electrical m aintenance men, who lived n ear the plant and who w ere paid straig h t time, w orked alternate Sunday m ornings.
I f a m inor rep air became necessary w hen the m ainte
nance engineer o r any of his assistants w ere in w eek-end charge, they w ould do the job, perhaps w ith the help of the electrical m aintenance m an. O n larg er repairs, they called in a few w orkm en of m iscellaneous c ra fts who lived nearby and who happened to be a t home. W h en oth er w eek-end supervisors found rep airs necessary, they w ere usually forced to telephone a m aintenance forem an who assem bled the necessary crew.
In case of a m ajo r breakdow n on S atu rd ay a ftern o o n o r Sunday, a general call fo r m aintenance m en w as broadcast and a tru c k dispatched to g ath er them in. A t such tim es, no strict lines betw een m aintenance c ra fts w ere draw n, the intention being to get as m any m echanics as possible w ho knew gas and corrosives well enough to avoid in ju ry and to tru s t th a t they could use a w rench, a pick, o r a chain block to get things going again.
W ith th e com ing o f the longer w eek-end, the tem po rary supervisors took charge of th e p lan t fo r th e full S atu rd ay and Sunday day shifts. In addition, th ere w as alw ays on du ty a t least one m aintenance m an w ho, w hen n o t engaged on some m inor rep air job, w as assigned to painting o r some sim ilar routine w ork.
Incidentally, the five-day week is one of th e m ost valuable innovations ever to befall th e m aintenance de
p artm en t in a seven-day plant, fo r it forces the d e p art
m ent to study and plan fo r each repair. T h e net result o f the five-day week on m aintenance should be an a p preciable low ering o f m aintenance costs.
T h e experience of being responsible fo r the operation and safety of the entire plant and o f having close con
tact w ith the process operators fo r a considerable tim e is a m ost valuable one fo r th e m em bers o f th e super
visory personnel privileged to share in it. T he five-day w eek has m ade such experience even m ore valuable.
122 Chemical & M etallurgical E ngineering — Vol.40,No.3
Improving A u to m a tic Control by
Recorder Chart Interpretation
By W. R. CLENDINNING
F o rm e rly W ith C. J. T agliabue M fg . Co.
P ro fessio n a l E n g in e er ( N . J ) E a s t O range, N. J.
I
N T H E A V E R A G E process plant, successful operation depends largely upon the accuracy w ith which the processes are carried out. T h is inherent necessity fo r accuracy has led to the developm ent of autom atic devices fo r the control of tem perature, pressure, flow, tim e and other variables— devices which are m ost useful w hen they supplem ent, not replace, carefu l hum an ob
servation. T h e design of industrial control instrum ents has follow ed the tren d of dem and and the plant engineer now has available m any “ stan d ard applications” fo r use w ith w ell-know n processes. B ut, w hen he steps across the bo rd er fro m the region of the know n to th a t of the unknow n, he will find little to help him.
In an effort to set u p certain basic principles of con
trol, to assist in devising new controls, and to sim plify the im provem ent and m ore effective use of th e old ones, a m ethod of c h a rt reading is described herein. Success
ful ch a rt in terp retatio n m ust be based on a consideration of all the in fo rm atio n a ch art can give, including the fac
to r of time. T h is tim e factor is the least well know n but the m ost im p o rtan t featu re o f satisfacto ry control.
T h e underlying m echanism of every control applica
tion consists of some m echanical o r electrical m eans fo r tu rn in g on o r off a source of supply a t a definite “ set”
point. T h is supply m ay be flowing w ater, steam , gas,
electricity, coal, air, a process fluid, or, in fact, anything which can be regulated to m aintain the desired process conditions. B ut the m ere fact th a t a control in stru m en t will regulate the flow is not a sure indication th a t actual control will result, as will be clear from w hat fo llo w s:
1. I f the effect of a change in the control supply re sults in an alm ost im m ediate response from the respon
sive elem ent, any type o f controller w hich will so reg u late the supply source will accom plish control.
2. If , how ever, a change in the supply of control m edium does not bring an im m ediate response from the responsive element, then a “ tim e lag ” exists, and the actual degree of control obtained will fluctuate m ore w idely th an the operating points of the instrum ent.
W e have an exam ple of this second condition in the sim plest form of space heating w here a num ber of large pipe coils are placed along the walls, w ith the responsive elem ent (th e rm o sta t) in the center of the room . In such a case th ere will be a considerable interval betw een the tu rn in g on of th e steam and the m om ent w hen its effect is first “ fe lt” by th e therm ostat. A s the heating continues the a ir tem p eratu re in the vicinity of the th erm o stat will gradually approach the set point, but by the tim e the steam is tu rn ed off, the tem p eratu re ad jacen t to the coils will be m uch too high. H ence, th e average tem perature will overshoot th e set point, both ascending and descend
ing, and the resu ltan t sw ings m ay be very wide.
T h is sim ple and fam iliar exam ple show s how time en ters into the problem o f controlling the tem perature of the a ir in a room . B y inference it also shows th a t the
F ig . 1— T y p ic a l C h a rts Illu s tra tin g (а ) P e rfe c t b a la n ce in a p p ara tu s a n d c o n tro lle r
(б ) A p p a ratu s h eats m o re ra p id ly th a n it cools (c ) G o o d c o n tro l i n a slow -heating a n d -cooling a p p ara tu s id ) P e rfe c t ra p id -a ctio n co n tro l
(e ) A p p a ratu s cools m o re ra p id ly th a n it h eats
( /) T h is c o n tro l w o u ld b e b e tte r w ith a fa ste r c o n tro lle r
V a rio u s S orts of A u to m a tic C o n tro l
(g) W h at h a p p en s w h en a fast c o n tro lle r is u sed on a slow a p p ara tu s
(h ) S u p p ly flu ctu atio n in th e a u x ilia ry system of F ig . 3 (£) P e rfe c t c o n tro l on v ery slow a p p ara tu s
( ;) W h at h a p p e n s in a p e rfe c t system w ith fa ilu re of su p p ly source
March, 1933— C hem ical & M etallurgical E ngineering 123
Set Point
Fig. 2— Six Classes of P ro b le m M et in C o n tro l a n d th e M ethods of S olving E ach
(а ) R a p id rise a n d slow f a ll; u se g ro u p (1) c o n tro ls ; p ro b a b le c h a rt ty p e, 16 (б ) R a p id rise a n d slo w er f a ll; use gro u p (1) o r (2) c o n tro ls ; p ro b a b le ch art type, 16, c o r d
(c) R a p id rise a n d e q u ally ra p id f a ll;
use only g ro u p (1) co n tro ls, p re fe r
a b ly fastest ty p e s ; p ro b a b le c h art ty p e, l a o r d
( d ) S lo w er rise th a n f a ll; u se g ro u p (2) c o n tro ls (in cre ase in su p p ly source sh o u ld be p ro v id e d ) ; p ro b a b le c h art type, “ as is,” le , a lte re d , Id
(e ) S erio u s tim e la g ; see tex t on tim e-lag a p p lic atio n s
( /) S erio u s tim e lag c o rre c te d b y aux
ilia ry so u rc e of su p p ly ; fo r c o n tro l, see tex t o n tim e-lag a p p lic a tio n s ; p ro b a b le c h art type, l i w ith steady su p p ly s o u rc e ; o r l /i w h en source v aries
Sef Point
Time in Seconds S e t P o in t
Opening-
tim e factor may likewise affect the control to be expected from any system.
A n understanding of the subject of ch art in te rp re ta tion is dependent, first, upon a prelim inary know ledge of the types of devices to be controlled and th eir characteris
tics; and, second, upon a general insight into the avail
able controllers w ith which to control them . F o r th a t reason we will discuss both a t some length, particularly w ith reference to their tim es of response and of action.
E q u ip m e n t T ypes
Typical of the devices in chemical plants w hich m ay give the greatest trouble in controlling are steam -jacketed kettles, air-d ry in g ovens and gas coolers and w ashers.
T he reason fo r the difficulty is largely assignable to the fact th a t such equipm ent is uniform ly slow to respond to a change in the source of (h e a t) supply: kettles, because of th eir very small ratio of heating surface to m ass;
ovens, because o f slow conduction and convection; and coolers and w ashers, because of the slow tra n sfe r of heat from air to w ater, and unfavorable physical contour.
In contrast to these slow -responding devices, several types of apparatus respond to changes in the supply source at a very satisfactory ra te of speed. A m ong these are horizontal, steam , atm ospheric vulcanizers; flow- control system s w ith oversized p u m p s ; and benzol and other light-oil stills. It is obvious th a t the treatm en t of these tw o gro u p s of devices m ust somehow differ if satis
factory control is to be had.
T h e principal construction featu res of autom atic con
trollers are known to m ost instrum ent users, b u t there
has been as yet no grouping of these in stru m en ts accord
ing to available differential settings, and according to speeds of action between th eir extrem e lim its of differen
tial. T h e differential of an instrum ent, it should be noted, is its range, expressed in the units controlled, betw een the points at w hich it fully opens and fully closes the control valve. T h is range is usually adjustable. T h e speed of action of a controller is the speed w ith w hich the m ech
anism responds to an increase or decrease o f tem perature (o r other condition) at the responsive elem ent, and w ithin the zone defined by the extrem e lim its of th e d if
ferential, as set. T h is velocity is n o t only affected by the design and ad ju stm en t of the instrum ent, but is (in the case o f tem p eratu re co n tro llers) lim ited by the speed w ith
C o n tro lle rs C lassed b y S peed an d D ifferen tial G ro u p (1) C o n tro llers, F ast-A ctlo n T ypes
Differential,*
Per Cent of Range
Type Controller Max. Min. Speed
A Air operated, vapor filled.. . . 5 1 Highest
B Do., mercury filled... 10 1.5 Hiph C Do., gas filled... 20 3 Fairly high D Do., expansion stem ... , 20 3 Fairly high
E Electric, thermocouple... Almost as desired Potentiometer type not faster than once in 10
F Contact thermometers and re- sec*
corder-controllers... , ,. Almost as desired High 0 Auxiliary-operated (steam or
water) using pilot v alv es... 20 1-5 High G ro u p (2) C o n tro llers, Slow -A ction T ypes H Vapor tension self-contained.. . . . 100 5(?) Slowest
I Type D above... 3 Can be made very slow
J Type E above... Can be made very slow
by infrequent con
tacting
K Type C above... 20 3 Can be slowed by pilot valve adjustment L Combined instruments (recorder-
controllers)... 50 1 Can be made very slow
* Accuracy of percentages not guaranteed.
124 Chem ical & M etallurgical E n g in eerin g— Vol.40,No.3
