INDUSTRIAL A X D ENGINEERING CHE-MISTRY
834
delivered into the open feed-water heater, thus simplifying the equipment by the omission of the flash tank and lowpressure drainer. Where an intermediate steam-pressure system, as a t 150 pounds per square inch, is not available, one of the flash tanks can be omitted; likewise the flash condenser can be omitted if there is insufficient exhaust steam for heating the boiler feed supply. The value of the heat recovery equipment in the example just given is shown by the following computations: The heat lost for each pound of boiler blow-off without heat recovery is the differencebetween the heat of the liquid at 450 pounds pressure and at 60" F., or 439 - 28 = 411 B. t. u. The heat recovered per pound is the difference between the heat of the liquid at 450 pounds pressure and of water at 70" F., or 439 - 38 = 401 B.t. u. The daily heat loss in boiler blow-off without heat recovery when operating 24 hours per day and burning fuel having 14,300B. t. u. and costing $5.00 per ton on thegrate, with a boiler efficiency of 75 per cent, is: 15,789 X 411 X 24 X 5 = ~36.20 14,300 X 0.75 X 2000
The value of heat recovered is: 15,789 X 401 X 24 X 5 14,300 X 0.75 X 2000
- ~35.40 -
On the basis of the assumptions made above, the returns from the equipment installed for recovering heat from the boiler bloy-down may be placed a t $35.40 per day, which would be very satisfactory for the investment involved. I n addition to the return on the investment as above computed, there is a perhaps even greater value in the fact that the concentration in the boiler is more readily controlled with continuous blow-off and therefore a better quality of steam is produced than when the boilers are blown off intermittently. Where continuous blow-off with heat recovery equipment is installed, the cost of blowing off boilers becomes relatively in-
VOl. 21, No. 9
significant, even with rather large percentages of boiler blowoff, as is demonstrated by the above figures. I n the example the net cost is equal to the difference between the value of the total daily blow, 836.20, and the value of the heat recovery, S35.40,or 80 cents per day. The latter represents the value of the difference between the heat of the liquid at 70" F. and at 60" F. The computations as to cost of blowing down do not take into account that there is a loss in availability for work in that the blow-off water is dropped from a high pressure to a low pressure without obtaining work from it. This decreases the amount of exhaust steam which may be used for heating feed water after the steam has produced a very considerable amount of power. The recovery of flashed steam a t 150 pounds pressure, as well as the recovery of the flashed steam from the low-pressure flash tank by the aid of a flash condenser through which the hot feed water is pumped on the way to the boiler, reduces such loss considerably. With this arrangement the only reduction in the amount of exhaust steam that may be used for heating feed water is the heat recovered by the heat exchanger plus the high-temperature drips from the flash condenser. It is therefore evident that the blow-off should be kept as small as possible, but nevertheless, where equipment is installed for recovering the heat from the blow-off water, the actual losses are not great, in any event, not of such proportion that the operator can afford to carry higher concentrations of mineral solids within the boiler than are conducive to good steaming conditions. Conclusion In conclusion there seems to be no question but that high pressure and high rating boilers may be installed with assurance of satisfactory operation with regard to feed-water problems, even nrhere it is necessary to use approximately 100 per cent make-up water, which is rather high in mineral solids, provided suitable equipment which is now available is installed and properly operated.
Mechanism of Formation of Calcium Sulfate Boiler Scale' Everett P. Partridge and Alfred H. White DEPARTMENT OF CHEMICAL
ENGINEERING, UNIVERSITY OF MICHIGAS,ASN ARBOR,MICH.
The early stages of calcium sulfate scale formation on a heated surface have been observed in an experimental apparatus utilizing the principle of the metallographic microscope. I t has been observed that bubbles of dissolved gas or steam formed on a heating surface in contact with a saturated solution of calcium sulfate cause the deposition of crystals a t the solid-liquid-vapor interfaces formed by the surface, the solution, and the bubbles. On the basis of the experimental work a new theory of scale formation on a boiler heating surface is presented. This theory states that the initial deposition of scale crystals takes place directly on the surface as the result of the formation of steam bubbles. If the substance so deposited has a negative solubility slope, the crystals will continue to grow by contact with the super-saturated liquid film 1 Received April 5 , 1929. This paper has been prepared from one section of a dissertation offered by Doctor Partridge in partial fulfilment of the requirements for the degree of doctor of philosophy in the University of Michigan. The work was supported by a fellowship established by the Detroit Edison Company, and will be published in a more complete form in a Bulletin of the Department of Engineering Research of the University of Michigan.
a t the heating surface. If the substance has a positive solubility slope, the crystals may either completely redissolve in the under-saturated liquid film a t the heating surface, or this tendency toward re-solution may be overbalanced by the rate of deposition of new crystals by the process of bubble evolution. The bubble evolution theory of scale formation discredits the "colloidal" theory of scale formation. It agrees with Hall's theory concerning the growth of scale, b u t goes further t h a n this theory in explaining the deposition in scale a t heating surfaces of substances with positive solubility slopes. The rate of scale formation is believed t o be chiefly a function of the rate of heat transfer across the boiler surfaces, and of the slope of the solubility curve of the scaling substance.
EFORE presenting a new theory of boiler scale formation it is necessary to review the salient points of the previous ideas on this subject. The two chief theories have been the one which the authors have called the
B
September, 1929
ISDCSYRIAL A,VD E,YGI,VEERISG CHEXISTRY
“colloidal” theory and the more recent theory presented by R. E. Hall. Theories of Scale F o r m a t i o n
The “colloidal” theory of scale prevention developed in a rather amorphous way. This theory apparently has its roots in the story told by Payen (7‘) after a visit to England in 1821, during which he observed that boiler operators added potatoes to their boilers to the amount of 2 per cent of the feed water, thereby preventing the formation of scale. “As in the case of so many good things,” he says, “this must be ascribed to chance. Some laborers on Watt’s machine, wishing to cook their potatoes in the boiler, forgot them, and were astonished when, 14 days later, as they started to clean the boiler in the customary toilsome manner, they saw the excellent effect of the forgotten potatoes.” Since this early example of scale prevention by colloidal methods, everything from hemlock bark to malted barley sprouts has been used in boiler compounds. I n recent years a theory has grown up to account for the undoubted effect of some sort which many organic and supposedly colloidal materials have upon scale formation. This theory, which for lack of a more definite appellation will be called in this paper the “colloidal” theory of scale deposition, has been described by E. RI. Partridge (5) (not the senior author of this paper), who states:
835
The ideas of Hall have been critically reviewed by Stumper (8),who insishs upon a more complex answer to the puzzles of scale formation and prevention, but credits the former with having made a fundamental contribution. Stumper mentions the idea of Karplus (d), that scale-forming salts necessarily deposit on the boiler surfaces in much the same way that silver deposits on a glass surface in mirror manufacture, and the radically opposed idea of some engineers that scale is only a secondary hardening of boiler sludge, and decides that the actual process is a combination of formation directly at the heating surfaces and of secondary crystallization. Stumper emphasizes our lack of knowledge of just how the initial nuclei of adherent crystalline scales become attached to the boiler surfaces. It is for just this question that the present authors offer an answer.
When, through concentration of the boiler water or lessened solubility a t the boiler temperature, the scale-forming salts come out of solution, they do not appear immediately as hard crystalline scale on the boiler parts. On first becoming insoluble, the scale-forming matter is suspended as minute particles throughout the boiler water. It has become an insoluble precipitate without the addition of chemicals, and whether or not it will form scale depends upon whether or not it will attach itself and crystallize into hard scale on the boiler parts.
The “colloidal” theory of scale formation has never been supported by experimental test. Four years ago it was rather decisively contradicted by Hall ( 2 ) , who found that calcium sulfate, calcium chromate, and calcium hydroxide, all of which decrease in solubility with increase in temperature, deposited from solution directly on the heating surface of a small experimental boiler, while calcium iodate and potassium sulfate, which increase in solubility with temperature increase, were deposited, not on the heating surface but chiefly in the colder portions of the boiler. A subsequent 42-day chemical balance test on a 545-horsepower boiler operating a t 150 pounds per square inch (10.6 kg. per sq. cm.) gage pressure on an untreated water high in calcium sulfate indicated that 89 per cent of the CaS04 entering the boiler deposited as anhydrite scale upon the evaporative surfaces, while only 3 per cent was found in the sludge obtained by filtration of the blowdown from the boiler, the remainder leaving the boiler in solution in the blow-down. The remarkably small amount of CaS04 appearing in the blowdown indicated a boiler mater very free from the suspended particles, which, according to the colloidal theory, are supposed to form scale by impinging at random upon the boiler heating surfaces. Hall’s theory of scale formation is, in brief (J): (1) The hard adherent scales are formed by crystallization from the saturated boiler water directly on the surfaces where found when the boiler is opened. They may or may not be contaminated by the incorporation of varying amounts of suspended material in the boiler water. (2) If S represents the solubility of a salt, a t temperature T, then deposition of solid phase occurs as adherent scale on heating surfaces when d S / d T , the slope of the solubility curve, is negative, but as non-adherent crystals in the cooler parts of the mass of solution when d S / d T is positive.
F i g u r e I-Cross-Section of Cell Used for Photomic r o g r a p h i c S t u d y of Scale Formation A-glass window B-experimental hot surface of highly chromium - . polished . plate C-electrical resistance heating coil mounted in alundum cement D-air insulating space around heating unit except on side covered by experimental hot surface E-cell solution F-feed solution inlet
Experimental Work
I n the investigations of the authors the formation of scale upon a heated metal surface in contact with calcium sulfate solutions boiling under atmospheric pressure has been studied by a method allowing continuous visual observation through the microscope and the photomicrographic recording of various stages in the process. This method is an adaptation of the ordinary metallographic microscope, the scales-forming metal surface being an electro-deposit of chromium highly polished to allow observation of surface changes by means of reflected light. APPARATUS-A cross section of the small cell used in the experiments is shown in Figure 1. This cell, made from brass, was equipped with a glass window, A , through which a microscope was focused on the surface of the polished chromium plate, B, which formed the front face of a small brass box internally heated by an electrical resistance coil, C, mounted in alundum cement. I n order to increase the heat flow across the observed surface, the heating unit was enclosed on five sides by the air space, D. The solution, E, in the cell was supplied through a feed inlet, F, from a
constant-level reservoir, whik: titi! steam produced was vxiteil through an outlet in the cell corer. Fignrc 2 sliou~sa pliotograph of the cell moioited togetlicr. wit,li a microscope and a small moving-pictnrc camera adqitctl for tnking individual exposnres at rrnnll t i m e iiitervals. For obt,:niiiing greater detsil tlie moviiie-pictiire cnnierii was red ivlien necessary by :in ordinary camera. Illumination \vas suplilied by the arc I:mp at t.lie left of t tioii. A fec&dntion reservoir and de-aeratiug . shown coniieeted to the cell. In placc of t,he elir~irriium-pl:itedsurface a piect: of c l m m e niokrl-iron? was used for some of tlre vork, :in11 proved rjnito rctory, the snrface rcinaiiiirig pract,icaliy untnrnislicd for the 2- or 3-hour rnns in contact with lroiling salt solutians. ~ ~ E ~ ~ I \ L E S I’RoCEDERE TAII AXn ( f R s E R v h T l i ~ s S - - ’ ~ ~ , , idifferent procedures \wre follouwl during t h e expcrinients. At first,, t.he rell was filled with :I solnt,ion of ciilcinrn sulfate snt,urated with respect to gypsum n t rooni temperature (npproxirnat,cly 20.50 p. p. m. CaSOd (G), and contnining dissolved gases in equilibrium with the laboratory atmosphere. After the current. to the heating coil had been turiit:d or!, the heating OS the filni of solution next to t,he experiiiiriitsl hot surface i:anscrl tile fonniibiiin of tiny gas brihliles on t,liir snsface. A s heating cont.inucd, these lini~blesgrew i i i siar iiritil their ~ I I I I W U I Ccaused ~ tlicni to pull IIIOSP fsiiin tlic nif*t2rl s n T S a w , Ei-e-ery t i m e that, such a hulilile ~k:t:ial~od itself, it lc,ft hehind it. an easily visible ring of tiny rryst;tls
The question at once presented by t.bese results was whether Iiirhble of steam formed a t an evaporative surface w~iild iirucluce the s m i e t:ffect a i a Iiubhle of dissolved gas. The rxpcrimeiital procadiire was inccordirrgly changed to correspmd nmi?dosely trr the nct,ii:rl process taking place in %: lioilcr. The cell wns filled iiiitiiilly with distilled water, nnd li inake-np srilnt,ion s:rtnrnt,ed \yitli respect to gypsum w i i s fed to tire ccll thniiigll t,lie de-aerating Iq’igitre 2 , where it was boiled gently in each of the three cr,nttiiners Irefore pnssing into the cell. From 2 to 3 hours were reqoised for the evaporation in tlie cell to bring t.he cell soliit,ion n p to satnrstion. The first evidences of ere il frmntion upon the experiirrental hot surfaces from tlie initially nnsatnrated and well de-aerated boiling snlntion u w e very fine lines of a circular or elliptical shape, as s h o w n in A of F i p r e 1. From these lines small crystals began to grow, as shown in the succeeding pictures of Figure 4, until eventually the metal surface was entirely covered with a layer of interlocking crystals forming a continuous deposit of scale. Subsequent experiments with higher rates of heat input to the apparatus gave, in some cases, deposits like circular jilacqnes, caused by the evolution in rapid succession of :t large nunrher of Imlihles from one point on the metal surface. l’hese i:lacqiies in their early st,ages consisted of eonccntric rings of crystals xhich rapidly grew t.ogether into a continnous ti
niRss.
While in some cifses the crystal rings deposited on the nrctal sitrfince during tlie experiment,s were partially destroyed by convection currents while in an carly stage of developnicnt, tlic deposits in general were sufficient,ly adherent t u the highly polislied surface to resist this action. Bubble Evolution Theory of Boiler Scale Formation
Tile fwmat.ion of the crystal rings a t the solid-liquidvapor interface beneath bnbhles of either dissolved gas or
Flaure Z--Phofo~l,raph of Experiments1 Appsrafus for Photomlcrographlc Study Of Scale Formation
upoii the inetal. These erystals subsequently grew from contar:t with the solution, forming long needles which interlocked and eventually produced a continuoils network, all t,mees of the original ring nuclei being covered up. Four stages i n this process of crystal deposition and powtl! as t l i e result of gas bnbhle evolution are shown in Figure 3. Similar tests were subsequentlg made with solutions diluted to approximat.ely 1500 p. p. m. CaSOr, slightly under the snturation valnes for gypsum arid for hemiliydrat,e a t 100” C;. (6). Under tliis conditiou the depiwtore of each linhble Sroin tlie expcriniental hot surface revealed a typical crystnl ring, but each ring rapidly redissolved, disappearing completely within a few seconds after the evolution of the bubble which had caused its formation.
* Enduro KAI, Eupplied by
the Central Alloy Sfrel Co.
stearn is not ditlicult of satisfactory explanation. The experimental evidence indicates that, siiice the deposits are in the form of rings, the bubbles are formed directly in contact with tlie heated surface. In the first ease the dissolved gases come out of solutivn a t t h i s point because of their decre‘ased solubility in the heated liquid film at the hot metal surface,while in the second caae the steam bubbles are formed by the sudden vaporization, a t points on the surface, of areas of the liquid film which are superheated by contact. with the hot metal surface. Each bubble forming in direct contact with the heated snrface creates a ring-shaped triple interface at which the solid surface, the solution, and the vapor of the bubbles are in contact. Within this interface the heated surface is in contact Rtth a gas film rather than with the normal liqnid film which exists outside the interface. Owing to the great. increase in resistance to heat Aow of tlre gas film over the resistance of the liquid film, the metal surface tends to overheat locally in t,he area within the interface, This local overheating causes increased evaporation into the bubble from the very thin edge of solution surrounding the interface. If the body of solution is saturated with calcium sulfate this thin edge of solution is already supersaturated as a result of the increased temperature in the liquid film a t the heated surface, and the effect of the local overheating herieatli the bubble is to cause rapid deposition of the excess dissolved material at the triple interface in direct contact with the heated surface. I n the experimental work the evolution of bubbles of dissolved gas was rather slow, allowing periods as long as 2 minutes for the growth of crystals at the triple interface, wliile the evolution of steam bubbles was extremely rapid,
:allowing only a fraction d ii secimd for the cry+tnldeposition. This is reflected in the difference br%w-eeii the lieavy riiigs of Figure 3 and tlic very fine rings of Figure 4. From the work xith solutions slightly ir,lderset,urated, it seems prolial~leto t,lie ni:tliors that, bubble evnlut,ion may (xiuse tho deposit,ioriof crystals of substances diich increase in solubility with incmase iii temperature, such xs cirlciurn carhonat.e, as well i i s of subst.anccs with :LiieEativc solutility elope, such as c:dcii:rn siilfatc. Wit,li hi& rates of heat transfer the act.ioii I~cncatha biibble would be cquivnleiit to a rapid drying-up of tlie tliiii edge of soliitian at the trilile interface. Crystrrls of sobstances with ii positire solubility slope deposited in t.liis ~ a mould y teiid to redissol\-e as soon as the bubble xhich deposited tlierrr l i d dcjrart,cd, since they would then be in contact with a liquid film sliglrtly undersaturated by elevat,iuii iir teinperatorc at the heated m e t d
surfare. It. is qiiitc jrossililc, Iiowever, that the rate of depsition l ~ bubble , evolution ~uouldbe greater tlmn the rate of re-solution with the conxrjuciit forinntion of scale at B berttiirg surface of a substance d l i a negative solubility slopr*, in contradictiuii t.o the second part of Ilall's theory, previuiisly stnted. Tliis hypothesis, ndvmced by the autliors, explains the formatimi of t,iiiii (:rrlr:iom carbonate soeles oii the ImLting s n r f h c ~ sof boilers intcni:illg t,re:itcd with soda ash, and of thin e d c i i i n i pliospliate scdes 011 the bcstiri# siirfnrcs of boiler siini1;irly treat,cii with trisodium i h s p h n t e . I t is hoped t o t,est t,lie validity of this hypt,Iiesis 113' adrlitioiial experiiiutiittii work in t,lic fiitim. TIE aotl~orshclieve that tlie fonnatioli of aiih?ilrite scale iii an iiiduatriril boiler uiider presslire is perfect,lg analogous t,o tlic formnt,ion of a gypsum smle in the exporinietital cell,
finalamerital factors, the rate at which crystal nuclei are &posited on the metal surface and the rate at which growth of t.hese crystal nuclei proceeds. The rate OS deposition of crystal nuclei is dependent, according to the authors' bubhle evolution theory, on the riite of boiling, or, in turn, upon tho rate OS heat transfer across the boiler heat,ing surfaces, other fact,ors being constant. While Rn increased rate of boiling may teiid somewhat t,o reniow scale crystals mechanically from the ineta1 surface, this effect is probably s~nallcumparcd mit,li the increased rate oS dcposition of new cryi;tals by hubhle evohition. Thc rete of growth of the crystals on tire metal surface is directly dependent upon the degree of supersatitration of the liquid with which they are in contact. This degree of supersaturation is a quaiitit,y rather hard to determine under the turhulcrit coriditioiis existingin a boiler tube, but it is probably a function of the slope of the solubility curve of the scaling suhst.ance and of the rate of lreiit transfer across the metal surface. The velocity of the hoiler water througli the tubes will have an effect in so far as i t reduces t.he temperatiire differential across the liquid film a t the hcatiiig surface and therefore tcitcls to reduce the dcgrcc of super-saturntioii produced iii watzr piissing from the body of boiler soluiion into this liquid film This effcct, is probably small compared witli that of the rate of heat transfer and that of t,lie solubility slope of the scaling substance. Seventy-five years ago Couste (1) observed that scale was, in general, thickest in regions of a boiler exposcd to the most severe heat, and suggested that calcium Figure J-Ray Stru~fmreof Hemihydrate Scale Formed i n Eiperimenral Boiler during sulfate, because of its decreasing solubiliiy 48-Hour Run at 15 Pounds Gaga Preds~re with increasing temperature, might be erThe authors do not wish, at the present time, to claim too pected t,o form scale upon strongly heated boiler surfaces groat generality for their new theory of scale formation. cven before the body of the boiler solution arrived a.t satucaScale may conceivably form upon a heating surface as the tion. This is checked by the recent report (9) that in one result of any act,ion which causes crystals of a siibstaiice with boiler driven at high ratings a thin, hard scale was found a negative solnhility slop to become attached to the surface. in water screen tubes exposed to severe radiation although In addition to the deposition of crystals by bubble evolu- no deposit WRR evident in the boiler proper. This indicates tion described in this paper, such crystal nuclei might be de- the effect of rate of heat transfer upon scale formation. In general, the over-all rate of scale formation v l i U probably posited by two other means: first, the accidental trappiiig of suspended crystals by surface irregnlwities; or second, be a function of the rate of heat transfer and the slope of the the action of such irregularities as promoters of crystalliza- solniility curve of the scaling substance. It is hoped to tion from a super-saturated liquid film at the heated surface. investigate experimentally the accuracy of this prediction, Such phenomena as the formation of calcium carbonate which is irnportaiit in view of the current trend toward higher scales at cooling surfaces and of calcium sulfate scales in the ratings and higher pressnres, particularly with respect to Ion-er parts of the tubes in forced-circulat,ion evaporators silicate scale formation concerning which there are as yet are obviously not dependent upon a process of bubble evolu- no reliable data. tion for the establishment of c tal nuclei. However, in a L i t e r a t u r e Cited steam boiler, which only functions as the result of t,he rapid (1) Courte. Ann. miner, 151 6 , 09 (1854). production of stcam bubbles a t its heating surfaces, the IIall, INO.E x o . C ~ a u .17, , 283 (1928). initial step in scale formation must lie almost exclnsively (2) (3) Hall e t at., Carnrgie Inst. Tech., Bull. 14. 17, 21 (1837), that which has here been reported on a.n experimental basis. 141 Karolus, W8rmc. 49. 561 (19201. A prediction wvhich follows from the above discussion is i 5 i p&il&, E. M.; f . i m . waic,workrA ~ ~11, 28.3 ~ ~(1924). ~ . , that scales formed upon a heating surface in contact with 16) Partridge, E. P., and White, f . Am. Cham. Soi., 61, 360 (1929). Payen, IXnglcrr polylrch. J., 10, 264 (1823). a snturated complex solution which is not boiling will coo- 17) IS) Sturnper, Arch. W ~ r m d r l .8, , 271 (1927); Chirnie indrririb, 10, 10 sist almost entirely of constituents whose soluhiIit,y slopes (19281. arc negative, while scales formed upon heating surfaces at (9) Union Electric Light and Power C o . , Serial Repi. 01 Prime Movers Committee on Eigher Steam Pressurea and Tenrpersfures, 1927-28. which rapid ebullition of the same solution is t.aldng place will be round to coiitaiti appreciable aniouiits of constituents whose solubility slopcs arc positive. More than $500,000,000 was spent in the United States last R a t e of Scale F o r m a t i o n year in drilling for oil and gas, according t o replics to questionsince ill each case the so1ubilit.y curve of the solid phase dcposited is similar ( 6 ) . Scale formation tests in an expcrimental boiler a t pressurcs up to I50 pounds per s r p x c inch (10.6 kg. per sq. cm.) gage pressure, described by t,lie authors in another paper (41, further support this vicw, sincc, at. different boiler temperatures arid pressures, scales coiit,aiiiing gypsum, hemihydrate, or anhydrite %?-erefornicd from talcium sirlfatc solutions wider corirlitions whicli were otherwise ideiitiral, Figure 5 shows interesting cross sect.iwisof a heinhydrate ?cnk formcd during a 4&hour run at 25 pounds per S ~ J U ~ iiieh V C (1.7 kg. per sq. em.) gage pressore. The radiating iiccdle cryatnls orieiii:iting at a common Focus mi the inner surface of t.lie scale are good supporting evidence Sor tlic t,heory ui scale g n ~ w t hSroin crystal nnclei deoosited in rims during thc process of bubble evolution.
The rate of Scale forniation on a boilcr heating surSace
is dependent,, Srom the vic%\-pointof the autliors, on two
naires sent out to the oil industry hy the American Petroleum Institute. The exact total of the figures on all the replies r a s $,503,3:12,000 which errs OIL the side of conservatism.
