839
Thermal Effects of Boiler Scale' Everett
P. Partridge and Alfred H. White
DLI.nnl-mEliT OF CIIF.a.c.,,. S i l t i i X B R X l h ' V . 1!NIYBISITY
New determinationsof thecoefficient of heat conductivity of calcium sulfate boiler scales made in an experimental boiler a t pressures up to 150 pounds per square inch gage are reported. The coefficientof heat conductivity measured by various investigators on nineteen hailer scales varied between 1.13 and 3.06 kilogram-calories per square meter per hour per meter thickness of scale per degree Centigrade, with a n average value of 2.15. According to Eherle and Holzhauer, the conductivity of boiler scales decreases with increase in porosity, and very porous silicate scales may have heat conductivity coefficients as low as 0.1, or one-twentieth of rhe average value for dense, non-porous sulfate scales. The effect of boiler scale upon heat utilization is very slight, probably amounting to not more than 3 per cent for a scale 2 mm. in thickness in a boiler operated a t high ratings. While scale is relatively unimportant from the standpoint of loss in heat utilization, it is very important from tile standpoint of temperature elevation in the metal of scaled heating surfaces. A t the high rates of heat input by radiation occurring in water walls and in frontrow tubes. thin scales formerly regarded as unimportant may cause failure due to overheating. A simple graphical solution of t h e heat flow equation is given for the rapid estimation of metal temperature increases caused by different rates of heat transfer, for scales of different thicknesses and heat conductivities.
OP MICHIOAN,ANN
ARBOR,MICII.
proper, suspended vitliin an air thermostat, fed by a snrtill pump, arid cqiiipped witli a hot t.uhe internally heated ti$ an electrical resistaiiee coil. Sdeain was bled off at coustant lire through a needle valve t o a condenser which dischnrged bo a weigliirig receiver. Tlrc h i l e r was a 2-foot ( T I - c m . ) lengt,h of 6-inch (15crii.) steam pipe, fitted wit11 cornpa:iion flanges. The boiler feed h i e and an exhaust line werc tapped into tht! top flanpe, wlrile the hobtorn flange cnrricd a specinl lilisg wliicti sr:rved t,o introduce tlic liot tube and ti~ermoeouplele& into the hiler. The air tlmmoskit was made of sheet steel with two spaced linings of transite to cut, down heat loss. I t was heated by cliroiucl A rrsistance coils and rapid circulation of air was obt,ained wit.li a motor-driven fair. T l i c trinperature of the tliermost,:Ltwas maintaiiied for eacli run nt the value corresponriiirg to tire prcssiire at which ttie boiler was to be operated, a liquid-se;iled inercitry cdunin being used to operate ii relay controlling tlic ciirreot to tht: lieatiirg coils. Tlie pump uscd for feeding the lioiler while in operatioti was lotined to t,lie nutliors hy R. E. Hiill. It Itas previously heen dcscri1ii.d Iiy him elsewhere ( 7 ) . Ttx hot tribe was an X-incll (20-cm.) lengt,li of stdel tubing oS iiieh ( 2 cinA out,sidc diiimetcr and l/g inch (3 mm.) wall tlrickiiess witli tlie upper end closed iiy wclding on a
Experimental Determination of Coefficient of Heat Conductivity of Boiier Scales
T
HE thermal conductivity coefficients of hoiler scales have been measured by Ernst (6),after reinoval from the boiler and while in contact with water at 30" C,; bv Eherle (8) aud Reutlinger (18) in an experimental boiler a t 100" C.; by Croft ( 2 ) in an experimental boiler at 170" C.; and by Eberle and Holahauer (4) after removal from the boiler and while dry, at various temperatures in the boiler range. The present authors have determined the conductivity of six scales formed from calcium sulfate SDlotions in an experimental boiler at various pressures from that of the atmosphere up to a gage pressure of 150 pmuids per square inch (10.6 kg. per sq. cui.). The experiniental conditions wo believed to approximate actual boiler operation more nearly than those of tlie previous determiiiations. The authors believe that the method utilized offers ixmsiderahle possibilities, iiot only for heat trarrsfer work on a laboratory scale, but also for the study of actual conditiosis in operating boilers. APPARATUSAND Pl3oCEl>URE-Tlie experilrittiital hoiler hits heen descrihcd in a previous paper on the soliilility of calcium 8ulSate up to 200" C. (11). A pbotogrsph of the equipment is sliown in Figure 1. For the work on calcium siilfate scale the boiler unit consisted essentially of the boiler
* Received April 2, 1929. This paper has been prepared from sections of r disiertatioii offered by Doctor Partridge i n pmitirl fulelrnent of t h e r e q u i ~ e m e n ilor ~ the degree 01 doctor o i philosophy i n the University of Michigan. The vork W ~ Ssupported h y IL ielioivrhip ~sfablirlicd by the Detroit Edison Company. and will be published in a more compiete form r Builrtin of the Department of ISngiiieering Research of the University of Michigan.
Figure I - E x p e r i m e n t a l
Boiler Unit Used in Research on Scale Formation
eapping disk, and t,lx lower eiid supplied with a flauge for rnounting in ttie sperial plug inserted in tile bottom flange of the boiler. Fourtecn Scet of KO. 18 chrome1 wire wcrc wound in R 11ouBIe lielix O ' / z inelics loug (16.5 ern.), which was mounted in aiundum cement withiii the t,iibe. The outor surSacc of t,lre tube was equipped with a tlleruiocouple clement of the plated-surface type (10). This was made by plating the entire tube with a tliiii 1;tyer of copper, over which a thin plate of nickel was deposited except for
Fiaurr J
Cumpilrison of Crosa-Sectional and Surface Appearance of Scalee Produced a t Various Prexrurcs in Erprrimentai Boiler
INDUSTRIAL ASD ESGINEERING CHEMISTRY
:842
tinuous non-porous boiler scales be taken as 2.0 but that until further information is available the possibility of values as low as 0.1 with extremely porous silicate scales should be recognized. Table 11-Coefficients
of T h e r m a l Conductivity of Boiler Scales
THERMAL T E M P . DEN- POROS- COKDUCTIVITY OF S I T Y O F ITY O F SCALESCALE COEFFICIENT TEST OF SCALE
OBSERVER
YEAR
Kn-cal. B. L. u .
-Kg./m.3 2 . 2 4 0,089 1 . 2 7 0.410 2 . 3 5 0.114 2 . 3 1 0.086 1 . 6 8 0.056 1 . 9 5 0.100 2.20 0.068 2.14 0.022 1 . 6 6 0.117 2.34 0.005
..
...
..
...
.. ..
.. .. .. .. .
I
...
...
... ...
... ... ...
Minimum Maximum Average
4.2.hr.m.-O C. 1.27 1.13 2.02 2.19 2.23 1.82 2.68 2.77 2.24 2.77 1.91 2.96 2.62 1.60 1.42 2.61 2.18 3.07 1.39 1.13 3.06 2.15
FL.?-hr,f:.." F . 0.85 0.76 1.34 1.47 1.50 1.22 1 80 1.86 1.50 1.86 1.28 1.98 1.75 1.07 0.95 1.75 1.46 2.06 0.93 0.76 2.06 1.43
" C
30 Ernst ( 5 ) 30 Ernst ( 3 ) 30 Ernst (5) 30 Ernst (5) 30 Ernst (5) 30 Ernst (5) 30 Ernst (5) 30 Ernst (5) 30 Ernst ( 5 ) 30 Ernst ( 5 ) 100 Eberle (3) 100 Reutlinger (12) 170 Croft ( 1 ) 100 Partridge and White 115 Partridge and White 130 Partridge and White 130 Partridge and White 148 Partridge and White 185 Partridge and White
(11) (11) (11) (11) (11)
(11)
1902 1902 1902 1902 1902 1902 1902 1902 1902 1902 1909 1910 1927 1928 1928 1928 1928 1928 1928
The values for the coefficients of heat conductivity of the non-porous boiler scales place them slightly below firebrick as insulating mate;ials. Scales of even tl,ese highest conductivities offer a decided resistance t o heat flow, and produce either one or both of the following effects: (1) a decreased over-all rate of heat transfer from the furnace side of the boiler heating surface t o the boiler water, reflected in a decreased utilization of the heat of the hot furnace gases in producing evaporation; (2) an increased temperature of the metal of the boiler heating surface on which the scale has formed. These effects will br discussed in the succeeding parts of this paper. Effect of Scale upon Heat Utilization
Although in present importance the heat utilization effect of boiler scale falls far below the metal temperature effect, it belongs first in a historical sense. When Graham (6) was experimenting with the construction of boilers in 1860, he concluded that the presence of '/x inch (1.6 mm.) of calcium sulfate scale had caused a decrease in heat utilization of 14.7 per cent in one of a pair of "breeches" boilers supposedly operated under identical conditions. The figure of 15 per cent loss for 1/16 inch (1.6 mm.) of scale was subsequently paraded down the years, receiving partial confirmation from the data of Schmidt and Snodgrass (13) in 1907. The high values of Schmidt and Snodgrass have been quoted as arguments in favor of boiler feed-water treatment, while the low values obtained in their tests and in the more accurate experiments of Eberle ( 3 ) and Reutlinger (12) hare been discreetly disregarded. The statement of Eberle in 1909, that "from the results of the experiments it is to be concluded that a scale deposit averaging 5.5 mm. in thickness and of average heat conductivity influences the heat utilization in a boiler but very little, so that the determination of this influence in a reliable manner by comparative boiler tests is not possihle," has been more recently supported by Hellemans ( 8 ) , and by Croft (1). A summary of the experimental data on the effect of scale upon heat utilization is given in Table 111. The values of Schmidt and Snodgrass (13) have been recalculated from
Vol. 21, No. 9
their published data, using logarithmic mean average temperature drops, and all values for the same thickness of scale have been averaged together. The fact that the estimates for heat loss due to scale decrease as we come down to the present time is probably due partially to the gradual development from boilers in which heat was transferred almost entirely by conduction and convection from hot gases, to boilers which a t the present time receive a very appreciable portion of their heat by direct radiation from the burning fuel, and from the furnace setting. With heat transfer only by conduction from hot gases, the increased resistance to heat flow due to the formation of a layer of scale causes an increased metal temperature and therefore a decreased temperature difference, and a correspondingly decreased rate of heat transfer across the gas film. With heat transfer by radiation alone the increased resistance due to scale formation causes an increased metal temperature, but this increase in temperature has a very slight effect on the rate of heat input to the metal surface, because this rate is dependent, not upon a direct temperature difference, but upon the difference of the fourth powers of the respective temperatures of the hotter and colder bodies. For example, if the average temperature of the burning fuel and boiler setting is taken as 2000" K., an increase of the boiler metal temperature from 500" K. to 800" K. due to the formation of scale will cause a decrease in heat transfer by radiation of only 2.25 per cent. With heat transfer both by conduction and by radiation the effect of scale upon heat utilization will vary between the limiting cases given above. It seems very definitely established that the loss due to scale of average conductivity and thicknesses up to 2 mm. will not exceed 3 per cent. Table 111-Effect
THICK
of Boiler Scale u p o n H e a t Utilization
THERMAL i
)ECREASE IN
H E A T OBSERVER YEAR UTILIZA-
N E S S OF
SCALE OFSCALE
M M m
1 0 0 1 1 1 1 2 2 2 3 1 5 2 2 5 5
6 5 8 0 3 5 8 0 3 8 3 9 5 5 5 1 1
I
TION
In 006 002 003 004 005 006 007 008 009 011 013 007 019 010 010 020 020
1.00 1.00 2.61 2.61 2.61 2.61
0.67 0.67 1.75 1.75 1.75 1.75
14,270 14,270 14,270 14,270 14,270 14,270 14,270 14,270 14,270 14,270 11,650 11,650 8,190 20,470 8,190 20,470
5250 5250 5250 5250 5250 5250 5250 5250 5250 5250 3900 3900 3015 7540 3015 7540
Per cenl 1 4 . 7 Graham(6) 1860 2 . 9 Schmidt and 1907 46 . 18 Snodgrass(13Ja
4.6 6.2 4.7 8.6 8.1 15.8 6.6 1 . 7 Eherle (3) 1909 Reutlinger ( 1 2 ) 1910 6.0 1927 1 . 6 7 Croft ( I ) 3.83 2.99 5.98
a Recalculated, b y the authors, from the original data with the use of logarithmic mean temperature drops, all values for different scales of a given thickness being averaged together.
Effect of Scale upon Boiler Metal Temperature
In boilers of contemporary design the front-row and waterwall tubes are exposed to severe radiation, with total heat input rates of the order of 200,000 to 300,000 kilogramcalories per square meter per hour (70,000 t o 100,000 R. t. u. per square foot per hour) ( 4 , 9, 14). Scale forming in these tubes will not materially reduce the rate of heat input, as previously pointed out, but will cause an increase in the metal temperature proportional to the increased resistance to heat flow from the metal to the water within the tubes. It is in just these regions, however, that scale forms first and grows most rapidly, as was noticed long ago by Couste
September, 1929
INDUSTRIAL AND E-VGIXEERING CHE;MISTRY
(2). The authors have discussed this matter in another paper on the mechanism of scale formation. For the present, the point should be emphasized that the heat transfer conditions which make for the most rapid growth of scale are precisely those which will cause the maximum increase in boiler metal temperature as the result of scaling.
The temperature drop across a continuous adherent layer of scale may be found by using the fundamental equation for heat flow in the following form: AT
AT
0 __ A8
L
Ks
Figure 4-Permissible Elevation of Boiler M e t a l T e m p e r a t u r e as a F u n c t i o n of Boiler Pressure The upper curve gives the actual boiler water temperature, the lower curve the increase in temperature which may be permitted in the metal heating surfaces before reaching a metal temperature of 480' C. (900' F . ) .
843
=
(g)
1 Ks
.L , -
QUAFTITY Temperature difference Heat units transferred per unit of area per hour
METRIC UNTS
ENGLISH UNITS F. Kg-cal. per sq. meter B . t . u. per sq. foot per hour per hr.
Thickness of scale Coefficient of thermal conductivity
Meters Feet Kg-cal per sq meter B. t u. per sq foot per hour per meter per hour per foot per ' C. per F.
c.
.
which shows that the temperature drop varies directly as the rate of heat transfer and the thickness of the scale, and inversely as the conductivity of the scale. Of these three factors, the coefficient of thermal conductivity has been discussed in the first part of this paper, the rate of heat transfer for severely irradiated surfaces has been mentioned in the present part of this paper, and the thickness of a scale deposit is a quantity permitting measurement-at least after the mischief is done. The temperatiire drop through a scale deposit may be conveniently obtained from the diagram of Figure 5 , which is a graphical solution of the heat flow equation given above. Starting n-ith a value for the rate of heat transfer as a t ,4, proceed horizontally t o the right until the proper value for the thickness of scale in millimeters is reached, as at B. At this point proceed vertically either upward or downward to the proper value of the conductivity, as a t C or C', then proceed horizontally to the right-hand scale, as at D or D ' , which will indicate the temperature drop through a scale deposit for the given values of the various factors. This diagram may also be used to solve for any other factor in the equation, provided values for the remaining three factors are given. It is quite definitely established that heat transfer rates of over 200,000 kilogram-calories per square meter per hour occur in front-row tubes of boilers today, and it is thought that values as high as 300,000 may actually be obtained a t
The safe maximum continuous temperature for boiler tubes of the customary low-carbon steel is probably not greater than 480" C. (900" F.). If from this limiting temperature there are subtracted the water temperatures corresponding to various boiler pressures, the lower curve of Figure 4 is obtained. This shows the allowable temperature increase of the boiler metal over the temperature of the boiler water, as a function of boiler pressure. In a clean boiler there is a small temperature drop across the fluid film on the water side of the tube. There are very few definite data concerning heat trnnsfer from metal t o boiling water (16), but the present authors believe that this temperature drop across the water film does not exceed 15" C. (27" F.) for the conditions of high water velocity, high fluidity, and rapid evaporation existing in tubes subjected t o severe radiation. As scale forms in a tube, the temperature drop from the surface of the scale t o the boiling water may possibly increase slightly, owing t o the increased roughness of the solid-liquid interface, but in the authors' opinion it will not exceed the value given above. Owing t o the method of formation of boiler scale there is no discontinuity between the metal and the deposited crystals, and hence no temperature drop a t this point would be expected, unless the scale should become loosened but not completely detached from the metal. I n the latter event the steam pocket formed under the loosened scale would interpose a large resistance to Figure 5-Graphical S o l u t i o n for T e m p e r a t u r e Drop t h r o u g h Boiler S c a l e heat flom, with resultant high temperature drop. This a s a F u n c t i o n of R a t e of H e a t Transfer, T h i c k n e s s of Scale, a n d Coefficient is one possihle explanation of the blistering of boiler Of Heat Conductivity Of L-thickness of scale in millimeters tubes. K-coefficient of heat conductivity of scale, in kg-cal. per mz.-hr -m -'C.
J
very high ratiiigs -~winiiiigtlie niore conservative figure of 200,000, and iieglecting tlie temperature drop from metal to boiling water. it is intPre4ng to see how much vale might be tolerated in boilers ope1ating a t various prezwres. From Figure 4 a boiler at 600 11ouiids pressure ha' a water temperature of 256" C. and therefore an allon-able margin of tube temperature increase of 221" C. before reaching the probable safe limit of 480" C. Thi- boiler would therefore tolerate. from Figure 5, approximately 2.25 inm. of a non-porous scale with a heat conductivity coefficient of 2 0, or
