INDUSTRIAL A N D ENGINEERING CHEMISTRY
458
The equation for compound sections on a cylindrical surface in terms of heat loss per square foot of outer surface is Ti
h =
rs log, K1
2 r1
-
re log,
Tz rg
-
rs log,
(6)
2
Heat Transmission in an Inclined Rapid Circulation Type Vacuum Evaporator'
r3
r2
+ T f 7
The various steps involved in the solution of a problem of this type will be illustrated by an example. It is desired to calculate the heat flow in B. t. u. per hour per
square foot of pipe surface through a compound insulation on an 8-inch pipe consisting of a first layer of Hi-Temp 1.5 inches thick, and a second layer of 85% Magnesia No. 1, 2 inches thick. The temperature of the pipe is 800' F., and the room temperature is 70' F. First, a canvas temperature of 120' F. is assumed. The total drop in temperature through the two coverings then is 800' 120" = 680' F. The next step is to assume the temperature drop through each insulation, in order to determine the conductivity value to use. No rule can be given for the first assumption, but it will be found that the second assumption can be calculated fairly closely. It is known from Fig. 3 that the drop through Hi-Temp should be less than the drop through Magnesia, so there is first assumed a drop of 300' F. through Hi-Temp and a drop of 380' F. through Magnesia. The temperature between the Hi-Temp and the Magnesia covering then is 800' - 300' = 500' F. The mean temperature of Hi-Temp is 8ooo + 5000 = 650" F. 2 and from Fig. 3, K1 = 0.676. The mean temperature of Magnesia is 5000 + l 2 O o = 310' F., and & = 0.509. 2 Now 71 = 4.3125 r2 = 5.8125 t'8 = 7.8125 73
log,
ra log,
Vol. 16, No. 5
By D. J. Van Marle BUFFALOFOUNDRY & MACHINECo., BIJFFALO, N. Y.
T
HE evaporator is of the rapid circulation type with inclined steam chest. Its body is cylindrical in shape, with conical bottom. T o this bottom the steam chest and downtake are bolted, inclined a t an angle of 45 degrees. At the lower end they are connected by a casting which forms the liquor space. The liquor enters the top of the liquor space, boils up through the tubes, strikes a baffle plate in the vapor body, and returns through the downtake. Steam enters through one of two inlet openings, one near the top and one near the bottom of the steam chest. Condensed steam is removed by means of a wet vacuum pump. At first an attempt was made to remove noncondensable gases from the steam chest through this same pump, but this proved unsatisfactory. With the steam entering a t the bottom of the steam chest, air removal was apparently incomplete, resulting in greatly reduced heat transmission. By carefully venting the steam chest directly into the vapor body, better andmore consistent results were obtained. Fig. 1 shows the arrangement of the evaporator with the exception of the upper steam inlet and the corresponding air vent a t the other end of the steam chest.
rz - = 2.33 71
2
= 2.31
r2
Substituting the values obtained above in Equation 3, so0 - 120 680 h = 2 33 2.31 - 3.45 4.54 =85.1B.t.u* __ 0.676 + 0 x 9 Substituting this value in the surface loss equation, there is obtained 272.5 X 85.1 = 55.3 Td = A64 85.1 15,6°.'8 This indicates that 50' F. temperature difference first chosen was too low, and there is next assumed a temperature difference of 55' F. or a canvas temperature of 125' F., since the canvas temperature will generally be fairly close to the canvas temperature first obtained by substituting in the surface loss equation. The temperature drop through Hi-Temp for the foregoing calculation was approximately 3.45 X 85.1, or 293.5" F., while the temperature drop through Magnesia was approximately 4.54 X 85.1, or 386.5' F. Since there has been assumed a canvas temperature increase of 5" F., there will be assumed a corresponding decrease in the drop through the coveing. or the temperature drop throughoHi-Temp as 292' F. and the drop through Magnesia as 383 F., with the temperature between the Hi-Temp and Magnesia covering as 508' "F. 800' 508" The mean temperature of Hi-Temp then is "
+
v'
+-
+ 4
654' F., and
K1
= 0.677.
The mean temperature of the Mag-
nesia covering is 5080 4- 12" = 216.5' F., and KZ = 0.512. 2 675 Then h= = 85.5 B. t. U. 2.33 2.31 0.678 + 0.512 Checking again for the canvas temDerature difference. 272.5 X 85.5 = 55.40 F, Td = 85.5 335 This value checks very well with the last value of 55" F. assumed, and the heat loss is 85.5 E. t. u. The total heat loss per square foot of pipe surface per hour is, then. 8.55 X 7'8125 = 154.8B. t. u. 4.3125 -
I
+
-
FIG.1-INCLINED RAPID CIRCULATION TYPEEVAPORATOR
It is realized that this method of venting causes a slight increase in the heat transmission, but this is estimated to be well within the experimental error of the tests as a whole. The heating surface consists of seven 3-inch 0 . d., 1Pgage copper 1
Received January 19, 1924.
I N D U S T R I A L A N D ENGINEERING CHEMISTRY
May, 1924
tubes, 4 feet 10.5 inches long, giving an interior tube surface of 25.3 square feet and a total heating surface, including the tube plates, of 28.35 square feet. MEASUREMENTS
Vacuum was measured in the vapor body just below the baffle plate with a mercury vacuum gage and reduced to a 29.92-inch barometer. A thermometer placed in the bottom of the downtake generally registered one or two degrees lower than the boiling temperature corresponding to the vacuum. Because loss of heat through radiation takes place in the downtake, the thermometric reading was disregarded and the boiling temperature was calculated from the absolute pressure in the vapor body. The steam pressure was determined with a Foxboro compound gage, which was calibrated against a mercury vacuum gage and a standard pressure gage-tester. I n most tests the steam entered the steam chest in superheated condition caused by throttling in the regulating valve. At the entrance to the vent tube the temperature was 5" to 10" F. below the temperature of saturated steam a t the indicated pressure. To determine any possible pressure drop between both ends of the steam chest, additipnal gages were placed in the steam inlet and vent in Tests 17, 18, and 19. These gages indicated no pressure drop at 8.25 pounds, 0.85 pound drop a t 9.7 pounds, and 1.46 pounds drop a t 11.8 pounds absolute pressure. The average steam temperature determined by these two gages checked within 1.5" F. with that calculated from the reading of the gage placed directly in the steam chest, at a point a little below the upper inlet. The liquor level in all tests was approximately 22 inches above the bottom of the lowest row of tubes. In the tests in which the steam entcred the bottom of the steam chest, the liquor surged and fluctuated over a range of 2 to 3 inches. After shutting down the evaporator the stationary level was about 6 inches less, indicating that in this type of evaporator a liquor gage registers more nearly the level in the downtake than in the tubes.
vacuum. In Table I are tabulated the results of tests made with the steam entering the steam chest a t the bottom. Table I1 shows the results where the steam entered the top. Although the temperature difference is not constant, it is so high that the heat transmission coefficient should be independent thereof, which would make it permissible to plot the heat transmission coefficient against the temperature level. If this is done the results of the tests in each set are found to lie very close to a straight line (Fig. 2). Above 10 pounds absolute pressure, with steam entering the top of the steam chest, a higher heat transmission is obtained, as is to be expected, because the steam sweeps down the tubes and tends to remove the film of condensate. In some of the tests a level was carried both below and above the average of 22 inches, but no material influence on the heat transmission could be observed if the variation in temperature level was taken into consideration (see Tests 13 to 16).
I100
900
a00
700
600
IIS
TABLE I-STEAM ENTERINO BOTTOM OF STEAM CHEST
Test 1 2 3 4 5
Steam Vacuum Pressure Iriches Lbs. Abs. Mercury 7 35 26.85 9 8 26.25 9 8 26.25 12 25 25.6 14 7 24.75
Steam Temp. OF. 179 192 192 203 212
Heat Transmission Boiling Temp. Feed Coefficient Point Diff. Temp. B . t . u./Sq.OF. OF. OF. Ft./Hr./O F . 116 63 54 620 122 70 70 680 122 70 58 695 128 75 65 780 135 77 66 885
Heat, transmission had to be calculated from the amount of water evaporated, because no facilities were available for determining the quality of the steam. Vapors mere passed through an entrainment separator, condensed in a surface condenser, and weighed. Both steam chest and vapor dome are subject to radiation losses, which would affect the weight of either condensed stearn or vapor. This radiation loss adversely influences the heat transmission calculated from the weight of condensed vapor, which partly offsets the slight increase caused by venting the steam chest into the vapor body. During each test two or more weights were taken, each over a 15-minute period, which was sufficient to obtain satisfactory checks. All readings are the average of three or four observations in each period.
120
ILS
ISS
130
100
105
TrrIPcRnTurcE LEVEL D C q a r r o FAR. FIG. 2
Another set of tests (Table 111) was then made at a temperature level of approximately 140" F., again with varying steam pressure, steam entering the top of the steam chest. Contrary to expectations, the heat transmission coefficient varies greatly with the temperature difference. T A B L E11-STEAM
ENTERING TOP
STEAM CHEST Heat Transmission Vacuum Steam Boiling Temp. Feed Coefficient Steam Pressure Diff. Temp. B . t . u./Soq. Inches T,emp. Point Test Lbs. Abs. Mercury F. O F . OF. O F . Ft./Hr./ F . 6 26.55 7.35 179 119 61 60 630 26.1 7 9.85 192.5 63 09 720 123.5 25.55 8 9.55 191 76 128.5 62.5 835 25.0 9 12.4 203.5 62 133 70.5 900 24.3 10 14.7 212 62 74 138 1020 23.95 11 16.95 219 140.5 78.5 75 1060 23.55 18.5 12 224 75 143 1140 81 26.2 130 9.75 192 63 123 69 680 14b 9.6 191.5 26.25 122 63 69,5 660 12,25 24.6 150 203 62 136 960 67 12.25 24.4 16d 203 62 137.5 995 65.5 a 12-inch level. b 30-inch level. c 18-inch level. d 27-inch level.
TESTS After starting the evaporator sufficient time was allowed to elapse for conditions to become constant. All tests were made with distilled water, colored with a little tannin extract to detect entrainment. The first two sets of tests were made with variable steam pressure but without regulation of the
459
Test 17 18 19 10 12
OF
TABLE111-STEAM ENTERING T O P O F STEAM CHEST (Temperature level approximately 140O F.) Heat Transmission Vacuum Steam Boiling Temp. Steam Feed Coefficient Pressure Inches Temp. Point Diff. Temp. B. t .u./Sq. Lbs. Abs. Mercury OF. OF. OF. OF. Ft./Hr./'F. 8.25 23.75 184 142 42 51 465 9.7 23 7 5 192 142 50 59 835 11.8 23.75 201 142 59 69 890 24.3 14.7 212 138 74 62 1020 23.55 224 18.5 143 81 75 1140
