PAUL W. KILPATRICK and EDWARD BREITWIESER Western Regional Research Laboratory, U. S. Department of Agriculture, Albany, Calif.
. Increasing Evaporator
I
. . . With Polished
Capacity
Tubes
Increased capacity and longer operating time could improve plant efficiency and increase profits F o u ~ i s cof evaporator tubes decreases heat transfer, lowers product quality, and requires frequent shut-down to clean the fouled tubes. Fouling also requires additional capital investment because equipment must be overdesigned to meet plant capacity. During cleaning of the 3-inch polished bore of a turbulent film evaporator, it was observed that "burn-on" of food products started a t the rougher surfaces. Therefore, the effect of tube bore surface finish on evaporator capacity was studied. One-inch (outside diameter) stainless steel tubes were used in a 9-foot, vertical, forced-circulation evaporator concentrating tomato paste from 15 to 327, solids. Evaporator capacity \vas increased about 80% by replacing the unpolished \vith polished bore tubes. Because of variable bore roughness of unpolished tubes, there may be a twofold (or larger) variation in evaporative capacity from tube to tube. Polished bore tubes, because of their smoother and more uniform bore surface, gave uniformly higher evaporative capacity. There was less apparent heat damage to the product and less tube fouling d u e to burned-on material when po!ished tubes replaced unpolished bore tubes in the evaporator. A much longer operating time before shutdown for clean-out of burned-on material may therefore be possible. These results should be generally applicable in other processing industries.
All tubes used were pickled finish outside and otherwise identical, except for bore finish. Tubes 1, 11, and 111 had commercial pickled finish bores. Tubes 3, 33, and 333 had polished finish bores (final polishing grit was No. 320). Pumps in the evaporator system were the positive displacement type. Feed pump, PF, and hot-concentrate removal pump, PC, were a precision metering, gear type, mechanically interlocked to a common variable speed drive. Recirculation pump. PG, was operated at a fixed speed to provide a mass flow rate of 7 5 pounds per second per square foot through the 0.87-inch bore of the 1-inch tube. Calibration was made with 327, solids content tomato paste flowing at 190' F. Feed Material. Canned tomato pas!e (30% solids) was obtained from a California processor. T o ensure uniform raiv material for the evaporator studies, the pack \yas from a production run of high pectin content tomato paste. taken during a 3-hour period. This pack \vas composited, stored a t 34' F., and prior to each run was thoroughly mixed with distilled water to a total solids
content of 15.0% (13.8' Brix a t 68' F.); it was maintained in thoroughly homogenized condition. This feed material probably had less fouling tendency than the original raw material from which it was made. Procedure. Prior to each run, the tubes \yere changed, conditioned, and the evaporation rate checked \vith distilled water. Conditioning consisted of the following steps : Tube bore and exterior \\'ere thoroughly cleaned with alkaline solution. T h e filled tube was treated for about 16 hours \vith acid (concentrated HSOB saturated with HZCrO1). Distilled lvater \vas pumped continuously through the tube, and at thr same time 50-p.s.i.g. steam \vas applied to the evaporator jacket to permit "boiling-out" for a period of not less than 48 hours. O n start-up. the evaporator sJ-stem \vas filled with feed materia!. and the recirculating liquor \vas concentrated to approximately 28' Brix \iith 20-p.s i.g. steam in the evaporator jacket. .After the induction period, about 1.5 to 2
PUMPS P B - BOOSTER PUMP P G - RECIRCULATION PUMP FOR CONSTANT MASS VELOCITY- G P F AND PC F E E D AND CONCENTRATE PUMPS INTERLOCKED FOR CONSTANT RATIO OF FEED CONCENTRATE. DE- CONDENSATE P U M P
Experimental
Equipment. -4forced-ci r c u l a t i o n , vertical tube evaporator system was set u p (see floxv diagram). T h e evaporator consisted of a 10-foot length of 16-gage (0.065-inch) seamless tubing (T\.pe 316 stainless steel; 1-inch in outside diameter) jacketed with a %foot length of 3-inch pipe. T h e tube was protected from entering steam by a concentrically located 1.5-inch sleeve. Aliowing for the steam packing glands. the effective length of the steam jacketed tube was 8.59 feet
CHECK V A L V E
? STEAM TRAP
Forced-circulation, vertical tube system was used to study effect of tube bore finish in evaporating tomato paste VOL. 53, NO. 2
FEBRUARY 1961
1 19
hours. evaporator steam pressure was increased and maintained constant at 40.0 + 0.5 p.s.i.g. for the duration of each tomato run. Vapor temperature in the vapor-liquid separator was instrument-controlled at 190” I 1’ F. T h e instrument-controlled recirculation pre2’ F. heater suppiied material a t 187” to the up-flow entrance of the evaporator tube. Both recycled and discharging concentrates were controlled at 29.8” Brix (32.070 solids) by adjusting feed pump drive speed. At the end of each tomato run, “burn-on” material was disintegrated by the acid treatment. and any residue was easily removed by flushing and fiber bristle brushing.
*
I I
2
3
4
5
6
7
8
3
, 0 8 1 , 2 1 1 1 4
3PERATING - I V E , HOURS
Evaporator capacity was consistently higher when polished bore tubes were used
Results and Discussion T h e induction period was excluded from all data. T h e over-all heat transfer coefficient r, used as a “yardstick” for comparison of experimental results. was evaluated from the relationship: C’ = Q/A.lt, B.t.u./hour/square foot/’ F.
Lvhere:
Q
=
LV
= pounds of water evaporated per
1140 W , B.t.u./hour
hour
A = 2.24 square feet At = average temperature
difference, F. 287 - ( t i t?)/2 tl = temperature of product entering evaporator tube, ’F. t? = temperature of product leaving evaporator tube, F.
+
Table I includes a resume of results obtained by eight 1-hour tests, four made prior to and four made after each tomato run. There was no significant difference bet\veen the before and after check tests with distilled water for Runs 1 to 6. Decrease of tube wall thickness from the polishing operation had no significant effect on the over-all heat transfer coefficient, as the average value of VHz0 was somewhat greater for the unpolished
Table I.
tubes. This larger value for unpolished tubes may be attributed to the somewhat larger surface area presented by the rougher bore finish. However, for a non-Xewtonian fluid such as tomato paste, increased bore roughness appeared to slow down the flow of the laminar layer contacting the metal, to such a n extent that burn-on increased as bore roughness increased. Tube polishing would tend to make the degree of bore roughness more uniform by removing deeper scratches and striations; but it could also add to the total roughness by increasing. the number of minute scratches. Apparently the degree of surface roughness is a major factor influencing burn-on and is more important than the total roughness. A4tthe end of R u n 7, the distilled water check tests showed a significant increase in L.;Itq for polished tube 333. This change is possibly attributable to erosion of the tube bore as a result of attrition by the fiber content in the tomato paste. Comparing Runs 6 and 7, however, shows that the type of erosion and its effect on the tube bore surface finish should have little if any effect on the over-all heat transfer coefficient when
Effect of Tube Bore Finish on Over-all Heat Transfer Coefficient Polished bore tubes retained a high over-all
Tube Bore
Finish
Pickled
Run
Tube
YO.
NO.
r.4n
1 3
1 11 111
125 242 171 179
167 312 217 232
88 187 123 133
3 33 333 333
318 310 356 340 328
378 415 410 414 401
240 261 292 298 264
5
Av. Polished
2 4 6 7 Av
.
...
...
r,b
Glib
U 1‘8>*
... ... ... ... ... ... ... 285 ...
rH2flc
730
555 633 639 445 690 448
...
528
a Average over-all heat transfer coefficient based on total pounds of evaporation during 14 Heat transfer coefficient at end of hours of operation on tomato following induction period. Average value of heat transfer coefficient 1, 14, and 32 hours of operation, respectively. when evaporator was operated on distilled water.
120
INDUSTRIAL AND ENGINEERING CHEMISTRY
operating on tomato. Consequently, it can be assumed that any erosive action by the fiber in the tomato paste increased the total amount of roughness but not the degree of roughness in the tube bore. Future experimental data may show that it is feasible to improve the surface finish of unpolished tubes by suitably controlled accelerated erosion. When polished bore tubes were used to evaporate tomato, the evaporator’s capacity was consistently higher than when pickled finish tubes \vere used (see graph and Table I). Polished bore tubes retained a high over-all heat transfer coefficient and showed but little fouling compared with unpolished tubes. Based on the average over-all heat transfer coefficient Cr,, the evaporator’s capacity was larger by a factor of about 1.8 with polished bore tubes. Runs with pickled finish bore tubes were stopped after about 15 hours of operation, when the product was obviously heat damaged severely, as evidenced by color and odor. However: when using polished bore tubes (Runs 2. 4: and 6), only lack of feed material limited the running time to about 13 hours. R u n 7 was made for a dual purpose: to determine how closely results could be checked and to determine if a polished bore tube could be satisfactorily operated for a n appreciably longer time than unpolished bore tubes. Runs 6 and 7 were duplicates! except lor the longer running time in the case of Run 7. The two runs checked within about 570. R u n 7 was terminated after 33 hours of continuous operation, as the product appeared heat damaged. At the end of 32 hours of operation, the over-all heat transfer coefficient for the polished bore tube was considerably higher than coefficients for pickled finish bore tubes after 14 hours of operation (Table I). Use of polished bore tubes might therefore decrease the necessary “clean-out” frequency by a factor of 2. Inspection of the tube bore after R u n 7 showed a continuous deposit of burnedon material, but the deposit did not appear nearly as heavy as in the case of Runs 1, 3 , and 5. Bores of the unpolished tubes showed a heavy (but nonuniform) continuous layer of burned-on material at the end of Runs 1, 3. and 5. T h e thickest deposits appeared as radial rings (or partial rings) with a somewhat thinner deposit between the rings. Ring spacing varied from a n estimated 0.23 to 1 inch and varied randomly throughout the steam-jacketed section of the unpolished tube bores. Polished bore tubes (at the end of Runs 2, 4. and 6) shcwed very little burn-on. This burn-on appeared to be a thin, spotty film confined to small discontinuous areas and apparently distributed a t random. RECEIVED for review September 6, 1960 ACCEPTED October 24, 1960
