INDUSTRIAL A N D ENGINEERING CHEMISTRY
May, 1924
Heat Transfer in Enamel-Lined Apparatus' By Emerson P. Poste ELYRIA ENAMELED PRODUCTS Co., ELPRIA,OHIO
T
HE data to be presented herein are the result of observations on a variety of types of equipment operated under a wide range of conditions. All the work has been done on commercial units, in some cases as tests in the factory before shipment, and in others in connection with field operation in the customer's plant. Typical cases are covered to illustrate what may be expected under various conditions, and from these and other data general conclusions are drawn. In computing the over-all coefficient of heat transfer from test data, the familiar equation K = -is used. H 0, K =: the "over-all coefficient of heat transfer"-namely, the number of heat units per unit area per unit of time per degree mean temperature difference C =: the total number of heat units transferred per unit of time H =: the area of the surface through which the heat is transferred 8, = the logarithmic mean temperature difference I n all cases the unit of time is taken as 1 hour and values for K are given both as B. t. u. per square foot per hour per degree Fahrenheit ( K E )and as kilogram calories per square meter per hour per degree Centigrade ( K F ) .
ENAMELED STEELUXITS The partition through which heat is transferred in enameled steel equipment is, as a rule, a relatively thin coat of enamel as applied by the wet process and a plate of steel, usually inch thick. The simplest case is that of a jacketed kettle used for heating water by means of steam. A unit actually tested was a kettle 30 inches in diameter with a jacket 46 inches high. It was equipped with a small side-propeller agitator near the bottom. The test involved heating water from 70" to 140" F. by steam a t slightly above atmospheric pressure. The over-all coefficient of transfer for various rates of agitation follows~ 11.D. m. Aaitator 0 103 190 335 385 408
I"F
IC E
480 600 742 655 600 600
96 120 148 131 120 120
The action of the agitator in causing an increase followed by a decrease is worthy of note. A second case of somewhat the same nature involved a jacketed kettle with a vertical agitator shaft centrally placed, carryiqg three sets of propeller blades. In addition t o the agitator there were two vertical, adjustable baffle plates half way from center to side of the kettle, by means of which liquid could be deflected towards or away from the walls. The test involved heating water from 70" to 200" F. by steam a t 10 pou ds pressure.
P,
BAFFLES Straight
R. p . m. Agitator
KF
KE
0 17 36 70
400 447 506 586 476 543 635 507 585 667
80 89 101 117 95 109 127 101 117 133
dowards wall Away from walls
1
R.eceived January 17, 1924.
17 36 70 17 36 70
469
Increased agitation raised the over-all coefficient of transfer in each case; breaking up of the rotating action of the liquid by the baffles increased the rate, more by deflecting away from, than towards, the walls. The heating capacity of a 36 X 36-inch jacketed kettle was determined for heating water through various temperature ranges, using steam as a heating medium. Gentle agitation by a side-propeller agitator was involved in each case. Heating Range
F. 40 t o 145 40 to 210 70 to 145 145 to 210
KF
KE
430 425 490 490
86 85 98 98
A jacketed kettle 3 feet in diameter and 9 feet deep was tested while pasteurizing milk. A side-propeller agitator was used a t 200 r. p. m. Heating milk from 40" to 145" F. by steam slightly above atmospheric pressure gave KF = 430 K E = 86 A case of milk pasteurization in a 54 X 36-inch jacketed kettle with a side agitator a t 600 r. p. m. and steam a t slightly above atmospheric pressure gave KF = 620 K E = 124 Another test involved the pasteurization of cream in a 33 X'6O-inch jacketed kettle by steam a t 10 pounds pressure, a side agitator being used: KF = 450 K E = 90 There is but scant information on the over-all coefficient of transfer when heating liquids by hot water in the jacket. One case on record covers heating water from 90" to 145" F. by means of water a t 200" 3'. KF = 350 K E = 70 Numerous determinations of the over-all coefficient of heat transfer from steam to boiling water have given rather uniform results. Agitation by mechanical means is of relatively small effect. Values average KF = 700 KE = 140 The lower value involved in the heating of a much less mobile mass is evident from a test on a jacketed kettle used for boiling shredded pineapple by steam a t 35 pounds with no agitation. KF = 165 K E = 33 The effect of the agitation of a thick liquid is shown in the case of the vacuum concentration of tomato pulp with steam a t 10 pounds. Good agitation was accomplished by a sidepropeller agitator. No agitation Good agitation
KF
KE
450
90 154
770
There is a wide range in results of tests on the use of jacketed kettles for cooling liquids, in view of differences in rate of flow of cooling medium. One test on the cooling of water by cold water flowing through the jacket of a 36 X 36-inch jacketed kettle with mild agitation gave Kp = 215 Kg = 43 Milk in a 54 x 36-inch unit was cooled from 145" to 110"F. by circulating water initially a t 80" F. Good agitation was produced by a side-propeller agitator a t 600 r. p. m. From 110" to 50" F. the milk was cooled by brine initially at 18" F, Water Brine
KF
KE
620 480
124
96
INDUSTRIAL A N D ENGINEERING CHEMISTRY
470
In another case, cream was cooled in a 33 x 60-inch unit with good agitation with a rate: K E = 420 KF 84 Another case of cooling milk by cold water with good agitation gave K E = 520 KF = 104 The data above include the extreme values that have been noted. In high-temperature work hot oil has been circulated in the jackets of steel tanks. Not a great deal of information is available. The heating of oils in such units has been accomplished with over-all coefficients of heat transfer ranging from K E = 13 to 24 KF = 65 t o 120 Under given conditions thorough agitation of the contents of the kettle has increased the value of K by about 10 per cent. Though of no practical value, tests on the rate of transfer from oil at 550" F. to water have been made. CONDITIONS Heating water Boiling water
KF 100
KE 20
150
30
Good agitation increased the values for heating water by about 30 per cent. A rather limited amount of information has been obtained from tests on double-pipe heat exchangers working as liquid heaters by steam, and as condensers with cold water in the outer space. With such units placed vertically and working as preheaters, over-all coefficients of heat transfer have varied from K E = 100 to 160 KF = 500 to 800 A narrower range of conditions has been involved in tests on this type of unit serving as a condenser. Results on record average K E = 140 K F = 700
ENAMELED CAST-IRON UNITS I n this type of apparatus heat is transferred through a partition involving relatively thick cast iron (0.5 to 1 inch) coated with the thick layer of enamel characteristic of the dry process. I n general, the units are smaller than those made of steel and no efficient means of agitation is involved. I n small open-top kettles, tested with steam a t 75 pounds pressure, and transmitting heat to boiling water, the over-all coefficient of heat transfer has been determined as averaging K E = 60 KF = 300 In a larger kettle, 36 inches in diameter, the over-all coefficient of heat transfer from steam to boiling water was found to be K F = 260
Kg = 52
A 3O-gal1onl cast-iron jacketed still was tested under various conditions, with results as follows: Atmospheric pressure: Top bare Top insulated Vacuum: Top bare Top insulated
-
K7?
KP
205 246
41 49
249 264
60 53
I n this case the evaporating capacity of the still as a whole was computed against the actual heating surface. Condensation of vapors in the top cut down the effective capacity more when working against atmospheric pressure than when under vacuum, owing to the higher outer surface temperature.
Vol. 16, No. 5
A larger still, 100 gallons, was tested at atmospheric pressure with the top bare, and the effective coefficient of heat transfer, calculated from the evaporating capacity of the unit, was Kp = 260 K E = 52 A cast-iron condenser was studied as to its rate of transfer while condensing steam at atmospheric pressure. KF = 230 K E = 46 COMPARISON OF WET AND DRYPROCESS ENAMELS I n general, the cast-iron units have a lower over-all coefficient of heat transfer than the steel. The question arose as to whether this is the result of thicker metal or thicker enamel in the cast-iron pieces. To answer this question, two like steel units were enameled, one by the wet process typical of steel apparatus, and the other by the dry process used on cast iron. The essential difference was the relative thickness of enamel. Under like conditions of testing rates of transfer were found to be as follows: CONDITIONS Steam t o water being heated Steam to boiling water
ENAMEL Wet Dry Wet Dry
KF 420 260 720 360
KE 84 52 144 72
The foregoing table shows that the over-all coefficient of transfer for the wet process enamel on steel is of the same order as in previous cases, while that of the dry process enamel on relatively thin steel is of the same order as when on thicker cast iron. The obvious conclusion is that the thickness of the enamel is the determining point in the case. SUMMARY Observations on a large number of commercial steelenameled units have led to the following over-all coefficients of heat transfer under the conditions named: OVER-ALL COEFFICIENT OF HSATTRANSFER Kg. Cal. per Hour B.t. u. per Hour per Sq. M. per per Sq. Ft. per
CONDITIONS Steam to water being heated Hot water t o water being heated Steam t o boiling water Steam t o a thick fruit product Cooling water by cold water a n d by brine Hot oil to oil being heated Hot oil to boiling water Steam to water being heated in tubular heaters Steam being condensed t o water in tubular condenser jacket
c.
" F.
400 t o 700 350 700 100
80 t o 140
200 t o 600 06 to 140 150 to 200
40 t o 120 13 t o 24 30 to 40
600 to 800
100 to 160
O
700
70 140 32
140
Velocities of liquids over heating surfaces as affected by agitation, differences in mobility, and specific heat have been involved in effecting the variations in values. The data on cast-iron units are more limited. Over-all coefficients of heat transfer have been determined as ranging from 260 to 350 kg. cal. per square meter per hour per degree Centigrade, or from 52 to 70 B. t. u. per square foot per hour per degree Fahrenheit. The thickness of the enamel coating, rather than the thickness of the metal, seems t o be the determining factor in the over-all coefficient of heat transfer. The values stated are based on actual operating conditions rather than theoretical considerations. The justification of this lies in the fact that the figures are used as the basis of designing new installations when rates of heat transfer for units must be predetermined, and, with proper allowances for variations and factor of safety, the results are satisfactory.
