I
ARTHUR I. MORGAN, Jr., and ROBERT A. CARLSON Western Regional Research Laboratory, U.
S. Department
of Agriculture, Albany, Calif.
Steam Injection Heating Steam iniector design principles for a fast, simple heating method to warm liquids to temperatures below their boiling points
WARMING
liquids to temperatures below their boiling point by introducing steam which totally condenses can be called injection heating. This method of heating has been used especially in plastics and in food processing, but it is now also used for killing microorganisms or inactivating enzymes, and for applying heat so that flash vaporization occurs when pressure is reduced. However, lack of design data has limited the use of this kind of heating; therefore, the work described here is devoted mainly to mechanism. Apparatus for heating to pasteurize or sterilize by steam injection has been the subject of a number of patents ( 7 , 3, 5-7, 9-77). Some of the devices have been known by such wade names as Rotojet, Mallorizer, Torpedo, Vacreator, LHT, Uperisator, and Califactor. The design described here has been presented ( 7 , 2). The only previous study of injection was incompletely published (8). Besides mechanism, this article presents designs and conditions of operation resulting in the most rapid heating possible. The extent and duration of the steam-liquid interface are critical for use with heat-sensitive and surface-active solutes or colloids. The uncondensed steam volume is given so that the size of the chamber for any liquid flow rate may be calculated for any duration of heat treatment.
Experimental
A variety of designs were tested. None showed any important advantage over the simplest arrangement. The liquid feed was moved by a positive displacement pump through entry A into a 3/4-inch stainless steel pipe 6 inches long, B. Steam enters tangentially through F. The combined condensate and feed warm thermocouple C, which sends a signal to a controller that regulates steam flow. The liquid passes through orifice D and flashes into E. T o take moving pictures, the steel injector body was replaced by glass tubing of various diameters. Liquid,
A
E
B
After testing several designs, the simplest type of steam injector was chosen vapor, and noncondensable gas were separately collected and measured. The motion pictures were taken through a shield with a high-speed camera. By means of a 120-cycle-persecond index mark, the film was edited to retain only sections taken at 1600 frames per second. In this way, the action could be viewed at one hundredth its actual speed.
Results The major observation was the extreme rapidity of the process. This is shown by the fact that the volume of the steam jet never exceeded 1% of the volume it would have shown at the prevailing pressure had it not been condensing as it emerged. The volume of a well regulated jet is less than 5 cc., although the steam is entering at sonic velocity through a 3/16-inch port. The steam entered the liquid in periodic jets. By study of individual frames of the high speed film, the growth and decay of the steam jet can be seen, although the satellite bubbles of gas make good definition impossible. The frequency of these jets was related
inversely to the temperature of the water. These frequencies, being close to middle C, are distinctly audible but not unpleasant. If there is no solid surface normal to the axis of the jet and the port is less than l j 4 inch in diameter, the sound intensity is moderate. The phenomenon does not resemble a vibrating closed water column, which would have a much higher frequency and rise in pitch with temperature. The rate of condensation of the steam was calculated from the observed Bow rates and the bubble area was measured on the photographs. The condensation rate is approximately predicted by the Langmuir equation ( 4 ) . This presumes that all gas molecules reaching the liquid surface condense, and all liquid molecules reaching the gas surface evaporate. The observed rate is therefore close to the theoretical maximum. For steam condensing on solid surfaces, the heat transfer coefficients are very seldom above 2500 B.t.u./hr. sq. ft. F. The corresponding coefficient for direct steam injection was found to be 350,000 B.t.u./hr. sq. ft. O F., measured as described above. VOL. 52, NO. 3
MARCH 1960
219
A simplified model of the process was developed, which assumes that each steam pulse is surrounded by a shell of liquid which it warms by pure conduction and into which it condenses. No heat transfer due to mixing of liquid between the shells of adjacent steam pulses is assumed. This model is shown here as a cartoon strip. The events in the same area are shown a t later times from left to right. Steam is assumed to enter only in the second, third, and sixth frames. This model fits the flow rates, pulse frequencies, and fluid properties observed in a well designed steam injector. The successful operation of a direct injection heater depends on four rules: (1) The liquid flow should be turbulent inside the injector body; (2) the excess of pressure in the injector above the saturated pressure for the attained temperature should be from 3 to 7 p.s.i.; (3) the liquid flow rate should be as independent as possible of the injector pressure; and (4) the system should be as free as possible of uncondensable gases. The Reynolds number (73) of the liquid must be over 3000, preferably over 10,000 for rapid condensation. This must be achieved by the use of a liquid cross section of the smallest possible diameter, especially if the liquid being heated is viscous. Laminar flow results in formation of a steam core due to swirl. This core produces instability when it reaches the discharge constriction. The difference between the hydrostatic pressure in the injector and the vapor pressure of the heated feed should be over 3 p.s.i., preferably over 7 p s i . This implies that the steam is usually 4’ to 9’ F. warmer than the liquid. The maximum pressure difference is fixed by the heat sensitivity of the liquid. This arises from the fact that the temperature of the steam in contact with the liquid depends on the pressure in the injector. The feed pump must be a constant displacement type, and musi be located close to the injector. The steam pulsations can act back on a head-sensitive pump and unbalance the inherently unstable system
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0 MILLISECOND
I
WATER
MILLISECOND
n t
2 MILLISECONDS
50
Figure 1.
Liberation of dissolved gases may cause appreciable loss of steam
The liberation of gas from the feed by heating, or the inclusion of uncondensable gas in the steam, results in an unpredictable two-phase flow through the downstream constriction. The amount of steam lost to the heating process by this means can be appreciable. This is shown in Figure 1, where Ap refers to the difference between the hydrostatic pressure and the saturated pressure corresponding to the liquid temperature. The presence of uncondensable gases greatly increases the duration of the gasliquid interface. As this surface becomes older. temperature-sensitive solutes from farthet away have time to arrive and be dena iurcd.
The form of the injecro;. should be such that no foulable solid wall separates steam from liquid. There should be no wall a t right angles to the axis of the jet. The size and number of entry ports are determined by the rate of steam flow required. The diameter of a single port should not exceed ‘/4 inch. As such ports are converging nozzles, the velocity 3 MILLISECONDS
4 MILLISECONDS
of steam inside the port cannot be greater than the sonic velocity-Le., about 1000 feet per second (72). Therefore if the steam pressure just upstream from the port can be kept at 175% or more of the pressure in the liquid, the steam flow will be independent of the liquid pressure. For any flow rate, the injector diameter which results in a turbulent Reynolds number is selected. A length is chosen which results in the desired retention time at the high temperature, allowing about 5 ml. per entry for steam volume. literature Cited (1) Brown, A. H., Kilpatrick, P. W., Lazar, M. E., U. S. Patent 2,636,430
Design Principles
5 MILLISECONDS
(April 28, 1953).
(2) Brown, A. H., Lazar, M. E. Wasserman, T., Smith, G. S., Cole, M. W.,
IND. ENG.CHEM.43, 2949 (1951).
( 3 ) deBethune. G. S. P.. U. S . Patent \
,
2,077,227 (April 13, 1937). (4) Glasstone, S., “Physical Chemistry,” 2nd ed., p. 1118, Van Nostrand New York, 1946. (5) Grinrod, G., U. S. Patent 2,170,195 (Aue. 22. 1939). (6)’ Hammer, B.’ W., Horneman, H. C., Parker, M. E. (to Sealtest System Laboratories), Ibid., 2,130,643 (Sept 20, 1938). (7) Hawk, L. R . (to Golden State, Ltd.). Zbid., 2,492,635 (Dec. 27, 1949). (8) Mohler, H., Chimia 6 , 212 (1952). ( 9 ) Morrow, P . W., Parscns, J. L. (to E. I. du Pont de Nemours & Co.), U. S. Patent 2,467,769 (April 19, 1949). (10) Rogers, C. E., Ibid., 2,122,954 (July
5. - , 19381. (11) Ursina, A . G., Brit. Patent 674,695 ~
t
t
STEAM
The simplified model assumes that each steam pulse is surrounded by a shell of liquid
220
100
INJECTOR PRESSURE PSlG
INDUSTRIAL AND ENGINEERING CHEMISTRY
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(June 25, 1952). (12) Vennard, J. K., “Elementary Fluid Mechanics,” 3rd ed., p. 102, Wiley, New York, 1954. (13) Zbid,,p.. 146.
RECEIVED for reiriruv April 8, 1955, ACCEPTED Ncveinber 20, 1959
