US1735980A - Process of evaporation - Google Patents

Description

Nov. 19, 1929. P. B. SADTLER I 1,735,980

PROCESS OF EVAPORATION Original Filed July 9, 1926 aflyam I 971g W4 11.

- Original application filed July 9, .1926,

Patented N av. 19, '1929 UNITED sm'rss PATENT OFFICE PHILIP 2B. SADTLER, or CHICAGO, I LI'NoIs, assrenon T0 swan-son EVAPOBATOR COMPANY, OF HARVEY, ILLINOIS, A CORPORATION" or ILLINOIS PROCESS OF EVAPORATION 1927. Serial My invention relates to methods or processes of evaporation.

It will be explained in conjunction wlth and particular reference to evaporatlng apparatus of the so-called vertical tube type wherein the liquid to be evaporated circulates upwardly through vertical tubes while the heating medium is applied to the outside of the tubes.

This application is a division of my copending application Serial No. 121,452; filed July 9, 1926.

Heretofore it has been the common prac- 'tice to provide tubes of relatively large diameter and short length or in coils or with abrupt bends and depend upon a relatively slow movement of the liquid into and through the same. Ordinarily natural circulation, that is, the circulation induced by the lifting effect of bubbles of vapor rising through the tubes has been depended upon to effect the heat transfer from the tubes to and through the liquid. Of course, the agitation and circulation of the liquid under such conditions is relatively sluggish, the transfer of heat is correspondingly slow and evaporation proceeds at a comparatively low rate.

The so-called film theory of heat transfer to and from metallic surfaces is now generally accepted. According to this theory the principal resistance to heat transfer from the heating medium, such for example as steam, on the outside of the evaporator. tubes to the tubes and from the inside surfaces of the tubes to the liquid within them is due to relatively stagnantfilms of fluid lying against the outside and inside walls of the tubes. The film on the inside of the tubes is composed of the liquid being heated. The film on the outside of the tubes is composed, for example, of condensed steam and air and other non-con- (lensable gases, and is usually called the steam. film. These films produce most of the temperature drop between the heating fluid, for example, steam, and the liquid being evaporated. Thus, ignoring the effects of the metal walls of the tubes, and of scale which may be present, the overall rate ofheattransfor from steam on'the outside of the tubes to the liquid on the inside of the tubes may be Serial No. 121,452. Divided and this application filed August 5,

represented by the following fo mula, whereis the steam film coefficient, and K is the liquid film coefficient:

U 1/K +1/K It has been recognized that increasing the velocity of the liquid flowing through the evaporator tubes would decrease the thickness of the stagnant liquid film and correspondingly raise the liquid film coefficient with the result that the rate of heat transfer is increased. Some attempts have been made to augment the velocity of the circulating ,liquid by indirect methods such as the provision of screw conveyors and paddle or propeller wheels and the introduction of buoyant agents such as air or other gases. Where viscous or easily scaling liquids have been treated, forced circulation has been provided by positive acting pumps. But with the adoption of all of these expedients to raise the liquid film coefficient, the practice has been to use large diameter short tubes or tubes offering such resistance to high velocity flow that only relatively low velocities and heat transfer to the treated liquid has been attained. 3 1

It is generally accepted that, when steam or similar heating media. are employed, the stagnant film on the outside or steam side of the evaporator tubes is composed of condensed steam and non-condensable gases,

such as air, which are always present in or accompany the steam. Although, as will be apparent from the above formula, the steam film coeflicient is a factor in the overall rate of heat transfer between the steam side and the liquid side of the tubes, yet under the common practice of employing large diameter tubes and slow circulation of liquid therethrough, the liquid film resistance has been so high that the steam film resistance was, or at least has been considered negligible in the ultimate result. However, by utilizing small diameter long tubes and high velocities such as I propose. the steam film resistance to heat transfer becomes a matter of real importance. Where heating media of the character of flue "treated liquid.

- Another object is to provide a process asesv have been used, the heating medium fllmresistance has been so high that the liquid film resistance was relatively unimportant. Consequently I- not onlyincrease the liquid film coefiicient by providing high velocity cireulation of the liquid thlmiliillll the tubes, but I raise the heating medium co-' eflicient by reducing the stagnant film of non- 1 process or method of operating apparatus for heating liquids to be evaporated.

Another object is to provide a process which will materially raise the liquid film coeflicient to increase the overall rate of heat subject to foanimg.

transfer from the heating medium to the whereby the film coeflicient of a heating medium may be raisedif desiredor necessary still further to raise the overall rate of heat transfer.

Another object is to reduce the formation of scale whenevaporating liquids subject to scale formation under heat.

,Another object is to reduce the formation of foamdurin the evaporation of liquids per- Anoth object is to provide a process mitting a reductiom of heatin area without decreasing'but in fact increasmg the rate of evaporation.

Qther purposes, objects and advantages will appear from the process or method hereis heated and boiled in a heatin element 9.

inafter described.

My improved process or method may be:

performed by the apparatus shown in the accompanying drawing, but, of course, other apparatus may be employed.

Inthedrawing: Y

Fig. '1 is an elevation and partial section of an evaporator;

Fi 2 is an enlarged'section on the line Fig. 3 isan enlarged vertical section of the top portion of the vertical tube heater' on the line3 .3ofFig.2.

According to my improved method or process the liquid to beItreated is positively forced at a high entrance or initial velocity into an unobstructed substantially straight high velocity zone composed of'heating tubes of relatively great length and small diam eter.; Augmented by the efiect of the steam generated during the passage ofthe liquid through the tubes'the velocity of the treated liuid becomes very high, the liquid film coe cient is raised and heat is imparted to the liquid at a rapid ratef The treated liquid is freely expelled or projected from the tubes into the evaporating chamber and, since the velocity increases 'as the liquid approaches the exit ends of the tubes, because of the high velocity-acceleratin eflfect of the boiling within the tubes, t e liquid film coeflicient and rateof heat transfer increase in the 'direction of expukion. In order still further to augment evaporation when vaporous heating media such as steam are employed,the,

heatmgmedium may be caused to flow along 7 the outside of the heating tubes in a, direction reverse to that of the treated liquid to remove from the tubes the condensed vapor and nQn cOndensabIegaSes, reducing the .film'.

thickness and raising the film coeflicient on the heating medium side of the tubes, parcoeflicient is highest.

The improved process or method is especially applicable for treating liquids that are vis-' cous, have high boiling points or carry sus- The evaporator chosen for an applicationof the "process has, a generally cylindrical metal body 5 which forms a dollecting chamticularly in the region where the liquid film 7 her in which separation of vapor and liquid takes place. This body has a cover 6 and. a bottom 7 A vapor pipe 8 communicates with this chamber through cover 6. This vapor pipe, as is common in evaporator practice, is for'the discharge of vapors driven off duringlthe evaporating process and for the applhlcation ofwhatever vacuum may be desira e.-'

The treated liquid in the evaporator body This heating element comprises, 1n general, a

long tubular section 10 anda relatively short enlarged cylindrical section 11. Tubular section 10 extends below body 5 and projects upwardly through bottomv 7 some distance into the evaporatmg chamber. It is provided with an annular flange 12 by which the heater maybe attached-to the evaporator body-by appropriate means such as bolts 13. A suit-- able packing ring 14 maybe employed to insure atight joint. Section 10 is tightly closed 1 The lower end of drum The lowerends of section provided with a downwardly extending annular flange 20' for thereception of a liquid inlet pipe to be hereinafter described.

A series of long and substantially straight tubes 22, which are open at both ends, extend through the steam drum and openings in the two tube sheets 15 and 19. The ends of these tubes are pressed, expanded or otherwise tightly secured to the tube sheets so that steam under pressure may be supplied to the drum outside of the tubes and cannot escape. In effect, the heater is like a Water tube boiler in that the treated liquid is inside of the tubes and the heating medium is on the outside thereof.

Steam is supplied to the drum in the space or chamber aroundthe tubes through a steam inlet 23-controlled by a valve 24. Excessive pressures can be avoided by providing a steam relief valve 25. The enlarged section 11 provides a chamber for the collection and separation of the water of condensation and the non-condensable gases which usually accompany steam. The condensate may be removed by a valved drain 26 located at the bottom of the collecting chamber, while the gases may escape through a valved gas vent pipe 27 at the top thereof. The side wall of. section 11 may be made of relatively thin sheet metal so that the additional function of an expansion joint may be performed by this section.

A steam deflector is located within the steam chamber of the drum to direct the flow of steam downwardly along the tubes. This deflector is in the form of a tube 30 which lies between the inner wall of drum section- 10 and the group of tubes. Its lower end fits within the inwardly projecting rim" 31 of flange 16 and, either by the tightness of- I downwardly in the steam chamber along and in contact with the tubes in a direction opposite to that of the flow of liquid therethrough and finally collects as condensate in' section 11. The condensate may be removed through drain 26 and the gases liberated through vent 27. If desired, the lower end of deflector tube 30 may project below flange 16 in order to provide a gas trap in the top of section 11.

4 Liquid to be treated is supplied by a valved liquid supply pipe 32 in the evaporator body. The liquid is withdrawn from the body through a liquid outlet pipe 33 which leads to a pump-34, such as a centrifugal pump. A valved inlet pipe 35 extends from the dis-- charge side of pump 34.- and has its upper end tightly fitted within and connected to flange 2O of bottom plate 19 so that the pump directly and positively forces the withdrawn liquid upwardly into the lower ends of the heater tubes.

A somewhat cone-shaped bafiie 36 is located above the open ends of tubes 22 in body 5, to disperse and diffuse the hot liquid which is projected from the tubes when the evaporator is in operation. The concentrated liq uid, or other products resulting from evapo ration, may be withdrawn through a valved outlet 37 in pipe 35.

" The heating tubes are made relatively long and of small diameter, for example, for ordinary evaporating processes, these tubes may be in the neighborhood of 6 to 10 feet long and to l inch inside diameter. The length of the tubes does not excessively increase the height of the evaporator body be- 1ejause the tubes may be largely outside of the ody.

In operating the evaporator according to my improved method, the liquid which "is withdrawn from the evaporator body, is positively pumped directly upward at high velocity into and through the long slender tubes. At the same time, the steam is directed downwardly outside of the tubes, op-

posite to the direction of the liquid flow therethrou'gh and in the same direction as the force of gravity acting upon the film of condensate and gases-which tends to cling to the tubes. Because the stea is directed downwardly by the steam de ector, condensation creates a downward current or flow of the steam along the tubes at a relatively high velocity. I The steam velocity is greatest nearthe upperendsof the heater tubes which is the region where the liquid velocityalso is the greatest. Consequently, the steam film coeflicient is highest in that region of the heater where the liquid film coeflicient is the highest.

The entrance velocity of the pumped liquid at the bottom ends of the tubes may, for exemple, be raised 'to the neighborhood of 5 to 7 or more feet per second as compared to an entrance velocity probably much. less than 1 to 2 feet per'second where natural circulation is employed. l/Vith this initial high velocity augmented by the lifting effect of the vapor created by the boiling the treated liquid, as steam is formed by evaporation while passing through the heating element, attains great speed and momentum as it flows upwardly through the tubes and is projected directly into the vapor space above the level of .the liquid in the evaporator body chamber. The high velocity of the liquid flow upwardly through the tubes reduces the stagnant liquid film on the inside of the tubes and also tends to remove and prevent the deposit and collection of scale so that the liquid film coefiicient is raised and the scale resistance to heat transfer is reduced. Consequently, the rate of heat transfer and evaporation are high and therefore high liquid velocities are entirely feasible, and excessive pump capacity and power consumption are not required.

The downwardly directed and high velocity flow of heating steam'carries along the condensate and non-condensable gases which tend to cling as a relatively stagnant and heat transfer resistant film on the outside of the, tubes, aiding the force ,of gravity to carry them-to the common collectmg and separat-' ing chamber at thebottom, where the condensate and gases are removed. Thus, the steamserves to sweep the film from the steam side of the tubes, resulting in a rise of the steam film coefiicient and increased heat transfer. And this higher steam coeflicient is par-' ticularly pronounced in the upper region of the tubes where the liquid film coeflicient is likewise highest.

The increase in the rate of heat transfer permits the use of a smaller area of heating surface for the treatment of the same or even more durable but more expensive metal may be employed for the tubes. Thus, nicke which for many purposes is by farthe most satisfactory metal because of its non-corrosive r 4 properties, may be used, although its cost ordinarily has been prohibitive with the prior processes and,-evaprators because of the relatively large heating surface required.

The extremely high velocity at which the,

treated 'liquidand vapors rise through the heating tubes prevents in a large measure the formation and retention of gas bubbles within the body 'of treated liquid. Further, if such'bubbles are formed they are broken by the excessive velocity with which the mass of liquid and vapor, issuing from the tubes, strikes the deflector. 36. It is the presence of these gas bubbles which constitutes the prin-r cipal agent in the formation of foam. Consequently, my hi h velocity process reduces the tendency to oam, because it reduces or prevents the formation and retention of gasbubbles within the body of liquid, and breaks any foam that may be formed by the action By inserting the heating element .as a unit from'the outside of the body and having all of the connections therefor entirely out side of the evaporating chamber, the place-' ment and removalof the heating element is a relatively easy matter and the number of openings and connections through the --wall of the evaporator body is reduced While yet retaining the desirable feature of introducing'the steam near the top of ,the heater. In

the event that the heater becomes clogged,

or for any other reason requires attention and repair, it may be rcmovedjfrom the evaporator body and a duplicatesubstituted therefor. Thus, continuous and eflicient operation maybe ins'ured without duplicating the relatively great expens'e'of 'provid ad-V ditional evaporator-bodies. i T

Having, described my invention whatI claim as new and desire to secure by'Letters Patent of the United States is 1. A process of concentrating a liquid by I evaporation which includes the steps of set-v ting said liquid in movement at 'a high velocity in a straight path, leading said liquid in the same directioninto and through a heating zone in an unobstructed relatively straight long and narrow path directly into the vapor space of a;collecting chamber, and

arnount sufiicient to boil structed relatively straight long and narrow high veldcity zone, applying to the liquid in said high velocity zone heat in amount sufiigreater Volumes of treated liquid that -cient to boil the liquid and therebyjaugment its velocity, and permitting unobst-riicted discharge from said high velocity zone directly into the vapor space of a-eollecting chamber ofthe liquid and its vapor caused by boiling.

'3. The method of operating anevaporator having an unobstructed substantially straight and relatively long and narrowtube through' which the liquid to be treated passes at high velqcity into the vapor space of a collecting chamber, which includes the steps of feeding the liquid at high cntrance velocityfto 'the tube, and applying to the exterior of the tube heat in amount sufiicient to'boil theli'qs a uid passing therethough.

4. The method of operating an evaporator having an unobstructed substantially 've rtical and straight and relatively lon'gand; narrow tube through which the liquid .tobe. treated passes at high velocity-directlyinto the vapor space of a collecting'gchainben,

which includes, feeding theliquidatfli'ighem having an unobstructed relatively straight long, andnarrow tube through which the liquid to b'e treated. passes at high velocity directly into the vapor space ofa collecting chamber, which includes, feeding the liquid at high entrance velocity-to the tube, and applying to the exterior of the tube vaporous' heat in amount suflicient to boil'the liquid passing therethrough;

6. The method of operating a concentrating evaporator, having an unobstructed relatively straight long and narrow tube through which the liquid to be treated-.flowsat high velocity, which includes, feeding the liquid at high entrance. velocity into the tube, applying tothe ,exterior of the tube a vaporous heating medium in amount sufficient to boil the liquid in its passage therethrough' and thereby progressivelyraise the liquid film coeflicient toward the discharge end of the tube, the vaporous heating-medium being rapidly passed along the tube in a direction reverse to that of the flow of liquid therethrough so that the higher heating medium coeflicient is in the region of the higher liquid film coefiicient, and permittin the liquid and the vapor produced-by its boi ing to be freely and directly discharged from the tube into the vapor space of acollecting chamber. 7. The method of operating a concentrating evaporator having unobstructed substantially vertical and straight and relatively long and narrow tubes for rapidly passing a liquid through a heating zone into the vapor space of a collecting chamber, which includes, moving the liquid into the tubes at a high entrance velocity, and passing along the outside of the tubes in the heating zone a, medium in amount sufii vaporous heatin cient to boil the liquid in the tubes and thereby raise the velocity of fiow to increase the liquid film coefiicient inthe region ofthe Vapor space ofthe collecting chamber, the heating medium being passed along the tubes in a direction reverse to that of the liquid flow therethrough torraise the heating medium coeflicient in the region of the vapor space of the collecting chamber.

In witness whereof, I hereunto subscribe my name this 3 day of August, 1927.

' PHILIP B. SADTLER.

Images