US 2490759 A
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Description (OCR text may contain errors)
Dec. 6, 1949 A. E. TYDEN 2,490,759
METHOD OF CLEANING SCALE Filed Oct. 6, 1943 If 4 in;
. i 36 i= 3 I 34 .l v as i 18 INVENTOR.
Patented Dec. 6, 1949 UNITED STATES PATENT- orrics I METHOD 0323mm scant: I
Arnold Eugn by mesne Application October s, 194:, Serial No.
In Sweden June 13, 1942 assignor,
to Bosenblad Corporaa corporation of New 3 Claims. (Cl. 159-23) The present invention relates to a method of maintaining surface heat exchangers substantially clean as regards incrustations when used for bringing ascale forming liquid in indirect heat exchange relation with another fluid for the purpose of heating, or evaporating such liquid, for instance, and particularly in the evapo ration of so called sulphite waste liquor from the cooking stage of manufacturing sulphite cellulose by the calciumbisulphite process.
Generally it is preferred to use chemical means for removing any incrustations formed and there have been proposed various means of effecting this in diiferent cases by interrupting the operation of the heat exchanger for a time and washing the heat exchange surface with a suitable chemical solution during such periods of inactivity. A drawback common to the known chemical methods is, however, the loss of time when the apparatus is inactive since this causes a decrease in the useful capacity of the apparatus which may be considerable.
In other cases satisfactory cleaning may be effected simply by washing with hot water or treatment with condensing steam and the heat exchange process in question consists in heating a scale forming liquid by means of relatively pure hot water or steam which causes practically no incrustations. It has also been proposed to effect the cleaning with the hot water or steam used as the heating medium so that this medium and the scale forming liquid are passed alternately on opposite sides of the heat exchange surfaces so that the incrustations caused by said liquid on either side during one period are dissolved and removed by the heating medium during a following period withoutthe necessity of interrupting the operation. This method, which is described in U. S. Patent No. 1,006,197, means obviously a great progress where it can be used with success but unfortunately in many cases the incrustations grow too rapidly or cannot be dissolved easily enough by such simple means to admit perfect cleaning in the manner mentioned.
It is an object of the present invention to provide a method of cleaning which combines the advantages of the known methods described and can be adapted to give good results independently of the composition of the incrustations while rendering interruption of the heat exchange process unnecessary.
A further objector the-invention is to realize this by using a suitable chemically active fluid,
as heating medium or adding chemicals to the same and performing the cleaning with such v similar method to the heat treatment of sulphite waste liquor, in a mode modified to suit that special case Further objects and advantages of the invention will be apparent from the following description taken in conjunction with the accompanying drawings, wherein:
The single figure of the drawing is a view partially in section diagrammatically illustrating the application of this invention to a conventional heat exchanger.
Thus, in accordance with the invention, for heating a scale forming liquid, there is prepared a hot fluid which contains suitable chemicals capable of removing the expected incrustations with relative ease, and such fluid and the liquid to be heated are caused to change paths with each other at intervals in the heat exchange apparatus so that the heating fluid is utilized for cleaning purposes, similarly as known.
It is obvious that the method according to the invention must be, more eifective than the known methods referred to and above all the new method has the advantage of being very easily modifled so as to meet the different requirements of each special case.
In cases in which it is possible to use rather small quantities of relatively inexpensive and not very aggressive chemicals in the heating fluid, there may continually be used a heating medium with a constant content of the desired chemicals. If, however, there are required very expensive or aggressive chemicals to obtain a suflicient cleaning effect, a considerable saving of such chemicals may be effected if such chemicals are supplied during relatively short periods, as needed to efiect cleaning, and another cheaper chemical is used at all other times, or no chemical at all is supplied to the heating medium during such other times.
Another way of saving chemicalsis to circulate the crust dissolving heating medium and as it leaves the heat exchanger in a cooled condition heat it again by suitable means and pass it back to the heat exchanger possibly together with a fresh supply of the same medium or any of its constituents, to serve repeatedly as heating medium.
According to the invention the heating medium proper may be provided in the form of a liquid or in the form of vapours and in each case naturally the chemicals chosen must be soluble a 3 in such liquid or in the'condensate of the vapours, respectively.
Often it may be advantageous to use more or less impure waste vapours from one or another process as heating medium and in case the heating medium proper has the form of vapours of any kind the chemicals to be added may consist of a gaseous matter which is mixed with the vapours and soluble in the'condensate thereof, or it may consist of a liquid or solution which is either sprayed intothe vapours or distributed as i a film over the surfaces conducting the vapours.
present or by other special chemicals during cer- Now, it is known in the art to use waste vapours containing SOz-gas as liberated from substantially degassed sulphite waste liquor when subjected to evaporation, a's-a medium for transmitting heat to such waste liquor in a further evaporation unit, for instance. However, the vapours generated under these. circumstances are but weakly acid and have no great cleaning effect and the simple utilization thereof remains, outside the scope of the present invention which moreover calls for the addition of extra, preferably concentrated, SOz-gas from another source if such vapours are supplied as heating medium, in order to establish a controlled suitable acidity as required to attain the desired increased cleaning effect.
By way of illustrating a practical application of this invention there is shown in the drawing a heat exchanger 10 having an outer shell H within which is positioned a plurality of tubes l2 secured in tube sheets l3 fixed to the shell H in spaced relation to the ends thereof. Tubes l2 communicate at one end with a space I4 and at the opposite end with a space I5,the spaces being closed by end walls 16 and I1 respectively.
An inlet conduit I8 for liquid to be treated in the heat exchanger It connects to a two-way valve l9 which in turnis connected by a conduit 20 with the space I5 and by a conduit 2| with the space 22 surrounding the tubes l2. An outlet conduit 23 for the treated liquid connects through a two-way valve 24' with a conduit 25 communicating withthe space M and a conduit 26 communicating with the space 22.
A vaporous heating medium such as steam may be supplied to the heat exchanger ll through a. conduit 2'! controlled by a valve 28, this conduit being connected to a two-way valve 29 communicating with space H through conduit 30 4 a ing medium may be discharged from the heat exchanger ll through a conduit 32 connected to the space 22 or conduit 33 connected to space ii, there being a two-way valve 34 provided for controlling the outlet through these two conduits. Valve 34 is connected by a conduit 35 to two-way valve 36 by which the exhaust heating medium may be directed through an exhaust line 31 or to the inlet 38 of a circulating pump 39 having an outlet 40. Outlet 40 connects with a heat exchanger 4| provided with an inlet 42 and an outlet 43 for conducting a heating medium therethrough, the heat exchanger 4| being also connected through conduit 44 with the supply conduit 21.
Since the heat exchanger I0 is utilized in the treatment of, a liquid having scale forming properties and since this invention is concerned with the removal of scale deposited on the internal surfaces of the heat exchanger, there is provided a pipe 45 controlled by a valve 45 connected to supply conduit 21 and through which a scale dissolving solvent may be introduced and mixed with the heating medium flowing through supply conduit 21. The solvent may. of course be supplied to pipe 45 from any desired source, such as a tank, pump or the like.
In order to permit proper drainage of the heat exchanger in when desired there is provided a drain pipe 41 connected to the bottom thereof and communicating with the space l5, this drain pipe being controlled by a valve 48 which would normally be closed but which may be opened for draining the contents of the heat exchanger.
In operation the valves l9 and 24 may be set as shown in the drawing, in which case liquid to be treated will flow through inlet conduit l8 and conduit 20 into space I5 thence upwardly through tubes l2 to space i4 and be discharged from the heat exchanger through conduit 25; valve 24 and outlet conduit 23. During this period scale will be deposited on the inner surfaces of tubes l2 and must be periodically removed therefrom in order to maintain the eiflciency of the heat exchanger at a relatively high level. A heating medium such as steam is supplied through conduit 21 and with the valves 29, 34 and 36 in the positions shown will supply steam through conduit 3| to the space 22 surrounding the tubes l2, thus heating these tubes and the liquid flowing therethrough. The heating medium in the form of vapor and condensate or condensate alone if complete condensation takes place, will be discharged from the space 22 through conduit 32, valve 34, conduit 35, valve 35 and conduit 31. The above described manner of operation will continue until the scale deposited on the inner surfaces of tubes I2 is suflicient to lower the efliciency of the heat exchanger a predetermined amount at which time the positions of valves I9, 24, 29 and 34 will be reversed to permit flow of liquid from conduit l8 through conduit 2| into the space 22 and from space 22 through conduit 26, valve 24 and discharge conduit23. Likewise, the heating medium from supply conduit 21 will pass through conduit 30 into space 14, through tubes I 2, space [5 and be discharged therefrom through conduit 33, valve 34, conduit 35, valve 36 and conduit 31. During this cycle of operation, a liquid scale solvent is introduced through conduit 45 into supply conduit 21 and mixed with the heating medium, thus supplying to space M a mixture of vapor and liquid which will flow through the tubes l2 anddissolve scale deposited on the walls thereof.
and with space 22 through conduit 3|. The heat- 74 This cycle of operation is continued until the scale deposited in the tubes I2 is removed at which time of course scale will have been deposited on the external surface of the tubes due to circulation of the liquid to be treated through space 22. At this time the valves I9, 24, 29 and 34 are again reversed to return to the original cycle of operation, during which time scale will be removed from exterior surface of tubes l2 and the liquid to be treated will pass through the interior of the tubes to again deposit scale thereon.
While the principles of this invention may be carried out by discharging the mixture of heating medium and solvent from the conduit 31, it is also contemplated that this mixture may be reheated and recirculated through the heat exchanger in order to utilize the solvent properties of the mixture without the necessity of continually adding additional solvent. For this purpose the valve 36 may be positioned to conduct the discharged heating medium and solvent through conduit 38 to circulating pump 39, the discharge 40 of this pump directing the mixture through heat exchanger 4| which raises the temperature thereof a predetermined amount whereupon the mixture flows through conduit 44 into supply conduit 21 and recirculates through the heat exchanger III. This reheating and recirculation may be carried out for each cycle of operation of the heat exchanger I or in other words may be utilized regardless of which path of flow of the heating medium through the heat exchanger is utilized.
It will thus be seen that by this invention there has been provided a relatively simple system of which one practical application thereof is illustrated in the drawing and in which scale may be removed from the surfaces of a heat exchanger in order to maintain the emcienc'y thereof at a relatively high level and in which this scale removing.
function may be accomplished without the neces sity of shutting down the system but while maintaining full capacity operation thereof. Likewise, means may also be provided to reheat and recirculate the heating medium which incorporates a scale solvent thus precluding the necessity of continually adding solvent and contributing to the economical operation of the system.
It will be obvious to those skilled in the art that various changes may be made in the invention without departing from the spirit and scope thereof and therefore the invention is not limited by that which is shown in the drawings and described in the specification but only as indicated in the appended claims.
Thus, what I claim and desire to secure by Letters Patent is: v
1 The method of treating a liquid tending to deposit scale in a system including a heat exchanger providing two diflerent passages for flow of fluids in heat exchange relation comprising the steps of passing said liquid through one of said passages, providing a vaporous heating medium, mixing with said medium a chemically active liquid scale solvent to provide a mixture including liquid and vapor in sumcient quantity to insure flow of the chemically active mixture in liquid 6 face of the other of said passages when passed therethrough whereby both the vapor and liquid solvent act to dissolve the scale and periodically alternating the paths of flow of the liquid to be treated and the chemically active heating medium through said passages to remove during any one cycle the scale deposited in one of said passages bythe liquid flowing therethrough during the preceding cycle.
2. The method of treating a liquid tending to deposit scale in a system including a heat exchanger providing two ditt'erent passages for flow of fluids in heat exchange relation comprising the steps of passing said liquid through one of said passages, providing a vaporous heating medium,
mixing with said medium a chemically active liquid scale solvent to provide a mixture including liquid and vapor in suiiicient quantity to insure flow of the chemically active mixture in liquid film form over substantially the entire wall surface of the other of said passages when passed therethrough whereby both the vapor and liquid solvent act to dissolve the scale, reheating and recirculating said mixture through the exchanger, and periodically alternating the paths of flow of the liquid to be treated and the chemically active heating medium through said passages to remove during any one cycle the scale deposited in one of said passages by the liquid flowing therethrough during the preceding cycle. I
3. The method of treating a liquid tending to deposit scale in a system including a heat exchanger providing two difierent passages for flow of fluids in heat exchange relation comprising the steps of passing said liquid through one of said passages, providing a vaporous heating medium, forming a chemically active mixture of liquid and vapor by mixing with said medium a chemically active scale solvent soluble in said medium to thereby form said mixture prior to the introduction of the mixture into said exchanger, and introducing the mixture into the exchanger in sufficient quantity to insure flow of the chemically active mixture in liquid film form over substantially the entire wall surface of the other of said passages when passed therethrough whereby both the vapor and liquid solvent act to dissolve the scale and periodically alternating the paths of flow of the liquid to be treated and the chemically active heating medium through said passages to remove during any one cycle the scale deposited in one of said passages by the liquid flowing I therethrough during the preceding cycle.
ARNOLD EUGEN TYDEN.
REFERENCES CITED The following references are of record in the flle of this patent:
UNITED STATES PATENTS fllm form over substantially the entire wall sur- OTHER REFERENCES Handbook of Chemistry, Lange, fifth edition.