US2796120A - Utilization of heat from condensate in indirectly steam heated multistage evaporators - Google Patents

Description

June 1 1957 c. J. LOCKMAN ETAL 2,796,120

UTILIZATION OF HEAT FROM CONDENSATE IN INDIRECTLY STEAM HEATED MULTISTAGE EVAPORATORS 3 Sheets-Sheet 1 June 18, 1957 c. J. LOCKMAN ETAL 2,796,120

vUTILIZATION OF HEAT FROM CONDENSATE IN INDIRECTLY STEAM HEATED HULTISTAGE EVAPORATORS 3 Sheets-Sheet 2 Filed April 10. 1951 June 1957 c. J. LOCKMAN ETAL 2,796,120

UTILIZATION OF HEAT FROM CONDENSATE' IN INDIRECTLY STEAM HEATED MULTISTAGE EVAPORATORS 3 Sheets-Sheet 3 Filed April 10. 1951 United States Patent UTELIZATION 9F HEAT FROM CONDENSATE IN KNDTRECTLY STEAM HEATED MULTISTAG EVAPURATORS Carl Johan Lockman, Stockholm, and Sverre Jenssen, Osirarstrom, Sweden, assignors, by mesne assignments, to Rosenblad Corporation, New York, N. Y., a corporation of New York Application April 10, 1951, Serial No. 220,172

2 Claims. (Cl. 159-47) This invention relates to a method and means for utilizing heat from condensate in indirectly steam heated multistage evaporators.

In evaporators of this kind it has been customary to allow the condensate produced from the heating medium in the heat exchangers of preceding stages to discharge into the heating spaces of the heat exchangers of succeeding stages so as to release flash vapor under the influence of the drop in pressure occurring from heat exchanger to heat exchanger and to obtain a correspondingly lower temperature. In this manner the condensate is passed through the evaporator from the first to the last stage so that from every succeeding stage the condensate formed therein will be discharged together with the condensate received from the preceding stage so that the total amount of condensate produced in the evaporator will be collected in and discharged from the last stage at the com paratively low temperature of the heating medium in that stage. This condensate may in turn be utilized for preheating the liquid to be evaporated, provided, of course, that this liquid is of lower temperature than the condensate, whereby the heat thus transmitted is returned to the evaporator with the liquid. It is, however, clear that owing to the low temperature of the condensate it is impossible, in this manner, to preheat the liquid to any considerable degree as compared with the temperature levels of the preceding stages and thus the possibility of recovering heat in that way remains comparatively limited.

It should be noted that the expressions preceding and succeeding, first and last, used here to indicate the relative positions of the different stages, relate to the direction of flow of the heating medium from stage to stage if not otherwise expressly stated.

The main object of the invention is to provide anew method of operation by means of which the liquid to be evaporated may be preheated to a temperature at most approaching the evaporation temperature of one of the preceding stages, preferably the first stage, this preheating being ettected substantially by means of condensate only, collected from that stage and from the succeeding stages.

In the new method, contrary to known methods above described, the condensate is passed and collected from succeeding stages to preceding stages, whereby in every preceding stage the condensate supplied from a succeeding stage will be brought into direct contact with the vaporous heating medium so as to be heated to at most approximately the temperature of the heating medium and then, together with the condensate produced from the heating medium in that stage, passed to the next preceding stage. In that way the condensate will be passed, heated and collected from stage to stage, preferably from the last to the first stage and ultimately discharged at a temperature close to that of the heating medium of the stage from which said condensate is discharged and utilized for heating the liquid to be evaporated, which in turn is passed to the last mentioned stage.

Assuming that the collected condensate is discharged 2 J96, llZ'll Patented June 18, 1957 from the first stage practically at the temperature of the heating medium therein, it will be possible to heat the liquid to a temperature at most, close to the evaporation temperature of said stage. Assuming also that the original temperature conditions for the liquid are not altered in relation to the compared prior art, it will be possible thereby to cool the collected condensate to a temperature lower than that of the condensate formed in the last stage, so that heat from condensate formed in the evaporator, not only in the first stage, but also in all succeeding stages, will be transmitted to the liquid during its heating. By supplying the heated liquid to the first stage, this heat will be completely returned to the evaporation process.

Further features of the invention will be explained in connection with the following description with reference to the accompanying drawings, in which:

Fig. 1 diagrammatically illustrates an embodiment of the invention as applied to a four-stage evaporator,

Fig. 2 shows a modification of a detail of Fig. 1 in section on a larger scale,

Fig. 3 shows part of a modified embodiment of the same evaporator as in Fig. 1 on a larger scale, partly in section,

Fig. 4 shows a further modification as applied to a three-stage evaporator of a special kind, and

Fig. 5 illustrate a modification of Fig. 3.

In Fig. 1 each of the four stages comprises a heat exchanger 1, 2, 3 and 4, preferably of the conventional tubular type (see Fig. 3), a steam separator 5, 6, 7 and 8, a steam trap 9, 10, 1'1 and 12 and a condensate collector 13, 14, 15 and 16 respectively. Within each stage the heat exchanger is connected to the associated steam separator through an outlet 17 for liquid and vapor, and to the associated condensate collector through an outlet 18 for heating medium condensate while the separator communicates with the associated steam trap through a pipe 19 for liquid. Each heat exchanger is provided with an inlet 20 for vaporous heating medium and an inletZl for liquid to be evaporated. The vapor spaces of the separators 5, 6 and 7 are each connected to the heating medium inlets 20 of the succeeding stage through pipes 22', 23 and 24, respectively. The steam traps 9 and II are each-adapted to discharge liquid to the inlets 21 of the succeeding stages through a pipe 25. The steam trap 12 is adapted to dischargeliquid through a pipe 26 to a receptacle 27 which through a pipe 2%, a pump 29, a check valve 30, a pipe 31, a preheater 32 and a pipe 33 communicates with the inlet. 21 of the heat exchanger 1. Steam is supplied to the system through a pipe 34 communicating with the inlet Ztl of the heat exchanger 1, and vapor is discharged from the system by being drawn ott lfrom separator 8; through a pipe 35, for instance to a condenser, not shown. The liquor to be evaporated is suppliedto the system through a pipe 36 communicating with the inlet 21 of heat exchanger 3 and discharged from the system by being drawn off from steam trap 10 through a pipe 37..

It will be clear to any person skilled in the art that so far. the evaporator is one of conventional type in which the pathof flow of the heating medium from stage to stage. will follow the order 1-2-3-4 whilst the path of flow of the liquid to be evaporated. will follow the stage wise order 34l-2. It will also be clear that in the evaporation of so called black liquor, if the evaporator is arranged to operate with its third stage at evaporation temperature close to the temperature of the liquid supplied, there will be no possibility of heating that liquid by means of condensate at the condensing temperature of the last stage and thus to recover heat from the condensate in this manner.

According to. the invention, therefore, the condensate produced in the evaporator 4 and discharged into the collector 16 is passed through pump 43, valve 46, check valve and pipe 33 into the pipe 23 so as to mix with the heating vapor entering the evaporator 3, and in this manner the condensate is heated to a temperature at most close to the temperature of the vapor. In evaporator 3 the heated condensate will be mixed with the condensate from the condensing vapor in that evaporator and both quantities of condensate will be collected in the collector 15. The condensate thus collected and having approximately the same temperature as the heating vapor is then passed through pump 44, valve 47, check valve 51 and pipe 39 into pipe 22 to be treated in stage 2 in the same manner as now described. The process is repeated once more by passing the collected condensate from the collector 14 through pump 45, valve 48, check valve 52 and pipe 40 into pipe 34 and the total amount of condensate formed in the entire evaporator will be collected in the collector 13 at a temperature at most close to the temperature of the heating medium of the first stage. the collector 13 the total amount of condensate is passed through pipe 41 and the surface heat exchanger 32 in heat exchange relation to the liquor which passes from the last to the first stage and thereby becomes heated to a temperature at most close to the evaporation temperature of the first stage. The condensate is ultimately discharged from the evaporator through valve 49 and outlet 42. Each valve 46, 47, 48 and 49 is controlled by.a float 53 in the associated collector 16, 15, 14 and 13, respectively, so as to operate in accordance with the condensate level in the collector. In order to make certain that the condensate passed from one stage into the heating vapor of the preceding stage quickly assumes a temperature at most close to that of the heating vapor, we prefer to introduce the condensate into the vapor in a finely divided state so as to mix intimately with the heating vapor, for instance as illustrated in Fig. 2 showing pipes 38, 39 and 40 connected to pipes 23, 22 and 34, respectively, and terminating in spray nozzles 54, of any suitable type.

Fig. 3 illustrates one example of a modified manner of mixing the condensate and the heating vapor. In this case the pipes 38, 39 and 40 terminate in the top space of the collectors 15, 14 and 13 respectively, in which they discharge the condensate against a baflie plate 55 thus producing a shower or spray of condensate which by causing the heating vapor collected in this space to condense will assume a temperature at most close to that of the vapor.

It should be understood that Fig. 3 merely shows one of the heat exchangers 1, 2 or 3 of Fig. 1 together with associated steam separator, modified condensate collector and pipes and that the general construction of the evaporator may be the same as in Fig. 1.

It is of course necessary that the outlet 18 for the condensate produced in the adjacent heat exchanger 1, 2 or 3 and the collector is sufliciently large to ensure that heating vapor together with the condensate will pass continuously into, and be collected in the associated collector in an amount sufiicient for the condensing and heating purposes. The baflie plate 55 is shown as an example only, and it should be understood that other means may be provided to distribute the condensate to be heated. Thus, for instance, a spray nozzle of the kind shown in Fig. 2 may also be utilized in the embodiment shown in Fig. 3.

Fig. 4 illustrates diagrammatically an embodiment of the invention in connection with an evaporator for the evaporation of scale forming liquid such as sulphite waste liquor in which the paths of flow of the heat exchanging media through the heat exchanger of each stage may be interchanged periodically by means of valves 191, 192, 193, 194 and 195, 196, 197, 19.8, respectively, in a similar manner to that described in Lockmans U. S. Patent No. 2,488,598, so that during each period or cycle the vaporous heating medium will be passed into those pas- From sages of the heat exchanger through which the liquid to be evaporated was passed during the preceding period or cycle and vice versa, whereby the heating medium supplied will be mixed with its condensate in recirculation as described in Lockmans pending U. S. patent application No. 206,752, filed Ianuary 19, 1951, now U. S. Patout No. 2,7345 65 for the purpose of increasing the quantity of condensate flooding those heating surfaces, which during the preceding period were exposed to the liquid, whereby scale formed on such surfaces will be dissolved and removed.

Each of the three stages has a heat exchanger 101, 102 and 103, respectively, each provided with separate passages 104 and 105 for alternately passing heating vapor and liquid in heat exchange relation. Condensate collected in the collector pertaining to heat exchanger 103 (stage 3) is recirculated through a pump 151, a pipe 152, a float-controlled valve 153 controlled by a float 180 and a pipe 154 and is mixed with the heating vapor supplied to the same stage in the pipe 203, for instance by means of a spray nozzle 54 according to Fig. 2. In the same way condensate from collector 160 is recirculated in stage 2 through pump 161, pipe 162, fioat controlled valve 163 and pipe 164 into supply pipe 202 for the heating vapor and in stage 1 from collector 170 through pump 171, pipe 172 and pipe 173 into supply pipe 201 for the heating vapor. In the full open position of valve 153 the pressure in pipe 156 exceeds the pressure in pipe 152. Consequently, no condensate from pipe 152 can pass to pipe 156 but the check valve prevents the condensate from flowing in the reverse direction. As condensate formed in heat exchanger 103 is collected in the collector 150 the float 180 will raise and the passage through valve 153 will be throttled until the pressure in pipe 152 exceeds the pressure in pipe 156 to a sufficient degree to cause an equal quantity of condensate as that newly formed in and discharged into the collector from the heat exchanger, to pass through check valve 155, pipe 156 and pipe 164 into the supply pipe 202 in which the condensate, together with the recirculated condensate, becomes mixed with and heated by the heating medium supplied to stage 2. The total amount of condensate passes through the heat exchanger 102 to the collector 160. From this collector condensate newly formed in heat exchanger 102 together with the condensate from stage 3 is passed through check valve 165, pipe 166 and pipe 173 into supply pipe 201 to be treated in stage 1 in the same way as the condensate passed to stage 2 is treated in the latter stage so that the total amount of condensate formed in the entire evaporator will ultimately arrive at the collector 170 at a temperature which at most is close to that of the heating medium in the first stage.

The preheating and recovery of heat may be elfected in the embodiment according to Fig. 4 in a similar manner as described with reference to Fig. 1. Similar parts of Figures 1 and 4 have the same reference characters.

However, the method described in connection with Fig. 4 is modified in comparison with that described with reference to Fig. 1 in as much as in the first mentioned case, the process involves the step of introducing passedover condensate together with recirculated condensate into the heating medium in order partly to utilize, for mixing the condensate with the heating medium, means which already are available in the system for quite another purpose, namely for dissolving the scale. The important similarity between the exemplified modifications of the method according to the invention is inherent in the fact that the passed-over condensate is heated by the heating vapor from the preceding stage before it is passed to the next preceding stage. In connection with the evaporator illustrated in Fig. 4 the invention would also be quite operative if the condensate, instead of being passed over to the pipes 164 and 173 respectively, were passed over to the collectors and respectively, in the manner shown and described in connection with Fig. 3.

Fig. 5 illustrates a modification of Fig. 4. In Fig. 5 a restricted passage or a manually controlled valve 205 is provided in the pipe 154 instead of the valve 153 in Fig. 4. In Fig. 5 the float-controlled valve 153 is provided in the pipe 152 and is adapted to open when the liquid level rises in the collector 150. The restricted passage or valve 205 is adjusted so as to permit the condensate from the collector 150 to pass into the pipe 203 in a suificient amount. Alternatively the restricted passage may consist of the spray nozzle 54, the discharge area of which may be dimensioned so as to provide sufficient flow resistance.

What we claim is:

1. A method of utilizing heat from heating medium condensate in pre-heating liquid to be evaporated in indirect heat exchange with condensable vaporous heating medium in a succession of evaporation stages, whereby every succeeding stage is heated by flash vapor released from the liquid from the immediately preceding stage, comprising the steps of mixing condensate from one stage in a finely divided state with the vaporous heating medium passing to the immediately preceding stage so as to condense a sufficient quantity of said vaporous heating medium to heat said condensate to a temperature at most approaching the same temperature as said vaporous heating medium, then mixing said heated condensate with the heating medium condensate thus formed, and with the condensate formed in the same preceding stage from the remaining part of said vaporous heating medium in heat exchange with the liquid evaporated with the last mentioned stage, subsequently passing the condensate thus collected in heat exchange relation to liquid to be evaporated which liquid has a temperature lower than that of the condensing temperature of the heating medium of the first mentioned stage so as to cool the collected condensate and heat the liquid and supplying the heated liquid to said preceding stage.

2. A method of utilizing heat from heating medium condensate in preheating liquid to be evaporated in indirect heat exchange with condensable vaporous heating medium in a succession of more than two evaporation stages, whereby every succeeding stage is heated by flash vapor released from liquid from the immediately preceding stage, comprising the steps of mixing condensate from one stage, preferably the last stage of the succession, in a finely divided state with the vaporous heating medium passing to the immediately preceding stage so as to condense a sufficient part of said vaporous heating medium to heat said condensate to a temperature at most approaching the same temperature as said vaporous heating medium, then mixing said heated condensate with the heating medium condensate thus formed and with the condensate formed in the same preceding stage from the remaining part of said vaporous heating medium in heat exchange with the liquid evaporated there, subsequently treating the condensate thus collected in said preceding stage in the same way in connection with the next preceding stage, repeating such treatment and collection of condensate from stage to stage, passing the condensate thus ultimately treated and collected from one ultimate stage of the treatment stages in heat exchange relation to liquid to be evaporated having a lower temperature than the condensing temperature of the heating medium of the first mentioned stage, preferably the last stage in the succession, so as to cool the ultimately collected condensate and heat the liquid and supplying the heated liquid to said ultimate stage.

References Cited in the file of this patent UNITED STATES PATENTS 643,702 Waterhouse Feb. 20, 1900 713,297 Goss Nov. 11, 1902 1,582,067 Moore Apr. 27, 1926 2,012,668 Jahn Aug. 27, 1935 2,193,483 Hinckley Mar. 12, 1940 2,262,519 Talton Nov. 11, 1941 2,535,777 Bickley Dec. 26, 1950 2,562,495 Hulme July 31, 1951 2,570,212 Cross Oct. 9, 1951 FOREIGN PATENTS 71,712 Sweden Dec. 5, 1928

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