US2001022A - Indirect heating and circulating system for sulphite digesters - Google Patents

Description

May 935., NNN-30W BZ INDIRECT HEATING AND CIRCULATING SYSTEM FOR SULPHITE 'DIGESTERS Filed Sept. 8, 1933 4 Sheets-.Sheet l May 'M5 H935., H, W. www UlZZ INIRECT HEATING AND CBCULATING SYSTEM FOR SULPHITE DIGESTERS Filed Sep. 5, 1953 4 Sheets-Sheei 2 INVENTO'R om@ W0( am ATTORNEYS May Mr? @935 QZZ lNDlREcT HEATING AND CIRCULATING SYSTEM FOR SULPHITE DIGESTERS Filed Sepc.

8. 1953 4 Sheets-Sheet 3 www May 14, 1935. H, w. How 2,001,022

INDIRECT HEATING AND CIRCULATING SYSTEM FOR SULPHITE DIGESTERS Filed Sept. 8, 1955 4 Sheets-Shee 4 INVENTOR wir w. @141W d CP a i ATTOR N EYS Patented May 14, 1935Vv .PATENT OFFICE INDIRECT HEATING AND CIRCULATING SYSTEM FOR SULPHITE DIGESTERS Harlan W. How, Aspinwall, Pa. Application September 8, 1933, Serial No. 688,672

27 claims.

This invention relates to an indirect heating and circulating system for sulphite digesters and more particularly to the construction and control of a system for withdrawing the sulphite liquor from the mixture of liquor and pulp in thev digester, heating the Withdrawn liquor so as to obtain theproper temperature Within the digester for the required length of timeand returning the heated liquor to the digester.

One of the principal diiculties encountered in digesters of this character is the eiect of unequal expansion strains on different .parts of the circulating system. Because of the corrosive action of the sulphite liquor it is necessary to make those parts of the heating and circulating system which come in contact with the sulphite liquor of an adequate acid resisting materialV such as chrome nickel stainless steel. Moreover, the digester, for reasons of economy, must be lined with acid resisting brick. Both to avoid expansion strains upon the lining and also to avoid internal expansion on the chrome nickel parts, the acid resisting qualities-- of which are greatly reduced by such strains, the present invention is designed to relieve ysuch expansion strains throughout the heating and circulating system of the digester and thereby reduce the expense of maintenance as well ,as the expense of replacing costly stainless steel parts..

Another object of the present invention is to provide a circulating system which provides for indirect heating of the' liquor thereby avoiding the direct application of steamwith the resultant dilution of the cooking acid and the release of sulphur dioxide, and also provides an adequate circulation of the liquor through the pulp in the digester and in which that part of the circulating system which is arranged within the digester is so designed as to withstand the upward pressure of the pulp in the digester which is occasioned 'by the buoyancy of the chips or pulp.

Another object of the present invention is to provide means whereby low pressure exhaust r steam can be utilized throughout the process of digestion by providing a. thermo-compressor for the exhaust steam which ,raises its temperature and pressure to that necessary for carrying .on the process. v

Another object of the present invention is to provide a suitable control system for supplying a constant flow of steam during each of the three periods of th process, which means avoidsy the necessity of any delicate control mechanism and can be readily atuatedby a time clock mechanism to provide the requisitev steam ilow` duringV each of the three periods.4 f y Another object of the present invention is to provide a" control system in which the tempera- Iture of the steam in the heater is always main- 5 tained a predetermined number of degrees higher than the sulphite liquor so that the heating of the sulphite liquor is gradual and uniform.

Another purpose of the present invention is to provide a heater for such a system in which' the 10 steam is distributed uniformly over the heating tubes and in which the amount 'of stainless steel required in"making the heater is reduced to a minimum.

Another purpose is to provide means for quickly 15 testing for leaks in the tubes of the heater or between the tubes and the tube sheet.

In the accompanying drawings:

Fig. l is a vertical fragmentary section through a digester having an indirect heating and circu- 20 lating system made in accordance with my invention.

Fig. 2 is an enlarged fragmentary section taken on line ,2 2, Fig. 1.

Fig. 3 is an enlarged vertical fragmentary sec- 25 tion taken on line 3 3, Fig. 2.

Figs. 4 and 5 are horizontal sections taken on the correspondingly numbered lines of Fig. 1.

Fig. 6 is a vertical section through the heater of the circulating and heating system, this sec- 30 tion being taken on line 6 6, Fig. 5.

Fig. '7 is a horizontal section through the heater l and`taken on line 1 1, Fig. 6.

Fig. 8 is a fragmentary section similar to' Fig. 6 and showing the manner in which the upper 35 header or dome of the heater can be removed and a ring substituted for the purpose of detecting leaks in the tubes or tube sheets of the heater.

Fig. 9 is a horizontal section rthrough the heater y taken on une 9 9, Fig. 6. 40

Fig. 10 is a side elevation of the heater for heating the sulphite liquor and showing, diagrammatically, the system for controlling the admission of steam to the heater during the three different periods of the operation of the digester and including a time clock mechanism for controlling the admission of steam during the three different periods.

Fig 11 isga verticalsection through the branch 50 steam lines to the heater and showing the orice vplates by means of which uniform steam ows are obtained during the three .diiferent periods of the digestion operation. y I

Fig. 12= is alongitudinal section through the 55 in Figs. 10 and 13. f

Fig. 13 is a view similar to'Fig. 10 showing an alternate form'of steam control to the heater, this control supplying steam to the heater at a constant temperature above the temperature of the liquor in the digester.A

Fig. 14 is a fragmentary vertical section through the shell of the digester, this section being'taken on line |4-I4, Fig. 5.

Fig. 15 is a vertical fragmentary section taken on line |5-l5, Fig. 1.

Similar reference numerals refer to similar parts in each of the several views'.

The digester, as illustrated in Fig. 1, consists of an upright tubular shell 2U which is made of steel plates or the like and formed at its upper end to provide an inlet opening 2| through which the chips are introduced into the digester. The lower end of the digester shell is conical and terminates in anoutletneck 22 which is valved and throughwhich the digested pulp is withdrawn` from the digester at the end of the cook. Steam connections 23 are also provided for assisting in the discharge of the pulp through the outlet neck 22 or for cleaning the digester. The entire interior of the digester is lined with an acid resistlng brick lining 24 and the digester shell is .sp-

ported in its upright position upon standards 25',

brackets 26 being secured to the lower conical part of the digester shell and having lower flat faces 21 which rest on the upper sides of the7 standards 25. The lower horizontal flat faces 21 of the several brackets 26 are all arranged in the same horizontal plane and it is apparent-that the i digester shell expands in opposite directions from this plane, that is, that part of the digester shell which is disposed above this plane expands upwardly, and that part of the digester shell 4which is disposed below this plane expands downwardly.

It is customary to heat the contents of the sulphite digester from 25 C. in a period of two hours. In the next three hours, the digester is heated from 100 C. to 135 C. The temperature rise during the first ytwo hours preferably proceeds at a constant rate and the same applies to the next three hours. The total time of cooking the sulphite pulp is 9 to 11 hours, or more, depending `on the quality of the pulp which is produced. After the digester reaches the temperature of 135 or 140 C. the temperature of the digester is kept constant during the third period of the cook and during this period, which proceeds for from 4 to 6 hours, only radiation losses are to lbe cared for.

In the standard method of heating digesters, high pressure steam is blown into the bottom of the digester and the temperature is held as near as possible to the schedule outlined above. Owing to the fact that the circulation in the digester is caused only by the steam, there necessarily is a great difference in temperature between the different parts of the mass of pulp in the digester which dierence causes local'overheating of the chips'or pulp in some parts and under cooking of the pulp in other parts. Moreover,` the admission of high pressure steam directly into the contents of the digester dilutes the cooking acid and also releases sulphur dioxide.

Diierent systems of digester circulation and outside or indirect heating of the liquor have been applied to sulphate digesters in the past and such indirect heating is probably more important in connection with sulphite pulp than with sulphate pulp because of the higher price of the form ,stresses while in service.

' 2,001,022 `thermal.compressor for the exhaust steam shown mer. However, indirect heating and circulating systems have not been extended rapidly into the sulphite industry due to the lack of adequate acid resisting materials from which to make the necessary circulating pulp, heater, piping and with supports for the piping within the digester. Sulphite liquor attacks nearly all metals, with the exception of lead and some of the new chrome nickel steels. Bronze has been used to some extent in digester acid piping, but this material is not strictly acid resisting. With the advent of the newer chrome nickel stainless steels and their use in digester service in the past several years, obstacles inthe path of improved direct heating and circulating systems for sulphite digesters have been removed.

However, it has been discovered that any severe l or continued strains or stresses set up in chrome nickel steels affect adversely the ability of the.

metal to withstand corrosion from sulphite liquor. For this reason, every part of the circulating and direct heating system should be so arranged and constructed that every part made of chrome nickel steel is subjected to minimum strains and It is one of the important objects of this invention to avoid, throughout, strains uponthe chrome nickel parts either through expansion strains or otherwise and, because of the relatively high cost of chrome nickel steels, to reduce the amount of chrome nickel parts employed to a minimum. The circulating system for withdrawing the sulphite liquor from the digester and returning it, after heating, to the digester, is preferably constructed as follows: l)

The sulphite liquor is withdrawn from the top of the digester through a strainer that encircles the interior of the digester near the brick lining. As best shown in Figs. 13, this strainer consists of a pluralityof lengths 30 of straight chrome nickel steel pipes which are provided with perforations 3| and arranged end to end in a circle about the interior of the digester. The sulphite liquor drawn through this strainer is Withdrawn from one side thereof through a vertical pipe indicated at 32 and, in order to secure a uniform withdrawal of the sulphite liquor on all'sides of the digester those strainer pipe sections 30 which are disposed adjacent the withdrawal pipe 32 preferablyhave fewer perforations 3| which are spaced further apart than the corresponding perforations 3| of those sections 30 disposed on the opposite side of the strainer ring. By this means' the frictional resistance to the liquor flow- -ing through the interiorof the strainer ring is compensated for by decreasing the free area of the strainer ring as it approached the discharge pipe 32, thereby securing uniform withdrawal of the sulphite liquor from all sides of the digester.

The adjacent ends of the several strainer sections 30 of the strainer ring are supported by brackets indicated generally at 33. Each of these brackets, as best shown in Fig. 2, is made of chrome steel and consists of a hollow block base portion 34 which is approximately the size of the bricks of the brick lining 24 and is provided with fianges 35 which are secured by means of rivets 36 to the outer steel shell 20 of the digester, thebricks of the lining 24 being set around and cemented to the block-like base 34 so as to secure an acid proof seal between the lining 24 and the bracket base. A webbed neck 31 projects outwardly from the block-like base 34 and supports the half 38 of a split ring support for the adjacent ends of the strainer sections 36. The other half 39 of the split ring support is secured to the stationary half 38 of the split ring support by a pair of bolts 40.

The vertical pipe 32 is connected to the strainer ring in any suitable manner and at its lower end this vertical pipe 32 and the succeeding pipe 4l are each loosely fitted and supported by a bracket 42. Each of these brackets consists of a sleeve 43 supported by a neck M from a blocklike support 45 which is secured to the outer steel shell 26 of the digester in the same manner as the block-like support 34 for the brackets 33. In order to support the pipe sections 32 and 4I on the brackets 42 each of these pipe sections is provided with a collar 46 which rests on the upper side of the sleeve t3 of each of these brackets.

From the lower end of the vertical pipe section l theliquor passes into a pipe lll which is held at its upper end in the lower bracket t2 and at its lower end is connected to an elbow d8. 'Ihis elbow, as best shownl in Fig. 14, connects with the small ilange i9 of a conduit '56, this conduit being square in cross section and passing through the side of the digester, its square form permitting bricks to be laid around it without cutting the bricks to provide a circular opening. 'I'he flange of the elbow 48 is preferably secured to the small A ilange 49 of the conduit 5I] by means of a plurality of lugs 5i and bolts 52 which pass through these lugs and through the flange on the elbow 58. On the outside of the digester the square conduit 50 is secured to a casting 53 by means of screws which pass through the outer flange 513 of the square conduit 56. The passage through this casting 53 is large enough to receive and pass the small flange 49 of the square conduit 50. To the outer end of the square conduit 5B is secured a valve 55 which passes the liquor to the inlet of a centrifugal circulating pump 56, this pump being driven through reducing gearing 5l by a motor 58. This pump, as best shown in Fig. 15, has its casing formed to provide supporting ears, the lower at faces of which are horizontally disposed in a plane which passes axially through the pump. These ears are secured to supports in any suitable manner and by so arranging these ears it is apparent that the upper part of the pump expands freely upwardly and the lower part expands freely downwardly so that at no time are any stresses or strains set up within the pump casing because of its expansion and contraction. The outlet of the pump 56 discharges into a conduit 59 which delivers the liquor to a heater indicated generally at 66 and later described in detail. From the heater the liquor passes through a valve 6l, check valve 62 into a square conduit 53 which is constructed and supported in the same manner as the square conduit '50 of the circulating system. From the conduit 63 the liquor is delivered to a downwardly directed pipe 611 the lower end of which is supported by a bracket 65 which is in general similar to the brackets 62. The pipe 64 is arranged to discharge downwardly against the outlet of the digester so that it tends to keep this outlet clean and prevent clogging. The downward direction of the heated liquor also causes its uniform distribution through the mass of pulp or chips upon rising through the mass of pulp or chips for recirculation.

The heater 68, as best shown in Figs. 6-9, includes a tubular steel casing 76, the upper end of which is hanged, as indicated at 'il and is also enlarecd to slidingly receive a sleeve lf2, a gland 'i3 being provided between the casing 16 and the sleeve 'l2 so as to prevent the loss of steam therebetween. The sleeve l2 provides an expansion joint for the upper end of the heater which permits the free relative expansion of the tubes and the casing. The upper end of the sliding sleeve 'l2 is formed to provide an undercut flange 14 and this flange is adapted to be secured to the corresponding ange l5 of an upper header 16. The anges 'I4 and 'l5 are recessed to receive an upper tube sheet 'll to which the upper ends of the tubes i8 are secured. The upper header 16, tube sheet 'H and sleeve l2 are clamped together by a plurality of bolts 'H9 which pass through a ring 8) tting around and into the upper side of the ange 'l5 of the header 16. The lower end of each of the bolts 'i9 passes through a lug 8| which its into the undercut side of the ange M of the sleeve l2. For economy the lugs 8| are preferably made by cutting sections from rings similar to the ring 89. The purpose of connecting the upper header i6 with the llange M of the sleeve 'l2 in the manner shown is to eiect economies in the amount of stainless steel required in making the heater. The header lB is in contact with the sulphite liquor and consequently is made of stainless steel. By providing the clamping ring 86 its flange l5 can be made very narrow thereby decreasing .thc amount of stainless steel required as compared with a structure in which a larger flange .Y

l5 was provided and the through this llange.

For the purpose of detecting any leaks in the tubes or tube sheet the upper head-er 76 and its ange T5 can be is similar in cross section to the flange l5. As best shown in Fig. 8, when this is done, the upper ends of the tubes F8 are exposed and consequently by admitting steam around the tubes any leak in the tubes or the tube sheet can be observed and repairs made. After the repairs are made the ring 15a is substituted by the header 'i6 and its lange 'l5 and the heater is again in condition for operation.

The sulphite liquor delivered by the pump outlet line 59 passes into a lower casting 85 of the heater 66, this casting having a vertical partition 86 which forces th sulphite liquor to pass up through the tubes 'i8 on one side of the heater, through the header 16, down through the tubes i8 on the other side of the heater and out through the valve 6l into the return line to the digester. The lower tube sheet 8l of the heater is clamped between the lower casting B5 and the shell 70 of the heater and for this purpose the lower end of the shell lll is provided withy a grooved ange 88 which receives a plurality of lugs 89 and the upper end of the lower casting 85 is provided with an undercut flange 99 which receives a split clamping ring 9i. The lugs 89 and split clamping ring 9i are drawn together by stud bolts 92, as best shown in Figs. 6, 7 and 9. The lower end of the lower casting 85 is closed by a circular plate 93 which is secured in position by a clamping ring Sli, this clamping ring being secured in position by stud bolts which pass through clamping lugs 96. The clamping lugs 96 are formed to engage the upper side of a grooved flange 97 at the lower end of the lower casting 85. In order to prevent any chips or pulp from entering the tubes 'F8 of the heater and clogging the same a screen 98 is suitably secured across the inlet chamber formed by the partition 66, this screen being arranged directly under the corresponding tubes i8 and being secured in any suitable manner to prevent bolts 'I9 passed directly substituted with a ring 15a which chips or pulp from passing upwardly through these tubes.

Steam is introduced into the heater so as to iiow around the outside of the tubes 18 and heat the sulphite liquor flowing through these tubes. For this purpose a vertical manifold is welded to one side of the heater shell 10, this manifold having a central inlet neck IOI. This manifold is preferably made of a half section of tubing, the ends being rounded and the edges being welded to the adjacent sides of the shell 10. In order to distribute the steam uniformly against the pipes 18 of the heater the shell 10 is provided with a series of vertical slots |02, these slots permitting the discharge of steam from all parts of the manifold |00 against the tubes 18 of the heater. The condensate is withdrawn through a condensate outlet pipe |03.

It is desirable for the purpose of operating economy to employ as much low pressure exhaust steam to the heater 60 as possible. However, the pressure of this exhaust steam is not sufficient to maintain the required temperatures in the digester and consequently the present invention proposes to employ a thermo-compressor, indicated generally at |09, by means of which low pressure steam can be compressed .by means ofhigher pressure steam discharged through a nozzle into a combining tube, the Acombinedhigh and low pressure steam being delivered to the heater. yThis thermo-compressor, as shown in Figs. and 12 consists of a body having an upper chamber I I0 which has a high pressure steam inlet connection I I I. The lower wall I I2 of this chamber I I0 forms a diaphragm between this chamberl of the combining tube I I3 which is contracted so as to form a venturi. The high pressure steam is jetted into thecontracted part of the combining tube by means of a nozzle I I4 in the diaphragm I I2. Low pressure steam is admitted to the upper end of the combining tube, to one side of the discharge.

end of the nozzle I|4 through a low pressure steam inlet II5. With such a thermo-compressor, using high pressure steam at a gage pressure of 100l lbs. and exhaust steam of 5 lbs. gage pressure, l lb. of high pressure steam will compress l lb. of the exhaust steam to 13 lbs. pressure but will onlycompress about 1/2 lb. of such exhaust steam to 20 lbs. gage pressure. Similarly, high pressure steam at 150 lbs. gage pressure will compress exhaust steam at lbs. gage pressure to about 32 lbs. gage pressure atthe rate of 2 lbs.

,of high pressure steam to l lb. of exhaust steam.

In pulp mills it is not unusual to find exhaust steam at lbs. gage pressure and while that pressure is sufficient to raise the temperature of the digester to a considerable degree it is not suiicient in temperature to heat the -liquor and pulp to the desired point. In such plants the live steam pressure is frequently 130 or 150 lbs. gage and is considerably higher in temperature than what is required to do the work. Great economy can be effected by using the live steam of high pressure to compress the exhaust steam, raising both the pressure and the temperature of the exhaust steam to a point where it can either heat the contents of the digester to the required degree or do more of the work required. Where it is impossible to do all the-work by means of exhaust steam and thermo-compressed exhaust steam the balance can be taken care of by the live steam.

Asv previously explained the process of digestion can be divided into three periods or cycles, the first period being for two hours in which the contents of the digester are heated from to 100 C. In the next three hours the digester is heated from |00o C. to I35 or |40 C. In the last period of four to six hours the temperature is maintained at to |40 C. and only radiation losses are to be cared for. In the system of control the admission of the different amounts of steam required during these periods is controlled by a time clock |20 which is shown as having a pneumatic control systemand is supplied with air from an air supply line I2 I. The steam outlet line |22 from the mixing tube I3 of the thermal compressor |09, is divided into three branches |23, |24 and |25, these branches having a common connection to the inlet neck I0 I of the heater 60. In the branch |25 is arranged a plate |26 having a small orifice |21; in the branch |23 is arranged a plate |28 havinga larger orifice |29; and in the branch |24 is arranged another plate |30 having a still larger orifice I3 I. In the branch |24 is also arranged a diaphragm valve |32 which preferably is reverse acting and is either open or closed depending upon whether the time clock |20 delivers air pressure to it Athrough the line |33. In a similar manner adiaphragm valve |34 i@ provided in the branch |23 and controlled from the time clock through a pressure line |35.'

A similar diaphragm valve |36 is arranged in the branch |25 and controlled from the time clock through a control line |31.

The object of the v thin plate orifices in the branch line is to secure a uniform quantity 'of steam delivered through the branches per unit of time regardless of changes in pressure on the discharge side of the orifice. A thin plate orifice placed in the steam line will pass a practically uniform quantity of steam per unit of' time regardless of the pressure on the discharge side of the orice as long as this pressure does not exceed .58 of the absolute pressure on the line side. It is, of course, assumed that the pressure on the line side is uniform. For example, if the pressure is 100 lbs. absolute the quantity of steam passed by any of the orifices will be uniform until the pressure on the discharge side reaches` 58 lbs. absolute.

In the cooking of wood chips in a digester for the manufacture of pulp in the three stages, as

stated, in the rst period the contents of the di gester are heated from approximately 77 F. to 212 F. in two hours, this being a rise of 67.5 per hour. It is desired to bring the temperature up to 212 in as short a time as possible and this time of two hours isl generally considered about the minimum. In the second stage or period the temperature is brought up to about 284 F. for a rise of about 24 per hour. This temperature is then held for the balance of the cook for four or more hours as the third period of the temperature control. Naturally this holding of the teme perature constant without the addition of any heat other than that lost by radiation requires but a small ow of steam. In the operation of this time and temperature schedule it is desirable that the temperature of the digester is brought up uniformly during each of the first two periods and maintains uniformly during the last period and consequently it is desirable that there be a uniform flow of steam to the heater during each period. This uniform flow or consumption of steam is desirable in order to make the operation of the boilers in the plant more uniform. By using thin plate orifices for controlling the steam flow to the heater an ideal method is provided for securing this uniform ow as all parts of the control system can be made of a simple design and no delicate cam work devices, such as are generally used in automatic timetemperature controls are required.

At the start of the cook all three of the reverse acting diaphragm valves H32, H34 and H30 are open and steam is admitted through all three branches H23, H213 and H25 to the heater. Inasmuch as a thin plate having an orice is incorporated in each of these branches the amount of steam delivered to the heater will be uniform regardless of variations of pressure in the heater, provided, of course, this pressure does not exceed .58 of the steam pressure on the line side. Since the thin plate orices deliver uniform quantities of steam the temperature in the digester is brought up uniformly to the desired 100 C. and the orifices are designed to bring the digester temperature up to 100 C. in a period of two hours. At the end of two hours the time clock H20 relieves the pressure in its control outlet line H33 thereby causing the reverse acting Adiaphragm valve H32 to close and cut off the flow of steam through the branch H 24 and its orifice plate H30. This diaphragm valve and the diaphragm valves in the other branches have no regulatingfunction but merely start and stop the full flow of steam to the orice. Steam is now delivered through the orifices H29 and H2'H in the branches H23 and H25 and these orifices are proportioned to deliver the steam in constant volumes to secure a temperature rise in the digester during the second period of three hours from 100 C. to 135 C. or 140 C. At the end of the second period the time clock H20 relieves the air pressure in the line H35, thereby closing the reverse acting diaphragm valve H32 and cutting off the flow of the steam through the orifice H 29.

During the balance of the cook the small amount of steam required for maintaining thel temperature of the digester at 135 to 140 C. is therefore supplied through the smallest orice H27 and this orifice is designed to deliver steam in uniform quantities sufficient only to take care of radiation losses. At the end of the cook the time clock mechanism H20 relieves the pressure, its outlet control line H31 thereby closing the diaphragm valve H36 and cutting oir all steam to the heater.

It is apparent that the thin plate orices employed permit of a very simple control system especially in combination with a time block, as shown. The rise iny the steam pressure in each case is on the discharge side of the orice and, of course, represents the steam pressure on the outside of the tubes of the heater. The rise in pressure is entirely automatic since as the teiniqperature of theliquor in the tubes of the heater rises the temperature of the steam must also rise and its pressure increase as the temperature difference between the liquor in the tubes and the steam on the outside of the tubes must be constant in order that the steam flow be uniform. In other words, all the steam that passes into/ the heater must be condensed and to condense the steam at the required rate a certain temperature difference is required. As the thin plate orices provide a constant flow the pressure in the heater is built up automatically to maintain the proper temperature difference required to condense the steam.

Another method of control is illustrated in the `modified form of the invention in Fig. 13. It is desirable, in some digester operations, that a definite temperature differential be at all times maintained betwene the liquor and the steam. This is particularly important during the initial stages when the liquor is comparatively cool and rises in temperature rapidly and, in the absence of means providing a constant temperature differential, rises rapidly until it approaches the temperature of the steam. In Fig. 13 is shown a control system in which thevsteam temperature is always maintained at a predetermined number of degrees higher than the temperature of the liquor so that at all times the steam consumption is uniform and the liquor is never subjected to excessive heat. To accomplish this a master thermostat H40 is arranged in the upper header l5 of the heater and has its sensitive parts located in the liquor so that it is responsive to the temperature of this liquor. This thermostat is supplied with air' pressure from a line HM The control line M2 from this master thermostat connects with a sub-thermostat H43 which 'is arranged in the steam inlet neck HUH of the heater and has its sensitive parts responsive to the steam temperature in the heater. This sub'- thermostat is supplied with air pressure from the air pressure supply line HM and its outlet control line HM controls a diaphragm valve M5 in the line H46 leading from the thermal compressor to,

the steam inlet neck HHlH of the heater. The diaphragm valve H 45 is arranged to gradually open and close as the pressure in the sub-thermostat outlet line H33 varies. The voutlet line H 42 from the master thermostat is arranged to vary the setting of the sub-thermostat H43 so that as the temperature of the liquor rises the master thermostat raises the setting of the sub-thermostat H43 so as to cause the sub-thermostat M3 to deliver increased air pressure to the diaphragm valve H25 and open this diaphragm valve, which is assumed to be reverse acting, and admit more steam to the heater. It is apparent bywthis means that any-desired diiTerential can be obtained by adjusting the setting of the master thermostat and it is apparent that by this master and sub-thermostat combination a `constant temperature differential is obtained between the steam and the liquor. l I

From the foregoing it is apparent that the present invention provides an indirect heating `and circulating system for sulphite digesters in which the cost of the installation is reduced to a minimum; 'all strains upon the necessary stain- .less steel parts are reduced to a minimum, the

acid resisting quantities of these parts being adversely affected when subjected to such strains in which a heater having arelatively small amount of heating surface can be employed and in which automatic control means are provided which are simple and reliable and are not liable to get out of order under conditions of severe and constant use.

I claim as my invention:

l. In a digester system, a digester shell, means for supporting said shell, means extending through said shell adjacent said supporting means for withdrawing and'retu'rning the liquor in said shelll and means for heating the withdrawn liquor.

2.In a digester system, a vertically disposed digester shell, a plurality of supports forv said shell adjacent its lower end, means extending ing the Withdrawn liquor.

- places of juncture of said 3. In a digester system, a vertically disposed cylindrical digester shell having a funnelshaped lower end, a plurality of outwardly projecting supporting brackets secured to the funnel shaped lower end of saidshell and having under faces arranged in the same horizontal plane, supporting standards engaging said under faces and lsupporting said brackets and shell, means extending through said shell in substantially said horizontal plane as said under faces for withdrawing and returning the liquor in said shell and means for heating the withdrawn liquor.

4. In a digester system, a vertically disposed digester shell having a discharge opening at its lower end, a plurality of supports for said shell adjacent its lower end, aV conduit extending vertically within said shell for withdrawing the liquor from the upper part of said shell, means for heating the withdrawn liquor, and means for returning the heated liquor, including a return conduit extending through the side of said shell, said conduits extending through said shell in substantially the Vsame horizontal plane as the supports and shell.

5. In a digester system, a vertically disposed digester shell, a strainer supported within the upperpart of said shell,a vertical conduit for withdrawing liquor from said strainer, said conduit including a plurality of lengths of pipe loosely fitted at their opposite ends in supporting brackets secured to the wall of said shell and the lower end of said conduit extending out through the wall of said Ashell adjacent its lower end,

means for heating the liquor so withdrawn and a return conduit for said heated liquor, said return conduit extending through said shell in substantially the same horizontal plane as said liquor withdrawing conduit.

6. In a digester system, a vertically disposed digester shell having an acid resistant lining, a strainer supported within the upper part of said shell, a vertical conduit for withdrawing liquor yfrom said strainer, said conduit including a nurnber of lengths of pipe, brackets for supporting said pipes, each comprising a block-like base portion secured to the inside of said shell and around which the lining is. built up and av ring portion loosely receiving and supporting the adjacent ends of the corresponding pipes, the lower end of said conduit extending out through the wall of said shell adjacent its lower end, means for heating the liquor so withdrawn and a return conduit for the heated liquor, said return conduit extending through said shell in substantially the same vertical plane as said liquor withdrawing conduit.

7. In a digester system, a vertically disposed digester shell having an acid resistant lining, a

strainer in the upper part of said shellvand consisting of a plurality of lengths of perforated pipe arranged end to end in generally circular arrangement, brackets supporting the ends of said sections, each bracket comprising a base portion secured to said shell and around which said lining is built up and a ring portion loosely receiving and supporting the adjacent ends of corresponding pipe lengths, means for withdrawing the liquor from said strainer, means for heating the withdrawn liquor and means yfor returning the heated liquor to said shell.

8. In` a digester system, a vertically disposed digester shell, a` strainer in thev upper part of said shell and consisting of a horizontally disposed perforated pipe of generally circular form, a conduit for withdrawing the liquor from one side of said strainer, means for heating the withdrawn liquor and means for returning the heated liquor to said shell, the perforations in said strainer being proportioned to provide greater free area through said strainer at places remote from said withdrawal conduit thereby to withdraw substantially equal amounts of liquor from all sides of the shell.

9. In a digester system, a vertically disposed digester shell having an acid resistant lining, a conduit extending through said shell and lining for withdrawing liquor therefrom, a conduit extending through said shell and lining and returning the withdrawn liquor to said shell, and means for heating the return liquor, one of said conduits at the place of passage through the shell and lining being of rectangular form in cross section to permit of readily building up the lining thereabout and being Valso provided with a flange adapted to be secured to said shell on the external side thereof.

10. In a digester system, a digester shell, a pump for withdrawing liquor from said shell, a heater connected to the outlet of said pump, a conduit for returning the heated liquor to said shell, and supporting means for said pump, the supported faces of said supporting means being disposed in a plane extending through the center of its inlet and outlet to relieve expansion strains on the inlet and outlet connections to said pump.

11. In a digester system, adigester shell, a pump for withdrawing liquor from said shell, a heater connected to the outlet of said pump, a heater outlet conduit for returning the heated liquor to said shell, and supporting means for said heater, the supported faces of said supporting,

means being disposed in a plane extending through the center of its inlet and outlet to relieve expansion strains on the inlet and outlet connections thereto.

12. In a digester system, a digester shell, an inlet and o utlet conduit for the liquor in said shell and having connections extending through the wall of said shell, supporting means for said shell, the supported faces of said supporting means being disposed in the same plane as said connections, a pump for withdrawing liquor from said outlet conduit, a heater connected to the outlet of said pump and having its outlet connected to said return conduit and supporting means for said pump and heater, the supported l faces of said last supporting means being disposed in a plane extending through the center of the corresponding inlet and outlet to relieve expansion strains on the inlet and outlet connections.

13. In a digester system having a digester shell and means for withdrawing and returning the liquor in said shell, a heater for said return liquor comprising a casing having an external flange at one end, tubes in said casing, a tube sheet at each end of said casing and seated against the corresponding end of said casing, a header at one end of said casing and having a relatively' small external flange, clamping means connected to the ange of the casing and hooking liquor comprising a casing having an external ilange at one end, tubes in saidcasing, a tube sheet at each end of said casing and seated against the corresponding end of said casing, a header at one end or said casing and having a relatively small grooved external iiange, clamping means having a tongue fitting in said groove and embracing said grooved flange, bolts extending through said clamping means and connected to the flange of casing to hold said header against the corresponding tube sheet, a header at the opposite end of said casing, means for passLng said liquor through said headers and tubes and means for admitting steam to said casing around said tubes.

15. In a digester system having a digester shell and means for withdrawing and returning the liquor in said shell, a heater for said return liquor, comprising a casing having an external flange at one end, tubes in said casing, a tube sheet at each end of said casing and seated against the corresponding end of said casing, a header atone end of said casing, and having a relatively small external flange, clamping members engaging the opposite outer faces of said flanges, bolts drawing said clamping members together to hold said header against the corresponding tube sheet, a header at the opposite end of said casing, means for passing said return liquor through said headers and tubes and means for admitting steam to said casing around said tubes.

1S. In a digester system having a digester shell and means for withdrawing and returning the liquor in said shell, a heater for said return liquor, comprising a casing having an external flange at one end, said flange being provided with a. groove on the side adjacent the body of said casing, tubes in said casing, a tube sheet at each end of said casing and seated against the corresponding end of said casing, a header at the flanged end of said casing and having a relatively small flange grooved on the side opposite to the grooved side of the casing flange, clamping lugs having tongues entering the groove of said casingflange, a clamping ring having a tongue entering the groove of said header flange, bolts connecting said lugs and ring and drawing said header against the adjacent tube sheet, a header at the opposite end of said casing, means for passing said return liquor through said headers and tubes and means for admitting steam to said casing around said tubes.

17. In a digester system having a digester -shell and means forwithdrawing and returning the liquor in said shell, a heater for said return liquor, comprising a casing, tubesheets at the ends of said casing, tubes between said tube sheets headers at the opposite ends of said casing, means for passing the return liquor through said headers and tubes and means for admitting steam to said casing comprisinga manifold secured against the outside of said casing, a steam inlet for said manifold and a plurality of openings in said casing and extending substantially the full length of said manifold to distribute the steam admitted to said casing.

18. As an article of manufacture, a testing ring for a' heater having a casing, a tube sheet, tubes .connected with said tube sheet and a header removably secured by its rim to said casing and holding said tube sheet seated against said easing, comprising a ring formed to correspond to the rim of said heater and thereby adapted to be substituted therefor.

19.v In a digester system having a digester shell, a steam heater and means for withdrawing liquor from said shell and passing it through said heater and returning it to said shell, means for supplying low and high presmre steam to said heater comprising a thermo-compressor, means for supplying low pressure steam to said thermo-compressor, means for supplying high pressure steam to said thermo-compressor to combine with said low pressure steam and compress said low pressure steam and meal s for conducting the combined low pressure steam to said heater.

2). In a digester system having a digester shell, a steam heater and means for withdrawing liquor from said shell, passing it through said heater and returning it to said shell, means for supplying low and high pressure steam to said heater comprising a thermo-compressor having a Venturi tube, means for admitting low pressure steam at the inlet end of said Venturi tube, means for jettf'ng high pressure steam into said Venturi tube from a point adjacent said low pressure steam inlet and meansfor conducting the steam from the outlet of said Venturi tube to said heater.

21. In a digester system having a digester shell, a steam heater and means for withdrawing liquor from said shell, passing it through said heater and returning it to said shell, steam supply means to said heater and means for controlling said steam supply to admit uniform volumes of steam per unit of time to said heater over an extended period of time.

22. In a digester system having a digester shell, a steam heater and means for withdrawing liquor from said shell, passing it through said heater and returning it to said shell, steam supply means tosad heater and means for controlling said steam supply to admit uniform volumes of steam per unit of time to said heater over an extended period of time comprising a thin wall in said steam supply line and having an orifice of a size predetermined to pass uniform volumes of steam to obtain the desired digestion action.

23. In a digester system having a digester shell, asteam heater and means for withdrawing liquor from said shell, passing it through said heater and returning it to said shell, steam supply means to said heater comprising a conduit having two branches, a thin wall in each of said branches, each of said walls having an orifice adapted to pass uniform quantities of steam per unit of time, and valve means in at least one of said branches for preventing the flow of steam through the corresponding orice and reducing the flow of steam to said heater.

24. In a digester system having a digester shell, a steam heater and means for withdrawing liquor from said shell, passing it through said heater and returning it to said shell, steam supply means to said heater comprising a conduit having two branches, a thin wall in each of said branches, each of said walls having an orifice adapted to pass uniform quantities of steam per unit of time, valve means in at least one of said branches for preventing the flow of steam through the corresponding orice and reducing the ow of steam to said heater and a time clock controlling said valve means.

25. In a digester system having a digester shell, a steam heater and means for withdrawing liquor from said shell, passing it through said heater and returning it to said shell, steam supply means to said heater comprising a conduit having three branches, a thin wall in each of said branches, each of said walls having an orice adapted to pass uniform quantities of steam per unit of larger sized orifices.

26. In a digester system having a digester shell, a heater and means for withdrawing liquor from said shell, passing it through said heater and returning it to said shell, means for supplying a heating medium to said heater and means for maintaining the temperature of said heating me- .dium a constant number of degrees higher than the temperature of said liquor. Y

27. In a digester system having a digester shelll a heater and means for withdrawing liquor from said shell, passing it through said heater and returning it to said Shell, means for supplying a heating medium to said heater and means for maintaining the temperature of said heating medium a constant number of degrees higher than the temperature of said liquor comprising subthermostat meansvresponsive to the temperature of said heating medium, master thermostat means responsive to the temperature of said liquor and controlling the setting of said sub-thermostat means, and means controlled by said sub-thermostat means for controlling the temperature of the heating medium in said heater. y

, HABLAN W.,HOW.

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