Abstract:
A direct contact steam injection heater body is placed directly in line and allows axial flow of stock (i.e. liquid or slurry) through a pipe. The steam injection heater includes a Mach diffuser having a plurality of steam diffusion holes. The Mach diffuser is mounted transverse to the axial flow of stock through the pipe and the heater body. High velocity steam is injected from the plurality of steam diffusion holes into the stock flowing through the heater body. An adjustably positionable cover in the Mach diffuser modulates the amount of steam added to the flowing stock by exposing a desired number of steam diffusion holes. Modulation is accomplished at constant steam pressure by an actuator that rotates the cover. The arrangement is able to efficiently heat large flows of viscous stock, such as slurries having suspended materials that tend to flocculate. The upstream surface area of the Mach diffuser is preferably free of steam diffusion holes to eliminate unnecessary plugging. The downstream surface area of the Mach diffuser is also preferably free of steam diffusion holes to lessen the probability of large steam bubble conglomeration. A deflector is preferably mounted directly in front of the upstream surface of the Mach diffuser to route the flow of stock towards the side surfaces of the Mach diffuser where the steam diffusion holes are located.

Description:
FIELD OF THE INVENTION 
     The invention relates to direct contact steam injection heaters that use full pressure steam. In particular, the invention relates to a direct contact steam injection heater for heating certain types of slurries which contain material that tends to flocculate. 
     BACKGROUND OF THE INVENTION 
     In direct contact steam injection heaters, steam is directly mixed into a flowing stock (e.g. liquid or slurry) being heated. Direct contact steam injection heaters are very effective at transferring heat energy to the flowing stock. They provide rapid heat transfer with virtually no heat loss to the atmosphere, and also transfer both the latent and the available sensible heat of the steam to the liquid. 
     The present invention was developed during ongoing developmental efforts by the assignee in the field of direct contact steam injection heaters. U.S. Pat. No. 5,622,655 entitled “Sanitary Direct Contact Steam Injection Heater And Method” by Bruce A. Cincotta et al., issuing on Apr. 22, 1997, and U.S. Pat. No. 5,842,497 entitled “Adjustable Direct Contact Steam Injection Heater”, by Brian Drifka and Bruce A. Cincotta, issuing on Dec. 1, 1998, represent some of the prior developments in direct contact steam injection heaters by the assignee, and are hereby incorporated by reference. 
     These types of direct contact steam injection heaters use full pressure steam (i.e. the full amount of steam pressure available), and modulate the amount of steam added to the flowing liquid or slurry by a nozzle and plug configuration. The steam exits through the nozzle under sonic choked flow conditions. The high speed steam from the nozzle shears the flowing liquid or slurry, and creates a homogeneous blend in a combining region located downstream of the nozzle. As heat is transferred, the steam condenses. 
     Another direct contact steam injection heater was developed by the assignee for heating purified water or other liquids in which steam bubbles tend to merge to create large steam bubbles prior to condensing. This direct contact steam injection heater is disclosed in U.S. patent application Ser. No. 09/112,499, entitled “Direct Contact Steam Injection Heater”, allowed Feb. 1, 2000, now U.S. Pat. No. 6,082,712, and incorporated herein by reference. This direct contact steam injection heater employs a Mach diffuser. The Mach diffuser injects a sonic velocity steam into the liquid stock through a plurality of relatively small steam diffusion holes. The Mach diffuser is generally coaxial with the heater body and resides within the inlet of a combining region. The purified water or other liquid flows in a radial direction through the inlet into the combining region and turns at an essentially right angle to flow through the combining region. The steam exits the coaxial Mach diffuser as small jets of steam injecting partially into the axial flow through the combining region. The velocity of the liquid flowing through the channel between the Mach diffuser and the combining region is maintained at a relatively high velocity (i.e., a relatively small flow area in the channel compared to the downstream portion of the combining region). 
     Although direct contact steam injection heaters are efficient and effective, stocks containing materials that flocculate tend to plug the heater if forced through bends and turns. Large flows of viscous stock cannot flow easily through the 90° turns presented by certain prior art devices. For example, the direct contact steam injection heaters with adjustably positionable combining tube are not well suited for certain applications because the adjustable combining tube introduces additional creases, folds and pockets into which flocculating materials can accumulate. In addition, many designs are not easy to disassemble for manual cleaning. 
     Further, large volume flows of viscous slurries are difficult to heat with prior art direct contact steam injection heaters. 
     SUMMARY OF THE INVENTION 
     The invention is a direct contact steam injection heater in which steam is introduced into a flow of stock that is flowing axially through a pipe. The heater is installed in line and allows continued axial flow of the stock so the stock flow is not required to negotiate sharp turns when passing through the heater. That is, the heater includes a heater body having a flowing stock inlet and a heated stock discharge outlet that are aligned to provide axial flow through the pipe and the heater body. Full pressure steam is introduced into the stock through a Mach diffuser that is mounted transverse to the axial flow through the heater body. The Mach diffuser has a plurality of steam diffusion holes through which small jets of steam are injected into the flowing stock. The small steam jets break apart easily in viscous slurries and disperse before the steam has a chance to conglomerate into large bubbles which can create “steam hammers” and lead to unwanted vibration within the heating system. Furthermore, small steam bubbles dissipate heat more efficiently and thereby prevent hot and cold spots in the flowing stock. 
     The Mach diffuser has an adjustably positionable cover. The adjustably positionable cover obstructs a selected amount of the steam diffusion holes in order to modulate the amount of steam discharged through the Mach diffuser into the flow of stock. The cover is preferably rotatable relative to the longitudinal axis of the transversely mounted Mach diffuser. 
     The Mach diffuser preferably has a cylindrical wall containing the steam diffusion holes. The cover is preferably a cylindrical wall nested inside the cylindrical wall of the Mach diffuser, although if desired the cover can be placed on the outside of the cylindrical wall. The preferred cover has an internal region within the cylindrical wall that receives steam passing into the heater. The cylindrical wall has at least one steam opening that enables steam to flow from the internal region in the cover through the exposed steam diffusion holes in the Mach diffuser and into the axial flow of stock. Preferably, there are two steam openings in the cover, each consisting of a longitudinal slot located on opposite sides of the cover. The longitudinal slots preferably have widths that substantially occupy one quarter of the circumference of the cylindrical wall of the cover. 
     The Mach diffuser has an upstream surface area and a downstream surface area on its cylindrical wall, each occupying substantially one quarter of the circumference of the transversely mounted Mach diffuser. The upstream and downstream surface areas do not contain steam diffusion holes. The side surface areas on the Mach diffuser contain the steam diffusion holes. The Mach diffuser is oriented in the heater body so that the upstream surface area faces into the axial flow of the stock. This orientation to prevents unnecessary plugging of the diffusion holes on the upstream surface. Preferably, a deflector is mounted upstream of the Mach diffuser. The deflector deflects the flow of stock around the upstream surface area on the Mach diffuser and towards the side surfaces of the Mach diffuser. This prevents flow directly into a fluid stagnation point on the upstream surface of the Mach diffuser. The deflector is preferably welded to the inside wall of the heater body so that it does not become dislodged in the face of heavy flows of viscous slurries. 
     As mentioned, the downstream surface does not contain steam diffusion holes. This configuration helps to prevent the unnecessary formation of large steam bubble conglomerations. Large steam bubble conglomerations would likely be generated if steam diffusion holes were present on the downstream surface area because flow around the transversely mounted Mach diffuser normally separates from the cylindrical surface on the downstream side of the Mach diffuser. 
     The amount of full pressure steam discharged through the Mach diffuser into the axially flowing stock is modulated by adjusting the position of the cover over a selected amount of steam diffusion holes. This adjustment is preferably accomplished electronically with a rotating actuator having a key that engages one end of the cover. When the actuator rotates the key, the cover is positioned to expose a generally proportional amount of steam diffusion holes in the Mach diffuser. Radial jets of steam then flow through the exposed steam diffusion holes into the axial flow of stock. 
     It should be apparent to those skilled in the art that the use of an actuator to rotate the cylindrical cover for the Mach diffuser is especially accommodating for large volume flows through pipes having relatively large diameters. The rotatable cover allows for generally consistent injection of steam across the entire length of the transversely mounted Mach diffuser. In addition, the stroke on a linear actuator may create installation problems. 
     Various other features, objects, and advantages of the invention will be made apparent from the following detailed description and the drawings. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a side elevational view showing a longitudinal cross-section of a direct contact steam injection heater having a Mach diffuser in accordance with the prior U.S. patent application Ser. No. 09/112,499, now U.S. Pat. No. 6,082,712. 
     FIG. 2 is an isometric view of an installed direct contact steam injection heater in accordance with the invention. 
     FIG. 3 is an assembly view of the heater shown in FIG.  2 . 
     FIG. 4 is a side view showing a cross-section of the steam injection heater shown in FIGS. 2 and 3 as it is installed in a pipe through which a flow of stock flows axially. 
     FIG. 5 is a view taken along line  5 — 5  in FIG.  4 . 
     FIG. 6 is a view of the Mach diffuser taken along line  6 — 6  in FIG.  4 . 
     FIG. 7 a  is a view similar to FIG. 6 further showing a deflector positioned upstream from an upstream surface area of the Mach diffuser, and also showing a cover aligned so that it does not obstruct steam diffusion holes in the Mach diffuser. 
     FIG. 7 b  is a view as shown in FIG. 7 a  with the cover rotated to partially close steam injection holes through the Mach diffuser. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Prior Art 
     FIG. 1 shows a direct contact steam injection heater  10  constructed in accordance with the prior U.S. patent application Ser. No. 09/112,499, now U.S. Pat. No. 6,082,712. The heater has a body  12  having a steam inlet  14 , a liquid inlet  16 , and a heated liquid discharge outlet  18 . Steam  20  flows into the heater  10  through steam inlet  14 , and then into a cover  24  located within a Mach diffuser  21 . As shown in FIG. 1, the Mach diffuser is mounted axially within the heater  10 . The steam flows into an internal region  22  within the cover  24  through opening  26 . The cover  24  is a cylindrical wall having a closed top  28  and an open bottom  30 . The Mach diffuser  21  includes a plurality of radial steam diffusion holes  32  that are arranged at least in part longitudinally along the cylinder wall defining the Mach diffuser  21 . The amount of steam supplied through the Mach diffuser  21  into the liquid  34  flowing through the combining tube  36  is modulated by linearly moving the adjustably positionable cover  24  as shown by arrow  27  to expose a selected number of steam diffusion holes  32  in the Mach diffuser  21 . 
     The Mach diffuser  21  is located within the upper end of a combining tube  36  such that small jets of steam are discharged radially into the flow of liquid  34  as the liquid is flowing through the combining region  38 . The width of the channel for liquid  34  flowing between the Mach diffuser  21  and the wall  40  of the combining region  38  is selected to optimize the axial velocity of liquid  34  flowing through the channel for enhanced mixing. The axial velocity of the liquid should be sufficient to continually wet the outer surface of the cylindrical wall of the Mach diffuser  21 , thus eliminating the likelihood that continuous large bubbles will generate from the small radial jets of steam flowing into the axial flow of liquid  34 . The preferred width of the channel between the Mach diffuser  21  and the inner wall  40  of the combining region  38  depends on the size of the heater  10 , and on the type of liquid  34  being heated, and the amount of steam being added. 
     The radial jets of high velocity steam shear the high velocity axial flow of liquid  34  in the channel between the Mach diffuser  21  and the inner wall  40 . The mixture flows axially downstream past the cone shaped Mach diffuser end cap  42  and into the combining region  38  to continue heat transfer and condensation of the steam. With this heater, steam bubbles within the combining region remain relatively small. Therefore, steam condensation within the combining region does not cause substantial vibrations even when heating difficult liquids, such as liquids having relatively small numbers of nucleation points, or insufficient surface tension (e.g., pure water). 
     Although this heater works well in most applications, the heater  10  cannot easily accommodate slurries or other viscous liquids containing materials that tend to flocculate. Such stock tends to clog narrow passages and does not flow easily around bends and turns. For example, paper pulp or oxide bauxite slurries tend to plug the heater shown in FIG.  1 . In these slurries, fibers or particles flocculate and excessive cleaning is required. 
     Present Invention 
     The invention as illustrated in FIGS. 2-7 b  is designed to accommodate large flows of slurries or other viscous liquids containing materials that tend to flocculate, such as suspended fibrous or particulate matter. In accordance with the invention, the heater  110  has a Mach diffuser  121  that is placed transversely in the heater body  112 . The heater body  112  is connected in line with a stock supply pipe  151 , FIG.  4 . 
     As shown in FIGS. 2-4, steam  120  flows into heater  110  through a steam inlet  114 , and into an internal region  152  (FIG. 6) defined by a cover  154  located within the Mach diffuser  121 . Steam  120  enters internal region  152  through an opening  156  located near steam pipe  182 . Cover  154  is a cylindrical wall having a closed end  158  and an open end  160 , FIG.  3 . Steam is supplied through the cover  154  via openings  162  (only one opening  162  is shown in FIG. 3, the other opening is directly opposite of the opening  162  that is shown) at essentially the full steam pressure available at the heater  110 . 
     As shown in FIGS. 3 and 4, the Mach diffuser  121  includes an open end  161 , a cylindrical wall  164  and a flanged base  166 . An internal region within the Mach diffuser  121  is defined by the base  166  and the cylindrical wall  164 . Cover  154  is preferably contained within the internal region of the Mach diffuser  121 . 
     The cylindrical wall  164  of the Mach diffuser  121  includes a plurality of radial steam diffusion holes  168 . The size and number of the steam diffusion holes is a matter of choice depending on the size of heater  110 . However, a diameter of about ⅛ of an inch is preferred for most stocks. Such a diameter is sufficiently small to facilitate the creation of relatively small radial jets of steam through the diffuser wall  164 , yet is not so small as to create other problems such as plugging or scaling. In addition, it is preferred that the Mach diffuser  121  be made of stainless steel, and that the cylinder wall  164  for the Mach diffuser have a thickness sufficient to avoid premature deterioration as steam passes through the plurality of steam diffusion holes  168  over extended periods of time. 
     The plurality of steam diffusion holes  168  are arranged at least in part longitudinally along the cylinder wall  164 . In this manner, the amount of steam supplied through the Mach diffuser  121  into the stock  126  flowing through the heater body  112  can be easily modulated by moving the adjustably positionable cover  154  to expose a selected number of steam diffusion holes  168 . The pattern of steam diffusion holes  168  in the Mach diffuser  121  as shown in FIG. 4 includes rows of steam diffusion holes  168 , with each row  170  being offset from the immediately adjacent rows in order to provide high hole density. 
     Referring now in particular to FIGS. 4 and 5, Mach diffuser  121  is attached to heater body  112  first by seating the Mach diffuser base  166  on a rim  171  located on an outwardly extending cylindrical flange  172  extending from heater body  112 . Next, the Mach diffuser end  161  engages a seal assembly  174  located at the steam inlet  114 , FIG.  4 . Seal assembly  174  is attached to heater body  112  by inserting a lip  176  on the seal assembly  174  into an outwardly extending steam inlet projection  114  of the heater body  112 . The interior surface  178  of seal assembly  174  is preferably sealed by a pair of O-rings  180  against the end  161  of the Mach diffuser  121 , although other types of sealing arrangements may be used. 
     The steam pipe  182  has a radially extending flange  184  that engages a flange  186  of seal assembly  174 . Preferably, flange  184  is not flush with the end  187  of steam pipe  182 , so that end  187  extends slightly outward from the surface  191  of the flange  186  on the seal assembly  174 . This interface preferably forms a stepped seal to prevent steam from escaping. Flange  186  and flange  184  are secured using bolts  188  extending through holes  190 , FIG.  3 . 
     An actuator  192  drives rotation of the cover  154  by rotating an actuator key  194 . The actuator  192  is shown in the drawings as a phantom box. One skilled in the art will recognize that the actuator  192  may be activated manually, pneumatically or electrically. Preferably, the operation of the actuator, if pneumatic or electric, is controlled by an electronic controller in response to a feedback signal from a downstream temperature sensor. The preferred actuator is a quarter turn actuator by Neles Jamesbury, and provides shaft rotation of 90°. The actuator key  194  has a shank  196  and a key head  198 . The key  194  engages the output shaft of the actuator  192  using means appropriate for the type of actuator provided. 
     The actuator  192  is mounted on an actuator plate  200  that is secured to the base  166  of the Mach diffuser. As shown in FIGS. 5 and 3, the actuator  192  is mounted to base  166  of the Mach diffuser using a pair of threaded bolts  202  that are screwed into apertures  204  located in the base  166  and the actuator plate  200 . Openings  206  and  208  are located in the actuator plate  200  and the base  166  of the Mach diffuser, respectively. The openings allow passage of actuator key  194  into the base  166  of the Mach diffuser for engagement with the cover  154 . Openings  206  and  208  are aligned with the longitudinal axis of rotation for the cover  154 . The actuator key head  198  engages the cover  154  at the top end  158  of the Mach diffuser where a key slot  210  is provided. Preferably, the key slot  210  is located in a disc-shaped end cap  212 , FIG. 5, that is rigidly attached (e.g., welded) to the top of the cover  154  at end  158 . 
     As shown in FIGS. 2 and 4, the heater body  112  is attached to the stock supply pipe  151  in such a manner that the longitudinal flow axis of heater body  112  is aligned with the longitudinal flow axis of the supply pipe  151 . The supply pipe  151  is fitted with flanges  216   a ,  216   b  that are designed to engage flanges  218   a ,  218   b , respectively, located on heater body  112 . The flanges  216   a ,  216   b  on the pipe extend radially from the cylindrical surface of the pipe  151 , and are preferably welded to the supply pipe  151 . Flanges  216   a ,  216   b  and  218   a ,  218   b  preferably have a stepped interface  120 . Flanges  216   a ,  216   b , and flanges  218   a ,  218   b  have apertures  122  provided therein through which bolts  224  are passed to secure the heater body  112  to the pipe  151 . 
     FIG. 6 illustrates the flow of stock  126  through the heater body  112 . Note that the flow wets the outer side surfaces of the Mach diffuser  121 . In the embodiment shown in FIG. 6, however, there is likely to be a stagnation point at the upstream surface  234 . The existence of a stagnation point is likely to cause unwanted accumulation of suspended materials on or near the Mach diffuser  121 . Therefore, it may be desirable to use a deflector  226  positioned upstream of the Mach diffuser, see FIGS. 7 a  and  7   b.    
     Referring now to FIGS. 7 a  and  7   b , a deflector  226  is preferably located within the heater body  112  in a position upstream from the Mach diffuser  121 . Preferably, deflector  226  is welded to inner surface  228  (FIG. 4) of the heater body  112  in order to secure the deflector  226  in a manner that is capable of withstanding pressure from the stock flow through the pipe  151 . The deflector  226  is preferably constructed from an angle-iron shaped piece of metal, such as stainless steel. A leading edge  230  of the deflector  226  is aligned with the central axis of the Mach diffuser  121 . The symmetric shape of deflector  226  deflects the flow of stock away from the upstream surface area  234  on Mach diffuser  121  and towards the side surface areas  233 ,  235 . Thus, deflector  226  helps to prevent materials suspended in the stock  126  from flocculating in steam diffusion holes  168  facing upstream or partially upstream, and also prevents the stock  126  from stagnating at the Mach diffuser  121  at the upstream surface area  234 . 
     The Mach diffuser  121  is mounted to the heater body  112  transversely to the longitudinal flow axis through heater body  112 . Upstream surface area  234  and downstream surface area  236  each occupy substantially one quarter or 90° of the circumference of the Mach diffuser  121 . Upstream surface area  234  is directly opposite downstream surface area  236 , and both are aligned so that the center of the 90° arc defining each area is substantially aligned with the longitudinal flow axis through heater body  112  and the pipe  151 . Steam diffusion holes are not present in the upstream surface area  234  and the downstream surface area  236 . Steam diffusion holes  168  are located in the arcs remaining between upstream surface area  234  and downstream surface area  236 , i.e., steam diffusion holes  168  are located in the side surface areas  233 ,  235 . 
     The cover  154  is preferably placed concentrically inside the Mach diffuser  121 , although one skilled in the art should realize the cover  154  may be placed concentrically around the outside surface of Mach diffuser  121 . Like Mach diffuser  121 , cover  154  has two areas of solid wall that each comprise substantially one quarter or 90° of the cover  154  circumference. Preferably, two longitudinal slots  162  are centrally positioned on each side of the cover  154 . Each slot  162  has a width substantially one quarter or 90° of the circumference of the cover  154 . The ends  158  and  160  of the cover  154  also have a solid wall around the entire circumference. Only the central portion  155  of the cover  154  are slotted. The perimeter defining the slots  162  on the cover  154  is substantially coextensive with the perimeter defining the area of the Mach diffuser  121  having steam diffusion holes  168  when the heater is in the fully open position. 
     In operation, the cover  154  is rotated to selectively cover steam diffusion holes  168  in the Mach diffuser  121  either partially, or completely. Steam  120  flows through the heater inlet  114  into an internal region within the Mach diffuser  121  through opening  156 , FIG.  4 . Steam flows from the internal region within the Mach diffuser into the flow of stock  126  by passing through the uncovered steam diffusion holes  168  in the Mach diffuser  121 . In FIG. 7 a , the cover  154  is shown in a completely open position, and all of the steam diffusion holes  168  are open. In FIG. 7 b , the cover  154  is shown in a partially closed position so that only a portion of the steam diffusion holes  168  are open. When the cover  154  is fully closed (not shown), the cylindrical wall  165  of the cover  154  covers all of the steam diffusion holes  168  in the Mach diffuser wall  164 , and no steam is allowed to flow through the Mach diffuser  121  into the flow of stock  126 . When the cover  154  is moved to an open or partially open position, steam within internal region  152  of the cover  154  flows through the exposed steam diffusion holes  168  of the Mach diffuser  121 . Steam flows through the respective steam diffusion holes  168  in the form of high velocity jets of steam  238  into the flow of stock  126  through the heater body  112 . 
     The inside diameter of the heater body  112  should match the inside diameter of the stock supply pipe  151 . It is desired that the velocity of the stock be sufficient to continually wet the outer side surfaces  233 ,  235  of cylindrical wall  164  of Mach diffuser  121 , thus eliminating the likelihood that continuous large bubbles will generate from the small jets of steam  238  into the flow of stock  126 . 
     The steam pressure within Mach diffuser  121  is sufficient so that the flow of steam through the steam diffusion holes is not hindered by the flow of stock  126 . As long as there is a sufficient pressure drop across the open steam holes  168 , the flow of steam  120  into stock  126  will remain stable. The flow rate of steam  120  is defined by the steam pressure and the accumulated flow area of the exposed steam diffusion holes  168 . As mentioned, the amount of steam  120  added to the flowing stock  126  is precisely modulated by adjusting the position of the cover  154  to expose the proper amount of steam diffusion holes  168 . 
     While the preferred embodiments of the invention has been shown in connection with FIGS. 2-7 a,b , it should be noted that the invention is not limited to these specific embodiments. For instance, while the drawings show a Mach diffuser  121  in a fixed position with respect to heater body and a selectively positionable cover  154 , there may be alternative methods for varying the number of steam diffusion holes that are exposed. These alternative methods should be considered to fall within the scope of the invention.