Title of Invention	A VAPORIZER FOR LOW TEMPERATURE LIQUID
Abstract	This invention relates to a vaporizer for low temperature liquid such as liquified natural gas, liquified nitrogen comprising an inflow header for flowing low temperature liquid; a plurality of outer heat exchange tubes communicated with the inflow header, each outer heat exchange tube extending in a direction perpendicular to the inflow header, an outside of the outer heat exchange tube coming into contact with a heating medium; an outflow header communicated with the inflow header by way of the plurality of outer heat exchange tubes for flowing vapor of the low temperature which is produced in the outer heat exchange tubes; a plurality of inner heat exchange tubes provided in at least respective inflow portions of the plurality of outer heat exchange tubes, each inner heat exchange tube forming an annular passage between an inside surface of the corresponding outer heat exchange tube and an outside surface of the inner heat exchange tube, the annular passage communicating with the inflow header for flowing the low temperature liquid. PRICE: THIRTY RUPEES

Title of Invention

A VAPORIZER FOR LOW TEMPERATURE LIQUID

Abstract

This invention relates to a vaporizer for low temperature liquid such as liquified natural gas, liquified nitrogen comprising an inflow header for flowing low temperature liquid; a plurality of outer heat exchange tubes communicated with the inflow header, each outer heat exchange tube extending in a direction perpendicular to the inflow header, an outside of the outer heat exchange tube coming into contact with a heating medium; an outflow header communicated with the inflow header by way of the plurality of outer heat exchange tubes for flowing vapor of the low temperature which is produced in the outer heat exchange tubes; a plurality of inner heat exchange tubes provided in at least respective inflow portions of the plurality of outer heat exchange tubes, each inner heat exchange tube forming an annular passage between an inside surface of the corresponding outer heat exchange tube and an outside surface of the inner heat exchange tube, the annular passage communicating with the inflow header for flowing the low temperature liquid. PRICE: THIRTY RUPEES

Full Text	The present invention relates to a vaporizer for low liquid such as liquified natural gas,liquified nitrogen. BACKGROUND ART This invention relates to an open rack type vaporizer for vaporizing low temperature liquid such as liquefied natural gas. liquefied nitrogen. Conventionally, vaporizers of open rack type have been known as a vaporizer for vaporizing such low temperature liquid. The open rack type vaporizer generally includes a lower header in which low temperature liquid is flowed, an upper header arranged in parallel with the lower header, and a great number of vertical heat exchange tubes connecting the lower and upper headers. Outside of the heat exchange tubes is flowed heating medium such as seawater so that the liquid flowing in the lower header is heated and vaporized by heat of the heating medium in each heat exchange tube. Thus obtained natural gas is recovered by the way of the upper header. In this arrangement, the lower header and a lower portion of the heat exchange tube come into direct contact with the lower temperature liquid. Consequently, the tem¬perature of these portions is very low. The formation of ice is liable to occur on an outside surface of these por¬tions. The ice becomes a heat insulation barrier, and hinders the heat exchange at the lower portion of the heat exchange tube, thereby lowering the temperature of these portions further and resulting in an exceedingly low temper¬ature condition. In the state that the lower portion of the heat exchange tube is cooled to an exceedingly low tempera¬ture, the contraction rate of the heat exchange tube becomes large. Accordingly, there is a likelihood that a slight deviation in the flow of heating medium or seawater causes a great contraction difference among heat exchange tubes, and finally results in local deformations in heat exchange tubes. Japanese Unexamined Patent Publication No. 4-217788 discloses provision of a heat insulator on a portion of a heat exchange tube except an outflow portion near an upper header to suppress heat transfer between the heat exchange tube and natural gas. The heat insulator is formed by providing a cylindrical insulating member on an inner sur¬face of a heat exchange tube, or by forming an insulating space in a heat exchange tube in which vaporized natural gas is contained. The heat insulator prevents an outside sur¬face of the heat exchange tube from being cooled to an terribly low temperature, thereby preventing an exceeding contraction. It is certain that the heat insulator hinders the heat exchange between low temperature liquid and heating medium. However, the vaporization efficiency of the heat exchange tube provided with the heat insulator may be sub¬stantially identical to that of the heat exchange tube which is not provided with the heat insulator but is likely to be formed with heat insulating ice on an outside thereof. However, the heat insulator provides a very low heat transfer rate. Accordingly, the thickness of the heat insulator is very significant. A slight difference in the thickness of the heat insulator causes a great difference in the heat transfer of the heat exchange tube. Specifically, if having a thickness slightly greater than a specified thickness, the heat insulator exceedingly insulates heat, resulting in an insufficient heat transfer between an out¬side and an inside of the heat exchange tube, consequently making it impossible to heat and vaporize low temperature liquid. Conversely, if having a thickness slightly smaller than a specified thickness, the heat insulator cannot insu¬late heat sufficiently, consequently resulting in ice forma¬tion as the conventional heat exchange tubes. For example. Fig. 25 shows a relationship between the thickness of the heat insulator containing natural gas in an insulating space formed in a heat exchange tube, that is. a radial space of the insulating space, and the thickness of ice providing equivalent insulation rate to the insulating space. It will be seen in Fig. 25 that a variation of 0.1 mm in the thick¬ness of the insulating space is equal to a change of 10 mm in the thickness of ice in the aspect of heat insulation. The heat insulator formed by providing the insulating member made of vinyl chloride or the like has a similar heat insulation relationship. In other words, a small variation in the thickness of the insulating member will cause a great change in the heat insulation. For this reason, in this vaporizer, it is necessary to manage the thickness of the heat insulator at the high preciseness. In production lines, however, it is very difficult to accomplish such high preciseness. Even if such high preciseness can be accomplished, its production costs will become very high. Further, even if the heat insulator can be made in the production line, its thickness will be liable to greatly change due to thermal deformation or other causes in the use. Alternatively, it is possible to expand the dimensional variation tolerance by using a heat insulator made of mate¬rial having relatively high heat transfer rate, e.g., metal, which has an increased thickness. In this case, however, there is a problem that a metal insulator is liable to corrode and change the quality of vaporized product gas. In view of these problems, the present invention has an object of providing a low temperature liquid vaporizer which can reliably prevent ice formation and ensure sufficient heat transfer even if a variation exists in the insulating thickness. DISCLOSURE OF THE INVENTION Accordingly, the present invention is directed to a low temperature liquid vaporizer comprising: an inflow header for flowing low temperature liquid; a plurality of outer heat exchange tubes communicated with the inflow header, each outer heat exchange tube extending in a direction perpendicular to the inflow header, an outside of the outer heat exchange tube coming into contact with a heating medi¬um; an outflow header communicated with the inflow header by way of the plurality of outer heat exchange tubes for flow¬ing vapor of the low temperature liquid which is produced in the outer heat exchange tubes; a plurality of inner heat exchange tubes provided in at least respective inflow por¬tions of the plurality of outer heat exchange tubes, each inner heat exchange tube forming an annular passage between an inside surface of the corresponding outer heat exchange tube and an outside surface of the inner heat exchange tube, the annular passage communicating with the inflow header for flowing the low temperature liquid. With this vaporizer, low temperature liquid is flowed from the inflow header into not only the inside of the inner heat exchange tube but also the annular passage between the outer and inner heat exchange tubes. In the annular pas- sage, the low temperature liquid in the vicinity of an inside surface of the outer heat exchange tube is vaporized by heat of heating medium which is flowed over an outside surface of the outer heat exchange tube. On the other hand, the low temperature liquid in the vicinity of an outside surface of the inner heat exchange tube flows up in the state of liquid phrase. The inner heat exchange tube has a lower temperature than the outer heat exchange tube. In this state, there is a great forced convection boiling in the annular passage owing to the fact of the flow of fluid in an axial direction of the outer and inner heat exchange tubes and the co-existing of the liquid and gaseous phrases. Accordingly, the heat transfer rate of the annular passage increases. In other words, comparing to the conventional heat insulator containing gaseous material, the annular passage has an increased heat transfer rate. Accordingly, the space distance of the annular passage can be increased. This will make it possible not only to prevent ice formation on the outside surface of the outer heat exchange tube but also to ensure sufficient heat transfer even if the space distance is slightly larger or smaller than a specified va1ue. It may be appreciated that either of the inside surface of the outer heat exchange tube and the outside surface of the inner heat exchange tube is formed with a plurality of projections, a free end of each projection coming into contact with the other surface. This construction will stably hold the outer and inner heat exchange tubes in a coaxial arrangement, and prevent deformation of the outer and inner heat exchange tubes, and assure efficient vapori¬zation, and further prevent the inner heat exchange tube from vibrating. In the case that the inside surface of the outer heat exchange tube is formed with the projections, the projec¬tions increase the inside surface area of the outer heat exchange tube, and thus enhance the heat transfer rate of the outer heat exchange tube with respect to the low temper¬ature liquid in the annular passage. The inner heat exchange tube may be made to be shorter than the outer heat exchange tube, and the inner heat ex¬change tube may be disposed in an inflow end portion of the outer heat exchange tube. In this construction, vaporized fluid can be sufficiently heated in the region of the outer heat exchange tube where the inner heat exchange tube is not provided, i.e., an outflow portion of the outer heat ex¬change tube . The inner heat exchange tube may be formed with an ejection hole in a wall thereof and provided with a closing member in the inner heat exchange tube at a position closer to the outflow header than the ejection hole. In this ^^^-^^f^ 2l}n, construction, the low temperature liquid flowed into the inner heat exchange tube is ejected in the annular passage through the ejection hole, and is mixed with vaporized fluid in the annular passage. Accordingly, the vaporization of low temperature liquid is enhanced. Also, a film of gas on an inside surface of the inner heat exchange tube is removed and the heat transfer to the low temperature liquid in the inner heat exchange tube is increased. It may be appreciated that the outer and inner heat exchange tubes are arranged in a vertical direction while the outflow header is above the inflow header, and an out¬flow end of the inner heat exchange tube is fixedly attached to the outflow header. The outer and inner heat exchange tubes are supported in a suspended state by the outflow header. Accordingly, respective lower end portions of the outer and inner heat exchange tubes are not fixedly held. This will prevent the outer and inner heat exchange tubes from receiving a great thermal stress due to thermal con¬traction. Further, in the above-mentioned vaporizers, if low temperature liquid is directly mixed with a heat adjusting liquid, impurity matters having a high solidifying tempera¬ture, e.g., methanol, contained in the heat adjusting liquid is liable to solidify and then close a passage. In view of the fact, it may be appreciated to provide a secondary header for flowing heat adjusting liquid, and use an inner heat exchange tube whose outflow end projects from an out¬flow end of the outer heat exchange tube and which communi¬cates with the secondary header, and is formed with a mixing hole in a wall thereof at a position which is closer to its outflow end than the closing member and not beyond the outflow end of the outer heat exchange tube. In this con¬struction, heat adjusting liquid is supplied from the sec¬ondary header, and is flowed into the inner heat exchange tube, and is ejected into the annular passage between the inner and outer heat exchange tubes, so that impurity mat¬ters having a high solidifying temperature is prevented from separating and solidifying. In this way, the low tempera¬ture liquid and the heat adjusting liquid are mixed in a good state, and an accurate temperature adjustment is thus attainab1e. It may be appreciated that an inflow end of the inner heat exchange tube is positioned in the inflow header. The low temperature liquid flowing in the inflow header is assuredly introduced into the inner heat exchange tube. Accordingly, this construction will eliminate the likelihood that when the flow of low temperature liquid flowing in the inflow header is small, heavy components of the low tempera¬ture liquid remain and deposit in the inflow header. Accordingly the present invention provides a vaporizer for low temperature liquid such as liquified natural gas, liquified nitrogen comprising an inflow header for flowing low temperature liquid; a plurality of outer heat exchange tubes communicated with the inflow header, each outer heat exchange tube extending in a direction perpendicular to the inflow header, an outside of the outer heat exchange tube coming into contact with a heating medium; an outflow header communicated with the inflow header by way of the plurality of outer heat exchange tubes for flowing vapor of the low temperature which is produced in the outer heat exchange tubes; a plurality of inner heat exchange tubes provided in at least respective inflow portions of the plurality of outer heat exchange tubes, each inner heat exchange tube forming an annular passage between an inside surface of the corrresponding outer heat exchange tube and an outsisde surface of the inner heat exchange tube, the annular passage communicating with the inflow header for flowing the low temperature liquid. BRIEF DESCRIPTION OF THE DRAWINGS Fig. 1 is a front view in cross section showing an essential part of a liquefied natural gas vaporizer as a first embodiment according to this invention; Fig. 2 is a cross sectional view taken along the line B-B in Fig. 1; Fig. 3 is a perspective view showing an overall con¬struction of the vaporizer; Figs. 4A to 4C are diagrams showing a flowing state of liquefied natural gas and natural gas in an annular passage defined in the vaporizer; Fig. 5 is a graph showing a relationship between the temperature of liquefied natural gas and specific enthalpy in the annular passage; Figs. 6 to 15 are graphs showing a change of tempera¬ture and ice thickness over the length of a heat exchange tube under specified conditions; Fig. 16 is a cross sectional view showing an essential part of a liquefied natural gas vaporizer as a second embod¬iment according to this invention; Fig. 17 is a front view in cross section showing an essential part of a heat exchange tube of a liquefied natu¬ral gas vaporizer as a third embodiment according to this invention; Fig. 18 is a front view in cross section showing an overall construction of the heat exchange tube of the third vaporize; Fig. 19 is a perspective view showing an overall con¬struction of the third vaporizer; Fig. 20 is a perspective view showing an arrangement of an upper portion of a heat exchange tube panel of the third vapor i zer ; Fig. 21 is a front view in cross section showing an upper portion of the heat exchange tube; Fig. 22 is a cross sectional view taken along the line C-C in Fig. 17; Fig. 23 is an enlarged cross sectional view showing a portion of a heat exchange tube where a closing member is provided; Figs. 24A and 24B are graphs showing a performance of a heat exchange tube; and Fig. 25 is a graph showing a relationship between a thickness of heat insulator of a conventional liquefied natural gas vaporizer and an equivalent thickness of ice. BEST MODE FOR CARRYING OUT THE INVENTION A first embodiment according to this invention will be described with reference to Figs. 1 to 5. It should be noted that the following embodiments will be described as a vaporizer for vaporizing liquefied natural gas (hereinafter referred to as "LNG"). This invention is applicable for vaporizing various low temperature liquids such as liquefied n i t rogen. As shown in Fig. 3, the vaporizer includes a plurality of lower headers (or inflow headers) 10 extending in paral¬lel with a horizontal direction. A specified number of lower header headers 10 are connected with a manifold 12 which has an LNG supply pipe 14. Further, the vaporizer includes a plurality of upper headers (or outflow headers) 16 arranged above the plurality of lower headers 10 in parallel with them. A specified number of upper headers 16 are connected with a manifold 18 which has an LNG discharge pipe 20 . Between the lower header 10 and the upper header 16 is provided a heat exchange panel including a number of outer heat exchange tubes 22 vertically extending in parallel with one another. As shown in Fig. 1, each outer heat exchange tube 22 connects an interior of the lower header 10 and that of the upper header 16. Further, an inner heat exchange tube 24 is provided in a lower portion of each outer heat exchange tube 22. The inner heat exchange tube 24 has a smaller diameter than the outer heater exchange tube 22. Between the outer heat exchange tube 22 and the inner heat exchange tube 24 is defined an annular passage 23 which communicates with the interior of the lower header 10. More specifically, as shown in Fig. 2. the inner heat exchange tube 24 is formed with a plurality of ribs or projections 25 on an outside surface thereof. The ribs 25 are arranged in a circumferential direction of the inner heat exchange tube 24. In this embodiment, four ribs 25 are formed. A free end of the rib 25 comes into contact with an inside surface of the outer heat exchange tube 22. and welded to an inside surface of the outer heat exchange tube 22 at least one portion, e.g.. a lower portion, to thereby fixedly connect the inner heat exchange tube 24 to the outer heat exchange tube 22. It should be appreciated that in this invention, the manner of fixedly connecting the inner heat exchange tube 24 to the outer heat exchange tube 22 is not 1 imited to the welding. For example, the inner and outer heat exchange tubes 22 and 24 may be fixedly connected with each other by a mechanical manner of placing the inner heat exchange tube 24 in the outer heat exchange tube 22 and drawing them together with each other. Further, the outer heat exchange tube 22 is formed with a number of fins 28 on an outside surface thereof to enhance the heat transfer. Heating medium 30 such as seawater is flowed over the outside of the outer heat exchange tube 22 in a downward direction as shown by the arrow A in Fig. 1. In Fig. 1, indicated at 26 are twisted heat transfer accelerators. These heat transfer accelerators 26 are placed in the inner heat exchange tube 24 and an upper portion of the outer heat exchange tube 22. In this inven¬tion, the heat exchange accelerator 26 may be omitted. Alternatively, other heat exchange accelerators may be pro-V ided. An operation of this vaporizer will be described. LNG supplied in the lower header 10 flows up in the inner heat exchange tube 24 and in the annular passage 23 which is defined between the inner heat exchange tube 24 and the outer heat exchange tube 22. The inflow rate into the inner heat exchange tube 24 and the inflow rate into the annular passage 23 depend on a pressure loss of LNG flowing in the inner heat exchange tube 24 and in the annular pas¬sage 23. Their respective flow rates become stable when the pressure at an inflow portion of the inner heat exchange tube 24 equals to that at an inflow portion of the annular passage 23 and that the pressure at an outflow portion of the inner heat exchange tube 24 equals to that at an outflow portion of the annular passage 23. The larger the space distance δ of the annular passage 23 (see Fig. 2) becomes, the more the inflow rate into the annular passage 23 be¬comes . Generally, the relationship between the temperature T of LNG and specific enthalpy i can be represented by a graph shown in Fig. 5. Specifically, in a first region where the LNG temperature T is smaller than the boiling point Tb of LNG or the temperature at which LNG transits to the gaseous phrase, i.e., natural gas (hereinafter, referred to as "NG"), there exists only LNG in the form of liquid. In a second region where the LNG temperature T is greater than the boiling point Tb but smaller than the dew point Td. there exist LNG in the form of liquid and NG in the form of gas in a mixed state. In a third region where the LNG temperature T is greater than the dew point Td, there exists only NG. A lower portion of the annular passage 23, i.e., an inflow portion into which LNG is flowed, corresponds to the first region. In this region, only LNG can be theoretically supposed to exit. Actually, as shown in Fig. 4A, near an inside surface of the outer heat exchange tube 22, LNG is partially boiled, so that bubbles occur because the LNG is subjected to heat transmitted through the outer heat ex¬change tube 22. This phenomenon is called "sub-cool boil¬ing" . An upper portion of the annular passage 23 corresponds to the second region where the LNG is substantially boiled in the vicinity of the inside wall of the outer heat ex¬change tube 22 as shown in Fig. 4B, consequently producing a mixed state of gas and liquid. In either case, the mixture of gas and liquid and upward streams of LNG and NG promote forced convection boiling, thereby increasing the heat transfer rate. Accord¬ingly, compared to the conventional vaporizer having the heat insulator containing entirely gaseous natural gas, the annular passage 23 provides a higher heat transfer rate. Accordingly, the space distance δ of the annular passage 23 (see Fig. 2) can be made larger. This will not need the severe management of the heat insulator thickness which has been required in the conventional vaporizer provided with the heat insulator having the heat insulating member at¬tached on the inside surface of the heat exchange tube or the insulating space containing natural gas. In this con¬struction, even if the space distance δ of the annular passage 23 is erroneously beyond a specified value, LNG can be safely heated and vaporized without forming ice because the high heat transfer rate is assured. If the inner heat exchange tube 24 is too short com¬pared to the outer heat exchange tube 22, LNG is liable to flow up to an upper end portion of the inner heat exchange tube 24, which results in ice formation at an intermediate portion of the outer heat exchange tube 24. On the con¬trary, if the inner heat exchange tube 24 is too long com¬pared to the outer heat exchange tube 22, NG is liable not to be sufficiently heated in an upper portion of the outer heat exchange tube 22 where the inner heat exchange tube 24 does not extend. For this reason, the length of the inner heat exchange tube 24 and the flow area of the annular passage 23 are set at such a value that the vaporization of LNG can be completed at the vicinity of the upper end of the inner heat exchange tube 24. Accordingly, the prevention of ice formation and sufficient heating of NG can be realized in a simplified construction. Expe r i men t Data Figs. 6 to 15 are graphs showing a change of tempera¬ture of each fluid and thickness of ice formed on the out¬side surface of the outer heat exchange tube over the length of heat exchange tube. These graphs are formed based on data of simulation experiments. Specifically, in Figs. 6 to 10, the ratio of the length of the inner heat exchange tube 24 to the outer heat exchange tube 22 is 50 : 100, whereas in Figs. 11 to 15. the ratio is 40 : 100. It should be noted that LNG at 222kg/h in Fig. 6 means that the total flow rate of LNG is 222 kilograms per hour, while LNG at 184.5 37.5 in the same graph means that the flow rate of LNG flowing into the inner heat exchange tube 22 is 184.5 kilograms per hour and the flow rate of LNG flowing into the annular passage 23 is 37.5 kilogram per hour, and these representations are identical for Figs. 7 to 15. The entire length of the outer heat exchange tube 22 is 10 meters. It can be clearly seen in Fig. 6 (wherein a space dis¬tance δ of 1.0 mm) and Fig. 7 (wherein a space distance δ of 1.5 mm) that ice formation is remarkable in an intermedi¬ate portion of the outer heat exchange tube 22, with the result that the outside surface temperature of the outer heat exchange tube 22 at this intermediate portion is local¬ly and remarkably lowered. This can be explained as fol¬lows: The small space distance δ of the annular passage 23 reduced the flow rate of LNG flowing in the annular passage 23; The LNG flowing in the annular passage 23 was vaporized in a lower portion or in an intermediate portion of the annular passage 23; Accordingly, the enhanced heat transfer was not attained that is provided by the forced convection boiling which occurs under the co-existing state of gaseous phase and liquid phase; and The LNG flowing in the inner heat exchange tube 24 was flowed out from the inner heat exchange tube 24 without being vaporized. On the other hand, as can be clearly seen in Fig. 8 (wherein a space distance δ of 2.0 mm). Fig. 9 (wherein a space distance δ of 2.5 mm), and Fig. 10 (wherein a space distance δ of 3.0 mm) that ice formation negligibly oc¬curred in an intermediate portion of the outer heat exchange tube 22, and no local reduction in outside surface tempera¬ture of the outer heat exchange tube 22 occurred. This can be explained as follows: A large space distance δ enabled the co-existing of liquid phase and gaseous phase in the annular passage 23 which causes the forced convection boil¬ing; The forced convection boiling increased the heat trans¬fer rate of the annular passage 23; Even in the case that some of the LNG flowing in the inner heat exchange tube 24 was not completely vaporized and flowed out from the inner heat exchange tube 24 still in the state of liquid, NG produced in the annular passage 23 enclosed such escaped LNG. As can be seen from Figs. 8 to 10, according to this invention, the sufficient heat transfer rate can be ensured even if the space distance S is varied in a relatively wide range, which can consequently prevent ice formation. In the case of Figs. 11 to 15 where the ratio of the length of the inner heat exchange tube to the outer heat exchange tube is 40 : 100, similarly, the temperature of the LNG flowing in the inner heat exchange tube 24 cannot be sufficiently raised in the region where the space distance δ is small because the flow rate of the LNG flowing in the inner heat exchange tube 24 is great and the heat transfer area is sma11. Further, ice formation is remarkably increased around a temperature region where enthalpy abruptly changes, i.e., a region where latent heat for vaporization is absorbed in the case of single component such as water, as shown in Fig. 5. However, the ice formation can be suppressed by widening the space distance δ to increase the flow rate of LNG flowing in the annular passage 23. in other words, reducing the flow rate of LNG flowing in the inner heat exchange tube 24. Although data for the case that the space distance δ is further increased were not obtained, it can be predicted that the heat transfer rate lowers in the case of the space distance δ being too large. This is because of the fact that a too-large space distance causes a whole separation of an LNG layer and an NG layer in the annular passage 23 as shown in Fig. 4C, consequently making it difficult to pro¬duce the forced convection boiling in the co-existing state of liquid phrase and gas phrase which increases the heat transfer rate. Accordingly, in this invention, the length of the inner heat exchange tube 24 and the space distance δ of the annular passage 23 are respectively set at such values that the enthalpy of. LNG flowing in the inner heat exchange tube 24 gently changes around the vicinity of the upper end of the inner heat exchange tube 24, e.g., the enthalpy change at point P in Fig. 5, under an operation pressure of LNG. In this way, ice formation can be reliably prevented with keeping the length of the inner heat exchange tube 23 as shor t as possible. Next, a second embodiment of the invention will be described with reference to Fig. 16. Be- -J In this embodiment, a plurality of fins 29 in the form of wave-like projections are formed on an inside surface of an outer heat exchange tube 22, instead of the ribs 25 in the first embodiment. A free end of the fin 29 comes into contact with an outside surface of an inner heat exchange tube 24. In this state, the inner heat exchange tube 24 is placed in the outer heat exchange tube 22. The fin 29 which serves as a projection for positioning the inner heat exchange tube 24 in the outer heat exchange tube 22 are formed on the inside surface of the outer heat exchange tube 22. Consequently, the surface area of the outer heat exchange tube 22 defining an annular passage 23 is remarkably increased, which thus raises the heat transfer rate from the outer heat exchange tube 22 to the annular passage 23. Accordingly, the space distance of the annular passage 23 can be made further wider, and the precision tolerance for the space distance can be increased. Next, a third embodiment of the invention will be described with reference to Figs. 17 to 25. In this embodiment, as shown in Fig. 17, an inner heat exchange tube 24 is longer than an outer heat exchange tube 22 in such a manner that a lower end portion 24a of the inner heat exchange tube 24 protrudes in a lower header 10. The inner heat exchange tube 24 is formed with inflow through holes 32 slightly above the lower end portion 24a. The inflow holes 32 are cut through a wall of the inner heat exchange tube 24. The inflow hole 32 is adapted for eject¬ing LNG from the inside of the inner heat exchange tube 24 into an annular passage 23. As shown in Fig. 21, an upper end portion of the inner heat exchange tube 24 goes through an upper header 16 by a specified distance. In this state, the upper end portion of the inner heat exchange tube 24 is fixedly connected to the upper header 16 in a suspended state. An outflow end of the inner heat exchange tube 24 communicates with an LPG supply header 42 in which a heat adjusting fluid is flowed. In this embodiment, liquefied petroleum gas (hereinafter, referred to as "LPG") is flowed. As shown in Figs. 17 and 23, the inner heat exchange tube 24 is formed with a plurality of ejection holes 34 in an intermediate portion thereof (an area E in Fig. 18). The ejection holes 34 are preferably vertically arranged apart by equidistance and cut through the wall of the inner heat exchange tube 24. The inner heat exchange tube 24 is fur¬ther provided with a closing member 36 right above an upper¬most ejection hole to close the inner heat exchange tube 24. The inner heat exchange tube 24 is further formed with a plurality of mixing holes 40 starting from right above the closing member 36. The mixing hole 40 is similarly cut through the wall of the inner heat exchange tube 24. As shown in Fig. 19, seawater as heating medium is supplied through a pipe 11 into a seawater manifold 13 from where seawater is branched out and distributed to respective seawater headers 15. The seawater supplied into the sea¬water header 15 is, as shown in Fig. 20, flowed into a trough 19 having a generally U-shape in cross section by way of a pipe 17. The seawater flowed into the trough 19 over¬flows from the trough 19 and falls down along an outside surface of the plurality of outer heat exchange tubes 22. It should be noted that the seawater supply system having the above structure is applicable to the first and second embodiments. An operation of the vaporizer as the third embodiment will be described next. LNG supplied into the lower header 10 is flowed into the inner heat exchange tube 24 through the lower end portion 24a. Since the lower end portion 24a protrudes inside the lower header 10, heavy components contained in the LNG is left in the lower header 10 as residue in a condensed state, even in the case where the load of LNG inside the lower header 10 is small, i.e., the flow rate of LNG i s sma 1 1 . The LNG flowed in the inner heat exchange tube 24 is blocked by the closing member 36 and ejected into the annu¬lar passage 23 through the ejection holes 34 or through the inflow holes 32 both of which are located below the closing member 36. The LNG ejected in the annular passage 23 is effectively mixed with NG already produced in the annular passage 23. At this time, even if the LNG undergoes boiling in the inner heat exchange tube 24 and a film of NG exists on an inside surface of the inner heat exchange tube 24, the ejection will take the NG into the annular passage 23. On the other hand, LPG flowed into the LPG supply header 42 flows down inside an upper portion of the inner heat exchange tube 24, and is blocked by the closing member 36 and ejected into the annular passage 23 through the mixing holes 40 which are located above the closing member 36. Consequently, the ejected LPG is effectively mixed with the NG in the annular passage 23. Fig. 24A is a diagram showing a temperature distribu¬tion over the height of the outer heat exchange tube 22. In the diagram: the line Ll indicates an average temperature distribution of a mixture of LNG rising up in the outer heat exchange tube 22 and LPG; the line L2 indicates a tempera¬ture distribution of seawater; the line L3 indicates a temperature distribution of the inner heat exchange tube 24; and the line L4 indicates a temperature distribution of the outer heat exchange tube 22. As can be seen from the diagram, LNG having a specified pressure (e.g., 30 kgG/cm ) and a temperature of about -160C is flowed from the lower header 10. As represented by the portion Lla, the LNG flowed from the lower header 10 is heated by the heat exchange with the seawater as heating medium as rising in the inner heat exchange tube 24 and the annular passage 23, and the temperature of the LNG conse¬quently increases. Subsequently, as represented by the portion Lib. the LNG starts the vaporization at about -90'C, and completes the vaporization at the point Lle. As represented by the portion Lld. the temperature of the LNG further increases. When the temperature of the LNG becomes about -100oC, LNG is ejected from the inner heat ex¬change tube 24 into the annular passage 23 through the ejection holes 34. and is mixed with a mixture of LNG and NG in the annular passage 23. and is further mixed with LPG ejected from the mixing holes 40. At this mixing point, the temperature of the LNG is about -20C. Accordingly, even if impurity matters having a high solidifying temperature, e.g.. methanol (whose solidifying temperature is about -125C), are contained in the LNG. such impurity matters (methanol) will not solidify, which accordingly eliminates the problem that a passage is closed due to a solidifica¬tion. It should be noted that the temperature of seawater ' supplied from the trough 19 is about 10 "C and that the temperature of NG in the upper header 16 is about 8"C. The line L5 in Fig. 24B shows a change in the thickness ' JM 20IQ DUPLICATE of ice formed on an outside surface of the outer heat ex-clange tube 22. The thickness of ice is no greater than about 2 mm even at a lowermost portion of the outer heat exchange tube 22. This thickness is negligible and will not adversely affect the heat transfer rate. The entire length H of the outer heat exchange tube 22 shown in Fig. 18 may be set as long as 10 meters. In Fig. 18, the upper header 16 and the lower header 10 are support¬ed by a support frame and the outer heat exchange tube 22 and the inner heat exchange tube 24 are supported in the suspended state. The lower end portion 24a of the inner heat exchange tube 24 is a free end. Also, a lower end portion of the outer heat exchange tube 22 is not held in a fixed state. Accordingly, there is not the likelihood that the heat exchange tubes 22 and 24 receive a great thermal stress due to thermal contraction. The present invention is not limited to the above-mentioned vaporizer, and other constructions can be used as follows: (D According to this invention, the flowing direction of LNG in an outer heat exchange tube is not limited. For example, the upper header may serve as an inflow port, and the lower header may serve as an outflow port, which is opposite to the arrangement of the foregoing embodiments. In this case, lower temperature liquid such as LNG is forci- DUPLICATE bly flowed down from the upper header through the heat exchange tubes to the lower header utilizing the weight. Alternatively, the inflow header and the outflow header may be arranged in the same horizontal plane and connected with each other by way of outer heat exchange tubes lying in the same horizontal plane. 2 In the first and second embodiments, the length difference between the outer heat exchange tube 22 and the inner heat exchange tube 24, i.e., a distance of the region where the inner heat exchange tube 24 is provided in the outer heat exchange tube 22, may be set in accordance with a target temperature of NG in the outflow header. Specifical¬ly, the higher the target temperature is, the longer the length difference is made to enable NG to be heated for a 1onger period. ® According to this invention, it may be appreciated to provide a sparger tube having a plurality of through holes in an inflow header. LNG is flowed into the sparger tube. This construction is advantageous in supplying LNG in the inflow header uniformly. EXPLOITATION IN INDUSTRY As mentioned above, in this invention, an outer heat exchange tube for communicating with an inflow header and an outflow header is internally arranged with an inner heat exchange tube in an inflow portion thereof. An annular passage for flowing a low temperature liquid is defined between the inner heat exchange tube and the outer heat exchange tube. The annular passage communicates with the inflow header. Accordingly, an axial flow of fluid in the annular passage and a co-existing of liquid phase and gase¬ous phase enhance forced convection boiling in the annular passage, thereby increasing the heat transfer rate. Hence, compared to the conventional vaporizer provided with the heat insulator containing gas, the space distance of the annular passage can be made wider. Accordingly, it is possible to increase the dimensional tolerance of the space distance which assures sufficient heat transfer and prevent ice formation on an outside surface of a heat exchange tube. Thus, the production of a low temperature liquid vaporizer can be simplified and the production cost can be reduced. » 7 JAN zm CJJJpLiCATE WE CLAIM: 1. A vaporizer for low temperature liquid such as liquified natural gas, liquified nitrogen comprising an inflow header for flowing low temperature liquid; a plurality of outer heat exchange tubes communicated with the inflow header, each outer heat exchange tube extending in a direction perpendicular to the inflow header , an outside of the outer heat exchange tube coming into contact with a heating medium; an outflow header communicated with the inflow header by way of the plurality of outer heat exchange tubes for flowing vapor of the low temperature which is produced in the outer heat exchange tubes; a plurality of inner heat exchange tubes provided in at least respective inflow portions of the plurality of outer heat exchange tubes, each inner heat exchange tube forming an annular passage between an inside surface of the corrresponding outer heat exchange tube and an outsisde surface of the inner heat exchange tube, the annular passage communicating with the inflow header for flowing the low temperature liquid. 2. The vaporizer as claimed in claim 1 wherein one of the inside surface of the outer heat exchange tube and the outside surface of the inner heat exchange tube is formed with a plurality of projections, a free end of each projection coming into contact with the outer surface. 3. The vaporizer as claimed in claim 2 wherein the inside surface of the outer heat exchange tube is formed with the projections. 4. The vaporizer as claimed in anyone of the claims 1 to 3 wherein the inner heat exchange tube is shorter than the outer heat exchange tube and the inner heat exchange tube is disposed in an inflow portion of the outer heat exchange tube. 5. The vaporizer as claimed in any one of the claims 1 to 3 wherein the inner heat exchange tube is formed with an ejection hole in a wall thereof and provided with a closing member in the inner heat exchange tube at a position closer to the outflow header than the ejection hole. 6. The vaporizer as claimed in claim 5 wherein the outer and inner heat exchange tubes are arranged in a vertical direction while the outflow header is above the inflow header, and an upper end of the inner heat exchange tube is fixedly attached to the outflow header. 7. The vaporizer as claimed in claim 5 or 6 wherein an outflow end of the inner heat exchange tube projects from an outflow end of the outer heat exchange tube projects from an outflow end of the outer heat exchange tube and communicates with a secondary header, for flowing heat adjusting liquid and the inner heat exchange tube is formed with a mixing hole in a wall thereof at a position which is closer to its outflow end than the closing member and not beyond the outflow end of the outer heat exchange tube. 8. The vaporizer as claimed in any one of the claims 1 to 6 wherein an inflow end of the inner heat exchange tube is positioned in the inflow header. 9. A vaporizer for low temperature liquid substantially as herein described with reference to the accompanying drawings.

Full Text

The present invention relates to a vaporizer for low liquid such as liquified natural gas,liquified nitrogen.
BACKGROUND ART
This invention relates to an open rack type vaporizer for vaporizing low temperature liquid such as liquefied natural gas. liquefied nitrogen.
Conventionally, vaporizers of open rack type have been known as a vaporizer for vaporizing such low temperature liquid. The open rack type vaporizer generally includes a lower header in which low temperature liquid is flowed, an upper header arranged in parallel with the lower header, and a great number of vertical heat exchange tubes connecting the lower and upper headers. Outside of the heat exchange tubes is flowed heating medium such as seawater so that the liquid flowing in the lower header is heated and vaporized by heat of the heating medium in each heat exchange tube. Thus obtained natural gas is recovered by the way of the upper header.
In this arrangement, the lower header and a lower portion of the heat exchange tube come into direct contact with the lower temperature liquid. Consequently, the tem¬perature of these portions is very low. The formation of ice is liable to occur on an outside surface of these por¬tions. The ice becomes a heat insulation barrier, and

hinders the heat exchange at the lower portion of the heat exchange tube, thereby lowering the temperature of these portions further and resulting in an exceedingly low temper¬ature condition. In the state that the lower portion of the heat exchange tube is cooled to an exceedingly low tempera¬ture, the contraction rate of the heat exchange tube becomes large. Accordingly, there is a likelihood that a slight deviation in the flow of heating medium or seawater causes a great contraction difference among heat exchange tubes, and finally results in local deformations in heat exchange tubes.
Japanese Unexamined Patent Publication No. 4-217788 discloses provision of a heat insulator on a portion of a heat exchange tube except an outflow portion near an upper header to suppress heat transfer between the heat exchange tube and natural gas. The heat insulator is formed by providing a cylindrical insulating member on an inner sur¬face of a heat exchange tube, or by forming an insulating space in a heat exchange tube in which vaporized natural gas is contained. The heat insulator prevents an outside sur¬face of the heat exchange tube from being cooled to an terribly low temperature, thereby preventing an exceeding contraction. It is certain that the heat insulator hinders the heat exchange between low temperature liquid and heating medium. However, the vaporization efficiency of the heat

exchange tube provided with the heat insulator may be sub¬stantially identical to that of the heat exchange tube which is not provided with the heat insulator but is likely to be formed with heat insulating ice on an outside thereof.
However, the heat insulator provides a very low heat transfer rate. Accordingly, the thickness of the heat insulator is very significant. A slight difference in the thickness of the heat insulator causes a great difference in the heat transfer of the heat exchange tube. Specifically, if having a thickness slightly greater than a specified thickness, the heat insulator exceedingly insulates heat, resulting in an insufficient heat transfer between an out¬side and an inside of the heat exchange tube, consequently making it impossible to heat and vaporize low temperature liquid. Conversely, if having a thickness slightly smaller than a specified thickness, the heat insulator cannot insu¬late heat sufficiently, consequently resulting in ice forma¬tion as the conventional heat exchange tubes. For example. Fig. 25 shows a relationship between the thickness of the heat insulator containing natural gas in an insulating space formed in a heat exchange tube, that is. a radial space of the insulating space, and the thickness of ice providing equivalent insulation rate to the insulating space. It will be seen in Fig. 25 that a variation of 0.1 mm in the thick¬ness of the insulating space is equal to a change of 10 mm

in the thickness of ice in the aspect of heat insulation. The heat insulator formed by providing the insulating member made of vinyl chloride or the like has a similar heat insulation relationship. In other words, a small variation in the thickness of the insulating member will cause a great change in the heat insulation.
For this reason, in this vaporizer, it is necessary to manage the thickness of the heat insulator at the high preciseness. In production lines, however, it is very difficult to accomplish such high preciseness. Even if such high preciseness can be accomplished, its production costs will become very high. Further, even if the heat insulator can be made in the production line, its thickness will be liable to greatly change due to thermal deformation or other causes in the use.
Alternatively, it is possible to expand the dimensional variation tolerance by using a heat insulator made of mate¬rial having relatively high heat transfer rate, e.g., metal, which has an increased thickness. In this case, however, there is a problem that a metal insulator is liable to corrode and change the quality of vaporized product gas.
In view of these problems, the present invention has an object of providing a low temperature liquid vaporizer which can reliably prevent ice formation and ensure sufficient heat transfer even if a variation exists in the insulating

thickness.
DISCLOSURE OF THE INVENTION
Accordingly, the present invention is directed to a low temperature liquid vaporizer comprising: an inflow header for flowing low temperature liquid; a plurality of outer heat exchange tubes communicated with the inflow header, each outer heat exchange tube extending in a direction perpendicular to the inflow header, an outside of the outer heat exchange tube coming into contact with a heating medi¬um; an outflow header communicated with the inflow header by way of the plurality of outer heat exchange tubes for flow¬ing vapor of the low temperature liquid which is produced in the outer heat exchange tubes; a plurality of inner heat exchange tubes provided in at least respective inflow por¬tions of the plurality of outer heat exchange tubes, each inner heat exchange tube forming an annular passage between an inside surface of the corresponding outer heat exchange tube and an outside surface of the inner heat exchange tube, the annular passage communicating with the inflow header for flowing the low temperature liquid.
With this vaporizer, low temperature liquid is flowed from the inflow header into not only the inside of the inner heat exchange tube but also the annular passage between the outer and inner heat exchange tubes. In the annular pas-

sage, the low temperature liquid in the vicinity of an inside surface of the outer heat exchange tube is vaporized by heat of heating medium which is flowed over an outside surface of the outer heat exchange tube. On the other hand, the low temperature liquid in the vicinity of an outside surface of the inner heat exchange tube flows up in the state of liquid phrase. The inner heat exchange tube has a lower temperature than the outer heat exchange tube. In this state, there is a great forced convection boiling in the annular passage owing to the fact of the flow of fluid in an axial direction of the outer and inner heat exchange tubes and the co-existing of the liquid and gaseous phrases. Accordingly, the heat transfer rate of the annular passage increases. In other words, comparing to the conventional heat insulator containing gaseous material, the annular passage has an increased heat transfer rate. Accordingly, the space distance of the annular passage can be increased. This will make it possible not only to prevent ice formation on the outside surface of the outer heat exchange tube but also to ensure sufficient heat transfer even if the space distance is slightly larger or smaller than a specified va1ue.
It may be appreciated that either of the inside surface of the outer heat exchange tube and the outside surface of the inner heat exchange tube is formed with a plurality of

projections, a free end of each projection coming into contact with the other surface. This construction will stably hold the outer and inner heat exchange tubes in a coaxial arrangement, and prevent deformation of the outer and inner heat exchange tubes, and assure efficient vapori¬zation, and further prevent the inner heat exchange tube from vibrating.
In the case that the inside surface of the outer heat exchange tube is formed with the projections, the projec¬tions increase the inside surface area of the outer heat exchange tube, and thus enhance the heat transfer rate of the outer heat exchange tube with respect to the low temper¬ature liquid in the annular passage.
The inner heat exchange tube may be made to be shorter than the outer heat exchange tube, and the inner heat ex¬change tube may be disposed in an inflow end portion of the outer heat exchange tube. In this construction, vaporized fluid can be sufficiently heated in the region of the outer heat exchange tube where the inner heat exchange tube is not provided, i.e., an outflow portion of the outer heat ex¬change tube .
The inner heat exchange tube may be formed with an ejection hole in a wall thereof and provided with a closing member in the inner heat exchange tube at a position closer to the outflow header than the ejection hole. In this
^^^-^^f^ 2l}n,

construction, the low temperature liquid flowed into the inner heat exchange tube is ejected in the annular passage through the ejection hole, and is mixed with vaporized fluid in the annular passage. Accordingly, the vaporization of low temperature liquid is enhanced. Also, a film of gas on an inside surface of the inner heat exchange tube is removed and the heat transfer to the low temperature liquid in the inner heat exchange tube is increased.
It may be appreciated that the outer and inner heat exchange tubes are arranged in a vertical direction while the outflow header is above the inflow header, and an out¬flow end of the inner heat exchange tube is fixedly attached to the outflow header. The outer and inner heat exchange tubes are supported in a suspended state by the outflow header. Accordingly, respective lower end portions of the outer and inner heat exchange tubes are not fixedly held. This will prevent the outer and inner heat exchange tubes from receiving a great thermal stress due to thermal con¬traction.
Further, in the above-mentioned vaporizers, if low temperature liquid is directly mixed with a heat adjusting liquid, impurity matters having a high solidifying tempera¬ture, e.g., methanol, contained in the heat adjusting liquid is liable to solidify and then close a passage. In view of the fact, it may be appreciated to provide a secondary

header for flowing heat adjusting liquid, and use an inner heat exchange tube whose outflow end projects from an out¬flow end of the outer heat exchange tube and which communi¬cates with the secondary header, and is formed with a mixing hole in a wall thereof at a position which is closer to its outflow end than the closing member and not beyond the outflow end of the outer heat exchange tube. In this con¬struction, heat adjusting liquid is supplied from the sec¬ondary header, and is flowed into the inner heat exchange tube, and is ejected into the annular passage between the inner and outer heat exchange tubes, so that impurity mat¬ters having a high solidifying temperature is prevented from separating and solidifying. In this way, the low tempera¬ture liquid and the heat adjusting liquid are mixed in a good state, and an accurate temperature adjustment is thus attainab1e.
It may be appreciated that an inflow end of the inner heat exchange tube is positioned in the inflow header. The low temperature liquid flowing in the inflow header is assuredly introduced into the inner heat exchange tube. Accordingly, this construction will eliminate the likelihood that when the flow of low temperature liquid flowing in the inflow header is small, heavy components of the low tempera¬ture liquid remain and deposit in the inflow header.

Accordingly the present invention provides a vaporizer for low temperature liquid such as liquified natural gas, liquified nitrogen comprising an inflow header for flowing low temperature liquid; a plurality of outer heat exchange tubes communicated with the inflow header, each outer heat exchange tube extending in a direction perpendicular to the inflow header, an outside of the outer heat exchange tube coming into contact with a heating medium; an outflow header communicated with the inflow header by way of the plurality of outer heat exchange tubes for flowing vapor of the low temperature which is produced in the outer heat exchange tubes; a plurality of inner heat exchange tubes provided in at least respective inflow portions of the plurality of outer heat exchange tubes, each inner heat exchange tube forming an annular passage between an inside surface of the corrresponding outer heat exchange tube and an outsisde surface of the inner heat exchange tube, the annular passage communicating with the inflow header for flowing the low temperature liquid.

BRIEF DESCRIPTION OF THE DRAWINGS
Fig. 1 is a front view in cross section showing an essential part of a liquefied natural gas vaporizer as a first embodiment according to this invention;
Fig. 2 is a cross sectional view taken along the line B-B in Fig. 1;
Fig. 3 is a perspective view showing an overall con¬struction of the vaporizer;
Figs. 4A to 4C are diagrams showing a flowing state of liquefied natural gas and natural gas in an annular passage defined in the vaporizer;
Fig. 5 is a graph showing a relationship between the temperature of liquefied natural gas and specific enthalpy in the annular passage;
Figs. 6 to 15 are graphs showing a change of tempera¬ture and ice thickness over the length of a heat exchange tube under specified conditions;
Fig. 16 is a cross sectional view showing an essential part of a liquefied natural gas vaporizer as a second embod¬iment according to this invention;
Fig. 17 is a front view in cross section showing an essential part of a heat exchange tube of a liquefied natu¬ral gas vaporizer as a third embodiment according to this invention;
Fig. 18 is a front view in cross section showing an

overall construction of the heat exchange tube of the third vaporize;
Fig. 19 is a perspective view showing an overall con¬struction of the third vaporizer;
Fig. 20 is a perspective view showing an arrangement of an upper portion of a heat exchange tube panel of the third vapor i zer ;
Fig. 21 is a front view in cross section showing an upper portion of the heat exchange tube;
Fig. 22 is a cross sectional view taken along the line C-C in Fig. 17;
Fig. 23 is an enlarged cross sectional view showing a portion of a heat exchange tube where a closing member is provided;
Figs. 24A and 24B are graphs showing a performance of a heat exchange tube; and
Fig. 25 is a graph showing a relationship between a thickness of heat insulator of a conventional liquefied natural gas vaporizer and an equivalent thickness of ice.
BEST MODE FOR CARRYING OUT THE INVENTION
A first embodiment according to this invention will be described with reference to Figs. 1 to 5. It should be noted that the following embodiments will be described as a vaporizer for vaporizing liquefied natural gas (hereinafter

referred to as "LNG"). This invention is applicable for vaporizing various low temperature liquids such as liquefied n i t rogen.
As shown in Fig. 3, the vaporizer includes a plurality of lower headers (or inflow headers) 10 extending in paral¬lel with a horizontal direction. A specified number of lower header headers 10 are connected with a manifold 12 which has an LNG supply pipe 14. Further, the vaporizer includes a plurality of upper headers (or outflow headers) 16 arranged above the plurality of lower headers 10 in parallel with them. A specified number of upper headers 16 are connected with a manifold 18 which has an LNG discharge pipe 20 .
Between the lower header 10 and the upper header 16 is provided a heat exchange panel including a number of outer heat exchange tubes 22 vertically extending in parallel with one another. As shown in Fig. 1, each outer heat exchange tube 22 connects an interior of the lower header 10 and that of the upper header 16. Further, an inner heat exchange tube 24 is provided in a lower portion of each outer heat exchange tube 22. The inner heat exchange tube 24 has a smaller diameter than the outer heater exchange tube 22. Between the outer heat exchange tube 22 and the inner heat exchange tube 24 is defined an annular passage 23 which communicates with the interior of the lower header 10. More

specifically, as shown in Fig. 2. the inner heat exchange tube 24 is formed with a plurality of ribs or projections 25 on an outside surface thereof. The ribs 25 are arranged in a circumferential direction of the inner heat exchange tube 24. In this embodiment, four ribs 25 are formed. A free end of the rib 25 comes into contact with an inside surface of the outer heat exchange tube 22. and welded to an inside surface of the outer heat exchange tube 22 at least one portion, e.g.. a lower portion, to thereby fixedly connect the inner heat exchange tube 24 to the outer heat exchange tube 22.
It should be appreciated that in this invention, the manner of fixedly connecting the inner heat exchange tube 24 to the outer heat exchange tube 22 is not 1 imited to the welding. For example, the inner and outer heat exchange tubes 22 and 24 may be fixedly connected with each other by a mechanical manner of placing the inner heat exchange tube 24 in the outer heat exchange tube 22 and drawing them together with each other.
Further, the outer heat exchange tube 22 is formed with a number of fins 28 on an outside surface thereof to enhance the heat transfer. Heating medium 30 such as seawater is flowed over the outside of the outer heat exchange tube 22 in a downward direction as shown by the arrow A in Fig. 1.
In Fig. 1, indicated at 26 are twisted heat transfer

accelerators. These heat transfer accelerators 26 are placed in the inner heat exchange tube 24 and an upper portion of the outer heat exchange tube 22. In this inven¬tion, the heat exchange accelerator 26 may be omitted. Alternatively, other heat exchange accelerators may be pro-V ided.
An operation of this vaporizer will be described.
LNG supplied in the lower header 10 flows up in the inner heat exchange tube 24 and in the annular passage 23 which is defined between the inner heat exchange tube 24 and the outer heat exchange tube 22. The inflow rate into the inner heat exchange tube 24 and the inflow rate into the annular passage 23 depend on a pressure loss of LNG flowing in the inner heat exchange tube 24 and in the annular pas¬sage 23. Their respective flow rates become stable when the pressure at an inflow portion of the inner heat exchange tube 24 equals to that at an inflow portion of the annular passage 23 and that the pressure at an outflow portion of the inner heat exchange tube 24 equals to that at an outflow portion of the annular passage 23. The larger the space distance δ of the annular passage 23 (see Fig. 2) becomes, the more the inflow rate into the annular passage 23 be¬comes .
Generally, the relationship between the temperature T of LNG and specific enthalpy i can be represented by a graph

shown in Fig. 5. Specifically, in a first region where the LNG temperature T is smaller than the boiling point Tb of LNG or the temperature at which LNG transits to the gaseous phrase, i.e., natural gas (hereinafter, referred to as "NG"), there exists only LNG in the form of liquid. In a second region where the LNG temperature T is greater than the boiling point Tb but smaller than the dew point Td. there exist LNG in the form of liquid and NG in the form of gas in a mixed state. In a third region where the LNG temperature T is greater than the dew point Td, there exists only NG.
A lower portion of the annular passage 23, i.e., an inflow portion into which LNG is flowed, corresponds to the first region. In this region, only LNG can be theoretically supposed to exit. Actually, as shown in Fig. 4A, near an inside surface of the outer heat exchange tube 22, LNG is partially boiled, so that bubbles occur because the LNG is subjected to heat transmitted through the outer heat ex¬change tube 22. This phenomenon is called "sub-cool boil¬ing" .
An upper portion of the annular passage 23 corresponds to the second region where the LNG is substantially boiled in the vicinity of the inside wall of the outer heat ex¬change tube 22 as shown in Fig. 4B, consequently producing a mixed state of gas and liquid.

In either case, the mixture of gas and liquid and upward streams of LNG and NG promote forced convection boiling, thereby increasing the heat transfer rate. Accord¬ingly, compared to the conventional vaporizer having the heat insulator containing entirely gaseous natural gas, the annular passage 23 provides a higher heat transfer rate. Accordingly, the space distance δ of the annular passage 23 (see Fig. 2) can be made larger. This will not need the severe management of the heat insulator thickness which has been required in the conventional vaporizer provided with the heat insulator having the heat insulating member at¬tached on the inside surface of the heat exchange tube or the insulating space containing natural gas. In this con¬struction, even if the space distance δ of the annular passage 23 is erroneously beyond a specified value, LNG can be safely heated and vaporized without forming ice because the high heat transfer rate is assured.
If the inner heat exchange tube 24 is too short com¬pared to the outer heat exchange tube 22, LNG is liable to flow up to an upper end portion of the inner heat exchange tube 24, which results in ice formation at an intermediate portion of the outer heat exchange tube 24. On the con¬trary, if the inner heat exchange tube 24 is too long com¬pared to the outer heat exchange tube 22, NG is liable not to be sufficiently heated in an upper portion of the outer

heat exchange tube 22 where the inner heat exchange tube 24 does not extend. For this reason, the length of the inner heat exchange tube 24 and the flow area of the annular passage 23 are set at such a value that the vaporization of LNG can be completed at the vicinity of the upper end of the inner heat exchange tube 24. Accordingly, the prevention of ice formation and sufficient heating of NG can be realized in a simplified construction. Expe r i men t Data
Figs. 6 to 15 are graphs showing a change of tempera¬ture of each fluid and thickness of ice formed on the out¬side surface of the outer heat exchange tube over the length of heat exchange tube. These graphs are formed based on data of simulation experiments. Specifically, in Figs. 6 to 10, the ratio of the length of the inner heat exchange tube 24 to the outer heat exchange tube 22 is 50 : 100, whereas in Figs. 11 to 15. the ratio is 40 : 100. It should be noted that LNG at 222kg/h in Fig. 6 means that the total flow rate of LNG is 222 kilograms per hour, while LNG at 184.5 37.5 in the same graph means that the flow rate of LNG flowing into the inner heat exchange tube 22 is 184.5 kilograms per hour and the flow rate of LNG flowing into the annular passage 23 is 37.5 kilogram per hour, and these representations are identical for Figs. 7 to 15. The entire length of the outer heat exchange tube 22 is 10 meters.

It can be clearly seen in Fig. 6 (wherein a space dis¬tance δ of 1.0 mm) and Fig. 7 (wherein a space distance δ of 1.5 mm) that ice formation is remarkable in an intermedi¬ate portion of the outer heat exchange tube 22, with the result that the outside surface temperature of the outer heat exchange tube 22 at this intermediate portion is local¬ly and remarkably lowered. This can be explained as fol¬lows: The small space distance δ of the annular passage 23 reduced the flow rate of LNG flowing in the annular passage 23; The LNG flowing in the annular passage 23 was vaporized in a lower portion or in an intermediate portion of the annular passage 23; Accordingly, the enhanced heat transfer was not attained that is provided by the forced convection boiling which occurs under the co-existing state of gaseous phase and liquid phase; and The LNG flowing in the inner heat exchange tube 24 was flowed out from the inner heat exchange tube 24 without being vaporized.
On the other hand, as can be clearly seen in Fig. 8 (wherein a space distance δ of 2.0 mm). Fig. 9 (wherein a space distance δ of 2.5 mm), and Fig. 10 (wherein a space distance δ of 3.0 mm) that ice formation negligibly oc¬curred in an intermediate portion of the outer heat exchange tube 22, and no local reduction in outside surface tempera¬ture of the outer heat exchange tube 22 occurred. This can be explained as follows: A large space distance δ enabled

the co-existing of liquid phase and gaseous phase in the annular passage 23 which causes the forced convection boil¬ing; The forced convection boiling increased the heat trans¬fer rate of the annular passage 23; Even in the case that some of the LNG flowing in the inner heat exchange tube 24 was not completely vaporized and flowed out from the inner heat exchange tube 24 still in the state of liquid, NG produced in the annular passage 23 enclosed such escaped LNG. As can be seen from Figs. 8 to 10, according to this invention, the sufficient heat transfer rate can be ensured even if the space distance S is varied in a relatively wide range, which can consequently prevent ice formation.
In the case of Figs. 11 to 15 where the ratio of the length of the inner heat exchange tube to the outer heat exchange tube is 40 : 100, similarly, the temperature of the LNG flowing in the inner heat exchange tube 24 cannot be sufficiently raised in the region where the space distance δ is small because the flow rate of the LNG flowing in the inner heat exchange tube 24 is great and the heat transfer area is sma11.
Further, ice formation is remarkably increased around a temperature region where enthalpy abruptly changes, i.e., a region where latent heat for vaporization is absorbed in the case of single component such as water, as shown in Fig. 5. However, the ice formation can be suppressed by widening the

space distance δ to increase the flow rate of LNG flowing in the annular passage 23. in other words, reducing the flow rate of LNG flowing in the inner heat exchange tube 24.
Although data for the case that the space distance δ is further increased were not obtained, it can be predicted that the heat transfer rate lowers in the case of the space distance δ being too large. This is because of the fact that a too-large space distance causes a whole separation of an LNG layer and an NG layer in the annular passage 23 as shown in Fig. 4C, consequently making it difficult to pro¬duce the forced convection boiling in the co-existing state of liquid phrase and gas phrase which increases the heat transfer rate.
Accordingly, in this invention, the length of the inner heat exchange tube 24 and the space distance δ of the annular passage 23 are respectively set at such values that the enthalpy of. LNG flowing in the inner heat exchange tube 24 gently changes around the vicinity of the upper end of the inner heat exchange tube 24, e.g., the enthalpy change at point P in Fig. 5, under an operation pressure of LNG. In this way, ice formation can be reliably prevented with keeping the length of the inner heat exchange tube 23 as shor t as possible.
Next, a second embodiment of the invention will be described with reference to Fig. 16.
Be- -J

In this embodiment, a plurality of fins 29 in the form of wave-like projections are formed on an inside surface of an outer heat exchange tube 22, instead of the ribs 25 in the first embodiment. A free end of the fin 29 comes into contact with an outside surface of an inner heat exchange tube 24. In this state, the inner heat exchange tube 24 is placed in the outer heat exchange tube 22.
The fin 29 which serves as a projection for positioning the inner heat exchange tube 24 in the outer heat exchange tube 22 are formed on the inside surface of the outer heat exchange tube 22. Consequently, the surface area of the outer heat exchange tube 22 defining an annular passage 23 is remarkably increased, which thus raises the heat transfer rate from the outer heat exchange tube 22 to the annular passage 23. Accordingly, the space distance of the annular passage 23 can be made further wider, and the precision tolerance for the space distance can be increased.
Next, a third embodiment of the invention will be described with reference to Figs. 17 to 25.
In this embodiment, as shown in Fig. 17, an inner heat exchange tube 24 is longer than an outer heat exchange tube 22 in such a manner that a lower end portion 24a of the inner heat exchange tube 24 protrudes in a lower header 10. The inner heat exchange tube 24 is formed with inflow through holes 32 slightly above the lower end portion 24a.

The inflow holes 32 are cut through a wall of the inner heat exchange tube 24. The inflow hole 32 is adapted for eject¬ing LNG from the inside of the inner heat exchange tube 24 into an annular passage 23. As shown in Fig. 21, an upper end portion of the inner heat exchange tube 24 goes through an upper header 16 by a specified distance. In this state, the upper end portion of the inner heat exchange tube 24 is fixedly connected to the upper header 16 in a suspended state. An outflow end of the inner heat exchange tube 24 communicates with an LPG supply header 42 in which a heat adjusting fluid is flowed. In this embodiment, liquefied petroleum gas (hereinafter, referred to as "LPG") is flowed.
As shown in Figs. 17 and 23, the inner heat exchange tube 24 is formed with a plurality of ejection holes 34 in an intermediate portion thereof (an area E in Fig. 18). The ejection holes 34 are preferably vertically arranged apart by equidistance and cut through the wall of the inner heat exchange tube 24. The inner heat exchange tube 24 is fur¬ther provided with a closing member 36 right above an upper¬most ejection hole to close the inner heat exchange tube 24. The inner heat exchange tube 24 is further formed with a plurality of mixing holes 40 starting from right above the closing member 36. The mixing hole 40 is similarly cut through the wall of the inner heat exchange tube 24.
As shown in Fig. 19, seawater as heating medium is

supplied through a pipe 11 into a seawater manifold 13 from where seawater is branched out and distributed to respective seawater headers 15. The seawater supplied into the sea¬water header 15 is, as shown in Fig. 20, flowed into a trough 19 having a generally U-shape in cross section by way of a pipe 17. The seawater flowed into the trough 19 over¬flows from the trough 19 and falls down along an outside surface of the plurality of outer heat exchange tubes 22. It should be noted that the seawater supply system having the above structure is applicable to the first and second embodiments.
An operation of the vaporizer as the third embodiment will be described next.
LNG supplied into the lower header 10 is flowed into the inner heat exchange tube 24 through the lower end portion 24a. Since the lower end portion 24a protrudes inside the lower header 10, heavy components contained in the LNG is left in the lower header 10 as residue in a condensed state, even in the case where the load of LNG inside the lower header 10 is small, i.e., the flow rate of LNG i s sma 1 1 .
The LNG flowed in the inner heat exchange tube 24 is blocked by the closing member 36 and ejected into the annu¬lar passage 23 through the ejection holes 34 or through the inflow holes 32 both of which are located below the closing

member 36. The LNG ejected in the annular passage 23 is effectively mixed with NG already produced in the annular passage 23. At this time, even if the LNG undergoes boiling in the inner heat exchange tube 24 and a film of NG exists on an inside surface of the inner heat exchange tube 24, the ejection will take the NG into the annular passage 23.
On the other hand, LPG flowed into the LPG supply header 42 flows down inside an upper portion of the inner heat exchange tube 24, and is blocked by the closing member 36 and ejected into the annular passage 23 through the mixing holes 40 which are located above the closing member 36. Consequently, the ejected LPG is effectively mixed with the NG in the annular passage 23.
Fig. 24A is a diagram showing a temperature distribu¬tion over the height of the outer heat exchange tube 22. In the diagram: the line Ll indicates an average temperature distribution of a mixture of LNG rising up in the outer heat exchange tube 22 and LPG; the line L2 indicates a tempera¬ture distribution of seawater; the line L3 indicates a temperature distribution of the inner heat exchange tube 24; and the line L4 indicates a temperature distribution of the outer heat exchange tube 22.
As can be seen from the diagram, LNG having a specified pressure (e.g., 30 kgG/cm ) and a temperature of about -160*C is flowed from the lower header 10. As represented

by the portion Lla, the LNG flowed from the lower header 10 is heated by the heat exchange with the seawater as heating medium as rising in the inner heat exchange tube 24 and the annular passage 23, and the temperature of the LNG conse¬quently increases. Subsequently, as represented by the portion Lib. the LNG starts the vaporization at about -90'C, and completes the vaporization at the point Lle.
As represented by the portion Lld. the temperature of the LNG further increases. When the temperature of the LNG becomes about -100oC, LNG is ejected from the inner heat ex¬change tube 24 into the annular passage 23 through the ejection holes 34. and is mixed with a mixture of LNG and NG in the annular passage 23. and is further mixed with LPG ejected from the mixing holes 40. At this mixing point, the temperature of the LNG is about -20*C. Accordingly, even if impurity matters having a high solidifying temperature, e.g.. methanol (whose solidifying temperature is about -125*C), are contained in the LNG. such impurity matters (methanol) will not solidify, which accordingly eliminates the problem that a passage is closed due to a solidifica¬tion.
It should be noted that the temperature of seawater '
supplied from the trough 19 is about 10 "C and that the temperature of NG in the upper header 16 is about 8"C.
The line L5 in Fig. 24B shows a change in the thickness
* ' JM 20IQ DUPLICATE

of ice formed on an outside surface of the outer heat ex-clange tube 22. The thickness of ice is no greater than about 2 mm even at a lowermost portion of the outer heat exchange tube 22. This thickness is negligible and will not adversely affect the heat transfer rate.
The entire length H of the outer heat exchange tube 22 shown in Fig. 18 may be set as long as 10 meters. In Fig. 18, the upper header 16 and the lower header 10 are support¬ed by a support frame and the outer heat exchange tube 22 and the inner heat exchange tube 24 are supported in the suspended state. The lower end portion 24a of the inner heat exchange tube 24 is a free end. Also, a lower end portion of the outer heat exchange tube 22 is not held in a fixed state. Accordingly, there is not the likelihood that the heat exchange tubes 22 and 24 receive a great thermal stress due to thermal contraction.
The present invention is not limited to the above-mentioned vaporizer, and other constructions can be used as follows:
(D According to this invention, the flowing direction of LNG in an outer heat exchange tube is not limited. For example, the upper header may serve as an inflow port, and the lower header may serve as an outflow port, which is opposite to the arrangement of the foregoing embodiments. In this case, lower temperature liquid such as LNG is forci-
DUPLICATE

bly flowed down from the upper header through the heat exchange tubes to the lower header utilizing the weight. Alternatively, the inflow header and the outflow header may be arranged in the same horizontal plane and connected with each other by way of outer heat exchange tubes lying in the same horizontal plane.
2 In the first and second embodiments, the length difference between the outer heat exchange tube 22 and the inner heat exchange tube 24, i.e., a distance of the region where the inner heat exchange tube 24 is provided in the outer heat exchange tube 22, may be set in accordance with a target temperature of NG in the outflow header. Specifical¬ly, the higher the target temperature is, the longer the length difference is made to enable NG to be heated for a 1onger period.
® According to this invention, it may be appreciated to provide a sparger tube having a plurality of through holes in an inflow header. LNG is flowed into the sparger tube. This construction is advantageous in supplying LNG in the inflow header uniformly.
EXPLOITATION IN INDUSTRY
As mentioned above, in this invention, an outer heat exchange tube for communicating with an inflow header and an outflow header is internally arranged with an inner heat

exchange tube in an inflow portion thereof. An annular passage for flowing a low temperature liquid is defined between the inner heat exchange tube and the outer heat exchange tube. The annular passage communicates with the inflow header. Accordingly, an axial flow of fluid in the annular passage and a co-existing of liquid phase and gase¬ous phase enhance forced convection boiling in the annular passage, thereby increasing the heat transfer rate. Hence, compared to the conventional vaporizer provided with the heat insulator containing gas, the space distance of the annular passage can be made wider. Accordingly, it is possible to increase the dimensional tolerance of the space distance which assures sufficient heat transfer and prevent ice formation on an outside surface of a heat exchange tube. Thus, the production of a low temperature liquid vaporizer can be simplified and the production cost can be reduced.
» 7 JAN zm CJJJpLiCATE

WE CLAIM:
1. A vaporizer for low temperature liquid such as liquified natural gas, liquified nitrogen comprising an inflow header for flowing low temperature liquid; a plurality of outer heat exchange tubes communicated with the inflow header, each outer heat exchange tube extending in a direction perpendicular to the inflow header , an outside of the outer heat exchange tube coming into contact with a heating medium; an outflow header communicated with the inflow header by way of the plurality of outer heat exchange tubes for flowing vapor of the low temperature which is produced in the outer heat exchange tubes; a plurality of inner heat exchange tubes provided in at least respective inflow portions of the plurality of outer heat exchange tubes, each inner heat exchange tube forming an annular passage between an inside surface of the corrresponding outer heat exchange tube and an outsisde surface of the inner heat exchange tube, the annular passage communicating with the inflow header for flowing the low temperature liquid.
2. The vaporizer as claimed in claim 1 wherein one of the inside surface of the outer heat exchange tube and the outside surface of the inner heat exchange tube is formed with a plurality of projections, a free end of each projection coming into contact with the outer surface.

3. The vaporizer as claimed in claim 2 wherein the inside surface of the outer heat exchange tube is formed with the projections.
4. The vaporizer as claimed in anyone of the claims 1 to 3 wherein the inner heat exchange tube is shorter than the outer heat exchange tube and the inner heat exchange tube is disposed in an inflow portion of the outer heat exchange tube.
5. The vaporizer as claimed in any one of the claims 1 to 3 wherein the inner heat exchange tube is formed with an ejection hole in a wall thereof and provided with a closing member in the inner heat exchange tube at a position closer to the outflow header than the ejection hole.
6. The vaporizer as claimed in claim 5 wherein the outer and inner heat exchange tubes are arranged in a vertical direction while the outflow header is above the inflow header, and an upper end of the inner heat exchange tube is fixedly attached to the outflow header.
7. The vaporizer as claimed in claim 5 or 6 wherein an outflow end of the inner heat exchange tube projects from an outflow end of the outer heat exchange tube projects from an outflow end of the outer heat exchange tube

and communicates with a secondary header, for flowing heat adjusting liquid and the inner heat exchange tube is formed with a mixing hole in a wall thereof at a position which is closer to its outflow end than the closing member and not beyond the outflow end of the outer heat exchange tube.
8. The vaporizer as claimed in any one of the claims 1 to 6 wherein an
inflow end of the inner heat exchange tube is positioned in the inflow
header.
9. A vaporizer for low temperature liquid substantially as herein
described with reference to the accompanying drawings.

Documents:

924-mas-1995 abstract.pdf

924-mas-1995 claims.pdf

924-mas-1995 correspondence-others.pdf

924-mas-1995 correspondence-po.pdf

924-mas-1995 description (complete).pdf

924-mas-1995 drawings.pdf

924-mas-1995 form-1.pdf

924-mas-1995 form-26.pdf

924-mas-1995 form-4.pdf

924-mas-1995 others.pdf

924-mas-1995 petition.pdf

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Patent Number

190874

Indian Patent Application Number

924/MAS/1995

PG Journal Number

30/2009

Publication Date

24-Jul-2009

Grant Date

11-Mar-2004

Date of Filing

19-Jul-1995

Name of Patentee

KABUSHIKI KAISHA KOBE SEIKO SHO

Applicant Address

3-18 WAKINOHAMACHO, 1-CHOME, CHUO-KU, KOBE-SHI, HYOGO-KEN 651

Inventors:

#	Inventor's Name	Inventor's Address
1	KEIZO KONISHI,	C/O TAKASAGO SEISAKU SHO KABUSHIKI KAISHA, KOBE SEIKO SHO, 3-1 SHINHAMA 2-CHOME, ARAICHO, TAKASAGO-SHI, HYOGO-KEN,
2	KOHICHI HAYASHI;	C/O TAKASAGO SEISAKU SHO KABUSHIKI KAISHA, KOBE SEIKO SHO, 3-1 SHINHAMA 2-CHOME, ARAICHO, TAKASAGO-SHI, HYOGO-KEN,
3	KATSUFUMI TANAKA;	C/O TAKASAGO SEISAKU SHO KABUSHIKI KAISHA, KOBE SEIKO SHO, 3-1 SHINHAMA 2-CHOME, ARAICHO, TAKASAGO-SHI, HYOGO-KEN,
4	MASANORI TAKATA	C/O TAKASAGO SEISAKU SHO KABUSHIKI KAISHA, KOBE SEIKO SHO, 3-1 SHINHAMA 2-CHOME, ARAICHO, TAKASAGO-SHI, HYOGO-KEN,
5	ICHIRO SAKURABA;	C/O OSAKA GAS KABUSHIKI KAISHA, 1-2 HIRANOMACHI 4-CHOME, CHUO-KU, OSAKA-SHI, OSAKA-FU,
6	KOHICHI SHINKAI;	C/O OSAKA GAS KABUSHIKI KAISHA, 1-2 HIRANOMACHI 4-CHOME, CHUO-KU, OSAKA-SHI, OSAKA-FU,
7	YOSHINORI HISAZUMI	C/O OSAKA GAS KABUSHIKI KAISHA, 1-2 HIRANOMACHI 4-CHOME, CHUO-KU, OSAKA-SHI, OSAKA-FU,
8	MASANORI OKI	C/O OSAKA GAS KABUSHIKI KAISHA, 1-2 HIRANOMACHI 4-CHOME, CHUO-KU, OSAKA-SHI, OSAKA-FU,

PCT International Classification Number

F22B23/06

PCT International Application Number

N/A

PCT International Filing date

PCT Conventions:

#	PCT Application Number	Date of Convention	Priority Country
1			NA