Abstract:
A core-in-shell heat exchanger, a method of fabricating the core-in-shell heat exchanger, and a method of exchanging heat in a core-in-shell heat exchanger disposed on a slosh-inducing moving platform are described. The method of exchanging heat includes introducing a shell-side fluid into a shell of the core-in-shell heat exchanger and introducing a fluid to be cooled into each of one or more cores of the core-in-shell heat exchanger, the one or more cores being arranged along an axial length of the shell with a plurality of baffles disposed on either side of the one or more cores along the axial length of the shell to reduce slosh of the shell-side fluid. The method also includes draining excess shell-side fluid using a plurality of drains, at least two of the plurality of drains being disposed on opposite sides of one of the plurality of baffles.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application is a non-provisional application which claims benefit under 35 USC §119(e) to U.S. Provisional Application Ser. No. 61/987,164 filed May 1, 2014, entitled “LIQUID DRAINS IN CORE-IN-SHELL HEAT EXCHANGER.” 
     
    
     FIELD OF THE INVENTION 
       [0002]    This invention relates to an arrangement of drains in a core-in-shell heat exchanger that includes baffles separating the cores. The drainage of refrigerant from a heat exchanger shell was previously addressed by providing a single drain to flow liquid into a scrubber or flare system, for example. However, when the cores of the heat exchanger are separated by baffles, the single drain arrangement can have a negative effect on the efficiency of the heat exchanger operation. 
       BACKGROUND OF THE INVENTION 
       [0003]    Cryogenic liquefaction is commonly used to convert natural gas into a more convenient form for transportation and/or storage. Because liquefying natural gas greatly reduces its specific volume, large quantities of natural gas can be economically transported and/or stored in liquefied form. 
         [0004]    Transporting natural gas in its liquefied form can effectively link a natural gas source with a distant market when the source and market are not connected by a pipeline. This situation commonly arises when the source of natural gas and the market for the natural gas are separated by large bodies of water. In such cases, liquefied natural gas (LNG) can be transported from the source to the market using specially designed ocean-going LNG tankers. 
         [0005]    Storing natural gas in its liquefied form can help balance periodic fluctuations in natural gas supply and demand. In particular, LNG can be “stockpiled” for use when natural gas demand is low and/or supply is high. As a result, future demand peaks can be met with LNG from storage, which can be vaporized as demand requires. 
         [0006]    Several methods exist for liquefying natural gas. Some methods produce a pressurized LNG (PLNG) product that is useful, but requires expensive pressure-containing vessels for storage and transportation. Other methods produce an LNG product having a pressure at or near atmospheric pressure. In general, these non-pressurized LNG production methods involve cooling a natural gas stream through indirect heat exchange with one or more refrigerants and then expanding the cooled natural gas stream to near atmospheric pressure. In addition, most LNG facilities employ one or more systems to remove contaminants (e.g., water, mercury and mercury components, acid gases, and nitrogen, as well as a portion of ethane and heavier components) from the natural gas stream at different points during the liquefaction process. 
         [0007]    In general, LNG facilities are designed and operated to provide LNG to a single market in a specific region of the world. Because specifications for the final characteristics of the natural gas product, such as, for example, higher heating value (HHV), Wobbe index, methane content, ethane content, C 3 + content, and inerts content vary widely throughout the world, LNG facilities are typically optimized to meet a certain set of specifications for a single market. In large part, achieving the stringent final product specifications involves effectively removing certain components from the natural gas feed stream. 
         [0008]    One or more heat exchangers involved in the liquefaction process may be configured as core-in-shell heat exchangers that include one or more cores. In addition, when the LNG processing is done off shore, baffles are used between cores of the heat exchanger to address the sloshing of the shell-side liquid (refrigerant). Because the baffles cause a pressure gradient or horizontal pressure drop across the shell, the single drain configuration results in unequal refrigerant levels at different parts (related to different cores) of the heat exchanger. Unequal refrigerant levels can cause several problems. In a core in which the shell-side liquid level is too high (for example, because the core is farthest from the drain), efficient heat exchange, in the form of boiling of the refrigerant, may be suppressed. In a core in which the shell-side liquid level is too low, the core may not be in contact with the refrigerant and, as a result, heat exchange may not take place at all. The discussion of  FIG. 2  below illustrates some of the issues that arise from the prior drain configuration. 
         [0009]      FIG. 2  is a block diagram of a core-in-shell heat exchanger  200  according to the prior art.  FIG. 2  illustrates the prior approach to draining the shell. The heat exchanger  200  is assumed to be in an off-shore environment, for example, where baffles  1  are used to reduce sloshing of the shell-side fluid (refrigerant). The heat exchanger  200  includes three cores  2  with baffles  1  on either side of each core  2 . A weir plate  3  is used to separate the cores from the drain which includes a vortex breaker  4  that minimizes entrained vapor flow into the outlet drain line  5 . The baffles  1  are designed to create additional flow resistance within the heat exchanger  200  and dampen the effects of motion to prevent wave action building in the heat exchanger  200 , because the wave action could cause liquid to flood over the top of a core  2  or low liquid levels in a core  2  or liquid levels falling beneath a core  2 . However, Computational Fluid Dynamics (CFD) analysis, static calculations, and scale model testing have demonstrated that the pressure drop across the baffles  1  required to satisfactorily suppress the liquid motion results in excessive liquid stack-up and flooding in the core  2  located farthest from the weir plate  3 . In addition, the core  2  located closest to the weir plate  3  experiences low liquid levels. Due to the resistance to liquid flow caused by the baffles  1 , the liquid level in the outflow section experiences low enough liquid level that there is a high potential for gas ingestion into the liquid drain. This gas ingestion is a serious operational constraint and will severely restrict the downstream process equipment capacity and operation. That is, the variations in the liquid level resulting from the baffles  1  can have a deleterious effect on the thermal performance of the heat exchanger  200  and on the mechanical integrity of the heat exchanger  200  due to thermally induced fatigue stress which can shorten the life of the heat exchanger  200  and result in damage and a leaking heat exchanger  200 . 
       SUMMARY OF THE INVENTION 
       [0010]    According to an aspect of the invention, a method of exchanging heat in a core-in-shell heat exchanger disposed on a slosh-inducing moving platform includes introducing a shell-side fluid into a shell of the core-in-shell heat exchanger; introducing a fluid to be cooled into each of one or more cores of the core-in-shell heat exchanger, the one or more cores being arranged along an axial length of the shell with a plurality of baffles disposed one either side of the one or more cores along the axial length of the shell to reduce slosh of the shell-side fluid; and draining excess shell-side fluid using a plurality of drains, at least two of the plurality of drains being disposed on opposite sides of one of the plurality of baffles. 
         [0011]    According to another aspect of the invention, a method of fabricating a core-in-shell heat exchanger disposed on a moving platform includes arranging one or more cores along an axial length of a shell; arranging a plurality of baffles on either side of the one or more cores along the axial length of the shell; and disposing a plurality of drains in the shell, at least two of the plurality of drains being disposed on opposite sides of one of the plurality of baffles. 
         [0012]    According to yet another aspect of the invention, a core-in-shell heat exchanger includes a shell; one or more cores arranged along an axial length of the shell; a plurality of baffles arranged on either side of the one or more cores along the axial length of the shell; and a plurality of drains, at least two of the plurality of drains being disposed on opposite sides of one of the plurality of baffles. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0013]    The invention, together with further advantages thereof, may best be understood by reference to the following description taken in conjunction with the accompanying figures by way of example and not by way of limitation, in which: 
           [0014]      FIG. 1  is a simplified overview of a cascade-type LNG facility configured in accordance with one embodiment of the present invention; 
           [0015]      FIG. 2  is a block diagram of a core-in-shell heat exchanger according to the prior art; 
           [0016]      FIG. 3  illustrates a heat exchanger according to an embodiment of the invention; 
           [0017]      FIG. 4  illustrates a heat exchanger according to another embodiment of the invention; and 
           [0018]      FIG. 5  is a cross sectional view of the heat exchanger shown in  FIG. 4  along A-A. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0019]    Reference will now be made in detail to embodiments of the invention, one or more examples of which are illustrated in the accompanying drawings. Each example is provided by way of explanation of the invention, not as a limitation of the invention. It will be apparent to those skilled in the art that various modifications and variation can be made in the present invention without departing from the scope or spirit of the invention. For instance, features illustrated or described as part of one embodiment can be used on another embodiment to yield a still further embodiment. Thus, it is intended that the present invention cover such modifications and variations that come within the scope of the appended claims and their equivalents. 
         [0020]    As noted above, cryogenic liquefaction is used to convert natural gas to a more easily transportable form. As also noted, a core-in-shell heat exchanger used in the process, when performed off-shore, for example, includes baffles between the cores to suppress sloshing of the shell-side fluid (refrigerant). When the existing single-drain configuration is employed in this type of core-in-shell heat exchanger, the result may be unequal shell-side liquid levels and inefficient heat exchange with the core-side fluid. Embodiments of the method and system described herein relate to a drain arrangement that equalizes the shell-side liquid level to address the issues discussed above. 
         [0021]    The present invention can be implemented in a facility used to cool natural gas to its liquefaction temperature to thereby produce liquefied natural gas (LNG). The LNG facility generally employs one or more refrigerants (cryogenic fluids) to extract heat from the natural gas and reject to the environment. Numerous configurations of LNG systems exist and the present invention may be implemented in many different types of LNG systems. 
         [0022]    In one embodiment, the present invention can be implemented in a mixed refrigerant LNG system. Examples of mixed refrigerant processes can include, but are not limited to, a single refrigeration system using a mixed refrigerant, a propane pre-cooled mixed refrigerant system, and a dual mixed refrigerant system. 
         [0023]    In another embodiment, the present invention is implemented in a cascade LNG system employing a cascade-type refrigeration process using one or more predominately pure component refrigerants. The refrigerants utilized in cascade-type refrigeration processes can have successively lower boiling points in order to facilitate heat removal from the natural gas stream being liquefied. Additionally, cascade-type refrigeration processes can include some level of heat integration. For example, a cascade-type refrigeration process can cool one or more refrigerants having a higher volatility through indirect heat exchange with one or more refrigerants having a lower volatility. In addition to cooling the natural gas stream through indirect heat exchange with one or more refrigerants, cascade and mixed-refrigerant LNG systems can employ one or more expansion cooling stages to simultaneously cool the LNG while reducing its pressure. 
         [0024]    Referring now to  FIG. 1 , one embodiment of a cascade-type LNG facility in accordance with one embodiment of the present invention is illustrated. The LNG facility depicted in  FIG. 1  generally comprises a propane refrigeration cycle  30 , an ethylene refrigeration cycle  50 , and a methane refrigeration cycle  70  with an expansion section  80 . While “propane,” “ethylene,” and “methane” are used to refer to respective first, second, and third refrigerants, it should be understood that the embodiment illustrated in  FIG. 1  and described herein can apply to any combination of suitable refrigerants. The main components of propane refrigeration cycle  30  include a propane compressor  31 , a propane cooler/condenser  32 , high-stage propane chillers  33 A and  33 B, an intermediate-stage propane chiller  34 , and a low-stage propane chiller  35 . The main components of ethylene refrigeration cycle  50  include an ethylene compressor  51 , an ethylene cooler  52 , a high-stage ethylene chiller  53 , a low-stage ethylene chiller/condenser  55 , and an ethylene economizer  56 . The main components of methane refrigeration cycle  70  include a methane compressor  71 , a methane cooler  72 , and a methane economizer  73 . The main components of expansion section  80  include a high-stage methane expansion valve and/or expander  81 , a high-stage methane flash drum  82 , an intermediate-stage methane expansion valve and/or expander  83 , an intermediate-stage methane flash drum  84 , a low-stage methane expansion valve and/or expander  85 , and a low-stage methane flash drum  86 . 
         [0025]    The operation of the LNG facility illustrated in  FIG. 1  will now be described in more detail, beginning with propane refrigeration cycle  30 . Propane is compressed in multi-stage (e.g., three-stage) propane compressor  31  driven by, for example, a gas turbine driver (not illustrated). The stages of compression may exist in a single unit or two or more separate units mechanically coupled to a single driver. Upon compression, the propane is passed through conduit  300  to propane cooler  32 , wherein it is cooled and liquefied through indirect heat exchange with an external fluid (e.g., air or water). The stream from propane cooler  32  can then be passed through conduit  302  to a pressure reduction means, illustrated as expansion valve  36 A, wherein the pressure of the liquefied propane is reduced, thereby evaporating or flashing a portion thereof. The resulting two-phase stream then flows through conduit  304   a  into high-stage propane chiller  33   a . High stage propane chiller  33   a  uses the flashed propane refrigerant to cool the incoming natural gas stream in conduit  110 . 
         [0026]    The cooled natural gas stream from high-stage propane chiller  33   a  flows through conduit  114  to a separation vessel, wherein water and in some cases propane and heavier components are removed, typically followed by a treatment system  40 , in cases where not already completed in upstream processing, wherein moisture, mercury and mercury compounds, particulates, and other contaminants are removed to create a treated stream. The stream exits the treatment system  40  through conduit  116 . Thereafter, a portion of the stream in conduit  116  can be routed through conduit A to a heavies removal zone. The remaining portion of the stream in conduit  116  is combined with a stream in conduit G exiting the heavies removal zone illustrated. The combined stream can then enter intermediate-stage propane chiller  34 , wherein the stream is cooled in indirect heat exchange means  41  through indirect heat exchange with a propane refrigerant stream. The resulting cooled stream in conduit  118  can then be recombined with a stream in conduit B exiting heavies removal zone, and the combined stream can then be routed to low-stage propane chiller  35 , wherein the stream can be further cooled through indirect heat exchange means  42 . The resultant cooled stream can then exit low-stage propane chiller  35  through conduit  120 . Subsequently, the cooled stream in conduit  120  can be routed to high-stage ethylene chiller  53 . 
         [0027]    The combined vaporized propane refrigerant stream exiting high-stage propane chillers  33  and  33 A is returned to the high-stage inlet port of propane compressor  31  through conduit  306 . The liquid propane refrigerant in high-stage propane chiller  33 A provides refrigeration duty for the natural gas stream  110 . Two-phase refrigerant stream can enter the intermediate-stage propane chiller  34  through conduit  310 , thereby providing coolant for the natural gas stream (in conduit  116 ) and to streams entering intermediate-stage propane chiller  34  through conduits  204  and  310 . The vaporized portion of the propane refrigerant exits intermediate-stage propane chiller  34  through conduit  312  and can then enter the intermediate-stage inlet port of propane compressor  31 . The liquefied portion of the propane refrigerant exits intermediate-stage propane chiller  34  through conduit  314  and is passed through a pressure-reduction means, illustrated here as expansion valve  44 , whereupon the pressure of the liquefied propane refrigerant is reduced to thereby flash or vaporize a portion thereof. The resulting vapor-liquid refrigerant stream can then be routed to low-stage propane chiller  35  through conduit  316  and where the refrigerant stream can cool the methane-rich stream and an ethylene refrigerant stream entering low-stage propane chiller  35  through conduits  118  and  206 , respectively. The vaporized propane refrigerant stream then exits low-stage propane chiller  35  and is routed to the low-stage inlet port of propane compressor  31  through conduit  318  wherein it is compressed and recycled as previously described. 
         [0028]    As shown in  FIG. 1 , a stream of ethylene refrigerant in conduit  202  enters high-stage propane chiller  33   b , wherein the ethylene stream is cooled through indirect heat exchange means  39 . The resulting cooled ethylene stream can then be routed in conduit  204  from high-stage propane chiller  33   b  to intermediate-stage propane chiller  34 . Upon entering intermediate-stage propane chiller  34 , the ethylene refrigerant stream can be further cooled through indirect heat exchange means  45  in intermediate-stage propane chiller  34 . The resulting cooled ethylene stream can then exit intermediate-stage propane chiller  34  and can be routed through conduit  206  to enter low-stage propane chiller  35 . In low-stage propane chiller  35 , the ethylene refrigerant stream can be at least partially condensed, or condensed in its entirety, through indirect heat exchange means  46 . The resulting stream exits low-stage propane chiller  35  through conduit  208  and can subsequently be routed to a separation vessel  47 , wherein a vapor portion of the stream, if present, can be removed through conduit  210 , while a liquid portion of the ethylene refrigerant stream can exit separator  47  through conduit  212 . The liquid portion of the ethylene refrigerant stream exiting separator  47  can have a representative temperature and pressure of about −24° F. (about −31° C.) and about 285 psia (about 1,965 kPa). 
         [0029]    Turning now to ethylene refrigeration cycle  50  in  FIG. 1 , the liquefied ethylene refrigerant stream in conduit  212  can enter ethylene economizer  56 , wherein the stream can be further cooled by an indirect heat exchange means  57 . The resulting cooled liquid ethylene stream in conduit  214  can then be routed through a pressure reduction means, illustrated here as expansion valve  58 , whereupon the pressure of the cooled predominantly liquid ethylene stream is reduced to thereby flash or vaporize a portion thereof. The cooled, two-phase stream in conduit  215  can then enter high-stage ethylene chiller  53 . In high-stage ethylene chiller  53 , at least a portion of the ethylene refrigerant stream can vaporize to further cool the stream in conduit  121  entering an indirect heat exchange means  59 . The vaporized and remaining liquefied ethylene refrigerant exits high-stage ethylene chiller  53  through respective conduits  216  and  220 . The vaporized ethylene refrigerant in conduit  216  can re-enter ethylene economizer  56 , wherein the stream can be warmed through an indirect heat exchange means  60  prior to entering the high-stage inlet port of ethylene compressor  51  through conduit  218 , as shown in  FIG. 1 . 
         [0030]    The cooled stream in conduit  120  exiting low-stage propane chiller  35  can thereafter be split into two portions, as shown in  FIG. 1 . At least a portion of the natural gas stream can be routed through conduit E. While the remaining portion of the cooled natural gas stream in conduit  121  can be routed to high-stage ethylene chiller  53 , and then can be and cooled in indirect heat exchange means  59  of high-stage ethylene chiller  53 . 
         [0031]    The remaining liquefied ethylene refrigerant exiting high-stage ethylene chiller  53  in conduit  220  can re-enter ethylene economizer  56 , to be further sub-cooled by an indirect heat exchange means  61  in ethylene economizer  56 . The resulting sub-cooled refrigerant stream exits ethylene economizer  56  through conduit  222  and can subsequently be routed to a pressure reduction means, illustrated here as expansion valve  62 , whereupon the pressure of the refrigerant stream is reduced to thereby vaporize or flash a portion thereof. The resulting, cooled two-phase stream in conduit  224  enters low-stage ethylene chiller/condenser  55 . 
         [0032]    As shown in  FIG. 1 , a portion of the cooled natural gas stream exiting high-stage ethylene chiller  53  can be routed through conduit C to the heavies removal zone, while another portion of the cooled natural gas stream exiting high-stage ethylene chiller  53  can be routed through conduit  122  to enter indirect heat exchange means  63  of low-stage ethylene chiller/condenser  55 . The remaining portion of the cooled natural gas stream in conduit  122  can then be combined the first column vapor stream exiting the heavies removal zone in conduit D and/or may be combined with a stream exiting methane refrigeration cycle  70  in conduit  168 , for the resulting composite stream to then enter indirect heat exchange means  63  in low-stage ethylene chiller/condenser  55 , as shown in  FIG. 1 . 
         [0033]    In low-stage ethylene chiller/condenser  55 , the cooled stream (which can comprise the stream in conduit  122  optionally combined with streams in conduits D and  168 ) can be at least partially condensed through indirect heat exchange with the ethylene refrigerant entering low-stage ethylene chiller/condenser  55  through conduit  224 . The vaporized ethylene refrigerant exits low-stage ethylene chiller/condenser  55  through conduit  226  and can then enters ethylene economizer  56 . In ethylene economizer  56 , the vaporized ethylene refrigerant stream can be warmed through an indirect heat exchange means  64  prior to being fed into the low-stage inlet port of ethylene compressor  51  through conduit  230 . As shown in  FIG. 1 , a stream of compressed ethylene refrigerant exits ethylene compressor  51  through conduit  236  and can subsequently be routed to ethylene cooler  52 , wherein the compressed ethylene stream can be cooled through indirect heat exchange with an external fluid (e.g., water or air). The resulting cooled ethylene stream can then be introduced through conduit  202  into high-stage propane chiller  33 B for additional cooling as previously described. 
         [0034]    The cooled natural gas stream exiting low-stage ethylene chiller/condenser  55  in conduit  124  can also be referred to as the “pressurized LNG-bearing stream” As shown in FIG.  1 , the pressurized LNG-bearing stream exits low-stage ethylene chiller/condenser  55  through conduit  124  prior to entering main methane economizer  73 . In main methane economizer  73 , the methane-rich stream in conduit  124  can be cooled in an indirect heat exchange means  75  through indirect heat exchange with one or more methane refrigerant streams. The cooled, pressurized LNG-bearing stream exits main methane economizer  73  through conduit  134  and can then be routed into expansion section  80  of methane refrigeration cycle  70 . In expansion section  80 , the pressurized LNG-bearing stream first passes through high-stage methane expansion valve  81  and/or expander, whereupon the pressure of this stream is reduced to thereby vaporize or flash a portion thereof. The resulting two-phase methane-rich stream in conduit  136  can then enter high-stage methane flash drum  82 , whereupon the vapor and liquid portions of the reduced-pressure stream can be separated. The vapor portion of the reduced-pressure stream (also called the high-stage flash gas) exits high-stage methane flash drum  82  through conduit  138  to then enter main methane economizer  73 , wherein at least a portion of the high-stage flash gas can be heated through indirect heat exchange means  76  of main methane economizer  73 . The resulting warmed vapor stream exits main methane economizer  73  through conduit  138  and can then be routed to the high-stage inlet port of methane compressor  71 , as shown in  FIG. 1 . 
         [0035]    The liquid portion of the reduced-pressure stream exits high-stage methane flash drum  82  through conduit  142  to then re-enter main methane economizer  73 , wherein the liquid stream can be cooled through indirect heat exchange means  74  of main methane economizer  73 . The resulting cooled stream exits main methane economizer  73  through conduit  144  and can then be routed to a second expansion stage, illustrated here as intermediate-stage expansion valve  83  but could include an expander. Intermediate-stage expansion valve  83  further reduces the pressure of the cooled methane stream which reduces the stream&#39;s temperature by vaporizing or flashing a portion thereof. The resulting two-phase methane-rich stream in conduit  146  can then enter intermediate-stage methane flash drum  84 , wherein the liquid and vapor portions of this stream can be separated and can exit the intermediate-stage flash drum  84  through respective conduits  148  and  150 . The vapor portion (also called the intermediate-stage flash gas) in conduit  150  can re-enter methane economizer  73 , wherein the vapor portion can be heated through an indirect heat exchange means  77  of main methane economizer  73 . The resulting warmed stream can then be routed through conduit  154  to the intermediate-stage inlet port of methane compressor  71 , as shown in  FIG. 1 . 
         [0036]    The liquid stream exiting intermediate-stage methane flash drum  84  through conduit  148  can then pass through a low-stage expansion valve  85  and/or expander, whereupon the pressure of the liquefied methane-rich stream can be further reduced to thereby vaporize or flash a portion thereof. The resulting cooled, two-phase stream in conduit  156  can then enter low-stage methane flash drum  86 , wherein the vapor and liquid phases can be separated. The liquid stream exiting low-stage methane flash drum  86  through conduit  158  can comprise the liquefied natural gas (LNG) product. The LNG product, which is at about atmospheric pressure, can be routed through conduit  158  downstream for subsequent storage, transportation, and/or use. 
         [0037]    The vapor stream exiting low-stage methane flash drum (also called the low-stage methane flash gas) in conduit  160  can be routed to methane economizer  73 , wherein the low-stage methane flash gas can be warmed through an indirect heat exchange means  78  of main methane economizer  73 . The resulting stream can exit methane economizer  73  through conduit  164 , whereafter the stream can be routed to the low-stage inlet port of methane compressor  71 . 
         [0038]    Methane compressor  71  can comprise one or more compression stages. In one embodiment, methane compressor  71  comprises three compression stages in a single module. In another embodiment, one or more of the compression modules can be separate, but can be mechanically coupled to a common driver. Generally, one or more intercoolers (not shown) can be provided between subsequent compression stages. 
         [0039]    As shown in  FIG. 1 , the compressed methane refrigerant stream exiting methane compressor  71  can be discharged into conduit  166 . A portion of the compressed methane refrigerant stream exiting compressor  71  through conduit  166  can be routed through conduit F to the heavies removal zones, while another portion of the compressed methane refrigerant can be routed to methane cooler  72 , whereafter the stream can be cooled through indirect heat exchange with an external fluid (e.g., air or water) in methane cooler  72 . The resulting cooled methane refrigerant stream exits methane cooler  72  through conduit  112 , whereafter a portion of the methane refrigerant can be routed through conduit H to the heavies removal zones, while the remaining portion of the methane refrigerant stream can be directed to and further cooled in propane refrigeration cycle  30 . 
         [0040]    Upon being cooled in propane refrigeration cycle  30  through heat exchanger means  37 , the methane refrigerant stream can be discharged into conduit  130  and subsequently routed to main methane economizer  73 , wherein the stream can be further cooled through indirect heat exchange means  79 . The resulting sub-cooled stream exits main methane economizer  73  through conduit  168  and can then combined with stream in conduit  122  exiting high-stage ethylene chiller  53  and/or with stream in conduit D exiting the heavies removal zone prior to entering low-stage ethylene chiller/condenser  55 , as previously discussed. 
         [0041]    Several heat exchange means (e.g.,  39 ,  41 ,  45 ,  57 ) are shown and discussed above as being part of the LNG facility of  FIG. 1 . One or more of the heat exchangers in the LNG facility may be core-in-shell heat exchangers that include baffles, as discussed above. Embodiments of the invention are discussed below with reference to exemplary heat exchangers  300  ( FIG. 3) and 400  ( FIG. 4 ) that may be employed as one of the above-discussed heat exchange means. The heat exchangers  300 ,  400  according to embodiments of the invention reduce the hydraulic pressure drop due to cross flow while maintaining the slosh suppression via baffle plates (e.g.,  1  in  FIG. 2 ) by draining shell-side liquid in multiple locations as detailed below. 
         [0042]      FIG. 3  illustrates a heat exchanger  300  according to an embodiment of the invention. The view shown in  FIG. 3  is a cut-away view that exposes the inner components of the heat exchanger  300 . The heat exchanger  300  includes a shell  310  shown with two maintenance access manways  320 . Disposed in the shell  310  are three cores  330 , and six baffles  340 , one on each side of each core  330 . In alternate embodiment, different numbers of cores  330  (one or more) and baffles  340  may be used. Each drain pipe  350  places a drain  360 , which may include a vortex breaker  365  to prevent the formation of a vortex that entrains vapor in the liquid stream, at a specified height h from the bottom of the shell  310 . The drain  360  is continuously operating rather than being a manually operated maintenance drain, for example. As such, the drain  360  carries fluid out through the drain pipe  350  to the outlet drain line  370  when the fluid reaches the drain  360 . By positioning the drain  360  at a height h rather than at the bottom of the shell, a specified fluid level is maintained within the shell  310  (a refrigerant inventory is maintained in the shell  310  at process shutdown). 
         [0043]    The drain pipe  350  may be constructed in multiple sections and mounted within the shell  310  in a way that facilitates removal and replacement of sections with a different diameter or length of pipe. The diameter and length of the drain pipe  350  may be selected to take into account the internal velocity imposed on each drain  360  to ensure that adequate space is provided between the normal operating range of liquid level and the top of the drain pipe  350  (entrance to the drain  360 ). In this way, ingestion of vapor (generated from the refrigerant during heat exchange) into the drain pipe  350  does not occur. The vortex breaker  365  that may be employed at one or more drains  360  also works to minimize vapor ingestion into the drain  360  outlets in addition to minimizing vortex formation. The position of the drain pipe  350  may also be selected such that the velocity head (energy of the shell-side fluid due to its bulk motion, expressed as a length measurement) created by the liquid entering the drain  360  is maintained to be less than the distance between the drain  360  and the lowest point of the operating range of liquid levels included in the design margin. That is, if the drain pipe  350  length is too short such that the drain  360  is positioned too low within the shell  310 , the shell-side liquid (refrigerant) may drain out of the shell  310  even when it is (at the lowest point) in the normal range of acceptable liquid levels. The number of drains  360 , their location, and the height h of the drain pipes  350  are all variables that may be selected based on several factors. The arrangement according to the embodiment shown in  FIG. 3  facilitates adjusting the position of the drain  360  by simply adjusting the length of the drain pipe  350 . Thus, if conditions change and those conditions affect the operating range of liquid levels, the drain  360  height may be adjusted, based on adjusting the drain pipe length  350 , to account for the change. 
         [0044]      FIG. 4  illustrates a heat exchanger  400  according to another embodiment of the invention.  FIG. 5  is a cross sectional view of the heat exchanger  400  shown in  FIG. 4  along A-A. The components inside the heat exchanger  400  are shown, as are the opening in the shell  410  for the drain pipe  450 . That is, the views shown in  FIGS. 4 and 5  depict a transparent shell  410 . Two maintenance access manways  420  are shown for the shell  410 . Three cores  430  and six baffles  440 , one on each side of each core  430 , are also shown. In alternate embodiments, the number of cores  430  or baffles  440  may be different. Each drain pipe  450  connects to a drain  460 , which may include a vortex breaker  465  to prevent the formation of a vortex that entrains vapor in the liquid stream. Each continuously operating drain  460  is positioned at a predefined height d from the bottom of the shell  410 . The drain pipe  450  associated with each drain  460  channels the drained shell-side fluid (refrigerant) to an outlet drain line  470 . The number of drains  460 , their location, and height d within the shell  410  at which each drain  460  opening is positioned may be selected as discussed above with reference to  FIG. 3 . For example, the height d of the drain  460  may be selected to avoid ingestion of vapor into the drain pipe  450  and to ensure that drainage of the shell-side liquid does not occur with the shell-side liquid is within the normal operating range of liquid level. As another example, the height d may also be selected to ensure that a refrigerant inventory is maintained at process shutdown (i.e., drain  460  position is not too low on the shell  410 ). 
         [0045]    While the invention has been described in detail in connection with only a limited number of embodiments, it should be readily understood that the invention is not limited to such disclosed embodiments. Rather, the invention can be modified to incorporate any number of variations, alterations, substitutions or equivalent arrangements not heretofore described, but which are commensurate with the spirit and scope of the invention. Additionally, while various embodiments of the invention have been described, it is to be understood that aspects of the invention may include only some of the described embodiments. Accordingly, the invention is not to be seen as limited by the foregoing description, but is only limited by the scope of the appended claims.