Patent Publication Number: US-2021164619-A1

Title: Ambient Air Vaporizer with Icephobic/Waterphobic Treatment

Description:
CLAIM OF PRIORITY 
     This application claims the benefit of U.S. Provisional Application No. 62/942,446, filed Dec. 2, 2019, the contents of which are hereby incorporated by reference. 
    
    
     FIELD OF THE INVENTION 
     The present disclosure relates generally to heat exchangers for fluid processing and, in particular, to ambient air vaporizers for evaporating and superheating of cryogenic fluids. 
     BACKGROUND 
     Cryogenic fluids, that is, fluids having a boiling point generally below −150° C. at atmospheric pressure, are used in a variety of applications, such as mobile and industrial applications. Cryogenic fluids typically are stored and distributed as liquids to reduce storage volume and facilitate distribution. 
     Cryogenic fluids, such as nitrogen, oxygen, argon, natural gas, hydrogen, etc., are typically utilized in the gaseous phase. The low temperature in the liquid phase enables use of the heat energy of ambient air for cryogenic liquid evaporation and cold gas superheating. 
     An ambient air vaporizer is a heat exchanger intended for evaporating and superheating of cryogenic fluid. The ambient air vaporizer comprises a bundle of high finned tubes made from extruded aluminum profiles connected in series and in parallel. The cryogen flows inside the tubes, while the outer sides of the tubes are exposed to the warmer ambient air. As a result, the cryogen is heated as it flows through the tubes. An example of a system that uses such heat exchangers is provided in commonly assigned U.S. Pat. No. 6,799,429 to Drube et al., the contents of which are hereby incorporated by reference. 
     Ambient air contains water and moisture. As the air cools down on the heat exchanger profiles, this water condenses and freezes out on the heat exchanger surface. This icing considerably impairs the heat exchanger performance. 
     A nominal capacity of the ambient air vaporizer usually applies for 6 to 8 hours operation between complete defrosts. As a result, and depending on the expected climatic conditions, in a typical installation, the installed nominal capacity of the ambient air vaporizers used is usually 4 to 5 times larger than the requirements. For example, if a requirement is for 100 Nm 3 /h, the installed nominal capacity is 400 to 500 Nm 3 /h. 
     Besides the costs associated with additional capacity, the ambient vaporizers occupy a considerable space that increases with additional capacity. As a result, any reduction in the required capacity of an installation is welcome as the installation site area can be smaller. 
     SUMMARY 
     There are several aspects of the present subject matter which may be embodied separately or together in the devices and systems described and claimed below. These aspects may be employed alone or in combination with other aspects of the subject matter described herein, and the description of these aspects together is not intended to preclude the use of these aspects separately or the claiming of such aspects separately or in different combinations as set forth in the claims appended hereto. 
     In one aspect, an ambient air vaporizer includes a heat exchanger tube having a surface with an icephobic/waterphobic treatment. 
     In another aspect, an ambient air vaporizer includes an evaporating section and a superheating section, where the evaporating section includes an icephobic/waterphobic treatment and the superheating section includes an icephobic/waterphobic treatment. 
     In still another aspect, a method of treating an ambient air vaporizer includes the step of providing the surface of the ambient air vaporizer with an icephobic/waterphobic treatment. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       In describing the preferred example embodiments, references are made to the accompanying drawing figures wherein like parts have like reference numerals, and wherein: 
         FIG. 1  is a schematic view of an ambient air vaporizer in an embodiment of the disclosure; 
         FIG. 2  shows plots of temperature along a vaporizer at different times with a first cryogen fluid flow rate in an embodiment of the disclosure; 
         FIG. 3  shows plots of ice build-up along a vaporizer at different times with the first cryogen flow rate in an embodiment of the disclosure; 
         FIG. 4  shows plots of temperature along a vaporizer at different times with a second cryogen fluid flow rate in an embodiment of the disclosure; 
         FIG. 5  shows plots of ice build-up along a vaporizer at different times with the second cryogen flow rate in an embodiment of the disclosure. 
     
    
    
     DETAILED DESCRIPTION OF EMBODIMENTS 
     In accordance with the disclosure, an icephobic/waterphobic treatment is applied to the outer surface of one or more ambient air vaporizers. Such a treatment repels or sheds water and ice on the treated exterior surface(s) of the ambient air vaporizer. 
     An ambient air vaporizer that has been treated in accordance with an embodiment of the disclosure is indicated in general at  10  in  FIG. 1 . The ambient air vaporizer includes an evaporating section  12  of parallel heat exchanger tubes and a superheating section  14  of heat exchanger tubes arranged in series. The heat exchanger tubes of sections  12  and  14  each preferably include a finned construction, including, but not limited to, multiple fins (not illustrated in  FIG. 1 ) running parallel to the longitudinal axis of the heat exchanger tube and extending outwardly in a radial fashion, as is known in the art. 
     Evaporating section  12  features heat exchanger tubes  16   a - 16   d , which receive cryogenic liquid from inlet header  18 . The cryogenic liquid is warmed by ambient air acting on the exterior surfaces and fins (if present) of the heat exchanger tubes of section  12  and is vaporized so that vapor exits section  12  via the outlet header  20 . The vapor travels to superheating section  14  via line  22  and through the series heat exchanger tubes  24   a - 24   d  and is superheated, again using heat supplied by ambient air. 
     The number and configuration of the heat exchanger tubes presented in  FIG. 1  are presented as examples only. As another example only, the ambient air vaporizer may include a single heat exchanger tube with an inlet end portion that serves as the evaporating section and an outlet end portion that serves as the superheating section, or a number of such heat exchanger tubes. 
     An icephobic/waterphobic treatment is applied to the surfaces and fins (if present) of the heat exchanger tubes of evaporating section  12  and superheating section  14 . The icephobic/waterphobic treatment may include, as examples only, a paint coating (super hydrophobic coating, nano layers, sol-gel, etc.), laser engraving, anodizing and/or a mechanical treatment (during extrusion of the heat exchanger profiles or after). The surface treatment(s) may be applied to both the fins and/or any other exterior surface(s) of the heat exchanger tube(s). In addition, the icephobic/waterphobic treatment applied to the evaporating section may be the same as or different from the icephobic/waterphobic treatment applied to the superheating section. In an alternative embodiment, the icephobic/waterphobic treatment may be applied to only one of the sections. 
     Although the surface treatment becomes ineffective once an ambient air vaporizer is covered with ice, and also in the evaporating section of the vaporizer (which is colder than the superheating section of the vaporizer), due to a low temperature absorption mechanism, the icephobic/waterphobic surface treatment may be advantageous for at least two reasons: 
     (1) It considerably slows down ice creation in the superheating section of the vaporizer. The fluid in the ambient air vaporizer undergoes a gradual heating from its liquid temperature to a temperature that is only few degrees lower than that of the ambient temperature. A large portion of the vaporizer thus experiences relatively warm temperatures at which the treatment is effective. 
     (2) It facilitates the ice shedding during the defrosting period. When ice melts in one section of the vaporizer, then this exposed section receives more heat from the surrounding air and the tubes longitudinally conduct this heat to the ice covered sections. The surface treatment makes the ice shedding easier. 
     As an example only, If the water freezing temperature (or temperature at which ice starts to stick to the vaporizer surface) can be reduced from 0.01° C. to minus 10° C., then the vaporizer capacity for 24 hour operation at 20° C./75% RH can be increased by about 12%. 
     Data is presented in  FIGS. 2 and 3  for an ice sticking temperature set to minus 10° C. and 100 Nm 3 /h of nitrogen passing through vaporizer with an outer surface of 36 m 2  (vaporizer type SG50HF). Each plot is for the number of hours listed in the keys of  FIGS. 2 and 3 . Outlet temperature in 24 hours is 7.37° C. 
     Data is presented in  FIGS. 4 and 5  for an ice sticking temperature set to minus 10° C. and 111.7 Nm 3 /h of nitrogen passing through vaporizer with an outer surface of 36 m 2  (vaporizer type SG50HF). Each plot is for the number of hours listed in the keys of  FIGS. 4 and 5 . Outlet temperature in 24 hours is 7.37° C. 
     While the preferred embodiments of the disclosure have been shown and described, it will be apparent to those skilled in the art that changes and modifications may be made therein without departing from the spirit of the disclosure, the scope of which is defined by the following claims.