Abstract:
A vaporizer provides enhanced removal of non-vaporizable components from a liquid to be vaporized. Flow of vaporizing fluid through the vaporizer follows a circuitous path to allow the non-vaporizable components to adhere to surfaces in the vaporizer rather than leave in the vaporized flow.

Description:
FIELD OF THE INVENTION  
         [0001]    The present invention relates to vaporizers and methods of vaporizing, and more particularly to vaporizers for chemical vapor sterilization systems and for a method of vaporizing with sterilants.  
         BACKGROUND  
         [0002]    Vapor based chemical sterilization systems are a popular alternative to steam sterilization. They typically allow sterilization at lower temperatures than is possible with steam, thereby allowing sterilization of articles sensitive to high temperatures. Several such systems are commercially available, such as the STERRAD Brand hydrogen peroxide gas/plasma sterilization system.  
           [0003]    In this system, a sterilization chamber is brought to low pressures, approximately one Torr, and liquid hydrogen peroxide is admitted into the chamber and vaporized into the low pressure. The hydrogen peroxide vapor diffuses to articles placed within the chamber. After a time, an electromagnetic field or other means is employed to ignite a plasma of the hydrogen peroxide vapor and after the plasma inducing field is removed, the constituents reassemble to form oxygen and water. Such systems are more fully described in U.S. Pat. Nos. 4,643,876 and 4,756,882 which are incorporated herein by reference.  
           [0004]    Solutions of hydrogen peroxide and other liquid sterilants typically contain non-vaporizable constituents; for instance, in a typical 59% concentration hydrogen peroxide solution, trace quantities of chemicals such as transition metal salts and organic free radical scavengers, are present to stabilize the liquid solution. Upon vaporization of the hydrogen peroxide solution, these chemicals are left as solid particulates. If no effort is made to separate and collect these constituents, they may become deposited upon items in the sterilization chamber. Most of these constituents are harmless, and their presence is merely unsightly and/or perhaps provides a false impression that these sterilization processes were not complete. However, in some sterilization processes, these constitutes may either be harmful to the instruments and to the patient. Accordingly, it is desirable to remove such constituents prior to releasing the vaporized hydrogen peroxide or other sterilant to the sterilization chamber  
           [0005]    U.S. Pat. No. 6,106,772, which is incorporated herein by reference, by Kohler and Williams, addresses this problem by providing an impingement plate outside of the vaporizer in the chamber upon which the stream of hydrogen peroxide which is being vaporized impinges prior to the contacting the devices or load to be sterilized in the sterilization chamber. In this fashion, a portion of the non-vaporizable constituents adheres to the plate rather than depositing onto the load in the sterilization chamber. While such system provides a marked improvement over no control of non-vaporizable constituents, small amount of such non-vaporizable constituents may still deposit on the load in the sterilization chamber. Accordingly, it would be desirable to provide a system and method for collecting such constituents with a higher degree of efficiency.  
           [0006]    The STERRAD  200  brand hydrogen peroxide/gas plasma type sterilizer employs a vaporizer in which the vaporizing hydrogen peroxide follows a path formed by a series of annular fins in a cylindrical chamber creating a series of torus-like spaces, and wherein each fin has an opening therethrough offset from the opening in the adjacent fins whereby to provide a series of direction changes through the vaporizer.  
         SUMMARY OF THE INVENTION  
         [0007]    The present applicants have discovered that by providing a flow restriction, residence time within the vaporizer is enhanced and the efficiency of the vaporizer is also enhanced.  
           [0008]    A vaporizer according to the present invention vaporizes a sterilant from its liquid phase in a vapor phase sterilization system having a pressure below atmospheric pressure. The vaporizer comprises an inlet to receive the sterilant in its liquid phase, an outlet to discharge the sterilant in its vapor phase, a circuitous path between the inlet and the outlet to collect non-vaporizable ingredients of the sterilant, and a flow restriction.  
           [0009]    Preferably, the circuitous path comprises a plurality of baffles. The circuitous path can comprise an inner tube positioned concentrically within an outer tube, the circuitous path including a first portion in a first direction between the inner tube and the outer tube and a second portion in a second opposite direction through the inner tube. The circuitous path comprises at least one portion in which an effective cross-sectional area of the portion increases by at least 89% to decrease the speed of the sterilant passing therethrough. The circuitous path preferably comprises at least two turns, each of which are at least 90 degrees.  
           [0010]    The flow restriction can comprise an orifice having a cross-sectional area no greater than 44.1% of a cross-sectional area of the circuitous path immediately upstream of the orifice. Preferably, restriction can retain the vapor within the vaporizer for at least 17 milliseconds, and more preferably for at least 26 milliseconds.  
           [0011]    A method of providing a vapor phase sterilant to a sterilization chamber, according to the present invention, comprising the steps of creating temperature and pressure conditions within a vaporizer sufficient to vaporize the sterilant and admitting the sterilant, in its liquid phase, into the vaporizer and vaporizing the sterilant. The sterilant passes through a circuitous path where non-vaporizable components of the sterilant collect on surfaces forming the circuitous path. The sterilant, in its vapor phase, passes through a flow restriction which increases residency within the circuitous path and enhances efficiency of collecting non-vaporizable components. The vaporized sterilant passes out of the vaporizer.  
           [0012]    The non-vaporizable components can comprise stabilizing compounds for the liquid phase of the sterilant. The sterilant can comprise hydrogen peroxide.  
           [0013]    Preferably, at least 50%, and more preferably at least 75%, of the non-vaporizable components are removed from the sterilant prior to the step of passing the sterilant out of the vaporizer. 
       
    
    
     BRIEF DESCRIPTION OF THE EDRAWINGS  
       [0014]    [0014]FIG. 1 is a flow diagram of a sterilization system employing a vaporizer according to the present invention;  
         [0015]    [0015]FIG. 2 is a perspective view of a first embodiment of the vaporizer of FIG. 1;  
         [0016]    [0016]FIG. 3 is an exploded sectional view taken along lines  3 -- 3  of FIG. 2, in which the core is partially removed;  
         [0017]    [0017]FIG. 4 is an exploded sectional view taken along lines  3 -- 3  of FIG. 2, in which the core is not removed;  
         [0018]    [0018]FIG. 5 is a perspective sectional view taken along lines  3 -- 3  of FIG. 2;  
         [0019]    [0019]FIG. 6 is a sectional view of a second embodiment of a vaporizer according to the present invention;  
         [0020]    [0020]FIG. 7 is a sectional view of a third embodiment of a vaporizer according to the present invention;  
         [0021]    [0021]FIG. 8 is a sectional view of a fourth embodiment of a vaporizer according to the present invention;  
         [0022]    [0022]FIG. 9 is a sectional view of an outlet tube of a fifth embodiment of a vaporizer according to the present invention;  
         [0023]    [0023]FIG. 10 is a perspective view of the system of FIG. 1; and  
         [0024]    [0024]FIG. 11 is a side elevation view of the system of FIG. 10.  
     
    
     DESCRIPTION  
       [0025]    [0025]FIG. 1 illustrates in schematic format of a vapor phase sterilization system  10  and components for providing sterilant thereto. Liquid sterilant, such as a 59% solution of hydrogen peroxide and water, is stored within a reservoir  14 . A pump  16  and valve  18  control flow of sterilant  12  from the reservoir  14  to a vaporizer  20 . The vaporizer  20  connects to a sterilization chamber  22  through a manifold  24 . A vacuum pump  26  and a valve  28  provide means for drawing a vacuum on the chamber  22  and a vent valve  30  allow venting of the chamber  22  to atmosphere.  
         [0026]    Before admission of the sterilant  12 , a vacuum is drawn on the chamber  22  by the vacuum pump  26 . Typically, the vacuum is approximately 1 Torr. The vaporizer  20  is fluidly connected to the chamber  22  and is, therefore, effectively at the same pressure initially as the chamber  22  with the exception of the flow induced pressure drops therebetween. Liquid sterilant  12  enters the vaporizer through an inlet  32  and immediately begins vaporizing due to the low pressure and heated vaporizer therein.  
         [0027]    It travels a circuitous path  34  therethrough, such as created by a series of baffles  36  or other flow direction changing objects which provide a plurality of directional changes, thereby allowing the flow of vaporizing sterilant  12  to impinge upon surfaces  38  with the vaporizer  20  as it passes therethrough. Such impingement causes non-vaporizable components  40  in the sterilant  12  to deposit upon these impingement surfaces  38 . As a sterilant  12  exits the vaporizer  20  through its exit  42 , a fairly large proportion of the non-vaporizable components  40  are left adhered to the impingement surfaces  38  within the vaporizer. Thus, as the sterilant  12  travels through the manifold  24  into the chamber  22  it is relatively free of non-vaporizable constituents.  
         [0028]    [0028]FIG. 2 shows one embodiment of the vaporizer  20  according to the present invention. It comprises a housing  44  having a removable panel  46 . The housing  44  fits into a mounting bracket  48 . Threaded fittings  50  on the bracket  48  connect to lugs  52  and  54  on the housing  44  and panel  46  respectively and are held by means of nuts  56 . Handles  58  are provided on the panel  46  for removing the panel.  
         [0029]    Turning also to FIGS. 3 and 4, it can be seen that the entire housing  44  is insulated by a blanket  60 , which may comprise any suitable insulation. An electric heater  62  lies between the blanket  60  and the housing  44 . The spacers  64  between the housing  44  and the mounting bracket  48  also help to reduce heat loss from the housing  44 .  
         [0030]    A core  66  fits within the housing  44 . The core  66  comprises a cylinder  68  having an open end  70  and closed end  72  with a plurality of annular fins  74  extending radially therefrom. The fins  74  extend toward the housing  44  but do not actually touch the housing. A partition  76  having an annular lip  78  attaches to the cylinder closed end  72  and seals against the housing  44  by means of an O-ring  80 . A core heater  82  having a thermostat  84  and thermister  86  attached to the partition  76  to heat the core  66 . An insulating blanket  87  covers the heater  82 . All of this is enclosed by the removable panel  46  so that the core  66  can be easily removed for cleaning.  
         [0031]    The core cylinder  68  fits over an outlet tube  88  which extends into the housing  44 . The outlet tube  88  has an outside diameter slightly smaller than the inside diameter of the core cylinder  68  and has an open end  90  which sits adjacent to but does not abut the cylinder closed end  72 . The tight fit between the outlet tube  88  and core cylinder  68  creates a flow restriction  91 .  
         [0032]    A pair of liquid tubes  92  enter the housing  44  adjacent the partition  76  and are preferably attached through a fitting  94 .  
         [0033]    Turning also now to FIG. 5, a gasket  96  covers distal edges  98  of each of the fins  74  to seal the fins  74  against the housing  44 . A series of openings  100  through the fins adjacent the cylinder  68  are provided and are offset from each other on adjoining fins  74  so that the gases may flow past the fins  74  through the openings  100 , but in doing so make frequent directional changes. The fins  74  create a series of spaces or pockets  102  with an inlet pocket  104  adjacent the liquid tubes  92  and a terminal pocket  106  adjacent the cylinder open end  70 . Liquid entering the vaporizer  20  through the liquid tubes  92  is vaporized and flows along a circuitous path  108  through the openings  100  to the terminal pocket  106  and then into a space  110  between the cylinder  68  and outlet tube  88 . It enters the space  110  through the cylinder open end  70 . Flow then proceeds into the outlet tube  88  through its open end  90 . Along the way, such flow impinges upon many surfaces leaving behind deposits of non-vaporizable components  40 .  
         [0034]    [0034]FIG. 6 illustrates an alternative embodiment of the invention. This embodiment employs an outlet tube  112  having a reducing section  114  at the interface with the housing  44  and which leads to a smaller diameter section  116  within the cylinder  68 . Accordingly, a space  118  between the outlet tube  112  and cylinder  68  has a larger cross-sectional area than in the previous embodiment. This reduces the velocity of the flow in the space  118  and increases residence time so as to allow a higher portion of the non-vaporizable components  40  to come out of the sterilant  12 . The narrow diameter of the small diameter section  116  also enhances this effect as the cross-sectional area in the small diameter section  116  is less than the cross-sectional area in the space  118  or in a space  120  between the cylinder  68  and housing  44  thereby acting as a flow restriction.  
         [0035]    [0035]FIG. 7 illustrates a further embodiment in which an outlet tube  122  does not extend into the housing but has a high area ratio (greater than or equal to 3:1) reducing section  124  leading to a very narrow outlet portion  126  thereby providing a flow restriction. This large flow restriction substantially decreases the velocity in the remainder vaporizer  20  thereby allowing a longer residence time and a higher degree of separation of the non-vaporizable components  40  for a given size of the vaporizer  20 . Alternatively, the size of the vaporizer  20  can be reduced while maintaining the same level of efficiency in removing non-vaporizable components  40 .  
         [0036]    [0036]FIG. 8 illustrates the same concept but employs an orifice  128  rather than a reducing section to provide a flow restriction.  
         [0037]    [0037]FIG. 9 illustrates an outlet tube  130  having an orifice  132  in the middle thereof.  
         [0038]    TABLE 1 illustrates how a flow restriction can enhance the efficiency of the vaporizer  20  in collecting the non-vaporizable components  40 . It illustrates the performance difference of a vaporizer configured according to that shown in FIG. 6 having two different size small diameter sections  116 . By reducing the diameter by 50%, the collection efficiency was increased from 76% to 100%.  
                                                                                                 TABLE 1                       FLUID VELOCITY AND       RESIDENCE TIME IN THE VAPORIZER AT 70° C. WITH       TWO DIFFERENT SIZES OF OUTLET TUBES INJECTION       OF 15 ML OF 59 WT% HYDROGEN PEROXIDE SOLUTION                   0.75 in OD Tube                Cross   Average   Stabilizer   Residence       Space   Sectional   Velocity   Collected,   Time,       Number   Area in 2     ft/sec   g   milliseconds               120   1.8   158   2.67   26       118   4.7    59   0.06       116   0.4   747   0                    Total Collected, g   2.73       Measured from Solution, g   2.60       % Recovered   100            Ratio of (space 116/space 118) = 0.4/4.7 = 8.5%       Ratio of (space 118-space 120)/(space 120) = 161%                    1.5 in OD Tube                Cross   Average   Stabilizer   Residence       Space   Sectional   Velocity   Collected,   Time,       Number   Area in 2     ft/sec   g   milliseconds               120   1.8   245   1.96   17       118   3.4   127   0.02       116   1.5   286   0                    Total Collected, g   1.98       Measured from Solution, g   2.60       % Recovered   76 (75% in Space 120)            Ratio of (space 116/space 118) = 1.5/3.4 = 44.1%       Ratio of (space 118-space 120)/(space 120) = 89%          
 
         [0039]    The residence time of vapor retained in the vaporizer can be calculated according to the following equations:  
         t=(L/v ) ×1000,  
         v=(W ×144) / (ρ×A),  
         [0040]    where  
         [0041]    t=calculated residence time, milliseconds,  
         [0042]    L=measured length of flow path, ft, p 1  v=calculated vapor velocity, ft/sec,  
         [0043]    W=measured mass flow rate, lb/sec,  
         [0044]    ρ=calculated vapor density, (P ×MW)/(R ×T), lb/ft 3 ,  
         [0045]    P=measured upstream pressure in vaporizer, psia,  
         [0046]    MW=calculated vapor molecular weight, g/mole,  
         [0047]    R=gas constant, mmHg-l/mole ° K.,  
         [0048]    T=measured vapor temperature, ° K.,  
         [0049]    A=measured cross sectional area for flow in the vaporizer, in 2 .  
         [0050]    The 17 milliseconds residence time for the 1.5 inches OD tube can be calculated with the follow measured data.  
         [0051]    L=4.1 ft  
         [0052]    W=1.4×10 −3  lb/sec  
         [0053]    P=0.125 lb f /in 2    
         [0054]    T=343° K.  
         [0055]    A=1.75 in 2    
         [0056]    ρ=(P ×MW) / (R ×T)  
         [0057]    =(0.125 lb f /in 2 ×760 mmHg/atm ×25 g/mole ×28.32 l/ft 3 ) / (14.7 lb f /in 2 −atm ×62.36 mmHg-l/mole ° K. ×343° K. ×454 g/lb)  
         [0058]    =4.7×10 −4  lb/ ft 3    
         [0059]    v=W/(ρA)  
         [0060]    =(1.4×10 −3  lb/sec ×144 in 2  /ft 2  ) / (4.7×10 −4  lb/ ft 3 ×1.75 in 2 )  
         [0061]    =245 ft/sec  
         [0062]    t=(L/v) ×1000  
         [0063]    =4.1 ft ×1000 milliseconds/sec / 245 ft/sec =17 milliseconds  
         [0064]    [0064]FIGS. 10 and 11 illustrate the system  10  with the vaporizer  20  located atop the sterilizer chamber  22  and showing the manifold  24  leading from the vaporizer  20  into various locations into the sterilization chamber.  
         [0065]    After a number of cycles, a sufficient amount of non-vaporizable components  40  will become deposited on the components within the vaporizer  20  and it will be desirable to remove these deposits. Preferably, the housing  44  tapers slightly from where the panel  46  attaches to where the outlet tube  88  leaves so that when the panel  46  is removed the core  66  can be slid out of the housing  44  more easily. If it becomes stuck, the nuts  56  can be turned to drive the core  66  out of the housing  44 .  
         [0066]    The invention now being fully described, it will be apparent to one of ordinary skill in the art that many modifications and changes can be made thereto without departing from the spirit or scope of the invention as defined in the following claims.