Abstract:
A bubble trap having a cylindrically symmetrical chamber, a filter covering an outlet and offset from the axis of the chamber, an inlet with a plurality of vertically aligned orifices, the orifices arranged to produce circumferential flow within the chamber. Bubbles in the liquid coalesce on the filter and are removed. A method of removing bubbles is also disclosed.

Description:
The present invention relates to a bubble trap for removing gas bubbles and, in some embodiments, also solids from a flowing liquid. 
     BACKGROUND OF THE INVENTION 
     Bubble traps are commonly used where there is a need to remove gas bubbles from a flowing liquid, for example, in hydraulic circuits or in liquid chromatography. The presence of gas bubbles in the flowing liquid would be detrimental to the action of the functional parts of a hydraulic system or the integrity of the solid phase of a chromatography column. 
     Filters are commonly used where there is a need to remove solids from a flowing liquid, for example, in liquid chromatography. The solid may be, for example, an undissolved component of the chromatography column buffer or a solid present within the liquid sample. This is especially likely if the liquid sample is a biological sample. 
     In U.S. Pat. No. 4,806,135 a bubble trap is described for removal of emboli from blood prior to transfusion. The device allows blood to pass through a filter arranged so that solid components, such as red blood cells, pass through the filter but entrained bubbles do not. This separation is achieved by introducing the blood in a tangential direction relative to the axis of rotation of the chamber. The introduction of the blood is above the level of the filter and with a low velocity so the bubbles rise from the blood as the blood rotates within the body of the bubble trap before reaching a filter below the level of the fluid inlet. The filter can be a wide mesh filter as it is not required to trap bubbles. 
     Frequently there is a need to remove gas bubbles efficiently from a rapidly flowing liquid which U.S. Pat. No. 4,806,135 cannot perform. In other cases the removal of both gas bubbles and solids from a rapidly flowing liquid is required. Typically the removal of both gas bubbles and solid components from a rapidly flowing liquid has necessitated the use of a filter and a bubble trap arranged in series, each independently performing its function. 
     SUMMARY OF THE INVENTION 
     There is therefore a need for simple compact devices that can perform the removal of bubbles, or both bubbles and solids, from a rapidly flowing liquid. 
     The flow of the liquid is directed from the inlet to impinge upon the barrier which causes bubbles present within the flow of the liquid to coalesce on the surface of the barrier and then float to the top of the bubble trap. 
     When the bubble trap of the invention combines the functions of a filter with a bubble trap the barrier takes the form of a filter covering the outlet. The bubble trap may have a plurality of filters and outlets. The filter has openings of a diameter which prevent the passage of bubbles through the filter. The surface tension of the liquid covering the filter openings is quite strong and it requires a considerable trans-filter pressure differential to force the bubbles through the filter openings. The bubbles may then aggregate before they float free from the surface of the filter or be dislodged by the flow of the liquid onto the filter surface. 
     The upper and lower levels of the liquid within the chamber may be regulated by a sensor to be maintained above the top of the filter. 
     The bubble trap is arranged to avoid vortex formation in the liquid as vortex formation could lower the liquid level sufficiently to allow air through the filter. 
     Conveniently the chamber is generally cylindrical in which case vortex formation in the chamber can be avoided by positioning the filter, and optionally filter outlet, off the central axis of the chamber. 
     Preferably the inlet has one or more vertically aligned orifices directed to produce circumferential flow of the liquid. Impingement of the liquid upon the filter also acts to dislodge coalesced bubbles on the filter surface. 
     An inlet with a plurality of orifices provides circumferential flow of the liquid throughout the entire height of the filter such that the liquid impinges more effectively upon the filter and ensures good vertical distribution of liquid over a wide range of flow rates. 
     Conveniently, the one or more orifices are selected from slits, holes and nozzles or any of the types of orifice known to those skilled in the art. 
     Preferably the filter openings are less than 1.2  i m. A typical flow rate for the bubble trap would be 5-40 liters/min. A low liquid residence time is desirable, such a residence time is typically 30-3 seconds. The effective size of the opening is dependent upon several factors, for example, the viscosity of the liquid and the internal pressure of the bubble chamber. 
     Preferably if sterile filtration is required the opening size should be less than 0.22  i m. 
     Preferably the filter may be a removable cartridge filter. The size of the cartridge filter, and of the inlet, reduce the interior volume of the chamber so reducing the stagnant holding volume of the chamber. Such cartridge filters are widely available having a wide range of opening sizes and structural tolerances. Such filter cartridges are available in a number of standard lengths which can be used interchangeably within the limitation of the maximum size of the bubble trap chamber. For example, MILLIGARD™ Standard and MILLIGARD™ Low Protein-Binding Cartridge filters may be used, both manufactured by Millipore Corporation, US. Preferably these filters may be either 4″ or 10″ in height. 
     The presence of the filter in the bubble trap device results in a small interior volume to minimise the stagnant or holding volume. It is capable of sterile operation, if such is required, and has a minimum number of easy-to-produce components. The simple construction reduces the need for complex valving arrangements and associated hardware. The general design of the device is sanitary to enable cleaning of the device with, for example, sanitizing chemicals or steam. The replacement of the cartridge is possible whilst remaining sterile. 
    
    
     IN THE DRAWINGS 
     A preferred embodiment of the invention will now be described with reference to the accompanying drawing, in which: 
     FIG. 1 is a vertical cross-sectional view through a bubble trap in accordance with the invention. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     The bubble trap  10  comprises a cylindrical tube  20  secured between an upper securing plate  22  and a lower securing plate  24 . A gasket  26  is used to produce a fluid tight seal between the cylindrical tube  20  and the upper securing plate  22 . A gasket  26 ′ is used to produce a fluid tight seal between the cylindrical tube  20  and the lower securing plate  24 . The cylindrical tube  20  is preferably made from clear material such as TPX resin, glass or PERSPEX resin to allow the user to view the inside of the cylindrical tube  20 ; or may be made from stainless steel. The securing plates may be made from stainless steel. 
     The upper securing plate  22  and the lower securing plate  24  are held in place by connecting rods  27  each secured by an upper screw  28  and a lower screw  28 ′. In the preferred embodiment there are six connecting rods  27 , two of which are shown in FIG.  1 . 
     A cover  30  having a gas outlet  32  is sealed to the neck  34  of the upper securing plate  22 . The cover  30  is removably attached to the upper securing plate  22  by a clamp (not shown) securing an upper securing plate flange  23  to a cover flange  31 . A gasket  33  is used to produce a fluid tight seal between the cover  30  and the upper securing plate  22 . 
     In the preferred embodiment the neck  34  of the upper securing plate  22  has a large diameter which allows a hand to access the inside of the cylindrical tube  20 . The access may be, for example, to replace a filter cartridge  60 . The gas outlet  32  allows the exit of accumulated gas from the bubble trap  10 . If removal of solids is not required, the filter can be replaced by a barrier to present a surface for coalescence of the bubbles in the fluid flow. 
     Removably attached to the lower securing plate  24  is a base unit  40 . The base unit  40  has an inlet passage  42  and an outlet passage  44 . The base unit  40  is formed to provide an inlet fitting  46  mounted on the upper surface of the base unit  40  and arranged so that the inlet fitting  46  is off the central axis of the cylindrical tube  20  and is not in contact with the inner wall of the cylindrical tube  20 . The inlet fitting  46  has one or more jetting orifices  48 . The jetting orifices may be holes or slots or any means for directing fluid flow. The jetting orifices  48  are arranged so that the liquid flows tangentially in the cylindrical tube  20 . The delivery of the liquid may be pulsatile if a peristaltic pump (not shown) is used. 
     The base unit  40  is removably attached to the lower securing plate  24  by a clamp (not shown) securing a lower securing plate flange  25  to a base unit flange  62 . Sandwiched between the base unit  40  and the lower securing plate is a gasket  64  which makes the assembly fluid tight. 
     The base unit  40  is arranged so that the outlet passage  44  has a filter securing means  50  which allows a filter  60 , in this case a removable cartridge filter, to be secured to the base unit  40 . 
     In the preferred embodiment the filter  60  is off the central axis of the chamber. The filter securing means  50  is positioned so that the filter  60  is not in contact with the inner wall of the cylindrical tube  20 . This position of the filter  60  helps to avoid the formation of a liquid vortex within the cylindrical tube  20 . 
     A sensor (not shown) detects the upper and lower level of the liquid in the cylindrical tube  20  so that the level of the liquid is controlled to be above the filter  60  by controlling venting of accumulated gas from the outlet  32 . The inlet fitting  46  is arranged such that the jetting orifices  48  jet the liquid onto the filter  60  below the top of the filter  60 . 
     The bubble trap  10  has a low stagnant volume due to the arrangement of the inlet fitting  46  and the filter  60  and allows sanitary operation having no dead spaces. An advantage of the present embodiment is that there is only one inlet passage  42 , one gas outlet  32  and one outlet passage  44  consequently the need for complex valving and hardware arrangements is reduced 
     In operation the filter surface  66  does not allow bubbles present in the liquid flow to pass through the filter  60 . The bubbles coalesce on the filter surface  66  and then float to the surface of the liquid, exiting via the gas outlet  32 . Solid material present in the liquid is also filtered by the filter  60 . The present invention thus acts to remove both solids and gas bubbles from a liquid flow. 
     If no filtering is required, a solid flow barrier, optionally with surface texture to encourage bubble formation, can be located in the bubble trap instead of the filter, with the outlet positioned to allow liquid to exit the bubble trap.