Abstract:
A method of cleaning one or more electrodes ( 30 ) of an electrolytic chlorinator ( 10 ). The electrodes are immersed in water within a chamber ( 120 ). The method includes the steps of substantially stopping water flow through the chamber; supplying a volume of cleaning agent into the chamber; and agitating the water and the cleaning agent within the chamber to form a cleaning agent water mixture and to bring the cleaning agent water mixture into intimate contact with the one or more electrodes thereby cleaning the electrodes. According to preferred forms of the method, the agitation step includes activating the electrodes to liberate hydrogen and oxygen bubbles. The invention also provides an electrical driver ( 200 ) for controlling cleaning of the electrodes in accordance with the method, and an electrolytic chlorinator ( 10 ) including an agitator for agitating the water and the cleaning agent within the chamber to form a cleaning agent water mixture and to bring the cleaning agent water mixture into intimate contact with the one or more electrodes thereby cleaning the electrodes. The invention also provides an electrolytic chlorinator for having a housing ( 50 ) defining a chamber, an inlet ( 110 A) for water to flow into the chamber, and an outlet ( 110 B) for water to flow out of the chamber. Spaced electrodes ( 30 ) are arranged within the chamber for receiving power from a DC power supply to electrolyse the water. A cleaning agent retainer ( 160 ) is located within the chamber for preventing cleaning agent sinking from the chamber.

Description:
FIELD OF THE INVENTION 
       [0001]    The invention relates to electrolytic chlorinators. Certain electrolytic chlorinators are used to sterilize pool water. 
         [0002]    Throughout this specification, the term ‘pool’ includes in its ambit any kind of confined water body in which humans can be immersed, including spas, swim spas and Japanese-style immersion tubs. 
       BACKGROUND OF THE INVENTION 
       [0003]    Mono-polarity electrolytic chlorinators work by passing a direct electric current through a stream of the pool water. The electric current has the effect of temporarily converting anions (predominantly chlorine ions) associated with dissolved salts back into their elemental state. In this way elemental chlorine is intimately contacted with the water which has the effect of sanitising the water by killing undesirable biological entities. The electrodes are driven at a voltage such that the products of electrolysis are readily reabsorbed into the water. 
         [0004]    An unfortunate consequence of this electrolytic reaction is the conversion of cations (predominately calcium ions) back to their elemental state. This can cause calcium build-up on the cathode which creates an impedance that reduces the performance of the electrolytic cell. 
         [0005]    ‘Bipolarity’ chlorinators address the problem of calcium build-up by driving an alternating electric current between the electrodes. This is effective in reducing the calcium build-up but has serious drawbacks including the rapid erosion of the electrodes. This problem has in turn been addressed by expensive coatings seeking to reduce the erosion of the electrodes. The net result is that commercial ‘bipolarity’ chlorinators whilst effectively avoiding the problems associated with calcium build-up are relatively expensive and still have a less than satisfactory electrode life. 
         [0006]    Mono-polarity chlorinators can be operated effectively by periodically cleaning off the accumulated calcium. The usual approach involves removing the electrodes from the chamber and manually immersing the electrodes in a hydrochloric acid solution. This manual cleaning operation is both labour intensive and hazardous. The electrodes typically require cleaning on a weekly basis. The hydrochloric acid can burn the skin and is highly toxic. 
         [0007]    A conventional form of mono-polarity chlorinator includes electrodes packaged within a housing defining a chamber. The housing includes an inlet and an outlet for water flow through the chamber. The inlet and the outlet both project downwardly from the chamber so that the chamber is upwardly closed. This arrangement is a precaution in case of a failure resulting in the production of hydrogen and oxygen. By having the electrodes mounted in an upwardly closed chamber any hydrogen and oxygen so produced is readily captured and detected such that safety interlocks can deactivate the electrodes. 
         [0008]    International Patent Application No. WO 96/11166 describes one attempt to address the difficulties of cleaning the electrodes of such mono-polarity chlorinators. The international application describes cleaning the electrodes in situ by periodically stopping the water flow through the chamber and injecting a quantity of acid into the interior of the chamber. This arrangement has been found to be less than satisfactory in cleaning the electrodes. 
         [0009]    International Patent Application No. WO 2005/033015 describes an acid free approach to cleaning the electrodes. An ultrasonic transducer is used to cause ultrasonic vibrations throughout the liquid contained in the housing. 
         [0010]    It is an object of the invention to provide an improved chlorinator or at least provide an alternative in the market. 
       SUMMARY OF THE INVENTION 
       [0011]    The applicant has realised that the unsatisfactory results obtained with arrangements such as that shown in International Patent Application No. WO 96/11166 are related to the acid failing to satisfactorily contact the electrodes. Experiments using tracer dyes have shown that the injected acid is relatively dense and rather than mixing with the water in the chamber and intimately contacting the electrodes, most of the acid simply sinks and escapes from the chamber via either the inlet or the outlet without effectively acting on the calcium deposits. 
         [0012]    Accordingly a first aspect of the invention relates to an electrolytic chlorinator having cleaning agent retention means for preventing the acid, or other cleaning agent, sinking from the chamber. A second aspect of the invention relates to agitating the water and the cleaning agent within the chamber to form a cleaning agent water mixture and to bring the cleaning agent water mixture into intimate contact with the electrodes thereby cleaning the electrodes. 
         [0013]    In the first aspect of the invention there is provided an electrolytic chlorinator having:
       a housing defining a chamber, an inlet for water to flow into the chamber, an outlet for water to flow out of the chamber;   spaced electrodes arranged within the chamber for receiving power from a DC power supply to electrolyse the water; and   cleaning agent retention means within the chamber for preventing cleaning agent sinking from the chamber.       
 
         [0017]    The chlorinator may include a dedicated cleaning agent inlet. The cleaning agent retention means preferably includes an upwardly open receptacle, such as a trough, and is most preferably positioned at least approximately vertically downwardly from the cleaning agent inlet for receiving cleaning agent sinking from the cleaning agent inlet. The cleaning agent inlet may be an aperture in a wall portion partly defining the chamber and from which the cleaning agent retention means extends. The receptacle forming the cleaning agent retention means is preferably formed by an integrally formed portion attachable to the wall portion. The electrodes are preferably connectable to a DC power supply via apertures in the wall portion. The wall portion is preferably removable from a main body of the housing, the main body predominantly defining the housing. 
         [0018]    The cleaning agent retention means may include at least one closure for selectively substantially closing one or both of the inlet and the outlet. The or each closure is preferably a valve, most preferably a non-return valve. The valve may be biased to a closed position. Preferably each of the inlet and the outlet is provided with a respective closure. The closures may include like components, the components of each closure being differently arranged to respectively suit the inlet and the outlet. 
         [0019]    The chlorinator advantageously includes agitation means for agitating the cleaning agent and water within the chamber to remove cleaning agent from the cleaning agent retention means and to mix the cleaning agent and water within the chamber to form a cleaning agent water mixture and to bring the cleaning agent water mixture into intimate contact with the electrodes for cleaning the electrodes. Of course mixing does occur when the cleaning agent is received within the chamber via the cleaning agent inlet, but improved results have been achieved by providing agitation means. 
         [0020]    In either aspect the agitation means preferably includes the electrodes and an electrical driver including the DC power supply and a controller, the DC power supply being operatively connectable to the electrodes, the controller being configured to control the DC power supply to drive the electrodes, when flow through the chamber is substantially stopped, to liberate hydrogen and oxygen bubbles to agitate the water and the cleaning agent within the chamber. Most preferably the chlorinator includes an electrical driver in accordance with the second aspect of the invention. The controller may be operatively connectable to a pump for driving water through the chamber and configured to in use substantially stop the pump. 
         [0021]    The chlorinator may include a cleaning agent supply for supplying cleaning agent to the chamber. 
         [0022]    Preferably the inlet and the outlet are arranged in an in use lower portion of the chamber. Most preferably the inlet and the outlet are arranged to in use downwardly open from the chamber. 
         [0023]    In the second aspect of the invention there is provided a method of cleaning one or more electrodes of an electrolytic chlorinator wherein the electrodes are immersed in water within a chamber, the method including the steps of:
       substantially stopping water flow through the chamber;   supplying a volume of cleaning agent into the chamber; and   agitating the water and the cleaning agent within the chamber to form a cleaning agent water mixture and to bring the cleaning agent water mixture into intimate contact with the one or more electrodes thereby cleaning the electrodes.       
 
         [0027]    The cleaning agent is preferably injected into the chamber via a cleaning agent inlet. 
         [0028]    The agitation step preferably lasts for between 2 and 20 seconds, most preferably around 5 seconds and most preferably includes activating the electrodes to liberate hydrogen and oxygen bubbles. 
         [0029]    There is preferably a delay between the supply of cleaning agent and the agitation step. Preferably the delay is between 1 and 10 minutes, and most preferably is around 5 minutes. According to preferred forms of the invention, at least most of the cleaning agent so supplied is held in cleaning agent retention means within the chamber during the delay. 
         [0030]    Advantageously the method may include restarting the water flow through the chamber 1 to 10 minutes, and preferably about 5 minutes, after the agitation step. 
         [0031]    The cleaning agent may be an acid, and is preferably hydrochloric acid and most preferably is hydrochloric acid at a strength of about 30 percent prior to supply. 
         [0032]    The one or more electrodes may be cleaned by removing deposits on the electrodes that are the product of electrolysis. 
         [0033]    The second aspect of the invention also provides an electrolytic chlorinator having:
       a housing defining a chamber, an inlet for water to flow into the chamber, and an outlet for water to flow out of the chamber;   spaced electrodes arranged within the chamber for receiving power from a DC power supply to electrolyse the water; and   agitation means for mixing cleaning agent and water within the chamber to form a cleaning agent water mixture and to bring the cleaning agent water mixture into intimate contact with the electrodes for cleaning the electrodes.       
 
         [0037]    The second aspect of the invention also provides an electrical driver for driving an electrolytic chlorinator:
       the electrolytic chlorinator including spaced electrodes within a chamber;   the electrical driver including a DC power supply, for driving the electrodes, and a controller;   the controller being configured to control:   (i) the DC power supply,   (ii) a cleaning agent supply for supplying cleaning agent to the chamber, and   (iii) a pump for pumping water through the chamber;   to clean the electrodes in accordance with the method of the second aspect of the invention.       
 
         [0045]    The DC power supply is preferably configured to receive an AC mains supply and to convert power received therefrom to DC. For this purpose the DC power supply may include a transformer and a rectifier. The DC power supply preferably produces about 9 volts in the range of 20-25 amps DC current. 
         [0046]    The electrical driver may include a pump for pumping cleaning agent from the cleaning supply to the chamber. Desirably the components of the electrical driver may be mechanically joined, or packaged within a common housing, for sale as a single unit. 
         [0047]    Preferably the controller includes, or is connectable to, an interface and is configured to receive user input from the interface and to vary cleaning cycle parameters in response to the user input. Preferred forms of the controller include default cleaning cycle parameters. It is desirable that a user should be able to adjust the frequency of cleaning. 
         [0048]    The various aspects of the invention are complementary and each aspect may incorporate the features of the other aspects. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0049]    The figures illustrate electrolytic chlorinators according to preferred forms of the invention. 
           [0050]      FIG. 1  is a perspective view of an electrolytic chlorinator embodying various aspects of the invention; 
           [0051]      FIG. 2  is a perspective view of an inner end of an end plug; 
           [0052]      FIG. 3  is a perspective view of an outer end of an end plug; 
           [0053]      FIG. 4  is a partial vertical axial cross section view of the electrolytic chlorinator; 
           [0054]      FIG. 5  is an exploded view of the electrolytic chlorinator; 
           [0055]      FIG. 6  is an outer end view of the end plug; 
           [0056]      FIG. 7  is a side view of the end plug; 
           [0057]      FIG. 8  is horizontal axial cross section view of the end plug on the line C-C shown in  FIG. 6 ; 
           [0058]      FIG. 9  is an inner end view of the end plug; 
           [0059]      FIG. 10  is a partial vertical axial cross section view of the end plug on the line E-E in  FIG. 9 ; 
           [0060]      FIG. 11  is a schematic representation of the main body, the control means and the cleaning agent supply; and 
           [0061]      FIG. 12  is a vertical cross section view of an electrolytic chlorinator according to an alternative embodiment of the invention. 
       
    
    
     DETAILED DESCRIPTION OF THE EMBODIMENTS 
       [0062]    The electrolytic chlorinator  10  includes a main body  50  having a predominantly cylindrical form. The body  50  has a closed domed end  220  and an open end  230  which has an external thread  231  ( FIG. 5 ) about its outer periphery. An end plug  20  is receivable within the open end  230  of the main body  50  to define a chamber  120  within the main body  50 . In its operative position, main body  50  is mounted with its central axis horizontal. 
         [0063]    Inlet  110 A and outlet  110 B depend downwardly from the main body  50  at axially spaced locations in a vertical plane that includes the central axis of the main body  50 . The inlet  110 A and the outlet  110 B each include a short vertical cylindrical tubular body extending to a downwardly open end having an external thread about its periphery. As best illustrated in  FIG. 5 , each of the inlet  110 A and the outlet  110 B is adapted to sealingly communicate with a respective pipe end portion  80 . Each pipe end portion  80  includes an outwardly extending flange about its periphery. A respective collar  90  is received about each pipe end portion  80  and threadingly engages with the external thread about the periphery of the respective inlet/outlet  110 A,  110 B to compress an annular gasket  70  between the flange of the respective pipe end portion  80  and a respective end face of the inlet/outlet  110 A,  110 B. 
         [0064]    The end plug  20  is predominantly in the form of a hollow cylinder having a diameter slightly larger than its length and is in situ concentrically aligned with and received within the main body  50 . An end  21  of the end plug  20  is closed by an integral wall  190  which in this embodiment is substantially planar and perpendicular to the central axis of the end plug  20 . This closed end  21  is in use received within the main body  50 . The other end  22  of end plug  20  is open and in use projects outwardly from the main body  50 . 
         [0065]    As best illustrated in  FIG. 4  the end plug  20  includes an O-ring groove  180  in its outer cylindrical surface in which in use an O-ring  40  is received and forms a piston seal against the internal cylindrical surface of the main body  50 . The end plug  20  includes an outwardly extending peripheral flange  240  which is in use clamped against an end face of the main body  50  by the threading engagement of a collar  100  with the external thread  231  of the main body  50 . 
         [0066]    The end plug  20  includes electrode apertures  130  passing through the wall  190  at in use vertically and horizontally spaced locations. Electrodes  30  are mechanically supported in cantilever fashion by fasteners  270  (see  FIG. 4 ) passing through electrode apertures  130 . The electrodes  30  are electrically communicated with electrical driver  200  (see  FIG. 11 ) by electrical wires  280  connecting to portions of the fasteners  270  projecting outwardly beyond the electrode apertures  130 . 
         [0067]    The wall  190  of end plug  20  also includes an acid inlet  140  in the form of a cylindrical aperture having an internal thread as illustrated. In use gland  170  is received within and threadingly engaged with the internal thread of the acid inlet  140 . Through the gland  170  acid injection means (not shown) are provided for injecting acid into the chamber  120 . As illustrated in  FIG. 6 , the acid inlet  140  is positioned toward one side of the end plug  20 . An inner end of the acid inlet  140  is closed but for a small aperture  141  (see  FIG. 8 ). The aperture  141  is preferably drilled thereby allowing a common end plug moulding to be used for applications that do not require the acid inlet. 
         [0068]    As best illustrated in  FIG. 4  the end plug  20  includes cleaning agent retention means in the form of trough  160  extending from wall  190  to in use project into the chamber  120  and underlie the acid inlet  140 . The trough member  161  is an integrally formed piece attachable to the wall  190 , which is also integrally formed. The trough member  161  includes an upright wall portion  162  and a horizontal wall portion  260  extending from the wall portion  162  to an upright end wall  250 . The plug  20  includes an outwardly open groove  191  in which the wall portion  162  is frictionally fitted so that when the plug  20  is received in the body  50  the trough member  161  is captured and held in place. Ribs  163  about an outer surface of the wall portion  260  contact an inner surface of the body  50  to locate the trough member  161  during and after assembly. The trough  160  is defined by a wall portion  260  extending from the wall  190  to a short upright end wall  250 . Viewed from chamber  120 , the wall portion  260  has an arcuate cross-section which is concentric with and closely fits within the main body  50 . The end wall  250  is planar and parallel to the wall  190 . The end wall  250  terminates at a height of about 13 mm above the lower extent of the wall portion  260  being the maximum permissible height to avoid fouling the electrodes  30  (see  FIG. 4 ). In this embodiment the trough  160  has a capacity of about 15 mL. 
         [0069]    At one side of the trough  160  a guide  290  is formed by a small portion of the wall portion  260  extending 5 mm above the upper extent of the end wall  250 . As illustrated the other side of the wall portion  260  terminates at the same height as the end wall  250 . The guide  290  is arranged toward the same side of the end plug  20  as the acid inlet  140  to assist in capturing within the trough  160  a greater portion of the injected acid. 
         [0070]      FIG. 6  illustrates the layout of the electrode apertures  130  and the acid inlet  140  across a face of the wall  190  which is in use disposed outwardly from the chamber  120 . A sensor aperture  300  extends through the wall  190  and in use receives a sensor for detecting low salt levels or no water conditions. 
         [0071]    The electrodes  30  are made up of multiple, in this case seven, parallel spaced rectangular mesh sheets  31 . In use the mesh sheets  31  are vertically orientated. Sheets  31  are electrically connected to a DC power supply  310  (described below) so as to define two sets of interleaved sheets of differing polarity. The sheets  31  are held in relative disposition and mutually isolated by spacers  32 . 
         [0072]    In normal operation water flows into the chamber  120  via the inlet  110 A and flows out via the outlet  1108 . The illustrated electrolytic chlorinator  10  is operated as a mono-polarity chlorinator, i.e. a direct current from the DC power supply  310  is passed through the water between the electrodes  30 . 
         [0073]    As best shown in  FIG. 11 , the electrical driver  200  includes the DC power supply  310  and control means  320  and is operatively connected to an acid supply  210 , to the electrodes  30  and to a pool pump (not shown). The control means  320  includes a programmable microprocessor programmed to operate the chlorinator including to actuate the steps of a cleaning cycle (described below) and is thereby configured to control cleaning of the electrodes. In this embodiment the electrical driver  200  also includes an acid pump  340  which includes mechanical pump components  341 . The DC power supply  310 , the control means  320  and electrical pump components (not shown) are packaged in a common housing  201 . The mechanical pump components  341  are mechanically joined to an underside of the housing  201  for the electrical driver  200 , including housing  201  and mechanical pump components  341 , to be sold as a single unit. 
         [0074]    During the cleaning cycle the control means  320  is operative to deactivate the pool pump to stop the water flow through the chamber  120 . The control means  320  then activates an acid pump  340  for approximately 20 seconds to draw from the acid supply  210  and supply to the chamber  120  via the acid injection means about 20 ml of hydrochloric acid of 30% strength. A portion of the acid mixes with the water and immediately begins to act on any deposits on the electrodes. However, because the acid is denser than the water, most of the acid sinks rapidly and accumulates in the trough  160 . Thereafter the electrolytic chlorinator  10  remains dormant for approximately five minutes for the mixed portion of the acid to act on any deposits on the electrodes  30 . The electrodes  30  are then energised for five seconds by DC power supply  310  under the control of the control means  320 . This activation of the electrodes  30  in the absence of water flow results in electrolysis of the water which causes bubbles of hydrogen and oxygen to form on respective electrodes. As further bubbles form, bubbles are liberated to rise through the water to create an effective circulation and agitation of the water and acid within the chamber  120 . The electrodes  30  and electrical driver  200 , including the DC power supply  310  and control means  320 , thereby form the agitation means in this embodiment. 
         [0075]    It has been observed that the rising of bubbles from the electrodes  30  generates water flow in the area that it is needed most, i.e. immediately adjacent the surfaces of the electrodes  30 . This water flow within the chamber  120  defines a recirculating pattern including flow upwardly in line with the moving bubbles from electrodes  30  and downwardly along the vertical wall  190 . This vertically downward flow along the wall  190  impinges on the trough  160  and thus entrains the acid accumulated therein to form an acid water mixture within the chamber  120 . The recirculating pattern is completed by the water including entrained acid being upwardly drawn through the electrodes  30 . 
         [0076]    After the five second activation of the electrodes the chlorinator is again allowed to sit dormant for a further five minutes. During this period the acid water mixture within the chamber  120  is able to act on the deposits on the cathode surfaces. The pool pump (not shown) is then reactivated by the control means  320 . The acid water mixture, including dissolved calcium, is thus purged from the chamber  120  and flows back to the pool (not shown) via the outlet  110 B. The acid water mixture is of course of lesser strength than the injected acid and in transit to and upon arrival within the pool is rapidly dispersed and thus does not present a safety hazard. 
         [0077]    The electrical driver includes an interface  330  including an LCD display and keys by which a user may vary the operation of the control means  320 . Although other variations are possible, in this embodiment the control means  320  includes a default setting. A user can vary the default setting to change the frequency of cleaning to suit local conditions. For example, in the case of ‘hard’ water more frequent cleaning may be required. On the other hand, with soft water, the frequency of cleaning can be reduced thereby conserving acid and extending the electrode life. 
         [0078]    It is also envisaged that the control means be cooperable with, or indeed formed by, a computer, such as a PC, in which case the computer may form the interface. 
         [0079]      FIG. 12  illustrates an alternative embodiment of the invention including non-return valves  380 A and  380 B respectively mounted in inlet  110 A′ and outlet  110 B′. The valves  380 A, 380 B constitute selectively openable closures for preventing cleaning agent sinking from the chamber  120 .  FIG. 12  shows the valves in their closed position. 
         [0080]    Each valve  380 A,  380 B includes a cylindrical tubular body  382  spanned by set of radial spokes  384  at each end. Openings (not shown) between the spokes  384  allow water to flow through the valve when the valve is open. 
         [0081]    The body  382  of each valve  380 A,  380 B is co-axially aligned with the respective inlet  110 A′ or outlet  110 B′ in which it is mounted. An exterior of the each body  382  includes a tapered portion  396  which nests within a complementary tapered portion about the interior of the respective inlet or outlet. 
         [0082]    Each body  382  includes by a groove  392  extending circumferentially about its exterior and positioned axially between a thicker end of the tapered portion  396  and a peripheral flange  398  which extends circumferentially about the exterior of the body  382 . An O-ring seal  394  is carried in each groove  392  to bear against, and form a piston seal with, the interior of the inlet or outlet. 
         [0083]    The pipe end portions  80 , are held in place by collars  90  as in the previously described embodiment. Each pipe end portion overlies a respective peripheral flange  398  to retain a respective valve. 
         [0084]    The spokes  384  within each set of spokes converge to define a respective central hub  391  and are shaped to present a respective shallow conical surface to an interior of the respective body  382 . Within each valve  380 , a shaft  390  extends axially from the central hub  391  at one end of the body  382  to the central hub  391  at the other end. 
         [0085]    Each shaft  390  carries a spacer  386  and a valve member in the form of a silicon (or rubber) ‘flap’ 388 within the interior of the valve body  382 . The flap  388  is conically domed and resiliently flexible so as to be biased to the illustrated closed position wherein the flap  388  overlies, and is seated against, a shallow conical surface defined by a set of spokes  384  to close the openings between the spokes and thereby stop water flow through the valve. 
         [0086]    The valves are arranged so that pressure driving fluid in a ‘reverse flow’ direction (i.e. inward, toward the chamber, via the outlet  110 B′ and outward, away from the chamber  120 , via the inlet  110 A′) will tend to drive the valves to the closed position to prevent such flow; and that, conversely, flow in the ‘forward direction’ will tend to lift flaps  388  away from the spokes  384  to open the valves so that such flow is permitted. 
         [0087]    In operation of the chlorinator, the pool pump drives the water in the forward direction and thus the valves remain open. During a cleaning cycle, when the pump is stopped the valves  380 A,  380 B return to the closed position under there own bias and thus operate to prevent cleaning agent sinking from the chamber  120 . 
         [0088]    As illustrated the valves  380 A and  380 B include like components (body  382 , spokes  384 , shaft  390 , flap  388  and spacer  386 ). The components of each valve are arranged differently to respectively suit operation within the inlet  110 A′ and the outlet  110 B′. 
         [0089]    Within valve  380 A, which is mounted within the inlet  110 A′, the flap  388  is mounted adjacent the spokes  384  at the outer end of the body  382  (i.e. the end furthest from the chamber  120 ). The flap  388  is held in place by spacer  386  spacing the flap  388  from the hub  391  at the inner end of the valve  380 A. As such an inward flow (i.e. flow toward the chamber  120 ) lifts flap  388 . 
         [0090]    Within valve  380 B, which is mounted within the outlet  110 B′, the relative positions of the flap  388  and the spacer  386  is reversed whereby inward flow is prevented. 
         [0091]    The use of common components within the valves of course has advantages including improved economies of scale and stock control. 
         [0092]    It will be understood that the invention disclosed and defined in this specification extends to all alternative combinations of two or more of the individual features mentioned or evident from the text or drawings. All of these different combinations constitute various alternative aspects of the invention.