WO2007141563A1

WO2007141563A1 - Fluid sterilisation apparatus

Info

Publication number: WO2007141563A1
Application number: PCT/GB2007/050250
Authority: WO
Inventors: Alexander Rollo Spowart
Original assignee: Uv Energy Limited
Priority date: 2006-06-09
Filing date: 2007-05-10
Publication date: 2007-12-13
Also published as: GB0611406D0

Abstract

An apparatus for fluid sterilisation comprises one or more radiant members arranged in series within a housing (70) that is conformed to the topography of the radiant members, defining a fluid conduit on either side of the plane of the radiant members. In one embodiment, the housing comprises a scalloped surface (72) and the radiant members are UV glow discharge tubes.

Description

Fluid Sterilisation Apparatus

The present invention relates to a fluid sterilisation apparatus, with particular utility to UV sterilisation of water .

Cylindrical lamps operating in the glow discharge mode are the basis of modern efficient fluorescent lighting. Such tubes first appeared about 1938. For fluorescent tubes for lighting, a doped glass envelope is employed to stop the escape of harmful UV-C at 254nm from the internal glow discharge.

The history of UV generation and the discoveries of its uses are well known. Johann Ritter discovered UV via its chemical reactions in 1801. Niels Finsen over the period 1860-1904 became the father of UV therapy in medicine following the use of UV in the therapeutic treatment of rickets. The first papers on the lethal bacteriological effectiveness of UV appeared in 1880. Fig. Ia shows the curve of relative biological effectiveness of UV radiation for destroying DNA, where the percentage kill (y-axis) is plotted against the wavelength of radiation (x-axis) . This curve peaks at 265nm, and has a full width half maximum width of about 53nm. This spectrum is now recognised to be essentially identical to the UV absorption spectrum of nucleic acid. In 1857 Siemens reported on filamentary UV creation via charge plates, micro discharges, nanosecond discharges, "Entladung", an approach subseguently to dominate ozone production, and well reviewed by Kogelschatz. By 1932 the Coblentz Congress had defined the three regions of the UV action spectrum.

Now, disinfection of air or water by short wavelength UV principally employs the dominant 254nm line from a noble gas mercury glow discharge tube. For UV sterilisation/treatment purposes using glow discharge tubes the outer light emitting phosphor layer is not present in the cylindrical tube, and the UV simply escapes through a UV transparent guartz/silica envelope .

Medium pressure arc lamps (MPAL's), operating at several atmospheres pressure (as compared with the vacuum pressures of glow discharge tubes) currently dominate the high-volume UV sterilisation industries.

A typical high guality unit of this type is the Heraeus Amba 5639X. This is a compact unit about a foot long, but reguiring heavy copper ballasts and high voltage igniter circuitry, and operating at very high temperatures of around 800 Centrigrade . MPAL's are electrically inefficient in that a IkW unit actually consumes 2kW of electrical power at 5p/kWhr. Such a tube is said to sterilise about 20 cubic metres of water per hour at an electrical cost of about 1000 GBP per annum. These deliver diffuse UV spectra due to the temperature and pressure broadening of their collision spectra, and require massive copper ballasts.

Other long-known methods of UV generation include electrodeless bulbs, where an external microwave generator is used to excite the discharge. This approach was in use for factory lighting by the 1940's. Amalgam and excimer lamps as well as solid state and gas lasers can also be used to generate UV.

In the water purification industry various directives control the degree of sterilisation which must be achieved, for example in Europe the EEC Bathing Waters Standard, EU 1976, sets a limit of 2000FC/100ml, and a further drinking water standard of total inorganic carbon (TOC) of 3mg/l.

Currently, it is many times cheaper for a water utility provider to purify water through a chlorination process, but this brings with it health and safety issues, particularly following inadvertent high chlorine dosing into the network. Because of its high costs, commercial pressure means that UV sterilisation is only generally used when absolutely necessary.

However, if its running costs could be decreased and the electrical efficiency increased, UV sterilisation would become increasingly attractive, because it does not bring with it the health and safety issues to do with monitoring the levels of chlorine in water.

Indeed, as health and safety guidelines are continually being monitored and modified and as the effects of chlorine in the water supply is still an area of active research, a move away from chlorine may even in the future become imperative.

UV generating tube types are defined by their electrical operating characteristics, which in turn are largely determined by the internal gas pressure. All UV generating tubes have to utilise UV transmitting materials because all normal glasses heavily absorb short wave UV radiation. The basic physics and design of light emitting fluorescent tubes, the UV emitting sterilisation version, their calibration and use, are well known per se. However, there is a need for more efficient structures and for more efficient power supply thereof.

According to a first aspect of the present invention there is provided an apparatus for fluid sterilisation, comprising one or more radiant members arranged in series within a housing that comprises a first and a second surface conformed to the topography of the or each radiant member to define a fluid conduit on both sides of the plane of the radiant member or members.

"Sterilisation" in the context of this invention is taken to mean any process that rids a fluid of bacteria or other micro-organisms. A "radiant member" is an object that emits electromagnetic radiation, while the topography of the radiant members is defined as a surface formed to fit with the shape of each individual radiant member.

Preferably, a series of adjacent radiant members within a housing that comprises at least one scalloped surface in a spaced fitted arrangement with the radiant members .

A "scalloped surface" is one that undulates in a fitting arrangement with the radiant members. It may include flat portions provided between each undulated portion .

Preferably, the housing comprises first and second scalloped surfaces, arranged at opposing sides of the radiant members.

Preferably, the housing comprises a fluid inlet and a fluid outlet, arranged such that fluid to be sterilised is flowable from the inlet to the outlet across a longitudinal axis of each radiant member and around each radiant member.

Preferably, the scalloped troughs are interposed between successive radiant members and are shaped so as to increase turbulence in the fluid flow.

Preferably, the or each radiant member comprise a UV glow discharge tube. Preferably, the or each radiant member further comprises a containment member around the UV glow discharge tube.

Preferably, the UV glow discharge tube is removable from the apparatus without affecting the fluid flow.

Preferably, a UV sensor means is provided within each containment member .

Preferably, the housing is formed from one or more extruded members.

Preferably, the or each extruded member comprises aluminium or duralumin.

Preferably, the inside walls of the containment member comprises a coating of titanium dioxide.

Preferably, the diameter of the UV tubes is 15mm and the distance between the tubes and the walls of the extrusion is 6mm.

Preferably, the radiant members are arranged in a substantially planar fashion.

Preferably, the series of adjacent radiant members is held in a vertical arrangement, in use. Preferably, a plurality of columns of adjacent radiant members are served by the same fluid inlet and fluid outlet .

Preferably, each column of radiant members acts as a self-contained conduit for fluid flow.

Preferably, the radiant members are arranged in a curved fashion.

Preferably, the fluid to be sterilised, in use, is water or any other liquid.

Preferably, the apparatus comprises an electronic ballast, said electronic ballast being adaptable to operate at a predetermined frequency, said predetermined frequency being an optimum operation frequency that corresponds to a maximum radiation intensity for varying ballast frequencies at constant tube power .

The ballast is specified by a method of selecting an operating frequency of a ballast for powering a gas discharge lamp, comprising the steps of measuring the intensity of emitted radiation at constant tube power for varying ballast frequencies, and selecting an operating frequency that corresponds to a maximum radiation intensity.

The ballast is operated according to a method of operating a gas discharge lamp comprising operating a ballast at a predetermined selected frequency, said predetermined selected frequency beinq an operatinq frequency that corresponds to a maximum radiation intensity for varyinq ballast frequencies at constant tube power .

Preferably, the operatinq frequency is chosen to be within a ranqe havinq its lower bound at the maximum frequency .

Preferably, the qas discharqe lamp comprises a UV tube operable in the qlow discharqe mode.

Preferably, the operatinq frequency is chosen to be between fifty-five and fifty-eiqht kilohertz.

The present invention will now be described, by way of example only, with reference to the accompanyinq drawinqs, in which:

Fiq. Ia shows a curve of relative bioloqical effectiveness for UV radiation of DNA;

Fiq. Ib shows qeneral UV tube discharqe characteristics, wherein the qlow discharqe operates in the abnormal qlow reqion 16;

Fiq. 2 shows the dominant 254nm peak in the UV emission from a qlow discharqe tube; Fig. 3 shows the UV emissions from a modern medium pressure tube;

Fig. 4 shows the spectral power distributions of a glow discharge;

Fig. 5 shows the spectral power distributions of a medium pressure discharge;

Fig. 6 shows the variation of the luminous efficacy of UV generation at 254nm for varying diameters of glow discharge tubes;

Fig. 7 shows an existing geometry of fluid treatment devices;

Fig. 8 shows a proposed flat plate design for a discharge lamp;

Fig. 9 shows a proposed geometry for a UV sterilisation apparatus according to a first embodiment;

Fig. 10 illustrates design considerations in the structure of the geometry of Fig. 9;

Fig. 11 shows a proposed geometry for a UV sterilisation apparatus according to a second embodiment;

Fig. 12 shows an exploded view of the apparatus of Fig. 11; Fig. 13 shows a further end view of the apparatus of Fig. 11;

Fig. 14 shows a proposed modular sterilisation unit according to a further embodiment; and

Fig. 15 shows an adaptation to the design of the unit of Fig. 14;

Fig. 16 shows a further possible design modification;

Fig. 17 shows an output stage of a ballast circuit;

Fig. 18 shows a graph of the variation of UV intensity with ballast freguency; and

Fig. 19 shows a sample design ballast circuit.

Figure Ib illustrates the well known tube discharge characteristics, with the voltage across the electrodes (in kV) plotted against the current through the tube (in Amperes) . As is well known, the variation of the voltage eguates to different types of discharges, called the Townsend discharge 10, subnormal glow 12, normal glow 14, abnormal glow 16, and finally the arc discharge 18. The usual region for the operation of commercial glow discharge tubes is the abnormal region 16. The dominant 254nm peak in the UV emission from a glow discharge tube is shown in Figure 2, with the UV emissions from a modern medium pressure tube, specifically optimised for sterilisation are shown in Figure 3, where the germicidally efficient 254nm radiation is no longer a single dominant peak.

For comparison, the spectral power distributions of a glow discharge is shown in Figure 4, and the spectral power distributions of a medium pressure discharge is shown in Figure 5.

The present inventor examined a range of low pressure glow discharge tubes with gas fill pressures of around 7 Torr, and it was found the average electrical performance rating was 0.46W per centimetre of tube length. To be commercially useful in, for example, water sterilisation uses, we need an assembly of low pressure glow discharge tubes capable of generating say IkW of UV at 254nm. Clearly, from the above measurements, we need to use low pressure glow discharge tubes about 90cm long, giving an average electrical performance rating of about 75W per tube.

A range of low pressure glow discharge tubes of varying diameter were examined. The luminous efficacy of generating UV at 254nm was measured for each tube diameter by measuring the UV output in the 254nm peak as integrated counts and then normalising this to electrical power consumption in watts, the electrical power consumption having been measured by secondary standard grade instrumentation. The experimental results are shown in Figure 6, where the peak in efficiency can clearly be seen to occur at a tube diameter (egual to the inner diameter of the guartz tube) of around 15mm. Relatively small changes in plasma tube diameter are seen to give substantial reductions in 254nm UV luminous efficacy.

Attention was then turned to the existing geometry of fluid treatment devices. A typical existing geometry is shown in Figure 7. A plurality of UV glow discharge tubes 20 are arranged along the length of a treatment pipe 22. The fluid 24 to be treated flows down the length of the UV tubes 20. There is a need to have turbulent flow to ensure that the fluid reaches the UV tubes, and a degree of electrical overkill is necessary to ensure that full coverage is achieved.

To give a more uniform spread of UV radiation, it is possible to construct a discharge lamp with a flat plate outer surface, rather than a cylindrical outer surface. An example of how this might work is shown in Figure 8, wherein an outer casing 30 of the lamp comprises an upper and lower flat plate surfaces 32, 34, separated by an optimal distance of 15mm in accordance with the results shown in Fig. 6, and with electrodes 36, 38 being provided for the creation of the electric discharge. However, this design does not seem to be a commercially viable, due to the cost of large flat sheets of guartz, which are reguired for formation of the flat plate members. The flat plate design can be provided in the form of a single flat plate, or in a modular, panelled construction. Alternatively or in addition, a stacked structure can be provided, wherein the fluid to be sterilised is caused to flow on both sides of each layer in the stack. A housing (not shown in Fig. 8) can then be placed around the lamp having a planar surfaces above and below the flat plate of the lamp. In this way, the surfaces of the housing are conformed to the topography of the "radiant member" (i.e. the lamp) to define a fluid conduit on either side of the plane of the radiant member.

A similar "flat plate" effect can be achieved by the architecture shown generally in Figure 9. Here, it can be seen that a plurality of discharge tubes 40 are arranged side-by-side within a container 42 comprising an upper and a lower scalloped surface corresponding to the contours of the arranged tubes 40. Fluid to be treated is introduced at a first end 44, and cascades through the geometry in the sense of the direction shown by the arrow, that is, across or perpendicular to the longitudinal axes of successive tubes.

Again, the scalloped surfaces are conformed to the topography of the radiant members (glow discharge tubes in this example) to define a fluid conduit on either side of the plane of the radiant members. To maintain the intensity of the light at a level to ensure sterilisation the flow is confined to a distance of 6 mm from the tube wall. This distance has been experimentally found to be an ideal amount to give good UV sterilisation for the power of UV that is produced by a glow discharge tube.

And to create a commercially acceptable unit, the pressure drop is often reguired to be under 0.5 bar across the complete operating module.

If reguired the distance at which flow is confined could be reduced; however, a substantial reduction would increase pressure drop and may increase the probability of debris lodging in the unit.

To visualise the unit we imagine the lower half of the unit to contain the machined lower reflector profiles for tubes, fluid entrance and exit apertures, and the transverse holes for guartz thimbles, and the upper half assembly to include the upper reflector profile sections. The doubly-contained tubes go across the unit sealed in by appropriate compression gaskets. The top half of the assembly is simply lowered onto the lower half with a gasket all the way around the external perimeter. Electrical connections are external to the tube housing through ceramic connectors. The units can be made from relatively cheap and easily machined materials such as duralumin or other good strength aluminium, because we do not need the high temperature capability of the expensive and difficult to machine stainless steels of current designs. In Fig. 9, the UV tubes 40 are illustrated as being immediately adjacent each other. However, this does not have to be the case, and in some situations it may be desirable to have a spacing between each tube. After DNA is broken down by UV radiation, a small portion of the DNA can sometimes re-combine. The spacing between tubes allows this recombination, so that the recombined DNA is again exposed to the UV radiation of the next tube. Once the recombined DNA is broken down, further recombination is significantly reduced. Thus, a spacing between tubes can in some cases increase the sterilisation effect.

The spacing can be applied to each tube, or a selection thereof from within the array. It is to be understood that when the tubes are spaced further apart, the surface 42 is still considered to be "scalloped" in line with the definition given above.

The air unit will be much smaller due to the number of UV tubes needed, and our design work to date is preferably fitted at the outlet of the air conditioner, not the inlet as in current designs, because our positioning results in sterilisation after conditioning coils which with their positioning results in sterilisation after conditioning coils which with their potential cold surfaces and dampness can be bacteria- generators themselves.

The scalloped container 42 not only serves the purpose of constraining the fluid flow to the near optimum UV treatment thickness, but is also designed also to act as vortice generators at the scallop crests which are between successive tubes, to ensure that we do not have a laminar sheet flow regime along the outer walls of the channels, which could result in reduced UV sterilisation efficiency because in laminar flow the fluid nearest the outer wall has essentially zero flow velocity. This principle is sketched in Figure 10. as seen in Fig. 10a, when the scallop trough 50 is relatively deep with respect to the UV tube 51 and the scallop crests 52 are relatively pointed, turbulent flow, illustrated at 54 is generated. In contrast and as shown in Figure 10b, when the scallop trough 56 is relatively shallow, fitting closely to the UV tube 60 and the scallop crests 58 are relatively flat, then an unwanted laminar flow, illustrated at 62 is generated.

In the case of successive tubes being immediately adjacent each other, the scallop crest 52 is formed as a result of the shape of the housing. However, when successive tubes have a spacing therebetween as described above, the scallop crest 52 is formed specially to stand proud of the downstream flat portion of the housing in order to ensure the abovementioned turbulent effect.

A titanium oxide coating applied to the sculpted channels would also have the effect of roughening the surfaces of the channels, making a laminar flow regime less likely to be formed, as well as performing its known function of generating radicals under shortwave UV radiation. This reaction with the UV further enhances the effects of UV in fluid sterilisation and purification .

An embodiment of the design is shown in Figure 11. A sterilisation module is formed from an extrusion 70 formed, as mentioned, from duralumin or other good strength aluminium. The extrusion 70 comprises a scalloped portion 72 for housing the UV tubes and their containers (not shown), an inlet portion 74 and an outlet portion 76. The extrusion 70 is closed by first and second end cap members 78, 80. The first end cap member 78 comprises apertures 82,84 that correspond to the inlet portion 74 and outlet portion 76 of the extrusion 70 for the flow of fluid into and out of the system.

The extrusion 70 can be arranged at any chosen angle of inclination, but an arrangement wherein the UV tubes are vertically stacked is advantageous for draining of the unit when not in use, and also for space efficient packing and ease of construction. Flow could either be vertical up-flow or down-flow.

An exploded view of the module is shown in Figure 12. as can be seen, sealing gaskets 82 are provided to be interposed between each end cap member 78,80 and the side of the extrusion 70. Each UV tube (not shown) is housed within a protective glass tube 84, sealed to the extrusion 70 via O-rings 86. As shown by the view of Figure 13, the UV tubes 88 (comprising electrical connectors 90) can be withdrawn from the extrusion 70 through the end cap members 80 so that they can be cleaned, repaired or replaced without interfering with the flow of fluid through the extrusion 70.

It is to be appreciated that our designs could of course be used with UV tubes of types other than glow discharge, such as the intermediate pressure amalgam tubes .

The design of Figures 11 to 13 is modular in its nature. Figure 14 illustrates an example geometry, showing how a number of the modules can be combined. The number of tubes per module can be decreased if the module bank is made wider. The inlets and outlets are modified to feed into/from a central inlet 92 and outlet 94, with the fluid flow being in the direction indicated by arrows 96,98,100. The lower end of each module has a fluid entry aperture (not shown) .

There are a number of design options for a modular construction. According to a first possible design option, identical modules can be linked together in parallel . Each module to be self-contained and a header unit (or simply, "header") can be fitted to each module, the individual headers being supplied via a common inlet and outlet header. There is a design advantage in using an aluminium extrusion for each module, including its headers. It is also advantageous that each module is self contained and only common parts would be fabricated. According to a second possible design option, number of modules can be formed as a bank of the reguired size as one unit. This would mean internal separating sides would be scalloped on both faces. Internal sides could thus be thinner as they would not contain pressure. Different headers could then be designed to suit a bank of modules of different sizes.

Isolating valves could be provided within the unit of linked modules so that a particular module can be withdrawn from service. However, in most cases this will be considered to be prohibitively expensive.

In any case, UV tubes are located in secondary UV transmitting containment tubes (for example formed from guartz) and, as mentioned above, if the design of Figures 11-13 is used, it is possible to change UV tubes without draining the unit. In a modular embodiment, isolation and draining of a module or unit would be reguired to remove and clean the secondary containment tubes. However, end seals can be provided that allow a secondary containment tube to be removed as an assembly after the unit is drained. These arrangements provide for the drainage and cleaning of sediments and debris from module headers.

The tubes are housed within containers (not shown), to prevent the escape of harmful substances, for example the mercury used in the tubes, into the water supply. The design is modular and can for example be based on a l/2kw array of glow discharge lamps at 14mm inner diameter, or as near to this as can be achieved. The UV reactor modules can be cascaded, with flow couplers between each module. This "flat plate" effect geometry gives as near laminar flow through the UV reactor as possible .

The use of optimum glow discharge tubes, rather than arc lamps, means that we are operating with surface temperatures of just above room temperature rather than the near 1000 DEG C of MPAL arc lamps. Operating at the much lower temperatures of glow discharge gives cost reductions in housing materials, less stringent specs on gaskets and seals, reduced fouling of tubes, longer lives, whilst accepting their compromise of a larger volume eguipment package. Commercially this means that our design can be fabricated in, for example, aluminium rather than reguiring more expensive high temperature capable steels.

The characteristics of the flow around a structure such as that shown in Figure 14, including flow distribution, residence time and pressure drop can be modelled by a computational fluid dynamics (CFD) modelling technigue to investigate the operational limits defined to ensure that sterilisation will be complete. CFD allows the velocity and pressure vectors of a fluid flow to be calculated across a grid of locations within the fluid flow field and hence enables the actual velocity and pressure fields to be predicted.

CFD modelling carried out based on the design of Figure 14 has suggested that recirculation zones may occur at the far corners of the inlet and outlet sections, and at points between vertically adjacent tubes. Therefore, a modified design as seen in Figure 15 is deemed to be more efficient. The modified structure of Figure 15 comprises a first rounded corner 110 at the far end of the inlet header 112 and a second rounded corner 114 at the far end of the outlet header 116. In addition, the vertical spacing 118 between the tubes of each module is increased with respect to the spacing seen in Figure 14. The tighter tube-spacing at first sight appears beneficial for maximising the exposure of the bacteria to UV light, but in fact results in higher frictional losses on the flow and actually causes more dead zones to occur.

'Residence Time' is the term used to describe the length of the time period through the course of which fluid remains within the UV radiation produced by the device. Inadeguate residence time will result in incomplete sterilisation. Residence time may be gualified to give a probable description, e.g. the residence time is x seconds with a 95% probability meaning that 95% of the fluid will remain in the unit for the reguired time. The required residence time of water passing across the tubes is known based on the amount of UV radiation required to provide adequate sterilisation. For each module this residence time will be defined by the flow rate, the intensity of radiation from the tubes and the number of tubes in the module. Calculation of the residence time, along with the cross sectional area of the flow passage, enables determination of the maximum allowable volumetric flow rate through the module. Residence time is a function of the volumetric flowrate and the cross sectional area of the flow path. The design limits for adequate sterilisation are derived from the UV exposure time and intensity for specified organisms. The UV exposure is not only a function of residence time at any one-flow path, but the distance from the tube and the local velocity.

The first design challenge is to ensure that the flow entering a module is evenly distributed in velocity terms across the unit hence preventing areas of high velocity giving low local residence times (even if the average is deemed suitable) . In practice, fluid is fed in inlet and outlet headers from pipework specified by the end user. Different pipework configurations can have an effect on the flow distribution in the headers, and so the design can be customised for an individual end user. Of course, the inlet and outlet headers could comprise a relatively simple form of inlet, for example a rectangular aperture, and the customisation could be anything from a simple specification of the size of aperture. Headers may be either square or round in section. Square headers are easier to fabricate to match the rectanqular inlets to a module and can provide flat surfaces to join modules toqether . However, circular- section headers have the advantaqe of easy connection to incominq pipework, can be constructed from stock pipe materials, and drain easily. They also provide more efficient strenqth characteristics for pressure retention and so are qenerally to be preferred as compared with square headers .

Another important commercial consideration is that of diaqnostic instrumentation. UV sterilisation units on both waste and clean water applications run in unmanned locations which are remotely monitored via radio or telephone links to control rooms. These links have to report failures and loss of efficiency promptly to allow the correct level of service intervention or, indeed, to initiate a process shutdown to avoid contaminated water enterinq the supply.

Firstly, a UV intensity sensor is required so that the breakout of harmful radiation can be promptly detected. In one embodiment, a UV sensor can be provided for each UV tube, advantaqeously located within the secondary containment, so that it is kept away from the wet area, i.e. it is not immersed in fluid flow when the sterilisation apparatus is in use. It is also important to monitor secondary tube fouling, which causes a loss of radiation to the active area. This can be monitored per module, with one or two sensors at suitable selected locations rather than having one sensor per tube, as this would give a sufficient indication of the general performance of the module for fouling.

It is also desirable to have a flow measurement apparatus, to check for abnormally high or low flows. A high flow rate indicates a reduced residence time, and hence sterilisation efficiency, whereas a low flow rate could also indicate an inefficiency or worse, a leak in the apparatus . Flow rate monitoring may for example be accomplished using differential pressure sensors or ultrasonic flow sensors.

The series of tubes need not be arranged in parallel as has been illustrated thus far. Fig. 16 illustrates an alternative geometry, in which radiant members (UV tubes 120), again within containment tubes 122, are arranged in a curved fashion. The housing 124 is again conformed to the topography of the radiant members, as it follows the curve in which they are arranged, and fits around them to effectively constrain fluid flow to within a well defined region around the radiant members. Headers 126,128 are provided for the inlet/outlet of fluid, and can be of any suitable form.

The housing 124 is preferably formed from an aluminium or duraluminium extrusion, which means it can be easily deformed and set during manufacture. The housing 124 can be a deformed housing as seen in prior embodiments, that is, a planar housing that is deformed during a manufacturing process to take on a curved shape.

It is also to be appreciated that, as for the planar embodiments illustrated above, a curved geometry can also be modular, with successive modules nesting together. This may be of use for specialised applications where space for fluid sterilisation apparatus is limited or of an irregular shape.

Another aspect of the invention is the design of a freguency optimised ballast.

Fluorescent lamps can be considered as negative resistance devices, because as the current flow through the lamp is increased, more gas in the lamp becomes ionised, which drops the electrical resistance of the lamp and allows more current to flow. Therefore, a gas discharge lamp is provided with a ballast to regulate the current that is supplied to the lamp.

The simplest form of ballast is a resistor, but this is very energy inefficient and so is used in only a very few applications. A magnetic ballast uses an inductor which improves efficiency. However, the most common form of ballast is an electronic ballast, which uses electronic circuitry to give more advanced control of the current regulation. Electronic ballasts can be supplied which provide for different methods of starting a lamp, so that the most energy efficient ballast can be chosen based on the prospective use of the lamp which is to be powered. Electronic ballasts are generally smaller, lighter and more efficient than magnetic ballasts.

A lamp connected with a magnetic ballast will illuminate on each half cycle of the AC mains freguency that powers the lamp, thus for a 50Hz mains power supply, a lamp with a magnetic ballast will flicker at 10OHz. An electronic ballast, however, transforms the freguency of the power, typically up to about 2OkHz.

According to accepted wisdom, if one wishes to increase the intensity of radiation emitted from the lamp, the power supplied must be increased, that is, a greater current must be supplied to the lamp.

Figure 7 shows a simplified form of an output stage of an electronic ballast. It can be thought of as being eguivalent to a simple LCR circuit fed by input voltage 36, comprising inductor 30, capacitance 32 and the lamp represented as resistance 34. A high performance ballast should provide the lamp with preheat current in the cathodes for a specified time to bring them to the correct temperature before ignition. During the preheat time the lamp voltage has to be low enough to ensure that ignition will not occur prematurely. At the end of preheat a high voltage is reguired to ignite the lamp and from then on the reguired current should be supplied to the lamp for operation at the correct power. These requirements are satisfied by selecting appropriate values for the frequency and magnitude of the input voltage, and for L and C. For preheat and ignition, the lamp is not conducting and the circuit is reduced to a series L-C. During its operation, that is, after ignition, the lamp is conducting and the circuit is an L in series with a parallel R-C.

As mentioned above, the intensity of the radiation output from a gas discharge lamp is known to depend only on the input power. However, the inventor has now realised that in fact, the intensity of radiation does actually vary according to the ballast frequency and that therefore, a particular lamp will have an associated ballast frequency at which the intensity of emitted radiation will be optimised.

Individual electrons in the plasma are subjected to an AC field accelerating them first in one direction and then another. Some will experience bouncing off very heavy ions, others capture etc. Some will reach mercury ions and interact in complex ways - mercury has a number of isotopes, and standard tubes contain argon with a trace of neon to assist ignition. There are therefore many interlocking energy levels available for excitation within the plasma. The efficiency of UV creation must depend on how many electrons arrive at a time to excite the mercury ions . In a very low frequency field the electrons have considerable time to accelerate in a given direction before the field reverses . In a very high frequency field they are much more static because the field reverses so quickly. Hence the electron mean free path must be frequency dependent, and so the UV creation efficiency may be time and frequency dependent .

A specific example is illustrated in Fiqure 18. The inventor investiqated the UV intensity at the 254nm wavelenqth emitted from a qlow discharqe tube. This wavelenqth is of interest as havinq a qermicidal effect and is important for UV sterilisation of fluid such as water or air. The tube used for this example was a GH036T5 (16mm diameter) UV tube. The power was held at a constant level, and the UV intensity was measured as a count rate in units of 10³ counts/sec, and plotted aqainst varyinq ballast frequencies.

It can clearly be seen from this example that a maximum UV intensity is obtained for a ballast frequency of just over 56.25kHz. It is to be noted from the qraph that the UV intensity drops sharply as this value is decreased, but drops slowly as this value is increased. Therefore, to take into account any variations in the supplied ballast frequency, an ideal operatinq ranqe is defined with a lower bound at the ideal frequency, say from 55 to 58 kHz, as it is preferable to err on the side of havinq too hiqh a frequency rather than risk the frequency droppinq too low.

Current thinkinq does not accept that a relationship exists between the ballast frequency and the intensity emitted from a qas discharqe tube, that is, the qraph of Figure 18 would be expected if asked to simply be a straight line .

Therefore, when designing a ballast circuit for a gas discharge lamp, the desired operating freguency can be determined based on the investigation of these parameters. This holds true for all gas discharge lamps, with the specific example of a UV glow discharge lamp given as an example only.

The operation of a gas discharge tube with this freguency optimised ballast provides for a more energy efficient gas discharge lamp. Returning to the example of Figure 18, two freguency optimised tubes were tested versus an industry standard UV tubes, the Philips T8 Germicidal. The output of our tubes with the new ballast system was found to be 2.7 times greater than the Philips T8 group at eguivalent distances with the same wall plug current, and the actual UV irradiance under the 253.7 nanometer peak was 27,500 i_WcπT² at 3mm from the outer guartz sleeve of the tube in shell geometry (a 15 mm tube in a 25mm guartz envelope) .

As mentioned above, an electronic ballast typically transforms the freguency of the power up to about

2OkHz. However, the freguency optimised ballast of the present invention invloves operation at much higher freguencies - 55-58kHz in the above example alone. International Rectifier are a supplier of ballast ICs. They also provide ballast design software, which reduces design time for a ballast IC by performing the complex iterative procedure required to optimise the operating points and component values of the ballast circuit. Given an input of various lamp operational parameters, the software produces a schematic, a bill of materials listing all component values, and winding specifications for the inductors.

This ballast design software was used to model a circuit for a frequency optimised ballast according to the abovementioned specific example, and the results are shown in Figure 19. The design is based around International Rectifier's IR2156 ballast control IC, with a power factor correction IC L6561 from STMicroelectronics .

It will be appreciated again that the examples shown in Figures 18 and 19 are not intended to limit the scope of the invention .

There are many advantages given by the invention which give it a strong commercial applicability. Firstly, the invention is of a modular nature and so independent parts can be manufactured separately which reduces overall costs. The material used for the extrusion is of low cost, and because the tube diameters and ballast frequencies are optimised, the operating costs are reduced. The optimised treatment geometry gives an effective fixed thickness layer or can be variable to cope with variable water quality. Reduced service costs are also seen in the ease and speed of tube changes. The tubes themselves are relatively low cost and come with a reduced weight because of the lack for a heavy iron ballast. Also the system of the invention provide a reduced EMI and line interference . Furthermore, an improved reliability is seen because the power circuits make use of high performance integrated circuits, and are fully freguency optimised.

Various improvements and modifications can be made to the above without departing from the scope of the invention .

Claims

1. An apparatus for fluid sterilisation, comprising one or more radiant members arranged in series within a housing that comprises a first and a second surface conformed to the topography of the radiant members to define a fluid conduit on either side of the plane of the radiant members.

2. The apparatus of claim 1, wherein the housing comprises at least one scalloped surface in a spaced fitted arrangement with the radiant members .

3. The apparatus of claim 2, wherein the housing comprises first and second scalloped surfaces, arranged at opposing sides of the radiant members.

4. The apparatus of claim 2 or claim 3, wherein the housing comprises a fluid inlet and a fluid outlet, arranged such that fluid to be sterilised is flowable from the inlet to the outlet across a longitudinal axis of each radiant member and around each radiant member .

5. The apparatus of any of claims 2 to 4, wherein scalloped troughs are interposed between successive radiant members and are shaped so as to increase turbulence in the fluid flow.

6. The apparatus of any preceding claim, wherein the or each radiant member comprise a UV glow discharge tube.

7. The apparatus of claim 6, wherein the or each radiant member further comprises a containment member around the UV glow discharge tube.

8. The apparatus of claim 7, wherein the UV glow discharge tube is removable from the apparatus without affecting the fluid flow.

9. The apparatus of claim 7 or claim 8, wherein a UV sensor means is provided within each containment member .

10. The apparatus of any preceding claim, wherein the housing is formed from one or more extruded members.

11. The apparatus of claim 10, wherein the or each extruded member comprises aluminium or duralumin.

12. The apparatus of any of claims 7 to 11, wherein the inside walls of the containment member comprises a coating of titanium dioxide.

13. The apparatus of any of claims 10 to 12 when dependent from claim 6, wherein the diameter of the UV tubes is 15mm and the distance between the tubes and the walls of the extrusion is 6mm.

14. The apparatus of any preceding claim, wherein the radiant members are arranged in a substantially planar fashion .

15. The apparatus of claim 14, wherein the series of adjacent radiant members is held in a vertical arrangement, in use.

16. The apparatus of claim 14 or claim 15, wherein a plurality of columns of adjacent radiant members are served by the same fluid inlet and fluid outlet.

17. The apparatus of claim 16, wherein each column of radiant members acts as a self-contained conduit for fluid flow.

18. The apparatus of any of claims 1 to 13, wherein the radiant members are arranged in a curved fashion.

19. The apparatus of any preceding claim, wherein the fluid to be sterilised, in use, is water or any other liguid.

20. The apparatus of any preceding claim, comprising an electronic ballast adaptable to operate at a predetermined freguency, said predetermined freguency being an optimum operation freguency that corresponds to a maximum radiation intensity for varying ballast freguencies at constant tube power.