GB2389848A

GB2389848A - Disinfecting a liquid using a UV lamp

Info

Publication number: GB2389848A
Application number: GB0213909A
Authority: GB
Inventors: Jonathan Mcclean
Original assignee: Hanovia Ltd
Current assignee: Hanovia Ltd
Priority date: 2002-06-17
Filing date: 2002-06-17
Publication date: 2003-12-24
Anticipated expiration: 2022-06-17
Also published as: GB2389848B; GB0213909D0

Abstract

The liquid is irradiated with ultraviolet light as it flows through a disinfection chamber 5. The elongate axis of the UV lamp 14 is inclined at an angle of at most 85{(preferably 20{-70 ) and the lamp is operated at a linear power density of at most 108 (preferably 27.5-78) Watts per centimetre. The UV source 14 comprises a transparent envelope (figure 2, 21) with an electrode (figure 2, 23 and 25) at one end. The level of deposition of the electrode over the sleeve inside the lamp envelope is monitored 15. Preferably the apparatus includes parallel medium pressure UV lamps 14 having sleeve 13 wipers 17.

Description

It r W Disinfection Apparatus and Method of Operating W Disinfection

Apparatus The present invention relates to the field of W disinfection apparatus and methods for

operating such apparatus.

UV radiation is generally considered to be radiation having wavelengths in the range from 200 rim to 300 rim and has proved itself as a useful disinfection means against many biological contaminants of the fluid supplies, such as the potable water supply.

Further, since the W radiation itself does not have any harmful biproducts, it is quickly overtaking chemical disinfection means such as chlorine.

in order for W radiation to be safely used, it is important to ensure that the fluid is treated with a sufficient W dose. Typically, doses from 25 mJ/cm2 to 100 mJ/cm2 are used. The UV dose (D) imparted by a W disinfection system varies as a multiple of the power density of the lamp (I), which is expressed in terms of the power per unit length, and the residence time (I) (the time when the fluid is irradiated by the lamp), thus: D aIx t In addition to the residence time and the lamp power, the transmittance of the fluid also determines the dose required as a higher intensity is necessary for a fluid with a lower transmittance than one with a higher transmittance, for a given residence time, to achieve the same dose.

As it is desirable to minimise the footprint of the W disinfection system, it is undesirable to increase the residence time in order to achieve a sufficient dose. Thus, generally, the power is increased. To reduce the pressure drop or 'headless' of the system, there is a tendency to use the minimum number lamps possible and run them at

( 2 high powers, typically lamps are run at powers in excess of 275 Watts per inch of arc length, where power density = lamp power/lamp arc length.

The applicants have found that, against the general teaching, it is highly undesirable to! run lamps at these high power densities.

Generally the lamps comprise at least one electrode which is sealed at an end of an elongate quartz lamp envelope. The W disinfection system generally comprises a disinfection chamber for the flow of fluid therethrough. The lamps are generally placed within quartz sleeves or thimbles which are sealed to the chamber such that the lamp may be replaced without the lamp contacting the liquid.

A lamp run at a high power has a higher internal temperature than a lamp run at a lower power. Thus, the quartz sleeve or thimble experiences a larger thermal gradient the higher the operating power of the lamp. Hence, lamps which are operated at higher power densities cause failure of the sleeve more quickly than those operated at lower power densities as heat cycling is a cause of a failure mode of quartz. As the sleeves are generally connected to the W disinfection chamber, replacement of the sleeve is time! consuming and costly since the disinfection system needs to be shut down.

As previously mentioned, the device is immersed in fluid and hence the sleeve will foul over time due to contaminants in the fluid. It the Amp is run at a high power density, typically over 280 Watts per inch (110 Watts per cm), then the temperature of the sleeve is increased thus baking contaminants onto the sleeve. The higher the temperature, the harder it is to remove the contaminants.

W radiation at wavelengths of less than 240 rim can cause the conversion of nitrate in the fluid into nitrite which is highly undesirable. To avoid this problem, the quartz sleeve is preferably doped with a suitable dopant which causes the quartz to filter out these undesirable low W wavelengths. A suitable dopant is titania. However, doped quartz absorbs W. Using a higher power density lamp means that more energy from the W light is absorbed by the sleeve than if a lower density lamp was used. Energy

absorbed by the sleeve may eventually cause failure to the sleeve since the absorbed energy must be eventually dissipated.

Also, when the lamp is operated, the body of the lamp swells. The quartz of the lamp! envelope is quite elastic and can cope with a degree of expansion. If the power density is too high, the extent of swelling in the lamp body creates excess stress at the pinch at either end of the lamp and the molybdenum seals fail causing vacuum loss and failure of the lamp.

A further failure mechanism is also enhanced when the lamp is run a too high a power.

As previously mentioned, the lamp envelope is quartz, which is a supercooled fluid which is naturally trying to return to a crystalline state. At normal temperatures, this rate of return is infinitesimally slow. This process is known a devitrification.

Cristabolite forms on the inside surface of the lamp envelope as a product of this devitrification process. Cristabolite is opaque to W light and thus affects the output of the lamp. Also, Cristabolite has a higher coefficient of expansion than quartz, thus massive differential stresses and compressive loads can be set up within the quartz if the lamp is operated at high power densities which lead to cracks appearing on the inside of! the sleeve and eventually failure of the sleeve. I Finally, the applicants have surprisingly discovered a further detrimental effect of running the lamps at too high a power. During operation, the electrodes inside the lamp eject particles onto the inside of the lamp envelope causing the fouling of the envelope.

This effect degrades the lamp performance as it shortens the arc length of the lamp. If the arc length of the lamp is shortened then the volume of water treated by the lamp decreases. Lamps which are operated at higher power densities foul more quickly than those operated at lower power densities, and also fail more rapidly due to a number of causes such as quartz devitrification and lamp seal failure.

Thus, in a first aspect, the present invention provides a W disinfection apparatus comprising a disinfection chamber for the flow of liquid therethrough and at least one W source, said W source being configured to operate at a linear power density of at

( 4 most 108 Watts per centimetre (275 Watts per inch) and provided with its elongate axis forming an angle of at most 85 to the flow direction of the liquid.

More preferably the lamps are operated at linear power densities of at most 98 Watts per centimetre (260 Watts per inch), more preferably at most 78 Watts per centimetre (200 Watts per inch).

The lamps are preferably operated at a minimum power density of 15.7 Watts per cm (40 Watts per inch), more preferably 23.6 Watts per cm (60 Watts per inch), even more preferably 27.5 Watts per cm (70 Watts per inch).

Previously, lamps have been placed perpendicular to the fluid flow direction. However, this arrangement is disadvantageous for a number of reasons.

If the lamps are placed perpendicular to the flow direction, the length of the lamps are limited to the width of the chamber so only lamps with a length which is equal to or less than the width of the chamber can be used. Since the linear power density of the lamps should be kept below a maximum, restricting the length of the lamps is highly undesirable as it results in more lamps being required to deliver a sufficient radiation dose which in turn increases the headless of the system.

When the lamps are placed perpendicular to the direction of fluid flow, it is difficult, if not impossible, to deliver a sufficient radiation dose to the fluid which flows at the edges of the disinfection chamber. This is because, as previously mentioned, the applicants have discovered that evaporation of electrodes causes fouling of the quartz lamp envelope which results in the arc length of the lamp shortening during use. If the lamps are placed perpendicular to the flow, shortening of the arc length of the lamps causes serious problems since fluid flowing at the edges of the chamber will not receive a sufficient dose as the lamps age. The use of High Frequency (HF) power supplies (also known as switched mode power supplies) can reduce, but not eliminate this phenomena.

( 5 The lamps are generally provided with a wiper which moves along the lamp during use.

The wiper is preferably an O-ring which circumscribes the lamp. Generally, a void is provided at either end of the sleeve to position the wiper mechanism away from the burning lamp. This permanent reduction in the arc length also prevents the fluid at the edges of the chamber from being properly irradiated when the lamps are arranged perpendicular to the direction of fluid flow.

Also, when a fluid is irradiated with W radiation, the lamps output UV radiation with a range of wavelengths. Particularly, so called 'germicidal' UV is outputted, which provides the disinfection effect on the fluid, and so called 'longwave' UV. Longwave UV transmits further than germicidal W and may repair DNA which has been damaged by germicidal W. thus at least partially reversing the disinfecting effect of the germicidal W. This effect was discussed in Oguma et al at the IWA 2002 World Congress, Melbourne, Australia, April 2002. This problem is exacerbated by arranging the lamps perpendicular to the direction of fluid flow.

Finally, since the lamps are oriented perpendicular to the flow direction, they must also be removed perpendicular to the flow direction. As space is open limited for such installations, the amount of volume required around that chamber to remove the lamps must also be taken into account.

Thus, to address each of the above problems, the lamps are placed non perpendicular to the flow direction of the fluid.

Arranging the lamps so that they are non-perpendicular to the flow means that the length of the lamps is not dictated by the width of the chamber, thus longer, lower power density lamps may be used.

Also, since the lamps are oriented with a component parallel to the flow direction of the liquid, it is possible to position the lamps so that the fluid flowing at the edges of the chamber is irradiated even when the arc length of the lamp is shortened due to evaporation of the electrodes or due to wiper.

Finally, since the lamps are not perpendicular to the flow direction, they can be removed from the disinfection chamber at this angle and hence less space is required around the disinfection chamber for the removal and insertion of lamps.

Preferably the elongate axis of the W source forms an angle of between 20 and 70 to the flow direction of the liquid, and even more preferably, forms an angle between 40 and 50 to the flow direction of the liquid.

The ends of the lamps may also be placed in recesses within the disinfection chamber in order to further compensate for arc shortening and to allow positioning of a wiper mechanism. Preferably, the W source emits light along at least 90 % of its length.

In a preferred embodiment, a monitoring means is provided for said at least one W source, said monitoring means being provided to monitor the UV output from said source. Monitor cameras have previously been used to monitor the performance of the W source or sources. However, up to now, they have been directed to monitor the centre of the elongate source, since conventionally, this has been believed to be the place which needs to be monitored. Due to the geometry of the chamber and the lamps, it is often necessary for monitor cameras to be placed on the walls of the chamber and at the end of the source.

The applicant has realised that it is more important to monitor the region of the source where the electrodes are located. Thus, the monitoring means is preferably directed to monitor the deposition of the electrode over the inside of the lamp envelope.

Thus, in a second aspect, the present invention provides a W disinfection apparatus comprising at least one W source, said W source comprising a transparent envelope and an electrode located at an end of said envelope, said apparatus further comprising

monitoring means configured to monitor deposition of the electrode over the inside of said envelope.

Preferably, the monitoring means is configured to monitor W output from the 5% of the envelope that is closest to the electrode. The 5% being 5% of the total length of the envelope. The monitoring means is preferably arranged perpendicular to the longitudinal axis of the W source.

Generally more than one W source will be provided. In this configuration, a separate monitoring means is preferably provided for each W source.

Typically, between 1 and 12 lamps are provided in each chamber. Thus, if twelve lamps are provided, twelve monitor cameras will preferably be used. If a plurality of lamps are provided, they are preferably arranged parallel to one another.

During continual use, fluid flows past the sleeves or thimbles, debris may stick to the outside of the sleeves reducing their transparency. Thus, preferably, a wiper is provided i for the sleeve or sleeves.

More preferably, the wiper is automated and moves along the length of the sleeve or thimble during use of the disinfection chamber. The wiper preferably extends around the whole circumference of the sleeve or thimble.

If a monitoring means is provided, the wiper may also be positioned to wipe the monitoring means to ensure that it does not become fouled.

The W sources are preferably medium pressure mercury sources. However, low pressure lamps may also be used.

In a third aspect, the present invention provides, a method of operating W disinfection apparatus, said disinfection apparatus comprising an elongate W source, said method

( 8 comprising operating the W source at a linear power density of at most 108 Watts per centimetre (275 Watts per inch) and passing fluid to be disinfected past said elongate UV source, said UV source being provided with its elongate axis at an angle of at most 85 to the direction of fluid flow.

In a fourth aspect, the present invention provides a method of monitoring a W source in a disinfection apparatus, the source comprising a substantially transparent envelope and an electrode located at the end of said envelope, said method comprising the steps of monitoring the level of deposition of the electrode over the inside of the envelope.

The present invention will now be described with reference to the preferred non-limiting embodiments in which: Figure 1 is a schematic diagram of a W disinfection apparatus having a W source in accordance with an embodiment of the present invention; Figure 2 is a schematic of a W source of the type used in the apparatus of Figure 1; and Figure 3 is a schematic of a W source and monitor camera arranged in accordance with an embodiment of the present invention.

The W disinfection apparatus of Figure I comprises a disinfection chamber 1 which is in the form of a pipe. The pipe is connectable at its ends 3 and S to a water system.

However, any fluid may be used. In this particular example, water flows through the pipe in the general direction denoted by arrow 7, but may also flow in the other direction. The pipe itself generally comprises stainless steel and will have a diameter of approximately from 10.2 cm to 121.9 cm (approximately 4 inches to 48 inches).

The chamber is provided with two cup-shaped projections 9 and 11, both of which are shaped to receive and accommodate the ends of sleeve 13. As an alternative or in

r 9 addition to cup shaped projections, inserts may be cut into the body of the chamber which facilitate the arrangement of a plurality of lamps and monitors etc. Sleeve 13 is an elongated tube which is sealed at either end to chamber 5. Sleeve 13 is provided at an angle of approximately 45 to the general flow direction of the fluid 7.

The sleeve 13 houses W lamp 14 and thus W lamp 14 is also provided at an angle of approximately 45 to the general flow direction of the fluid 7. The lamp will be described in more detail with reference to Figure 2.

By placing the lamp 14 at an angle to the direction of fluid flow has a number of advantages. Placing the lamp 14 at an angle means that a longer lamp can be used since the length of the lamp does not have equal that of the diameter of the tube of the pipe.

Thus, a longer, lower power density lamp may be used. Also, using a longer lamp which is placed non perpendicular to the direction of fluid flow increases the residence time of the chamber.

The lamp 14 is monitored via W monitoring means that in this example is W camera 15. W camera 15 is provided within projection 9 of pipe 1 and is positioned to measure radiation from the end of W lamp 14. The reason for the position of this camera will be described in more detail with reference to Figure 2. The camera itself will be described in more detail with reference to figure 3.

Finally, the W apparatus comprises a wiper 17 which is configured to move along sleeve 13 in the direction of arrow 19 in order to clean sleeve 13. Although not shown, the wiper 17 is also configured to wipe camera 15. During use, the wiper 17 moves under the control of a wiper drive mechanism (not shown) to avoid blocking output from the W lamp 14, this drive mechanism is housed in projections 9 and 11.

Figure 2 schematically illustrates W lamp 14. W lamp 14 comprises a quartz envelope 21, the two ends of the quartz envelope 21 are shown. Each end of the quartz envelope is provided with a tungsten electrode 23 and 25. Sealing means 27 and 29 are provided at the ends of the envelope 21. The tungsten electrodes 23 and 25 are formed

( 10 from thoriated tungsten. The sealing means 27 and 29 are made from molybdenum foil.

Tungsten is provided at the outside of the sealing pieces 27 and 29 to form electrical contacts. The lamp is a so-called medium pressure lamp. In this particular example, the mercury pressure in the lamp when the lamp is switched off is 20mBar, when the lamp is on, the pressure is 4Bar. Typically, the lamp is operated a potential of between 950 to 1800 Volts and a current of 4 to 8.5 Amps.

During use, the tungsten electrodes 23 and 25 get hot and splutter tungsten onto the inside of lamp envelope 21 in regions 31 and 33. As deposition areas 31 and 33 increase in length due to continuous use of the lamp, the arc length of the lamp shortens.

This effect is exacerbated by continuously starting and stopping the lamps.

If the arc length starts to shorten, then fluid flowing at the edges of the pipe may not receive a sufficient UV dose. Hence, in Figure 1, camera 15 is positioned in order to monitor the extent of the deposition areas 31 and 33.

Running the lamp at a relatively low power density, for example, between 27.5 to 51.2 Watts per centimetre (70 to 130 Watts per inch) means that the extent of deposition areas 31 and 33 are minimised.

Running at a low power density also has the advantage in that the lamp sleeve 13 should last much longer. The power density inside the lamp affects the heat of the lamp, the higher the power density the hotter the lamp. As the fluid flowing pass the 1arnp sleeve 13 is generally relatively cold, the hotter lamp 14 sets up a temperature gradient within the quartz sleeve 21. Running the lamp at a lower power density means that the quartz sleeve experiences a lower temperature gradient and hence will last longer.

Figure 3 is a schematic of a preferred monitor camera which can be used as monitor camera I S in figure 1. The monitor has a main body 51 and a connecting collar 53. The connecting collar 53 is used to connect the sensor to the side of projection 9 of chamber 1.

( W light from the lamp 14 inside the chamber 1 is detected by fused silicon probe 55 which is located within connection collar 53. The output from the fused silicon probe 55 is passed through an attenuation filter 57 and onto first mirror 59 which is located with the body S 1 of the sensor. First mirror 59 has a coating which allows the mirror to only reflect light within a certain wavelength range. The reflected light is then passed through a second attenuating filter 61.

The light which passes through filter 61 is reflected off second mirror 63 which is also configured only to reflect light within a certain wavelength range. Typically, the wavelength range for both of the first and second mirrors will be from 240 to 280nm.

The light is then reflected onto photo-diode 65 which outputs an electric signal dependent on the intensity of radiation incident on the photodiode. The electric signal is then fed into signal box 67 for amplification and conditioning of the signal This electrical signal is then fed out of main body 51. The signal may be analogue or digital.

In figure 3, the monitor I S is positioned to examine the end of the lamp 14. As the electrode evaporates, deposits build up on the inside of the lamp envelope. The monitor 15 monitors the extent of these deposits so that the lamp 14 can be replaced before the arc length shortens to an unacceptable level.

Claims

CLAIMS:

1. A W disinfection apparatus comprising a disinfection chamber for the flow of liquid therethrough and at least one elongate UV source, said UV source being configured to operate at a linear power density of at most 108 Watts per centimetre (275 Watts per inch) and provided with its elongate axis forming an angle of at most 85 to the flow direction of the liquid.

2. A W disinfection apparatus according to claim 1, wherein the elongate axis of the UV source forms an angle of between 20 and 70 to the flow direction of the liquid.

3. A W disinfection apparatus comprising at least one W source, said W source comprising a substantially transparent lamp envelope and an electrode located at an end of said envelope, said apparatus further comprising monitoring means configured to monitor deposition of the electrode over the inside of said lamp envelope.

4. A W disinfection apparatus according to any preceding claim, wherein the W source is configured to operate at a linear power density of at most 98 Watts per centimetre (260 Watts per inch).

5. A UV disinfection apparatus according to any preceding claim, wherein the W source is configured to operate at a linear power density of at most 78 Watts per centimetre (200 Watts per inch).

6. A W disinfection apparatus according to any preceding claim, wherein the W source is configured to operate at a linear power density of at least 15.7 Watts per cm (40 Watts per inch).

7. A W disinfection apparatus according to any preceding claim, wherein the W source is configured to operate at a linear power density of at least 27.5 Watts per cm (70 Watts per inch).

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8. A W disinfection apparatus according to any preceding claim, wherein the W source emits light along at least 90 % of its length.

9. A W disinfection apparatus according to any preceding claim, comprising monitoring means configured to monitor W output from said source.

10. A W disinfection apparatus according to claim 9, wherein said W source comprises a substantially transparent lamp envelope and a electrode located at an end of said envelope, said monitoring means being configured to monitor deposition of the electrode over the inside of said envelope.

1 1. A W disinfection apparatus according to either of claims 3 or 10, wherein the monitoring means is configured to monitor W output from the 5% of the sleeve which is located closest to the electrode.

12. A W disinfection apparatus according to either of claims 3, 10 or 11, wherein the monitoring means is arranged substantially perpendicular to the elongate axis of the W source.

13. A W disinfection apparatus according to any preceding claim, wherein said W source is provided within a sleeve and a wiper is provided for said sleeve.

14. A W disinfection apparatus according to claim 13 when dependent on any of claims 3 or 9 to 13, wherein the wiper is configured to additionally wipe the monitoring means.

15. A W disinfection apparatus according to any preceding claim, comprising a chamber, said chamber having projections for receiving the ends of said at least one W source.

16. A W disinfection apparatus according to claim 1 S when dependent on either of claims 13 or 14, wherein said projections house a mechanism for moving said wiper along said sleeve.

( 14

17. A W disinfection apparatus according to any preceding claim comprising a plurality of W sources.

18. A W disinfection apparatus according to claim 17, wherein said sources are arranged substantially parallel to one another.

l 9. A W disinfection apparatus according to either of claims 17 or 18, when dependent on any of claims 3, 9 or 10, wherein a monitoring means is provided for each of the W sources.

20. A W disinfection apparatus according to any of claims 17 to 19 when dependent on claim 13, wherein each W source is provided in a sleeve and a wiper is provided for each sleeve.

21. A W disinfection apparatus according to claim 20 when dependent on claim 19, wherein each wiper is also configured to wipe the monitoring means of each W source.

22. A W disinfection apparatus according to any preceding claim wherein said W source is a medium pressure W lamp.

23. A method of operating W disinfection apparatus, said disinfection apparatus comprising an elongate W source, said method comprising operating the W source at a linear power density of at most 108 Watts per centimetre (275 Watts per inch) and passing fluid to be disinfected past said elongate W source, said W source being provided with its elongate axis at an angle of at most 85 to the direction of fluid flow.

24. A method of monitoring a W source in a disinfection apparatus, the source comprising a substantially transparent lamp envelope and an electrode located at the end of said lamp envelope, said method comprising the steps of monitoring the level of deposition of the electrode over the sleeve inside of said lamp envelope.

( 15

25. A W disinfection apparatus as substantially hereinbefore described with reference to any of the accompanying drawings.

26. A method as substantially hereinbefore described with reference to any of the accompanying drawings.