WO2004088706A2

WO2004088706A2 - Ultraviolet lamp

Info

Publication number: WO2004088706A2
Application number: PCT/GB2004/001357
Authority: WO
Inventors: James Lucas; Ahmed Ibrahim Al-Shamma'a
Original assignee: University of Liverpool
Current assignee: University of Liverpool
Priority date: 2003-04-01
Filing date: 2004-03-31
Publication date: 2004-10-14
Anticipated expiration: 2005-10-01
Also published as: GB0307505D0; WO2004088706A3

Abstract

There is disclosed an ultraviolet ('UV') lamp comprising a microwave-resonant cavity (2), a microwave source (6) arranged to supply microwave power to the cavity, an envelope (4) containing gas, the envelope comprising material which is at least substantially transparent to UV radiation at an emission frequency of the gas and projecting into the resonant cavity (2) whereby microwave energy is coupled into the envelope (4) providing an electrical field with the envelope (4), the microwave source (6) being driven at a power which is such that in a discharge zone within the resonant cavity (2) plasma discharge takes place, while in a UV emission zone of the envelope (4) outside the cavity (2) the kinetic energy imparted to an electron due to acceleration by the electrical field is less than the ionisation energy of the gas.

Description

DESCRIPTION

ULTRAVIOLET LAMP

The present invention is concerned with ultraviolet lamps and with applications

thereof.

Low-pressure mercury discharge lamps are used at present for germicidal

applications. Mercury vapor in an elongate glass envelope is excited by a longitudinal

electric field and so caused to emit ultraviolet radiation (UV). UV in the germicidal

region from 200 to 280 nm results in the inactivation of most microorganisms. When

using mercury discharge lamps the germicidal effect is principally due to an emission

line at 254 nm.

Mercury low pressure discharge lamps, while very widely used, suffer from

significant disadvantages. Most importantly, their power is limited to approximately 30

Watts/metre. Additionally the presence of electrical connections and electrodes through

the fused glass forming the envelope gives a limitation on the lifetime of such lamps.

Medium and high pressure mercury lamps produce output powers greater than 30W/m,

but suffer from high temperatures and low efficiencies.

Furthermore there is a recognised need for a powerful UV source for use in the

production of ozone. Exposure of oxygen to UV of appropriate wavelength (the 254 nm

line produces ozone but a wavelength of 185 nm is more suitable) produces ozone, which

is itself a powerful germicidal agent. Increased UV power is also desirable when using UV for water sterilisation. The inventors have devised an ultraviolet lamp which makes use of microwave

power for excitation of gas in an envelope. Very brief details of the physical

construction, and some aspects of the performance, of such a lamp have been described

in a paper by Al-Shamma'a, Pandithas and Lucas (J. Phys. DAppLPhys. 34(2001) 2775-

2781) entitled "Low-pressure microwave plasma ultraviolet lamp for water purification

and ozone applications".

The lamp has been the subject of considerable experimental and theoretical study

by the inventors and it is now recognised that effective and reproducible operation of the

lamp requires that the physics of the lamp be understood in detail and that this understanding be applied to the choice of operating conditions of the lamp.

Broadly stated, an object of the present invention is to provide an effective

ultraviolet lamp using microwave excitation.

An additional or alternative object of the present invention is to provide for the

efficient operation of an ultraviolet lamp using microwave excitation.

In accordance with a first aspect of the present invention there is an ultraviolet

("UV") lamp comprising a microwave-resonant cavity, a microwave source arranged to

supply microwave power to the cavity and an envelope containing gas, the envelope

comprising material which is at least substantially transparent to UV radiation at an emission frequency of the gas and projecting into the resonant cavity whereby

microwave energy is coupled into the envelope providing an electric field within the

envelope, the microwave source being driven at a power which is such that in a discharge zone within the resonant cavity plasma discharge takes place, while in a UV emission

zone of the envelope outside the cavity the kinetic energy imparted to an electron due to

acceleration by the electric field is less than the ionisation energy of the gas .

Specific embodiments of the present invention will now be described, by way of

example only, with reference to the accompanying drawings in which: -

Figure 1 illustrates a UV lamp embodying the present invention;

Figure 2 is similar to Figure 1 except that part of a lower section of the lamp is

cut away to expose certain internal details;

Figure 3 illustrates a resonant cavity forming part of the lamp in section,

electrical field lines within the cavity being shown;

Figure 4 is a graph of experimental data showing how power due to microwave

excitation is dissipated in Xenon gas;

Figure 5 is a perspective illustration of a gas-containing envelope of a further UV

lamp embodying the present invention;

Figure 6 is perspective illustration of still a further UV lamp embodying the

present invention;

Figure 7 is a schematic illustration of a switching circuit for providing a

modulated power input to the lamp;

Figures 8 and 9 are schematic illustrations of doubler circuits used to drive a

magnetron used with the lamp;

Figure 10 is a schematic illustration of a complete circuit used to drive the lamp; Figure 11 is a schematic illustration of an ozone generator incorporating the

lamp;

Figure 12 is a schematic illustration of a food treatment arrangement for utilizing

ozone from the generator illustrated in Figure 11;

Figure 13 is a representation of the chemical reaction that occurs during the purification of water using UV and the photocatalyst titanium dioxide;

Figure 14 (a) is a schematic illustration of the interior of an arrangement for

water treatment;

Figure 14 (b) is a graph of experimental data showing how the kill rate of E.coli varies with intensity of UV used; and

Figure 15 is a schematic illustration of a modified atmosphere food packaging

line.

A source of UV radiation, referred to herein as a "UV lamp", will firstly be described. Briefly explained, the lamp uses microwave energy to create a UV emissive

plasma. Microwave energy is coupled into a resonant cavity 2. A gas-containing

envelope projects into the cavity and surface wave excitation takes place therein. The

envelope contains, in this embodiment, a mixture of argon and mercury at low pressure

which, when stimulated by the microwaves emits radiation including UV.

Looking at the lamp in more detail, a microwave source is schematically indicated by a box 6 in Figure 1. The microwave source in this embodiment is a

magnetron. The unit used in trials to date has been of a type known for use in a microwave oven, operating at 2.45 GHz and providing a power up to IkW. This is a well

known, widely available and inexpensive type of unit. Other types of microwave source

could be used. At higher magnetron frequencies, theory predicts that the UV output

power per unit length increases as the square of the frequency as shown in the following

equation

P_out = k. freq². d² where d is the diameter of the cylindrical envelope 4 and k is a normalising constant. As

industry is now aiming to provide a 5.8GHz magnetron, this may be the preferred source

when high powers are required. A low loss, high power, electric cable 7 conducts the

magnetron's output signal to the resonant cavity 2. Alternatively a waveguide structure

may be used to transport the microwaves to the resonant cavity along with an appropriate launcher. In the illustrated exemplary embodiment the cable is connected to a loop

antenna 8 within the cavity 2, the antenna serving to couple microwaves into the cavity.

The resonant cavity 2 is formed by a conductive container 10. In the illustrated

embodiment this has walls of copper construction. The cavity is a tunable short-gap or

re-entrant type operating in the TE₁₀₀ (transverse electric field) mode, although other cavity geometry may be chosen to suit the particular application. In order to obtain

resonance precisely at the magnetron frequency, and to provide the desired high Q factor,

the cavity has a tuning stub 12 at its base and also a fine tuning screw 14 in its side wall,

both projecting an adjustable distance into the cavity 2. It must be understood that the

illustrated construction has been used for experimental trials and the facility for adjustment of the cavity characteristics has been useful in this context. Such adjustment

may be. dispensed with in production versions of the lamp by virtue of optimised design

of the cavity. Field lines within the cavity have been calculated using commercial

modeling software and are represented at 16 in Figure 3.

The discharge tube 4 has a portion which projects into the resonant cavity. The

remainder of the discharge tube is exposed to provide UV emission to the exterior. The

wall of the discharge tube 4 is formed of a dielectric material. In the present embodiment

it consists of a fused silica tube (chosen for its transparency to the relevant UV frequencies, among other factors) containing argon and mercury vapor at a pressure of

approximately 1 Pa (0.01 Torr).

In the illustrated embodiment the cavity has dimensions of 100 mm diameter and

80 mm length. The envelope 4 is inserted approximately 10 mm into a gap in the cavity,

and is 25 mm in diameter. However the materials and dimensions used in this

experimental arrangement may be altered. The position of the end of the discharge tube

in the cavity is adjustable to give maximum transfer of microwave power thereto, but

this adjustment too is likely to be dispensed with in a production version, once the

design has been optimized.

In operation microwave excitation of the contents of the discharge tube produces a plasma therein. This plasma is found to be very stable and entirely reproducible. A

standing wave pattern is observed in the light intensity along the length of the discharge

tube (as a pattern of bright and dark regions) and is interpreted by the inventors as an indication that a surface wave, which propagates in the tube, carries the microwave

power along the tube away from a coupling region within the cavity, and simultaneously

provides the field required to sustain the plasma. A plasma column can propagate waves

such that the electric field has a velocity parallel to the tube axis, which has a maximum

value at the discharge tube / plasma interface. Such "surface waves" are known in the

academic literature. The plasma discharge can be sustained by the surface wave under

standing or traveling wave modes but it is the latter which allows an increase in power

per unit length to be achieved. The microwave power flow associated with such a wave

is partly in the plasma and partly in the dielectric tube. The relative proportions of the

power depend on the microwave frequency, tube diameter, lamp constituent pressure and

power absorbed per unit length.

The lamp can be thought of as comprising a pair of coupled resonators, one

formed by the cavity 2 and the other formed by the envelope 4.

A particularly important advantage of the present lamp, as compared with

conventional mercury discharge lamps, is that in the former the electric field acting on

the UV emissive material is transverse, whereas in the latter it is longitudinal. This

transverse field is provided by virtue of the aforementioned surface wave. The transverse

field generated in the present lamp can have a large potential gradient - ie a large electric

field strength - because of the small dimension across which the field is created - the

transverse dimension of the discharge tube. Compare this with the conventional mercury

discharge lamp in which the field is generated between electrodes at either end of the tube, the large separation of the electrodes creating a relatively small electric field. This

transverse field direction is one of the factors which enables the present lamp to produce

greater power per unit length than the conventional lamp.

Remarkably it is found that microwaves are confined by the cavity 2 and the envelope 4. That is, the lamp does not emit significant microwave energy to its surroundings. This is an important safety factor.

The physics of UV generation within the discharge tube have been extensively

studied by the inventors, using among other tools a unique computer model of "Monte

Carlo" type. The principles will now be explained. Generation of a plasma (ionised gas)

is necessary to the function of the lamp since it is charged particles which are excited by the microwaves, being accelerated by the electric field. Because the field is rapidly time

varying it is the free electrons of the plasma which, due to their low mass, are sufficiently

accelerated by the field to receive the majority of the energy. The more massive positive

ions achieve relatively low velocities in the time varying field. The desired emission of UV results largely from inelastic collisions between electrons and the mercury in the

discharge tube, bound electrons in the mercury being promoted by the collisions to

higher orbitals and releasing, upon their relaxation to lower orbitals, a photon of UV

light. Several factors affect the frequency of these desirable collisions:-

1. the majority of collisions are inelastic. That is, most collisions do not result

in energy being imparted to the mercury to achieve the required excitation by electrons.

This is not necessarily problematic in itself since inelastic collisions do not result in substantial loss of energy by the electrons.

2. mercury has "metastable states" - very long lived states in which bound

electrons are in elevated orbitals. The excitation within the discharge tube leads to

mercury atoms adopting metastable states. A mercury atom in a metastable state is not

available for UV production. Over time, due to the long lifetime of the metastable states, the tendency is for the proportion of mercury particles in such states to increase as is

experienced with conventional dc or ac lamps. The frequency of collisions which yield UV photons consequently tends to decrease over time, as the population of mercury

atoms available for UV production decreases.

3. the collision cross sections of the particles are energy dependent.

4. excessively energetic collisions cause ionisation of the mercury, rather than the desired excitation of electrons from one bound state to another. Hence, with respect

to the frequency of UV producing collisions, it is not desirable that the electron

population should be so energetic as to cause a high frequency of ionising collisions.

However it is necessary for ionising collisions to take place in order to create the plasma.

These considerations are potentially conflicting.

The construction of the lamp presented herein, as well as the manner in which it is operated in accordance with the present invention, permit these potentially problematic

and conflicting factors to be reconciled.

With reference to item 4 in the above list, it should be understood that two different regimes prevail within the envelope 4. The portion of the envelope 4 received in the resonant cavity 2 is subject to a relatively strong electric field. Electron energy in

this region is consequently relatively high. Specifically the electron energy in this region

is sufficient to provide frequent ionising collisions. Plasma generation is therefore

concentrated particularly in this region of the discharge tube. The remaining portion of

the discharge tube 2 is subject to a weaker but uniform electric field. Operating conditions are chosen such that in this outer portion of the discharge tube 2 the electron

energy is suitable to promote the desired collisions yielding UV photons. The energy

required to ionise a mercury atom is in the region of 12.6eV. Hence the energy

distribution of free electrons in the outer portion of the discharge tube should be such as

to provide only a small proportion of free electrons with this much energy.

In Figure 4 the relative significance of different modes of dissipation of energy

can be seen over a range of average electron energies E/N. Figure 4 relates to xenon

rather than mercury, although the principle is the same for either material. The present

invention makes possible use of xenon as the UV emissive material, in place of mercury,

a point which will be considered further below. Xenon has an emission line at 172 nm. At low energies, losses due to elastic collisions, e.g. with the walls of the discharge tube,

predominate. At high energies, power dissipation due to ionisation becomes significant.

Energy also goes into excitation of bound electrons into higher states, not yielding the

desired radiation at 172 nm. Between these two extremes of low and high energy is a desirable condition in which power usage in the exposed part of the discharge tube is

predominantly accounted for by excitation of the xenon to provide the required 172 nm radiation. Suitable selection of the input microwave power allows this desirable

condition to be achieved.

The problem of metastable states is addressed, in accordance with the present

invention, by providing a time varying or "modulated" microwave input power. The

inventors have found that by modulating the input power, the proportion of atoms in

metastable states is greatly reduced, even without any reduction in the average power

input to the lamp. In the lamp system currently under consideration the power

modulation takes the form of pulsing - rapidly and repeatedly switching the power on

and off. Circuitry for providing this pulsed input power will be described below.

The frequency of the power modulation is chosen to be high enough that the

plasma discharge is sustained through the low power part of the cycle. The required

frequency can be determined empirically. It is observed that too low a frequency causes

the luminous gas to retreat along the discharge tube - that is, the luminosity is not

observed in the part of the tube remote from the resonant cavity 2. A modulation

frequency in the region of 1MHz has been determined by the inventors to be highly

suitable. Frequencies down to 100 KHz have been successfully used in trials. However

the appropriate frequency depends on such factors as lamp design and lower - or indeed

higher - frequencies may be used in practice.

The mark space ratio of the power modulation has a bearing on the incidence of

metastable states. The inventors have determined that a ratio of 1:3 (i.e. high power

periods one third as long as the intervening low power periods) is highly suitable. The combination of power modulation and appropriate choice of power level

makes it possible to operate the lamp at high efficiency and high operating power

(>300W/m). In trials the illustrated lamp has been found to be 80 percent efficient in

converting microwave energy into UV radiation. The 20 percent loss is due to a variety

of causes including elastic and metastable collision losses, reflection, dielectric heating

and UV absorption and conversion.

The inventors have observed a further advantage of input power modulation

which is that the germicidal effect of the lamp is improved. For a given average input

power, a lamp having a modulated input is found to have a higher germicidal (e.g.

bacterial) "kill rate". The inventors have conjectured that this is because the high

instantaneous power provided by virtue of the modulation causes, where the UV output

is directed onto a surface to be sterilised, some ablation and boiling at the surface which

results in microorganisms being de-activated.

Running the magnetron on a modulated power input also increases its working

life.

Power modulation also provides for control of the lamp's operating temperature.

It is found that the proportion of energy in different spectral lines varies with

temperature. If operated at 30 degrees centigrade the illustrated mercury-based lamp is

found to emit 90 percent of the UV radiation at 254 nm and 10 percent at 185 nm. At an

operating temperature of 60 degrees centigrade it produces 40 percent at 254 nm and 60

percent at 185 nm. As noted above, lamps embodying the present invention need not necessarily use

mercury as the UV emissive material. Conventional electrical discharge lamps have

difficulty in producing the electron energy needed to ionise gases such as xenon. The

much stronger electrical field provided by the present lamp removes this limitation,

however, and makes the use of a broader range of materials possible. The UV emissive

material can be chosen to provide a required frequency. Light produced by xenon at

172 nm is particularly effective in production of ozone, making this a potentially

important example.

The concentration of UV emissive material - i.e. its pressure - can also be

increased as compared with conventional discharge lamps due to the greater electric field

strength, offering increases in UV power output.

Whereas the lamp illustrated in Figures 1 to 3 has a tubular discharge envelope

2, another of the advantages of the type of lamp presented herein is that the envelope can

be designed with any number of different shapes to suit a specific application. Figure 5

illustrates an alternative shape 20 for the envelope having a narrow stem 22 for insertion

into the resonant cavity and a broad, shallow UV emissive portion 24. The small depth

of this portion (perpendicular to the plane of the paper) results in the desired high

potential gradient.

Another variant of the lamp design is illustrated in Figure 6 and differs from the

version shown in Figure 1 in having two resonant cavities 26, 28, both supplied with

microwave energy, arranged at either end of a gas envelope 30, providing higher UV output power. Another possible variant (not illustrated) would have a set of discharge

envelopes being driven from a single resonator. Careful cavity design and positioning

of the envelopes would be required to ensure maximal power transfer to the envelopes.

A circuit 50 for switching the power input to the lamp to provide the required

power modulation is illustrated in Figure 7. The circuit is capable of handling 1900 volts.

It uses a DC to DC converter 52 receiving a 5 V DC supply to provide a 12 V DC supply

for the switching circuit itself The high voltage switch is controlled by a signal generator

54 which provides a logic signal to the input of an opto-isolator 56. The signal generator

is used, in this prototype system, for the sake of flexibility. A more simple oscillator

could of course be substituted. The output of the opto-isolator 56 is fed to an input of a

high speed, integrated circuit MOSFET/IGBT driver 58 whose output is connected

through a 10 ohm resistor 60 to the gate of a high voltage IGBT (integrated gate bipolar

transistor) 62. The IGBT serves to switch the voltage applied to the magnetron. The

component used in trials is capable of handling 2500 volts although a series combination

of zener diodes 64 clamps voltage across the IGBT at no more than 1900 volts.

Two or more circuits of this type can when necessary be cascaded to handle

larger voltages, the opto - isolator inputs being connected - typically in series - to switch

concurrently. Two stage circuits have been used to control a magnetron voltage of 3kV.

A refinement would be to add a resistor/capacitor network across each IGBT to aid

voltage sharing.

Circuitry used to provide the high voltage required to drive the magnetron is illustrated in Figures 8 and 9. Figure 8 shows a voltage doubler circuit 70 for use with

a single phase A.C. supply which is applied to primary windings 72 of a step-up

transformer 74 whose secondary windings 76 are connected on one side directly to earth and on the other side via a doubler capacitor 78 and diode 80 to earth. During the

capacitor charging time there is no voltage to the magnetron 82. Rather than take a path

through ground and up to the plate of the magnetron, the current swings up through the

diode. The voltage across the capacitor rises with the transformer secondary voltage to

the peak supply voltage (2800 volts). As the transformer secondary voltage begins to

decrease from its maximum positive value the diode prevents further capacitor discharge and the doubler capacitor 78 remains at the peak supply voltage.

Subsequently the transformer secondary (output) voltage swings into the negative half-cycle and increases in a negative direction to the negative peak of the supply voltage

(2800 volts). The transformer secondary and the charged doubler capacitor are now

essentially two EMFs in series. The 2800 volts across the transformer winding adds to

the 2800 volts stored in the capacitor and the sum voltage of 5600 volts is applied to the magnetron cathode to drive the magnetron.

There are two fundamental characteristics of this high voltage output that should

be noted. First, because a voltage doubler is also a rectifier, the output is a DC voltage.

Second, the resulting output voltage that is applied to the magnetron tube is actually a

pulsed DC voltage. This is because the doubler generates an output only during the

negative half-cycle of the transformer's output (secondary) voltage. So, the magnetron tube is, in fact pulsed on and off at the supply frequency (e.g. 50Hz, in the case of a

domestic UK mains supply).

To remove the supply frequency pulsing of the high voltage, a three phase supply

can be used with the circuit 100 illustrated in Fig. 9. Similarly to the previous voltage

double circuit, this uses a step-up transformer 102 one side of whose secondary 105 is

led to ground via a doubler capacitor 104 and diode 106. A bleeder resistor 108 is

incorporated in the circuit 100, in parallel with the double capacitor 104. The inclusion

of a second diode 109, connected in series between the capacitor 104 and the output 112

through which high voltage is supplied to the magnetron, prevents the common voltage

point from affecting the charge/discharge cycle to the voltage doubler and so enables use

with a three phase supply.

A substantially constant DC high voltage output is provided by the doubler

circuit 100. The entire magnetron drive circuit 120 is schematically indicated in Fig. 10,

incorporating the high voltage switching circuit 50 and the three phase doubler circuit

100 to drive the magnetron, labelled 122 in this drawing. The three phase supply is

indicated at 124.

The lamp can be applied in sterilisation and in killing bacterial growth and

mould. The high power it provides allows more effective UV sterilisation than was

possible with electrical discharge lamps.

Exposure of microbiological systems to UV light within the germicidal region

from 200 to 280nm wavelength results in their inactivation. Hence one manner of use of the lamp is to expose material to the lamp's UV output. Water sterilisation can be

achieved in this manner. The same technique can be used with some foodstuffs,

particularly those such as sliced meat which present a flat surface. Direct UV treatment does not work in shadowed areas and so is less well suited for example to use with bread,

in view of its porous crumb structure.

Ozone gas can be generated by use of UV light and has been considered for use

in food sterilisation. Doses in the region of 2-5 parts per million produce significant

micro-biocidal effect even in short exposure times consistent with modern high speed

production lines. The lamp described above may thus be used, for food treatment

purposes, either (1) by direct exposure of the foodstuff to UV, (2) by generating ozone

for food treatment or (3) through a combination of UV exposure and ozone generation.

Figure 11 illustrates an ozone generator utilizing the lamp 200. An enclosure

201 is provided around the envelope 202 of the lamp and is formed of UV opaque material. Aluminium is used in the illustrated example.

A gas in-feed 204 receives gas from a source 206 and supplies it to the enclosure

interior, wherein exposure to UV from the lamp produces ozone. The gas used in the

present embodiment is air although other gases including pure oxygen could be used.

Ozone-bearing gas leaves tlie enclosure through outlet 208 and passes via a flow valve 210 and ozone sensor 212 to a spray outlet 214. A computer or other electronic control

unit 216 receives inputs from various sensors via a bus 218 and controls the process

accordingly. Other inputs to the control unit 216 come from a pressure sensor 220 and a UV sensor 222 within the enclosure 201, the latter serving to indicate whether the lamp

is functioning. A sparking system for lamp starting is also indicated at 224. In tests,

ozone concentrations of 50-100 parts per million have been achieved using air at

pressures in the range 0.1-0.3 Mpa (1-3 Bar). Higher concentrations could be achieved,

if necessary, using oxygen.

Figure 12 is a schematic representation of an arrangement for utilizing the ozone

in food sterilisation. Ozone-bearing gas from the generator illustrated in Figure 11 is

passed through an input conduit 250 to a spray hood 252 disposed adjacent to, and directed toward, a confined food chamber 254. As well as containing food, the chamber

has an ozone sensor 256 which passes on indication of the ozone level to control electronics including an ozone concentration display 258, enabling the process to be

controlled in dependence upon the measured ozone concentration in the vicinity of the

food. The food chamber may form part of a conveyor on a food packaging line.

Particular advantages stem from incorporating the lamp and a photocatalyst, such

as titanium dioxide, into a water purification system.

The lamp is placed in a water pipe with water to be treated travelling through the

pipe. The rate of flow of the water to be treated is dictated by the quality of the water and

the hydraulic design. However, the flow rate at which the water is treatable is directly proportional to the amount of germicidal UV produced by a UV source. The water is

initially filtered and then exposed to the UV. light. The treatment of fine residual particulates is preferably achieved by combining the action of UV with the oxidising and reducing actions of UV irradiated titanium dioxide. The emitted UV alone will directly

kill microorganisms but the combination of UV and photocatalyst will remove other

micro contaminants and increase the kill rate of microorganisms. For compact systems

lamps having an output of at least IkW per single lamp are desirable.

Figure 14(a) represents the interior of such a water pipe and shows the envelope

275 of the UV lamp which is axially mounted within the tube. The direction of water

flow is indicated by arrow 277. Ti0₂ particles 279 are confined by upstream and downstream protective members 281, 283. The pipe is filled with water 285.

Figure 13 shows the chemical reaction that occurs due to the photocatalyst.

Acting as a semiconductor when exposed to UV, the titanium dioxide supplies an

electron current consisting of electrons in the conduction band and holes in the valence band. The electrons reduce material and the holes oxidise material, hence providing a

strong action for the decomposition of organic particulates and other water contaminants

(pesticides, oil, oxide of sulphur and nitrogen etc.). The necessary electric field is

provided by the microwaves.

In waste water processing the large organic particulates are removed by the use

of filters. The availability of inexpensive UV sources allows a combination of UV and titanium dioxide photocatalyst to be used for the removal of fine organic particulates

from the water, which would cause discoloration and odour if not removed. The reaction

rate is destruction of 1.0m mole/(m³kW) of organic particles. This process enhances the water quality and permits further recycling of the water to be obtained, lowering the processing cost.

A prototype of this technique has been successfully completed in the laboratory

using a combined action of UV to kill E. Coli bacteria and a combination of UV and

titanium dioxide to oxidise and reduce fine particulate matter not captured by the

filtration system. The 99.9% kill rate for E. Coli requires 66 J/m² (see Figure 14(b)) and

since the above described embodiment of the present invention produces 1000 J/m²¹ the

target of 66J/m² is achieved almost instantaneously. The reaction of particulates with titanium dioxide is slower. However, the kill rate for E. Coli was doubled in the

presence of titanium dioxide for the same strength of UV irradiation.

Existing water purification systems require the order of 5000W of germicidal UV

light. Hence if conventional mercury discharge lamps are used then a cluster of at least

200 lamps with a length of 1 metre are required to purify drinking water at the rate of 400m³/hr (140 litres/sec), while with the microwave plasma system described above it

requires no more than 5 lamps to achieve the same rate of purified water. This gives

many benefits to the water system engineers for example less electrical cabling, easy

access to lamps for cleaning, electrical connections only for the top end and these are not

immersed in water.

Particular advantages stem from incorporating the lamp and/or ozone generator in a modified atmosphere packaging apparatus. Such apparatus provides an atmosphere

other than ambient air inside a food package and can extend shelf life and improve the

environment of the food while inhibiting bacterial growth. Modified atmosphere packaging is well known to those skilled in the food packaging art. The atmosphere used

is selected to suit the particular food but typically contains some proportion of

C0₂, N₂ and O₂. Non-permeable packaging films can be used to ensure that the modified

atmosphere pack remains stable and sealed against entry of ambient air. Suitable micro-

biocidal treatment of the food prior to packaging is called for and chlorine washing, a

currently used technique, is expected to be discontinued for organic produce.

A modified atmosphere packaging apparatus utilizing the lamp is illustrated in

highly schematic form in Figure 15 and has a set of food trays 300 which are moved

from right to left on a conveyor. At filling stations 302 and 304 food is placed in the

trays, which are subsequently passed beneath an ultraviolet lamp 306 of the type already

described herein. The lamp is mounted above the conveyor and is similar to the lamp

illustrated in Figure 1, having an elongate cylindrical envelope which extends

transversely to the conveyor. A reflector 308 maximizes UV light utilization. After

passing beneath the lamp 306 in a guarded area 312 of the apparatus, a top web is applied

to the trays from a roller 314 and then sealed onto them by a sealing unit 316. The

process takes place under the required modified atmosphere. A photocatalyst may be

used in combination with the UV lamp to aid the process and is particularly useful for

packaging fruit and vegetables. The photocatalyst may be titanium dioxide.

Claims

1. An ultraviolet ("UV") lamp comprising a microwave-resonant cavity, a

microwave source arranged to supply microwave power to the cavity and an envelope

containing gas, the envelope comprising material which is at least substantially

transparent to UV radiation at an emission frequency of the gas and projecting into the resonant cavity whereby microwave energy is coupled into the envelope providing an

electrical field within the envelope, the microwave source being driven at a power which

is such that in a discharge zone within the resonant cavity plasma discharge takes place,

while in a UV emission zone of the envelope outside the cavity the kinetic energy imparted to an electron due to acceleration by the electrical field is less than the

ionisation energy of the gas .

2. An ultraviolet lamp as claimed in claim 1 wherein power from the microwave

source is modulated at a frequency high enough to sustain plasma discharge, thereby

reducing the incidence of metastable excited states in the gas.

3. An ultraviolet lamp as claimed in claim 2 wherein power from the microwave

source is modulated at a frequency greater than lOOKHz.

4. An ultraviolet lamp as claimed in claim 2 or claim 3 wherein power from the

microwave source is pulsed.

5. An ultraviolet lamp as claimed in any preceding claim wherein the envelope

has a longitudinal dimension and a shorter transverse dimension, the electrical field being transverse. 2004/088706

23

6. An ultraviolet lamp as claimed in any preceding claim wherein the envelope

comprises fused silica.

7. An ultraviolet lamp as claimed in any preceding claim wherein the gas

comprises mercury or xenon.

8. An ultraviolet lamp as claimed in any preceding claim wherein a standing

surface wave is formed along the length of the envelope in use.

9. An ultraviolet lamp as claimed in any preceding claim comprising two microwave resonant cavities each receiving microwave energy from a microwave source,

the envelope projecting into both cavities and being dimensioned such that surface waves

coupled into the envelope form the two cavities constructively interfere.

10. An ozone production apparatus comprising a lamp as claimed in any preceding claim.

11. An ozone production apparatus as claimed in claim 10 wherein the lamp is

housed in a gas enclosure having an input connectable to a supply of oxygen-bearing gas,

an internal region in which the oxygen-bearing gas is exposed to UV from the lamp, and an output connectable to a point of ozone usage.

12. An ozone production apparatus as claimed in either claim 10 or claim 11,

further comprising a photocatalyst.

13. A modified atmosphere packaging apparatus comprising an ozone

production apparatus as claimed in claim 10, claim 11 or claim 12.

14. A modified atmosphere packaging apparatus as claimed in claim 13 wherein the output of the ozone production apparatus is led to a nozzle through which ozone is output toward a conveyor by which food is carried past the nozzle.

15. A modified atmosphere packaging apparatus as claimed in claim 14, further comprising an ozone detector in the vicinity of the nozzle and conveyor.

16. A water purification apparatus comprising a lamp as claimed in any one of claims 1 to 9.

17. A water purification apparatus as claimed in claim 16, further comprising a photocatalyst.

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