AIR TREATMENT APPARATUS

Field The present application relates to an air treatment apparatus and in particular to an apparatus and method for the removal of airborne pollutants or impurities such as micro-organisms, smoke particles or odours from air by means of exposure to anti-pathogenic agents generated by an atmospheric plasma discharge. The plasma discharge and the agents generated by the same are responsible for the killing of bacteria, viruses and other pathogens in the air. The plasma discharge and the generated agents are also responsible for the dissociation of other pollutants present in the air.

Background Of The Invention The use of plasma discharges and the anti-pathogenic agents generated by these in many applications involving sterilising and cleaning air is well known. Devices comprising plasma discharges have been designed for a great variety of domestic and industrial applications. All depend on the by-products of the plasma discharge, namely anti-pathogenic agents, and their great oxidizing potential to kill micro-organisms and oxidize other organic particles and materials. Depending on the application, some of these agents are generated by means of ultraviolet radiation or electrical discharge to convert atmospheric oxygen and nitrogen into various anti-pathogenic species, which can be highly effective at destroying organic atmospheric contaminants.

State of the art air disinfection devices consist of either filtering apparatus or chemically reactive systems.

A common approach to removing micro-organisms and other small airborne organic particles, such as smoke, obviously include direct filtration of the air. Various type of filter including so-called High Efficiency Particulate Air (HEPA) filters (defined as removing 99.97% of particles of 0.3 micron size) and electrostatic HAF (High Airflow, electret) filters capable of similar performance at higher airflows are commonly used. Although effective in some situations, such filters suffer from the disadvantages that trapped (and potentially infective) material remains on the filters, necessitating frequent changes of filter and remaining a hazard until the filters are replaced. This is a particular problem where the air being filtered is humid. In addition, such filters are incapable of removing small viral particles.

An alternative approach to air disinfection is the use of air ozonizers. These devices generate significant levels of ozone which attacks organic matter. Ozone is, however, highly toxic at high concentrations and it is increasingly clear that even at much lower concentrations it is irritant, being particularly linked with asthmatic complaints in those chronically exposed to it. In many territories there are strict statutory limits on the concentration of ozone to which members of the public and employees at a place of work may be exposed. In the UK, the Health and Safety Executive recommendation (EH38) is that the exposure limit to ozone should be 0.1 ppm (0.2 mg m<-3> ) as an 8- hour time-weighted average concentration, with a short-term exposure limit of 0.3 ppm (0.6 mg m<-3> ) as a 15-minute time-weighted average concentration.

Although undoubtedly effective at high concentrations, there is considerable evidence that ozone is ineffective as a biocide or in oxidising organic contaminants at concentrations that are safe for chronic human exposure (Dyas et al, 1983, J Clin Pathol 36: 1 102-1 104; Berrington and Pedlar, 1998, J Hosp Infect 40: 61 -65; Esswein et al, 1994, Appl Occup Environ Hygiene 9: 139-146).

US patents No. 7,449,053 and 8,21 1 ,374 directed to an air filtration device and an air cleaning device respectively, attempted to address some of these issues with ozone.

Use of plasma radicals for sterilisation of air filter medium is also known. See for example US patent No. 2004/0184972 A1 . This document proposed that an upstream plasma discharge can generate active radicals which flow upstream to a medium filter and kill any bacteria or virus trapped by the filter. The filters only capture particles potentially allowing bacteria population to grow over them. US 2004/0184972 A1 is such that the plasma radicals kill virus and bacteria trapped by the filter. This document provides, in addition, for a radical destruction filter for the destruction of excess radicals after the filter medium. These radicals are toxic to humans. It is appreciated that failures of said filter may result in release of toxic gases into living spaces.

However there still remains a need for an more efficient means of removing airborne pollutants such as organic particles, micro-organisms and odours from air in forced airflow circulation systems, without the use of filter mediums or the release of potentially hazardous levels of chemicals into an enclosed environment, and whilst minimizing air flow resistance.

Summary Accordingly, the application provides an air treatment apparatus as detailed in claim 1 . Advantageous embodiments are provided in the dependent claims.

Brief Description Of The Drawings

The present application will now be described with reference to the accompanying drawings in which:

Figure 1 is a plan view showing in schematic form components of an air treatment apparatus provided in accordance with the present teaching; and

Figure 2 is a schematic representation of an air flow through an apparatus such as shown in Figure 1 .

Detailed Description Of The Drawings

The current teachings relate to using a plasma discharge field to effectively sterilise air of micro-organisms or pathogens or remove airborne contaminants and particles in such a way that the air is only transiently exposed to high concentrations of anti-pathogenic agents produced by the plasma discharge and is returned to the environment with the level of anti-pathogenic agents reduced to acceptable levels for safe exposure of those living or working in the immediate environment.

This is best described with reference to figure 1 , which shows in schematic form an exemplary arrangement which may be advantageously employed within the context of the present teachings. An air treatment apparatus 100 comprises a housing 1 1 1 within which a conduit 101 provides a fluid communication path between an inlet 102 and an outlet 103. The form and dimensions of these components may vary dependent on the application environment or volume of air intended to be treated. It will be appreciated that air enters the housing through the inlet 102, proceeds in the direction of the arrows and exits at outlet 103.

The housing or casing 1 1 1 is dimensioned to support and retain operating elements of the apparatus such as a power supply 1 10 and other elements as will be described hereinafter. Controls 108 for operation of the apparatus can be mounted outside the housing 1 1 1 for ease of user accessibility. The housing 1 1 1 is generally in the form of a box, cabinet, casing or the like. The housing 1 1 1 may be formed of any convenient material.

The type and specifics of the controls 108 are not important are used for turning the apparatus on and off, setting a timer for the apparatus etc. User inputs made at the controls can directly affect the generation of a plasma discharge and anti-pathogenic agents in the apparatus 100.

The acceleration of an air flow through the housing is effected by provision of at least one air flow impeller 104, most conveniently an electrically-driven fan arranged to efficiently draw air from the inlet 102 to the outlet 103 through the conduit 101 . In practice, the impeller 104 is capable of running at a relatively wide range of flow rates. As shown in figure 1 , the conduit 101 preferably follows a non-rectilinear path as this requires less turbulence to be generated in the air flow by the impeller 104 when compared to a linear conduit.

The apparatus also includes at least one plasma discharge unit or generator 109 mounted inside the conduit 101 . The discharge unit 109 is desirably provided proximal to the air inlet 102 such that air assisted into the conduit by the impeller 104 will pass over the discharge unit. The region between the air impeller and the generator defines an upstream stage 105 of the conduit 101 . The region between the discharge unit 109 and the outlet 103 defines a downstream stage of the conduit.

The discharge unit 109 is arranged for generating plasma with an inactivating zone proximal to the discharge unit. This inactivating zone will extend both into the upstream and downstream stages of the conduit but operably the direction of the induced air flow effected by the impeller causes the inactivating zone to extend further into the downstream stage that into the upstream stage. By locating the discharge unit in the fluid path, air introduced into the housing will pass into the inactivating zone and subsequently particles or constituents of the air will be exposed to the anti-pathogenic agents created by the plasma discharge.

The discharge unit 109 is desirably orientated to extend transverse to the direction of the air flow. The unit 109 provides a baffle such that air introduced into the fluid path will encounter an obstacle in the form of the unit 109 that serves to obstruct the flow of air in the conduit 101 . The positioning and shape of the discharge unit 109 is important as will be explained in more detail with reference to figure 2. Specifically, the position and shape of the discharge unit 109 ensures that a substantial portion of air flowing through is subjected to anti- pathogenic agents produced by the plasma field of the discharge unit 109.

Although a single discharge unit 109 is shown in the conduit 101 in figure 1 , it will be appreciated that multiple discharge units 109 can be included therein in any suitable configuration. For example, an array of discharge units may be distributed across the airflow path in the upstream stage 105. Furthermore two or more discharge units may be provided in series such that air passing through the fluid path will sequentially encounter each of the individual generators and experience successive exposure to reactive species generated by the generators.

The restricted anti-pathogenic agent concentration in the inactivating zone, created by the discharge unit 109, is sufficient to effectively inactivate airborne pollutant material entrained in the air flow. Furthermore the concentration of anti-pathogenic agents decays sufficiently outside the inactivating zone so that the concentration of anti-pathogenic agents in the cleaned air expelled from the outlet 103 of the apparatus 100 is at a physiologically acceptable level.

The inner surface of the conduit 101 may optionally be coated with a conductive material. A midstream stage 106 and a downstream stage 107 which is coupled to the midstream stage 106 may also being be coated with a conductive material from inside. It can be understood that the anti-pathogenic agents are transported from the upstream stage 105 passed the inactivating zone to the outlet 103. The combined length of the midstream and downstream stages stage is greater than the length of the upstream stage and is selected so as to exceed the path length required for neutralisation of the produced anti- pathogenic agent. In this way it is possible to ensure that the air exiting the device at outlet 103 contains anti-pathogenic agents at a harmless level.

While Figure 1 shows identifiable midstream 106 and downstream stages

107 it will be appreciated that these could be collectively termed a downstream stage. The treated air from the inactivating zone passes through this downstream stage and the duration of time spent in this downstream stage is sufficient to ensure that spontaneous break down of the anti-pathogenic agents is such that the concentrations levels of the anti-pathogenic agents within the air are sufficiently reduced to meet health and safety requirements.

As is known to those skilled in the art, anti-pathogenic agents produced by plasma generated spontaneously by the discharge unit 109, break down. The half-life in air is dependent on a variety of factors including temperature and concentration but is generally at least several minutes or hours. However, this half-life is generally significantly shortened by humidity and by the presence of oxidizable substrates, solid surfaces and specific catalysts.

It can be appreciated that the three stages of the conduit 101 can be of uniform width and shape or formed from a single unitary body but most likely formed individually, possibly of different width and shape, and joined together. The configuration of the stages of the conduit 101 can be chosen as appropriate by those skilled in the art. The most important consideration is the length of the conduit 101 (in particular mid stage 106), which must be of sufficient length to restrict or reduce the anti-pathogenic agents level in the air to a safe level by the time the air exits the apparatus at outlet 103.

Advantageously the conduit is comprised of metal, for example steel or aluminium, or a plastics material (or GRP) impregnated and/or coated with metallic material, suitable to suppress radio frequency (RF) interference resulting from the dielectric barrier discharge and is suitably earthed. The RF disturbance may interrupt the effective performance of other electrical components in the circuit.

Furthermore, the conduit 101 is sealed from other components and wiring of the apparatus such as the transformer 107 (the operation of which will be explained below) to ensure the forced airflow is directed through said upstream 105, midstream 106, and downstream 107 stages of the conduit 101 without any interference by such components. Specifically, air entering the apparatus at inlet 102 is only obstructed by the discharge unit(s) 109 in the conduit prior to exiting at the outlet 103. In addition, deflectors 1 10 are preferably provided at the corners of the conduit 101 to minimise interference with the airflow as well as to ensure that air is not trapped in the corners. These deflectors also reduce the gas pressure at the corners. It will be appreciated that deflectors 1 10 are not required if the conduit is suitable shaped i.e., rounded corners. Even in such a curvaceous path, the air still experiences a tortuous path through the housing.

In addition, the sealing of conduit such that the air in the conduit 101 does not interact with other components of the apparatus 100 (other than the discharge unit 109) ensures that these other components are not damaged or contaminated by exposure to plasma or anti-pathogenic agents in the air.

Turning now to the operation of the discharge unit 109, this unit 109 is formed and arranged so that an inactivating zone is contained within the conduit 101 i.e. does not extend outside of the confines of the conduit 101 . The inactivating zone is the combination of all upstream 105, mid 106, and downstream 107 stages because anti-pathogenic agents tend to disperse in the whole volume and cause deactivation of micro-organisms therein.

In addition to the particular benefit of providing rapidly decaying anti- pathogenic agents, low power coronal discharge plasma units 109 also have significant safety benefits in the case of any possible apparatus malfunctions, maintenance operations etc. Power to provide a suitable plasma generating dielectric barrier discharge (i.e., to power the discharge unit 109) is suitably provided by transformer 1 10 providing a low power high-voltage alternating current. Preferably, the transformer 107 is setup for a duty cycle of 5 minutes ON and 5 minutes OFF. This means that the discharge unit 109 only generates or discharges plasma in 5 minute blocks. This duty cycle of 5 minutes ON/OFF is chosen in order to control the excess production of plasma and anti- pathogenic agents by the discharge unit 109 in the apparatus 100. It will be appreciated that any duty cycle can be chosen as appropriate by a user via the controls 108. The rate of airflow through the conduit can also be chosen by a user via the controls 108.

It will be appreciated that the voltage and current parameters of the discharge unit 109 required to achieve a dielectric barrier discharge will depend principally on the nature of the dielectric used, as further discussed herein below. In general operating voltages below 1 kV are not practical, and preferably there is used an operating voltage in the range from 1 to 6 kV, most desirably from 3 to 5 kV, for example about 4 kV. It will be appreciated that the current required to maintain the dielectric barrier discharge is significantly less than that required to initiate it. The current (and hence power) of plasma generator units is normally expressed in terms of the starting current. There should be used a (starting) current in the range from 1 to 10 mA, preferably at least 3 mA. The power of the unit will of course depend on the voltage and current combination. Restriction of the power of the unit helps to ensure that the inactivation field is contained within the conduit 101 . In this connection it will be appreciated that a somewhat higher power unit might, in principle, be used with a larger conduit 101 . The power should generally be not more than 50 watts, and is preferably at least 4 watts. Typically the power is in the range from 10 to 40 watts. These power levels have in particular been found to be convenient with an apparatus unit having a conduit volume of the order of 0.02 to 1 .0 m3.

Even with such low power dielectric barrier discharge units 109 it has been found possible to achieve well contained localized highly inactivating concentrations of anti-pathogenic agents sufficient to inactivate a very wide range of airborne pollutants or pathogens.

Advantageously the transformer 107 is provided with an anti-surge and/or anti-spike device(s), in order to minimize transient excursions of the output voltage above the normal level which could result in temporary extension of the inactivation zone outside of the conduit 101 and/or generation of excessively high anti-pathogenic agent levels.

Importantly, the transformer 1 10 is located outside of the conduit 101 to minimize the risk of exposure to plasma/anti-pathogenic agents and possible breakdown in the course of use of the apparatus 100. As previously mentioned, this also ensures that the transformer does not interfere with the flow of air in the conduit 101 .

Although not shown in figure 1 , an AC supply is also provided in the apparatus 100. Preferably, this is co-located with the transformer 1 10 such that it does not obstruct airflow in the conduit 101 . A wide range of frequencies may be used in the AC supply to the low power dielectric barrier discharge device, and indeed somewhat higher frequencies may safely be used than is possible with conventional high power plasma generators. Conveniently an AC supply with a frequency in the range from 50 to 1000 Hz may be used.

Turning now to figure 2, this shows the operation of the plasma discharge unit 109 within conduit 101 . In particular, discharge unit 109 is showing obstructing the airflow 201 passing from the upstream stage 105 of the conduit 101 . It will be appreciated that the sizing and placement of the discharge unit with respect to the conduit is quite important. The conduit should not be smaller than a volume required to contain the inactivation zone of the plasma discharge generator(s) 109, and not so large that substantially the whole of the airflow 201 does not pass through said inactivating zone 202 in the course of its transit through the conduit 101 . It can be seen from figure 2 that the discharge unit 109 is placed such that substantially all of the airflow 201 passes through the inactivating zone 203. As can be seen from the figure 2, the shape of the discharge unit is important in directing the airflow 201 around the discharge unit 109 through highly inactivating zones 203 and into inactivating zone 202, which is less inactivating than zone 203. As previously mentioned, the inactivating zone 202 is a volume surrounding the discharge unit 109 containing an elevated concentration of anti-pathogenic agents, sufficient to substantially inactivate airborne pollutants or pathogens. The inactivating zone 202 is situated on the outer surface of the discharge unit 109 and in a downstream direction. The total inactivating zone (combination of inactivating zones 202 and 203) is the combination of all upstream, mid, and downstream stages as anti-pathogenic agents tend to disperse in whole volume and cause deactivation of microorganisms.

As can be seen from figure 2, airflow 201 is passed over and under the discharge unit 109 and not passed through it i.e., there are no air gaps within the discharge unit that allow air to enter or that could potentially hold a build-up in anti-pathogenic agents.

Various forms of low power discharge units 109 are known in the art but these are conventionally associated with ozone generators. In accordance with the present teaching a discharge unit is provided that is is desirably used with a solid dielectric 205 to provide a dielectric barrier discharge which the present inventors have found provides a more consistent and reliable plasma generation performance. Various geometries are also possible. However, for the present application, a generally tubular geometry, with a tubular dielectric with generally tubular electrodes 204 on the inner and outer faces thereof is preferred. This tubular shape works to direct the airflow 201 around the discharge unit 109 as shown in figure 2. It will be appreciated that plasma will be generated at both electrodes 204. Preferably there is used a generally mesh form electrode 204, providing a coil, in order to maximize the areas of dielectric surface at which plasma is generated. In this connection it will be appreciated that substantially "closed" meshes are less desirable as these reduce the exposed dielectric surface. On the other hand excessively "open" meshes are generally less efficient in the amount of plasma generated for a give size of unit. Most preferably the electrodes 204 are perfectly coaxial cylindrical mesh where the dielectric 205 is sandwiched between them.

Preferably the low power discharge unit comprises concentric tubular metal gauze electrodes 204 separated by a dielectric.

In a highly preferred embodiment, the low power discharge plasma unit 109 comprises tubular stainless steel gauze electrodes 204. (Whilst various other suitable electrode materials are known in the art, stainless steel is particularly convenient due to inter alia its resistance to corrosion and to oxidative and other damage from the plasma discharge.) The purpose of gauze electrodes 204 is to maximize the surface available for the dielectric barrier discharge and hence generation of plasma. However, other factors, such as the effects on the electromagnetic field generated, particularly hysteresis effects relating to the generation and collapse of the field during the 50 Hz cycle of the alternating current, also influence the choice of gauze and the fineness of the mesh. In a preferred embodiment the gauze on the outer electrode 204 is coarser than that of the inner electrode 204 as this favours the production of plasma on the outer, rather than inner, electrode 204. In a more preferred embodiment, the mesh count of the inner electrode 204 is from 50 to 30 ^* 45 to 25 (per inch or 25.4 mm) and that of the outer electrode 204 is 35 to 20 ^* 40 to 20. In a particularly favoured embodiment, the mesh count of the inner electrode 204 is 40 ^* 34 (per inch or 25.4 mm) using a 38 swg wire (0.15 mm diameter) and that of the outer electrode is 24 ^* 28 using a 30 swg wire (0.3 mm diameter).

It is also desirable for effective dielectric barrier discharge to take place that the mass of the electrodes 204 be substantially balanced, i.e. to differ by not more than 20%, preferably not more than 10%. This is especially significant in the case of the aforementioned tubular configuration discharge unit 109.

It will also be appreciated that the power of the dielectric barrier discharge plasma unit 109 is related to the size of the electrodes 204. In general it is preferred that each of the mesh electrodes 204 should have an area in the range from 25 to 100 cm2, preferably from 40 to 90 cm2. Typically examples of a coil provided in accordance with the present teaching is one having a length of 5.5cm or 13cm and having a diameter of 2.9 cm diameter. Such coils will have exemplary areas of 50 and 1 18 cm2 respectively. The power consumed by the plasma coils is in the range 10 to 20 W resulting in power density of 0.2 to 0.4 W/cm2 and 0.08 to 0.16 W/cm2 for each coil. Desirably a plasma generator comprising a coil per the present teaching will operate with power density values less than 1 W/cm ² and typically in the range from 0.1 to 0.5 W/cm2.

It will be appreciated that with a solid dielectric, the generation of a plasma through the use of a dielectric barrier discharge is very much dependent on the thickness of the dielectric, and especially at lower voltages, as used in accordance with the present teachings, it is necessary to minimize the thickness of the dielectric. It will also be understood, though, that the discharge unit must be strong enough to avoid damage by the substantial stresses encountered inside a plasma region.

Plasma generation occurs during the negative half cycle of the alternating current, at each electrode 204 in turn. During the corresponding positive half cycle there is a tendency for resident anti-pathogenic agents to be broken down, but this is a slower process than generation, and in any case the airflow 201 removes anti-pathogenic agents from the discharge area 203 as it is formed. This leads to a net production of anti-pathogenic agents. The electrochemistry of such methods of plasma production is known in the art.

It should be understood that the apparatus of figure 1 can be mounted for operation on a wall. However, any supporting structure that is sufficient to hold the apparatus 301 could be used instead of the wall. An inlet and the outlet should preferably be both disposed on the underside of the apparatus and are also spaced apart for maximum efficiency. This ensures that air exiting from outlet is not simply returned to the apparatus via the inlet.

The present design is free from any filters i.e., it does not uses air filters such as HEPA filters for air sterilization. The positioning of the inlet and outlet at the underside of the device minimising the particle collection at the outlet. The air flow speed plays a very important role in deciding the performance and efficiency of the device. It matters how someone is choosing the trajectory and air flow speed inside the apparatus. This is related to the parameter called residence time of the particles. It is defined as the ratio of reactor volume to the gas flow in a plasma volume, τ in sec = Reactor Volume in liters/Gas Flow in litre per sec. For instance, if the plasma volume is fixed then we can see that residence time is decreases with increase in the gas flow. Hence in this situation the particles will escape the plasma volume without getting treated by the anti-pathogenic agents produced therein. Also, by changing operating power, and plasma temperatures one can significantly reduce the particle residence time within the conduit and inter-particle collisions.

Furthermore, as will be understood by those skilled in the art, the restricted concentration of anti-pathogenic agents produced at the plasma discharge unit can be controlled by tuning or adjusting discharge power, dielectric barrier width, electrode configuration, airflow, and applied frequency.

Typical electrical operation parameters of the apparatus are in the range of some kV ignition voltage from line frequency to several MHz and power consumption of some W per dm2 electrode area. For high power input the gas temperature reaches up to 150 C. Various forms of impeller may be used. Conveniently there is used an electric fan running at a speed of the order of 2000 to 4000 rpm. A range of different flow rates may conveniently be obtained for a given fan speed by simply changing the fan blade angle.

Furthermore, although shown in a mounted position, the apparatus can also be used in an upright position, on a secure level surface such that air enters and exits the apparatus horizontally.

The apparatus is designed to provide substantially complete inactivation in a single pass-at least at typical pollutant loading levels in enclosed working, residential, transportation, recreational and like environments, whilst restricting anti-pathogenic agent emissions to physiologically acceptable levels. Naturally higher than average pollutant loadings and/or more resistant pollutants, may require somewhat lower maximum air flow rates and/or multiple passes through the apparatus, than are required for other cases.

Similarly effective inactivation and anti-pathogenic agent containment within the apparatus, may be achieved with a relatively wide range of residence times of the airflow within the conduit of the apparatus.

The inactivating effect of the apparatus of the present teachings may be used for inactivating a wide range of pollutants, including inter alia microbiological pollutants such as airborne bacteria, viruses and fungal spores, smoke, and various volatile organic compounds, in a wide range of situations so as to improve the quality of the air. Exemplary testing of an apparatus provided in accordance with the present teaching showed efficacy in killing both bacteria and virus. Table 1 shows exemplary data for bacteria testing whereas Table 2 shows similar results for viral testing.

Table 1

Table 2

In both examples a device per the present teaching with the plasma generator operating at power densities less than 1 W/cm ² achieved >Log 5 kill rate for all classes of bacteria or viral matter. Situations in which the anti-microbial applications of the invention are especially useful include hospitals, food preparation areas, laboratories and locations with limited ventilation, where air may be re-circulated. Storage of sterile instruments and materials in an atmosphere sterilised by means of the invention may extend their shelf life, with considerable consequent savings. The invention provides a means of supplying a unit for such storage with sterile, dry air capable of maintaining the sterility of stored instruments for extended periods. One particularly useful application is in flood-damaged buildings, where removal of fungal spores from the air can minimize subsequent growth of mould and development of rot in the fabric of the building, with significant reduction in damage and costs of repair. In another application, the apparatus may be installed in ducting or pipework carrying a flow of air, such as may for example be used in an air-conditioning system.

It will be appreciated that in general where airborne pollutants or pathogens are being removed from a room or other more or less enclosed space, the amount of treatment required will depend on the nature of the pollutant/pathogen, and possibly also the burden or loading thereof in the air. Whilst there may in principle be used multiple passes to progressively reduce the pollutant/pathogen loading, it is a particular advantage of the invention that the relatively high anti-pathogenic agent concentrations which can be achieved with apparatus of the invention within the contained inactivation zone, can usually provide substantially complete inactivation within a single pass, thereby minimizing the number of air changes required. Typically where it is desired to remove bacterial pollutants there should be provided at least 5 air changes per hour, whilst in the case of locations with moderate to high tobacco smoke loadings, it may be desirable to provide at least 10 or more air changes. The total airflow required to treat a room may be readily determined from the volume of the room and the number of air changes required. Whilst it might in principle be possible to achieve higher flow rates with larger sizes of apparatus, it is generally preferred to achieve them by using multiple apparatus units. In this connection it will be appreciated that more than one discharge plasma unit may be mounted in the same conduit, provided the inactivation zones of all the generators are contained within the conduit. Furthermore, more than one discharge unit may be powered by one (common) transformer, albeit the total power of the transformer will then be divided between the discharge units.

The words comprises/comprising when used in this specification are to specify the presence of stated features, integers, steps or components but does not preclude the presence or addition of one or more other features, integers , steps, components or groups thereof.