EP0879081A2

EP0879081A2 - Filtering

Info

Publication number: EP0879081A2
Application number: EP96935048A
Authority: EP
Inventors: David Ingles
Original assignee: Filtec UK Ltd
Current assignee: Filtec UK Ltd
Priority date: 1995-10-27
Filing date: 1996-10-18
Publication date: 1998-11-25
Also published as: AU7314896A; GB9522053D0; WO1997015376A2; GB2306346A; WO1997015376A3

Abstract

A filter unit (22) for fitting in a filter housing (12, 14), said filter unit comprising a plurality of elements (34, 36, 38, 40) including at least one filter element (34, 36) and one hydrophobic element (38) and scrim means (42, 44) substantially enclosing said elements, opposed edge regions of said scrim means being joined together such that said elements are maintained in a predetermined juxtaposed relationship by said scrim means. The assembly of the elements into a filter unit with the elements maintained in the desired relationship by the scrim means facilitates the assembly of filter apparatus incorporating those elements.

Description

FILTERING The invention generally relates to filtering and particularly, but not exclusively, to bacterial/viral filter apparatus for inclusion in respiratory apparatus in anaesthesia and ventilation support conditions.

Known filter apparatus designed for filtering bacteria and/or viruses typically comprises a moulded housing which is connectable via opposing ports to the respiratory apparatus and the patient. A filter medium is positioned within the housing so that all of the gases inhaled and exhaled by the patient passes through the filter medium. Typically, the filter medium is covered on each side by a scrim. A heat and moisture exchanger (HME) is usually provided for heating and moisturising gas inhaled by the patient utilising heat and moisture collected from the exhaled gases.

The filter media in common use are pleated paper and electrostatic non-woven felt. There are a number of problems associated with electrostatic filters. In use, it is possible for loose electrostatic fibres (or fragments thereof) to be released from the surface of the media, the fragments being mainly generated during a needle punching operation in which the fibres are punched through a scrim material. These loose fibres or fragments represent a potential hazard to both machinery and patients.

Another problem associated with electrostatic filters is their susceptibility to the accumulation of moisture within the filter material which reduces its filtration ability. This results in an increased penetration of bacteria/viruses over the lifetime of the product.

The internal geometry of known filter apparatus provides a further problem. The electrostatic filter medium is conventionally presented in the form of a flat element which is trapped at its edge between the two halves of the moulded housing. The medium is presented as a flat sheet disposed perpendicular to the ports. The problem arises if there is an accumulation of liquid within the housing when it is orientated with the ports vertical. In this condition the filter medium is presented as a flat, horizontal sheet which can easily become covered by a film of liquid increasing the resistance to breathing.

Yet another problem associated with known filter apparatus is that the various components housed in the housing (filter medium, scrim and HME) , have to be placed concentrically on top of each other between the two halves of the housing which are subsequently joined together by a welding process. It will be appreciated that all of the layers should be trapped between the two halves of the housing to avoid bypass of the gas flow and considerable care is required during the assembly and welding processes to ensure that each layer is firmly trapped between the halves of the housing.

An additional problem with existing filters is that moisture carried in the breath exhaled by the patient tends to condense on the internal surface or surface of the housing forming droplets which can fall onto the filter medium which, in the case of electrostatic filters in particular, reduces filtration ability.

The invention includes a filter unit for fitting in a filter housing, said filter unit comprising at least four elements which include at least one filter element and at least one hydrophobic element, said elements being disposed in a predetermined juxtaposed relationship such that the or each said filter element and the or each said hydrophobic element is disposed intermediate the outermost of said elements and opposed edge regions of at least said outermost elements being joined together such that each of said elements is maintained in said predetermined juxtaposed relationship.

Preferably, said elements comprise a plurality of filter elements separated one from the other by spacer means having substantially no filtration capacity relative to said filter elements.

Preferably, the elements comprise a hydrophobic element arranged to substantially prevent the ingress of moisture to the or each said filter element at least from one side of the filter unit.

Advantageously, the filter unit comprises two said hydrophobic elements arranged to substantially prevent ingress of moisture to the or each filter element from each side of the filter unit.

Preferably, the or at least one said hydrophobic element is able to support a column of twenty to forty-five centimetres of water.

Preferably, the or at least one said hydrophobic element is able to support a column of substantially forty centimetres of water.

Preferably, the or at least one said hydrophobic element comprises non-woven microfibres.

The microfibres may be coated with or incorporate a hydrophobic additive.

Preferably, the microfibres have a diameter substantially in the range two to five microns.

Preferably, the microfibres are arranged to provide a fibre density substantially in the range five to one hundred grams per square metre.

Preferably, the elements comprise a heat and moisture exchange element.

Preferably, the heat and moisture exchange element is disposed adjacent said hydrophobic element, said hydrophobic element being disposed between said heat and moisture exchange element and the or each said filter element.

The invention also includes a method of manufacturing a filter unit to be fitted in a filter housing, the filter unit comprising at least four elements including at least one filter element and at least one hydrophobic element and the method comprising the steps of arranging said elements in juxtaposed relationship such that the or each said filter element and the or each said hydrophobic element is disposed intermediate the outermost of said elements and joining together opposed edge regions of at least said outermost elements such that said elements are maintained in said juxtaposed relationship.

It will be appreciated that by assembling the elements into a filter unit with the elements maintained in the desired relationship by a joint made at the periphery of the filter unit, assembly of filter apparatus incorporating those elements is facilitated and ensures that each of the elements can be reliably clamped within a two-part housing thereby avoiding the problem of gas by-pass.

By providing a hydrophobic element between the filter element or elements and the heat and moisture exchange element, the problem of moisture ingress into the filter element or elements can be substantially avoided. Furthermore the performance of the heat and moisture element is enhanced preventing the escape of moisture from the heat and exchange element in the direction of the filter element or elements.

The invention also includes a device comprising (i) a housing for housing at least one filter element and (ii) thermochromic means mountable in said housing, said housing being adapted such that when said thermochromic means is mounted in said housing at least a portion of said thermochromic means is visible whereby, in use, the temperature of gaseous matter passing through the housing indicated by said thermochromic means can be observed.

Preferably, said thermochromic means comprises a non¬ woven material carrying a thermochromic pigment.

The provision of thermochromic means provides the advantage that, in use, the temperature of gaseous matter passing through the filter as indicated by the thermochromic means can readily be monitored by observing the thermochromic means.

The invention includes a method of monitoring breathing of a patient connected to respiratory apparatus for use in anaesthesia or ventilation support, the method comprising the steps of providing thermochromic means in a flow path between the patient and said apparatus and observing the colour state of said thermochromic means.

Preferably the thermochromic means is included in a bacterial/viral filter apparatus. Preferably the thermochromic means is selected to have a first colour state indicating the range of temperature of exhaled breath and a second colour state indicating the range of temperature of gaseous matter to be supplied by said apparatus.

The first colour state may indicate a temperature in the range 30 through 35°C.

The second colour state may indicate a temperature in the range 20 through 25°C.

The changing colour state of the thermochromic means which results from the difference in temperature of the inhaled and exhaled gases allows visual monitoring of respiration.

The invention also includes a housing for at least one filter element wherein the surface energy of at least an internal surface of the housing is adapted such that, in use, liquid condensing from gaseous matter passing through said housing is substantially prevented from forming droplets on said surface.

The housing may be made of a material which comprises a surfactant. Alternatively, the or at least one said internal surface is coated with a surfactant.

The modification of the surface energy of a surface or surfaces of the housing provides the advantage that any liquid which condenses in the housing tends to form a film over the surface(s) rather than distinct droplets thereby minimising the accumulation of liquid on the filter element(s) housed therein.

The invention also includes filter apparatus comprising a housing and filter means disposed in said housing, said filter means defining at least one convex surface and being arranged such that the or a first said convex surface is generally opposed to a port provided in said housing.

Preferably, the filter means defines a second said convex surface arranged to be generally opposed to a second port provided in said housing.

The provision of a convex filter surface directed towards a port of the housing gives the advantage that a gas stream entering the housing via the port tends to be deflected over the convex surface providing a more efficient utilisation of the filter by making use of the whole area thereof rather than a region aligned with the port. Furthermore the surface area of the filter means is increased as compared with a flat filter means having the same diameter and since resistance to flow is inversely proportional to the surface area of the filter means, the flow resistance is decreased. It will be understood that for a given filter size it is desirable that flow resistance is decreased.

The invention also includes filter apparatus comprising a housing, a plurality of filter elements disposed in said housing and spacer means separating said filter elements from one another, said spacer means having substantially no filtration capacity relative to said filter elements.

Preferably, at least one said filter element comprises an electrostatically charged material.

Preferably, said spacer means comprises respective layers of material disposed between said filter elements.

The use of multiple filter elements provides the advantage that an increase in filtration performance for a given amount of filter material can be obtained. The separation of the filter elements by spacer means having substantially no filtration capacity relative to the filter elements further enhances the filtration performance of the filter elements.

The invention also includes a filter unit comprising a plurality of elements which elements include at least one filter element and a plurality of hydrophobic elements, said hydrophobic elements being arranged such that, in use, ingress of moisture to the or each said filter element is substantially prevented.

The filter unit may comprise two said hydrophobic elements, wherein the or each said filter element is disposed intermediate said hydrophobic elements.

Preferably, said elements are disposed in a predetermined juxtaposed relationship, said elements being maintained in said predetermined juxtaposed relationship by a joint at the periphery of said filter unit.

In order that the invention may be well understood, some embodiments thereof, which are given by way of example only, will now be described with reference to the accompanying drawings in which: Figure 1 which is a schematic view of filter apparatus; and

Figure 2 is a schematic view of an alternative filter unit for use with filter housing shown in Figure 1.

The filter apparatus 10 shown in Figure 1 is for filtering bacteria and/or viruses and is intended for inclusion in respiratory apparatus of the kind used in anaesthesia and ventilation support conditions.

The filter apparatus 10 comprises a two-part housing 12, 14 which has opposed ports 16, 18 to which flexible tubes (not shown) can be connected. The tube connected to the port 16 passes to the patient and the tube connected to the port 18 passes to atmosphere or apparatus for dispensing a gaseous mixture to be administered to the patient. As shown the ports each have an internal and external taper for use in forming a connection with such tubes. The tapers and tubing are preferably adapted such that the port 16 can only be connected with a tube which passes to the patient and the port 18 can only be connected to tubing leading to the respiratory apparatus.

The two parts 12, 14 of the housing are plastics mouldings. The preferred material is a transparent acrylic such as Oroglas (Reg'd Trade Mark) . The housing is treated so that moisture contained in breath exhaled by the patient is substantially prevented from forming droplets on the internal surfaces 20 of the part 12. This is achieved by coating the surfaces 20 with a surfactant (surface active agent) . Alternatively, the surfactant can be included in the material from which at least part 12 of the housing is made. The effect of the surfactant is to modify the surface energy of the housing material so as to prevent the formation of droplets of liquid (rain out) . Any liquid which condenses on the modified surfaces will tend to form a film over the whole surface rather than distinct droplets. A preferred surfactant is polyethoxylated alcohol obtainable as Volpo Grade 26 from the Croda company of Goole, England.

A filter unit 22 is disposed in the housing. The parts 12, 14 have respective circumferentially extending ribs 24, 26 arranged such that when the parts 12, 14 are mated together, an edge region of the filter unit is clamped therebetween for maintaining the filter unit in position. In Figure 1 the housing is shown part assembled. It will be appreciated that when the parts 12, 14 are pushed firmly together the edge of the filter unit 22 will be nipped between the ribs 24, 26.

The housing part 14 comprises a port 28 which can optionally be connected with gas monitoring equipment. A projection 30 is provided for holding a cap 32 for the port 28 when the port 28 is connected to such monitoring equipment.

The filter unit 22 comprises a plurality of elements 34, 36, 38, 40, 42, 44. The elements 34-40 are enclosed by the elements 42, 44 which comprise two generally circular pieces of scrim material. The elements 34-40 and the scrim elements 42, 44 are cut to fit between the ribs 24, 26 and are assembled in juxtaposed relationship ie. concentrically one upon the other. The opposed edge regions of the elements 34-44 are joined together so that the elements 34-44 are maintained in their juxtaposed positions.

The elements 34-40 of the filter unit include two filter elements 34, 36 comprising electrostatically charged fibres in the form of a non-woven wadding which in terms of its structure can be likened to a plate of spaghetti. It will be appreciated that by forming the elements 34-44 into a unit joined at its periphery such that the filter elements 34, 36 are enclosed by the scrim elements 42, 44, it is not necessary for the fibres to be needle punched onto a scrim material and thus damage to the fibres is avoided. Furthermore, any loose fibres are retained within the unit by the scrim elements 42, 44 and therefore, in use, will not be inhaled by the patient or drawn into respiratory apparatus connected to the filter.

The filter elements 34, 36 are separated by spacer means in the form of a spacer element 46 having opposed convex faces. The filter element 34 is arranged against one of said faces so as to define a convex filter surface which is directed towards the port 18. The filter element 36 is arranged against the other convex face so as to define another convex filter surface which is directed towards the port 16.

The spacer element 46 is made of a very porous material which has substantially no filtration capacity relative to the filter elements 34, 36, ie it has substantially no capacity for filtering bacteria or viruses. Preferably the spacer element is made of a polypropylene non-woven wadding. In addition to assisting in providing the filter unit with outer surfaces having a generally convex shape, the advantages of which will be described hereinafter, the spacer element provides an improved filtration performance for a given amount of filter material. It has been found that the use of multiple filter elements physically separated by spacer elements having substantially no filtration capacity relative to the filter elements actually improves the performance of the filter elements and it is to be understood that although the embodiment has only two filter elements 34, 36, further filter elements separated one from another by respective spacer elements may advantageously be included in the filter element as required.

The element 38 is made of a hydrophobic material and is disposed between the filter elements and the port 16. The hydrophobic element may have some filtration capacity. Details of the preferred materials for, and construction of, the hydrophobic material are provided hereinbelow.

The element 40 is preferably made of a hygroscopic material which will retain heat and moisture from the patient's exhaled breath and is disposed adjacent the hydrophobic element 38 and between that element and the port 16. In a non-preferred embodiment, the element 40 may comprise a foam or corrugated paper treated with hygroscopic salts.

It will be appreciated that the hydrophobic element 38 is arranged to prevent the ingress to the filter elements 34, 36 of any liquid moisture in the patient's exhaled breath, condensation that may collect in the housing or excessive patient secretions. Thus the filtration ability of the elements 34, 36 is not reduced by accumulated moisture and their filtration capacity is substantially maintained over the lifetime of the product. It will also be appreciated that by acting as a barrier to the penetration of moisture into the filter elements, the hydrophobic element ensures that moisture is more effectively trapped in the heat and moisture exchange element 40 enhancing the moisture exchange capability of the filter apparatus.

The scrim material of the scrim elements 42, 44 is impregnated with a thermochromic pigment to define thermochromic means. As the patient breathes in, relatively cold gases (typically having a temperature of 20 to 25°C) are drawn through the filter apparatus and as the patient exhales, relatively warm gases (typically having a temperature of 30 to 35°C) pass back through the filter. The thermochromic pigment is selected to have two distinct colour states, one indicating the lower temperature range and one indicating the higher temperature range. Accordingly as the patient breathes, the scrim material changes colour indicating the change of temperature of the gases passing through the filter apparatus 10. It will be appreciated that since the housing is made of a clear plastics material, the change of colour of the scrim material can readily be observed allowing respiration to be monitored visually.

It will be understood that it is not essential that the housing is made of a transparent or semi¬ transparent material. Alternatively, in a non- preferred embodiment, windows may be provided in a housing made of an opaque material to allow at least a portion of the thermochromic scrim pieces to be observed.

An effective thermochromic pigment sold as 'chameleon pigment' can be obtained from Victor Plastics Ltd in England. This pigment is blue when the gas temperature in the housing is substantially in the range 30 through 35°C and turns clear when the temperature is in the range of 20 through 25°C.

An alternative filter unit 50 suitable for fitting in the two-part housing 12, 14 shown in Figure 1 will now be described with reference to Figure 2.

The filter unit 50 comprises a filter element 52 comprising electrostatically charged fibres in the form of a non-woven wadding. On each side of the filter element 52 there is a scrim element 54 consisting of a spun-bond polypropylene.

The filter unit 50 further comprises two hydrophobic elements 56. Suitable materials for and details of the construction of the filter elements 56 are provided hereinbelow. The filter element 52 and scrim elements 54 are disposed between the hydrophobic elements such that, in use, ingress of liquid moisture to the filter element 52 from both sides is substantially prevented.

The outermost elements 58 of the filter unit each consist of a spun-bond polypropylene scrim material. The outermost scrim elements 58 effectively encapsulate the other elements 52-56 of the filter unit.

The elements 52-58 of the filter unit 50 are cut so as to be generally circular for fitting between the ribs 24, 26 of the filter housing and are assembled in juxtaposed relationship. The elements 52-58 are maintained in that juxtaposed relationship by a joint 60 at the periphery of the filter unit.

It will be appreciated that the outer faces of the filter unit are convex. The convex outer faces are obtained by having a relatively thicker centrally disposed element, in this embodiment the filter element 52, and tightly compressing the joined edges of the element 52-58.

A suitable production method for forming the filter units 22, 50 is an ultrasonic cut and weld technique. However, other methods for forming a joint at the periphery of the filter unit may be used. Where the joint is formed by ultrasonic welding it is preferred that the joint 60 is made through the opposed edges of each of the elements of the filter unit as indicated in Figure 2. However, the various elements of the filter units 22, 50 may be cut and arranged to allow the joint to be made substantially only between the opposed edge regions of the outermost elements of the filter unit.

It will be appreciated that the above-mentioned method of manufacturing the filter unit has the advantage that the various elements are assembled in the required relationship and maintained in that relationship as a unit for assembly into the housing. This facilitates assembly of the filter apparatus 10 and ensures that each of the elements is trapped between the ribs 24, 26 to avoid bypass by the gas flow.

As mentioned above, advantages are obtained by providing the filter units 22, 50 with convex outer faces. Firstly, the filter unit is less prone to occlusion by formation of a film of liquid as described in connection with the flat filter element of the known filter apparatus. Secondly, the surface area of the filter unit is larger than a flat filter element having the same diameter. Since resistance to flow is inversely proportional to the surface area of the filter unit, the flow resistance is reduced for a unit having a given diameter. It will be understood that it is important that filter apparatus for filtering bacteria/viruses should provide the maximum resistance to penetration by bacteria/viruses and at the same time a minimum resistance to through flow of gas to facilitate breathing.

A further advantage afforded by the convex surfaces of the filter unit and their orientation relative to the ports 16, 18, is that gas entering the housing via the ports tends to be deflected radially to the edges of the filter unit. This improves the efficiency of the filter unit by utilising substantially the whole area of the filter unit rather than just the central position.

It is to be understood that the convex surface formation of the filter unit may advantageously be obtained by having a spacer element or elements which have convex faces and arranging the elements of the filter unit about those faces. However, generally convex filter surfaces can alternatively be obtained by tightly compressing the joined edges of the multi- element filter unit.

The hydrophobic elements 38, 56 are preferably made of non-woven microfibres consisting of polypropylene fibres coated with, or incorporating, a proprietary fluorocarbon preparation. It will, however, be appreciated that a variety of melt processable fibre forming plastics may be used to form the hydrophobic element and the hydrophobic additive may take the form of surfactants, silicones or waxes. The fibres should have a diameter of between two and five microns and are preferably arranged to provide a fibre density of between five and one hundred grams per square metre.

Preferably, the hydrophobic element or elements has a limited hydrophobicity. The skilled person will be aware that the hydrophobicity of the materials can be compared by generating a column of water above the filter element and measuring the height of the column; the height of the column that the element can support is taken as a measure of hydrophobicity.

An electrostatic filter element is able to support a water column of up to fifteen centimetres whereas a pleated glass filter element is able to support a column of at least fifty centimetres. The hydrophobic element or elements of the filter units 22, 50 should be able to support a column substantially in the range of twenty to forty-five centimetres and thus is characterised as possessing intermediate or limited hydrophobicity. It will be appreciated that the hydrophobic elements should possess sufficient hydrophobicity to prevent the passage of liquid at normal breathing pressures but should also be of low enough hydrophobicity to allow liquid to pass through should the breathing pressure become dangerously elevated.

It will be appreciated that it is not essential that all of the above-described features of the embodiment are included in filter apparatus and various of the features by themselves may advantageously be included in filter apparatus or a housing for at least one filter element. Furthermore, although the various features are intended primarily for inclusion in filter apparatus for use in connection with respiratory apparatus, some of those features may advantageously be included in gas filtration apparatus for more general use.

Claims

CLAIMS :

1. A filter unit for fitting in a filter housing, said filter unit comprising at least four elements which include at least one filter element and at least one hydrophobic element, said elements being disposed in a predetermined juxtaposed relationship such that the or each said filter element and the or each said hydrophobic element is disposed intermediate the outermost of said elements and opposed edge regions of at least said outermost elements being joined together such that each of said elements is maintained in said predetermined juxtaposed relationship.

2. A filter unit as claimed in claim 1, wherein said outermost elements comprise a scrim material.

3. A filter unit as claimed in claim 1 or 2, wherein said elements comprise a plurality of filter elements separated one from the other by spacer means having substantially no filtration capacity relative to each of said filter elements.

4. A filter unit as claimed in claim 3, wherein said spacer means defines opposed convex faces, a first said filter element being arranged against a first of said faces to define a first convex filter surface and a second said filter element being arranged against the second of said faces to define a second convex filter surface.

5. A filter unit as claimed in any one of claims 1 to 4, wherein the or each said hydrophobic element is arranged to substantially prevent the ingress of moisture to the or each said filter element at least from one side of the filter unit.

6. A filter unit as claimed in claim 5, comprising two said hydrophobic elements arranged to substantially prevent ingress of moisture to the or each filter element from each side of the filter unit.

7. A filter unit as claimed in claim 6, wherein the or at least one said hydrophobic element is able to support a column of twenty to forty-five centimetres of water.

8. A filter unit as claimed in claim 7, wherein the or at least one said hydrophobic element is able to support a column of substantially forty centimetres of water.

9. A filter unit as claimed in claim 6, 7 or 8, wherein the or at least one said hydrophobic element comprises non-woven microfibres.

10. A filter unit as claimed in claim 9, wherein said microfibres are coated with or incorporate a hydrophobic additive.

11. A filter unit as claimed in claim 9 or 10, wherein said microfibres have a diameter substantially in the range two to five microns.

12. A filter unit as claimed in claim 9, 10 or 11, wherein said microfibres are arranged to provide a fibre density substantially in the range five to one hundred grams per square metre.

1 . A filter unit a claimed in any one of the preceding claims, wherein said elements comprise a heat and moisture exchange element.

14. A filter unit as claimed in claim 6 when dependent on claim 5, wherein said heat and moisture exchange element is disposed adjacent said hydrophobic element, said hydrophobic element being disposed between said heat and moisture exchange element and the or each said filter element.

15. A filter unit as claimed in any one of claims 3 to 14 when dependent on claim 2, wherein said scrim material carries thermochromic means for indicating the temperature of gaseous matter which, in use, passes through the filter unit.

16. A filter unit as claimed in claim 15, wherein said thermochromic means has a first distinct colour state indicating a temperature in the range 30 through 35°C.

17. A filter unit as claimed in claim 15 or 16, wherein said thermochromic means has a second distinct colour state indicating a temperature in the range 20 through 25°C.

18. Filter apparatus comprising a housing housing a filter unit as claimed in any one of the preceding claims.

19. Apparatus as claimed in claim 18 when dependent on any claim dependent on claim 4, wherein said housing comprises opposed port means and said filter unit is arranged such that said first convex surface faces one of said port means and said second convex surface faces the other of said port means.

20. Apparatus as claimed in claim 18 or 19, wherein said housing is adapted such that liquid condensing from gaseous matter passing therethrough, in use, is substantially prevented from forming droplets on at least an internal surface thereof.

21. Apparatus as claimed in claim 20, wherein said housing is made of a material comprising surfactant means.

22. Apparatus as claimed in claim 20, wherein the or at least one internal surface of said housing is coated with surfactant means.

23. Apparatus as claimed in any one of claims 18 to 22, when dependent on claim 15, 16 or 17, wherein said housing is adapted such that at least a portion of said thermochromic means is visible.

24. Apparatus as claimed in any one of claims 18 to 23 wherein said housing is made of a transparent or semi-transparent plastics material.

25. Apparatus as claimed in any one of claims 18 to 24, wherein said housing comprises two parts, each said part comprising means adapted to clamp said filter unit in position when said parts are joined together.

26. A filter unit as claimed in any one of the preceding claims, wherein the or at least one said filter element is an electrostatic filter element.

27. A method of manufacturing a filter unit to be fitted in a filter housing, the filter unit comprising at least four elements including at least one filter element and at least one hydrophobic element and the method comprising the steps of arranging said elements in juxtaposed relationship such that the or each said filter element and the or each said hydrophobic element is disposed intermediate the outermost of said elements and joining together opposed edge regions of at least said outermost elements such that said elements are maintained in said juxtaposed relationship.

28. A device comprising (i) a housing for housing at least one filter element and (ii) thermochromic means mountable in said housing, said housing being adapted such that when said thermochromic means is mounted in said housing at least a portion of said thermochromic means is visible whereby, in use, the temperature of gaseous matter passing through the housing indicated by said thermochromic means can be observed.

29. A device as claimed in claim 28, wherein said thermochromic means has a plurality of distinct colour states according to the temperature of such gaseous matter.

30. A device as claimed in claim 29, wherein said thermochromic means has a first colour state indicating a temperature substantially in the range 20 through 25°C and a second colour state indicating a temperature substantially in the range 30 through 35°C.

31. A device as claimed in claim 28, 29 or 30, wherein said thermochromic means comprises a non-woven material carrying a thermochromic pigment.

32. Filter apparatus comprising a device as claimed in any one of claims 28 to 31 and at least one filter element housed in said housing adjacent said thermochromic means.

33. A method of monitoring breathing of a patient connected to respiratory apparatus for use in anaesthesia or ventilation support, the method comprising the steps of providing thermochromic means in a flow path between the patient and said apparatus and observing the colour state of said thermochromic means.

34. A method as claimed in claim 33, wherein said thermochromic means is included in a bacterial/viral filter apparatus.

35. A method as claimed in claim 33 or 34, wherein said thermochromic means is selected to have a first colour state indicating the range of temperature of exhaled breath and a second colour state indicating the range of temperature of gaseous matter to be supplied by said apparatus.

36. A method as claimed in claim 35, wherein said first colour state indicates a temperature in the range 30 through 35°C.

37. A method as claimed in claim 35 or 36, wherein said second colour state indicates a temperature in the range 20 through 25°C.

38. A housing for at least one filter element wherein the surface energy of at least an internal surface of the housing is adapted such that, in use, liquid condensing from gaseous matter passing through said housing is substantially prevented from forming droplets on said surface.

39. A housing as claimed in claim 38, wherein said housing is made of a material which comprises a surfactant.

40. A housing as claimed in claim 38, wherein the or at least one said internal surface is coated with a surfactant.

41. Filter apparatus comprising a housing as claimed in claim 38, 39 or 40, and at least one filter element housed in said housing.

42. Filter apparatus comprising a housing and filter means disposed in said housing, said filter means defining at least one convex surface and being arranged such that the or a first said convex surface is generally opposed to a port provided in said housing.

43. Apparatus as claimed in claim 42, wherein said filter means defines a second said convex surface, said second convex surface being arranged to be generally opposed to a second port provided in said housing.

44. Apparatus as claimed in claim 43, further comprising spacer means having opposed convex faces, said filter means comprising a first filter element arranged against one said face for defining said first convex surface and a second filter element arranged against the other said face for defining said second convex surface.

45. Apparatus as claimed in claim 44, wherein the edge regions of said filter elements are at least partially joined together whereby said spacer means is retained between said filter elements.

46. Apparatus as claimed in claim 44 or 45, wherein said spacer means comprises an element having substantially no filtration capacity relative to said filter elements.

47. Filter apparatus comprising a housing, a plurality of filter elements disposed in said housing and spacer means separating said filter elements from one another, said spacer means having substantially no filtration capacity relative to said filter elements.

48. Apparatus as claimed in claim 47, wherein at least one said filter elements comprises an electrostatically charged material.

49. Apparatus as claimed in claim 47 or 48, wherein said spacer means comprises respective layers of material disposed between said filter elements.

50. A filter unit comprising a plurality of elements which elements include at least one filter element and a plurality of hydrophobic elements, said hydrophobic elements being arranged such that, in use, ingress of moisture to the or each said filter element is substantially prevented.

51. A filter unit as claimed in claim 50, comprising two said hydrophobic elements, wherein the or each said filter element is disposed intermediate said hydrophobic elements.

52. A filter unit as claimed in claim 50 or 51, wherein said elements are disposed in a predetermined juxtaposed relationship, said elements being maintained in said predetermined juxtaposed relationship by a joint at the periphery of said filter unit.

53. A filter unit as claimed in claim 50, 51 or 52, wherein the or at least one said hydrophobic element is able to support a column of twenty to forty-five centimetres of water.

54. A filter unit as claimed in claim 53, wherein the or at least one said hydrophobic element is able to support a column of substantially forty centimetres of water.

55. A filter unit as claimed in any one of claims 50 to 5 , wherein the or at least one said hydrophobic element comprises non-woven microfibres.

56. A filter unit as claimed in claim 55, wherein said microfibres are coated with or incorporate a hydrophobic additive.

57. A filter unit as claimed in claim 55 or 56, wherein said microfibres have a diameter substantially in the range two to five microns.

58. A filter unit as claimed in claim 55, 56 or 57, wherein said microfibres are arranged to provide a fibre density substantially in the range five to one hundred grams per square metre.

59. A housing for at least one filter element, a filter unit or filter apparatus substantially as hereinbefore described with reference to Figure 1.

60. A method of monitoring breathing of a patient connected to respiratory apparatus for use in anaesthesia or ventilation support conditions or a method of manufacturing a filter unit to be fitted in a filter housing substantially as hereinbefore described with reference to Figure 1.