Heat recovering ventilators are particularly useful for ventilating rooms in domestic premises, for example kitchens, bathrooms, bedrooms and living rooms.

It is known to install heat recovering ventilators in an opening formed in a wall of a room to be ventilated, the opening extending from the inside of the room to the exterior of a building of which the room forms a part. It is relatively easy to form such openings using rotating cutting equipment such as drills which produce openings having a substantially circular cross-section. Furthermore, such openings may be readily lined with for example plastics tubes.

The size of opening required in a particular application is a function of the efficiency of any heat recovering heat exchanger located in that opening. That efficiency is in turn a function of the cross-sectional area of the heat exchanger in a direction transverse to the length of the opening. Conventionally heat exchangers have been of generally square or rectangular cross-section as it is relatively easy to make a heat exchanger from a series of identical spaced apart rectangular plates the edges of which are appropriately sealed together or left open so as to define the necessary flow passageways for the two fluids between which heat is to be exchanged. Unfortunately, square cross-section heat exchangers cannot efficiently use the cross-sectional area of a tubular opening and therefore to accommodate a heat exchanger of adequate size it is necessary either to form a rectangular section opening in which the heat exchanger is a sliding fit or to form a circular section opening the diameter of which is substantially larger than the length of any one side of the rectangular section heat exchanger. Attempts to overcome this problem have included structures in which the heat exchanger is formed from a roll of flexible material with fluid passageways being defined between successive layers of the roll, but in such structures a central section of the roll is left open to form a wide passageway for conveying air between one end of the heat exchanger and the other. Thus once again the cross-sectional area of the heat exchanger is substantially less than the cross- sectional area of the tubular opening in which the heat exchanger is located.

In one known heat recovering ventilator system a heat exchanger is located in an opening between two impellers which are independently driven. One impeller drives a first fluid such as air from one end of the heat exchanger to the other and the other impeller drives a second fluid in the opposite direction from the other end of the heat exchanger. Such an arrangement has proved to be less than ideal. Firstly, noise from ventilators is not welcome and given that one of the impellers is located at the end of the opening adjacent the ventilated room it is difficult to limit the level of noise penetrating the room. Furthermore the impeller located at the room end of the opening draws air from the room which can be contaminated with for example fat from a kitchen. Such contamination can rapidly reduce impeller efficiency. A further problem arises from the location of the second impeller at the end of the opening remote from the room. It is necessary to be able to service the unit which requires access to the impellers at both ends of the heat exchanger, and a structure which enables the replacement of the heat exchanger between the impellers. It is difficult to provide a unit which can be serviced without costly and time consuming effort by a servicing engineer.

Heat recovering ventilator systems are known in which a single motor drives an impeller system which drives both the first and second fluids through the heat exchanger. With such an arrangement if the single motor and impeller assembly is located at the end of the opening adjacent the room, the above mentioned noise and contamination problems arise. If the single motor and impeller system is located at the end of the opening remote from the room the above mentioned servicing problems arise. Furthermore, known arrangements relying upon a single motor have structures which result in flow paths which are tortuous and therefore present a high resistance to flow, in turn requiring relatively powerful motors and relatively large impellers to achieve adequate performance.

A heat recovering ventilator system is described in international patent application number W091/08425. In the described system heat exchange components are arranged substantially coaxially with the axis of a substantially tubular opening. In this arrangement a core area within the tubular opening is not utilise for heat exchange purposes and the flow paths are tortuous and of high resistance.

It is an object of the present invention to provide a heat exchanger which obviates or mitigates one or more of the problems outlined above.

According to the present invention, there is provided a heat exchanger for exchanging heat between first and second fluids, comprising a tubular housing of uniform substantially circular cross section in a direction perpendicular to a longitudinal axis of the housing, and a stack of parallel heat exchanger plates received within the housing and supported such that a space is defined between each pair of adjacent plates, each plate extending in a direction parallel to the longitudinal axis, seals being defined at edges of the plate such that one space of each adjacent pair of spaces communicate with an inlet and an outlet for the first fluid and the other space of the pair of spaces communicates with an inlet and outlet for the second fluid, and one inlet and one outlet being located at each end of the tubular housing, wherein the width of the plates varies with distance from the longitudinal axis of the housing such that edges of the plates extending parallel to the longitudinal axis abut an inner surface of the tubular housing.

The invention also provides a heat exchanger for exchanging heat between first and second fluids, comprising an elongate housing extending parallel to a longitudinal axis of the housing, and a stack of parallel heat exchanger plates received within the housing and supported such that a space is defined between each adjacent pair of plates, each plate extending in a direction parallel to the longitudinal axis, seals being defined at edges of the plates such that one space of each adjacent pair of spaces communicates with an inlet and outlet of the first fluid and the other space of the pair of spaces communicates with an inlet and an outlet for the second fluid, and one inlet and one outlet being located at each end of the tubular housing, wherein at least one pair of adjacent plates is formed from single strip of material which is folded.

The invention also provides a heat exchanger for exchanging heat between first and second fluids, comprising an elongate housing extending parallel to a longitudinal axis of the housing, and a stack of parallel heat exchanger plates received within the housing and supported such that a space is defined between each adjacent pair of plates, each plate extending in a direction parallel to the longitudinal axis, seals being defined at edges of the plates such that one space of each adjacent pair of spaces communicates with an inlet and outlet of the first fluid and the other space of the pair of spaces communicates with an inlet and an outlet for the second fluid, and one inlet and one outlet being located at each end of the tubular housing, wherein at least one pair of adjacent plates is formed from separate first and second strips of material, at least one edge of the first strip which extends parallel to the longitudinal axis overlapping and forming a seal with an edge of the second strip.

The invention also provides a heat exchanger for exchanging heat between first and second fluids, comprising an elongate housing extending parallel to a longitudinal axis of the housing, and a stack of parallel heat exchanger plates received within the housing and supported such that a space is defined between each adjacent pair of plates, each plate extending in a direction parallel to the longitudinal axis, seals being defined at edges of the plates such that one space of each adjacent pair of spaces communicates with an inlet and outlet of the first fluid and the other space of the pair of spaces communicates with an inlet and an outlet for the second fluid, and one inlet and one outlet being located at each end of the tubular housing, wherein the housing is a sliding fit in a carrier tube one end of which supports an electric motor, the carrier tube is a sliding fit in an outer support tube which in use extends between inner and outer faces of a wall, the motor is supported by the carrier tube so as to project from the outer end of the support tube, and wires for energising the motor extend along the length of and are supported by a wall of the carrier tube such that the wires are protected against contact with either the housing or the support tube.

Preferably, at least one pair of adjacent plates is formed from a single strip of material which is folded. The single strip of material may be pre-formed with one or more hinge-defining formations about which the strip is foldable.

Preferably, at least one pair of adjacent plates is formed from separate first and second strips of material, at least one edge of the first strip which extends parallel to the longitudinal axis overlapping and forming a seal with an edge of the second strip.

Edges of the strips which in the heat exchanger extend parallel to the longitudinal axis may be pre-formed. each pre-formed edge being inclined to the plate of which it forms an edge such that when the adjacent plates are placed together the pre-formed edges contact each other in an overlapping relationship.

The plates may be dimensioned such that the housing presses the overlapping plate edges together. Preferably, at least some of the overlapping plate edges are secured together with tape adhered to the outer surfaces of the overlapping plate edges.

Each plate may define projections which serve to maintain a desired spacing between the plate.

Preferably, the formations of adjacent plates cooperate to define seals along edges of the plates extending parallel to the longitudinal axis.

Preferably, welded seals are formed between edges of the plates which are not parallel to the longitudinal axis, the welded seals being formed between alternate pairs of plates at each inlet and outlet.

Preferably, each plate comprises a rectangular central section and triangular end sections extending from opposite ends of the rectangular central section, the apexes of the triangles remote from the central section at each end of the heat exchanger being aligned, and the seals being formed such that the aligned apexes are located between a respective inlet and outlet.

Preferably, the housing comprises end sections which extend over apexes of the triangles and define axially extending separators between the adjacent inlets and outlets.

Preferably, the tubular housing is a sliding fit in a carrier tube one end of which supports and electric motor, the carrier tube is a sliding fit in an outer support tube which in use extends between inner and outer bases of a wall, the motor is supported by the carrier tube so as to project from the outer end of the support tube, and wires for energising the motor extend along the length of and are supported by a wall of the carrier tube such that the wires are protected against contact with the housing or the support tube.

Preferably, the wires are embedded in grooves formed in the wall of the carrier tube.

The electric motor may drive an inlet and an outlet impeller, the inlet impeller driving air into the adjacent inlet, and the outlet impeller drawing air from the adjacent outlet.

Preferably, the impellers are mounted on opposite sides of a flange with which the impellers rotate, one impeller being located on one side of a partition in communication with the adjacent inlet and the other impeller being located on the other side of the partition in communication with the adjacent outlet, the flange overlapping edges of an opening in the partition, and the spacing between the flange and the overlapped edges being sufficiently small as to minimise leakage between opposite sides of the partition.

Preferably, the inlet impeller is positioned downstream of an inlet passage in which curved blades are arranged to cause rotation of air flowing through the inlet in the same direction of the direction of rotation of the inlet impeller.

Preferably, the electric motor is supported in a casing which is connected to the said one end of the carrier tube.

Preferably, the outer end of the support tube defines a lip the internal diameter of which is greater than the external diameter of the electric motor but less than the external diameter of the support tube, whereby the lip prevents the carrier tube projecting beyond the outer end of the support tube.

Embodiments of the present invention will now be described, by way of example, with reference to the accompanying drawings, in which; Figure 1 is a schematic representation of a known heat exchanger used in a heat recovering ventilator system located in a tubular opening in which peripheral portions of the tubular opening are not occupied by the heat exchanger; Figure 2 is a schematic representation of a further known heat exchanger in which a core area of an opening is not occupied by the heat exchanger; Figure 3 is a schematic representation of a heat exchanger according to the present invention; Figure 4 is a schematic representation of the heat exchanger of Figure 3 showing how adjacent plates in the heat exchanger may be formed; Figures 5a and 5b are schematic representations showing the shape of plates used in a conventional heat exchanger such as that represented in Figure 1; Figures 6a and 6b are schematic representations showing the shape of heat exchanger plates used in the embodiment of the present invention shown in Figure 3; Figure 7 is a schematic representation of a heat exchanger plate forming part of the heat exchanger of Figure 3 in plan view showing flow paths; Figures 8a and 8b schematically represent an impeller assembly used in association with the heat exchanger of Figure 3; Figure 9 is a schematic representation similar to that of Figure 4 showing details of formations which maintain separation and sealing between adjacent plates; Figure 10 is a schematic representation of a plate used in the heat exchanger of Figure 3 and incorporating edge formations used to enable the plates to be welded together; Figure 11 is a schematic representation of a plate used in the heat exchanger of Figure 3 showing plate separation formations in the form of raised ribs; Figures 12 and 13 are schematic representations showing how a stack of plates may be formed by folding a single strip of material; Figures 14a to 17b are schematic representations of different hinge formations which may be used to facilitate folding of a single strip of material so as to form a stack of adjacent plates.

Figure 18 is a schematic representation showing a seal formed in a heat exchanger of the present invention; Figure 19 is a schematic representation of an alternative sealing arrangement for two impellers of the same general type is illustrated in Figures 8a and 8b ; Figures 20a and 20b are schematic representations of a stator used with the impellers of Figure 19; Figure 21a is a schematic representation of an end view of an alternative plate structure of a heat exchanger in accordance with the present invention; Figure 21b is an enlarged view of part of Figure 21a showing in detail the interconnection of adjacent heat exchanger plates; Figure 22 is a plan view of one plate of the heat exchanger of Figures 21 a and 21b; Figure 23 is a plan view of another plate of the heat exchanger of Figures 21 a and 21b ; Figure 24 is a perspective view of a heat recovering ventilator system in accordance with the present invention; Figure 25 is a axial section through the assembly of Figure 24 after its insertion into an opening in a wall; Figures 26 and 27 are partly exploded views of the assembly of Figure 24; Figure 28 is a perspective view of a heat exchanger in accordance with the present invention which is incorporated in the assembly of Figure 24; and Figure 29 is an end view of the heat exchanger of Figure 28 inserted in a carrier tube.

Referring to Figure 1, a heat exchanger 1 of generally rectangular cross- section is received within an opening defined by a tube 2. Portions of the cross- section of the tube 2 which are not occupied by the heat exchanger are identified by inclined parallel lines. It will be noted that a large proportion of the cross-section of the tube 2 is not occupied by the heat exchanger 1.

Figure 2 shows an alternative known heat exchanger 1 which again is mounted inside a tube 2. In the case of Figure 2, the heat exchanger is formed by rolling up a strip of material and maintaining spaces between adjacent turns of the roll so as to provide fluid flow passageways. A central portion of the cross-section of the tube 2 which is identified by inclined parallel lines is left open to provide a through-flow passageway of low resistance and thus once again a substantial proportion of the cross-sectional area of the tube 2 is not occupied by the heat exchanger 1.

Figure 3 is a schematic representation of an embodiment of the present invention. In the arrangement of Figure 3 the heat exchanger 1 is formed from a stack of parallel plates 3 retained within a tube 2. The plates 3 reduce in width with distance from a central axis of the tube 2 such that the edges of the plates 3 extending parallel to the longitudinal axis about an inner surface of the tube 2. Thus the only portions of the cross-sectional area of the tube 2 which are not occupied by heat exchanger are the areas 4 which are identified by parallel inclined lines. It is immediately apparent from a comparison of the arrangements shown in Figures 1,2 and 3 that a structure in accordance with the present invention occupies a substantially greater proportion of the cross-sectional area of a tubular opening as compared with the prior art arrangements.

Referring to Figure 4, one method for achieving a structure of the type illustrated in Figure 3 is illustrated. In Figure 4, a stack of nine plates 3 is formed from a single strip of material by folding that strip along fold lines 5, each plate extending between a respective pair of fold lines. Adjacent plates are separated by portions 6 of the strip which are defined between adjacent pairs of folds 5 and abut the inner surface of the tube 2. The strip of material is formed from a flexible plastics sheet and when folded is a slightly oversized sliding fit within the tube 2, the strip material being sufficiently flexible to enable the act of sliding the structure into the tube 2 to slightly compress the assembly so that the folds 5 are pressed against the inner wall of the tube 2.

Adjacent pairs of plates 3 define a series of spaces and end edges of the plates 3 are interconnected as described below such that spaces 7 extend between an extracted air inlet and an extracted air outlet and spaces 8 extend between an incoming air inlet and an incoming air outlet, one inlet and one outlet being provided at each axial end of the heat exchanger.

The individual plates are interconnected so that at each end of the heat exchanger the adjacent inlet and outlet are arranged on opposite sides of a diameter of the tube 2 which is parallel to the plane of the plates 3. Each of the plates 3 could as in a conventional heat exchanger be rectangular in plan view as illustrated in Figure Sa. In Figure 5a, the side edges which in the heat exchanger would be parallel to the longitudinal axis of the tube 2 are indicated by lines 9 and the axially end edges are represented by lines 10 and 11. A line 12 represents the boundary between an inlet (corresponding to the location of the line 10) and an outlet (corresponding to the location of the line 11). Dimension A represents the width of the inlet and outlet and dimension B the overall width of the heat exchanger. As shown in Figure 5b, the heat exchanger has a height C. Thus the area of the heat exchanger through which all inlet air must pass is the dimension A multiplied by the dimension C.

Referring now to Figures 6a and 6b, plates having a shape which may be used to advantage in accordance with the present invention are shown. Such plates incorporate a central substantially rectangular section 13 and at each end of that rectangular central section a triangular end section 14. The lines 9,10,11 and 12 represent essentially the same elements as in Figure 5a. It will be seen that for the same overall width B of the plates the area of full flow corresponds to the dimension C multiplied by the dimension D. Thus, by shaping the plate so as to have triangular end sections for a given plate width the full flow area can be relatively increased.

This reduces the resistance of flow by as much as 50% or more. Although in Figure 6b, the plates are shown as all being of uniform width, it will be appreciated that in the arrangement of Figure 3 the width of any one plate will be a function of the distance of that plate from the longitudinal axis of the tubes. The same considerations apply however with regard to the increase in flow area achieved by using a plate having triangular end edges.

Figure 7 is a plan view of a plate such as that shown in Figure 6a indicating the location of inlet edges 10 and outer edges 11. It will be seen that air flows in the direction indicated by arrow 15 from one inlet to the outlet with which that inlet is in communication, and air flows in the direction indicated by arrow 16 between the other inlet and the other outlet. Seals must be formed between the longitudinal edges 9 of the plate and the supporting tube and seals must be formed between selected edges 10 and 11. For example, assuming two plates were superimposed one upon the other and a flow represented by line 15 passed between those two plates whereas the flow represented by line 16 passed above and below those two plates, the adjacent edges represented by lines 10 at the top left hand corner of Figure 7 would be welded together and the adjacent edges represented by line 11 at the bottom right hand corner of Figure 7 would be welded together. Flow could pass between the two plates therefore only from the inlet the position of which is represented by line 10 at the bottom left hand corner of Figure 7 to the outlet the position of which is represented by line 11 at the top right hand corner of Figure 7.

Referring now to Figures 8a and 8b, a double impeller fan system is illustrated which is as described in greater detail below is positioned at one end of the heat exchanger structure as described in Figure 3. An electric motor 17 drives an upper impeller 18 which communicates with the inlet edges 10 of the heat exchanger 1 and a lower impeller 19 which communicates with the outlet edges 11 of the heat exchanger 1. The impellers 18 and 19 rotate together and are arranged on opposite sides of a rotating flange 20. The impeller extends through an opening in a partition 21 so as to overlap an edge 22 of that opening. The overlap between the flange 20 and the partition 21 leaves a relatively small gap through which only minimal leakage will occur. The impeller is mounted on a structure described in greater detail below which extends through an opening in a wall 23. Air is drawn into the impeller 18 through an opening 24 in a casing of the impeller and air is ejected by the impeller 19 through an opening 25 in the impeller casing. That casing defines a supply impeller volute 26 and an extract impeller volute 27. The flow of incoming air is indicated by arrows 28 and the flow of extracted air as indicated by arrows 29.

Referring to Figure 9, this is a view similar to that of Figure 4 but showing pre-formed moulded formations 30 and 31 which serve both to appropriately locate adjacent layers of the folded plate structure and to improve sealing at the periphery of the folded structure. In the arrangement of Figure 9, two separate folded structures are provided which are symmetrical about a diameter of the tube 2, the two folded structures being interengaged at the top of Figure 9 by formations 30 and 31 and at the bottom of Figure 9 by overlapping folded sections 32 and 33 of the two structures.

The folds from which the overlapping sections 32 and 33 extend are formed such that in their free state the sections 32 and 33 would extend outwards, the tube serving to press the two sections 32 and 33 inwards such that they interengage and form a seal.

Referring to Figure 10, this is a view similar to that of Figure 7 but showing end flaps 34 and 35. These end flaps enable selected pairs of adjacent plates to be welded together by forming a weld between the flaps 34 and 35 of adjacent plates. It will be appreciated that in any stack of three plates, the flap 34 of the intermediate plate of the three will be welded to one of the adjacent plates whereas the flap 35 of the intermediate plate will be welded to the corresponding flap of the other adjacent plate.

Referring to Figure 11, this is a view of one of the plates similar to that of Figure 7 but showing pre-formed ribs 36 which maintain separation between central portions of two adjacent plates. The ribs 36 shown in full lines in Figure 11 are inclined to the ribs of each adjacent plate the positions of which are indicated in broken lines. The arrow 37 indicates the direction of flow above the plate shown in full lines in Figure 11 whereas the broken line 38 shows the flow path beneath the plate shown in full lines in Figure 11.

Figures 12 and 13 show details of pre-formed hinge formations. A single sheet of material 39 has pre-formed within it fold formations 40 extending downwards from the strip and fold formations 41 extending upwards from the strip. The formations are such that folds are relatively easy to form in a direction which closes the gap defined by each of the formations and thus upwardly directed formations are suited for folding in one direction relative to the initial plane of the strip 39 whereas downwardly directed formations are suitable for forming folds in the opposite direction.

Figure 13 shows a structure which can be formed from a strip with pre-formed fold lines as shown in Figure 12. It will be noted that not all of the formations do in fact form fold lines. Some of the formations define ribs which maintain separation between adjacent plates. Formations can be manufactured such that some are relatively thin so that they fold relatively easily and some are relatively thick such that they do not fold relatively easily and can maintain appropriate separation between adjacent plates.

Referring to Figures 14a to 17b, Figure 14a shows two hinge-forming formations which enable the illustrated strip to be folded in the direction of the arrows into the configuration shown in Figure 14b. Similarly, Figures 15a, 16a and 17a show formations which can be used to form folded structures as shown in Figures 15b, 16b and 17b respectively.

Referring to Figure 18. formations which can be provided on the end edges of the heat exchanger are illustrated for two mutually adjacent pairs of plates. In each pair, an upper plate is indicated by reference numeral 42 from which a portion 43 depends downwards and a further portion 44 extends outwards. A lower plate of each pair is indicated by line 45, that lower plate incorporating an upstanding portion 46, a portion 47 extending parallel to the plane of the plate 45, a downwardly extending portion 48, and an outwardly extending portion 49. The mutually adjacent portions 44 and 49 can be readily welded together. Such a structure establishes spaces 50 and 51, the spaces 50 communicating with a first flow path and the space 51 communicating with a second flow path.

Referring to Figure 19, an alternative impeller structure to that illustrated in Figures 8a and 8b is illustrated. The structure defines a recess 52 which receives a motor body (not shown), the motor driving two impellers 53 and 54 which are arranged on opposite sides of a flange 55. The flange is received in a slot defined between portions 56 and 57 project from the partition wall indicated in Figure 8a by numeral 21. Such an arrangement reduces the leakage rate between the two separate compartments in which the impellers 53 and 54 are housed as compared with the arrangement of Figure 8a and 8b.

Figures 20a and 20b illustrate a stator structure 58 into which the impeller assembly of Figure 19 may be introduced. The stator 58 supports three (or any other prime number) of blades 59 which will cause air flowing into the impeller structure to rotate in the direction of rotation of the adjacent impeller blades. The direction of rotation of the air flow is indicated by arrows 60.

Referring now to Figures 21a and 21b, an alternative heat exchanger plate assembly is shown in which individual plates are formed from separate strips of material the edges of which overlap to provide the necessary seals. The heat exchanger housing tube 2 receives within it twelve heat exchanger plates 3 arranged in two sets of six, the assembly being symmetrical about the plane indicated by line 61. Each plate has an upstanding portion 62 adjacent its peripheral edge which as shown in greater detail in Figure 21b supports a flexible downwardly extending portion 63. Formations 64 (Figure 21b) provide a stable surface against which the upper edge of the immediately adjacent plate 3 bears. In Figure 21b, the portions 63 are shown in their free state which they will adopt if a series of plates is simply stacked one upon the other. Such a stack of plates may then be pushed into a tubular housing so that the portions 63 are pressed down against each other in an overlapping relationship so as to form a secure seal. Alternatively, before the stack of plates is inserted into a tubular housing, an adhesive tape may be wrapped around or secured on part of the assembly so as to hold the overlapping portions 63 in close overlapping engagement. The portions 63 thus form a structure similar to an array of roof tiles, resiliency in the portions 63 ensuring that a reliable seal is achieved.

Figures 22 and 23 show two of the plates which may be components in a stack of plates such as that shown in Figure 21 a. It will be seen that the plates carry numbers which are moulded in plastics strips from which the plates are made so as to make it easier to correctly assemble a stack of plates. The plates are pre-formed with ribs 65 which serve to maintain appropriate separations between adjacent plates and which extend in the direction of flow on the side of the plates from which they project. Each elongate side of each plate has an upstanding rib 66 from which depends a skirt 67. The ribs 66 provide a degree of sealing between adjacent plates.

The angle at which the skirts 104 depend from the plates is determined by the relative position of the plates with the stack as will be appreciated from the representation of the stack shown in Figure 21 B. Each skirt 67 has formed within it a plurality spaced apart ribs 68, the ribs 68 of adjacent plates interengaging to assist in the proper location of one plate upon the other. The skirts 67 of adjacent plates overlap and are pressed against each other so as to provide good sealing.

Referring now to Figure 24 and 25, an embodiment of a heat recovery ventilator in accordance with the present invention will now be described. In Figure 24, an outer leaf 69 of a cavity wall and an inner leaf 70 of that wall has an opening 71 drilled through it. That opening 71 is lined with an outer support tube 72 which is secured in position by, for example, resilient seals 73 each of which engages in a respective leaf of the wall. The support tube 72 has at its inner end a housing 74 which fits flush against an inner face of the inner leaf 69 of the wall. The support tube 72 receives as a sliding fit a carrier tube 75 (Figure 29) which in turn receives within it as a sliding fit a heat exchanger 76 (Figures 28 and 29). The outer end of the carrier tube 75 supports a motor casing 77, power being supplie to the motor via wires 78 which are embedded in grooves formed in the watt of the carrier tube 75. An end edge 79 of the heat exchanger engages with a divider 80 which in turn engages with an internal cover 81. A filter 82 may be retained on the cover 81 by an apertured cover plate 83.

As best seen from Figure 25, the carrier tube 75 and motor casing 77 define a tapering tubular assembly which can be pushed readily into the support tube 72, the angle of taper being indicated in Figure 25 by angle 84. Thus after removal of the cover 81, the heat exchanger 76 can be simply pulled out for replacement or maintenance (for example, simply washing in soapy water). Furthermore, the carrier tube 75 can be pulled out of the support tube 72 so as to make the motor accessible for servicing. The outer end of the support tube 72 defines an inwardly extending flange (not shown) against which the outer end of the carrier tube 75 bears, the flange being demensional so as to enable the motor casing 77 to project beyond the end of the support tube 72 but to prevent the outer end of the carrier tube 75 projecting from the end of the support tube 72.

In many circumstances the outer end of the support tube will not be readily accessible. If it is, an external bezel 85 (Figures 26 and 27) can be fitted around the outer end of the support tube 72 so as to improve the external appearance of the assembly.

It will be appreciated that as the wires 78 are embedded in the wall of the carrier tube 75 it is possible to slide the heat exchanger 76 into the carrier tube 75 without risking damage to the wires 78 and similarly it is possible to slide the carrier tube 75 into the support tube 72 without risking damage to the wires 78. The heat exchanger 76 supports ribs 86 in which slots 87 are formed, those slots being aligned with the position of the embedded wires 78 so as to ensure that the heat exchanger 76 can only be inserted into the carrier tube 75 in an appropriate orientation.