Air-to-air heat exchanger for ventilation systems with two countercurrent air flows disposed inside a cylindrical housing, a first air flow circulating inside the heat exchanger inside closed pipes, while the second air flow is in spaces between the pipes and cylindrical housing, and a fan moving the countercurrent air flows and disposed at one end of the cylindrical housing, with the fan including concentric inner and outer rings separated by a wall for moving air in opposite directions, a bunch of straight, parallel pipes whose end elements at the fan side are tightly gathered together, in the end of a cylindrical wall and, on the opposite side, in the end of a cylindrical pipe fitting, and between end elements, taper into middle sections between which are spaces, and a sleeve lining the inner wall of the housing at the middle sections and constricts the inner diameter of the housing.

BACKGROUND OF THE INVENTION

The invention relates to an air-to-air heat exchanger for ventilation systems.

The insulation values required of building envelopes under current building regulations relative to heated buildings are such that controlled ventilation is a necessity. Such ventilation serves to transport stale air, i.e. air carrying pollutants and moisture, to the outside, replacing it with fresh, oxygen-filled exterior air. This used to be accomplished by means of regular ventilation via windows and doors, although this often cannot be done to a sufficient degree, for example when residents are absent for any length of time. Furthermore, such ventilation replaces the heated air on the inside with cold air from the outside, which in turn requires energy to heat the fresh air. For reasons of cost and ecology, this is not desirable.

Various types of ventilation installations based on heat recovery have long been in use. Part of the energy content of the heated waste air is transferred to the incoming fresh air. This is generally achieved using cross-flow heat exchangers or rotary heat exchangers, both of which are very complex in their construction, and hence relatively expensive. Reverse air ventilators are another variant, where a fan changes its direction of rotation at regular intervals, and therefore the direction in which air is conveyed, ensuring that a proportion of stale indoor air is first blown outside and then replaced with fresh air drawn inwards.

Counterflow heat exchangers are disclosed in, for example, DE 10 2006 051 903 A1, DE 10 2006 035 531 A1, DE 10 2005 045 734 A1, DE 10 2005 035 712 A1 and DE 10 2004 046 587 and EP 2 077 428 A2. German patent application DE 10 2008 058 817 A1, which represents the closest prior art, discloses an air-to-air heat exchanger operating according to the countercurrent principle where a first air flow is guided inside closed pipes whilst a second air flow, which flows counter to the first air flow, is located in an intermediate space between the pipes and the cylindrical exterior housing. For the purpose of moving the countercurrent air flows there is a fan disposed on one end of the cylindrical housing, comprising an inner ring and an outer ring disposed concentrically around the inner ring to transport air in the opposite direction. The spaces occupied by the outer ring and the inner ring are separated from each other by a cylindrical wall. In one embodiment, the pipes leaving the fan initially diverge conically, then run parallel and finally converge conically. In this way the second air flow circulates around the pipes in the intermediate space between the pipes, thereby allowing an efficient exchange of heat.

The design of this heat exchanger is relatively complex, however, and correspondingly costly to produce. In addition, further ways of improving the efficiency of this heat exchanger are still being sought.

SUMMARY OF THE INVENTION

Hence it is a task of the present invention to provide design improvements to the above-described heat exchanger disclosed in DE 10 2008 058 817 A1, in particular by simplifying its design to make production more cost-effective. Furthermore, the optimised heat exchanger should be as effective, or even more so, than the state-of-the-art heat exchanger.

These tasks are solved according to the invention by means of an air-to-air heat exchanger.

The heat exchanger according to the invention comprises a bunch of straight parallel pipes, the end elements of which, on the side closest to the fan, are enclosed in the ring-shaped end of the cylindrical wall separating the spaces occupied by the outer ring and the inner ring, whilst the end elements of the pipes at the opposite side are enclosed in the end of a corresponding cylindrical pipe fitting. The end elements are disposed very close together, with no space inbetween, so that no air can circulate between adjacent end elements. Between the end elements, the pipes taper to form middle sections, between which there are intermediate spaces inside the bunch. This allows the countercurrent air, that is the second air flow, to enter into the bunch and circulate freely around the middle sections of the pipes. In the vicinity of these middle sections the inner wall of the housing is lined with a sleeve or cup-shaped insulating insert which can simply be inserted or pushed inside the housing, for example. The insulating insert reduces the inner diameter of the housing, thereby constricting the flow cross-section. The second air flow, which is directed through the outer ring, is sucked inwards by this constriction so that it necessarily circulates around the pipes and re-exits the bunch of pipes radially behind the insulating insert. The insulating insert serves primarily for thermal insulation but may additionally function as acoustic insulation.

The guiding of the first air flow inside the pipes is simplified in that straight and parallel pipes can be used. These need only be fitted with correspondingly contrived end elements which are disposed very close to each other and are suitable for enclosure in the cylindrical wall or opposite pipe fitting. Bonding, welding or similar can be used to seal off adjacent end elements with respect to each other. The pipes themselves can be made from pipes drawn in the customary manner, for example, whose ends are simply widened and shaped such that they can be assembled in the above-described manner without intermediate spaces. It is also possible to bring the end elements together at each end of the bunch of pipes to form a single one-piece component so that adjacent end elements are separated from each other by separating walls, forming a cross-section with a honeycomb structure.

In one preferred embodiment of this invention at least part of the end elements is provided with a polygonal cross-section. The sides of the polygons then form the contact and separating surfaces of the end elements.

This polygonal cross-section of the end elements is preferably a hexagon.

Further, at least the end elements at each end of the bunch are preferably contrived as a single piece made from an injection-moulded part and form a honeycomb structure inside this injection-moulded part.

Inside this injection-moulded part, the end elements may preferably be fitted with connecting elements to receive pipe sections forming middle sections. The injection-moulded part may also comprise the ring-shaped end section of the cylindrical wall or pipe fitting which encloses the end elements.

Further, the middle sections of the pipes preferably have a structured inside and/or outside surface. This creates turbulence which improves the heat transfer between the air flows. Such structures can be formed, for example, by beading or projections on the pipe surfaces.

According to another embodiment of the invention, the end elements are tightly connected with each other, with the ring-shaped end of the cylindrical wall and/or with the end of the opposite cylindrical pipe fitting by means of bonding or welding.

Further, the insulating insert preferably has end surfaces with a sloping cross-section, via which the inner diameter of the insulating insert transitions into the larger inner diameter of the adjacent inner wall sections of the housing. Starting from the fan, the second air flow therefore flows through the outer ring, comes into contact with one of these sloping end surfaces at one end of the insulating insert and is pressed via the latter towards the inside of the bunch of pipes. The insulating insert extends along a longitudinal section of the bunch which is sufficient to ensure an efficient heat exchange between the air flows. At the end of the insulating insert the flow cross-section widens again via a sloping end surface which guides the second air flow back outside around the cylindrical pipe fitting enclosing the pipe end elements opposite the fan.

Further, the inner wall of the housing preferably has a structured surface. This creates turbulence which ensures improved heat transfer between the air flows.

According to another preferred embodiment the insulating insert is made of expanded plastic.

DETAILED DESCRIPTION

The heat exchanger10shown inFIGS. 1 and 2is an air-to-air heat exchanger with a cylindrical housing12which is open at either end. At one end of the housing12(on the left of the figures) a fan14is inserted inside housing12. The axis of rotation of fan14corresponds to the pipe axis of housing12. The term “cylindrical” in relation to housing12is not intended to designate a perfectly cylindrical shape, but rather deviations from this are possible, such as a polygonal cross-section for example.

Fan14is driven by a motor16positioned on its centre axis. The space around motor16forms an inner ring18around which an outer ring20is arranged, which encloses inner ring18. Inner ring18and outer ring20are separated by a cylindrical wall22. Outer ring20is delimited towards the outside by the inside wall24of housing12.

Airflows are transported in opposite directions inside inner ring18and outer ring20. The air flow inside inner ring18will be referred to below as the first air flow, by means of which the air is transported out of housing12at the end of heat exchanger10where fan14is disposed. A second air flow is transported inside outer ring20, by means of which the air at the end fitted with the fan is transported into heat exchanger10.

For the purpose of moving these opposite air flows, the impeller of fan14extends radially outwards into outer ring20. Blades26of inner ring18are positioned opposite blades28of outer ring20so that when fan14rotates, the air in rings18,20can be moved in opposite directions, creating the first and second air flows as described above.

The arrangement of fan14, inner ring18, outer ring20and the separating cylindrical wall22is substantially disclosed in German patent application DE 10 2008 058 817 A1.

Inside heat exchanger10, the opposite air flows exchange heat with each other but are transported inside separate structures so that the two air flows cannot mix with each other. These structures comprise a central pipe bunch30comprising a plurality of straight, parallel pipes32. The end elements34of these pipes32have a cross-section which differs from the cross-section of middle sections36of pipes32between end elements34. Whilst in this instance, middle sections36have a circular cross-section, end elements34are widened to form a polygonal cross-section. In the top views inFIGS. 3 and 4, one can see that this polygonal cross-section is a regular hexagon. Due to this shape, end elements34can be gathered into a regular honeycomb structure as can easily be seen inFIGS. 3 and 4. No spaces remain between end elements34because these end elements34are disposed close together, so that air cannot circulate between them.

End elements34may be grouped together as one piece in a single injection-moulded plastic part with a cross-section having a honeycomb structure. This injection-moulded part can also comprise other connecting components such as connecting pieces for receiving pipe sections forming middle sections36. The ends of the middle sections can be bonded or inserted onto or into these connecting pieces.

As can be seen inFIG. 1in particular, middle sections36of pipes32are not positioned close to each other inside bunch30. Rather, there are spaces40between these middle sections36, inside which air can circulate.

At the end associated with fan14, end elements34of pipes32are tightly enclosed in the ring-shaped end42of cylindrical wall22which separates inner ring18and outer ring20from each other. The term “tight” is used here to mean that air cannot flow along the side of pipe bunch30, but instead, a flow-proof enclosure of bunch30is ensured inside the ring-shaped end42of wall22. In the same manner, the opposite end elements34on the side of heat exchanger10opposite fan14(on the right inFIG. 1) are enclosed in the end44of a cylindrical pipe fitting46, whose diameter is approximately the same as that of cylindrical wall22. This enclosure of pipe bunch30is flow-proof too, i.e. cylindrical pipe fitting46encloses pipe bunch30such that no air can flow past bunch30. It is understood that at this end too, end elements34are enclosed such that they are flow-proof. The ring-shaped end42of cylindrical wall22and/or the end44of a cylindrical pipe fitting46may also be formed by the injection-moulded part which encloses the honeycomb structure to form end elements34, as described above.

The cylindrical pipe fitting46is open at the end of heat exchanger10furthest from fan14. As indicated inFIG. 1by arrows A, air is sucked through this opening by the operation of fan14and into the first air flow, it passes through end elements34into pipe bunch30, is guided into pipes32and passes through end elements34closest to fan14into the space occupied by inner ring18, where it flows through fan14and finally exits housing12. The first air flow is thus transported through pipe bunch30inside heat exchanger12.

In contrast, the opposite second air flow is transported through housing12in such a way that it enters pipe bunch30and circulates around the middle sections36of the individual pipes32so that heat exchange can take place. In detail, the second air flow is drawn in from the left (arrow B) through the outer blades28of fan14into the outer ring20, is transported past a constriction48in the inner wall24of housing12inwards between pipes32of pipe bunch30and, at the end of pipe bunch30, is transported outward again around cylindrical pipe fitting46so that, finally, the second air flow exits radially outwards.

The constriction48is formed by a cylindrical insulating sleeve50which lines the inner wall24of housing12in the vicinity of middle sections36and reduces the inner diameter of housing12. It is also conceivable to form the constriction48by means of one or several cupped insulating shells adjoining the inner wall of housing12. The ends of this insulating sleeve50are formed by end surfaces52,54with a sloping cross-section, via which the inner diameter of insulating sleeve50transitions into the larger inner diameter of adjacent inner wall sections of housing12. End surfaces52,54guide the second air stream radially inwards or outwards. Starting from outer ring20, the second air flow comes into contact with a first end surface52of insulating sleeve50, via which the second air flow is guided between tubes32of bunch30. The inner cross-section of insulating sleeve50is usefully only slightly larger than that of pipe bunch30, so that bunch30is enclosed inside insulating sleeve50. Hence the second air flow circulates between pipes32inside insulating sleeve50. At the end of bunch30the second air flow is guided via the closing end surface54back to the outside via cylindrical pipe fitting46.

Insulating sleeve50offers the additional advantage of a reduction in energy losses towards the outside. In addition to thermal insulation, insulating sleeve50can also provide acoustic insulation. An insulating shell offers the same advantages. Insulating sleeve50is made for example from expanded plastic such as polystyrene or polypropylene. Other plastics such as PVC are also suitable.

Inner wall24of housing12can have a structured surface, i.e. be provided with beading or such like so that the second air flow is made turbulent. This delivers an additional improvement in the heat transfer. Further, middle sections36of pipes34may have a structured inner and/or outer surface. These structures may, for example, be formed by beading or projections on said surfaces. The structures may also serve to redirect the flow of air.

Pipes32may be standard drawn pipes with a cylindrical cross-section which are widened at their ends into a polygonal cross-section to form end elements34. These must then merely be grouped together and welded or bonded together to form the pipe bunch.

To ensure a good seal with the ring-shaped enclosure formed by the end42of the cylindrical wall22and the end44of pipe fitting46, the cross-sections of the end elements34on the outside of bunch30may differ from those on the inside of bunch30. This can be seen clearly inFIGS. 2 and 4. Here there are outer end elements such as the ones designated at56, which differ from the hexagonal cross-section of the other end elements34in the middle of bunch30. A rounded outer surface58of this end element56serves in this case as a contact surface with the cylindrical wall22or pipe fitting46. The outer contours of the outer end elements34thus form a circular circumference of bunch30at each end, which can be enclosed with precision inside cylindrical wall22and pipe fitting46.