EP1361406A2

EP1361406A2 - Heat exchanger

Info

Publication number: EP1361406A2
Application number: EP03252876A
Authority: EP
Inventors: George Wilson
Original assignee: Smiths Group PLC; Vent Axia Group Ltd
Current assignee: Vent Axia Group Ltd
Priority date: 2002-05-08
Filing date: 2003-05-08
Publication date: 2003-11-12
Also published as: JP2004003838A; GB0210434D0; GB0310384D0; GB2389173A; CN1495393A; CA2428239A1; US20040031599A1; EP1361406A3; KR20030087561A

Abstract

A multi-plate dual flow path heat exchanger includes a plurality of stacked plates (70) each having a central section (71) provided with a number of longitudinal channels (72) separated by upstanding zig-zag walls (75), the channels (72) being provided on their floors (72A) with ridges (74), the floors and the ridges being of undulating form along the flow paths. Shallow ribs (75) extend laterally across the flow paths and in combination with the undulations assist in disruption of boundary layer flow.

Description

This invention relates to heat exchangers.
The invention is more particularly concerned with heat exchangers for use in building ventilation systems.
Heat exchangers are used in building ventilation systems to transfer heat from warm air extracted from the building to cold air supplied to the building. In this way, the amount of energy needed to maintain the temperature within the building can be minimized.
A common form of heat exchanger used in building ventilation systems comprises a stack of thin parallel plates spaced from one another to form two separate flow paths between alternate pairs of plates. The warm air is supplied along one path and a part of its heat is conducted through the thickness of the plates to the cold air supplied along the other path.
The ideal heat exchanger should have a high efficiency of thermal transfer, preferably above about 90% and should produce only a low back pressure so as to reduce energy expenditure by the fans used to pass the air through the exchanger. The exchanger should also have a low leakage between the two air paths and be easy to manufacture at low cost.
One example of a heat exchanger is described in GB 0121865.0
It has proved difficult to produce heat exchangers having a high efficiency and a low leakage without a relatively high back pressure. An important factor in increasing the efficiency of heat exchangers is the reduction in boundary layer effect.
It is an object of the present invention to provide an alternative heat exchanger.
According to one aspect of the present invention there is provided a heat exchanger including a plurality of plate members stacked parallel above one another to define two separate fluid flow paths between alternate pairs of adjacent plate members, the plate members having an undulating surface along the fluid flow path sufficient to reduce the boundary layer effect and each plate member having a series of laterally-extending surface formations along the flow paths.
The surface formations are preferably spaced at intervals of between about 20mm and 35mm and, in particular, are preferably spaced at intervals of about 25mm. The surface formations are preferably shallow ribs. The plate members are preferably textured, such as with an orange-peel texture in the manner produced by coarse sand casting. Each plate member may have a plurality of support members distributed over its surface and formed from material of the plate members. The support members may be elongate projections extending parallel to the direction of fluid flow. The support members of one plate member are preferably located close to but not in alignment with support members of an adjacent member so that the support members do not nest with one another but so that contact of the support members with adjacent plate members provides vertical support in a stack of plate members. The plate members preferably have a plurality of substantially straight longitudinally-extending ridges, the ridges preferably being arranged in groups separated from one another by support ridges of zigzag shape, the support ridges being higher than the straight ridges and arranged out of phase with zigzag ridges in adjacent plates. The heat exchanger preferably has triangular regions at opposite ends providing adjacent inlet and outlet faces inclined relative to one another and meeting at an apex. The exchanger preferably has at least one elongate resilient member extending vertically along the apex and having fingers interdigitated between plate members, the exchanger having elongate clamping members extending along opposite sides of the or each resilient member and compressing the or each resilient member therebetween.
According to another aspect of the present invention there is provided a plate member for a heat exchanger according to the above one aspect of the invention.
A heat exchanger assembly according to the present invention, will now be described, by way of example, with reference to the accompanying drawings, in which:
Figure 1 is a schematic plan view of the assembly;
Figure 2 is a perspective view of the heat exchanger unit;
Figure 3 is a perspective view of a side panel of the exchanger housing;
Figure 4 is a plan view of a lower type of heat exchanger plate;
Figure 5 is a plan view of an upper type of heat exchanger plate;
Figure 6 is an elevation view showing an edge part of a heat exchanger plate to an enlarged scale;
Figure 7 is a simplified longitudinal elevation view showing how the support peaks on the plates are positioned;
Figure 8 is a simplified lateral elevation view showing how the support peaks on the plates are positioned;
Figure 9A and 9B are simplified plan views of A and B type plates respectively showing the relative positions of the support peaks;
Figure 10 is a simplified plan view illustrating the out-of-phase nature of the zigzag walls on the A and B type of plates;
Figure 11 is a sectional side elevation view of the exchanger showing how the edges of the plates locate with the side panels;
Figure 12 is a perspective view of a foam sealing strip used in the exchanger;
Figures 13 and 14 Figures 13 and 14 are perspective views of two clamp strips used with the foam strip of Figure 12; and
Figure 15 is a simplified elevation view illustrating a step in the assembly of the foam strips.
With reference first to Figures 1 and 2, the heat exchanger assembly has an outer housing 1 with two inlets 2 and 3 and two outlets 4 and 5 located at four corners of the housing. A heat exchange unit 6 is located in the housing 1 and defines two separate air flow paths 7 and 8 through the housing. The first flow path 7 extends from the inlet 2 through the exchange unit 6 to the outlet 4 in the opposite corner and, in use, receives warm air exhausted from a room. The second flow path 8 extends from the other inlet 3 to the other outlet 5 and, in use, receives cold air from outside. The exchange unit 6 operates to transfer heat from the air flowing along the first flow path 7 to air flowing along the second flow path 8 so that the fresh air supplied to the building is warmed. The assembly includes two conventional electric fans 10 and 11 located in the housing 1 at the two outlets 4 and 5 to draw air along the respective flow paths 7 and 8.
The heat exchange unit 6 is of the counter-flow type having two parallel, vertical sides 61 and 62 and four end faces 63 to 66 providing the two inlets and outlets. The unit 6 has a horizontal base 67 and top 68. Operation of the two fans 10 and 11 causes warm air drawn in through the inlet 2 of the housing to flow in the inlet face 63, through the unit 6 and out of the diagonally opposite outlet face 65, from where it flows to the outlet 4. Cold air drawn in through the inlet 3 passes in the inlet face 64, through the unit 6 and out of the diagonally opposite outlet face 66, from where it passes to the outlet 5.
With reference now also to Figures 3 to 11, the heat exchange unit 6 comprises a parallel stack of forty-seven, six-sided heat exchanger plates 70, in twenty-three pairs and one single plate. Other exchangers may have different numbers of plates. Typically, the plates are about 300mm wide and about 650mm long between the apexes. The plates 70 are contained within a base panel 12, a top panel 13, and two side panels 14 and 15. The heat exchanger plates 70 are vacuum formed from a thin sheet of carbon-loaded uPVC of a black colour, which has a high thermal conductivity and is an efficient thermal radiator. The plates 70 are moulded with surface formations that act to enhance heat transfer and support the plates with one another. The heat exchanger plates 70 are of two different types: a lower type A and an upper type B. These are joined with one another in pairs having four sides sealed together by welding and two diagonally opposite sides open for inlet and outlet of air. The pairs of joined plates A and B are stacked one above the other. The space between the upper surface of the lower plate A in a pair and the lower surface of the upper plate forms a part of the first flow path 7. The space between the upper surface of the upper plate and the lower surface of the lower plate in an adjacent pair of the stack forms a part of the second flow path 8. The configuration of the lower type of plate 70A will now be described with reference to Figure 4.
The plate 70A has a main section 71 of rectangular shape divided into eight parallel, longitudinal channels 72 separated from one another by upwardly-projecting walls 73 of triangular profile and a zigzag configuration. The walls 73 serve to support and space adjacent plates from one another in a manner that will become apparent later. Extending along each channel 72 are five parallel ridges 74 equally spaced from one another across the width of each channel. The ridges 74 have a triangular profile but are only about half the height of the walls 73. The lower edges of the ridges 74 are contiguous with one another, with the peaks of the ridges being separated from one another by valleys of triangular section, as shown in Figure 6. The ridges 74 are straight when viewed from above but the floor 72A of the channels 72 and the ridges have an undulating profile along their length forming a series of about fourteen hills and valleys, as shown in Figure 7. The peak-to-peak height of the undulations is about 0.5 mm. The ridges 74 serve to channel air smoothly along the channels 72 and increase the surface area of the plate 70A contacted by the air. The walls 73 and ridges 74 also increase the longitudinal stiffness of the plates. The undulating floor 72A of the channel 72 has been found to be particularly important in helping to reduce boundary layer effects by increasing the buffeting of air between the plates as it flows along the channels.
The channels 72 are also interrupted by a series of fifteen ribs 75 extending laterally across the width of the plate. The ribs 75 are shallow compared with the ridges 74, only being no more than 1 mm high and extend across both the ridges and the walls 73. The spacing between adjacent ribs 75 is between about 20mm and 35mm and is preferably about 25mm. The purpose of the ribs 75 is also to reduce boundary layer effects by increasing disturbance of air flow at intervals. Without a similar formation, a boundary layer will build up over a distance of about 32mm so the spacing of the ribs is preferably chosen to be slightly less than this.
Each channel 72 also includes fourteen support members or peaks 80 spaced along the channels. The peaks 80 are of substantially rectangular shape when viewed from above, being about 9mm long and 1mm wide, and have a triangular profile. The peaks 80 project upwardly on the ridges 74 and, in particular, are formed equally spaced from one another alternately on the second and fourth ridges across each channel 72. The purpose of the peaks 80 is to maintain the spacing between adjacent plates 70, in particular, to maintain the spacing at about 3mm.
As shown in Figures 6 and 11, the edges 81 and 82 of the rectangular section 71 have an inner boundary wall 83 and a longitudinal depression 84 of semicircular profile extending along their length about halfway across the width of the edge. The upper surface of the edges 81 and 82 is welded to the upper plate 70B in a manner described in more detail later.
At opposite ends of the main section 71, the plate 70A has an inlet and outlet section 90 and 91, both of triangular shape. One side 92 of the inlet section 90 is closed by welding to the upper plate 70B; the other side 93 is open. The surface of the inlet section 90 is ribbed with shallow, parallel ribs 94 extending laterally of the plate and generally transversely to the direction of air flow. The inlet section 90 also has six higher raised walls 95 extending perpendicular to the open side 93 and forming a continuation of the zigzag walls 73. These ribs 94 and walls 95 act to channel air entering the open side 93 substantially evenly across the row of ends of the channels 72. The ribs 94 also introduce a small amount of turbulence into the air flow.
The outlet section 91 similarly has a closed, welded side 96 and an open side 97. The outlet section 91 also has ribs 98 and walls 99 to help channel air emerging from the channels 72 to the open side 97 of the section.
All the ridges, walls and other formations on the plate 70A are formed by moulding from the material of the plate so that the thickness of the plate is constant over its surface and each formation on one surface of the plate has a corresponding inverted formation on the opposite surface. The entire upper and lower surfaces of the plate are textured with a granular, orange peel texture. This texture is preferably produced directly in the vacuum forming mould tool by leaving this as a rough, coarse sand-cast finish. This texture has been found further to discourage the formation of boundary layers on the plates.
The upper type of plate 70B (Figure 5) has similar surface formations on its upper surface, which are given the same number as the formations for plate 70A with the addition of a prime'. The plates 70B have a pattern of zigzag walls 73' identical with the walls 73 except that they are out of phase with one another. In this way, the walls 73 and 73' in adjacent plates cross one another and support the plates relative to one another, as illustrated in Figure 10. The ridges 74' on the plate 70B extend in alignment with the corresponding ridges on the lower plate. The distribution of the peaks 80', however, is slightly different from those on the lower plate 70A in that they are aligned laterally but are displaced longitudinally by a distance equal to a peak length, as shown in Figures 6 to 8. This displacement is sufficient to ensure that the peaks 80 and 80' do not nest with one another but the spacing is sufficiently close that the column of peaks provides some vertical strength to the stack of plates 70.
The triangular left and right sections 90' and 91' of the upper plate 70B are similar to those of the lower plate 70A except that the upper surface of the left section 90' is configured to provide an outlet whereas the right section 91' is configured to provide an inlet. Different ones of the sides 92', 93', 96' and 97' are open and closed and the internal ribs 94', 98' and walls 95', 99' act to channel air from the open side 96' via the ends of the channels 72' to the open side 92'.
The two plates 70A and 70B in each pair are welded together around four sides. The edges 81' and 82' of the upper plate 70B along the sides of the rectangular section 71' are flat and are welded to the edges of the lower plate 70A along opposite sides of the semicircular depression 84 so that the open side of the depression is closed and sealed, thereby forming it into an air-filled longitudinal seal. At the same time, the closed sides 91 and 92 of the lower plate 70A are welded to the sides 91' and 92' of the upper plate 70B. The pairs of plates 70 are held together with one another in a stack by means of the bottom panel 12, top panel 13 and side panels 14 and 15. The side panels 14 and 15 (shown most clearly in Figures 3 and 11) are imperforate and moulded of a rigid, black ABS plastics material with twenty-two parallel slots 100 extending horizontally along their length. The width of the slots 100 is selected so that the welded edges 81 and 82 of the pair of plates are retained as a tight push fit, with the semicircular formation 84 on the lower plate 70A providing an effective seal against passage of air around the edges of the plates. The spacing of the slots 100 provides accurate spacing between adjacent pairs plates; accurate spacing between the A and B plates of a pair is ensured by the surface shapes of the lower A plate.
The unit 6 is assembled by clipping the side panels 14 and 15 into the base panel 12 and then sliding a pair of heat exchange plates 70A and 70B into the slots 100 along the side panels. When all the pairs of plates 70 have been slid into position, the top panel 13 is clipped onto the upper edge of the side panels 14 and 15. The top panel 13 has a series of recesses 180 on its lower surface located in positions corresponding to the peaks 80' on the upper plate 70B of the stack. The peaks 80' are received in the recesses 180 so as to ensure that the peaks do not space the plate 70B away from the top panel 13 and allow too great a proportion of air to flow between the plate and the top panel.
With the plates 70 stacked together, the open edges 93 and 93' of the lower and upper plates 70A and 70B are welded to the respective upper and lower plates of adjacent pairs, so that air cannot flow between the upper plate of one pair and the lower plate of the adjacent pair at the face 63. Similarly, the edges 97 and 97' are welded together at the face 65.
Because there is a transition at each apex 101 in the stack of plates 70, between the extracted and supply air flows, it is particularly important that this region is effectively sealed to prevent leakage between the two paths 7 and 8. This is achieved by means of two foam sealing strips 102, as shown in Figure 11, cut along one edge with a series of short cuts 103 extending at right angles to the edge (as shown in Figure 12). The number of cuts 103 is equal to the number of plates 70 in the stack. The strips 102 are assembled on either side of the apex 101 in the manner shown in Figure 15 50 that fingers 104 of the strip between each cut 103 extend between the plates 70 at the apex 101. Two clamping strips 105 and 106, as shown in Figures 13 and 14 are then positioned along opposite sides of the foam strips 102, as shown in Figure 15, and are clamped together so as to compress the foam strips into an effective seal with the plates 70.
Similar foam strips (not shown) are used at the corners 110 to 113, where the exchanger plates 70 project from the slots 100 in the side panels 14 and 15. Vertical clamping strips 114 are used to compress the foam strips and hold them in place so as to reduce leakage of air along the slots 100.
The arrangement of the present invention enables a heat exchanger of high efficiency to be provided without a high back pressure. The arrangement can also reduce cross leakage between the two air flows.

Claims

A heat exchanger including a plurality of plate members (70) stacked parallel above one another to define two separate fluid flow paths between alternate pairs of adjacent plate members (70) characterised in that each plate member (70) has an undulating surface along the respective fluid flow path sufficient to reduce the boundary layer effect and in that each plate member (70) has a series of laterally-extending surface formations (75) along the flow paths.
A heat exchanger according to Claim 1 characterised in that each laterally-extending formation (75) is in the form of a shallow rib.
A heat exchanger according to Claim 1 or 2 characterised in that each formation is no more than 1mm in height.
A heat exchanger according to any one of the preceding claims characterised in that the surface of each plate member (70) is textured.
A heat exchanger according to any one of the preceding claims characterised in that each plate member (70) is formed with a plurality of channels (72) defined between upwardly projecting walls (73) and providing floors (72A) therebetween.
A heat exchanger according to Claim 5 characterised in that each channel (72) is provided with a plurality of equi-spaced ridges (74) upstanding from the floor (72A) and extending in parallel and longitudinally along the channel between the walls (73) and being of lower height than the walls, the ridges (74) and the floor (72A) of each channel (72) undulating along the length thereof.
A heat exchanger according to Claim 6 characterised in that the undulations in the floor (72A) and the ridges (74) are of shallow and multiple form to provide a plurality of hills and valleys along each channel (72).
A heat exchanger according to Claim 5 and any claim dependent thereon characterised in that the upwardly projecting walls (73) bounding the channels are of zig-zag form in the longitudinal direction of the plate members (70).
A heat exchanger according to Claim 8 characterised in that the zig-zag formations are offset as between one plate member (70) and an adjacent plate member (70).
A heat exchanger according to Claim 6 and any claim dependent thereon characterised in that support members (80) are provided on at least one ridge (74) in each channel (72) and are upstanding therefrom and are adapted to support an adjacent plate member.
A heat exchanger according to Claim 6 characterised in that the support members (80) are disposed at spaced intervals longitudinally of the ridges (74).
A heat exchanger according to Claim 11 characterised in that the support members (80) are formed of the material of the plate member and are in the form of elongate projections extending parallel to the direction of fluid flow.
A heat exchanger according to any one of Claims 10 to 12 characterised in that support members (70) are provided on more than one ridge (80) and the support members (80) on one ridge (74) are offset from those on the other ridge (74).
A heat exchanger according to any one of Claims 10 to 13 characterised in that the support members (80) on one plate member (70) are offset from the support members (80) on an adjacent plate member (70).
A heat exchanger according to any one of the preceding claims characterised by triangular regions (90, 91) at opposite ends providing adjacent inlet and outlet faces inclined relative to one another and meeting at an apex (101).
A heat exchanger according to Claim 14 characterised by at least one elongate resilient member (102) extending vertically along the apex (101) and having fingers (104) interdigitated between plate members, the exchanger having elongate clamping members (105, 106) extending along opposite sides of the or each resilient member (102) and compressing the or each resilient member (102) therebetween.