GB2311845A - Ventilation heat recovery heat exchanger - Google Patents

Abstract

Adjacent plastic twin-wall boards 11 of a stack of such boards are separated by foam spacers/seals 12. In use stale air is passed through the flutes of the twin-wall boards and is in heat exchange with fresh air flowing through cavities defined between adjacent boards spaced apart by the seals/spacers. The seals/spacers are preferably made of double sided, self adhesive, compressible plastic foam strips. The stack of twin-wall boards is in use assembled into a ventilation unit which comprises boxes housing fans 33 for forced flow of stale and fresh air through respective flow paths in the stack.

Description

HEAT EXCHANGER FOR A HEAT RECOVERY VENTILATION SYSTEM.

This invention relates to air-to-air or gas-to-gas heat exchangers most typically used in building ventilation systems.

It is a common requirement that buildings should be ventilated with specified rates of change of air. Moreover there are requirements that the overall rate of loss of heat from a building should be kept within limits. To meet these joint requirements an apparatus for transferring heat from outgoing waste air to incoming fresh air is commonly used. A heat exchanger with four ports is commonly stationed at a location where stale air ducts can be routed close to fresh air ducts.

In such an arrangement fans mounted in one or more boxes are used to both extract stale air and draw in fresh air.

There are two design factors which have an overriding effect on the efficiency of heat exchangers. The first of these is the total area of the heat exchange laminae which form barriers between the warm and the cool gases. The second factor is the geometry of the heat exchanger. Crossflow heat exchanger cubes, where the stale air stream flows at right angles to the fresh air stream are well known and are most commonly used because of the simplicity of manufacture.

Complex analysis, however, shows that because of inevitable non-uniformities of temperatures across the cube, the efficiency achievable with cross flow designs is limited.

Counter-flow designs of heat exchanger are also well known, but for economic reasons, are not so commonly used for ventilator systems. The counter-flow concept, as shown schematically in Figure 3, allows opposing streams of gas to be maintained in close proximity, separated only by the heat exchange laminae, over longer lengths than is possible with simple crossflow geometries. The arrangement depicted in Figure 1 shows a practical solution to the problem of keeping the airflows separate as they enter and leave the heat exchanger stack. The heat exchanger stack is made up of a plurality of semi-rigid twin-wall boards separated by spacer seals. Figure 2 shows a typical arrangement of spacer seals which serve to separate the two flows and also keep the boards flat. The details of materials and mechanical arrangements for a practical realisation of this geometry is the essence of this invention.

It is not universally understood that the bulk thermal conductivity of the heat exchanger material is often of less importance than the resistance of the associated skin effect.

Economy in manufacture can be gained by the use of plastic, rather than metal. The use of plastic enables large heat exchangers to be made which are both efficient and inexpensive. A high performance heat exchanger will have a large cross-sectional area, orthogonal to the flow, which will result in low pressure drops for high flow rates, longer dwell time and maximum area for the heat exchange process. The dimension along the flow is maximised to provide large areas for heat exchange.

The heat exchanger described may be used for building ventilation, vehicle ventilation or for various industrial plants and processes. In the description that follows, gas flow streams are referred to as stale and fresh air. This is a reference to the application of this invention to ventilation systems. The use of the terms 'stale and fresh air' can serve as 'labels' for gas flows in applications other than ventilation.

The invention described here is a means of designing and manufacturing gas-to-gas or air-to-air heat exchangers, typically for use in ventilation systems. In particular this invention relates to heat exchangers which comprise a plurality of semi-rigid twin-wall plastic boards, otherwise known as fluted plastic boards.

The twin-wall plastic boards would commonly be manufactured from propylene ethylene copolymer, but many other plastics would be suitable, depending upon the nature of the gases and the temperatures involved. In the heat exchanger a plurality of twin-wall boards are spaced apart by seals made from compressible plastic foam strips to make a stack. Ideally the plastic foam spacer seals are self-adhesive on both faces.

Individual boards are combined in a stack to make up an essentially monolithic block, as shown in Figure 1, and are therefore easy to handle in later stages of manufacture, installation and service.

Compressible foams of various materials are suitable for the spacer seals. A closed cell PVC foam with uniform compressibility of a sufficiently high modulus that will overcome any tendency for the clamped boards to curl is ideal.

The dimensions of the flutes in the twin-wall plastic board and the thickness of the spacer seals lie in the region of lmm to 20mum. The dimensions of the stack will determine its efficiency, flow, and pressure drop characteristics. A size 1200mm long, 5OOmm wide and 500mm deep would be typical of a high performance ventilator for a home or office.

A heat exchanger stack, as described here, would best be combined on a modular basis with a set of fan boxes, channels and trays to make up an inexpensive and easily dismantled ventilation unit. An example of how this may be done, and the underlying concepts, are illustrated in the following figures.

Figure 1 shows in perspective a diagrammatic representation of the direction of flows in a heat exchanger stack.

Figure 2 shows a diagrammatic representation of the spacer seals in place on an individual twin-walled heat exchanger board.

Figure 3 shows a diagrammatic representation of counter-flow in a heat exchanger.

Figure 4 shows in perspective the position of the channels with respect to the heat exchanger stack.

Figure 5 shows a diagrammatic cross sectional view of a heat recovery ventilator unit complete with heat exchanger stack, fan boxes, channels and trays.

Figure 2 shows a typical arrangement of spacer seals fixed onto an individual twin-wall board 11. The spacer seals 13 serves to separate stale air from fresh air. The spacer seals 14 serve to maintain separation between adjacent twin-wall boards within acceptable limits. The spacer seals 12 confines the fresh air flow to the stack. Stale air flows straight through the flutes in the twin-wall board.

The pattern of the plastic foam seal strips must accurately correspond on each of the twin-wall boards that make up the stack. To achieve the required precision in manufacture, the pattern is screen printed, stencilled or marked by a similar means.

Figure 4 shows the position of the four channels 15 on the heat exchanger stack so that fresh air is guided to and from the spaces between the twin-wall boards.

Figure 5 shows that faces of the channels 15 are positioned coplanar with the ends of fan boxes 16 and 17 to form flat peripheral surfaces onto which the stale air inlet tray 18 and the stale air outlet tray 19 can be readily sealed.

The fan boxes 16 and 17 contain fans 20 and are provided with ports 21 and 22 which respectively connect to the ends of channels 15 or connect with the open top of the trays 19 and open underside of tray 18. The geometry is quite complex, but it is possible to see that the fan in box 16 provides forced flow through the waste air flutes in the plastic boards and the fan box 17 provides forced flow through the fresh air spaces 30 between the twin-wall boards.

In the example shown in Figure 5 four sets of threaded rods and nuts 23 are used to draw the fan boxes together, compressing the heat exchanger stack and clamping the channels 15 rigidly into position. This clamping operation requires temporary jigging to ensure that the peripheral surfaces are planar and that the channels 15 seal adequately to the heat exchanger stack.

During operation, condensation from the waste air may take place within the water resistant flutes 10. For this reason the orientation of the heat exchanger should be such that the flow of the waste air draws any moisture that is present down through the flutes to be collected for drainage purposes in the water resistant outlet tray 19.

A feature of the design shown in Figure 5 is that the dimensional tolerances of all the main components of the apparatus are not critical. The compressibility of the heat exchanger stack allows the fan boxes 16 & 17 to be clamped onto the channels 15 locking all these components into place.

The open edges of the trays 18 & 19 are clamped onto the peripheral surfaces so provided. Compressible gaskets are trapped between mating surfaces of all these components to prevent leakages.

The irregularities at the surfaces 31 indicated in Figure 4 where the channels 15 abut the heat exchanger stack are eliminated using a strip of semi-rigid plastic strip (not shown) bonded by a mastic compound which fills any gaps formed by residuals of the flutes which result from being their being cut to size. The one or more flutes at the outer edges of the stack are blocked using a plastic "L section extrusion, or by using one of many other obvious methods. The purpose of blocking off the ends of the outer flutes is to effect thermal insulation at the outer faces 32 of the heat exchanger block.

The fan boxes 16 and 17 typically house fans 33 of centrifugal design and filters 34 to prevent dust build up within the heat exchanger stack.

The fan boxes 16 and 17 are so arranged with ports and chambers 21 and 22 to deliver and collect air from the heat exchanger stack. The arrangement of clamping the fan boxes 16 and 17 onto the heat exchanger stack enables large, inexpensive and efficient heat exchangers to be manufactured and used without the need of expensive and bulky cabinets to house all the associated components. Moreover ventilation units of this design are readily disassembled for installation in spaces with limited access.

Another example of this invention is an arrangement in which the fresh air is guided using just two of the channels 15.

This is in principle the same arrangement as shown in Figure 4 but with two of the four channels 15 closed off from circulation. These two channels would ideally be on alternate sides to maximise the air path. However, crossflow cube and diamond heat exchangers, which have the commonest geometric arrangements for heat exchangers, may be economically manufactured using the materials and methods described above and these are merely a special case of the above with shortened path.

Claims (9)

1. A heat exchanger unit as shown in Figure 5 in which a counter-flow heat exchanger is made up from a stack of a plurality of semi-rigid twin-wall plastic boards1 also known as fluted plastic boards, spaced apart by double-sided selfadhesive PVC or polyethylene foam strips serving as spacer seals to guide the fresh and the stale air separately into channels and trays connecting with ports in the two fan boxes.

2. A heat exchanger unit as described in claim 1 wherein the dimensions of the flutes in the plastic board and the spacer seal thickness fall in the region of lmm to 20mm.

3. A heat recovery ventilation unit as in claim 1 where fan boxes, channels and trays are clamped around the heat exchanger stack in a modular arrangement for ease of assembly and disassembly.

4. A heat recovery ventilation unit as in claim 3, where the orientation is such so that moisture is drawn down in water resistant flutes by gravity and gas flow to be collected in the tray for subsequent drainage.

5. A heat recovery ventilation unit as in claim 3 wherein the assignation of stale and fresh air paths through the stack may be may be interchanged.

6. A heat exchanger unit as described in claim 1 which is made up from a plurality of semi-rigid twin-wall plastic boards held in a stack by an arrangement of self adhesive plastic foam spacer seals that are aligned with reference to marks printed on the plastic boards.

7. A heat exchanger unit as described in claim 1 wherein the double sided self-adhesive plastic PVC foam spacer seals are replaced by single sided self-adhesive plastic foam spacer seals used with or without the application of extra adhesive during assembly or where the plastic foam may be other than PVC.

8. A heat exchanger unit as described in claim 1 wherein the self-adhesive PVC foam spacer seals are replaced by non selfadhesive plastic foam spacer seals used with the application of adhesive to surfaces during assembly or where the plastic foam may be other than PVC or polyethylene.

9. A heat exchanger unit using the materials and methods described in claims 1,2,6,7 and 8 but configured in the well known crossflow cube or diamond geometries.

9. A heat exchanger using the materials and methods described in claims 1,2,6,7 and 8 but with the well known crossflow cube or diamond geometries.

Amendments to the claims have been filed as follows 1. A heat exchanger unit in which a counter-flow or crossflow heat exchanger is made up from a stack of a plurality of semi-rigid twin-wall plastic boards, also known as fluted plastic boards, spaced apart by plastic foam strips, held in place by adhesive, serving as spacer seals to guide two separate gas streams, typically the fresh and the stale air streams of a ventilation system, separately into channels and trays that connect with ports in two fan boxes.

2. A heat exchanger unit as described in claim 1 wherein the dimensions of the flutes in the plastic board and the spacer seal thickness fall in the region of lmm to 20mm.

3. A heat exchanger unit as in claim 1 where the fan boxes, channels and trays are clamped around the heat exchanger stack in a modular arrangement for ease of assembly and disassembly.

4. A heat exchanger unit as in claim 3, where the orientation is such so that moisture is drawn down in water resistant flutes by gravity and gas flow to be collected in one of the trays for subsequent drainage.

5. A heat exchanger unit as in claim 3 wherein the orientation and assignation of the flows through the stack with respect to the vertical may be changed depending upon the application.

6. A heat exchanger unit as described in claim 1 which is made up from the plurality of semi-rigid twin-wall plastic boards held in the stack by an arrangement of the plastic foam spacer seals that are aligned with reference to marks printed on the plastic boards.

7. A heat exchanger unit as described in claim 1 wherein the plastic foam spacer seals are manufactured from either single or double sided self-adhesive coated and are used with or without the application of extra adhesive during assembly.

8. A heat exchanger unit as described in claim 1 wherein the plastic foam spacer seals are made from strips of PVC, polyethylene or other suitable compressible sealant strip.