GB2156961A - Spiral heat exchanger - Google Patents

Abstract

A spiral heat exchanger consists of two sheets of heat exchange material, such as stainless steel, wound together to define two inter-wound spiral flow passages 6 and 7. The thicknesses of these flow passages are normally maintained by studs 4 welded on one or both of the strips. In accordance with the present invention, the studs 4 are elongated in the direction of flow, indicated by the arrow 8, so as to reduce the resistance to flow presented by the studs. In the arrangement shown in Figure 5, the studs 4 are more gently tapered on the downstream end than on the upstream end. <IMAGE>

Description

SPECIFICATION Spiral heat exchanger This invention relates to spiral heat exchangers.

A spiral heat exchanger, as that term is normally understood, comprises two sheets or strips of heat exchanger material, usually corrosion resistant metal such as stainless steel or titanium, wound together to define two interwound passages of substantially spiral form.

The flow of heat exchange media in the flow passages may be concurrent or countercurrent in the respective spirals, or one the media may flow axially across the spiral over all or part of the length thereof.

The thickness (or flow gap) of the flow passage, i.e., the spacing between the sheets or strips, is normally maintained by cylindrical studs of the requisite height welded to one or both of the sheets or strips and abutting the other sheet or strips when the exchanger is wound. The flow gaps for the two media are often different and this may be catered for by the use of studs of different heights in the two flow passages.

In view of the comparatively high operating pressures which may be encountered, and the method of manufacture of the heat exchanger, the studs have to be quite closely spaced. These studs present a restriction to fluid flow in the respective flow passages, and therefore lead to an increase in the pressure drop across the heat exchanger. This increase can be quite significant in terms of energy consumption.

In accordance with the present invention there is provided a spiral heat exchanger having studs on one or both of the sheets or strips to maintain the space in between the sheets or strips, and wherein the studs are elongated in the direction of flow to reduce the effective pressure loss.

The studs may for instance be cylindrical on the upstream end and tapered on the downstream end. In an alternative, both ends of the studs may be tapered and the tapering may be more gradual on the downstream end. Such tapering preferably terminates in a radiused extremity to facility manufactu re.

The pressure loss across an individual stud is not a direct consequence of the drag force exerted by the fluid on the stud and vice versa, but is rather a consequence of the production of viscous fluid friction. Consequently, the pressure loss is the result of the irreversible conversion of fluid energy into internal energy via viscous friction.

If the fluid is assumed to be of constant density, then the fluid energy component which becomes reduced is the pressure energy.

Viscous friction is generated as a result of two factors, namely viscous drag and form drag. The former is due to the relative motion of fluid layers within the boundary layer adjacent and in contact with the stud surface. The latter is due to the formation of turbulent eddies within the body of the fluid away from the stud surface.

Viscous friction occurs whenever a viscous fluid moves relative to a solid body suspended within the fluid. The fluid friction on the body surface results in viscous drag. For Newtonian fluids, the viscous drag force is proportional to the viscosity and the rate of shear.

The viscous drag force on the leading edge or half of the body may be decreased by tapering the leading surface. Despite increasing the surface area, the overall drag force may be reduced because the rate of shear is reduced also. If the leading edge is extended too much, then the increase in area will eventually lead to an increase in the drag force.

Form drag occurs primarily on the trailing half or edge of the stud due to the separation of the boundary layer from the stud surface, resulting in the formation of eddies which dissipate fluid energy through viscous friction.

Over the trailing half or edge of the stud, the pressure is increasing, hence the pressure gradient is positive. The transfer of the momentum out of a small element within the boundary layer is assisted by the pressure gradient as well as the viscous friction. The combined effect may deplete the momentum to such an extent that reversal of the direction of flow occurs. At the point of flow reversal the boundary layer effectively separates from the body surface.

The effect of viscous friction is thus to try and keep the direction of flow that of the bulk fluid, while the positive pressure gradient tries to reverse it.

The stability of the separated layer is low, and turbulent eddies are set up as the velocity profile is accelerated to the bulk fluid velocity. Also the layer which has undergone flow reversal must separate or be reversed again. The net effect is that more fluid pressure energy is irreversibly converted to fluid internal energy via viscous friction.

The production of eddies can be substantially, if not totally, avoided by preventing boundary layer separation. This can be achieved by reducing the positive pressure gradient by extending or elongating the trailing edge of the stud. This reduces the rate of momentum transfer to a boundary layer element due to the pressure gradient but increases the rate of transfer due to viscous friction by creating a larger surface area.

The bulk fluid is thus able to transfer sufficient momentum to layer below, to prevent flow reversal and hence boundary layer separation.

Form drag is normally much greater than viscous drag, so that the tapering of the trailing half is thus of more importance than that of the leading half in reducing the irreversible pressure loss.

The invention will be further described with reference to the accompanying drawing, in which: Figures 1, 2 and 3 show successive stages in the manufacture of a spiral heat exchanger; Figure 4 shows one form of stud for use in a spiral heat exchanger in accordance with the invention; and Figure 5 shows an alternative form of stud.

Turning first to Figure 1, there is shown a mandrel 1 having attached thereto sheets or strips 2 or 3 of stainless steel or other corrosion resistant metal of a thickness appropriate for heat exchange use, e.g. 2 mm. Each of the strips 2 and 3 is provided with a series of studs 4 welded onto the sheet in a predetermined, normally regular, arrangement and at a spacing in a predetermined, normally regular, arrangement and at a spacing to ensure that the studs can perform their function to maintain the flow thickness between the strips 2 and 3 during manufacture and use of the spiral heat exchanger.

To manufacture the heat exchanger, the mandrel 1 is rotated in the sense indicated by the arrow 5 so that the strips 2 and 3 become interwound around the mandrel and are held spaced apart by the studs 4 as illustrated in Figures 2 and 3. Figure 2 shows the position shortly after commencement of winding, and Figure 3 shows how the two sheets become wound together in coaxial spirals to define two inter-wound flow passages 6 and 7. It will be understood that the flow passages have their thickness or flow gap maintained by the studs 4.

It is conventional practice in spiral heat exchangers for the studs 4 to be of cylindrical form, and in order to reduce the flow resistance they engender, it is proposed in accordance with the invention that the studs should be elongated in the direction of flow, whether this be along the strips in the spiral direction or across the spiral.

Figure 4 shows one form of such an elongated stud 4. The flow direction is illustrated by the arrow 8 and it will be seen that the stud 4 of Figure 4 is substantially cylindrical on the upstream end and tapers gradually on the downstream end to a tip 9. The tip 9 is preferably radiused rather than sharp in order to facilitate manufacture.

Turning now to Figure 5, the flow direction is again illustrated by the arrow 8 and the stud 4 illustrated in Figure 5 is tapered to an upstream extremity 11 and a downstream extremity 12, but the tapering on the downstream side is less sharp than on the upstream side. The effect of this is that the length x between the tip 11 and the zone of greatest width of a stud is less than the length Y between the zone of greatest width and the tip 12.

Again, the tips 11 and 12 are preferably radiused rather than sharp.

It will be appreciated that the studs 4 on the strips 2 and 3 may be of different lengths to provide for different thicknesses of the flow passages 6 and 7. Also, it is possible that all the studs may be provided on one of the strips 2 and 3, by welding them to both sides and it is even possible for studs to be provided on both sides of both strips.

It will be appreciated that the details of the supply and discharge arrangements for the two flow passages are omitted from the present description, as they form no part of the present invention.

Various modifications may be made within the scope of the invention.

Claims (6)

1. A spiral heat exchanger (as hereinbefore defined), having studs on one or both of the sheets or strips to maintain the spacing between the sheets of strips, and wherein the studs are elongated in the direction of flow to reduce the effective pressure loss.

2. A spiral heat exchanger as claimed in claim 1, in which the studs are semi-cylindrical on the upstream end and tapered on the downstream end.

3. A spiral heat exchanger as claimed in claim 1, in which the studs are tapered on both the upstream and downstream ends.

4. A spiral heat exchanger as claimed in claim 3, in which the downstream end is more gradually tapered than the upstream end.

5. A spiral heat exchanger as claimed in claims 2, 3, or 4, in which the or each taper terminates in a radiused extremity.

6. A spiral heat exchanger substantially as hereinbefore described with reference to the accompanying drawing.