Prior Art Discussion There are a large number of different types of heat exchangers in general use. In shell-in-tube type heat exchangers one fluid passes through tubes extending through a shell and another passes through the shell. Heat is transferred between the fluids as they flow through the tubes and the shell. In plate type heat exchangers a plurality of plates are sandwiched together and heat transfer occurs across the plates. One advantage of plate heat exchangers is that they can be readily opened and the plates separated for cleaning.

Conventional heat exchangers are generally expensive. Further, most are not suitable for use with fluids which may contain solid/particulate material such as waste products. Plate heat exchangers can be used for such fluids, however they regularly become clogged and must be opened for cleaning. Such clogging also leads to a reduction in heat transfer efficiency.

Another problem with existing heat exchangers is that a limit is imposed on efficiency because of the boundary layer of little or no fluid movement where the fluid contacts a heat exchanger wall. An approach to reducing depth of the boundary layer is to increase flow rate. However, this results in higher pressures in turn leading to a requirement for thicker (and therefore less efficient) walls and for higher energy input. Higher flow rates also decrease the residence time for heat

transfer. Thus, for a given heat exchanger construction the major control parameter is fluid flow rate, chosen for a desired trade off of boundary layer thickness and residence time.

The invention is directed towards providing an improved heat exchanger.

SUMMARY OF THE INVENTION According to the invention, there is provided a heat exchanger comprising at least two fluid pathways, wherein the pathways are at least partially separated by a flexible membrane.

In one embodiment, the membrane is of plastics material.

In another embodiment, the membrane is of plastics film material.

In a further embodiment, the heat exchanger further comprises a flow controller for causing contra-flow in adjacent pathways.

In one embodiment, the flow controller comprises a one-way valve for each pathway.

In another embodiment, the flow controller applies pulses to fluids in the pathways to cause the contra-flow.

In a further embodiment, the flow controller propagates waves in a liquid to cause the contra flow.

In one embodiment, the flow controller propagates waves in an external liquid surrounding at least one pathway.

In another embodiment, the flow controller propagates waves in an internal heat transfer liquid.

In a further embodiment, the membrane is sufficiently flexible to allow transfer of pressure from a pathway to an adjacent pathway via the membrane.

In one embodiment, the pathways comprise tubes of flexible membrane material immersed in an external liquid contained within a housing.

In another embodiment, the external liquid is one of the heat transfer liquids.

In a further embodiment, the heat exchanger comprises an actuator for applying a wave pattern to the membrane.

In one embodiment, the actuator comprises an ultrasound generator.

In another embodiment, the membrane is of metal material.

In another aspect, the invention provides a method of operating a heat exchanger as defined above, comprising the steps of alternately in cycles:- directing a first fluid through a first pathway while preventing inlet of fluid into an adjacent second pathway, and directing flow of fluid through the second pathway while preventing inlet of fluid into the first pathway.

In one embodiment, at least one fluid is a liquid slurry.

In another embodiment, the flow is controlled by propagating waves in the first fluid, and the second fluid is an external fluid flowing externally of the pathways.

DETAILED DESCRIPTION OF THE INVENTION Brief Description of the Drawings The invention will be more clearly understood from the following description of some embodiments thereof, given by way of example only with reference to the accompanying drawings in which:- Fig. 1 is a cross-sectional diagram showing a heat exchanger of the invention; Figs. 2 (a) and 2 (b) are diagrams illustrating operation of the heat exchanger in different stages; Fig. 3 is a plot illustrating phases of operation of the heat exchanger ; and Figs. 4 (a) and 4 (b) are diagrams of large suspended particle movement of the prior art and of the invention respectively; Fig. 5 is cross-sectional diagram across the directions of flow of a heat exchanger of another embodiment; and Figs. 6 (a) and 6 (b) are diagrams of water movement within a heat exchanger pathway.

Description of the Embodiments

Referring to Fig. 1 a heat exchanger 1 of the invention is shown in cross-section along the directions of flow. A bottom pathway 2 has an inlet 3 and an outlet 4, and a top pathway 5 has an inlet 6 and an outlet 7. The inlets 3 and 6 have one-way valves 10 and 12 respectively. The pathways 2 and 5 are separated by a heat transfer membrane 20 of light gauge plastics material. The opposed sides of the pathways 2 and 5 comprise films 21 and 22 respectively of a similar plastics material.

In operation, applying a pulse of liquid under pressure to the inlet 3 causes the valve 10 to open and causes liquid to flow, as shown in Fig. 2 (a), from the inlet 3 to the outlet 4. The flexible membrane 20 allows pressure to be transmitted to the pathway 5 causing the valve 6 to close and causing fluid already in the pathway 5 to flow out through the outlet 7.

Removing the pressure pulse from the inlet 3 and applying a pulse of liquid under pressure to the inlet 6 causes the valve 12 to open and causes the fluid to flow as shown in Fig. 2 (b), i. e. from the inlet 6 to the outlet 7. The flexible membrane 20 allows pressure to be transmitted to the pathway 2, causing the valve 10 to close and causing fluid already in the pathway 2 to flow out through the outlet 4.

Thus, applying alternate pulses to the inlet 3 of the pathway 2 and to the inlet 6 of the pathway 5 causes a counter current to flow as illustrated by the arrows with pressure being transferred through the membrane.

The counter-current flow phases are shown in graphical form in Fig. 3.

The flow at the inlet 3 to the pathway 2 is illustrated by the upper line while the flow at the inlet 6 to the pathway 5 is illustrated by the lower line. It will be noted that the input flows to the inlet 3 and to the inlet 6 are out of phase with each other as shown in Fig. 3.

However, since fluid flows out through both of the outlets 4 and 7 in both cycles (i. e. at the same time) the output flows are continuous.

As indicated by the arrows F of Figs. 2 (a) and 2 (b) the liquid in the pathways 2 and 5 flows with a transverse component orthogonal to the general plane of the membrane 20.

Referring to Fig. 4 (a) a large suspended particle or agglomeration 25 can clog a heat exchanger pathway in the prior art. However, in the invention as shown in Fig. 4 (b) the membrane flexibility prevents such problems from occurring.

The heat exchanger of the invention is particularly suitable for heat exchange to/from slurries because of the lateral particle flow aspects. Another advantage for slurries is that because the heat transfer walls are flexible they do not become clogged and so little or no cleaning is required.

The pulses to control contra flow as described may be applied by pumps connected to pump fluid into the inlets. Alternatively, the pulses may be applied by waves of an external liquid within which the heat exchanger is immersed. For example, referring to Fig. 5 a heat exchanger 28 comprises three pathways 30 formed by tubes of thin plastics film. These carry a liquid A in one direction, say into the plane of the page.

A larger tube of thin plastics film 31 surrounds the tubes 30 and carries a liquid B in the opposite direction, out of the plane of the page. The tube 31 is in turn immersed in activating liquid C is contained within an insulated outer rigid container 32.

A wave generator 35 causes a series of waves to propagate in the activating liquid C.

These waves cause the outer tube 31 to develop a wave pattern, causing the liquid B to flow in this direction. This pulse closes one-way valves (not shown) in the tubes 30. When a wave is applied in the opposite direction to the actuating liquid C, a one-way valve for the liquid B closes. Thus, by applying waves to the activating

liquid C the flow is controlled in the tubes 30 and 31 in a manner akin to that described above referring to Figs. 2 (a) and 2 (b). Because the tubes 30 are completely surrounded by the liquid B there is a very high heat transfer surface area. Also, the activating liquid C provides an external counter balance to support the membranes 30 and 31, preventing them from encountering sharp movements or deflections which could damage them.

It will be appreciated that an arrangement such as shown in Fig. 5 provides a simple and effective way of applying the contra-flow pulses and also physical counter supporting forces for the membranes.

Referring again to the lateral flow component, Figs. 6 (a) and 6 (b) illustrate the flow patterns. For a deep pathway, a deep water wave pattern 40 of the membrane results in circular flow of water particles 41, with decreasing diameters away from the membrane. In a shallow water scenario as shown in Fig. 6 (b) the patterns 46 become elliptical away from the membrane 45. These flow patterns demonstrate that there is a much reduced boundary layer and what boundary layer there might be is decoupled from the drift flow rate of the fluid through the pathway. Because boundary layer is decoupled, the flow rate can be chosen as desired in the knowledge that it is not necessary to increase it to minimise the boundary layer. Instead the operator can control the parameters of : membrane wave height, membrane wave frequency, and imposed drift speed (flow rate) to suit the particular application.

It is even possible to have zero flow rate, as efficient heat transfer is still achieved.

The membrane may be of more stiff material with an actuator applying a wave pattern. For example, an ultrasound actuator may apply a"vibratory"wave pattern to a metal sheet membrane.

The heat exchanger of the invention may take many different forms, with any desired number of heat-transfer membranes in a wide variety of configurations which would be readily appreciated by those skilled in the art. Also, where the pulses are applied by propagation of waves in a liquid, the waves may be propagated in one of the heat transfer liquids. Also, a heat exchanger of the invention may be used for heat transfer in gases instead of liquids or between a liquid and a gas.

It will be appreciated that the invention provides a heat exchanger which: is of simple and inexpensive construction; is very efficient because of the thin boundary layer and use of thin membranes; does not become clogged when used with slurries.

The invention is not limited to the embodiments described but may be varied in construction and detail.