GB2510794A

GB2510794A - Heat recovery systems

Info

Publication number: GB2510794A
Application number: GB1213220.5A
Authority: GB
Inventors: Bohdan Majchrowski
Original assignee: Individual
Current assignee: Individual
Priority date: 2012-07-25
Filing date: 2012-07-25
Publication date: 2014-08-20
Also published as: GB201213220D0

Abstract

A heat exchanger comprises a first tube 12 through which a first fluid flows located within a second tube 14 with a gap 16 between an outer surface of the first tube and an inner surface of the second tube. A second fluid flows through the gap and a coil 18 located in the gap imparts non linear flow of the second fluid. The coil may be helical, may be fixed to the exterior of the first tube or may be fixed to the inner surface of the second tube. A gap may be provided between the outer radius of the coil and the inner surface of the second tube or between the inner radius of the coil and the outer surface of the first tube. The first tube and coil may be manufactured for thermal conductive material. The exterior of the second tube may comprise thermal insulation. The first tube may be connected in-line with a waste water pipe or an air duct for heat recovery. Recovered heat from waste water may pre-heat boiler feed water or stored in an insulated tank for later use. A corkscrew baffle (50, fig 6) may be located in the first tube.

Description

Title: Heat recovery systems

Description:

This invention relates to heat recovery systems, and in particular, but without limitation to heat recovery systems for waste water pipes.

In recent years it has become customary for buildings to be fitted with energy-saving or recuperation technologies to reduce their energy requirements and carbon footprints. Whilst it is possible to reduce the energy requirements of a building by providing better insulation, and to reduce a building's reliance on energy from non-renewable sources, there are, nevertheless, still aspects of a building's design that are inherently energy wasteful.

One aspect of energy efficient building design that could benefit from improvement is in the recovery of energy from waste heat from waste water and air pipes/ducts. For example, when hot waste water is sluiced to a drain, the heat in the water in it is lost, which is wasteful. The same is also true of waste air pipes, such as kitchen extractor units that expel hot air from above a cooker or hob to the exterior of the building, without recovering the heat from the expelled air.

To address this problem, a number of heat recovery systems have been proposed and which are already in use. These systems aim to recover the waste heat from water pipes and air ducts using a heat exchanger.

One example of such a known system comprises a pair of concentric pipes with a space therebetween through which a heat transfer fluid (often water, or a mixture of water and an anti-freeze, such as ethylene glycol, and/or an anti-corrosion solute) can flow. The heat transfer fluid is usually recirculated around a continuous loop, using an electric pump, so that recovered heat can be put to use elsewhere.

In such a known system, waste water flows through the interior of the inner pipe and makes contact with the interior sidewall of the inner pipe, which heats up. The exterior surface of the inner pipe is cooled by the heat transfer fluid so that heat is transferred from the waste water to the heat transfer fluid. The heated/warmed heat transfer fluid can be used, for example, to pre-heat the cold water entering a boiler to reduce the temperature differential between the boiler's hot and cold sides, and hence reduce the amount of work that the boiler needs to do to heat the water to a desired temperature. Such systems are well known, and offer some improvements in overall energy efficien cy.

A known problem with heat exchangers of the concentric pipe type described above is that the heat transfer fluid is pumped continuously and so only spends a relatively small proportion of its time in contact with the heat transfer components of the heat exchanger. As such, the amount of heat recovered can be quite low. One solution is to slow the flow rate of the heat transfer fluid, but this can lead to "hot spots" developing in the recirculation loop, especially where the heat exchanger is only in operation (that is, with hot/warm waste water flowing through it) for relatively short periods at a time.

A known alternative type of heat recovery heat exchanger comprises a central pipe through which the waste water flows, around which pipe, a separate, relatively narrow gauge heat transfer fluid pipe is coiled. Typically, the inner pipe and the heat transfer fluid pipes are made of a high thermal conductivity material, such as copper tubing, to maximise the heat transfer rate from the waste water to the heat transfer fluid. The main advantage of such a configuration is that the length of the heat transfer pipe in contact with the inner pipe is considerably longer than for a concentric pipe design, meaning that the time that the heat transfer fluid is in in the "active" part of the heat recovery circuit is much longer, thereby permitting higher flow rates for a given rate of heat transfer.

The main disadvantages of such a heat exchanger are that the narrow gauge heat transfer fluid pipe can be susceptible to blockages and can increase the pressure of the heat transfer fluid circuit for a given flow rate. Also, because the heat is transferred from pipe-to-pipe, as opposed to across a single wall, as in the concentric pipe design, the heat transfer rate can be inherently lower, especially where intimate contact between the inner pipe and the heat transfer fluid pipes is inefficiently intimate.

A need therefore exists for an alternative and/or improved heat exchanger that addresses one or more of the above problems.

According to the invention, there is provided a heat exchanger comprising a first tube through which, in use, a first fluid can flow, the first tube being located within a second tube with a gap between an outer surface of the first tube and an inner surface of the second tube, the gap forming a conduit through which, in use, a second fluid can flow, characterised by a coil located in the gap.

The provision of a coil in the gap can serve to cause the second fluid to flow around the inner pipe as it moves along the gap: the coil may act as a deflector or baffle to induce non-linear flow of the second fluid along the heat exchanger. In a preferred embodiment of the invention, the coil serves to create a coiled or helical pathway through which the second fluid can flow, thereby allowing direct fluid communication of the first and second fluids with opposite sides of the sidewall of the first tube, whilst at the same time, increasing the time that the second fluid remains in contact with the first tube. The use of a coil can, therefore, increase the time that the second fluid is in contact with the first pipe. Because the first and second fluids are able to come into contact with opposite sides of the sidewall of the first pipe, the rate of heat transfer can be improved, compared to a pipe-to-pipe interface.

The coil is preferably a helical coil. The coil is preferably affixed to the exterior surface of the first tube by welding, brazing, soldering or crimping. The coil may optionally be affixed to the inner surface of the second tube by any suitable means. In a possible embodiment of the invention, there may be a gap between the outer radius of the coil and the inner surface of the second tube, which gap may serve to provide a fluid bypass in the event of a blockage. Preferably, however, there will be no gap between the coil and the outer tube to ensure that all of the second fluid travels through the heat exchanger in a non-linear manner.

The first tube and coil are preferably manufactured from a high thermal conductivity material, and preferably also from a relatively corrosion resistant material, such as copper.

The exterior of the heat exchanger may comprise thermal insulation, such as lagging or a polymeric foam skin to retain as much heat as possible within the heat exchanger to increase its heat transfer performance.

The heat exchanger preferably comprises an inlet and an outlet for both the first and second tubes. The inlet and outlet of each respective set are preferably located at spaced apart locations on the heat exchanger. The inlet of the first tube is preferably located at the opposite end of the heat exchanger to the inlet of the second tube (and vice-versa) to create a contraflow of the first and second fluids.

The heat exchanger can conveniently be fitted in-line with a waste outlet, and preferably comprises push-fit, screw-fit or other industry-standard connectors to facilitate connecting it.

A preferred embodiment of the invention shall now be described, by way of example only, with reference to the accompanying drawings in which: Figure 1 is a first perspective view of an embodiment of a heat exchanger in accordance with the invention; Figure 2 is a second perspective view of the heat exchanger of Figure 1 with the second tube partially retracted; Figure 3 is a close-up view of the heat exchanger of Figure 2; Figure 4 is a close-up view of the heat exchanger of Figure 2 viewed from the opposite side to Figure 3; Figure 5 is a schematic sectional view of a second embodiment of the invention; and Figure 6 is a schematic side view of a baffle for the heat exchanger of any of Figures ito 5 The exemplary heat exchanger 10 according to the invention as shown in the drawings comprises a first, inner tube 12 through which waste water can flow and a second, outer tube 14 surrounding the inner tube 12 with a gap 16 therebetween. Disposed within the gap 16 is a helical

S

coil of copper tubing 18 that is wound or crimped tightly onto the inner tube 12 and which is secured to the inner tube 12 at intervals, by spot welds 20. In other embodiments of the invention, the coil can be retained to the inner or outer tube using a continuous join, and/or by other means, such as crimping, gluing, brazing, soldering etc. Waste water enters the inner tube 12 via a first open end forming an inlet 22 to which a conventional screw-fitting connector 24 is sealingly connected. The opposite end of the inner tube forms a waste water outlet 26 to which a second screw-fitting connector 24 is sealingly affixed. The heat exchanger 10 can thus be fitted in-line with a waste water pipe (not shown) by removing a section of the waste water pipe (not shown) of an appropriate length and by fitting the heat exchanger 10 in its place.

Heat transfer fluid enters the gap 16 between the outer surface of the inner pipe 12 and the inner surface of the outer pipe 14 via pipe fitting 28 and exits the gap 16 via another similar or identical pipe fitting 28. As such, a heat transfer fluid, such as water, can enter and leave the heat exchanger 10 via the pipe fittings 28.

The heat transfer fluid is contained within the gap 16 by a pair of step-down pipe fittings 30 that seal the ends of the outer tube 14 to the inner tube 12 to form a closed-ended cavity between the inner 12 and outer tubes 14.

It will be apparent that the heat transfer fluid is constrained, by the helical coil 18 to move through the gap 16 along a helical path, as indicated schematically by arrow 32, and that it makes contact with the outer sidewall of the inner tube as it flows through it. As such, the maximum possible surface area of the inner tube 12 comes into contact with the heat transfer fluid, and the length of the flow path of the heat transfer fluid is extended by virtue of it being constrained to move helically over the external sidewall of the inner tube 12, as opposed to if the helical coil 18 where not present, in which case, the heat transfer fluid would otherwise flow substantially linearly between the pipe fittings 28.

It will be appreciated that the heat exchanger 10 can be used in any orientation, although a vertical orientation is preferred so that the waste water (not shown) falls down the tube as it passes through it. The advantage of allowing the waste water to fall down the inner tube 12 is that it forms splashes and tumbles so as to form a film of water on the inner surface of the inner tube 12, whereas if the heat exchanger 10 were mounted horizontally, the waste water would run along a portion of the tube only, and may not come into contact with as great an area thereof.

It will be appreciated that the invention potentially offers a number of improvements over existing heat exchangers, for example, the coil 18 can increase the heat transfer area of the inner tube 12 as well as directing the heat transfer fluid to flow around the warm waste pipe and to extend the time that the cold water is in contact with the waste pipe.

Although the coil 18 has been shown in the form of a circular cross-section copper pipe of 4mm outer diameter, different geometries may be used as the function of the coil 18 is to deflect heat transfer fluid. The coil 18 could be made from metal or a wire, or even from a plastic strip of a suitable cross-section and size.

The coil 18 can be welded, braised, silver soldered or even glued to the wall of the waste pipe.

The inner 12 and outer tubes 14 of the illustrated embodiment are both made from copper, although the outer pipe could be made from different material, such as plastic. If the outer pipe is made from plastic, then the coil 18 could be attached to the inner surface of the outer tube 14, and could be formed by extrusion or moulding.

The heat exchanger 10 can be manufactured in different sizes to suit different requirements.

In the illustrated example, the inner tube 12 has been chosen to match a 54mm diameter, standard domestic waste water pipe, although to accommodate commercial and/or industrial requirements, the size could be increased.

The dimensions of the gap 16 can be designed to suit different flow and pressure requirements depending on individual applications.

A further possible modification to the design of the heat exchanger is shown in Figures Sand 6, in which a baffle 50 has been fitted within the inner tube 12 to deflect the waste water 52 as flows through the inner pipe 12. The baffle 50 is formed from a strip of metal that has been twisted to form an auger or corkscrew shape. The baffle 50 nests within the inner tube 12 and there may or may not be a gap between the edges of the baffle 50 and the interior sidewall 54 of the inner tube 12.

Where there is a gap between the baffle 50 and the inner tube 12, the baffle 50 serves to cause water 52 falling through the inner tube 12 to be shed radially outwardly, or for be spun outwardly, to form a film that flows down the interior sidewall 54 of the inner tube 12.

Alternatively, where there is no gap between the baffle 50 and the inner tube 12, the baffle serves to form a helical flow path for the waste water 52 in much the same way as the coil 18 does for the cooling fluid flowing in the gap 16 between the inner 12 and outer 14 tubes.

By such action, the amount of waste water 52 that does not come into contact with the active parts of the heat exchanger as if passes therethrough is minimised.

Of course, the configuration of the baffle 50 need not be a corkscrew, as shown, but could take other forms, such as a series of cones or planar deflector plates that deflect the water 52 radially outwardly. The main advantage, however, of using a corkscrew-type baffle 50 is that the waste water 52 is induced into following a helical path through the inner tube which not only ensures that a greater proportion of it comes into contact with the interior sidewall 54 of the inner tube, but also extends the length of the flow path, thereby increasing the time that the water 52 reamins within the heat exchanger as it passes through it.

The heat exchanger of the invention may form part of a heat recovery system with the first tube connected in-line with a waste water pipe or an air duct, and the heat transfer fluid being arranged to flow through a heat recovery circuit of, for example, a boiler, to pre-heat water at the cold side of the boiler. The heat transfer fluid could be arranged to flow through a heat recovery and storage system, whereby heated heat transfer fluid is retained in a thermally insulated tank so that the heat can be used at a later point in time.

The invention is not restricted to the details of the foregoing embodiment, which is merely exemplary of the invention. For example, the heat exchanger's dimensions and materials of manufacture could be changed to suit different needs, the connectors and fittings could be different and/or arranged differently, the overall configuration of the heat exchanger need not necessarily be straight, and, of course, the system could be used for recovering waste heat from air, rather than waste water by suitable adaption and/or modification.

Claims

Claims: 1. A heat exchanger comprising a first tube through which, in use, a first fluid can flow, the first tube being located within a second tube with a gap between an outer surface of the first tube and an inner surface of the second tube, the gap forming a conduit through which, in use, a second fluid can flow, characterised by a coil located in the gap.
2. A heat exchanger according to claim 1, wherein the coil is adapted to induce, in use, non-linear flow of the second fluid.
3. A heat exchanger according to claim 1 or claim 2, wherein the coil forms a helical pathway in the gap through which the second fluid can flow.
4. A heat exchanger according to claim 3, wherein the coil comprises a helical coil.
5. A heat exchanger according to any preceding claim, wherein the coil is affixed to the exterior surface of the first tube by welding, brazing, soldering or crimping.
6. A heat exchanger according to any preceding claim, wherein the coil is affixed to the inner surface of the second tube.
7. A heat exchanger according to any preceding claim, wherein a gap is provided between the outer radius of the coil and the inner surface of the second tube, or between the inner radius of the coil and the outer surface of the first tube.
8. A heat exchanger according to any preceding claim, wherein the first tube is manufactured from a high thermal conductivity material.
9. A heat exchanger according to any preceding claim, wherein the coil is manufactured from a high thermal conductivity material.
10. A heat exchanger according to any preceding claim, wherein the exterior of the heat exchanger or the second tube comprises thermal insulation.
11. A heat exchanger according to any preceding claim, comprising an inlet and an outlet for both the first and second tubes, the inlet and outlet of each respective set being located at spaced apart locations on the heat exchanger, and wherein the inlet of the first tube is located at an opposite end of the heat exchanger to the inlet of the second tube.
12. A heat exchanger according to claim 11, further comprising push-fit, screw-fit or other industry-standard connector affixed to the inlets and outlets.
13. A heat recovery system comprising a heat exchanger according to any preceding claim.
14. A heat recovery system as claimed in claim 13, wherein the first tube is connected in-line with a waste water pipe.
15. A heat recovery system as claimed in claim 13, wherein the first tube is connected in-line with an air duct.
16. A heat recovery system as claimed in any of claims 13, 14 or 15, wherein the heat transfer fluid is arranged to flow through a heat recovery circuit.
17. A heat recovery system as claimed in claim 16, wherein the heat recovery circuit is used to pre-heat water at the cold side of a boiler.
18. A heat recovery system as claimed in claim 16, wherein the heat recovery circuit is connected such that the heated heat transfer fluid is retained in a thermally insulated tank so that the heat can be used at a later point in time.
19. A heat recovery system according to any preceding claim, further comprising a baffle located within the interior of the first tube.
20. A heat recovery system as claimed in claim 19, wherein the baffle has an auger or corkscrew shape.
21. A heat recovery system according to claim 19 or claim 20, wherein the baffle is nested within, and is in contact with, the first tube to form a helical flow path for the first fluid.
22. A heat recovery system according to claim 19 or claim 20, further comprising a gap between the baffle and an interior side wall of the first tube.
23. A heat exchanger substantially as hereinbefore described, with reference to and as illustrated in the accompanying drawings.