EP0385700A1

EP0385700A1 - Heat exchange unit, heat exchange system, method of improving heat exchange efficiency, and refrigeration circuit

Info

Publication number: EP0385700A1
Application number: EP90302039A
Authority: EP
Inventors: Michael John Nunnerley
Original assignee: Individual
Current assignee: Individual
Priority date: 1989-02-28
Filing date: 1990-02-27
Publication date: 1990-09-05
Also published as: GB2228563A; GB8904571D0

Abstract

A heat exchange system comprises an inner pipe (6) of heat conductive material connected at one end to a supply of cold water at mains pressure, and at the other end to a hot water delivery system. Extending around the inner pipe (6) is an outer pipe (8), through which a primary supply of heated water is fed by a pump (P1). As cold water flows through the pipe 6 in one direction, and heated water flows through the pipe (8) around the pipe (6) in the opposite direction, heat is transferred to the cold water supply.

In improve heat transfer efficiency, helical formations are provided within the pipe (6) to cause the cold water to rotate (spin) about the longitudinal axis of the pipe as it travels from the inlet to the outlet, causing the colder, heavier component to move radially outwardly into contact with the inner walls of the pipe (6), whilst formations within the pipe (8) cause a similar rotational movement of the heated water, causing the colder component thereof to flow radially outwardly under the action of centrifugal forces, and for warmer component to flow radially inwardly into contact with the outer surface of the pipe (6), maximising the temperature differential across the pipe (6) and thus heat transverse efficiency.

The invention may alternatively be used in a refrigeration system.

Description

In a conventional heat exchange system, heat is transferred from first fluid supply to a second fluid supply through the intermediary of a heat exchange member. The first fluid supply may be hot gases, intended to heat a liquid (such as water), or both fluid supplies may be liquids such as water. Thus where it is desired to provide a supply of heated liquid, the first liquid supply may be a primary supply, which is heated from remote source of heat, and from which heat is transferred to the second liquid supply.
Alternatively where it is desired to provide a supply of cooled fluid, the second fluid may be the primary supply, which is cooled by means of a remote refrigeration unit, and by which heat is withdrawn from the first fluid supply.
The invention will be described hereinafter primarily in relation to a heat exchange system in which the first fluid supply is a supply of hot water heated by an external heating means (which may be gas, electricity, solid fuel) and the second fluid supply is a supply of cold water at mains pressure, the temperature of which is desired to raise to provide a supply of hot water. Additionally however, the invention may be utilised when the first fluid supply is provided at a high temperature, such as in relation to a district heating system, or conceivably by natural means.
It is also to be appreciated that the invention may be applied to systems in which the first and second fluid supplies are other than water, and where the first fluid supply is a different fluid from the second. Whilst the invention is primarily applicable to liquids, it may have applications to gaseous heat exchange system or liquid/gaseous exchange systems.
In the conventional heat exchange system, the heat exchange member is elongate and circular in cross-section, and may be coiled into a helix. Difficulty is encountered in that in the flow of one or both said fluid supplies produces a temperature differential which is effective to reduce the heat flow through the heat exchange member, and this reduces the efficiency of the heat exchange system. In particularly, where on one side of the heat exchange member there is a supply of cold water, and on the other side a supply of hot water, the rate of flow of heat through the heat exchange member is proportional to the temperature differential. However as the cold water arms, and the hot water cools, the temperature gradient is reduced.
Conventional means for reducing such temperature gradient fall provide means effective to cause turbulence to the water flow, which to some extent has been successful.
According to this invention there is provided a heat exchange unit comprising:

[a] a first fluid supply;
[b] a second fluid supply;
[c] an elongate heat exchange member of circular cross-section between said supplies whereby heat exchange between said supplies may be effected; characterised in that

The invention is particularly useful in a system for heating cold water at mains pressure from hot water which is heated by external heating means. In this manner, energy required to produce the requisite centrifugal forces in the supply or supplies subject to spinning may be derived in part at least by a reduction in the water pressure.
According to this invention there is also provided a heat exchange system comprising:

[a] an inner tube of a heat conductive material such as metal;
[b] means for connecting the inner pipe at one end to a pressurised water supply and at the other end to a heated water delivery system;
[c] an outer pipe extending around said inner pipe; and
[d] means for providing a flow of heated water through said outer pipe, characterised in that
- [1] means is provided in the first pipe to cause water to travel about the axis of the pipe as it flows along the pipe whereby a cooler component of the water flows along a radially outer path more closely adjacent to the wall of the pipe and a warmer component of the water flows along a radially inner path more closely adjacent to the axis of the pipe, said rotational movement of the water being produced by the pressure of the water supply; and
- [2] means is provided in the second pipe to cause the liquid to travel about the axis of the pipe as it flows along the pipe whereby a cooler component of the liquid flows along a radially outer path more adjacent to the axis of the pipe.

According to this invention there is also provided a method of improving the heat exchange efficiency of a heat exchange unit comprising an elongate heat exchange member of circular cross-section, through which liquid to be heated is fed, involving the step of providing in said heat exchange member formations such as to cause liquid flowing through the heat exchange member to rotate about the axis of the heat exchange member as it flows along the heat exchange member to maximise the temperature differential across the heat exchange member, said rotation being effected by a pressure differential between the inlet to said heat exchange member and to the outlet thereof.
Whilst the invention has been described above in relation to a heat exchange unit for use in providing a supply of heated fluid, it will of course be appreciated that the principles of the invention may be utilised in a heat exchange unit for use in the provision of a cool fluid.
Thus according to this invention there is also provided a refrigeration circuit comprising a first pipe, a second pipe, the first pipe extending within the second pipe, means to cause a primary fluid to flow along the first pipe, and means to cause a secondary fluid to flow along the second pipe, the primary fluid being subjected to a refrigeration operation, characterised in that means is provided to cause one or both of the primary and secondary fluids to spin as they flow through their respective pipes about the longitudinal axis.
There will now be given a detailed description,to be read with reference to the accompanying drawings, of a heat exchange system which is the preferred embodiment of this invention, and which has been selected for the purposes of illustrating the invention by way of example.
In the accompanying drawings:

FIGURE 1 is a schematic view of the heat exchange system which is the preferred embodiment of the invention;
FIGURE 2 is a schematic sectional view through part of a heat exchange unit of the preferred embodiment;
FIGURE 3 is a detailed view of the preferred embodiment of the invention;
FIGURE 4 is a view illustrating in detail part of the heat exchange unit,
FIGURE 5 is a graph illustrating temperature changes occurring in the preferred embodiment during use;
FIGURES 6a to 6h view illustrating the use of the invention in hot water systems of different types; and
FIGURES 7 and 8 are respective plan and vertical sectional views illustrating the use of the invention in a different type of heat exchange system.

The heat exchange system which is the preferred embodiment of this invention is for use as part of water heating system, in which it is desired to utilise a primary water supply A, which is heated to a high temperature by an external heating source H, to heat a secondary water supply B constituted by water at mains pressure and temperature, for domestic use.
The preferred embodiment comprises a heat exchange unit 5 comprising a heat exchange element in the form of an elongate internal pipe 6, conventionally of a heat conductive material such as metal (e.g. copper), and an elongate external pipe 8 extending around the pipe 6. The primary water supply is ducted through the pipe 8, flowing through the annular space between the pipes 6 and 8, whilst the secondary water supply is ducted through the pipe 6, the pipe 6 thus providing a cylindrical heat exchange surface between the primary and secondary supplies.
Provided on the pipe 6, both internally and externally of the heat exchange member, is a helical ribbing 10, said helical ribbing comprising an interior component 10a and an exterior component 10b.
Thus as cold water flows through the pipe 6, it is subjected by the helical component 10a to a helical spinning about the longitudinal axis of the pipe 6, whilst the primary water supply A is subjected to a similar helical spinning by the component 10b.
In this manner as the primary water supply A flows along the pipe 8, portions thereof adjacent to the heat exchange member 6, and which by virtue of having partaken in a heat exchange operation, and in consequence being of reduced temperature, and thus of higher density, are thrown radially outwardly of the pipe, to be replaced by relatively warmer water.
Similarly in the pipe 6, portions of the secondary supply which have been partaken in a heat exchange operation will be replaced by colder, more dense water, flowing radially outwardly.
In other words the colder, more dense component of both flows of water is moved radially outwardly, whilst the warmer component is moved radially inwardly, maximising the temperature differential across the heat exchange member.
As is seen from Figure 1, the heat exchange unit comprising the pipe assembly 6/8 is located in a generally conventional hot water tank T, which is lagged and maintained filled with water. The primary water supply A is fed by a pump and is heated by a primary heater H whilst the secondary water supply is provided directly from the mains cold water supply, and is connected on the return side to the domestic hot water delivery system.
In the preferred embodiment, shown in more detail in Figures 3 and 4, the heat exchange unit 5 is located in a main store of hot water, in the form of a water tank T. The outer pipe 8 is open at its upper end, adjacent to the region at which the inner pipe 6 enters the outer pipe, as best seen in Figure 4. The unit 5 then extends in generally coil form within the tank, the pipe 6 exiting from the pipe 8 at a lower portion.
The lower portion of the pipe 8 extends to a pump P1 to a primary heat input device, in the form of a boiler B, where the primary water supply is heated, and returned to the tank by pipe 15 entering the tank at the bottom. Extending from the tank is a central heating system C, water being drawn off by a pump P2 and circulated through conventional central heating radiators R prior to returning to the tank T.
At its lower or inlet end the pipe 6 is connected to a supply S of water at mains pressure, such water passing through the heat exchange unit 5 and being connected at its outlet end O to a hot water delivery system.
In use, on demand water is caused to flow through the outlet O, such as by turning on an appropriate tap at a place where hot water is desired, such as in a sink or bath. Flow of water into the heat exchange unit is detected by a flow switch F, which activates pump P1. As the cold water flows into the heat exchange unit 5, it is heated, and exits as hot water through the outlet O. If desired a thermostatic mixing valve V may be provided,so that hot water exiting from the heat exchange unit 5 maybe mixed in appropriate proportions with cold water bypassing the heat exchange unit 5, to ensure a substantially constant temperature at the outlet O.
During use, the pump P1 draws water from the upper part of the tank T through the pipe 8, to heat water flowing in the pipe 6, the spinning motion produced by the water as it flows through the two pipes maximising heat exchange efficiency of the unit 5, and ensuring that water exiting from the heat exchange unit 5 is at or adjacent to a desired temperature.
In the case of the pipe 8, the energy required to cause the primary, heated supply of hot water to spin as it travels along the pipe 8 over the heat exchange element, is produced by the pump P1, whilst in the case of the water flowing through the pipe 6, the energy required to cause rotation about the longitudinal axis of the pipe 6 is produced by the pressure of the mains supply, a pressure reduction being detectable between the inlet and the outlet.
Figure 5 illustrates an experiment, involving measurement of the temperature T1 of the water in the tank T, the temperature T2 of water returned to the tank through the pipe 15 (the boiler B being inoperative), and the temperature T3 of water exiting the heat exchange unit 5, without any mixing with the cold water through the valve V taking place.
Water in the tank T, having a volume of 210 litres, was heated to 85° C, the temperature of water entering the heat exchange unit (the main water temperature) being 12° C, and with the boiler inoperative to apply further heat to the system, water was drawn from the tank (without any mixing with cold water) at a rate of 60 litres per minute.
In Figure 5 the temperature changes during the first 40 seconds should be ignored, since during this time the temperatures are effected by the steady state conditions, involving the temperature of the water in the pipe 6 being at the store temperature, and the heat exchange unit itself being of store temperature. Thereafter however, it can be seen that the temperature T3 of water exiting the tank is substantially constant, despite a continuing drop in store temperature T1, and indeed the temperature T3 falls only a small amount whilst 250 litres of water are drawn through he heat exchange unit 5. Over the whole of the range the difference between the temperature T3 of water and the temperature T2 at which water is returned to the boiler through the pipe 15 remains small and substantially constant, indicating an exceedingly high level of heat exchange efficiency by the use of the unit 5.
In conventional manner,the tank T is connected to a feed and expansion tank E, and is provided with an internal immersion heater I and interior tank thermostat X.

Figures 6a to 6h illustrated different configurations involving the use of the invention, in heater installations of different types. In all cases, the installations comprise a heat exchange unit shown schematically as 5, but inf act as described in relation to the detailed description in Figures and 4.
Figure 6a illustrates a conventional domestic heater installation involving the use of an electric immersion heater I, the pump P1 being utilised to draw water from the store through the unit 5, and return such water to the bottom of the tank T adjacent to the heater I.
Figures 6b illustrates an installation appropriate for use with a gas/oil boiler B at a rating below 100,000 btu (30 kw), with an optional electric immersion heater, whilst Figure 6c illustrates an installation for boilers in excess of 30 kw.
Figure 6d illustrates a system appropriate for use with a solid fuel boiler operating at a rating of below 12 kw, whilst Figure 6e illustrates an installation suitable for use with a solid boiler having a rating of above 12 kw.
Figure 6f illustrates an installation which may be a combined solid fuel and gas/oil boiler with an optional electrical immersion heater, whilst Figure 6g illustrates an installation suitable for use with a combination of two solid fuel boilers.
Figure 6h illustrates an installation suitable for use with a low head system, which may comprise a gas or boiled boiler.

Whilst the invention has been described above in a heating installation in which the primary fluid supply (i.e. that from which the second fluid supply derives heat) is in the form of a supply of heated water, the characteristics of the invention may be utilised in deriving heat for input to a fluid supply where the primary supply is afforded directly by a heating installation, e.g. a gas boiler.
Thus there is illustrated in Figures 7 and 8, a boiler installation comprising an outer casing 20, and a central catalytic burner tube 22. Extending helically around the burner tube for a substantial height-wise distance is a heat exchange unit comprising twin coils 5a and 5b, each in the form of an inner pipe 6 provided with interior helical formations 10a, an exterior helical formations 10b, and an exterior pipe 8. A supply M of cold water at mains pressure may be fed through the pipe 6, being caused to rotate about the longitudinal axis of the pipe 6 by the helical formations 10a, causing the colder, more dense component of the water to adopt a position close to exterior walls of the pipe 6, and thus to participate in heat exchange with the pipe 6, whilst returning water drawn from a main store Th of water flows back along the pipes 8, being heated by the pipe 8, the exterior of which being heated by hot gases produced by the burner tube 22, and in turn giving heat to water flowing into the unit 5 through the pipe 6.
Preferably the pipe 8 is provided on its exterior with helical ribs 10d, which do not effect any spinning operation of heat exchange medium, but instead increases the surface area exposed externally of the pipe 6, to maximise heat transfer to the cold water flowing through the pipe.
The features disclosed in the foregoing description, or the following claims, or the accompanying drawings, expressed in their specific forms or in the terms or means for performing the desired function, or a method or process for attaining the disclosed result, may, separately or in any combination of such features, be utilised for realising the invention in diverse forms thereof.

Claims

1. A heat exchange unit (5) comprising:

[a] a first fluid supply (A);

[b] a second fluid supply (B);

[c] an elongate heat exchange member (6) of circular cross-section between said supplies whereby heat exchange between said supplies may be effected;
characterised in that
means (10) is provided to cause at least one of the supplies to spin as it travels along the heat exchange member.

2. A unit according to Claim 1 wherein the heat exchange member (6) is provided with helical formations (10a) to cause the second fluid supply to spin as it flows along the heat exchange member.

3. A unit according to one of Claims 1 and 2 wherein the heat exchange member (6) is located in a pipe (8) through which the first supply (A) flows, means (10b) being provided to cause the first fluid supply to spin as it flows along said pipe (8) over the heat exchange member (6).

4. A unit according to Claim 3 wherein said means is provided by helical formations (10b) on the exterior of the heat exchange member (6) and/or the interior of said pipe (8).

5. A unit according to any one of the preceding claims wherein the heat exchange member (6, 8) is provided by an elongate pipe, preferably being coiled.

6. A unit according to any one of Claims 3, 4 and 5 comprising means P to cause or to allow flow of the first fluid (A) along the heat exchange member (6), and to allow or cause flow of the second fluid supply (B) along the heat exchange member in a direction opposite to that of the first fluid supply.

7. A unit according to any one of the preceding claims wherein the primary supply (A) is liquid heated by an external heating means (H, B), and the second fluid supply (B) is liquid [such as water] at mains pressure.

8. A heat exchange system comprising:

[a] an inner pipe (6) of a heat conductive material such as metal;

[b] means for connecting the inner pipe (6) at one end to a pressurised water supply (S) and at the other end (10) to a heated water delivery system;

[c] an outer pipe(8) extending around said inner pipe; and

[d] means (P) for providing a flow of heated water through said outer pipe, characterised in that

[1] means (10a) is provided in the first pipe (6) to cause water to travel about the axis of the pipe as it flows along the pipe whereby a cooler component of the water flows along a radially outer path more closely adjacent to the wall of the pipe and a warmer component of the water flows along a radially inner path more closely adjacent to the axis of the pipe, said rotational movement of the water being produced by the pressure of the water supply; and

[2] means (10b) is provided in the second pipe (8) to cause the liquid to travel about the axis of the pipe as it flows along the pipe whereby a cooler component of the liquid flows along a radially outer path and a warmer component of the water flows along a radially inner path more adjacent to the axis of the pipe.

9. A heat exchange system according to Claim 8 wherein said means for providing a flow of heated liquid through said outer pipe is provided by a pump (P1) operative to cause water to flow through the pipe to a boiler (B).

10. A system according to Claim 9 wherein said pump (P) is operative to draw liquid through said outer pipe from a main store (T) thereof, and to return the liquid to the main store after passing through the boiler (B).

11. A method of improving the heat exchange efficiency of a heat exchange unit comprising an elongate heat exchange member (6) of circular cross-section, through which liquid to be heated is fed, involving the step of providing in said heat exchange member formations (10a, 10b) such as to cause liquid flowing through the heat exchange member to rotate about the axis of the heat exchange member as it flows along the heat exchange member to maximise the temperature differential across the heat exchange member (16), said rotation being effected by a pressure differential between the inlet to said heat exchange member and to the outlet thereof.

12. A refrigeration circuit comprising a first pipe or tube, a second pipe or tube, the first pipe extending within the second pipe, means to cause a primary fluid to flow along the first pipe, and means to cause a secondary fluid to flow along the second pipe, the primary fluid being subjected to a refrigeration operation, wherein means is provided to cause one or both of the primary and secondary fluids to spin as they flow through their respective pipes about the longitudinal axis.