WO2013104885A1

WO2013104885A1 - Heat transfer unit and a heat exchanger

Info

Publication number: WO2013104885A1
Application number: PCT/GB2012/053201
Authority: WO
Inventors: Dumitru Fetcu
Original assignee: Econotherm Uk Limited
Priority date: 2012-01-12
Filing date: 2012-12-20
Publication date: 2013-07-18
Also published as: GB2499975A; EP2802833A1; GB201200442D0

Abstract

A heat transfer unit (100) and a heat exchanger (200, 300) comprising a heat transfer unit (100) is disclosed, for transferring heat between a first medium and a second medium. The unit (100) comprises a reservoir (101) and a heat transfer fluid disposed within the reservoir (101) which is arranged to exchange heat with the first medium. The unit (100) further comprises a chamber (103) and a plurality of fluid transfer ducts (102) which extend between the reservoir (101) and the chamber (103) and which are arranged to communicate the heat transfer fluid between the reservoir (101) and the chamber (103). The unit (100) further comprises a flow duct (105) along which the second medium is arranged to pass, at least a portion of the flow duct (105) being arranged in thermal communication with an interior of the chamber (103), such that during use, the second medium is arranged to exchange heat with the heat transfer fluid.

Description

HEAT TRANSFER UNIT AND A HEAT EXCHANGER

The present invention relates to a heat transfer unit and a heat exchanger. A heat pipe is a hermetically sealed, evacuated tube comprising a working fluid in both the liquid and vapour phase. When one end of the tube is heated the liquid turns to vapour upon absorbing the latent heat of vaporization. The hot vapour subsequently passes to the cooler end of the tube where it condenses and gives out the latent heat to the tube. The condensed liquid then flows back to the hot end of the tube and the vapori2ation-condensation cycle repeats. Since the latent heat of vaporization is usually very large, considerable quantities of heat can be transferred along the tube and a substantially uniform temperature distribution can be achieved along the heat pipe.

Referring to figure 1 of the drawings, there is illustrated a known heat exchanger 10. The exchanger 10 typically comprise two chambers 11 , 12, which are separately arranged to receive a fluid between which heat transfer is desirable. The chambers 11, 12 are separated by a separation plate 13, and a plurality of heat pipes 14 are arranged to extend between the chambers 11 , 12, through the separation plate 13, to transfer heat between the chambers 11 , 12 and thus the fluids. In this respect, by passing a relatively hot fluid through one chamber 11 , via the respective inlet 11a and outlet 11b, the heat pipes 14 can transfer the heat absorbed from the hot fluid to the other chamber 12 wherein a relatively cold fluid may pass, via the respective inlet 12a and outlet (not shown), to subsequently absorb the heat from the heat pipes 14. The relatively cold fluid is typically passed through the chamber 12 under high pressures to ensure a suitable extraction of heat from the heat pipes. This necessitates a thick separation plate 13 and side walls of the second chamber 12 to withstand the high forces. A problem with using the high pressure fluid however, is that the fluid is found to leak from the second chamber 1 into the first chamber 11 and as such, the fluid in the first chamber can become contaminated by the fluid from the second chamber.

It is known to utilise heat exchangers to harvest heat from exhaust gases for example, to generate electricity. During so-called organic Rankine cycles, the heat extracted from an industrial exhaust gas in one chamber of the exchanger is used to heat an oil in the adjacent chamber of the exchanger. The oil undergoes a liquid-vapour phase change at a temperature lower than a water-steam phase change for example, and as such allows heat recovery from lower temperature sources. However, such oils are typically expensive and can decompose if exposed to too high a temperature. Moreover, any leakage of the oil from the cooler chamber into the hotter {exhaust gas) chamber presents a fire hazard and so it is necessary to ensure that the oil cannot pass between chambers of the heat exchanger.

It is also known to provide so-called annular heat pipes 20 in which the cooling fluid is arranged to pass within a flow duct 21 , along a longitudinal axis, through a heat pipe jacket 22, such that the heat transfer occurs by the vaporisation/condensation cycle of the working fluid located in an annular space 23 defined between the flow duct 21 and the outer jacket 22, as illustrated in figure 2 of the drawings. The jacket 22 may be welded to the flow duct 21 to minimise any contamination of the heated fluid by the cooling fluid. However a problem with these annular heat pipes is that heat transfer area at the exterior of the jacket 22 is small in comparison with the heat transfer area associated with the flow duct 21 when heat exchange is required between a liquid and a gas. For example, in situations whereby a liquid is passed along the flow duct, to extract heat from a hot exhaust gas which is arranged to pass over the jacket 22, then it is found that the annular heat pipe 20 only provides for a minimal exchange of heat.

We have now devised an improved heat transfer unit and an improved heat exchanger.

According to a first aspect of the present invention, there is provided a heat transfer unit for transferring heat between a first medium and a second medium, the unit comprising a reservoir and a heat transfer fluid disposed within the reservoir which is arranged to exchange heat with the first medium,

the unit further comprising a chamber and a plurality of fluid transfer ducts which extend between the reservoir and the chamber and which are arranged to communicate the heat transfer fluid between the reservoir and the chamber

the unit further comprising a flow duct along which the second medium is arranged to pass, at least a portion of the flow duct being arranged in thermal communication with an interior of the chamber, such that during use, the second medium is arranged to exchange heat with the heat transfer fluid. The reservoir and fluid transfer ducts provide for an increased heat transfer area of the unit and therefore provide for an improved exchange of heat between the first and second medium. Moreover, the flow duct may be welded or otherwise bonded to the chamber to suitably seal the flow duct to the chamber.

Preferably, the reservoir and chamber are substantially elongate and the fluid transfer ducts, reservoir and chamber are preferably disposed in substantially the same plane, such that the unit comprises a substantially planar unit.

Preferably, the flow duct extends along a longitudinal axis of the chamber, such that the interior of the chamber which is arranged in fluid communication with the heat transfer ducts, comprises an annular space. The flow duct preferably comprises an inlet for receiving the second medium into the duct and an outlet through which the second medium may pass out from the duct. Preferably, the duct comprises a plurality of turbulence promoters disposed between the inlet and the outlet thereof for encouraging a turbulent flow of the second medium along the flow duct, to increase the transfer of heat across a wall of the flow duct.

Preferably, the flow duct further comprises at least one expansion sleeve disposed within the wall thereof which permits the duct to expand and contract. The expansion sleeve preferably comprises a pleated portion of the side wall, such as a bellow. In an embodiment of the present invention, at least one of the reservoir and chamber of the heat transfer unit preferably comprises a cooler for cooling the fluid disposed within the respective reservoir or chamber. The cooler preferably comprises a cooling duct which extends within the reservoir or chamber and is arranged to communicate a cooling fluid between an inlet and an outlet of the cooling duct.

In a further embodiment of the present invention, the heat transfer unit preferably comprises a cooler disposed within the reservoir and the chamber. The coolers preferably separately comprise a cooling duct which extends within the reservoir and chamber. The cooling ducts are preferably arranged to communicate a cooling fluid between an inlet and an outlet thereof and the outlet of one cooling duct is preferably coupled to the inlet of the other cooling duct. The coolers are thus arranged to reduce the working temperature of the fluid and in this situation, provide for an increased capacity for heat absorption.

According to a second aspect of the present invention, there is provided a heat exchanger for exchanging heat between a first medium and a second medium, the exchanger comprising a heat exchanging housing having an inlet for receiving the first medium into the housing at a first temperature and an outlet through which the first medium may pass at a second temperature,

the exchanger further comprising at least one heat transfer unit of the first aspect, the reservoir and at least a portion of the heat transfer ducts of the unit being arranged to extend within the housing, such that during use, the fluid within the reservoir can exchange heat with the first medium.

The exchanger preferably comprises a plurality of heat transfer units orientated in a substantially parallel arrangement. Preferably, the or each heat transfer unit is orientated , substantially perpendicular to a passage of the first medium within the heat exchanger housing.

Preferably, the inlet to the or at least two flow ducts is coupled to an inlet manifold and the outlet from the or at least two flow ducts is coupled to an outlet manifold. Alternatively, the outlet from the flow duct of one heat transfer unit may be coupled to the inlet to flow duct of a transfer unit.

Embodiments of the present invention will now be described by way of example only and with reference to the accompanying drawings, in which:

Figure 1 is a longitudinal sectional view along a known heat pipe heat exchanger;

Figure 2 is longitudinaf sectional view along a known annular heat pipe;

Figure 3 is a longitudinal sectional view along a heat transfer unit according to an embodiment of the present invention; Figure 4 is a transverse sectional view across the heat transfer unit illustrated in figure 3 taken along line A-A; Figure 5 is a longitudinal sectional view along a heat exchanger according to an embodiment of the present invention;

Figure 6 is a transverse sectional view of the heat exchanger illustrated in figure 5, taken along line S-B; and

Figure 7 is a transverse sectional view of a heat exchanger according to a further embodiment of the present invention.

Referring to figures 3 and 4 of the drawings, there is illustrated a heat transfer unit 100 according to an embodiment of the present invention for transferring heat between a first and second medium (not shown). The unit 100 comprises an elongate reservoir 101, which is substantially circular in cross-section, and a plurality of heat transfer ducts 102 which extend from spaced apart locations upon the reservoir 101 , in a substantially parallel configuration, to an elongate chamber 103, which is similarly substantially circular in cross-section. During use, the unit 100 is orientated such that reservoir 101 is disposed at a vertical height below the chamber 103. The reservoir 101 is arranged to hold a heat transfer fluid 104, such as water and the volume of the heat transfer fluid 104 within the unit 100 is such that when orientated for use, the fluid 104 extends partly along the heat transfer ducts 102, toward the chamber 103.

The chamber 103, reservoir 101 and transfer ducts 102 extend in substantially the same plane, such that the heat transfer unit 100 comprises a substantially planar unit. The unit 100 further comprises a flow duct 105, which may be circular in cross-section and which is arranged to extend through the chamber 103 along a longitudinal axis thereof, such that a wall 106 of the flow duct 105 extends in thermal communication with an interior of the chamber 103. The flow duct 105 therefore defines an annular space 107 within the chamber 103 which is sealed to the wall 106 of the flow duct 105 by virtue of the bonding or welding of the flow duct 105 proximate the inlet 108 and outlet 109 of the flow duct 105, where the flow duct 105 enters and exits the chamber 103, respectively. The heat transfer ducts 102 are arranged in fluid communication with the reservoir 101 and the annular space 107 within the chamber 103, and are arranged to communicate the heat transfer fluid 104 between the reservoir 101 and the chamber 103.

In a further embodiment of the present invention, which is not illustrated, the reservoir 101 comprises a cooler (not shown) which is arranged to cool the heat transfer fluid 104 within the reservoir 101. The cooler (not shown) comprises a cooling duct (not shown), which extends along the reservoir 101 and which comprises an inlet (not shown) disposed at one end of the reservoir 101 and an outlet (not shown) disposed at an opposite end of the reservoir 101. The cooling duct (not shown) is arranged to communicate a cooling fluid (not shown) between the inlet and the outlet thereof, within the reservoir 101 , so that the cooling fluid can absorb heat associated with the fluid 104 within the reservoir 101 and thus cool the fluid 104 within the reservoir 101.

In a further embodiment which is also not illustrated, the reservoir 101 and chamber 103 separately comprise a cooler (not shown) which are separately arranged to cool the heat transfer fluid 104 therein. The coolers (not shown) separately comprise a cooling duct (not shown) which is arranged to communicate a cooling fluid (not shown) between an inlet (not shown) and the outlet (not shown) thereof, within the respective reservoir 101 and chamber 103, so that the cooling fluid (not shown) can absorb heat associated with the heat transfer fluid 104 within the respective reservoir 101 and chamber 103 and thus cool the fluid 104. In this embodiment, the outlet (not shown) of the cooling duct (not shown) within the reservoir 101 is coupled to the inlet (not shown) of the cooling duct within the chamber 103 so that the cooling fluid (not shown) can circulate from the cooler (not shown) disposed within the reservoir 101 to the cooler disposed within the chamber 103.

The coolers (not shown) are arranged to cool the fluid 104 within the transfer unit 100 to reduce the working temperature of the fluid 104 and thus increase the capacity of the fluid 104 to absorb heat and thus improve the heat exchange. Referring to figures 5 and 6 of the drawings, there is illustrated a heat exchanger 200 according to an embodiment of the present invention which incorporates the above described heat transfer unit 100. The exchanger 200 comprises a housing 201 having an inlet 202 for receiving a first medium, such as hot exhaust gases (not shown) from an industrial process, into the housing 201 , and an outlet 203 through which cooled exhaust gases (not shown) for example may exit the housing 201. The exchanger 200 further comprises a plurality of heat transfer units 100 which are orientated in a substantially vertical plane, substantially parallel to each other, and substantially perpendicular to the passage of exhaust gases (not shown) through the housing 201. The features of the heat transfer units 100 of the exchanger 200 illustrated in figures 5 and 6 have been referenced using the same numerals as the heat transfer unit 100 illustrated in figures 3 and 4, however the separate units 100 have been distinguished using a respective label a-f. The reservoir 101 a-f and at least a portion of the heat transfer ducts 102a-f of each unit 100a-f are arranged to extend within the housing 201. The chamber 103a-f of each unit 100a-f however is disposed outside of the housing 201 and the flow duct 105a-f associated with each unit 100a-f is arranged to pass a cooling fluid, such as oil (not shown). In the embodiment illustrated in figure 5 and 6 of the drawings, the flow ducts of each unit 100 are connected in a series arrangement by a plurality of connecting ducts 204, which connect the outlet of one unit with the intet of an adjacent unit. In this respect, the cooling fluid (not shown) is arranged to pass in a serial manner through the flow duct 105 of each unit 100. Referring to figure 7 of the drawings, there is illustrated a heat exchanger 300 according to an alternative embodiment of the present invention. The exchanger 300 of the alternative embodiment is substantially the same as the exchanger 200 of the first embodiment and so like features have been referenced with the same numerals but increased by 100. The units 100a-f are arranged in two groups namely an upstream group 305 and a downstream group 306, whereby, the inlet 108d-f to the flow ducts 105d-f associated with the upstream group 305 are coupled to an inlet manifold 307. The outlet 109d-f of the flow ducts 105d-f of the upstream group 307 are coupled to a transfer manifold 308 which is arranged to transfer the flow of cooling fluid (not shown) to the inlet 108a-c of the flow ducts 105a-c of the downstream group 306. The outlet to an outlet manifold 309. This embodiment provides for an increased flow rate of cooling fluid (not shown) through the chambers 103a-f of the units 100a-f and therefore provides for an increased heat exchange between the hot exhaust gases and the cooling oil, for example.

In a further embodiment which is not illustrated, the inlet to each flow duct of each unit may be coupled to an inlet manifold and the outlet of each flow duct may be coupled to an outlet manifold, such that the flow ducts of each unit become arranged in a parallel arrangement. It is envisaged that this further embodiment would further increase the heat exchange between the hot gases and the cooling oil, for example.

During use of the above described heat exchangers 200, 300, the hot exhaust gases (not shown) for example, are arranged to pass into the exchanger housing 201, 301 via the inlet 202, 302. As the hot gases pass over the reservoirs 101a-f and the portion of the heat transfer ducts 102a-f disposed within the housing 201, 301, the heat transfer fluid 104a-f is arranged to change from a liquid state to a vapour state upon absorbing the latent heat of vaporisation. The vapour subsequently passes along the heat transfer ducts 102a-f to the respective chamber 103a-f whereupon the vapour condenses to form a condensate on the wall 106a-f of the respective flow duct 105a-f. As the vapour condenses, it gives up the heat associated therewith to the wall 106a-f and ultimately the fluid passing through the flow duct 105a-f, for example oil (not shown), which therefore becomes heated on passing through the chamber 103a-f. In this manner, the exhaust gases (not shown) become cooled by the heat exchanger 200, 300 whereas the oil (not shown) becomes heated.

The portion of the flow ducts 105a-f which extend within the chamber 103a-f of each unit 100a-f comprises an expansion sleeve 110 (as illustrated in figure 3 of the drawings) which may for example comprise a pleated portion of the side wall, or a bellow, which enables the duct to expand and contract within the chamber 103a-f as the flow duct 105a-f heats and cools and therefore serves to relieve any stresses from forming at the weld between the flow duct 105a-f and the respective chamber 103a-f which may otherwise cause the flow duct 105a-f or the chamber 103a-f to fracture and thus cause a leakage of the heat transfer fluid 104a-f or the oil (not shown), for example. The portion of the flow ducts 105a-f which extend within the chamber 103a-f of each unit 100a-f further comprises a plurality of baffles or vanes 111 which promote a turbulent flow of the oil (not shown) within the flow duct 105a-f and thus improve the transfer of heat between the duct wall 106a-f and the oil (not shown).

In each of the above described embodiments, the extent to which the heat transfer fluid extends along each transfer duct 102a-f is substantially the same in accordance with Pascal's Law. Accordingly, in the event that the heating of the unit 100a-f by the gases becomes localised upon the unit 100a-f, such that more fluid 104a-f in one duct 102a-f changes to the vapour state than in other transfer ducts 102a-f, then the fluid level will fall in each of the transfer ducts 102a-f equally. The provision of the reservoir 101 within each unit 100 thus ensures that each transfer duct 102 remains operable to transfer heat to the annular space 107 within chamber 103 of the unit 100. From the foregoing therefore, it is evident that the heat transfer unit provides for an improved exchange of heat between two media and further minimises any contamination of one medium with the other.

Claims

A heat transfer unit for transferring heat between a first medium and a second medium, the unit comprising a reservoir and a heat transfer fluid disposed within the reservoir which is arranged to exchange heat with the first medium, the unit further comprising a chamber and a plurality of fluid transfer ducts which extend between the reservoir and the chamber and which are arranged to communicate the heat transfer fluid between the reservoir and the chamber the unit further comprising a flow duct along which the second medium is arranged to pass, at least a portion of the flow duct being arranged in thermal communication with an interior of the chamber, such that during use, the second medium is arranged to exchange heat with the fluid.

A unit according to claim 1, wherein the reservoir and chamber are substantially elongate.

A unit according to claim 1 or 2, wherein the fluid transfer ducts, reservoir and chamber are disposed in substantially the same plane, such that the unit comprises a substantially planar unit.

A unit according to any preceding claim, wherein the flow duct extends along a longitudinal axis of the chamber, such that the interior of the chamber which is arranged in fluid communication with the heat transfer ducts, comprises an annular space.

A unit according to any preceding claim, wherein the flow duct comprises an inlet for receiving the second medium into the duct and an outlet through which the second medium may pass out from the duct.

A unit according to claim 5, wherein the fiow duct comprises a plurality of turbulence promoters disposed between the inlet and the outlet thereof for encouraging a turbulent flow of the second medium along the flow duct, to increase the transfer of heat across a wall of the flow duct.

A unit according to any preceding claim, wherein the flow duct comprises at least one expansion sleeve disposed within a wall thereof which permits the duct to expand and contract.

A unit according to any preceding claim, in which at least one of the reservoir and chamber comprise a cooler for cooling the heat transfer fluid. A unit according to any preceding claim, in which the reservoir and chamber comprise a cooler for cooling the heat transfer fluid.

A unit according to claim 8 or 9, in which the or each cooler comprises a cooling duct which extends within the reservoir and/or chamber and which is arranged to communicate a cooling fluid between an inlet and an outlet of the cooling duct. A unit according to claim 9 and 10 in which the outlet of one cooling duct is coupled to the inlet of the other cooling duct.

A heat exchanger for exchanging heat between a first medium and a second medium, the exchanger comprising a heat exchanging housing having an inlet for receiving the first medium into the housing at a first temperature and an outlet through which the first medium may pass at a second temperature,

the exchanger further comprising at least one heat transfer unit according to any of claims 1 to 7, the reservoir and at least a portion of the heat transfer ducts of the unit being arranged to extend within the housing, such that during use, the fluid within the reservoir can exchange heat with the first medium.

An exchanger according to claim 12, comprising a plurality of heat transfer units according to any of claims 1 to 11 orientated in a substantially parallel arrangement.

An exchanger according to claim 12 or 13, wherein the or each heat transfer unit is orientated substantially perpendicular to a passage of the first medium within the heat exchanger housing.

An exchanger according to any of claims 12 to 14, as appended to claim 5, wherein the inlet to the or at least two flow ducts is coupled to an inlet manifold and the outlet from the or at least two flow ducts is coupled to an outlet manifold. An exchanger according to any of claims 12 to 14 as appended to claim 5, wherein the outlet from the flow duct of one heat transfer unit is be coupled to the inlet to flow duct of a further heat transfer unit.