GB2479867A

GB2479867A - A Heat Pipe Heat Exchanger for Condensing a Vapour

Info

Publication number: GB2479867A
Application number: GB1006863A
Authority: GB
Inventors: Dumitru Fetcu
Original assignee: Individual
Current assignee: Individual
Priority date: 2010-04-26
Filing date: 2010-04-26
Publication date: 2011-11-02
Anticipated expiration: 2030-04-26
Also published as: EP2564144A2; GB2479867B; GB201006863D0; WO2011135334A3; US20130075064A1; WO2011135334A2

Abstract

A heat exchanger 10 for condensing a vapour to a condensate has a first heat exchanging chamber 11 and a second heat: exchanging chamber 12, and may be separated from each other by a separation plate 21. An array of heat pipes 25 are arranged to extend from within the first heat exchanging chamber to within the second heat exchanging chamber, and provide heat transfer between chambers. The first heat exchanging chamber has an inlet 15 for receiving a coolant into the chamber and an outlet 16 through which the coolant can exit the first chamber, and the coolant is arranged to pass over at least a portion of the heat pipes in the chamber. The second heat exchanging chamber has an inlet 19 for receiving a vapour into the chamber, and an outlet 23 through which the condensate can exit the second chamber after cooling from passing over the heat pipe array. The heat pipes include fins 28, 29, and these may be arranged in a helical path around the heat pipe/pipes.

Description

Heat Exchanger The present invention relates to a heat exchanger and particularly, but not exclusively to a heat exchanger comprising heat pipes.

A heat pipe is a hermetically sealed evacuated tube typically comprising a mesh or sintered powder wick and a working fluid in both the liquid and vapor phase. When one end of the tube is heated the liquid turns to vapor upon absorbing the latent heat of vaporization. The hot vapor 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 vaporization-condensation cycle repeats. Since the latent heat of vaporization is usually very large, considerable quantities of heat can be transported along the tube and a substantially uniform temperature distribution can be achieved along the heat pipe.

Traditional methods of providing a condensate from a vapour utilise the so-called "shell and tube" condenser, which comprise a pressure vessel, namely the "shell", and a plurality of tubes which extend therethrough and which are arranged to transfer a liquid coolant between opposite ends of the vessel. There are a variety of shell and tube type condensers, such as the U-tube heat exchanger and the straight-tube heat exchange, but they all operate on the same principle which is illustrated schematically in figure 1 of the drawings with reference to a U-tube heat exchanger 1. Referring to figure 1, the coolant is caused to flow within and along the tubes 2 within the shell 3 between an inlet 4 and an outlet 5, and the vapour to be condensed is passed into the shell 3 via an inlet 6. The vapour subsequently becomes cooled as it passed over the tubes 2, and ultimately condensed to a liquid which is passed out from the shell 3 via an outlet 7.

A problem with this arrangement however, is that even though the heat transfer area on the inside of the tube, namely the liquid side, is substantially the same as the heat transfer area on the outside of the tube, namely the vapour side, the heat transfer coefficient on the liquid side is much less that on the vapour side. As a result, it is necessary to include many tubes to provide the required effective heat transfer area on the liquid side, which therefore increases the complexity and weight of the condenser.

In addition, the heat transfer coefficient on the cooling side is largely dependent on the velocity of the liquid through the tube. It is found that increasing the liquid velocity causes the heat transfer coefficient to increase. As a result, it is common to utilise high liquid velocities to increase the heat transfer coefficient on the liquid side. However, this requires large pumps and an increase in liquid pressure within the heat exchanger.

The heat transfer across the tube will also be influenced by the material of the tube and the thickness of the tube wall. Accordingly, brass and copper are typically utilised since these materials provide a good thermal conductivity, and the tube wall is kept as thin as possible to ensure a suitable transfer of heat across the tube wall. Brass and copper however, are expensive materials and owing to the reduced thickness of the tube wall, it is found such tubes are sensitive to thermal stresses, particularly during the start-up and shut-off periods when there is a significant change in temperature.

We have now devised an improved heat exchanger comprising a plurality of heat pipes, which alleviates the above-mentioned problems.

In accordance with the present invention as seen from a first aspect, there is provided a heat exchanger for condensing a vapour to a condensate, the exchanger comprising a first heat exchanging chamber, a second heat exchanging chamber and an array of heat pipes which are arranged to extend from within the first heat exchanging chamber to within the second heat exchanging chamber; the first heat exchanging chamber comprising an inlet for receiving a coolant into the chamber and an outlet through which the coolant can exit the first chamber, the coolant being arranged to pass over the portion of the heat pipes which extend within the first chamber; the second heat exchanging chamber comprising an inlet for receiving the vapour into the chamber and an outlet through which the condensate can exit the second chamber, the vapour being arranged to pass over the portion of the heat pipes which extend within the second chamber; wherein the portion of at least one of the heat pipes within the first heat exchanging chamber comprises a fin arranged in contact therewith which is arranged to increase thermal transfer between said portion of the heat pipe and the coolant.

The provision of at least one fin increases thermal transfer between said portion of the heat pipe and the coolant to compensate for the reduced heat transfer coefficient of the coolant within the first heat exchanging chamber compared with the heat transfer coefficient of the vapour within the second heat exchanging chamber.

The portion of each of the heat pipes within the first heat exchanging chambers comprises a fin. Preferably, the or each fin is arranged to extend in a substantially helical path around the portion of the at least one or each heat pipe within the second heat exchanging chamber.

The portion of at least one of the heat pipes within the second heat exchanging portion comprises a fin arranged in contact therewith, which is arranged to increase thermal transfer between said portion of heat pipe and the coolant. Alternatively, each of the heat pipes within the second heat exchanging portion comprises a fin arranged in contact therewith which is arranged to increase thermal transfer between said portion of the heat pipe and the coolant. Preferably, the or each fin is arranged to extend in a substantially helical path around the portion of the at least one or each heat pipe within the second heat exchanging chamber.

In the conventional shell and tube condenser, the condensate film which develops upon the tubes acts as a thermal barrier which prevents further vapour from becoming suitably cooled. This film typically increases in thickness until the force of gravity overcomes the surface tension of the condensate upon the tube and thus causes the condensate to move off the tube. The provision of the helical fins on the portion of the heat pipes in the second heat exchanging chamber provides for an additional surface for the film to develop and thus serves to reduce the overall thickness of the film upon the surface of the heat pipe. In addition, the helical fin serves to channel the condensate off the heat pipe.

Preferably, the number of turns of the or each fin per unit length around the or each heat pipe in the first heat exchanging chamber is greater than number of turns of the or each fin per unit length around the or each heat pipe in the second heat exchanging chamber.

The first and second heat exchanging chambers are preferably separated by a separation plate. The plurality of heat pipes are preferably supported within the heat exchanger by the support plate which is coupled to the heat pipes intermediate opposite ends thereof. Accordingly, the separation plate obviates the requirement to support the heat pipes at their free end and as such the free ends of the heat pipes can be left uncoupled and spaced from respective first and second chamber walls. This therefore enables the heat pipes to expand and contract along their length during use thereby minimizing thermal stresses upon the heat pipes.

The first and second heat exchanging chambers preferably further comprise at least one baffle for directing the flow of liquid and vapour, respectively, over the respective portions of the heat pipes.

Preferably, the inlet and outlet of the first heat exchanging chamber are arranged to pass the coolant into and out of the first chamber in a direction which is substantially transverse to a longitudinal axis of the heat pipes.

Preferably, the inlet and the outlet of the second heat exchanger are arranged to pass the coolant into and out of the second chamber in a direction which is substantially parallel to a longitudinal axis of the heat pipes.

An embodiment 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 through a conventional U-tube shell and tube type condenser; Figure 2 is a longitudinal sectional view of the heat exchanger according to an embodiment of the present invention; Figure 3 is a transverse sectional view of the heat exchanger of figure 2, at position A-A; and, Figure 4 is a transverse sectional view of the heat exchanger of figure 2, at position B-B.

Referring to figure 2 of the drawings, there is illustrated a longitudinal sectional view through a heat exchanger 10 according to an embodiment of the present invention. The heat exchanger 10 comprises a first heat exchanging chamber 11 and a second heat exchanging chamber 12. Each chamber 11, 12 comprises a substantially cylindrical housing 13, 14 having substantially the same diameter, which are mounted one on top of the other such that a longitudinal axis of the first chamber 11 extends in a substantially collinear relationship with a longitudinal axis of the second chamber 12 and thus the heat exchanger 10.

The first chamber 11 of the heat exchanger 10 is disposed above the second chamber 12 and comprises an inlet 15 and an outlet 16 which are disposed within an arcuate side wall of the housing 13. The inlet and outlet 15, 16 of the first chamber 11 are spaced longitudinally of the chamber 11 and are arranged to enable a coolant such as water, to pass into and out from the chamber 11, respectively. The first chamber 11 further comprises a passage 17 which extends along the first chamber 11 substantially along the longitudinal axis thereof. The passage 17 is defined by a substantially cylindrical wall 18 which seals the interior of the first chamber 11 from the passage 17, and extends from an opening 19 disposed in an upper end wall 20 of the first chamber 11 to an upper region of a separation plate 21.

The separation plate 21 comprises a first aperture 22 disposed substantially at the centre thereof which is arranged to align with the cylindrical wall 18 defining the passage 17, such that the wall 18 extends substantially around a periphery of the first aperture 22. The second chamber 12 is secured to the underside of the separation plate 21 and thus the first chamber 11, and comprises an outlet 23 disposed substantially upon the longitudinal axis of the chamber 12, within a lower end wall 24 thereof. The first aperture 22 disposed within the separation plate 21 serves as an inlet to the second chamber 12, such that the vapour to be condensed, for example steam, can pass into the second chamber 12 from the opening 19 disposed in the upper end wall 20 of the first chamber 11 through the passage 17 and into the second chamber 12.

The heat exchanger 10 further comprises a plurality of substantially linear heat pipes 25 which extend from within the first chamber 11, through an array of second apertures 26 disposed within the plate 21 around the first aperture 22, and terminate in the second chamber l2so as to enable heat to be transferred between the chambers 11, 12. The heat pipes 25 extend substantially parallel to the longitudinal axis of the first and second chambers 11, 12 and are configured in a substantially arcuate arrangement of rows of heat pipes 25, as illustrated in figures 3 and 4 of the drawings, the radius of curvature of each arcuate row being centered substantially on the longitudinal axis. In this manner each chamber 11, 12 comprises a plurality of arcuate rows of heat pipes 25, having different radii of curvature.

The heat pipes 25 are supported within the heat exchanger 10 by the separation plate 21 by a series of collars 27 which separately extend within each of the second apertures 26 and which further serve to seal the heat pipes 25 to the separation plate 21 such that the interior of the first and second chambers 11, 12 remain isolated from each other. The longitudinal ends of the heat pipes 25 are uncoupled and spaced from the upper end wall 20 of the first chamber 11 and the lower end wall 24 of the second chamber 12, such that the heat pipes 25 are free to expand and contract and thus relieve any thermal stresses which would otherwise develop during use of the heat exchanger 10.

The portion of each of the heat pipes 25 which extend in the first chamber 11 comprise a helical fin 28 disposed around the outer surface thereof which extend substantially along the length of the portion of the respective heat pipe 25 within the first chamber 11. The fins 28 comprise a metallic strip which extends away from the outer surface of the respective heat pipe 25, in direction which is substantially perpendicular to the longitudinal axis of the respective heat pipe 25. The fins 28 are found to increase the transfer of heat between said portion of the heat pipes 25 and the coolant to compensate for the reduced heat transfer coefficient of the coolant within the first heat exchanging chamber 11, compared with the heat transfer coefficient of the vapour within the second heat exchanging chamber 12.

The portion of each of the heat pipes 25 which extend in the second chamber 12 similarly comprise a helical fin 29 disposed around the outer surface thereof which extend substantially along the length of the portion of the respective heat pipe 25 within the second chamber 12. The fins 29 similarly comprises a metallic strip which extend away from the outer surface of the respective heat pipe 25, in direction which is substantially perpendicular to the longitudinal axis of the respective heat pipe 25. The number of turns of the fins 28 per unit length around the heat respective pipe 25 in the first heat exchanging chamber 11 is arranged to be greater than in the second heat exchanging chamber 12 to ensure that the increase in heat transfer coefficient associated with the coolant in the first chamber 11 due to the presence of the fins 28, is not offset by the provision of fins 29 on the portion of the heat pipes 25 in the second chamber 12.

The first chamber 11 of the heat exchanger 10 further comprises a plurality of baffles 30 which extend across the first chamber 11 substantially transverse to the longitudinal axis of the heat exchanger 11, and serve to direct the flow of coolant back and forth across the portion of the heat pipes 25 within the first chamber 11, as the coolant flows along the chamber 11 between the inlet 15 and the outlet 16. The second chamber 12 similarly comprises a plurality baffles 31 which extend across the second chamber 12, substantially transverse to the longitudinal axis of the heat exchanger 10, and serve to direct the flow of vapour back and forth across the portion of the heat pipes 25 within the second chamber 25, to ensure that the vapour becomes cooled and thus condenses within the second chamber 12.

In use, hot gas from an industrial process (not shown) for example, is passed into the passage 17 of the first chamber 11 through the opening 19 disposed in an end wall 20 of the first chamber 11. The gas subsequently passes into the second chamber 12 where it becomes redirected radially outwardly of chamber 12, back and forth across the heat pipes 25 by the baffles 31. As the hot gas passes across the heat pipes 25, it gives up the heat associated therewith to the heat pipes 25 thereby causing the temperature of the heat pipes 25 to rise. The hot gas thus becomes cooled in moving along the second chamber 12 between the inlet 22 and the outlet 23.

The increase in temperature of the portion of the heat pipes 25 within the second chamber 12 becomes transferred to the portion of the heat pipes 25 within the first chamber 11, which subsequently becomes cooled by the flow coolant therein. The intimate contact and increased surface area of the portion of the heat pipes 25 in the first chamber 11, due to the presence of the fins 28, provides an efficient removal of heat from the heat pipe 25, such that the portion of the heat pipes 25 in the second chamber 12 can further absorb the heat from the gas and thus cool the gas. This increased surface area provides for an increased effective thermal coefficient and thus a reduced length of the portion of the heat pipe 25 in the first chamber 11 compared to the conventional tube of the shell and tube condenser.

As the gas becomes cooled along the second chamber 12, it condenses upon the fins 29 associated with the portion of the heat pipes 25 in the second chamber 12, and a film of liquid (not shown) develops thereon. The fins 29 provide an alternative, or at least an additional surface to the surface of the respective heat pipe 25, for the film (not shown) to develop and therefore the ensures that the thickness of the film (not shown) does not exceed a threshold whereby it would insulate the respective heat pipe 25 from further hot gases and thus reduce the ability of the heat pipes 25 to suitably condense the hot gas. Furthermore, the helical nature of the fins 29 upon the portion of the heat pipes 25 within second chamber 12 provides a run-off slope for the condensate along the respective heat pipes 25 and out of the chamber 12 through the outlet 23.

From the foregoing therefore, it is evident that the heat exchanger provides for a more efficient condensation of a vapour.