US3250324A - Heat exchanger having extended heat transfer surfaces - Google Patents

Description

y 1966 J. G. c. HICKS 3,250,324

HEAT EXCHANGER HAVING EXTENDED HEAT TRANSFER SURFACES Filed June '1. 1964 United States Patent '0 3,250,324 HEAT EXCHANGE-R HAVING EXTENDED HEAT TRANSFER SURFACES John Gordon Clayton Hicks, Rugby, England, assignor to The English Electric Company Limited, London, England, a British company Filed June 1, 1964, Ser. No. 371,459 Claims priority, application Great Britain, June 11, 1963, 23,165/63 1 Claim. (Cl. 165-451) This invention relates to heat exchangers, for transferring heat between a flowing gaseous first fluid and a second fluid.

According to the invention, such a heat exchanger includes at least one duct for the second fluid and at least one extended heat-transfer surface arranged on the outside of said duct or ducts in direct heat-transfer relationship therewith and in the flow path of said first fluid, the extended heat-transfer surface or surfaces having a plurality of discontinuous surfaces substantially in line with each other across said flow path, and a plurality of continuous portions of extended heat-transfersurface being arranged between some of said discontinuous surfaces so as to interrupt the succession of the discontinuities in said surfaces.

According to a preferred feature of the invention, where the extended heat-transfer surface comprises a number of first plate members generally parallel to each other, said discontinuities comprising perforations in the first plate members, said continuous portions comprise unperforated second plate members, each of which is arranged between two of said first plate members and parallel thereto.

Preferably, the said continuous portions of extended heat-transfer surface are arranged at intervals less than half of the shortest wave-length of an audible wave in said first fluid.

One form of heat exchanger according to the invention will now be described by way of example and with reference to the accompanying drawing, which is a sectional elevation of a portion of an element of the heat exchanger.

The heat exchanger has a number of heat-exchange elements, each comprising a number of parallel liquidcarrying tubes 10 and a number of substantially fiat metal plates or fins 11 arranged at right angles to the axes of the tubes 10. Spacers 12 are provided around the tubes 10 between the plates 11. Only a few of the plates 11 are shown in the drawing.

The majority of the plates 11 have a depressed portion 11A formed by forcing a portion of the metal down- Wardly (as seen in the drawing), parallel to the plate, and so creating a hole 11B in the plate in order to improve its heat transfer characteristics. The depressed portion 11A of each plate is partly separated from the plate so that there is communication between the gas above and below the plate (as seen in the drawing) around the separated sides of the portion 11A and through the hole 11B. The remainder of the plates 11, indicated at 11C, are plain plates with no depressed portion, that is to say each plate 11C lies in a single plane. The plates 11 are arranged in the heat-exchange element so that the holes 11B are substantially in line one above the other.

In operation, hot liquid is passed through the tubes 10 as indicated by the arrow A and a relatively cool gas (for example air) is passed through the element, parallel with the plates 11, horizontally as seen in the drawing and at right angles to the plane of the drawing. Heat transfer occurs from the liquid to the gas through the walls of the tubes 10, spacers 12 and plates 11. The depressed portions 11A, in those plates which have them,

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cause shedding of any accumulated boundary layer in the gas, and serve to increase the turbulence of the gas so as to improve the rate of heat transfer.

It has been found that in the absence of the plain plates 11C, standing waves parallel to the tubes 10 are set up in the gas due to gas vibrations, when the gas fiow velocity exceeds a certain value. The presence of the depressed portions 11A in the gas stream gives rise to vortex-shedding in the wake of each depressed portion. The presence of a vortex street so formed may give rise to the standing waves mentioned above as well as causing vibration of the plates. Such standing waves depend for their existence on the communication beforementioned between the gas space on one side of a plate and that on the other, and their frequency and therefore their wave-length depends on the gas flow velocity.

The maximum possible wave-length of the standing waves is limited to approximately twice the distance between the plane plates 11C. The plain plates are so spaced that this distance is less than half of the shortest wave-length of an audible standing wave that would occur at a predetermined maximum gas flow velocity in the absence of the plain plates 11C. In other words, due to the presence of the plain plates, a standing wave of sufficient wave-length to be audible cannot form when the gas flow is no greater than this predetermined maximum value.

In heat exchangers where radial or helical fins, attached to the tubes and having holes to increase gas turbulence, are used instead of plates for heat transfer to or from a gas, or where there are fins or plates having holes and depressed portions of different shapes from those shown in the drawing, plain fins or plates, or plain portions thereof, may be provided at suitable intervals to prevent vibration, in the same way as are the plates 11C.

Heat exchangers according to the invention may normally be used in any position and are not confined to the position, or directions of liquid and gas flow, shown in the drawing.

The spacing of the plain plates 11C may be so arrangedthat not all vibration of the plates is prevented but only that at audible frequencies.

The fluid flowing past the plates can be any gaseous fluid, whether dry or not, and it may be arranged to flow in any desired direction with respect to the extended heat transfer surface in contact with it (such as the plates 11). For example, if for each plate 11 there is substituted a number of parallel strips, in one plane and separated by gaps, the gas flow may be in a general direction inclined to the plane of the strips, and it may then flow both across the strips through the gaps and along the strips.

In such a case, whatever the direction of gas flow, the strips in successive parallel planes may be staggered in such a way that the longest possible straight gas flow path intersecting the planes is less than half the shortest wave-length of an audible standing wave that would occur at a predetermined maximum gas flow velocity in the absence of such staggering. Thus any tendency for vortices shed from the edges of the strips to set up such a standing wave is avoided, in the same way as, in the example described above, by the plain plates 11C.

It should be noted that it is not always necessary in such a case that the plain plates should act as complete bafiies: in other words the vibrations which it is required to prevent can in some cases be avoided by providing partial baflies. In a configuration such as that described in the previous paragraphs, this might be achieved, for example, by providing, instead of plain plates 11C, strips extending between the discontinuous regions of the fins or plates, leaving gaps between the strips.

What I claim as my invention and desire to secure by Letters Patent is:

A heat exchanger for transferring heat between a first flowing gaseous fluid and a second fluid, comprising a plurality of parallel tubes for the second fluid and -a plurality of parallel flat plates spaced apart and arranged transversely on the outside of said tubes in direct heat exchange therewith, and adapted to receive said first fluid flowing laterally in a flow path across the surfaces of said plates, said plates including a plurality of .plain plates, each said plain plate being separated from the next plain plate in a direction parallel to the tubes by a plurality of further said plates and each said further plate having an elongated bent-out portion extending in a direction parallel to the-plate, joined thereto at each end of said bent-out portion and extending in a direction transverse to said flow path, the spacing between each plain plate and the said next plain plate being less than half the shortest wavelength of an audible wave in said first fluid so as to prevent the formation of standing au- 5 dible waves in said first fluid in a direction parallel-to the 3,135,320 6/1964 Forgo 165151 ROBERT A. OLEARY, Primary Examiner.

5 CHARLES SUKALO, Examiner.

N. R. WILSON, Assistant Examiner.

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