Stock material or miscellaneous articles – Structurally defined web or sheet – Nonplanar uniform thickness material
Patent
1994-02-08
1995-12-12
Loney, Donald J.
Stock material or miscellaneous articles
Structurally defined web or sheet
Nonplanar uniform thickness material
428167, 428183, 428184, 428192, 2611122, B32B 328
Patent
active
054748321
DESCRIPTION:
BRIEF SUMMARY
BACKGROUND OF THE INVENTION
This invention relates to packing elements for heat exchange and mass transfer between liquid and gas phases of fluid, for use in, for example, cooling towers. In particular this invention relates to regular or ordered packing elements formed from formable sheet material and having corrugations.
A common object in the design of any cooling tower pack is to provide a pack with efficient heat exchange and mass transfer characteristics. In fulfilling this object it is important to maximise the surface area to volume ratio of liquid on the pack and hence to effect an even distribution of fluid over the pack. Uniform wetting is easily achieved in self-wetting woven wire fabric elements where capillary forces act between the fibres. One drawback of such elements is that they are expensive to produce. In contrast, elements made from formable sheet material such as Polyvinylchloride (PVC), Polythene etc. are much cheaper to produce. However, designing a packing element from sheet material which achieves a uniform distribution of liquid over the surface of the sheet and thus efficient mass transfer from bulk liquid to film, is not a trivial matter, especially since capillary forces do not act between the liquid and sheet element as in the fabric elements.
Forming the sheets with folds or corrugations represents the most common method of increasing the surface area. Liquid flowing over the sheets is spread laterally either by inclining the corrugations at an angle to the vertical or by forming the sheets with secondary perturbations, or by incorporating in the sheet a combination of both.
Regular packing elements consisting of corrugated sheets fixed together to form an array of parallel vertical passageways, such as in GB-A-2,093,967 and EP-A-28545, have a number of advantages over other known packing arrangements.
In a conventional cooling tower, liquid is sprayed onto the pack from an array of horizontal plan sprays located above the pack. Packs whose passageways are disposed at an angle to the vertical have shadowed areas at the entrance of the pack which cannot be wetted effectively, and in the case of vertically falling liquid cannot be wetted at all. Thus, an advantage of packs having vertically standing passageways is that every unit area of the entry faces has an equal probability of being wetted.
In packs with slanted corrugations the liquid tends to concentrate into channels defined by the corrugations, resulting in a non uniform distribution of liquid over the pack. Furthermore, liquid flowing over the corrugations is for part of the time suspended from above, increasing the probability of the liquid becoming dissociated from the surface of the sheet. This loss of liquid results in a loss of thermal performance. In contrast, packing elements having corrugations vertically aligned do not suffer from either of these intrinsic problems.
In natural-draft cooling towers operating in counter-flow mode the air flow is governed by the relation between the buoyancy forces and the pack impedance. A pack with vertical passageways will present less impedance to the air flow than packs which are inclined to the vertical, because the passageways run parallel to the air flow direction. By the same argument, packs with horizontally disposed passageways present less impedance to the air flow in cross-flow cooling towers.
A still lower impedance is achieved for packs in which both the cross-sectional geometry of the corrugations forming the passageways and their cross-sectional area remain constant over the length of the pack. The impedance is further reduced if the passageways formed by the corrugations are linear.
A lower impedance results in a lower pressure drop along the length of the packing element for a given air flow and, therefore, in a natural draught cooling tower, a larger air flow for a given buoyancy induced driving force. This results in a more efficient heat exchange and mass transfer between the liquid and gas phases across the boundary layer for several reasons. Air flowing
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patent: 4657711 (1987-04-01), Wigley
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Loney Donald J.
National Power PLC
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