EP0311794A1

EP0311794A1 - Packing grid for water cooling towers

Info

Publication number: EP0311794A1
Application number: EP88114856A
Authority: EP
Inventors: Antonio Mosiewicz
Original assignee: SPIG INTERNATIONAL per Impianti Generali SpA Soc
Current assignee: SPIG INTERNATIONAL per Impianti Generali SpA Soc
Priority date: 1987-10-13
Filing date: 1988-09-12
Publication date: 1989-04-19
Anticipated expiration: 2008-09-12
Also published as: IT8722239A0; DE3870795D1; IT1222883B; ES2031199T3; ATE75846T1; EP0311794B1

Abstract

A packing grid for a water cooling tower, having a reticular structure in which the modular element is of hexagonal configuration, the reticular formation being of honeycomb structure and being provided with reinforcement strips.

Description

This invention relates to apparatus for obtaining contact between a gas and a liquid, consisting of a vertically disposed chamber through which the liquid descends by gravity from the top to the bottom to split into minute droplets by its impact against the packing material, whereas the gas flows through the chamber from the bottom upwards to thus countercurrently encounter the descending liquid subdivided into minute droplets. The invention zalso relates to apparatus for obtaining contact between a gas and a liquid in which while the liquid descends by gravity to split into droplets, the gas flows through the containing chamber horizontally (cross flow).
In particular the invention relates to packing grids for water cooling towers in which the cooling gas is air in open circuit, the air flowing through the tower either by natural draught or by forced draught obtained by a suction fan or blower.
Various types of packing grids are known, constructed of material resistant to the prevailing conditions. In particular, thermoplastic synthetic polymers such as polypropylene, polyethylene, polystyrene and polyester and vinyl resins are suitable. The characteristics of these materials are excellent for application in this field because of their inalterability, lightness combined with a certain mechanical strength, and simplicity and economy in manufacturing the grid, particularly by injection moulding.
Grids of this type have a structure consisting of rectangular or square cells of various dimensions, and may be provided with reinforcement strips (ribs) if the slats which delimit the individual cells are very thin and of unsuitable shape to give the grid the necessary mechanical strength and dimensional rigidity.
That amount of the horizontal cross-section which is occupied by the slats defining the individual cells plus the reinforcement ribs must be kept within optimised limits as a fraction of the empty air flow passage section.
On the one hand the pressure drop in the ascending air flow must be kept within very low values whereas on the other hand the total solid cross-section must be sufficient to provide an adequate impact surface for the water in free fall from above. For this reason the grids disposed one above another along the vertical axis are staggered relative to each other and are suitably spaced apart vertically.
It has now been surprisingly found that a water cooling tower packing grid of honeycomb configuration, ie consisting of hexagonal cells, provides excellent cooling tower operating conditions in terms of heat transfer efficiency and air flow pressure drop.
The grid according to the invention can be constructed of a thermoplastic material suitable for injection moulding. Particularly suitable materials are polypropylene, polyethylene and various vinyl polymers. The grid has a square outer periphery of standard side dimensions so that it can also be installed in existing cooling tower.
The grid according to the invention, shown schematically in Figure 1, is installed horizontally. Figure 2 is a detailed partial view of the honeycomb structure with its reinforcement ribs.
The horizontal section through the strips forming the contours of the hexagonal cells plus the horizontal section through the reinforcement ribs represents the solid surface, ie the impact surface against which the the water droplets collide during their free gravity fall from above. The optimum area of this solid surface is between 20% and 30%, and preferably of the order of 25%, of the total grid area.
Taking account of the dimensional limits imposed by injection moulding, the optimum side lengths of the hexagonal cells are of the order of 3-4 cm. This means that there are no individual flat sections (strips) or reinforcement ribs of considerable size which could result in troublesome liquid accumulation. In this respect, the optimum condition is for the impact surfaces to be as free as possible of stagnant liquid which would reduce the splashing effect during the droplet impact against them. The width of the strips and of the reinforcement ribs therefore does not exceed 6 mm and is preferably between 3 and 5 mm.
The configuration and position of the hexagonal cells in the grid is not symmetrical about the vertical axis passing through its centre. In this manner by mounting the grids one on another in the tower with each grid rotated through 90° about the preceding grid, the solid elements (strips, ribs) are given the necessary offsetting with respect to the preceding grid.
An important structural parameter is the distance in the direction of the tower vertical axis between one grid and the next. This distance is between 10 and 40 cm and is preferably of the order of 20 cm for the type of grid according to the invention.
The aforesaid optimum dimensions result in maximum heat transfer consequent on high liquid dispersion in the form of very small droplets, while at the same time providing very low air flow pressure drops.
In practice the pressure drop can be kept equal to or lower than the pressure drop obtained with rectangular or square cells of much larger dimensions but of consequent less heat transfer efficiency.
This better performance of the grid according to the invention is explained by the fact that in the honeycomb structure each hexagon vertex is a point of convergence of only three segments with vertex angles (or segment convergence angles) of 120°, whereas in the case of known grids with square or rectangular cells four segments converge with a convergence angle of 90°.
The reinforcement ribs of the grid according to the invention are disposed generally in the form of a rectangular lattice. They necessarily create conditions of reduced efficiency because they form convergence points for four segments and convergence angles of 120°, 90° and 60° as shown in Figure 1.
However the reinforcement ribs required for the hexagonal honeycomb structure according to the invention are generally sufficiently spaced apart from each other to not compromise the excellent characteristics of the structure.
By way of example, the diagrams of Figures 3 and 4 show operating data obtained for a water cooling tower packed with packing grids according to the invention, together with comparison data for the same tower under the same operating conditions but packed with grids of known type.
The grid according to the invention has the structure shown in Figures 1 and 2 with a hexagon side of 35 mm, a distance 1 of 155 mm, a distance 1′ of 145 mm, a strip thickness s of 4 mm, a reinforcement rib thickness s of 4 mm, and a grid size excluding the support edge of 612 x 612 mm. The grid of known type is shown diagrammatically in Figure 5, and comprises rectangular cells of inner dimensions 190 x 60 mm and a strip width of 12 mm. In the diagram of Figure 3 the vertical axis represents the heat transfer efficiency expressed in kaV/L and the horizontal axis represents the air flow velocity. The line B refers to the grid of the invention and the line A refers to the grid of known type. The better efficiency of the grid according to the invention appears at all air flow velocities.
In Figure 4, in which the horizontal axis represents the air flow velocity in m/sec and the vertical axis represents the water gauge pressure drop for a tower 3 m high, the pressure drops obtained during the tests are shown by the line B for the grid of the invention and by the line A for the grid of known type.
As the air flow velocity in industrial plants under normal operating conditions is between 2 and 2.8 m/sec it can be seen that the grid according to the invention also behaves as well as or better than the grid of known type from this aspect.

Claims

1. A horizontally disposed packing grid for a water cooling tower of the countercurrent flow type with an ascending air flow or of the cross draught type with orthogonol flows, characterised by having a square periphery and a reticular honeycomb structure, the strips forming the contour of the hexagonal cells and any reinforcement ribs having a width in the horizontal plane not exceeding 6 mm and such that their cross-sectional area in said plane represents between 20% and 30% of the total cross-sectional area of the grid.

2. A packing grid as claimed in claim 1, characterised in that the hexagonal cells have a side of between 30 and 40 mm.

3. A grid as claimed in claim 1, wherein the honeycomb arrangement is asymmetrical about the central vertical axis of the grid.

4. A water cooling tower provided with the packing grid claimed in one of the preceding claims, wherein the grids are disposed one above another spaced apart by 100-400 mm, each being positioned rotated through 90° relative to the preceding grid.