US3400917A

US3400917A - Cooling towers

Info

Publication number: US3400917A
Application number: US528741A
Authority: US
Inventors: Richards Derek Jess Wilton
Original assignee: Central Electricity Generating Board
Current assignee: Central Electricity Generating Board
Priority date: 1965-02-23
Filing date: 1966-02-21
Publication date: 1968-09-10
Anticipated expiration: 1985-09-10
Also published as: GB1141831A

Description

Sept. 1963 D. J. w. RICHARDS COOLING TOWERS 3 Sheets-Sheet 1 Filed Feb. 21. 1966 Sept. 10, 1968 J. w. RICHARDS COOLING TOWERS 5 Sheets-Sheet 2 Filed Feb. 21. 1966 P 10, 1958 D. J. w. RICHARDS 3,400,917

COOLING TOWERS 3 Sheets-Sheet 5 Filed Feb. 21. 1966 United States Patent() 14Claim s. (Cl. 261-29) ABSTRACT OF THE DISCLOSURE A cooling tower comprises a hollow structure of circular section in horizontal planes and externally of uniform convex aerofoil form in all diametral planes. Conveniently it is semi-ellipsoidal, the major axis of' the ellipsoid coinciding with or being slightly above the ground plane. The structure has apertures forming an air inlet around the base. At thetop there is an .aperture forming an air outlet. The aerofoil section is chosen so that the flow separation point due to a horizontal wind occurs downstream of the outlet aperture. Typically the base diameter is between 3 and 5 times the height of the tower and the air outlet aperture is between 0.4 and 0.6 of the diameter of the base of the tower.

This, invention relates to cooling towers.

It isythe present practice to make such towers of hyperbolic form, that is to say, they are hollow structures of hyperbolic cross-section in any diametral plane and circular in any horizontal plane. The water is cooled by the updraught induced through the tower, the air entering the tower around the base thereof and leaving through the open top. Most commonly such towers are iwet cooling towers in which the water tobe cooled comes in contact with the air. For some power stations, however, dry cooling towers may be preferred in which the air is drawn over radiators through which the water to be cooled is circulated. This avoids the need for the make-up water required in wet cooling towers; great quantities of make-up water are required for large power stations and typically half a million gallons per hour might berequired for a 2000 mw. station. t

Some hyperbolic cooling towers however suffer from certain disadvantages in that the aerodynamic behaviour is adversely affected by external winds. It has been found that in a dry, cooling tower with theradiators arranged around the periphery at the base, in windy conditions, the air-flow through the radiator, elements situated in the windward and leeward sectors of the tower is only slightly increased whereas the air-flow through the elements situl ated in the remaining side sectors is substantially reduced. This effect is associated with the circumferential disposition of the radiator elements, and is only partially alleviated by the suction induced by air-.flow across the top of the tower. Also, under certain conditions, it has been noted that the external wind did notcompletely pass over the tower. This effect may be due to the fact that the tower more particularly investigated had a smaller heightto-diameter ratio than has been the usual practice with conventional smaller'wet cooling towers; obviously however, a further increase of diameter to increase the cooling capacity is undesirable.

The latter disadvantage applies both to wet and dry cooling towers. The present invention is applicable to both types of cooling tower.

According to this invention, a cooling tower is of circular section in horizontal planes and externally is of uniform convex aerofoil form in all diametral planes, the air inlet being around the base of the tower and the 3,400,917 Patented Sept. 10, 1968 ice air outlet being through an aperture at the top, the aerofoil section being such that the flow separation point due to a horizontal wind occurs downstream of the outlet aperture.

Preferably the tower is of ellipsoidal form, the tower constituting externally a semi-ellipsoid with the circular diametral planes through the major axes of the ellipsoid coinciding with or above the ground plane.

The above described form of construction is applicable to both wet and dry cooling towers.

In a wet cooling tower, to obtain the maximum efficiency, it is necessary to ensure that the inflowing air can reach all the packing, including the innermost without becoming completely saturated and preferably therefore means are provided for directing the air from two or more (typically three) different inlet levels to separate concentric regions in the tower. Separate water distribution systems and packing may be provided for said concentric regions. These distribution systems and packing may be arranged at the same or different levels within the tower or may be arranged at different levels outside the tower around the base thereof. Such a wet cooling tower may operate with a natural draught or with a forced draught or means may be provided for assisting the draught.

In a dry cooling tower, for reasons which are discussed below, the radiators are preferably arranged in a conical surface inside the tower, the cone having an upright axis coincident with the central axis of the tower, the base of the cone lying above, preferably just above, the level of the air inlet around the periphery of the tower, and the conical surface sloping downwardly towards the center of the tower. As explained below, this reduces the possibilities of a vortex at the base of the tower. In a preferred errangement, the apex of the cone is at the base of the tower, but the surface may in practice he a frustum of a cone. The conical surface may be fluted to reduce the velocity of airflow through the radiators for a given mass how of air through the tower.

To explain why a conical surface for the radiators is preferred, it is necessary to consider other possible arrangements. The radiators may be arranged in what is known as a delta manner around the base of the tower. In this construction, considering a. short portion of the periphery, instead of putting a radiator directly across the length of air inlet opening, two radiators are arranged jutting out from the ends of this portion of the inlet so that, in plan, these two radiators form two sides of a triangle having its base as a chord across a short arcuate portion of the periphery. By arranging the radiators around the base in this manner, for a given velocity through the radiators, the mass how of air through the tower is increased compared with a simple arrangement in which the radiators lie on the plane of the periphery of the tower. The delta system is thus the preferred arrangement if the radiators are around the base of the tower.

In still air conditions the air-flow Q through radiator elements of a tower having radiators arranged in a 60 delta manner around the circumference of the tower is:

Q =21rDHv (1) where D=the base diameter of tower H =the height of the air inlet v ,'=the air velocity normal to the radiator elements in still air conditions.

As has previously been explained, with the radiators arranged around the base of the tower, the performance is affected by external winds. We have found in model experiments that external winds do not greatly influence the velocity distribution of the airflow normal to radiator elements which are arranged in a horizontal plane across the tower above the air inlet.

However, if the radiator elements were situated in a horizontal plane across the tower at the top of the inlet, and if v were assumed to be constant, then the airflow Q through the radiators would be:

It follows from Equations 1 and 2 that a horizontal arrangement of radiators across the tower can provide, for a given value of v the same or a greater air-flow than a construction using radiators around the periphery of the tower if H/D is equal to or less than 1/8.

The horizontal arrangement however has the disadvantage that it gives rise to a deceleration of the air beneath the radiators during the radial entry of the air. This can be seen from the following equation, which is derived from continuity considerations assuming that v is constant:

a n I l D where E =the average horizontal velocity at the air inlet .:the average horizontal velocity beneath the radiators at radius r of the tower.

The deceleration of the air as the centre of the tower is approached causes the boundary layer of air close to the floor of the tower to separate forming a horse shoe-shaped vortex. This vortex has been found to be unstable and is the cause of a considerable energy loss in the air-flow. However, if a fiat cone arrangement of the radiator elements is adopted the height of the elements from the floor of the tower is given by:

where h =the height of the elements from the floor at radius r of the tower.

Then the corresponding variation of the horizontal velocity of the air beneath the radiators is given by:

Therefore, the ave-rage horizontal velocity of the air beneath the radiators is constant and the horseshoeshaped vortex and its associated energy loss is less likely to be present.

It will be appreciated that many factors enter into the determination of the optimum form of an ellipsoidal tower. In order to avoid the entrainment of external wind by the leeward rim of the tower, the exit diameter and the height-to-base diameter ratio of the tower are interrelated.

In practice the base diameter might be between 3 and 5 times the height of the tower and more typically between 4 and 5 times. The exit diameter of the tower is related to the internal air-flow exit loss by:

where The uniformity factor, f, in Equation 1 is defined as where V =rnean square exit velocity v average exit velocity For f of the order of unity; is follows from Equation 6 that the exit diameter-should be' about half the base diameter for N -20. Consequently the downstream edge of th'e'leew'ard rim is situated-atabout three-quarters of the base diameter.

In practice the air outlet aperture mightbe between 0.4 and 0.6, and typically betwe'en'0.45 and 0.55, of the diameter of the base of the tower; this ratio as previously explained will be dependent on the height-to-base diameter ratio. i V

A number of embodiments of the invention are illustrated in the accompanying drawings in which:

FIGURE 1 is a diagrammatic vertical section through a dry cooling tower;

FIGURE 2 is a diagrammatic vertical section through a wet cooling tower;

FIGURES 3 to 7 are part sections through alternative constructions of wet cooling towers. 7

Referring to FIGURE 1, the tower is formed of an external shell 10 of ellipsoidal form having an air inlet 11 extending around the base of the tower at the periphery and having a central circular air outlet 12 at the top. The tower may be considered externally as a solid of revolution formed by cutting an ellipse in half along its major axis and rotating the half ellipse about its minor axis which is vertical. The external surface of the tower is smooth so as to ensure that the air-flow, due to external winds, passes over the tower and is not entrained in the exit aperture 12. In other words, the separation of the external air-flow from the surface of the shape should occur downstream of the position of the leeward rim in the absence of internal airflow from the exit. The required smooth external surface may conveniently be obtained by using an arched structure or a geodetic dome with lightweight cladding panels, e.g. aluminium panels or plastic-coated steel sheet.

In the embodiment illustrated in FIGURE 1 the exit aperture 12 has a diameter about 0.47 of the base diameter. The radiator elements, for the reasons previously explained, are arranged in a conical surface 13 extending from the air inlet almost to the centre of the tower. The air inlet extends substantially continuously around the base of the tower, being interrupted only by the necessary supports 14 which might typically be arranged at 10 intervals.

FIGURE 2 illustrates a wet cooling tower. This has an outer ellipsoidal shell 20 similar to the shell 10 of FIGURE 1 with an air inlet 21 and air outlet 22. In a very large cooling tower, such as might be used for a modern electrical power station, it is desirable to ensure that air can reach the packing in the centre of the tower without previously becoming saturated. For this reason, the incoming air at different'levels is taken to separate concentric regions within the tower. In the construction of FIGURE 2, there are three

concentric regions

23, 24 and 25. Air entering the top of the air inlet 21 is guided through packing 26, in the region 23, through which water from a distribution system indicated diagrammatically at 27 falls. This water is collected on the surface 28 forming the lower boundary of the air path and is returned to the condensers by outlet pipes (not shown). Air entering below the surface 28 passes either abovea dividing surface 29 to the region 24or below the surface 29 t0 the innermost region 25. The

regions

24 and 25 have

separate distribution systems

30, 31 and packing 32, 33.

In the arrangements of FIGURE 2, like FIGURE 1, the tower operates with a natural draught. FIGURE 3 illustrates a modification of the arrangement of FIGURE 2 in which fans 35 are provided for assisting or providing the draught through the tower. FIGURE 4 illustrates another modification of FIGURE 2 in which" the water distribution system and packing for all the three concentric regions are arranged in horizontal planes at 36, 37 respectively, the tower having a conical base 38 forming the lower boundary for the inlet air passages. The arrangement of FIGURE 4 may be modified, as shown in FIGURE 5, to have fans 39 for giving assisted or forced draught.

FIGURE 6 illustrates a wet cooling tower in which an ellipsoidal shell 40 is provided similar to the shell of FIGURE 1 but in which the inlet air is divided by horizontal partitions 41 and 42 into three streams which pass through

separate packing systems

43, 44 and 45 stacked at different levels inside the tower around the periphery. The air from the top level, above the partition 41, can pass upwardly through the packing 43 and so up the tower. The air at the two lower levels passes in a generally horizontal direction through the packing 44 and t 45 and so in towards the centre of the tower. In the central part of the tower, the floor 46 slopes upwardly towards the centre to improve the air-flow characteristics. The water falling through the lowest packing 45 falls down to a sump 47; the water from the packing 43 and 44 is removed through outlet pipes indicated at 48 and 49. In the embodiment illustrated, forced draught fans 50 are provided.

FIGURE 7 illustrates a modification of FIGURE 6 in which three separate water distribution and packing systems 51, 52, 53 are arranged, one above the other, outside the base of the cooling tower, the air passing in a generally horizontal direction through all three systems into the tower around the periphery thereof. In this particular embodiment also, fans 54 assist or provide draught through the tower.

I claim:

1. A wet cooling tower comprising a hollow structure of circular section in horizontal planes and externally of uniform convex aerofoil section in all diametral planes, which structure is provided with an air inlet around the base of the structure and has an aperture forming an air outlet aperture in the convex surface at the top thereof, the structure aerofoil section being such that the flow separation point due to a horizontal wind occurs downstream of the outlet aperture, in combination with a water distribution system and packing and means for directing air from diiierent levels in said air inlet to separate concentric regions in the structure.

2. A wet cooling tower as claimed in claim 1 wherein the structure is externally of ellipsoidal form in diametral planes.

3. A cooling tower as claimed in claim 1 wherein the water distribution systems and packing for said concentric regions are arranged at diiferent levels outside the aerofoil section structure around the base thereof.

4. A cooling tower as claimed in claim 1 wherein said outlet aperture has a diameter between 0.4 and 0.6 of the diameter of the base of the tower and wherein the base diameter of the tower is beween 3 and 5 times the height of the tower.

5. A cooling tower as claimed in claim 1 wherein separate water distribution systems and packing for said concentric regions are arranged concentrically within the tower.

6. A cooling tower as claimed in claim 5 wherein the water distribution systems and packing for said concentric regions are arranged at different levels within said structure.

7. A dry cooling tower comprising a hollow structure of circular section in horizontal planes and externally of uniform convex aerofoil form in all diametral planes, with the base diameter between 3 and 5 times the height of the tower, which structure has apertures forming an air outlet at the top, the aerofoil section of the structure being such that the flow separation point due to a horizonal wind occurs downstream of the outlet aperture, and having radiators arranged in an approximate conical plane inside the tower, the cone having an upright axis coincident with the central axis of the tower and the base of the cone lying above the level of the air inlet around the periphery of the tower, and the conical surface sloping downwardly towards the center of the tower.

8. A cooling tower as claimed in claim 7, wherein the apex of the conical surface is at the base of the tower.

9. A cooling tower as claimed in claim 7, wherein said conical surface is a frustum of a cone.

10. A cooling tower as claimed in claim 7 wherein said outlet aperture has a diameter between 0.4 and 0.6 of the diameter of the base of the tower.

11. A cooling tower as claimed in claim 7 wherein the structure is externally of semi-ellipsoidal form.

12. A cooling tower comprising a hollow structure of circular section in horizontal planes and externally of wholly convex aerofoil section in all diametral planes, which structure is provided with an air inlet around its base and has an aperture forming an air outlet at the top, the diameter of the outlet aperture being between 0.4 and 0.6 of the diameter of the base of the tower and the diameter of the base of the tower being between 3 and 5 times the height of the tower.

13. A cooling tower as claimed in claim 12 wherein the tower structure is a semi-ellipsoid with the circular diametral plane through the major axis of the ellipsoid horizontal.

14. A cooling ower as claimed in claim 13 and having forced draught means for forcing air through the tower from the inlet to the outlet.

References Cited UNITED STATES PATENTS 1,099,599 6/1914 Hartnell 5280 1,142,809 6/1915 Grace. 1,383,039 6/ 1921 Uhde. 2,494.057 1/1950 Reed -128 XR 2,907,554 10/1959 Heller. 3,175,960 3/1965 Kassat 165-122 XR 3,268,217 8/1966 Goitein. 3,322,409 5/1967 Reed.

FOREIGN PATENTS 677,166 12/ 1963 Canada. 320,505 10/ 1929 Great Britain.

HARRY B. THORNTON, Primary Examiner.

TIM R. MILES, Assistant Examiner.