Cooling towers

Abstract: A counterflow cooling tower is provided in which heat is exchanged between water on heat transfer media (10) and air flowing through the tower. The lower boundary (64) of the said media (10) slopes downwards towards the bottom (14) of the tower from an air entry (28) side to a far side of the tower so as to form a sloped interface. The air flow is therefore distributed more efficiently through all areas of the said media (10). Thus the vertical height requirement of the tower may be reduced. The sloped interface can be formed by using trapezoidal fill sheets (58) or serpentine tubes (72).

Description: This invention relates to a counterflow cooling tower comprising heat transfer media in the form of fill sheets comprising the features according to the preamble of claim 1. The design of counterflow cooling towers is a well developed technology. In one style of counterflow cooling tower which is widely applied in commercial installations heat transfer media comprising a plurality of fill sheets are mounted vertically and slightly spaced so as to provide vertical air channels between adjacent sheets, and ambient air is passed upwardly while heated water is flowed downwardly on the surface of the sheets to effect heat exchange to cool the water. The fill sheets have been relatively flat parallelograms, usually rectangles, of relatively impervious material with surface embossments to keep adjacent sheets spaced apart and to distribute a film of water on each surface. Such sheets have been arrayed in a horizontal bank with the lower edges positioned horizontally and held generally parallel and even with one another at a uniform distance above a water sump or pool in the bottom of the cooling tower so as to provide a rectangular plenum for air to enter beneath the fill sheets, as shown, for example, in US-A-3,132,190 to Engalitcheff Jr. Above the bank of fill sheets a water distribution network is located and balanced so as to deliver substantially uniform rates of water flow to all portions of the fill sheets; and above that an air outlet is located. Rarely do counterflow heat exchangers depart from such an arrangement, although there has been a design for such a unit wherein a pack of multiple fill units is shaped to form a sloping bank of interconnected uniform symmetrical fill sheets arranged with the sheets perpendicular to the entering air flow, as shown in US-A-3,983,190 to Norbäck (equivalent to DE-A-2 505 617) on which the preamble to claim 1 is based. The term "heat transfer media" is used throughout this specification to refer to the means for carrying liquid or water or means through or over which liquid or water is passed to be cooled by air flowing into the cooling tower. While it is sometimes possible to provide for a natural air draft through such a counterflow cooling tower, it is far more common that air is propelled mechanically. This may be by either forced draft, where one or more fans drive air from one or more sides of the cooling tower into the plenum below the fill sheets, or induced draft, where one or more fans are mounted above the fill sheets and water distributor and draw air through the plenum and media. In all such instances the air enters the plenum horizontally and therein gradually turns upwardly to pass between the media. FR-A-1271857, FR-A-2215597 and FR-A-2270539 disclose other known cooling devices. FR-A-2270539 discloses a counterflow cooling tower comprising heat transfer media arranged in a vertical bank, so as to be wetted by descending water which is contacted by air entering from one side of said cooling tower beneath the media through an air entry way formed in said one side, such air moving sideways into engagement with a lower extremity of the media and thereafter passing generally upwardly through the media, and a plenum space beneath said media which receives the air entering via said entry way, said plenum space having an interface with the lower extremity of said media, said heat transfer media having an upper extremity extending substantially horizontally. FR-A-1271857 describes a forced draft counterflow cooling tower in which the fill sheets of the heat transfer media are positioned some distance above the air entry way in the side of the tower, such that the air is flowing vertically when it encounters them, and are aligned parallel to the entering air flow, in order to ensure that the liquid to be cooled flows down the wall opposite the air entry way so as to eliminate problems caused by drops of liquid falling from the fill sheets. FR-A-2215597 discloses another type of counterflow cooling tower in which the heat transfer media comprises sheets arranged in a sloping bank and in which the channels formed in the sheets for liquid and air flow are inclined at varying angles to the horizontal. In these known cooling towers, a number of factors such as fan shrouding, the dynamics of air flow through the fan, and distance from the air inlet contribute to an uneven distribution of air flowing between the media. In the past this has usually been compensated by maximizing the vertical dimension and space within the plenum, orienting the lowermost media to be perpendicular to the initial horizontal direction of air entering the cooling tower and/or sometimes positioning control vanes within the plenum. Even so in prior cooling towers the air entering the plenum has tended to move horizontally beyond the media adjacent the air entry way resulting in an area of media close to the entry way that is relatively starved for vertically moving air and a consequent reduction in efficiency. This has been particularly true of forced draft towers employing centrifugal fans which heretofore tended to thrust air with great force toward a far side of the plenum. Moreover, it has been found that the overall height of a cooling tower is limited by practical considerations such as the need to reduce risk of wind damage, vertical structural restraints and restrictions at the location of installation (space between floors and maximum height for buildings) and the visual impact of a cooling tower respecting its surroundings. Another important factor applies to factory constructed cooling towers which are shipped from a manufacturing plant to an installation site along rail lines and roadways with vertical limits imposed by either the carrier equipment or bridges, and the like, along the route. Accordingly, it is a primary object of the present invention to enable a counterflow cooling tower to be of relatively smaller vertical height. It is also an object of the present invention to devise a counterflow cooling tower with a more efficient distribution of air flow through all areas of the media contained therein. According to the present invention, there is provided a counterflow cooling tower comprising heat transfer media in the form of fill sheets arranged in a vertical bank and defining a plurality of vertically extending air channels passing therethrough, the media being wetted by descending liquid which is contacted by air entering from one side of said cooling tower beneath the media through an air entry way formed in said one side, and a plenum space beneath the media which receives air entering via the entry way, in which the heat transfer media associated with said air entry way in said one side of the tower has a lower extremity which defines a sloping interface with the plenum space declining downwardly from the top of the air entry way and extending across the entire width of such media, whereby the air entering through said air entry way moves substantially horizontally into engagement with said lower extremity of the media and thereafter passes generally upwardly through the air channels in the media, characterised in that: the fill sheets are arranged in the region of said lower extremity so that the air flow channels defined thereby also extend horizontally in a direction which is parallel to the direction of air flow through said air entry way; and in that the upper extremity of said media is arranged to be substantially horizontal such that the depth of said media increases as the distance from said air entry way increases. Briefly the present invention allows for a reduction in the overall height of a counterflow cooling tower by organizing heat transfer media in a non symmetrical arrangement therein so that a lower boundary of the media is inclined downwardly toward the bottom of the tower from an air entry side toward a far side of the cooling tower, so as to form a sloped interface with a triangular air plenum from which the horizontally entering air tends to move substantially upwardly upon reaching the media. Stated differently, the present invention provides for arranging the media within a cooling tower to slope downwardly from an air entryway, thereby obstructing incremental layers of air moving horizontally into the plenum below the media and turning the incremental air flow upwardly as each layer reaches media interfacing the plenum. The preferred form of media is fill sheets of trapezoidal shape suspended so as to be parallel to the direction of entering air flow. Other advantages of the present invention will become apparent upon reading the following detailed description of an embodiment of the invention in conjunction with the drawings, wherein: FIGURE 1 is an illustrative side elevation of an induced draft counterflow cooling tower embodying the present invention and containing trapezoidal fill sheets, with an end removed to show interior parts; FIGURE 2 is an illustrative side elevation of a preferred embodiment of the present invention comprising a forced draft counterflow cooling tower with a centrifugal fan and trapezoidal fill sheets; FIGURE 3 is an illustrative side elevation of an induced draft counterflow cooling tower having symmetrical construction with air entry from the opposite sides; and FIGURE 4 is a view of the rectangular fill sheet stock, marked for cutting to produce two trapezoidal fill sheets of the present invention. An induced draft counterflow cooling tower is illustrated in Figure 1 wherein a plurality of heat transfer media generally 10 are suspended within a generally rectangular enclosure comprising vertical walls 12, a bottom basin 14 (or sump) and an overhead cowling 16 shaped to form a transition cover extending from the walls 12 to an exhaust fan ring 18 which surrounds and supports a multibladed fan 20 driven by an electric motor 22. Normally the motor 22 is mounted on the exterior of the cooling tower so as to be clear of the highly humid air discharged therefrom; however the motor 22 may be located within the tower or above the fan discharge. As illustrated the motor 22 is supported on the cowling 16 and is drivingly connected to the fan 20 by an endless belt 24, or the like, extending between sheaves on the motor shaft and fan shaft. As shown by the arrows A in Figure 1, air is drawn horizontally through an entry way 28 in only one side wall into a plenum space 36 within the cooling tower and moves upwardly between the media 10 and exits through the fan ring 18. The bottom of the cooling tower comprises an enclosed basin or sump 14 to receive a descending flow of water that is sprayed on the media 10 from an overhead distribution network comprising a water main 38, distribution pipes 40 and spray nozzles 42. Water is discharged from the basin 14 through a discharge conduit 48. A plurality of mist eliminator baffles 50 is positioned above the media 10 and may be conveniently supported above the water distribution pipes 40. A preferred form of media 10 in the present invention is non-rectangular fill sheets 58 having trapezoid shaped faces 60 which sheets are suspended in the cooling tower so as to have a horizontal upper edge 62 and a sloped, non-parallel lower edge 64 which extends from a short side edge 66 downwardly, to an opposite parallel and longer side edge 68. A plurality of such fill sheets 58 are suspended within the cooling tower with the short side edges 66 disposed toward an air entry side so that the lower edges 64 decline away from the top of an air entry way 28 toward the basin 14 thereby forming a sloped boundary and interface respecting the plenum space 36. Thus it will be understood that the present invention involves a novel construction of cooling tower which is formed by orienting the interface between heat transfer media and air plenum to slope downwardly from the top of the air entrance side of the cooling tower thereby resisting continued horizontal movement of increments or layers of air and turning such increments of air to move upwardly through the media. Further the present construction involves increasing the depth of the media as the distance from the air entrance increases (by maintaining the media top substantially at a horizontal level) thereby progressively lengthening both the horizontal and vertical air paths and hence increasing the flow resistance for successive increments or layers of air entering the cooling tower. This results in a relatively greater amount of air flow upwardly at the air entrance to the cooling tower as compared to towers having a horizontal interface between media and plenum and an even depth of media across the tower. It is to be noted that the fill sheets 58 are oriented to position the faces 60 parallel to the entering air flow (as designated by the arrows "A"). In the illustrated embodiment there are a plurality of substantially identical parallel fill sheets 58 (only one such sheet being visible from the side) each extending the full vertical depth of the media. It is to be understood, however, that such media may be constructed of stacked layers (not shown) wherein the upper layer (or layers) may comprise rectangular sheet components arranged vertically, and either parallel or crosswise to the lowermost fill sheets, so long as the latter present a sloping interface with the air plenum 36. Preferably the sloping interface is obtained by employing non-rectangular fill sheets and orienting a diverging edge 64 downwardly across the plenum 36, and while trapezoidal faces 60 as shown are preferred, it is possible to utilize a triangular fill sheet (not shown). Preferably the mist eliminator baffles 50 are mounted crosswise to the fill sheets (as shown) so as to function to both deflect and guide discharge air away from the intake air space and collect air entrained moisture droplets. However, baffles 50 may be repositioned if air is to be deflected in a different direction. The water distribution spray nozzles 42 may be adjusted to supply relatively lesser quantities of water adjacent the fill sheet short side edges 66 as compared to the longer side edges 68 to partially balance the effect of the relative cooling paths of increments of water descending along the fill sheet faces 60 when appropriate to fill height and density conditions. However, that effect is also at least partially balanced by a relatively lower resistance to air flow, and hence greater rate of air flow, across the shorter cooling paths. Because the preferred fill sheets have sloped lower edges 64 there is a tendency for some of the descending water to temporarily collect in the form of beads which run down the lower edge rather than to drop into the plenum space 36. In excessive amounts and circumstances such beading on adjacent fill sheets could bridge the space between sheets and interfere with air flow distribution. To counteract such a possibility the lower edges 64 of interdigitating fill sheets are alternately offset a small vertical distance (as shown by a phantom line 70 in Figure 1. Further details of fill sheets generally 10 are shown in Figure 4. Formed rectangular stock 80 of plastic film is available as the usual type of rectangular fill sheet in a wide range of dimensions. Such stock usually is embossed with a uniform pattern of contours 82, which define pathways and generally increase the surface area across which water is spread in the cooling tower, and one or more stiffening ridges 84 and margins. Also the stock is embossed to include a plurality of spacer projections 86 extending in two directions perpendicular to the plane of the sheet which projections nest when the sheets are stacked but serve to keep adjacent sheets spaced apart when suspended in a cooling tower in an offset interdigatating manner. The rectangular stock 80 is cut at an angle, as shown by the dashed line in Figure 4, between points on opposite sides equally spaced from the longer edges thereof. The specific dimensions and the slope angle of the non-parallel edge may be adapted to the size limitations of a cooling tower. Figure 2 illustrates a preferred embodiment of the present invention utilizing trapezoidal fill sheets 58'' in a forced draft counterflow cooling tower having a centrifugal fan 94. The fill sheets 58 are aligned with the incoming air propelled by the fan 94 through a transition duct 96. It has been found that the horizontal distribution of air from such a fan 94 is improved if the fan shaft 98 is slightly elevated and the fan discharge aimed through a gradually flared transition duct 96 which expands to approximately the dimensions of the inlet side 28'' of plenum 36''. Another advantage of aligning the fill sheets 60'' with the forceful air streams from a centrifugal fan 94 is that the lower edges 64'' of the sheets have a lessened tendency to flutter and thereby air flow between the sheets is facilitated. An example of a forced draft (centrifugal fan) counterflow cooling tower having exterior width and length dimension of 1.2 m x 1.82 m (48 x 71 3/4 inches) (exclusive of fan and entry duct) and height of 2.00 m (78 3/4 inches), containing seventy eight trapezoidal fill sheets suspended lengthwise, can supply approximately 50.7 tonnes (55.9 tons) of cooling capacity by cooling 38.1 m³ per hour (167.7 gallons of water per minute) from 35°C (95°F) inlet to 29.5°C (85°F) discharge using 444 m³ per minute (15670 cubic feet per minute) of 26°C (78°F) ambient air supplied by a centrifugal fan driven with a 3.7 kw (5 horsepower) motor. This is believed to represent a height reduction of 200 mm (8 inches) and a capacity increase of more than 7% over prior cooling towers containing the same number and total surface area of fill sheets. A further operating savings is obtained with the present invention in that by reducing the height of the cooling tower there is a reduction in the vertical distance that the warm water must be lifted to the water distribution pipes and nozzles with a consequent and significant saving of energy required to drive water pumps. Figure 3 illustrates an induced draft cooling tower provided with two air entry ways 28a and 28b at opposite walls 12'' of the cooling tower and two banks of fill sheets 10a and 10b above plenum spaces 36a and 36b. As is readily seen the cooling tower is generally symmetrical about a central vertical plane and parts corresponding to those described in connection with Figure 1 are denominated by the same reference character with a double prime notation. It is also possible to construct a forced draft embodiment of a symmetrical unit similar to the induced draft embodiment of Figure 2.

Claims: A counterflow cooling tower comprising heat transfer media (10) in the form of fill sheets (58) arranged in a vertical bank and defining a plurality of vertically extending air channels passing therethrough, the media being wetted by descending liquid which is contacted by air entering from one side of said cooling tower beneath the media through an air entry way (28) formed in said one side, and a plenum space (36) beneath the media (10) which receives air entering via the entry way, in which the heat transfer media (10) associated with said air entry way in said one side of the tower has a lower extremity (64) which defines a sloping interface with the plenum space declining downwardly from the top of the air entry way and extending across the entire width of such media, whereby the air entering through said air entry way moves substantially horizontally into engagement with said lower extremity (64) of the media and thereafter passes generally upwardly through the air channels in the media, characterised in that: the fill sheets (58) are arranged in the region of said lower extremity so that the air flow channels defined thereby also extend horizontally in a direction which is parallel to the direction of air flow through said air entry way; and in that the upper extremity (62) of said media is arranged to be substantially horizontal such that the depth of said media increases as the distance from said air entry way (28) increases. A counterflow cooling tower as claimed in claim 1 wherein said sheets (58) are trapezoidal in shape. A cooling tower as claimed in claim 1 or 2 wherein the bottom edges of adjacent fill sheets (58) are staggered sufficiently to lessen the contact between water beads forming along said bottom edges. A cooling tower as claimed in any of claims 1, 2 or 3 including a plurality of mist eliminator baffles (50) located above said fill sheets (58) and oriented to be substantially perpendicular thereto. A counterflow cooling tower as claimed in any preceding claim, wherein said fill sheets are arranged so that the air flow channels defined thereby extend in a horizontal direction which is parallel to the direction of air flow through said air entry way (28) for the entire depth of said media. A counterflow cooling tower as claimed in any preceding claim, wherein said media is composed of vertically stacked layers of fill sheets and at least the fill sheets in the lowest layer are arranged so that the air flow channels defined thereby also extend in a horizontal direction which is parallel to the direction of air flow through said air entry way (28). A counterflow cooling tower as claimed in any preceding claim further comprising a second air entry way (28b) formed in the opposite side of the cooling tower through which air may enter, said second air entry way having further heat transfer media (10b) in the form of fill sheets (58b) associated therewith, said further media defining a second plenum space thereunder, wherein the further heat transfer media has a lower extremity which defines a sloping interface with the second plenum space declining downwardly from the top of the second air entry way and extending across the entire width of said further media, and the further media having an upper extremity arranged to be substantially horizontal such that the depth of said further media increases as the distance from said second air entry way increases, said further fill sheets defining vertically extending air channels passing therethrough and being arranged in the region of said lower extremity so that the air flow channels also extend horizontally in a direction which is parallel to the direction of air flow through said second air entry way.

Inventors: GARRISH BRYAN F, GARRISH, BRYAN F.
Applicants: BALTIMORE AIRCOIL CO INC, BALTIMORE AIRCOIL COMPANY, INC.
Application year: 1989
Main classification: F28C1
IPC codes: F28C1, F28D5