CA2060531A1 - Cooling tower with different resistance to gas flow - Google Patents
Cooling tower with different resistance to gas flowInfo
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- CA2060531A1 CA2060531A1 CA 2060531 CA2060531A CA2060531A1 CA 2060531 A1 CA2060531 A1 CA 2060531A1 CA 2060531 CA2060531 CA 2060531 CA 2060531 A CA2060531 A CA 2060531A CA 2060531 A1 CA2060531 A1 CA 2060531A1
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Abstract
ABSTRACT
A crossflow gas-liquid cooling tower including an upper and a lower liquid crossflow cooling section, both including substantially all film fill and/or splash fill, the total of the fill components of the upper liquid crossflow cooling section having a gas velocity head resistance substantially higher than that of the total of the fill components of the lower liquid crossflow cooling section.
A crossflow gas-liquid cooling tower including an upper and a lower liquid crossflow cooling section, both including substantially all film fill and/or splash fill, the total of the fill components of the upper liquid crossflow cooling section having a gas velocity head resistance substantially higher than that of the total of the fill components of the lower liquid crossflow cooling section.
Description
A-54044/DJB 2 0 6 0 5 3 ~
COOLING TOWER WITH DIFFERENT RESISTANCES TO GAS FLOW
Cross-Reference to Re~ated A~lications ~his application is a Continuation-in-Part of my co-pending application entitled COOLING TOWER WITH
MULTIPLE FILL SECTIONS OF DIFFERENT TYPES, Serial No.
07/451,464 filed December 15, 1989.
Back~round of the Inventio~
Conventional cool$ng towers of the counterflow type employ a generally horizontal fill with an air opening below the lower surface of the same. Counterflow fills of the film type have a relatively good heat transfer coefficient. The air is drawn from below the fill and out the tower by a fan, or the draft from a high stack, positioned above the fill. When the distance between the fill and base of the tower is relatively small, the ;; air must be drawn from the surrounding into the tower ~ at a relatively high velocity and, when it reaches a `~ position below the fill, it i8 forced to turn abruptly at a sharp angle to proceed upwardly through the fill.
This contributes to resistance to air flow and requires ~, high fan power requirements. On the other hand, by building the tower of relatively high supporting columns, the velocity of the incoming air i5 somewhat reduced but the overall height of the tower is substantially increased. Among the disadvantages of ~ such height increases are increased pumping head, -j structural wind loads, and general appearance.
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COOLING TOWER WITH DIFFERENT RESISTANCES TO GAS FLOW
Cross-Reference to Re~ated A~lications ~his application is a Continuation-in-Part of my co-pending application entitled COOLING TOWER WITH
MULTIPLE FILL SECTIONS OF DIFFERENT TYPES, Serial No.
07/451,464 filed December 15, 1989.
Back~round of the Inventio~
Conventional cool$ng towers of the counterflow type employ a generally horizontal fill with an air opening below the lower surface of the same. Counterflow fills of the film type have a relatively good heat transfer coefficient. The air is drawn from below the fill and out the tower by a fan, or the draft from a high stack, positioned above the fill. When the distance between the fill and base of the tower is relatively small, the ;; air must be drawn from the surrounding into the tower ~ at a relatively high velocity and, when it reaches a `~ position below the fill, it i8 forced to turn abruptly at a sharp angle to proceed upwardly through the fill.
This contributes to resistance to air flow and requires ~, high fan power requirements. On the other hand, by building the tower of relatively high supporting columns, the velocity of the incoming air i5 somewhat reduced but the overall height of the tower is substantially increased. Among the disadvantages of ~ such height increases are increased pumping head, -j structural wind loads, and general appearance.
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-2- 2060~31 Conventional crossflow towers comprise a relatively thin vertical fill section with the water being fed from an overhead source and the air being drawn therethrough from air inlets at the side of the tower. Since there is no necessity for the air to make radical changes of direction in the fill and the air inlet is spaced along the entire height of the fill, the overall air pressure losses are usually less than those of a conventional counterflow tower as set forth above.
A crossflow cooling tower is inherently less efficient with respect to heat transfer than a counterflow tower based on a unit of fill. Another disadvantage of the crossflow cooling tower is that the water is loaded onto the top of the relatively thin crossflow fill section.
There is a maximum water load beyond which the water will not redistribute effectively because it will start gushing in a steady stream through the tower. When this maximum water load is exceeded in a crossflow tower of the film fill type, the water will not cling to the fill, leading to relatively poor heat transfer between the air and water. Also, resistance to the transversely flowing air is substantially increased requiring excessive fan power. This problem of water loading cannot be effectively overcome by widening the fill in the direction of air flow because there is a limiting factor on cooling efficiency relative to the thickness of the fill. A major factor in this limit is that the fan power for the longer air path through the fill disproportionately increases in comparison to the advantages to be attained by easing the above water load problems.
Corrugated film type fill is relatively efficient in either a counterflow or crossflow cooling tower. When 20~3~
A crossflow cooling tower is inherently less efficient with respect to heat transfer than a counterflow tower based on a unit of fill. Another disadvantage of the crossflow cooling tower is that the water is loaded onto the top of the relatively thin crossflow fill section.
There is a maximum water load beyond which the water will not redistribute effectively because it will start gushing in a steady stream through the tower. When this maximum water load is exceeded in a crossflow tower of the film fill type, the water will not cling to the fill, leading to relatively poor heat transfer between the air and water. Also, resistance to the transversely flowing air is substantially increased requiring excessive fan power. This problem of water loading cannot be effectively overcome by widening the fill in the direction of air flow because there is a limiting factor on cooling efficiency relative to the thickness of the fill. A major factor in this limit is that the fan power for the longer air path through the fill disproportionately increases in comparison to the advantages to be attained by easing the above water load problems.
Corrugated film type fill is relatively efficient in either a counterflow or crossflow cooling tower. When 20~3~
utilized in conventional (horizontal) counterflow type tower, variations in the direction of the corrugations will affect the ease or difficulty of gas and liquid passage in a similar manner. For example, by disposing the corrugations at a relatively vertical inclination, the gas path is eased for lower power requirements but so is the liquid path leading to relatively poor film formation and low liquid residence time. On the other hand, by disposing the corrugations at a relatively horizontal inclination, the gas must travel through a tortuous path which greatly increases the fan power requirements.
The invention applies to conventional wet cooling towers wherein direct contact occurs bet~een liquid and gas (air), as distinguished from so-called wet/dry cooling tower~ wherein a conventional wet cooling section is combined with a dry cooling section, with a common structure. The dry cooling section utilizes tubes to convey liquid through a gas (air) stream, and the tubes are cooled by contact with the gas.
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U. S. Pat. No. 3,917,764 discloses a liquid-gas cooling tower which combines advantages of the counterflow and crossflow cooling towers. 5pecifically, that patent describes a cooling tower with a film fill section having an incline principal plane formed of a number of sheets mounted for the passage of gas and liquid. This sloping film fill section spreads the liquid gravitating onto its upper surface into a thinner, more uniform film on the lower surface. Splash-type fill is disposed inboard and/or outboard of the sloping fill. Corrugated and other types of film fill are disclosed. Figure 3 discloses multiple spaced, stacked sloping film fill sections, each extending only part of the total vertical :~,.
, _4_ 2~60~31 distance across the gas inlet but across the total liquid supply area. Figure 4 discloses multiple side-by-side sloping film fill sections extending across the entire gas path but only partially across the liquid supply path.
U.S. Pat. No~ 3,450,393 discloses a crossflow cooling tower including a stack of two film fill sections with a thin high density fill disposed over a low density film fill section which performs the cooling. The upper fill section is used to provide an even diffusion of water falling from the distribution pan. The corrugated sheets are perpendicular to the direction of air flow through the lower section and so block the passage of air. At column 2, lines 63-69, the patent specifies that the air does not pass through the channels of this high density film.
U.S. Pat. No. 4,460,521 discloses another use of fills of different density, in this instance, all of the splash fill type in a crossflow cooling tower. In one embodiment, a sloped section of high density splash fill is flanked by triangular sections of low density splash fill. In another embodiment, outboard high density splash fill is mounted adjacent to inboard low density splash fill.
U.S. Pat. No. 3,707,277 discloses a combination cross-flow and counterflow tower in which the water gravitating from the crossflow section is isolated from the air inlet in the lower portion of the tower. The fill sections are served by independent air streams.
Another combined fill application is disclosed in U.S.
Pat. No. 4,317,785. That patent describes a cooling ~5~ 206~53~
tower with a number of film fill box-like sections arranged in a stair-step configuration progressing with the highest section at the outboard end of the fill area and the lowest section at the inboard end. The remainder of the tower available for water distribution is filled with splash fill. Air travels horizontally through the film fill boxes.
U.S. Pat. No. 4,592,877 discloses a combination of crossflow and counterflow. Figure 3 of that patent discloses two sloping film fill sections of the type described in U.S. Pat. No. 3,917,764 flanking a horizontal film fill section, all disposed above splash fill sloping generally inboard to define a partially open chamber.
U.S. Pat. No. 4,826,636 discloses a multi-level fill crossflow cooling tower with stair-stepped fill.
U.S. Pat. No. 3,322,409 discloses the advantage of high water loading for ice reduction on air inlet faces of a cooling tower.
U.S. Pat. No. 4,934,663 discloses a cooling tower with high density sloping film fill flan~ed by low density upper and lower film fill. In a specific embodiment, the sheets form integral units with upper and lower film fill sections of generally triangula~ configuration and the fi}m fill formed of adjacent crossing corrugated sheets.
U.S. Pat. No. 2,394,755 and No. 4,737,321 disclose means for attaining uniform velocity air flow through the fill section of a cooling tower.
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U.S. Pat. No. 4,781,869 discloses a multilevel cooling with separate counterflow cooling sections served by separate water distribution systemshaving equal airflow distribution between the systems.
U.S~ Pat. No. 4,421,303 discloses means outside the fill region for throttling air flow in a natural draft cooling tower during periods of excessively high air flow (i.e. windy conditions) to reduce excessive drift loss from the tower resulting from that high air flow.
A unit similar to Figure 4 of pending U.S. application Serial No. 07/451,464 has been built and used commercially.
Summary of the Invention and Objects It is an object of the invention to provide a liquid-gas contact tower which uses multiple cooling sections having different types of fill which maximize the cooling effects at the outboard portion of the tower.
It is another object of the invention to provide a high efficiency cooling tower with a split gas inlet feeding to an upper liquid cooling section of substantially higher gas flow resistance in velocity heads than that of a lower liquid cooling section.
It is one specific object of the invention to provide such a tower using optimum high water loading in the outboard section of the tower and low water loading on the inboard section.
It is another specific object of the invention to provide a tower with multiple stacked, sloping film fill sections of variable lengths to maximize efficiency.
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Other objects and features of the invention will be apparent from the following description ta~en in conjunction with the appended drawings.
In accordance with the above ob~ects, in one embodiment, the present invention relates to a crossflow cooling tower with an upright sidewall defining a common gas inlet opening, typically extending the full height of the tower, bisected by an imaginary horizontal plane between an upper gas inlet opening and a lower gas inlet opening. An upper liquid crossflow cooling section, including splash fill, film fill, or both, is disposed ; between the upper gas inlet opening and gas outlet opening at the same level as the upper gas inlet opening. It has a substantially higher gas flow resistance in velocity heads than a lower liquid crossflow cooling section disposed between the lower gas inlet opening and gas outlet opening, at substantially the same level as the upper gas inlet opening, resulting in an increase of tower efficiency. (The term "velocity head" refers to standard velocity head as defined in the cooling tower industry.) The liquid from the upper cooling section directly contacts the air in the lower cooling section. In a particularly effective cooling tower, this system is combined with features disclosed in my co-pending patent application Serial No.
07/451,464. Specifically, the liquid supply means formed of an outboard liquid supply section which supplies liquid at a flow rate per distribution area substantially higher than an inboard liquid supply section. At least one outboard film fill section is disposed directly below the outboard liquid supply section while a different inboard fill section of the film fill or splash fill type or both is disposed directly below the inboard liquid supply section.
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-8- 2060~31 Brief Description of the Drawings Figure 1 is one embodiment of the present invention illustrating uneven water loading for a generally upright sheet film fill section in which the upper film has a higher gas velocity head resistance than the lower fill.
Figures 2 and 3 are other embodiments of the invention with multiple stacked sloping film fill sections in which the upper fill sections have higher gas velocity head resistance than the lower fill sections.
Figures 4 and 5 are two other embodiments of the tower of the present invention.
~etailed Description of the Preferred Embodiments In general, the present invention concerns the utilization of a crossflow tower with an upright gas inlet wall defining a gas inlet opening bisected between an upper gas inlet opening and a lower gas inlet opening. Typically, the gas inlet opening extends substantially the entire vertical height of the cooling section of the tower. The term "bisected" refers to an imaginary horizontal plane bisecting the gas inlet wall and dividing the gas into such upper and lower gas openings. (As the gas is normally air, the terms will be used interchangeably.) An upper liquid crossflow cooling section is disposed between the upper gas inlet opening and gas outlet opening, at substantially the same level as the upper gas inlet opening to create substantially horizontal flow. Correspondingly, a lower liquid crossflow cooling section is disposed between the lower gas inlet opening ,. :.
-9- 2060~3~
and the gas outlet opening at substantially the same level as the lower gas inlet opening. "Gas outlet opening" refers to the plenum chamber, typically the open space below a fan in a fan-driven tower.
The upper and lower liquid cooling sections are defined to include substantially all fill serving to cool the liquid, including film fill, splash fill, or both, but excluding components serving other functions (e.g. drift eliminators), all disposed between the gas inlet and outlet openings.
The upper liquid cooling section has gas flow velocity ; head resistance substantially higher than that of the lower liquid cooling section resulting in a higher gas velocity in the lower cooling section. Suitably, the gas velocity head resistance of the upper liquid cooling section is at least about 110% (preferably at least about 150%) of the lower gas flow velocity head resistance. For convenience, the difference in gas flow velocity head resistance between the upper and lower cooling sections will be designated "the velocity head resistance differential".
If there is a variation of fill in the upper cooling section in terms of fill density or type (e.g. film fill or splash fill), the combined total of the ~as flow velocity head resistance of such fill is added and averaged (over the cross-sectional area of the upper or lower gas inlet opening) to determine the velocity head resistance. This can be approximated for the fill prior to insertion into the tower. A similar calculation is performed for the fill in the lower liquid cooling section. It is possible that either the upper or lower liquid cooling section would have variable velocity head ... .
-lO- 2060~1 resistance as a function of elevation within each section or in distance inboard and outboard across the cooling section so long as the upper liquid cooling section has a composite average velocity head resistance substantially higher than that of the lower liquid cooling section. The drift eliminator walls, common to the upper and lower cooling sections, are not considered in the calculations of the difference in velocity head resistance.
The lower liquid cooling section is in open communication with the upper cooling section. Thus, a substantial portion, typically 50 to 100%, of liquid gravitating from the upper liquid cooling section contacts the gas flowing through the lower cooling section in cooling relationship.
The gas velocity head resistance can be determined for a tower with generally uniform liquid loads throughout, or with different liquid loads, as described below.
In a preferred embodiment, a system of the foregoing type is combined with multiple different outboard and inboard fill sections, preferably with high water loadinq on the outboard side of the tower as described in my co-pending application Serial No. 07/451,464, incorporated herein by reference. As used herein, the term "outboard" means closer to the gas inlet opening, and the term "inboard" means farther from the gas inlet opening and closer to the gas outlet opening. The outboard fill section includes at least some ilm fill and may be totally film fill. Alternatively, it may include some fill voids or splash fill. The inboard fill section includes some splash fill or film fill, or both.
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The combination of high effective fill density and high resistance, best exemplified by film fill, in the outboard portion of the upper cooling section, together with high water loading in the outboard portion, produces particularly efficient overall cooling. This combination results in maximum upper pre-cooling of the outboard water resulting in minimal heating of the lower air stream by that water, while at the same time encouraging increased gas velocity in the lower cooling section due to the differential in gas flow resistance.
This combination of cool and high velocity gas through the lower cooling section makes that gas well suited for optimizing the cooling of the lower water as it leaves the tower, particularly the lightly concentrated inner water. This is important as the driving force caused by the temperature differential between the gas and liquid is at a minimum in this lower portion because the liquid has already been partially cooled in the upper cooling section.
Although useful in gas contacting towers and the like, such towers will be designated "cooling towers" herein, as it would most commonly be utilized for cooling liquid, such as water, by inducing the surrounding gas, air, into the tower to contact and thereby cool the water.
Referring to Figure 1, a cooling towqr, broadly denoted by the number 10, is illustrated comprising two similar cooling sections, disposed on opposite sides of a central plenum chamber. For simplicity of description, only the section illustrated on the left-hand portion of the drawing will be described. This section includes an upright side wall 12 having a gas inlet opening 14 : .:
extending along a major portion of the side wall and defined by a framing portion of the same.
An outboard fill section 16 is mounted adjacent to gas inlet opening 14 and extends the full vertical distance of gas inlet opening 14. To the right or inboard of outboard fill section 16 is mounted inboard fill section 18 also extending the full vertical distance of gas inlet opening 14. As illustrated, both fill sections are mounted at approximately a 10- angle off the vertical extending with the outermost upper corner outboard of the outermost lower corner. This is conventional sloping to accommodate water drift conditions. However, if desired, these sections may be vertically disposed.
A cool liquid basin 20 is disposed below fill sections 16 and 18 to receive liquid gravitating therefrom.
Means is provided for supplying gravitating liquid to an upper portion of the tower. As illustrated, such means comprises an open top perforated distribution pan or tray 22 which includes an outboard liquid supply section 22a and an inboard liquid supply section 22b.
Outboard section 22a supplies gravitating liquid at a flow rate per distribution area onto outboard fill section 16 substantially higher than inboard fill section 22b onto inboard fill section 18. As illustrated, this is accomplished by the use of an open tray 22 with a larger nozzle or port area in section 22a than in 22b. Such large nozzle or port area may include larger sized nozzles or a larger number of nozzles so long as the higher flow rate is accomplished. Another way to accomplish the same objective, not shown, is to partition the outboard liquid supply section 22a from the inboard li~uid supply section 22b and to provide a -13- 2~6~531 higher flow of water to the outboard section. Another embodiment would be to use spray nozzles rather than a distribution pan with a higher volume of spray on the outboard section compared to the inboard section. This could be accomplished by using higher pressure at the outboard section or a large number of spray nozzles.
At least 5% to 10% more water at the outboard section compared to the inboard section accomplishes increased efficiencies. However, it is pre~erable that the water load on the outboard section be at least 50~ to 100~
more than the water loading on the inboard section to accomplish the beneficial effects as described hereinafter.
The dividing line between the outboard liquid supply section 22a and the inboard liquid supply section 22b may vary depending upon the characteristics of the underlying fill. ~or most applications, the outboard liquid supply section 22a extends from 20% to 50% and preferably about 30% to 40% of the total area across the liquid supply means, pan 22 as illustrated.
Preferably, the effective fill density of the outboard section is substantially higher than that of the inboard section. In one embodiment, both sections are of the film fill type including vertically disposed sheets parallel to the direction of gas flow. More particularly, a preferred type o~ fill is described in U.S. Pat. 4,934,663. Crossing corrugated sheets with ridges and grooves are disposed so that ridges of alternate sheets cross and abut against the ridges disposed between alternate sheets to form channels between them. As described in such application, the channels have a constantly varying width from 0 at the contact points between the sheets to a maximum of twice -14- 2~60531 the distance between the ridgec and grooves of individual sheets. The sheets are vertically disposed to provide an essentially vertical path to liquid gravitating from tray 22. The sheets also define a path A between the gas inlet and outlet openings.
An embodiment of Figure 1 where fill sections 16 and 18 are both comprised of adjacent parallel vertical sheets as described above, effective fill density refers to the number of sheets per unit length of fill thickness.
Here, the high density fill preferably includes about 1.2S to 3 times the number of sheets per unit length compared to the low effective density fill and preferably from 1.5 to 2 times for optimum performance.
For example, Munters 25060 could be used as low density fill and Munters 12060 as high density fill. Suitable dense film fill in the outboard section includes an average distance between adjacent sheets no greater than about 25mm.
Conventional splash fill, not shown, can be used in place of the inboard film fill section 18 described above. Conventional splash fill includes generally horizontal slats disposed on stringer wires spaced apart from each other at the same elevation and above and below them. Such splash fill provides splash surfaces for gravitating water from tray 22 to thereby disperse the water for more efficient cooling contact with the incoming air. In this instance, effective fill density of the inboard fill section refers to cooling efficiency comparable to that of film fill section 18. For a combin~tion of inboard film and splash ~ill the effective fill density is defined solely by the film fill.
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. ---15- 2060~31 A cooling region is defined as the area below the full horizontal extent from the outboard to the inboard side of the tower directly below the liquid supply means 22 and extending substantially the full vertical distance of the gas inlet opening 14. As illustrated in Figure 1, this cooling region is in the form of a parallelogram.
In composite, the outboard fill section 16 and inboard fill æection 18 extend substantially the entire outboard to inboard distance of the cooling region. In certain applications, some area of the outboard or inboard fill sections may be devoid of fill. However, at least some fill i~ present in both the outboard and inboard sections of the cooling regions.
For maximum efficiency, outboard fill section 22a should extend at least about 20% to 30% across the cooling region but no greater than about 50% to 60%. For maximum efficiency, it extends about 30% to 40% across.
Correspondingly, the remainder of the horizontal area of the cooling region is comprised of the inboard section and a thin layer adjacent to the air inlet, if the thin layer is utilized. The inboard extent of outboard fill section 16 should be approximately the same as the inboard extent of outboard liquid supply section 22a.
Referring again to Figure 1, suitable pumping structure, not shown, is operably coupled to basin 20 for moving liquid through line 24 for delivery of the water to equipment for cooling and returning the same to tray 22 through valve combination 26. If desired~ a portion of the liquid removed in line 24 may be recycled to tray 22 for further cooling prior to delivery to equipment, : .
-16- 206~531 or liquid from the equipment can bypass tray 22 directly into basin 20.
Referring again to Figure 1, the vertical stack 28 is secured to the top face of tower 10 and extends upwardly from a central plenum chamber 30 to define an upper outlet opening 28a for gas (air) exiting therefrom.
fan 32 within stack 28, coupled to a suitable source of power for actuation, causes currents of air to be drawn through fill sections 16 and 18 generally along path A
and forced upwardly through chamber 30 into stack 28 for discharge through the upper portion of the latter.
Alternatively, the cooling tower may be operated by natural induction by the elimination of fan 32 in which case air would be induced to flow through the tower by means of natural convection of the warm exhaust air rising through chamber 30 and stack 28.
A drift eliminator wall 34 is disposed across the path of air exiting from sections 16 and 18 in a generally upright position to prevent gravitating water from being carried as a spray into the plenum ~hamber. Wall 34 may be of any conventional type such as including a series of spaced inclined baffles to prevent significant quantities of liquid droplets from escaping into the plenum chamber.
Referring again to Figure 1, an important aspect of the invention comprises the use of an upper liquid cooling section with a gas velocity head resistance substantially higher than that of a lower liquid cooling section. In the embodiment illustrated in Figure 1, an imaginary horizontal plane bisects inlet opening 14.
The portion of outboard fill section 16 and inboard fill section 18 above that plane comprises the upper liquid .~
-17- 2060~31 cooling section because it constitutes substantially the entire resistance to air flow between the gas inlet and outlet openings (except for the drift eliminators). It has a velocity head resistance substantially higher than the velocity head resistance of the lower liquid cooling section below that plane.
One way to accomplish the velocity head resistance differential in the embodiment of Figure 1 is to use a corrugated film fill in the upper section which is more closely packed than corrugated film fill in the lower section, e.g. by 50~, resulting in an approximate 2 to 1 ratio of velocity head resistance of the upper section to the lower section. This is analogous to the effective film fill differentiation between low density fill and high density fill in the outboard film fill section 16 and the inboard film fill section 18, respectively as described above.
In general, the relationship between gas velocity head resistance and effective fill density is as follows.
It can generally be said that a cooling tower with a higher effective fill density will usually have a higher resistance to air flow through that fill. The effective fill density is represented by the factor KaV/L, wherein "V" is the total fill volume per amount of water, "L", to be cooled (sometimes factor RaV/L is represented as KaY/L if just one horizontal square foot of a tower is isolated, and then the water is considered in terms of its flow, L, per square foot). A tower with large volume "V" can normally have a high overall effective fill just due to the size of the structure. Even comparing two cross flow towers of equal height "Y" but different overall volume, there is a larger overall :
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-18- 2060~31 KaY/L for the larger volume tower, for a given amount of water and air, other factors being equal. In like manner, there i8 more resistance to air flow through a larger tower than there is through a small tower, other thinqs being equal, so there is a direct connection between "Y" in KaV/L and air flow resistance. Some fills have greater turbulence inducing features than others, and this turbulence will usually result in a higher transfer coefficient "xn. Greater turbulence also means higher friction loss in velocity heads. Some fills will have a greater area of wetted surface per cubic foot, "a", and this makes them more effective in cooling. The greater surface, "a", normally means that there is more physical surface acting on the air stream, either as the air runs into such surface or as it flows past it. Such contact will be reflected in air flow resistance.
A fill with high Ka can utilize a small volume, V, for a given amount of water flow, L, thus concentrating that water in a small volume. Such water concentration in a small volume also tends to be reflected in high resistance to air flow through that volume.
From the above, it can be seen that a fill whose individual components contribute to a high KaV/L, and which is placed in the upper outer region of a tower to optimize cooling of a high volume of water as typified by application Serial No. 07/451,464, can be expected to provide high resistance to upper air flow from these same components. This combination of effects will promote the advantages described in the present application. Film fill is particularly well suited to provide these features.
-19- 206as3l In like manner, it can also be said that in general, creating resistance to upper air flow in fill within the cooling section will normally result in greater fill effectiveness, as well. High gas flow resistance in drift eliminators or air inlet structure does not enhance cooling effectiveness in any reasonably measurable amount.
Referring to Figure 2, another embodiment utilizing uneven water loading is illustrated with a different fill configuration. Here, a series of three sloping stacked fill sections 36, 38 and 40 are illustrated of the type described in U.S. Pat. 3,917,764. They extend from upper outboard ends toward lower inboard ends.
Preferably they include vertically disposed sheets of the corrugated type as described above which are parallel to the direction of airflow through the gas inlet openings. As such fill sections may be installed in the same general type of tower as illustrated in Figure 1, only the cooling region of the tower is shown.
The inclination of the film fill assemblies 36, 38 and 40 are illustrated 45 to the vertical. However, it should be understood that this angle may be varied substantially between 20 and 70 to the vertical depending upon the requirements of a particular tower.
It is important that the major portion, and, preferably, essentially all of the air entering inlet opening 14 be prevented from bypassing film fill sections 36, 38 or 40. If a major opening around the film fill assemblies were provided, the air would take this path. To prevent this, perforated membrane means comprising perforate pans or grids 42 and 44 are provided having openings large enough to permit liquid gravitating from assembly 40 and 3~ to pass through and contact the upper surface -20- 2060~3~
of the next lower film fill section. On the other hand, pans 42 and 44 provide barriers to gas flow. To accomplish this, pans 42 and 44 preferably extend from the lower edge of a film fill section to the upper edge of the one directly below the same. If desired, conventional splash fill 46 may be provided in the spaces between the film fill section in the form of horizontal splash plates or decks. Such splash type fill provides increased ga~ liquid contact efficiency with respect to that portion of the tower. In this instance, the effective fill density is defined herein solely by the film fill.
In this embodiment, the outboard film fill sections 36, 38 and 40 preferably extend the same distance inboard as does outboard liquid supply section 22a. Like the embodiment of Figure 1, the inboard fill section may be of any conventional fill type having a lower density than the fill sections in the outboard portion of the tower in a ratio described with respect to the embodiment of Figure 1. As illustrated, the inboard fill section comprises splash fill 48.
The embodiment of Figure 2 is illustrated with three tiers of sloping film fill section. However, it should be understood that one, two or four or more such tiers may be employed.
The embodiments of Figures 1 and 2 share the features of higher outboard than inboard water loading and higher effective fill density outboard than inboard. It has been found that increasing the water load on the outboard film fill increases the efficiency of the tower. This is believed due to the beneficial effect of high water loading at the outboard side of the tower ..
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where it contacts the most efficient type of fill and the coolest air. Another advantage of using higher water loading in the outboard portion of the tower is that it tends to more nearly flood that portion of the tower. This creates a condition where icing of outer tower cooling surfaces in cold winter weather is reduced. Also, such high water loading correspondingly reduces water loading in the inboard portion of the tower adjacent to the drift eliminators. This reduced water loading results in reduced carry-over of objectionable drift into and through the drift eliminators.
In the embodiment of Figure 2, the tower may be divided by an imaginary horizontal plane bisecting the gas inlet opening at the left. The composite gas velocity head resistance in the upper liquid cooling section above that plane is substantially higher than that o~ the lower liquid cooling section below that plane. In the embodiment of Figure 2, this is readily accomplished by using a closely packed fill for upper fill section 36 in comparison to lower fill section 40. In this manner, the total gas velocity head resistance of the upper fill section is substantially higher than that of the lower section. Fill 36 could be Munters 12060 and fill 40 could be Munters 19060. Fill 38 could be either 12060 or 19060.
Referring to Figure 3, three superposed, spaced, sloping film fill sections 50, 52 and 54 are illustrated of the same general type as shown in Figure 2. In addition, a fourth lower spaced sloping film fill section 56 is illustrated. All film fill sections 50, 52, 54 and 56 have principal planes inclined at angles from 20~ to 70 to the vertical and pr~ferably parallel to each other . .
. . :, in spaced superposed relationship. Sections 50, 52 and 54 extend only partway inboard of the tower to a maximum extent of about 50% (about 30% as illustrated) of the depth of the liquid cooling region, generally designated by the dimension B. Preferably such sections only extend about 30% to 40% of the distance. The same fill sections each also extend less than 50% (about 20% as illustrated) of the vertical gas intake distance of gas inlet opening 14. In contrast, sloping film fill section 56 extends essentially from the outboard side of the tower a distance of at least about 90% of the liquid cooling region and preferably approximately the total distance. Perforated pans 58, 60 and 62 extend between the superposed sloping film fill sections and accomplish the same purpose as perforated pans 42 and 44 described with respect to the embodiment of Figure 2.
Low density splash fill is preferably disposed in the cooling region between distribution pan 22 and the top surface of sloping film fill 56. Such fill is of relatively low density compared to that of sloping film fill sections 50, 52, 54 and S6 as described above. A
thin layer of this fill can be installed adjacent to the air inlet wall. The splash fill can be ignored in calculating the velocity head resistance differential because its effect is canceled by its presence in equal amounts in the upper and lower cooling sections.
one advantage of the configuration of Figure 3 is that the tower tend~ to act as a counterflow tower. Very cold air is contacted on the outer surface where the ~ir enters. With the high outer effective fill density due to the three fill levels 50, 52 and 54 and with a high outer water loading, there is substantial heat transfer , :
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-23- 2~60531 from this outer upper water due to the usual large temperature differential in this region, even with the problems noted in the third paragraph of the Background of the Invention section. There has been significant cooling of thi~ outer water by the time it reaches the lower fill section. This maintains the air entering the portion of the tower below section 56 from being excessively heated. The water entering the inboard region of the lower film fill is slightly warmer since it does not contact a high density film fill layer for cooling it. However, there is better heat transfer in the cool air flowing across the lowest part of the tower since it contacts this warmer water. Therefore, there is a greater driving force between this water and the lower air. The net effect is that the cooled water temperature leaving the tower is only slightly affected but the amount of film fill requlred is reduced since a ma~or portion of the film fill in sections 50, 52 and 54 is eliminated. Also, the installation of the film fill, particularly in the retro-fitting of an existing tower, is easier since there is ready access to the upper film sections through the open air inlets.
The embodiment of Figure 3 is most effective in combination with uneven water loading wherein the flow at the outboard section of the tower is substantially higher than the inboard section as described above. In addition, this configuration using multiple short outboard sloping film fill sections above a film fill section extending substantially across the cooling region of the tower has unique performance advantages where the water loading is unifor~ across the distribution pan.
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In this e~bodiment, the imaginary horizontal plane cuts the tower so that the upper liquid cooling section includes short film fill sections 50, 52 and 54 and the lower liquid cooling section includes long film fill section 56. The velocity head resistance differential i8 accomplished with substantially higher density fill in the shorter film fill sections than the longer one in an analogous manner to the embodiment of Figure 1.
By way of example, sections 50, 52 and 54 are Munters film fill No. 12060 and Section 56 is Munters No. 19060 with about half the pressure drop.
Referring to Figure 4, the cooling section of a tower which combines some of the features of Figures 1-3 is illustrated. In this cooling tower generally designated by the number 100, is illustrated including an upright sidewall having a gas inlet opening 102 which is divided by a imaginary bisecting horizontal plane to form gas inlets to an upper cooling section 104 and a lower cooling section 106. An outboard film fill section 108 is mounted adjacent to the upper portion of gas inlet opening 102 and extends approximately the entire elevation of upper cooling section 104 and inwardly one-fourth the distance of the cooling section. To the right or inboard of outboard film fill section 108 is a sloping film fill section 110 extending from the upper inboard edge of film fill 108 to the bottom of upper cooling section 104 adjacent to drift eliminator wall 112 at its inboard end. Film fill section 110 is of the same general type as the sloping film fill sections of Figures 2 and 3. In an alternative embodiment, film fill section 110 is eliminated and replaced with splash fill or no fill at all.
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~ . -- -25- 20~0531 Sloping film fill section 114 is disposed in lower cooling section 106 extended from the inboard lower corner of film ~ill section 108 to the inboard side of cooling section 106 at drift eliminator wall 112. Fill Section 114 could also extend from outboard corner of film fill section 108 adjacent to gas inlet opening 102.
A cool liquid basin 116 is disposed below the cooling sections to receive gravitating liquid therefrom.
Liquid is supplied to the upper portion of the tower in the form of an open top perforated distribution pan or tray 118 including an outboard liquid supply section 118a and an inboard liquid supply section 118b.
Outboard liquid supply section 118a may be supplied gravitating liquid at a flow rate per distribution area onto outboard film fill section 108 substantially higher than onto inboard film fill section 110. As illustrated, this is accomplished by the use of an open tray 118 with a larger nozzle or port area in section 118a than in section 118b. This tray distribution may be the same type described as with respect to the embodiment of Figure 1.
To the inboard side of drift eliminator 112 is a central plenum chamber 120 which in composite defines the outlet gas opening from both the upper and lower cooling sections. A fan 122 within stack 124, coupled to a suitable source of power actuation, causes currents of air to be drawn through the cooling tower upwardly through chamber 120 and out the stack for discharge.
The above embodiment film fill section 108, alone or in combination with optional film fill section 110, has a substantially higher gas velocity head resistance than the lower film fill section 114. As illustrated, this 2~0531 would be ~rue if the resistanca per unit depth of fill were the same for the two film fill sections because of the greater depth of film fill section 108. It is noted that the spaces in the upper and lower cooling sections not filled with film fill could be filled with splash fill or no fill without affecting this composite substantially higher velocity head resistance of the upper film fill section compared to the lower film fill section. In this manner, substantially higher velocity air will flow through the lower cooling section 106.
A mode of calculating velocity head resistance is illustrated with respect to a typical tower built in accordance with Figure 4. Film fill section 108 is four layers (air travel) of Munters No. 19060 film fill. The inner sloping film fill sections 110 and 114 are each a single layer of Munters No. 12060 film fill. The fill resistances are as published, by Munters in curves for the film fill dated July, 1986. The lower air velocity is 20% higher than the upper air velocity. This results in fill losses of 50 velocity heads in the upper and 30 velocity heads in the lower fill section. One velocity head at 400 ft/min. standard air velocity equals 0.01 in. water gauge. By adding the other fixed losses of inlet, drift eliminator and fan plenum, plus the effect of consequent difference in air velocity, the lower losses are the same as the overall upper losses due to the differences in air velocity. The outer fill layer ha~ a 25% higher water load than the inner layer, in GPM/sq.ft. The height of the upper fill is the same as the height of the lower fill. The effective fill density of the outer film f~ll section 108 is higher than that for the inner fill density in sections 110 and 114. The upper film fill has an effective film density, KaY/L, of two. The upper and lower inner film fill , , -27- 2060~31 sections have effective fill density (KaY/L), respectively, 0.45 and 0.5, but they total about half that of the outer film fill.
Referring to Figure 5, another embodiment o~ the invention is illustrated which is similar in overall structure to Figure 4. Accordingly, like parts will be designated with like numbers. In this instance, the film fill for the entire tower constitutes a single sloping film fill section 126 of the type illustrated in Figure 3 extending from the upper outboard corner of the upper cooling section to the lower inboard corner of the lower cooling section. The velocity head resistance differential is accomplished by forming the portion of the fill in the upper cooling section of 126a of a substantially higher fill density than that of the film fill 126b in the lower cooling section. In this instance, the higher capacity (KaY/L) fill 126a may be located under a higher water loading from section 118a ~accomplished by a greater number of nozzles than in section 118b). Also, since upper film fill section 126a has a higher resistance to air flow than lower film fill section 126b, this forces higher velocity air to flow through the lower cooling section 106 than the upper cooling section 104. In an alternative embodiment, not shown, fill 126 may be stepped as illustrated in Pat.
No. 4,317,785. Splash fill may be placed above or below film fill section 126.
In all of the illustrated embodiments a large amount of heat is "flashed off" in the outer cooling section even though there is a lower air velocity. The resultant higher velocity flow in the lower section is used for better cooling of inboard sections for a net overall gain in cooling. A low inner water load further assists .:
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by reducing the amount of heat compared to uniform water loading, avoiding heating of the lower gas.
Other alternative configurations may be used in accordance with the present invention. For example, other suitable film fill may be used as the outboard film fill section in the tower below the outboard liquid supply section of liquid cooling. Also, different numbers and thicknesses of stacked sloping film fill may be used compared to the ones depicted in Figures 2 and 3. In some cases, it may be desirable to install a thin low density fill layer, or even leave a void, in the area adjacent to the air inlet face. This area could typically be 1-2 meters deep, or even a little more in special cases. In all cases, water load may be uniform, or have different loading.
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The invention applies to conventional wet cooling towers wherein direct contact occurs bet~een liquid and gas (air), as distinguished from so-called wet/dry cooling tower~ wherein a conventional wet cooling section is combined with a dry cooling section, with a common structure. The dry cooling section utilizes tubes to convey liquid through a gas (air) stream, and the tubes are cooled by contact with the gas.
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U. S. Pat. No. 3,917,764 discloses a liquid-gas cooling tower which combines advantages of the counterflow and crossflow cooling towers. 5pecifically, that patent describes a cooling tower with a film fill section having an incline principal plane formed of a number of sheets mounted for the passage of gas and liquid. This sloping film fill section spreads the liquid gravitating onto its upper surface into a thinner, more uniform film on the lower surface. Splash-type fill is disposed inboard and/or outboard of the sloping fill. Corrugated and other types of film fill are disclosed. Figure 3 discloses multiple spaced, stacked sloping film fill sections, each extending only part of the total vertical :~,.
, _4_ 2~60~31 distance across the gas inlet but across the total liquid supply area. Figure 4 discloses multiple side-by-side sloping film fill sections extending across the entire gas path but only partially across the liquid supply path.
U.S. Pat. No~ 3,450,393 discloses a crossflow cooling tower including a stack of two film fill sections with a thin high density fill disposed over a low density film fill section which performs the cooling. The upper fill section is used to provide an even diffusion of water falling from the distribution pan. The corrugated sheets are perpendicular to the direction of air flow through the lower section and so block the passage of air. At column 2, lines 63-69, the patent specifies that the air does not pass through the channels of this high density film.
U.S. Pat. No. 4,460,521 discloses another use of fills of different density, in this instance, all of the splash fill type in a crossflow cooling tower. In one embodiment, a sloped section of high density splash fill is flanked by triangular sections of low density splash fill. In another embodiment, outboard high density splash fill is mounted adjacent to inboard low density splash fill.
U.S. Pat. No. 3,707,277 discloses a combination cross-flow and counterflow tower in which the water gravitating from the crossflow section is isolated from the air inlet in the lower portion of the tower. The fill sections are served by independent air streams.
Another combined fill application is disclosed in U.S.
Pat. No. 4,317,785. That patent describes a cooling ~5~ 206~53~
tower with a number of film fill box-like sections arranged in a stair-step configuration progressing with the highest section at the outboard end of the fill area and the lowest section at the inboard end. The remainder of the tower available for water distribution is filled with splash fill. Air travels horizontally through the film fill boxes.
U.S. Pat. No. 4,592,877 discloses a combination of crossflow and counterflow. Figure 3 of that patent discloses two sloping film fill sections of the type described in U.S. Pat. No. 3,917,764 flanking a horizontal film fill section, all disposed above splash fill sloping generally inboard to define a partially open chamber.
U.S. Pat. No. 4,826,636 discloses a multi-level fill crossflow cooling tower with stair-stepped fill.
U.S. Pat. No. 3,322,409 discloses the advantage of high water loading for ice reduction on air inlet faces of a cooling tower.
U.S. Pat. No. 4,934,663 discloses a cooling tower with high density sloping film fill flan~ed by low density upper and lower film fill. In a specific embodiment, the sheets form integral units with upper and lower film fill sections of generally triangula~ configuration and the fi}m fill formed of adjacent crossing corrugated sheets.
U.S. Pat. No. 2,394,755 and No. 4,737,321 disclose means for attaining uniform velocity air flow through the fill section of a cooling tower.
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U.S. Pat. No. 4,781,869 discloses a multilevel cooling with separate counterflow cooling sections served by separate water distribution systemshaving equal airflow distribution between the systems.
U.S~ Pat. No. 4,421,303 discloses means outside the fill region for throttling air flow in a natural draft cooling tower during periods of excessively high air flow (i.e. windy conditions) to reduce excessive drift loss from the tower resulting from that high air flow.
A unit similar to Figure 4 of pending U.S. application Serial No. 07/451,464 has been built and used commercially.
Summary of the Invention and Objects It is an object of the invention to provide a liquid-gas contact tower which uses multiple cooling sections having different types of fill which maximize the cooling effects at the outboard portion of the tower.
It is another object of the invention to provide a high efficiency cooling tower with a split gas inlet feeding to an upper liquid cooling section of substantially higher gas flow resistance in velocity heads than that of a lower liquid cooling section.
It is one specific object of the invention to provide such a tower using optimum high water loading in the outboard section of the tower and low water loading on the inboard section.
It is another specific object of the invention to provide a tower with multiple stacked, sloping film fill sections of variable lengths to maximize efficiency.
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Other objects and features of the invention will be apparent from the following description ta~en in conjunction with the appended drawings.
In accordance with the above ob~ects, in one embodiment, the present invention relates to a crossflow cooling tower with an upright sidewall defining a common gas inlet opening, typically extending the full height of the tower, bisected by an imaginary horizontal plane between an upper gas inlet opening and a lower gas inlet opening. An upper liquid crossflow cooling section, including splash fill, film fill, or both, is disposed ; between the upper gas inlet opening and gas outlet opening at the same level as the upper gas inlet opening. It has a substantially higher gas flow resistance in velocity heads than a lower liquid crossflow cooling section disposed between the lower gas inlet opening and gas outlet opening, at substantially the same level as the upper gas inlet opening, resulting in an increase of tower efficiency. (The term "velocity head" refers to standard velocity head as defined in the cooling tower industry.) The liquid from the upper cooling section directly contacts the air in the lower cooling section. In a particularly effective cooling tower, this system is combined with features disclosed in my co-pending patent application Serial No.
07/451,464. Specifically, the liquid supply means formed of an outboard liquid supply section which supplies liquid at a flow rate per distribution area substantially higher than an inboard liquid supply section. At least one outboard film fill section is disposed directly below the outboard liquid supply section while a different inboard fill section of the film fill or splash fill type or both is disposed directly below the inboard liquid supply section.
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-8- 2060~31 Brief Description of the Drawings Figure 1 is one embodiment of the present invention illustrating uneven water loading for a generally upright sheet film fill section in which the upper film has a higher gas velocity head resistance than the lower fill.
Figures 2 and 3 are other embodiments of the invention with multiple stacked sloping film fill sections in which the upper fill sections have higher gas velocity head resistance than the lower fill sections.
Figures 4 and 5 are two other embodiments of the tower of the present invention.
~etailed Description of the Preferred Embodiments In general, the present invention concerns the utilization of a crossflow tower with an upright gas inlet wall defining a gas inlet opening bisected between an upper gas inlet opening and a lower gas inlet opening. Typically, the gas inlet opening extends substantially the entire vertical height of the cooling section of the tower. The term "bisected" refers to an imaginary horizontal plane bisecting the gas inlet wall and dividing the gas into such upper and lower gas openings. (As the gas is normally air, the terms will be used interchangeably.) An upper liquid crossflow cooling section is disposed between the upper gas inlet opening and gas outlet opening, at substantially the same level as the upper gas inlet opening to create substantially horizontal flow. Correspondingly, a lower liquid crossflow cooling section is disposed between the lower gas inlet opening ,. :.
-9- 2060~3~
and the gas outlet opening at substantially the same level as the lower gas inlet opening. "Gas outlet opening" refers to the plenum chamber, typically the open space below a fan in a fan-driven tower.
The upper and lower liquid cooling sections are defined to include substantially all fill serving to cool the liquid, including film fill, splash fill, or both, but excluding components serving other functions (e.g. drift eliminators), all disposed between the gas inlet and outlet openings.
The upper liquid cooling section has gas flow velocity ; head resistance substantially higher than that of the lower liquid cooling section resulting in a higher gas velocity in the lower cooling section. Suitably, the gas velocity head resistance of the upper liquid cooling section is at least about 110% (preferably at least about 150%) of the lower gas flow velocity head resistance. For convenience, the difference in gas flow velocity head resistance between the upper and lower cooling sections will be designated "the velocity head resistance differential".
If there is a variation of fill in the upper cooling section in terms of fill density or type (e.g. film fill or splash fill), the combined total of the ~as flow velocity head resistance of such fill is added and averaged (over the cross-sectional area of the upper or lower gas inlet opening) to determine the velocity head resistance. This can be approximated for the fill prior to insertion into the tower. A similar calculation is performed for the fill in the lower liquid cooling section. It is possible that either the upper or lower liquid cooling section would have variable velocity head ... .
-lO- 2060~1 resistance as a function of elevation within each section or in distance inboard and outboard across the cooling section so long as the upper liquid cooling section has a composite average velocity head resistance substantially higher than that of the lower liquid cooling section. The drift eliminator walls, common to the upper and lower cooling sections, are not considered in the calculations of the difference in velocity head resistance.
The lower liquid cooling section is in open communication with the upper cooling section. Thus, a substantial portion, typically 50 to 100%, of liquid gravitating from the upper liquid cooling section contacts the gas flowing through the lower cooling section in cooling relationship.
The gas velocity head resistance can be determined for a tower with generally uniform liquid loads throughout, or with different liquid loads, as described below.
In a preferred embodiment, a system of the foregoing type is combined with multiple different outboard and inboard fill sections, preferably with high water loadinq on the outboard side of the tower as described in my co-pending application Serial No. 07/451,464, incorporated herein by reference. As used herein, the term "outboard" means closer to the gas inlet opening, and the term "inboard" means farther from the gas inlet opening and closer to the gas outlet opening. The outboard fill section includes at least some ilm fill and may be totally film fill. Alternatively, it may include some fill voids or splash fill. The inboard fill section includes some splash fill or film fill, or both.
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The combination of high effective fill density and high resistance, best exemplified by film fill, in the outboard portion of the upper cooling section, together with high water loading in the outboard portion, produces particularly efficient overall cooling. This combination results in maximum upper pre-cooling of the outboard water resulting in minimal heating of the lower air stream by that water, while at the same time encouraging increased gas velocity in the lower cooling section due to the differential in gas flow resistance.
This combination of cool and high velocity gas through the lower cooling section makes that gas well suited for optimizing the cooling of the lower water as it leaves the tower, particularly the lightly concentrated inner water. This is important as the driving force caused by the temperature differential between the gas and liquid is at a minimum in this lower portion because the liquid has already been partially cooled in the upper cooling section.
Although useful in gas contacting towers and the like, such towers will be designated "cooling towers" herein, as it would most commonly be utilized for cooling liquid, such as water, by inducing the surrounding gas, air, into the tower to contact and thereby cool the water.
Referring to Figure 1, a cooling towqr, broadly denoted by the number 10, is illustrated comprising two similar cooling sections, disposed on opposite sides of a central plenum chamber. For simplicity of description, only the section illustrated on the left-hand portion of the drawing will be described. This section includes an upright side wall 12 having a gas inlet opening 14 : .:
extending along a major portion of the side wall and defined by a framing portion of the same.
An outboard fill section 16 is mounted adjacent to gas inlet opening 14 and extends the full vertical distance of gas inlet opening 14. To the right or inboard of outboard fill section 16 is mounted inboard fill section 18 also extending the full vertical distance of gas inlet opening 14. As illustrated, both fill sections are mounted at approximately a 10- angle off the vertical extending with the outermost upper corner outboard of the outermost lower corner. This is conventional sloping to accommodate water drift conditions. However, if desired, these sections may be vertically disposed.
A cool liquid basin 20 is disposed below fill sections 16 and 18 to receive liquid gravitating therefrom.
Means is provided for supplying gravitating liquid to an upper portion of the tower. As illustrated, such means comprises an open top perforated distribution pan or tray 22 which includes an outboard liquid supply section 22a and an inboard liquid supply section 22b.
Outboard section 22a supplies gravitating liquid at a flow rate per distribution area onto outboard fill section 16 substantially higher than inboard fill section 22b onto inboard fill section 18. As illustrated, this is accomplished by the use of an open tray 22 with a larger nozzle or port area in section 22a than in 22b. Such large nozzle or port area may include larger sized nozzles or a larger number of nozzles so long as the higher flow rate is accomplished. Another way to accomplish the same objective, not shown, is to partition the outboard liquid supply section 22a from the inboard li~uid supply section 22b and to provide a -13- 2~6~531 higher flow of water to the outboard section. Another embodiment would be to use spray nozzles rather than a distribution pan with a higher volume of spray on the outboard section compared to the inboard section. This could be accomplished by using higher pressure at the outboard section or a large number of spray nozzles.
At least 5% to 10% more water at the outboard section compared to the inboard section accomplishes increased efficiencies. However, it is pre~erable that the water load on the outboard section be at least 50~ to 100~
more than the water loading on the inboard section to accomplish the beneficial effects as described hereinafter.
The dividing line between the outboard liquid supply section 22a and the inboard liquid supply section 22b may vary depending upon the characteristics of the underlying fill. ~or most applications, the outboard liquid supply section 22a extends from 20% to 50% and preferably about 30% to 40% of the total area across the liquid supply means, pan 22 as illustrated.
Preferably, the effective fill density of the outboard section is substantially higher than that of the inboard section. In one embodiment, both sections are of the film fill type including vertically disposed sheets parallel to the direction of gas flow. More particularly, a preferred type o~ fill is described in U.S. Pat. 4,934,663. Crossing corrugated sheets with ridges and grooves are disposed so that ridges of alternate sheets cross and abut against the ridges disposed between alternate sheets to form channels between them. As described in such application, the channels have a constantly varying width from 0 at the contact points between the sheets to a maximum of twice -14- 2~60531 the distance between the ridgec and grooves of individual sheets. The sheets are vertically disposed to provide an essentially vertical path to liquid gravitating from tray 22. The sheets also define a path A between the gas inlet and outlet openings.
An embodiment of Figure 1 where fill sections 16 and 18 are both comprised of adjacent parallel vertical sheets as described above, effective fill density refers to the number of sheets per unit length of fill thickness.
Here, the high density fill preferably includes about 1.2S to 3 times the number of sheets per unit length compared to the low effective density fill and preferably from 1.5 to 2 times for optimum performance.
For example, Munters 25060 could be used as low density fill and Munters 12060 as high density fill. Suitable dense film fill in the outboard section includes an average distance between adjacent sheets no greater than about 25mm.
Conventional splash fill, not shown, can be used in place of the inboard film fill section 18 described above. Conventional splash fill includes generally horizontal slats disposed on stringer wires spaced apart from each other at the same elevation and above and below them. Such splash fill provides splash surfaces for gravitating water from tray 22 to thereby disperse the water for more efficient cooling contact with the incoming air. In this instance, effective fill density of the inboard fill section refers to cooling efficiency comparable to that of film fill section 18. For a combin~tion of inboard film and splash ~ill the effective fill density is defined solely by the film fill.
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. ---15- 2060~31 A cooling region is defined as the area below the full horizontal extent from the outboard to the inboard side of the tower directly below the liquid supply means 22 and extending substantially the full vertical distance of the gas inlet opening 14. As illustrated in Figure 1, this cooling region is in the form of a parallelogram.
In composite, the outboard fill section 16 and inboard fill æection 18 extend substantially the entire outboard to inboard distance of the cooling region. In certain applications, some area of the outboard or inboard fill sections may be devoid of fill. However, at least some fill i~ present in both the outboard and inboard sections of the cooling regions.
For maximum efficiency, outboard fill section 22a should extend at least about 20% to 30% across the cooling region but no greater than about 50% to 60%. For maximum efficiency, it extends about 30% to 40% across.
Correspondingly, the remainder of the horizontal area of the cooling region is comprised of the inboard section and a thin layer adjacent to the air inlet, if the thin layer is utilized. The inboard extent of outboard fill section 16 should be approximately the same as the inboard extent of outboard liquid supply section 22a.
Referring again to Figure 1, suitable pumping structure, not shown, is operably coupled to basin 20 for moving liquid through line 24 for delivery of the water to equipment for cooling and returning the same to tray 22 through valve combination 26. If desired~ a portion of the liquid removed in line 24 may be recycled to tray 22 for further cooling prior to delivery to equipment, : .
-16- 206~531 or liquid from the equipment can bypass tray 22 directly into basin 20.
Referring again to Figure 1, the vertical stack 28 is secured to the top face of tower 10 and extends upwardly from a central plenum chamber 30 to define an upper outlet opening 28a for gas (air) exiting therefrom.
fan 32 within stack 28, coupled to a suitable source of power for actuation, causes currents of air to be drawn through fill sections 16 and 18 generally along path A
and forced upwardly through chamber 30 into stack 28 for discharge through the upper portion of the latter.
Alternatively, the cooling tower may be operated by natural induction by the elimination of fan 32 in which case air would be induced to flow through the tower by means of natural convection of the warm exhaust air rising through chamber 30 and stack 28.
A drift eliminator wall 34 is disposed across the path of air exiting from sections 16 and 18 in a generally upright position to prevent gravitating water from being carried as a spray into the plenum ~hamber. Wall 34 may be of any conventional type such as including a series of spaced inclined baffles to prevent significant quantities of liquid droplets from escaping into the plenum chamber.
Referring again to Figure 1, an important aspect of the invention comprises the use of an upper liquid cooling section with a gas velocity head resistance substantially higher than that of a lower liquid cooling section. In the embodiment illustrated in Figure 1, an imaginary horizontal plane bisects inlet opening 14.
The portion of outboard fill section 16 and inboard fill section 18 above that plane comprises the upper liquid .~
-17- 2060~31 cooling section because it constitutes substantially the entire resistance to air flow between the gas inlet and outlet openings (except for the drift eliminators). It has a velocity head resistance substantially higher than the velocity head resistance of the lower liquid cooling section below that plane.
One way to accomplish the velocity head resistance differential in the embodiment of Figure 1 is to use a corrugated film fill in the upper section which is more closely packed than corrugated film fill in the lower section, e.g. by 50~, resulting in an approximate 2 to 1 ratio of velocity head resistance of the upper section to the lower section. This is analogous to the effective film fill differentiation between low density fill and high density fill in the outboard film fill section 16 and the inboard film fill section 18, respectively as described above.
In general, the relationship between gas velocity head resistance and effective fill density is as follows.
It can generally be said that a cooling tower with a higher effective fill density will usually have a higher resistance to air flow through that fill. The effective fill density is represented by the factor KaV/L, wherein "V" is the total fill volume per amount of water, "L", to be cooled (sometimes factor RaV/L is represented as KaY/L if just one horizontal square foot of a tower is isolated, and then the water is considered in terms of its flow, L, per square foot). A tower with large volume "V" can normally have a high overall effective fill just due to the size of the structure. Even comparing two cross flow towers of equal height "Y" but different overall volume, there is a larger overall :
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-18- 2060~31 KaY/L for the larger volume tower, for a given amount of water and air, other factors being equal. In like manner, there i8 more resistance to air flow through a larger tower than there is through a small tower, other thinqs being equal, so there is a direct connection between "Y" in KaV/L and air flow resistance. Some fills have greater turbulence inducing features than others, and this turbulence will usually result in a higher transfer coefficient "xn. Greater turbulence also means higher friction loss in velocity heads. Some fills will have a greater area of wetted surface per cubic foot, "a", and this makes them more effective in cooling. The greater surface, "a", normally means that there is more physical surface acting on the air stream, either as the air runs into such surface or as it flows past it. Such contact will be reflected in air flow resistance.
A fill with high Ka can utilize a small volume, V, for a given amount of water flow, L, thus concentrating that water in a small volume. Such water concentration in a small volume also tends to be reflected in high resistance to air flow through that volume.
From the above, it can be seen that a fill whose individual components contribute to a high KaV/L, and which is placed in the upper outer region of a tower to optimize cooling of a high volume of water as typified by application Serial No. 07/451,464, can be expected to provide high resistance to upper air flow from these same components. This combination of effects will promote the advantages described in the present application. Film fill is particularly well suited to provide these features.
-19- 206as3l In like manner, it can also be said that in general, creating resistance to upper air flow in fill within the cooling section will normally result in greater fill effectiveness, as well. High gas flow resistance in drift eliminators or air inlet structure does not enhance cooling effectiveness in any reasonably measurable amount.
Referring to Figure 2, another embodiment utilizing uneven water loading is illustrated with a different fill configuration. Here, a series of three sloping stacked fill sections 36, 38 and 40 are illustrated of the type described in U.S. Pat. 3,917,764. They extend from upper outboard ends toward lower inboard ends.
Preferably they include vertically disposed sheets of the corrugated type as described above which are parallel to the direction of airflow through the gas inlet openings. As such fill sections may be installed in the same general type of tower as illustrated in Figure 1, only the cooling region of the tower is shown.
The inclination of the film fill assemblies 36, 38 and 40 are illustrated 45 to the vertical. However, it should be understood that this angle may be varied substantially between 20 and 70 to the vertical depending upon the requirements of a particular tower.
It is important that the major portion, and, preferably, essentially all of the air entering inlet opening 14 be prevented from bypassing film fill sections 36, 38 or 40. If a major opening around the film fill assemblies were provided, the air would take this path. To prevent this, perforated membrane means comprising perforate pans or grids 42 and 44 are provided having openings large enough to permit liquid gravitating from assembly 40 and 3~ to pass through and contact the upper surface -20- 2060~3~
of the next lower film fill section. On the other hand, pans 42 and 44 provide barriers to gas flow. To accomplish this, pans 42 and 44 preferably extend from the lower edge of a film fill section to the upper edge of the one directly below the same. If desired, conventional splash fill 46 may be provided in the spaces between the film fill section in the form of horizontal splash plates or decks. Such splash type fill provides increased ga~ liquid contact efficiency with respect to that portion of the tower. In this instance, the effective fill density is defined herein solely by the film fill.
In this embodiment, the outboard film fill sections 36, 38 and 40 preferably extend the same distance inboard as does outboard liquid supply section 22a. Like the embodiment of Figure 1, the inboard fill section may be of any conventional fill type having a lower density than the fill sections in the outboard portion of the tower in a ratio described with respect to the embodiment of Figure 1. As illustrated, the inboard fill section comprises splash fill 48.
The embodiment of Figure 2 is illustrated with three tiers of sloping film fill section. However, it should be understood that one, two or four or more such tiers may be employed.
The embodiments of Figures 1 and 2 share the features of higher outboard than inboard water loading and higher effective fill density outboard than inboard. It has been found that increasing the water load on the outboard film fill increases the efficiency of the tower. This is believed due to the beneficial effect of high water loading at the outboard side of the tower ..
. .
.~
. .
;.
where it contacts the most efficient type of fill and the coolest air. Another advantage of using higher water loading in the outboard portion of the tower is that it tends to more nearly flood that portion of the tower. This creates a condition where icing of outer tower cooling surfaces in cold winter weather is reduced. Also, such high water loading correspondingly reduces water loading in the inboard portion of the tower adjacent to the drift eliminators. This reduced water loading results in reduced carry-over of objectionable drift into and through the drift eliminators.
In the embodiment of Figure 2, the tower may be divided by an imaginary horizontal plane bisecting the gas inlet opening at the left. The composite gas velocity head resistance in the upper liquid cooling section above that plane is substantially higher than that o~ the lower liquid cooling section below that plane. In the embodiment of Figure 2, this is readily accomplished by using a closely packed fill for upper fill section 36 in comparison to lower fill section 40. In this manner, the total gas velocity head resistance of the upper fill section is substantially higher than that of the lower section. Fill 36 could be Munters 12060 and fill 40 could be Munters 19060. Fill 38 could be either 12060 or 19060.
Referring to Figure 3, three superposed, spaced, sloping film fill sections 50, 52 and 54 are illustrated of the same general type as shown in Figure 2. In addition, a fourth lower spaced sloping film fill section 56 is illustrated. All film fill sections 50, 52, 54 and 56 have principal planes inclined at angles from 20~ to 70 to the vertical and pr~ferably parallel to each other . .
. . :, in spaced superposed relationship. Sections 50, 52 and 54 extend only partway inboard of the tower to a maximum extent of about 50% (about 30% as illustrated) of the depth of the liquid cooling region, generally designated by the dimension B. Preferably such sections only extend about 30% to 40% of the distance. The same fill sections each also extend less than 50% (about 20% as illustrated) of the vertical gas intake distance of gas inlet opening 14. In contrast, sloping film fill section 56 extends essentially from the outboard side of the tower a distance of at least about 90% of the liquid cooling region and preferably approximately the total distance. Perforated pans 58, 60 and 62 extend between the superposed sloping film fill sections and accomplish the same purpose as perforated pans 42 and 44 described with respect to the embodiment of Figure 2.
Low density splash fill is preferably disposed in the cooling region between distribution pan 22 and the top surface of sloping film fill 56. Such fill is of relatively low density compared to that of sloping film fill sections 50, 52, 54 and S6 as described above. A
thin layer of this fill can be installed adjacent to the air inlet wall. The splash fill can be ignored in calculating the velocity head resistance differential because its effect is canceled by its presence in equal amounts in the upper and lower cooling sections.
one advantage of the configuration of Figure 3 is that the tower tend~ to act as a counterflow tower. Very cold air is contacted on the outer surface where the ~ir enters. With the high outer effective fill density due to the three fill levels 50, 52 and 54 and with a high outer water loading, there is substantial heat transfer , :
.; . , .- , -. -.. . .
-:~. , :
-23- 2~60531 from this outer upper water due to the usual large temperature differential in this region, even with the problems noted in the third paragraph of the Background of the Invention section. There has been significant cooling of thi~ outer water by the time it reaches the lower fill section. This maintains the air entering the portion of the tower below section 56 from being excessively heated. The water entering the inboard region of the lower film fill is slightly warmer since it does not contact a high density film fill layer for cooling it. However, there is better heat transfer in the cool air flowing across the lowest part of the tower since it contacts this warmer water. Therefore, there is a greater driving force between this water and the lower air. The net effect is that the cooled water temperature leaving the tower is only slightly affected but the amount of film fill requlred is reduced since a ma~or portion of the film fill in sections 50, 52 and 54 is eliminated. Also, the installation of the film fill, particularly in the retro-fitting of an existing tower, is easier since there is ready access to the upper film sections through the open air inlets.
The embodiment of Figure 3 is most effective in combination with uneven water loading wherein the flow at the outboard section of the tower is substantially higher than the inboard section as described above. In addition, this configuration using multiple short outboard sloping film fill sections above a film fill section extending substantially across the cooling region of the tower has unique performance advantages where the water loading is unifor~ across the distribution pan.
.
, -24- ~06~53~
In this e~bodiment, the imaginary horizontal plane cuts the tower so that the upper liquid cooling section includes short film fill sections 50, 52 and 54 and the lower liquid cooling section includes long film fill section 56. The velocity head resistance differential i8 accomplished with substantially higher density fill in the shorter film fill sections than the longer one in an analogous manner to the embodiment of Figure 1.
By way of example, sections 50, 52 and 54 are Munters film fill No. 12060 and Section 56 is Munters No. 19060 with about half the pressure drop.
Referring to Figure 4, the cooling section of a tower which combines some of the features of Figures 1-3 is illustrated. In this cooling tower generally designated by the number 100, is illustrated including an upright sidewall having a gas inlet opening 102 which is divided by a imaginary bisecting horizontal plane to form gas inlets to an upper cooling section 104 and a lower cooling section 106. An outboard film fill section 108 is mounted adjacent to the upper portion of gas inlet opening 102 and extends approximately the entire elevation of upper cooling section 104 and inwardly one-fourth the distance of the cooling section. To the right or inboard of outboard film fill section 108 is a sloping film fill section 110 extending from the upper inboard edge of film fill 108 to the bottom of upper cooling section 104 adjacent to drift eliminator wall 112 at its inboard end. Film fill section 110 is of the same general type as the sloping film fill sections of Figures 2 and 3. In an alternative embodiment, film fill section 110 is eliminated and replaced with splash fill or no fill at all.
.
--. .-~; , .. ;. .
~ . -- -25- 20~0531 Sloping film fill section 114 is disposed in lower cooling section 106 extended from the inboard lower corner of film ~ill section 108 to the inboard side of cooling section 106 at drift eliminator wall 112. Fill Section 114 could also extend from outboard corner of film fill section 108 adjacent to gas inlet opening 102.
A cool liquid basin 116 is disposed below the cooling sections to receive gravitating liquid therefrom.
Liquid is supplied to the upper portion of the tower in the form of an open top perforated distribution pan or tray 118 including an outboard liquid supply section 118a and an inboard liquid supply section 118b.
Outboard liquid supply section 118a may be supplied gravitating liquid at a flow rate per distribution area onto outboard film fill section 108 substantially higher than onto inboard film fill section 110. As illustrated, this is accomplished by the use of an open tray 118 with a larger nozzle or port area in section 118a than in section 118b. This tray distribution may be the same type described as with respect to the embodiment of Figure 1.
To the inboard side of drift eliminator 112 is a central plenum chamber 120 which in composite defines the outlet gas opening from both the upper and lower cooling sections. A fan 122 within stack 124, coupled to a suitable source of power actuation, causes currents of air to be drawn through the cooling tower upwardly through chamber 120 and out the stack for discharge.
The above embodiment film fill section 108, alone or in combination with optional film fill section 110, has a substantially higher gas velocity head resistance than the lower film fill section 114. As illustrated, this 2~0531 would be ~rue if the resistanca per unit depth of fill were the same for the two film fill sections because of the greater depth of film fill section 108. It is noted that the spaces in the upper and lower cooling sections not filled with film fill could be filled with splash fill or no fill without affecting this composite substantially higher velocity head resistance of the upper film fill section compared to the lower film fill section. In this manner, substantially higher velocity air will flow through the lower cooling section 106.
A mode of calculating velocity head resistance is illustrated with respect to a typical tower built in accordance with Figure 4. Film fill section 108 is four layers (air travel) of Munters No. 19060 film fill. The inner sloping film fill sections 110 and 114 are each a single layer of Munters No. 12060 film fill. The fill resistances are as published, by Munters in curves for the film fill dated July, 1986. The lower air velocity is 20% higher than the upper air velocity. This results in fill losses of 50 velocity heads in the upper and 30 velocity heads in the lower fill section. One velocity head at 400 ft/min. standard air velocity equals 0.01 in. water gauge. By adding the other fixed losses of inlet, drift eliminator and fan plenum, plus the effect of consequent difference in air velocity, the lower losses are the same as the overall upper losses due to the differences in air velocity. The outer fill layer ha~ a 25% higher water load than the inner layer, in GPM/sq.ft. The height of the upper fill is the same as the height of the lower fill. The effective fill density of the outer film f~ll section 108 is higher than that for the inner fill density in sections 110 and 114. The upper film fill has an effective film density, KaY/L, of two. The upper and lower inner film fill , , -27- 2060~31 sections have effective fill density (KaY/L), respectively, 0.45 and 0.5, but they total about half that of the outer film fill.
Referring to Figure 5, another embodiment o~ the invention is illustrated which is similar in overall structure to Figure 4. Accordingly, like parts will be designated with like numbers. In this instance, the film fill for the entire tower constitutes a single sloping film fill section 126 of the type illustrated in Figure 3 extending from the upper outboard corner of the upper cooling section to the lower inboard corner of the lower cooling section. The velocity head resistance differential is accomplished by forming the portion of the fill in the upper cooling section of 126a of a substantially higher fill density than that of the film fill 126b in the lower cooling section. In this instance, the higher capacity (KaY/L) fill 126a may be located under a higher water loading from section 118a ~accomplished by a greater number of nozzles than in section 118b). Also, since upper film fill section 126a has a higher resistance to air flow than lower film fill section 126b, this forces higher velocity air to flow through the lower cooling section 106 than the upper cooling section 104. In an alternative embodiment, not shown, fill 126 may be stepped as illustrated in Pat.
No. 4,317,785. Splash fill may be placed above or below film fill section 126.
In all of the illustrated embodiments a large amount of heat is "flashed off" in the outer cooling section even though there is a lower air velocity. The resultant higher velocity flow in the lower section is used for better cooling of inboard sections for a net overall gain in cooling. A low inner water load further assists .:
`: ~
by reducing the amount of heat compared to uniform water loading, avoiding heating of the lower gas.
Other alternative configurations may be used in accordance with the present invention. For example, other suitable film fill may be used as the outboard film fill section in the tower below the outboard liquid supply section of liquid cooling. Also, different numbers and thicknesses of stacked sloping film fill may be used compared to the ones depicted in Figures 2 and 3. In some cases, it may be desirable to install a thin low density fill layer, or even leave a void, in the area adjacent to the air inlet face. This area could typically be 1-2 meters deep, or even a little more in special cases. In all cases, water load may be uniform, or have different loading.
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`: ,
Claims (6)
1. A crossflow cooling tower for contacting liquid and gas comprising at least one upright sidewall defining a gas inlet opening means receiving substantially all the gas flowing into the tower and bisected between an upper gas inlet opening and a lower gas inlet opening, a gas outlet opening, an upper liquid crossflow cooling section including substantially all fill, selected from the group consisting of film fill, splash fill, or both, disposed between said upper gas inlet opening and said gas outlet opening at substantially the same level as said upper gas inlet opening, means for supplying gravitating liquid to said upper liquid crossflow cooling section, a lower liquid crossflow cooling section including substantially all fill, selected from the group consisting of film fill, splash fill, or both, disposed between said lower gas inlet opening and said gas outlet opening at substantially the same level as said lower gas inlet opening, said lower liquid crossflow cooling section being in open communication with said upper liquid crossflow cooling section to cause a substantial portion of liquid gravitating therefrom to contact the gas from said lower gas inlet opening in cooling relationship, the total of the fill components of the upper liquid crossflow cooling section having a gas velocity head resistance substantially higher than that of the total of the fill components of said lower liquid crossflow cooling section.
2. The crossflow cooling tower of Claim 1 in which the gas velocity head resistance of said upper liquid crossflow cooling section is at least about 110% of the gas velocity head resistance of said lower liquid crossflow cooling section.
3. The crossflow cooling tower of Claim 1 in which said liquid supply means includes adjacent outboard and inboard liquid supply sections, said outboard liquid supply section including means for supplying gravitating liquid at a flow rate per distribution area substantially higher than said inboard liquid supply section.
4. The crossflow cooling tower of Claim 3 in which said upper liquid crossflow cooling section including an outboard fill section disposed directly below said outboard liquid supply section and an inboard fill section disposed directly below said inboard liquid supply section, said outboard fill section having a substantially higher gas velocity head than the gas velocity head of said inboard fill section.
5. The crossflow cooling tower of Claim 4 in which said outboard film fill section comprises film fill.
6. The crossflow cooling tower of Claim 4 in which said outboard film fill section has a higher effective fill density than said inboard fill section.
Applications Claiming Priority (2)
| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| US65023191A | 1991-02-04 | 1991-02-04 | |
| US07/650,231 | 1991-02-04 |
Publications (1)
| Publication Number | Publication Date |
|---|---|
| CA2060531A1 true CA2060531A1 (en) | 1992-08-05 |
Family
ID=24608038
Family Applications (1)
| Application Number | Title | Priority Date | Filing Date |
|---|---|---|---|
| CA 2060531 Abandoned CA2060531A1 (en) | 1991-02-04 | 1992-02-03 | Cooling tower with different resistance to gas flow |
Country Status (1)
| Country | Link |
|---|---|
| CA (1) | CA2060531A1 (en) |
-
1992
- 1992-02-03 CA CA 2060531 patent/CA2060531A1/en not_active Abandoned
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