GB1604389A

GB1604389A - Cooling towers

Info

Publication number: GB1604389A
Application number: GB35518/77A
Authority: GB
Original assignee: Film Cooling Towers 1925 Ltd
Current assignee: Film Cooling Towers 1925 Ltd
Priority date: 1978-05-31
Filing date: 1978-05-31
Publication date: 1981-12-09

Description

(54) IMPROVEMENTS IN OR RELATING TO COOLING TOWERS (71) We FILM COOLING TOWERS (1925) LIMITED, a British Company of Chancery House, Parkshot, Richmond, Surrey, do hereby declare the invention for which we pray that a patent may be granted to us, and the method by which it is to be performed, to be particularly described in and by the following statement:- The present invention relates to water cooling towers and to methods of operating water cooling towers.

In the evaporative cooling of water by contact with air in cooling towers warm saturated air generally leaves the cooling tower discharge stack at near 100% relative humidity. This warm air discharge from the cooling tower mixes with the surrounding air and cools to ambient temperature.

Under summer conditions, the cooling of this discharge air takes place at or below the saturation level of the surrounding air and little or no visible plume occurs above the cooling tower stack. In winter conditions of air temperature, however, the water vapour content of the air discharge from the tower rises above the saturation level of the surrounding air during the cooling process, condensation of the discharge water vapour takes place and moisture in the discharge is precipitated as micro-droplets, These droplets result in fogging above the stack and extensive and undesirable visible plumes.

Methods for reducing the visible plume discharge from cooling towers have been proposed. Generally, such known methods seek to reduce the relative humidity of the air discharge from the cooling tower, for example, by providing a stream of warm dry or relatively dry air for discharge from the tower with the saturated or near saturated air discharge from the packing section.

In one known method a warm dry air stream is produced by inducing a flow of air across a heat exchanger to which the water to be cooled by the tower is initially fed. The water discharged from the heat exchanger is subsequently distributed over an evaporative packing section from which a saturated or near saturated air stream is discharged.

The two air streams produced are then discharged from the tower.

However, it has been found that such methods do not substantially reduce visible plume because the two air streams do not efficiently mix. It has therefore been proposed that the tower be designed to ensure that the two air streams meet "head-on" to enhance mixing.

The applicants have found that even where the two air streams are arranged to flow in opposite directions so that they collide within the tower the discharge from the tower may still be stratified into separate streams of warm dry air and saturated air, the saturated air causing plume in some atmospheric conditions. Furthermore, it is not always possible or commercially viable to redesign a cooling tower so that the two air streams can be arranged to flow in opposite directions.

It is an object of the present invention to provide a cooling tower and a method of operating a cooling tower in which a warm dry air stream and a saturated air stream can be effeciently mixed before being discharged to atmosphere.

According to a first aspect of the present invention there is provided a method of operating a water cooling tower having an evaporative packing section and a heat exchanger, comprising the steps of directing water to be cooled to the packing section so that the water flows through the packing section, inducing a first stream of air through the packing section to cool the water, the first stream of air being arranged to flow through said packing section in cross flow to the flow of water, inducing a second stream of air through the heat exchanger in indirect heat exchange relationship with a fluid flowing through the heat exchanger, wherein said first and second streams of air are induced by at least one centrifugal fan mounted within the cooling tower between the packing section and the heat exchanger such that said first stream is arranged to be substantially parallel to and in the opposite direction to the second stream whereby said first and second streams collide upstream of said fan, the fan being arranged to mix the first and second streams of air and to discharge the mixed air streams to atmosphere.

With a method of the invention the warm dry of relatively dry airstream from the heat exchanger is arranged to converge with the saturated or near saturated airstream from the packing section to bring about a pre luminary mixing of the two airstreams. tow- ever, the two airstreams are then further mixed by the centrifugal fan which discharges the mixed airstreams to atmosphere. In this manner efficient mixing of the two airstreams is achieved. Furthermore, as the fan arranged to mix the airstream is also the fan which induces the airstreams there is little extra cost in providing a cooling tower for operation in this manner.

If even more efficient mixing of the two airstreams is required or found to be necessary turbulence or swirl may be induced in the airstreams before they are mixed by the fan. For example, turbulence inducing means such as vanes or dampers may be mounted upstream of the fan.

In an embodiment, the fluid within the heat exchanger is the water to be cooled by the tower which may be fed to the heat exchanger initially and subsequently directed to the packing section.

According to a further aspect of the present invention there is provided a water cooling tower comprising an evaporative packing section, water distribution means arranged above said packing section, at least one centrifugal fan arranged adjacent said packing section to induce a first stream of air therethrough in cross flow to the flow of water, an indirect heat exchanger mounted on the opposite side of the fan to the packing section such that the fan is arranged to induce a second stream of air through the indirect heat exchanger, said first and second streams of air being substantially parallel and in opposite directions such that said first and second streams are arranged to collide within the tower upstream of the fan, and wherein the centrifugal fan is arranged to mix the first and second streams of air and to discharge the mixed stream to atmosphere.

Preferably, means for inducing turbulence in the streams of air are mounted downstream of the heat exchanger andlor of the packing section. For example, said turbulence inducing means are adjustably mounted vanes or dampers. In an embodiment, dampers movable between an open position and a closed position are mounted downstream of the heat exchanger such that they isolate the heat exchanger in their closed position. lhese dampers can be arranged to induced turbulence in said second airstream in their open position.

During summer months when ambient air temperatures are high the embodiment of the cooling tower described above can be operated with the dampers closed so that the heat exchanger is isolated. The heat exchanger is preferably also shut down. The air discharged from the evaporative packing section, which will be saturated or very close to saturation, is discharged directly to atmosphere by the fan but will produce little or no visible plume discharge from the tower because of the high ambient temperature. When ambient temperatures are low, for example, in winter months, the dampers are opened and a warm heat exchange medium is induced to flow through the heat exchanger. The fan then induces the airstreams through both the packing section and the heat exchanger, mixes the air discharged from both the packing section and the heat exchanger and discharges the mixture to atmosphere.The air discharged from the packing section will be saturated or nearly saturated whilst the air discharge from the heat exchanger will have been heated without any increase in its moisture content and will therefore be at a lower humidity but higher temperature than the ambient air.

The air mixture discharged by the fan from the tower will thus be substantially below saturation level and thus, as the air mixture is cooled by the ambient air there will be either a minimal or no excess moisture so that there will be either a minimal visible plume or no visible plume discharged from the tower.

The heat exchange medium for the heat exchanger can be chosen as required. In a preferred embodiment the heat exchange medium is provided by a percentage of the water to be cooled by the tower.

In an embodiment further adjustable dampers are arranged at the air intake of the packing section to control air flow through the packing section.

Preferably, a water distribution system is provided and is arranged to selectively distribute water to both said distribution means of the packing section and said heat exchanger. The water distribution system may include control means for controlling the ratio of water distributed to the packing section to water distributed to the heat exchanger.

When the water to be cooled is distributed to both the packing section and the heat exchanger it is possible to directly mix the cooled water discharged from the packing section and the heat exchanger. Alternatively, the water discharged from the heat exchanger can then be fed to the packing section for further cooling. It is also possible to feed all the water to be cooled to the heat exchanger initially and then to feed the water charged from the heat exchanger to the packing section for further cooling. In all of these modes of operation the air dis charged from the heat exchanger will be warmed dry or relatively dry air whereas the air discharged from the packing section will be saturated or near saturation.

An embodiment of the present invention will hereinafter be descnbed, by way of example, with reference to the accompany mg arawmgs, m wnicfl:- Figure 1 is a psychometric chart illustrating the effect on visible plume discharge of evaporative cooling at high or low ambient air conditions, Figure 2 is a diagrammatic vertical section through an induced draught cooling tower of the present invention, and Figure 3 is a psychometric chart illustrating a typical mode of operation of the tower of Figure 2 whereby water flow passes firstly through an indirect heat exchanger then through an evaporative section of the tower under the same duty conditions as case "C" of Figure 1.

Referring to the drawings, Figure 1 shows a standard psychometric chart in which the properties of air and water mixtures are represented graphically. In this chart absolute humidities grains of water/lb of air) are plotted against dry bulb temperatures (CF) and lines of constant relative humidity and wet bulb temDeratures added. Curve S (marked 100%) gives the absolute humidities of saturated air at various temperatures and is known as the "saturation line". Any cooling of air and water mixtures which takes place to the left of this saturation line can occur only with the precipitation of water contained in the cooling air.

The line marked B shows the cooling line of air which leaves the stack of a cooling tower at 100% relative humidity with a wet bulb temperature of 84.2"F (point Bis). The air cools along the line B to an embient wet bulb temperature at point B2 of 69.0 F dry bulb and 72% of relative humidity; the cooling line does not pass to the left of the saturation line S and little or no fogging or plume discharge occurs.

The line marked C shows the normal cooling of air discharged from the cooling tower stack at near 100% relative humidity and 67.7"F wet bulb temperature (point Cl) and cooling to an ambient wet bulb temperature of 34 dry bulb and 90% relative humidity at point C2. Due to the super-saturated nature of the discharge air and the low ambient temperature most of the cooling takes place to the left of the saturation line S; moisture in the discharge air condenses as microdroplets, causing fogging and extensive plume. The excess moisture in the air and extent of fogging is indicated at "A".

Figure 2 shows a vertical section through an induced draught cooling tower which can be operated to reduce the fogging indicated on Figure 1. The tower shown in Figure 2 has a housing 2 in which an evaporative packing 4 is mounted. Conveniently, the packing 4 may be moulded from plastics material. The open front face 6 of the housing adjacent the packing 4 is provided in conventional manner wi a plurality of inlet louvres (not shown) to allow ingress of air to the packing 4. The face of the packing 4 remote from the inlet loutres is provided in conventional manner with a drift eliminator (not shown).

Hot water to be cooled is fed to a water inlet 8 which is arranged to discharge into a distribution pan 10 arranged substantially horizontally and above the packing 4. The cooled water from the packing 4 is channelled into a collection tank 12 which is arranged at a level beneath the packing 4. In the embodiment illustrated the tank 12 is actually arranged downstream of the pack ing 4.

An induced draught centrifugal fan 14 is mounted in the housing 2 downstream of the packing 4. The fan 14, which is arranged in cross flow to the direction of air flow through the packing 4, is arranged to discharge air into a substantially vertical discharge outlet 16. The fan 14 is driven by a motor 18 by way of a belt drive. The motor 18 is mounted externally of the housing 2 on rails 20 for ease of movement.

The open rear face 22 of the housing 2 is provided with a plurality of horizontally mounted adjustable louvre-type dampers 24 which may be moved from the fully open position shown in Figure 2 to the fully closed position indicated by dotted lines in Figure 2. In their fully closed position the dampers 24 close the rear face 22 of the housing 2 to flow of air.

An indirect heat exchanger 26 in the form of a bank assembly of finned tubes is supported by the housing 2 externally of the rear face 22. Means (not shown) are provided to supply a heat exchange fluid to said heat exchanger 26.

A plurality of adjustable louvre-type dampers 28 are also provided on the front face 6 of the housing 2. These dampers 28 are movable between the fully open position shown in Figure 2 and a fully closed position indicated in dashed lines in Figure 2.

It will be seen that if the dampers 28 on the front face 6 of the housing 2 are positioned in their fully open position and the dampers 24 on the rear face 22 are positioned in their fully closed position the cooling tower shown in Figure 2 can be operated in conventional manner to cool water. Thus, the fan 14 is driven to induce a substantially horizontal flow of air through the packing 4, the air discharged from the rear face of the packing 4 being discharged by the fan 14 through the outlet 16. Water to be cooled is fed to the distribution pan 10 by the inlet 8 and the water flows under gravity down through the packing 4 so that it is cooled by evaporation. The cooled water is collected in the tank 12.As discussed above with reference to Figure 1, this mode of operation is acceptable at high ambient temperatures as the saturated or near saturated air discharged causes little or no fogging to occur adjacent the outlet 16.

If the ambient temperatures drop, for example, below 45"F, the mode of operation of the tower shown in Figure 2 can be varied so that there is still little or no flogging. For example, the dampers 28 are moved to a partially opened position and the dampers 24 are moved to an open position so that the fan 14 now also induces a horizontal air flow through the heat exchanger 26 and hence to the outlet 16. Due to the reduction in air rate thus caused through the packing 4, the static pressure drop therein is reduced and the total air flow is increased at constant fan speed.

At the same time a heat exchange fluid is caused to flow through the heat exchanger 26. Conveniently, this fluid is a percentage of the water delivered by the inlet 8 which can be diverted to the heat exchanger 26 by control means (not shown).

The cooled water discharged from the heat exchanger 26 can be discharged directly into the tank 12 or it can be fed to the distribution pan 10 for a further cooling by the packing 4.

Figure 3 illustrates the effects on visible plume discharge when the tower is operated in this manner. Figure 3 assumes that the heat load situation is identical to that shown in Figure 1. With the dampers 24 open the air flow will be spilt so that approximately 50% flows through the packing 4 and approxl mately 50% flows through the heat exchanger 26. The heat load is likely to be greater through the packing 4 and as illustrated is assumed to be 75% of the total.

Air enters both the packing 4 and the heat exchanger 26 at the ambient conditions of 34"F and 90% humidity. It leaves the packing 4 at 71.3QF saturated, point Cl and the heat exchanger 26 at 66off dry bulb and approximately 27.5% humidity, point Dl.

The air flows from the heat exchanger 26 and the packing 4 collide in the fan plenum and are mixed by the fan 14 both in the fan plenum and in the outlet 16. The air at condition Dl is therefore humidified along the line E whilst the air at condition Cl is dehumidified along the line G to the final discharge air condition of 61.7"F dry bulb and humidity of 67%, point Fl. The discharge air then cools outside the tower along the line F back to the ambient condition C2 without an excess of moisture to cause precipitation of droplets and fogging.

In the operational mode described above a percentage of the water delivered by the inlet 8 is diverted into the heat exchanger 26 to act as the heat exchange medium. If required, all of the water delivered by the inlet 8 can be fed to the heat exchanger 26.

If the necessary cooling can be effected by the heat exchanger 26 alone, the water discharged from the heat exchanger 26 can be channelled into the tank 12. In this case the dampers 28 on the front face 6 of the housing 2 are closed to shut off air flow through the packing 4. If the heat exchanger 26 cannot cool the water sufficiently the water discharged therefrom can be fed to the distribution pan 10 for further cooling b the packing 4. In this case the dampers 2 will be either fully or partially open. The ratio of the air flow through the packing 4 to that through the heat exchanger 26 can be varied as required by movement of the dampers 24 and28.

It will be appreciated that the actual operational mode of the cooling tower will depend both upon the ambient temperature and upon the actual cooling duty required.

However, it will be seen that the cooling tower described above can always be operated to produce, at most, a minimal visible plume discharge.

The dampers 24 and 28 can be operated either manually or automatically. It required the dampers 24 and 28 could be operated by a control device (not shown) sensitive to ambient temperatures. Similarly, valved conduits (not shown) could be associated with the water inlet 8 and with the outlet of the heat exchanger 26 whereby the path taken by the water to be cooled could be varied as necessary. The water control valves could, of course, be operated manually or automatically. Again, automatic operation of the water control valves could be achieved by way of a control device sensitive either to ambient temperatures or to the position of the dampers.

As indicated above, the heat exchanger 26 can employ any suitable,heat exchange medium. It the water to be cooled does not act as the heat exchange medium the heat exchanger will act as a separate source of heat for providing warm dry air for mixing with the saturated or near saturated air discharge from the packing. If the heat exchanger is used with a heat exchange fluid other than the water to be cooled the heat exchanger could be used continuously during all ambient conditions, with the dampers 24 simply being used to adjust the air flow through the heat exchanger. This mode of operation would be advantageous if the heat exchange fluid had a minimum operational temperature below which there would be a risk of solidification.The cooling of the heat exchange fluid could be reduced under low ambient conditions by control of the air flow to maintain the minimum required temperature in the heat exchange fluid.

The applicants have found that in the embodiment shown in Figure 2 the centrifugal fan 14 mixes the air flows from the packing 4 and the heat exchanger 26 efficiently so that the air discharged through outlet 16 is mixed and not stratified. It is important that there is no stratification of the two air flows if pluming is to be reduced.

The mixing of the two air flows can be further enhanced by mounting the dampers 24 to be vertically pivotable (not illustrated) so as to introduce swirl in the air flow to the plenum chamber of the fan 14 to enhance mixing. Alternatively or additionally, further dampers or vanes (not shown) could be mounted in the housing 2 between the heat exchanger 26 and the fan 14 to introduce turbulence into the air flow to enhance the mixing. Similarly, further dampers or vanes (not shown) could be mounted between the packing 4 and the fan 14. For example, the drift eliminator (not shown) could be designed to introduce the required degree of turbulence to the air flow through the packing.

It will be appreciated that in the tower illustrated warm dry air can be collided with the saturated or near saturated air from the packing. The two air flows are then mixed by way of the fan so that the air discharged to atmosphere is efficiently mixed without stratification so that visible plume is effectively reduced. If the mixing produced by the fan is found to be insufficient turbulence inducing means may be provided upstream thereof to enhance mixing.

A fan assisted natural draught tower is described and claimed in our copending application No. 8102710 Serial No.

1604390 which was divided from the present application.

WHAT WE CLAIM IS: 1. A method of operating a water cooling tower having an evaporative packing section and a heat exchanger, comprising the steps of directing water to be cooled to the packing section so that the water flows through the packing section, inducing a first stream of air through the packing section to cool the water, the first stream of air being arranged to flow through said packing section in cross flow to the flow of water, inducing a second stream of air through the heat exchanger in indirect heat exchange relationship with a fluid flowing through the heat exchanger, wherein said first and second streams of air are induced by at least one centrifugal fan mounted within the cooling tower between the packing section and the heat exchanger such that said first stream is arranged to be substantially parallel to and in the opposite direction to the second stream whereby said first and second streams collide upstream of said fan, the fan being arranged to mix the first and second streams of air and to discharge the mixed air streams to atmosphere.

2. A method as claimed in Claim 1, further comprising the step of inducing turbulence in one or both of said air streams upstream of the fan.

3. A method as claimed in any preceding Claim, wherein the water to be cooled initially flows through the heat exchanger and is subsequently directed to the packing section.

4. A water cooling tower comprising an evaporative packing section, water distribution means arranged above said packing section, at least one centrifugal fan arranged adjacent said packing section to induce a first stream of air therethrough in cross flow to the flow of water, an indirect heat exchanger mounted on the opposite side of the fan to the packing section such that the fan is arranged to induce a second stream of air through the indirect heat exchanger, said first and second streams of air being substantially parallel and in opposite directions such that said first and second streams are arranged to collide within the tower upstream of the fan, and wherein the centrifugal fan is arranged to mix the first and second streams of air to discharge the mixed stream to atmosphere.

5. A cooling tower as claimed in Claim 4, wherein means for inducing turbulence in the streams of air are mounted between the heat exchanger and the fan.

6. A cooling tower as claimed in Claim 4 or 5, wherein means for inducing turbulence in the streams of air are mounted between the packing section and the fan.

7. A cooling tower as claimed in Claim 5 or 6, wherein said turbulence inducing means are adjustably mounted vanes or dampers.

8. A cooling tower as claimed in any of Claims 4 to 7, further comprising a water distribution system arranged to selectively distribute water to said water distribution means of said packing section or to said heat exchanger.

9. cooling tower as claimed in Claim 8, wherein said water distribution system is also arranged to selectively distribute water to both said water distribution means and said heat exchanger.

10. A cooling tower as claimed in Claim 9, wherein the water distribution system further comprises control means for controlling the ratio of water distributed to the packing section to water distributed to the heat exchanger.

11. A cooling tower as claimed in any of Claims 4 to 7, further comprising a water distribution system arranged to feed water to said heat exchanger, and means feeding

**WARNING** end of DESC field may overlap start of CLMS **.

Claims

**WARNING** start of CLMS field may overlap end of DESC **. to maintain the minimum required temperature in the heat exchange fluid. The applicants have found that in the embodiment shown in Figure 2 the centrifugal fan 14 mixes the air flows from the packing 4 and the heat exchanger 26 efficiently so that the air discharged through outlet 16 is mixed and not stratified. It is important that there is no stratification of the two air flows if pluming is to be reduced. The mixing of the two air flows can be further enhanced by mounting the dampers 24 to be vertically pivotable (not illustrated) so as to introduce swirl in the air flow to the plenum chamber of the fan 14 to enhance mixing. Alternatively or additionally, further dampers or vanes (not shown) could be mounted in the housing 2 between the heat exchanger 26 and the fan 14 to introduce turbulence into the air flow to enhance the mixing. Similarly, further dampers or vanes (not shown) could be mounted between the packing 4 and the fan 14. For example, the drift eliminator (not shown) could be designed to introduce the required degree of turbulence to the air flow through the packing. It will be appreciated that in the tower illustrated warm dry air can be collided with the saturated or near saturated air from the packing. The two air flows are then mixed by way of the fan so that the air discharged to atmosphere is efficiently mixed without stratification so that visible plume is effectively reduced. If the mixing produced by the fan is found to be insufficient turbulence inducing means may be provided upstream thereof to enhance mixing. A fan assisted natural draught tower is described and claimed in our copending application No. 8102710 Serial No. 1604390 which was divided from the present application. WHAT WE CLAIM IS:

1. A method of operating a water cooling tower having an evaporative packing section and a heat exchanger, comprising the steps of directing water to be cooled to the packing section so that the water flows through the packing section, inducing a first stream of air through the packing section to cool the water, the first stream of air being arranged to flow through said packing section in cross flow to the flow of water, inducing a second stream of air through the heat exchanger in indirect heat exchange relationship with a fluid flowing through the heat exchanger, wherein said first and second streams of air are induced by at least one centrifugal fan mounted within the cooling tower between the packing section and the heat exchanger such that said first stream is arranged to be substantially parallel to and in the opposite direction to the second stream whereby said first and second streams collide upstream of said fan, the fan being arranged to mix the first and second streams of air and to discharge the mixed air streams to atmosphere.

water discharged from said heat exchanger to said distribution means of said packing section.

12. A cooling tower as claimed in any of Claims 4 to 1 I, wherein a plurality of adjustable dampers are mounted downstream of the heat exchanger and upstream of the fan to isolate the heat exchanger from the fan in their closed position.

13. A cooling tower as claimed in Claim 12, wherein said dampers are arranged to induce turbulence to the second stream of air in their open position.

14. A method of operating a water cooling tower substantially as hereinbefore described with reference to the accompanying drawings.

15. A water cooling tower substantially as hereinbefore described with reference to and as illustrated in Figure 2 of the accompanying drawings.