This invention relates to a method and apparatus for mitigating or abating plumes from cooling towers. Cooling towers are typically used to cool working fluids used in industrial processes, for instance to cool the cooling water of a power station, which is in turn used to cool condensers. The cooling water must be returned to an appropriate temperature before it is re-circulated through the condenser, and this may be achieved by rejecting heat from the cooling water into ambient air via a cooling tower.

Wet cooling towers employ evaporative cooling processes to reject heat from the fluid to be cooled, typically water. This naturally has the consequence of increasing the humidity of the air discharged from the cooling tower, and under certain conditions may result in the formation of condensate droplets as the warm, humid discharged air from the cooling tower mixes with the cooler ambient air. This phenomena varies in intensity depending on the conditions, and can result in a visible plume of water condensate which varies in height depending on the conditions. If the ambient temperature is low or the ambient humidity is high the condensation effect tends to be stronger and cooling tower plumes will be larger and more visible.

Such cooling tower plumes can be considered unsightly and may be equated with pollution. Plumes can cause low lying fog in the vicinity of the cooling tower, which may result in icing of roads where conditions result in freezing of the moisture in the plume. Condensation of moisture on objects in the vicinity of the tower may result in accelerated corrosion and failure. The presence of mist in the air is further generally considered objectionable to people within the vicinity of the tower. It is therefore desirable to control and/or reduce the occurrence and size of cooling tower plumes.

The prior art describes a number of methods by which cooling tower plumes may be abated. US 6,663,087 describes a heat exchanger and condenser for use in a cooling tower which uses ambient air to cool, and thereby condense moisture from, air from the cooling tower before it is discharged.

US 6,247,682 describes a heat exchanger which, in addition to cooling water by an evaporative process, rejects some of the heat from the water via a dry heat exchange process to increase the temperature and decrease the relative humidity of an air flow. The air flow heated in this way is subsequently mixed with the moist air used for evaporative cooling before the mixture is discharged, thereby reducing the relative humidity of the discharged air and abating plumes.

US 4,076,771 and US 3,899,553 describe arrangements for wet-dry cooling towers in which separate wet cooling and dry cooling sections are provided, which cool water by evaporative cooling and without evaporation respectively. Air from the dry cooling section will be warm and have a lower relative humidity than ambient, and air from the wet cooling section will be warm and have a higher relative humidity than ambient. The proportions of dry and wet cooling may be controlled, and the air discharged from both sections mixed to abate plume formation.

Plume abated cooling towers of the prior art require costly additional heat exchangers either to provide dry heat to air, or to reduce the temperature of the discharged air. Prior art approaches based on reducing the temperature of the discharged air cannot reduce the humidity of the discharged air to less than 100% relative humidity.

Prior art approaches to plume abatement based on adding dry heat to the discharged air draw typically use the fluid to be cooled as a source of heat. The heat in the fluid to be cooled generally comes from heat sources in which the transfer of heat is imperfect, leading to loss of heat. The fluid to be cooled also loses heat as it is transported to the heat exchanger of the dry section of the cooling tower. The present applicant has identified that such heat losses are undesirable.

According to a first aspect of the present invention, a cooling tower is provided comprising a plume abatement system which includes a burner that heats ambient air drawn from outside the cooling tower to produce heated air, wherein the plume abatement system is arranged to deliver the heated air to mix with the air within the cooling tower.

The plume abatement system may include a fan arranged to supply ambient air to the plume abatement system.

The ambient air supplied to the plume abatement system may be used by the burner as combustion air, and the remaining ambient air may be media air which is heated by the combustion products of the burner to become the heated air. The fan may be arranged to produce air flow throughout the plume abatement system, drawing ambient air from outside the cooling tower and introducing heated air into the cooling tower.

The fan may be an axial fan mounted within a duct.

The duct and fan may be arranged in a substantially vertical orientation, and ambient air may be drawn into the duct in a downward direction.

The fan may be positioned between the ambient air intake of the plume abatement system and the burner.

The burner may be located coaxially with the duct.

The burner may be oriented to fire downwards in a substantially vertical direction.

The plume abatement system may be arranged to mix the combustion products from the burner with the media air drawn from outside the cooling tower, thereby producing heated air.

The burner, fan and duct may be arranged such that the flow of media air cools and insulates the duct from the heat of the flame produced by the burner. The plume abatement system may further comprise an output duct arranged to introduce heated air into the cooling tower.

The output duct may deliver heated air into the cooling tower from a plurality of nozzles.

The output duct may split into at least two branch ducts. The nozzles may be provided on the branch ducts.

The burner fuel may be a hydrocarbon fluid. The burner fuel may be a hydrocarbon gas.

The plume abatement system may further comprise: a main fuel pipe for delivering fuel to the burner, and an automatic safety shut off valve arranged to stop fuel flow in the main fuel pipe when the burner is not lit.

The plume abatement system may further comprise a pilot igniter which is supplied with fuel from upstream of the automatic safety shut off valve and arranged to ignite the burner.

The cooling tower may further comprise a plenum room positioned between a wet cooling section and an air output of the cooling tower, wherein the plume abatement system is arranged to introduce heated air into the plenum room. The burner may be positioned within the envelope of the plenum room.

The cooling tower may further comprise a cooling tower fan, wherein the cooling tower fan is positioned between the wet cooling section and the air output of the cooling tower.

According to a second aspect of the invention, there is provided a method for use in abating plumes from cooling towers comprising the steps of: burning fuel within a burner to heat ambient air drawn from outside the cooling tower, thereby producing heated air; introducing the heated air into the cooling tower to mix with the air within the cooling tower, thereby transferring the heat from the heated air to the air within the cooling tower.

The ambient air may be heated by mixing with the combustion products from the burner.

The burner may be positioned within the envelope of the cooling tower.

The heated air may be introduced to the cooling tower between a wet cooling section and an air output of the cooling tower.

A fan may be used to drive the flow of ambient air and heated air. The fan may be a ducted axial fan. The fuel may be a hydrocarbon gas.

The heated air may be introduced to the cooling tower by a branched duct comprising a plurality of nozzles.

A cooling tower fan may be used to drive air flow from the cooling tower.

The method may be used response to the ambient air temperature and/or humidity.

By way of an example, an embodiment of the present invention will be described hereinafter in more detail.

Figure 1 is a schematic diagram of a cooling tower according to an embodiment of the present invention; and

Figure 2 is a schematic diagram of a plume abatement system according to an embodiment of the present invention.

Figure 1 shows a cooling tower comprising: a wet cooling section 20, plenum room 2 and cooling tower stack 4. The wet cooling section 20 comprises a hot water pipe 6, water nozzles 5, drip eliminator 7 and basin 21. The plenum room 2 is provided with a plume abatement system 8, and the cooling tower stack is 4 provided with a cooling tower fan 1.

The cooling tower of Figure 1 is for cooling hot water, for instance the water from a condenser in a thermal power station. The hot water conveys heat from the condenser into the cooling tower via the hot water pipe 6, and is sprayed from water nozzles 5 in the wet cooling section 20 of the cooling tower. Air at ambient temperature and humidity is drawn from outside the cooling tower into the wet cooling section 20 of the tower through the inlet port 3. The hot water sprayed from the water nozzles 5 forms droplets and rejects heat into the air by evaporative processes and by conduction. The droplets are collected at the bottom of the wet cooling section in a basin 21 at a reduced temperature for re-use in the condenser external to the cooling tower. The drip eliminator 7 is a mesh which allows the air to pass through while assisting in preventing the droplets of water from entering the plenum room 2. Despite this, some droplets will pass through the drip eliminator 7 into the plenum room 2.

The cooling tower fan 1 draws air in through the inlet port 3 and discharges the warm air from the top of the cooling tower stack 4. The inlet port 3 is remote from the output of the cooling tower stack 4 thereby avoiding mixing of the discharged warm air with the intake air, which would result in a reduction in the efficiency of the cooling tower. Although the embodiment shown in Figure 1 includes a fan, an arrangement in which the natural draft created by the stack effect is used to draw air through the cooling tower without a cooling tower fan is also envisaged. Similarly, the cooling tower stack 4 is optional, and may not be required in certain arrangements. Alternative arrangements in which at least one fan is provided at the inlet port to blow air through the cooling tower are also envisaged.

The evaporative cooling of the hot water within the wet cooling section 20 results in an increase in temperature and humidity of the air discharged from the wet cooling section 20, which tends to result in condensation as air C1 discharged from the cooling tower mixes with the ambient air which is at a lower temperature. A plume results which can vary in intensity and size depending on the ambient conditions including the ambient relative humidity and temperature.

In order to abate the intensity and size of the plume, the plenum room 2 is provided with a plume abatement system 8, which adds heat to the air discharged from the wet cooling section 20 thereby increasing the temperature and decreasing the relative humidity of the air C1 discharged from the cooling tower. The plume abatement system 8 adds heat directly to the air of the plenum room 2 using a hydrocarbon gas fuelled burner 11.

Figure 2 shows the plume abatement system, comprising a fan 9, filter 10, burner 1 , main air duct 12, branch ducts 13, nozzles 14, and fuel pipe 15. The fuel pipe 15 is provided with an automatic safety shut off valve 16. A pilot igniter 17 is provided, fuelled from the fuel pipe 15 upstream of the automatic shut off valve 16. A pilot safety shut-off valve 19 is provided on the fuel line to the pilot igniter 17. The fan 11 is a coaxial fan mounted within the main air duct 12, and is powered by an electric motor. The fan 11 draws ambient air C2 from outside the cooling tower through the filter 10 into the main air duct 12. Some of the ambient air C2 drawn into the main air duct 12 is used as combustion air C3 by the burner 11 , with the remaining media air C4 passing around the burner 11. The burner 11 is fuelled by hydrocarbon gas via a fuel pipe 15, and is coaxially mounted within the main air duct 12.

In order to ensure that substantially all of the heat output from the burner 11 is added to the air within the cooling tower, the burner 11 is positioned within the envelope of the cooling tower being on the interior side of the plenum room wall 18. The burner 11 is vertically oriented and directs the flame C5 and resulting combustion products C6 downwards. The media air C4 mixes with the combustion products C6 and is heated by the flame C5 to produce heated air C7. Although the combustion products C6 of the burner 11 include water vapour, the heated air C7 is of increased temperature and has a relative humidity lower than the air leaving the wet cooling section 20.

Controlling the relative proportions of media air C4 and combustion products C6 allows the temperature of the heated air C7 to be controlled. Adding more media air C4 to the combustion products C6 results in a greater volume of heated air C7 air at lower temperature. The increased volume of heated air C7 results in greater flow rates from the nozzles 14 which results in improved mixing with the cooling tower air. Reducing the temperature of the heated air C7 may further reduce unwanted heat losses from the heated air C7, for instance to the duct 12 and plenum room wall 18. The media air C4 passing around the flame C5 produced by the coaxially located burner 11 further cools the walls of the duct 12 and insulates the walls from the combustion products C6 thereby preventing the duct 12 experiencing high temperatures. Increasing the flow rate of the media air C4 will tend to decrease the temperature of the walls of the duct 12 for a given heat output at the burner 11. The automatic safety shut off valve 16 is arranged to shut off the flow of fuel to the burner 11 when the burner is not lit. The fuel pipe 15 introduces gas to the burner nozzle (not shown) within the burner 11. To ignite the burner 11 , an automatic pilot gas igniter 17 is disposed through the wall 18 of the plenum room 2 and the main air duct 12, into the firing zone of the burner 11. The pilot gas igniter 17 uses fuel taken from the fuel pipe 15 upstream of the automatic safety shut off valve 16, and is provided with a pilot safety shut-off valve 19. The heated air C7 flows in the main duct 12 through a wall 18 of the plenum room 2, and subsequently into ducts 13 which branch from the main duct 12. The branch ducts 13 are provided with a plurality of nozzles 14 through which the hot air C7 is discharged into the plenum room 2 to mix with the warm humid air of the cooling tower. The resulting air mixture is of increased temperature and decreased relative humidity, and water is less likely to condense from this air mixture when it is discharged from the cooling tower (compared with discharged air to which hot air from the plume abatement system has not been added).

An apparatus and method have been described for directly providing adding heat from a burner into cooling tower air before it is discharged, thereby increasing the temperature and decreasing the relative humidity of the discharged air. The increased temperature and decreased relative humidity result in reduced condensation when the discharged air mixes with the ambient air, the consequence of which is an abatement of the visible plume from the cooling tower.

The embodiments described above are given by way of examples only, and various other modifications will be apparent to persons skilled in the art without departing from the scope of the invention, as defined by the appended claims.

The direct introduction of heat according to an embodiment of the present invention results in less heat loss than delivering heat indirectly obtained from other sources of combustion as disclosed by the prior art. In addition, an embodiment of the present invention is of lower cost and is more straightforward to retrofit to an existing cooling tower.