Patent Publication Number: US-2012037338-A1

Title: Evaporative cooling device

Description:
The present invention relates to an evaporative cooling device, that is not liable to harbour the bacteria responsible for various types of legionnaires disease, and which has good heat-exchange capacity. 
     Legionella pneumophilia is an aerobic bacterium, common in nature, that can cause a potentially fatal Pneumonia type infection when inhaled into the lungs on an aerosol. Evaporative condensers and evaporative cooling towers are one possible source of such Legionella-containing aerosols. Outbreaks tend to occur when the condenser or cooling tower becomes heavily infected with the bacteria. On the other hand, if the equipment cannot allow growth of Legionella within it, no infections will occur, even if some aerosols are produced. Conditions that favour growth of Legionella are volumes of stagnant water, especially if nutrients such as dust and products of decomposition are present. Such conditions can be present in conventional cooling towers and evaporative condensers, especially if they are shut down for a period of time. 
     Evaporative condensers are perhaps the most effective method of rejecting heat of condensation to the atmosphere. However, use of evaporative condensers is hindered by the cost of providing regular treatment of the cooling water with biocide and the need to inspect the condenser at regular intervals to ensure that bacteria are not growing in it. Widespread adoption of such coolers would produce significant reductions in power required to operate refrigeration systems and would reduce the carbon footprint of systems using them in preference to air-cooled condensers. 
     It is an object of the present invention to provide an evaporative cooler that does not contain stagnant cooling water and which would therefore not require dosing with biocide to prevent the growth of bacteria. 
     An evaporative cooling device has been described in our previous patent application GB2027529, wherein cooling water flows downwardly over a nest of heat exchange tubes and is entrained into an upwardly flowing air flow, which breaks the cooling water into droplets. Some of the cooling water evaporates into the airflow, thereby producing a cooling effect. Air is introduced through a perforated plate situated below the tube nest. The tubes are randomly placed with respect to the perforations with the result that liquid droplets entrained by the upwardly flowing gas are intercepted quickly by the tubes and form a mass of aerated water though which bubbles rise. This restricts the height of the water mass, for a given air velocity. This, therefore, restricts the efficiency of the heat exchanger. 
     Other evaporative cooling devices, where water flows down over heat exchange tubes in counter-current to an upwardly flowing gas are disclosed in patent specifications U.S. Pat. No. 4,969,507, U.S. Pat. No. 4,434,112, CN101338981 and US2006/0192305. However, these specifications do not use a restrictor to direct the upward gas flow. 
     It is an object of the present invention to improve the heat-exchange efficiency of such evaporative coolers. 
     Generally speaking, the present invention is based on the discovery that improved efficiency can be obtained by providing substantially straight upwardly directed gas flow channels through the restrictor and the heat exchange elements. 
     In particular the present invention provides an evaporative cooler for cooling a fluid contained in an array of spaced heat exchange elements by heat transfer to a cooling liquid flowing downwardly around the elements in counter current flow to an upwardly flowing gas; which comprises
         a casing housing the array of spaced heat exchange elements, the gas flowing upwardly through the casing, and humid gas leaving the casing at an upper end;   a restrictor comprising a series of apertures located below the array of spaced heat exchange elements; the spaces between the heat exchange elements being aligned with the apertures, such as to provide substantially straight upwardly directed gas-flow channels through the heat exchange array gas being drawn into the casing through the restrictor apertures and producing a pressure drop sufficient to inhibit loss of cooling liquid from a lower end of the cooler; and   a coalescer located above the heat exchange array, such that liquid droplets entrained in the upward gas flow are captured and coalesced to a size whereby coalesced liquid droplets falls down onto the heat exchange array.       

     Because the restrictor apertures and the spaces between the heat exchange elements are aligned to provide unencumbered upwardly directed gas-flow channels, for a given air velocity, liquid droplets may be projected upwardly to a considerable greater height, generally up towards the upper end of the array of heat exchange of elements or into the coalescer itself. This results in a flow of liquid in the form of a liquid film or large liquid droplets downwardly over substantially the full height of the heat exchange array. This gives good heat transfer efficiency between the heat exchange elements and the cooling liquid and also allows a higher heat exchange array to be used. 
     Generally, the upwardly directed gas channels have a straight unencumbered height up to at least 50% of the heat exchanger array depth, particularly at least 80%, especially at least 90% and more preferably at least 95% of the depth. 
     However, in a preferred embodiment, the elements in the last 20% of the height, preferably 10%, particularly 5% (and particularly the elements in the uppermost rows) of the height are positioned in the channels, such that the upwardly moving entrained liquid droplets impinge upon these last heat exchanger elements, thereby removing some of the load from the coalescer. 
     The cooling liquid is preferably water and it has been found that the terminal velocity of large droplets of water in atmospheric air is of the order of 9 m/s. The gas is preferably air. If the inlet gas velocity is greater than this speed, then liquid droplets will not fall out through the restrictor apertures and therefore the cooling liquid will be retained within the casing. Preferably the air inlet velocity is at least 9 m/s, though high velocities should be avoided to minimise power consumption. 
     Cooling liquid evaporates, producing a cooling effect, and is lost from the system within the humid gas which leaves the upper end of the casing. Means are therefore generally provided to introduce make-up liquid into the casing. In order to promote the even distribution of liquid across the cross-section of the casing, the liquid may be introduced into the gas flow via a series of sprays positioned at a lower end of the casing, particularly below the array of heat exchange elements or below the restrictor. In this way a fine mist of liquid is drawn into the cooler and the liquid is well distributed within the cooler. This also has the advantage that the fine mist is easily drawn up to a significant height in the heat exchanger array, without excessive gas pressure drop. 
     The casing is generally rectangular in cross-section, open or with a gas inlet at a lower end and a humid gas outlet at an upper end. Gas is generally drawn through the casing by means of a fan located at the upper or a lower end of the casing. 
     The fluid to be cooled is contained within the array of heat exchange elements and is generally under pressure. The fluid may be a conventional volatile refrigeration fluid or could be a supercritical fluid, such as trans-critical carbon-dioxide. The heat exchange elements may take the form of any hollow duct, particularly tubes, hollow plates etc. These are generally arranged in a regular array in a spaced configuration. Hollow plates will usually be positioned substantially vertically and spaced apart to provide the upward gas-flow channels. Each plate may be formed from panels fused or welded together (with a peripheral seam) so as to provide cooling liquid flow channels; for example, in the form of pillow panels. The downwardly flowing cooling liquid will generally flow as a film over such hollow plates. A combination of hollow plates and tubes may be used. 
     The restrictor may comprise a series of slots created between spaced restrictor strips, and will be formed such that the slots match the spaces between the heat exchange elements. A single strip may be attached beneath each heat exchange element in a lowermost row e.g. by means of a spring clip. Alternatively, a strip may be attached below two or more adjacent lowermost heat exchange elements. They are constructed such as to avoid any stagnant pools of cooling liquid. Alternatively, the restrictor may comprise series of rows of holes. Generally, the restrictor apertures will be as wide as the spacing between the heat exchange elements. The width of the apertures is typically from 5 to 20 mm, e.g. 7 to 15 mm. However, the spacing of one could be from 50 to 200% of the other, especially from 75 to 150%. 
     The spaces between the heat exchange elements are aligned with the restrictor apertures, such as to provide substantially straight upwardly directed gas-flow channels through the heat exchange array. As mentioned previously, the channels need not extend the full height of the heat exchange array but will generally extend through at least half of it. The upwardly directed channels are not necessarily vertical, but may be inclined up to 10°, particularly up to 5° to the vertical and still provide effective entrainment of liquid droplets. In the same way, the channels may not necessarily be absolutely straight, provided the effective entrainment of liquid droplets to the upper regions of the heat exchange array is not substantially hindered. This construction has the effects that liquid droplets are projected high up into the array of heat exchange elements and therefore flow back downwards through substantially the full height of the heat exchange array. Also, in the present invention the liquid tends not to froth, but flows back downwardly in the form of large droplets of liquid or a film of liquid over the heat exchange tubes, and so improves the efficiency of the heat exchange process. This limits the pressure drop and reduces the fan power required for a given height of heat exchange array. The fan noise may be reduced. It also allows a higher heat exchange array (e.g. from 8 to 18 rows) to be used. Generally, the upwardly flowing air stream does not directly impinge on the heat exchange elements but the gas and entrained droplets flow upwardly past the elements. 
     As mentioned previously, make up liquid may be provided to compensate for evaporated liquid lost with the humid exiting gas. Make up liquid may also be provided to avoid a build up of dissolved solids within the cooling liquid. Means may be provided for removing cooling liquid from the lower end of the heat exchange array. 
     The make-up liquid may be provided using an arrangement such as that shown in GB2027529, where make-up water is only drawn into the cooler when it is in operation. However, biocide dosing of the small make-up tank may still be required by the authorities. It is therefore preferred to provide an alternative make-up arrangement. Preferably, the present invention employs a pressure operated liquid value, which supplies liquid into the cooler in response to a rise in pressure in the high pressure fluid in or connected to the heat exchange array and sensed by a pressure sensor. 
     The invention also relates to a corresponding process of cooling a fluid. 
    
    
     
       Embodiments of the present invention will now be described by way of example only, with reference to the attached drawing wherein. 
         FIG. 1  is a schematic cross-sectional elevation of a evaporative cooler according to the present invention. 
     
    
    
     Principally, the evaporative cooler of the present invention comprises a casing  1  enclosing an array  2  of spaced hollow heat exchange elements. A gas, typically air, is introduced at a lower end of the casing through a restrictor  3 . Entrained liquid droplets are captured in a coalescer  6 . 
     The spaces between the heat exchange elements  2  are regularly aligned one above the other; and in alignment with slots  3   a  between the restrictor strips  3  at the lower end of the casing. In this way, substantially straight upwardly directed gas-flow channels are provided through the restrictor and the entire height of the heat exchange array, whereby entrained liquid droplets may be projected upwardly (without impact on any of the heat exchange elements) into the coalescer  6 , where they coalesce to form a bulk liquid, which flows downwardly as a liquid film or as large droplets onto the surfaces of the heat exchange elements. Air is drawn into the bottom of the casing through the apertures between the restrictor strips and this produces sufficient pressure drop to inhibit loss of liquid water from the bottom of the casing. The restrictor apertures are arranged so that water falling under gravity is entrained in the incoming air at the restrictor and carried up past the heat exchange elements as droplets of a size  5  that can be lifted by the air stream. The restrictor apertures  3   a  are positioned so that the air stream does not directly impinge on the heat exchange elements, though some droplets in the air stream may wet the surface of the heat exchanger. 
     The heat exchanger typically is 2 m×2 m in plan and each row contains 56 tubes at a pitch of 36 m (and 57 slots). Each restrictor slot is 2 m long and 8 mm wide to provide an airflow through each slot in the order of 15 m/s for an airflow of 14 m 3 /s. Such a condenser is able to reject 650 kw of heat. 
     Droplets carried up past the heat exchange elements are coalesced in the coalescer  6  above the heat exchange array to form drops of sufficient size  7  to fall back down onto the heat exchanger elements, such that a film  4  of water flows down under gravity to the point of air entry. The coalescer may be of the honeycomb type, but is more preferably in the form of a knitted wire pad, which is an effective coalescer and minimises undesirable stagnant liquid volumes and therefore the growth of bacteria. Generally, the coalescer is spaced about 100-200 millimetres above the top of the heat exchange array. 
     Humid water-laden air is drawn upwardly by a fan  9  and discharged from an upper end of the evaporative cooler. Substantially all of the liquid droplets should have been removed from the exiting air. 
     A pipe  8  provides make-up water to the casing; alternatively this may be supplied as a spray either above the heat exchanger or into the air inlet below the heat exchanger. The flow of make-up water is controlled by a pressure operated water valve (not shown) that opens on rise of pressure in the high pressure fluid side of the refrigeration system sensed by a pressure sensor (not shown). Excess water is removed through a U trap  11  and exits through outlet  10 . 
     Preferably, all surfaces in contact with water are made from stainless steel to prevent formation of corrosion products. Because there are substantially no stagnant volumes of liquid water, the potential for bacterial growth is minimised or eliminated. 
     In use, duty is called for from the refrigeration system and the fan operates to draw cooling air through the cooler. The compressor of the refrigeration system connected to heat exchange array starts, and the pressure rises in the high pressure refrigerant fluid. The pressure rise is sensed and water is introduced into the cooler and distributes throughout the heat exchange array by the effect of the air flow. Droplets are carried upwards in the gas flow channels and are captured and coalesced in the coalescer; before flowing back down as a liquid film or large droplets. Water is largely prevented from leaving the lower end of the cooler due to the air velocity in the slots in the restrictor. 
     In an alternative embodiment (not shown), the top layer of heat exchange elements are off-set, so as to lie within the upwardly directed gas flow channels, so that entrained liquid droplets impinge against these last heat exchange elements, thereby taking some of the loading from the coalescer. 
     The evaporative cooler of the present invention may be used as a closed circuit liquid chiller; or in a refrigeration apparatus.