Abstract:
The heat exchange apparatus and method is provided with an indirect evaporative heat exchange section and a direct evaporative heat exchange section. An evaporative liquid is sprayed downwardly into the direct evaporative section to directly exchange heat from the evaporative liquid flowing across fill sheets. The evaporative liquid is then collected in a re-spray tray. The collected evaporative liquid is then sprayed onto an indirect evaporative heat exchange section to indirectly exchange sensible heat from a fluid stream flowing within a series of enclosed circuits comprising the indirect evaporative heat exchange section.

Description:
BACKGROUND OF THE INVENTION  
       [0001]     The present invention relates to a heat exchanger and a method of exchanging heat and, more particularly, to an evaporative heat transfer apparatus comprising a direct evaporative heat exchange section and an indirect evaporative heat exchange section.  
         [0002]     Evaporative heat transfer units comprising both direct and indirect heat transfer sections are disclosed in U.S. Pat. No. 5,435,382. This patent discloses a design that allows the collection of the evaporative liquid from the direct evaporative section and then pumping it upwardly to redistribute it over the indirect evaporative section. Two limitations exist with the prior art described in this patent. First, the evaporative fluid must be pumped upwardly from the collection basin located below the direct evaporative section for distribution over the indirect evaporative section. This means the indirect evaporative section must be located in the upper section of the heat exchange apparatus. While this arrangement provides benefits for accessibility of the indirect section after installation, it puts additional requirements on the apparatus structure to support the mass of the indirect section at higher elevations. Secondly, when desiring to maximize the thermal capability per apparatus plan area, the plan area occupied by the indirect heat transfer section subtracts from the plan area of the apparatus available for the vertical flow of the hot discharge air. The total apparatus airflow must then pass through this remaining smaller net discharge plan area. The air moving device size may also be smaller than optimum due to the reduced size of the net discharge plan area. Due to the need for both the indirect heat transfer section plan area and the net discharge plan area to occupy separate portions of the total apparatus plan area, neither area can be made as large as desired.  
         [0003]     A combined direct and indirect heat exchange apparatus is disclosed with the direct section located above the indirect section in U.S. Pat. No. 5,724,828. However, there still exists a problem with maintaining consistent and uniform spray water flow over the indirect section. No provision is made to account for the pull in of the evaporative liquid due to the horizontal flow of the inlet air stream. As the air moves into the unit, it pulls the outer edges of the evaporative liquid falling from the bottom of the direct section inwardly causing the effective wetted plan area available for the indirect section to be smaller than the plan area of the direct section overhead. Additionally, since the falling water is not pulled in uniformly over the entire plan area nor is the pull in consistent with varying fan power levels, the resulting water spray over the indirect section is not uniform. This distracts from the optimum performance that could be achieved with uniform distribution of the evaporative liquid over the entire indirect heat transfer section.  
         [0004]     U.S. Pat. No. 6,598,862 discloses a combined indirect and direct heat exchange apparatus wherein the indirect section is of smaller plan area than the direct evaporative section located above it. This application teaches that higher performance is achieved by not allowing any airflow through the indirect section and discounts the additive performance effect of this additional evaporative surface. This limits the size and capacity of the indirect section that can be used in a given plan area. As with other prior art designs, performance also suffers due to the inconsistent and non-uniform spray water loading at the top of the indirect evaporative section. Furthermore, this design teaches to accelerate the velocity of the falling evaporative liquid to at least 9.5 feet per second and up to 15 feet per second. The claimed purpose of these higher velocities is to improve the heat transfer coefficient of the falling evaporative liquid film over the outside surface of the coil. What impact, if any, this higher velocity liquid may have is limited to the top surface of the coil only. Once the liquid hits the top surface, the flow energy is dissipated and the flow through the rest of the coil is the same as it would be if the evaporative liquid had an initial velocity of zero.  
       SUMMARY OF THE INVENTION  
       [0005]     Accordingly, it is an object of the present invention to provide an improved heat exchange apparatus and method including a direct evaporative heat exchange section and an indirect evaporative heat exchange section.  
         [0006]     It is also an object of the present invention to provide a heat exchange apparatus and method including a direct evaporative heat exchange section above an indirect evaporative heat exchange section, wherein an intermediate collection of evaporative fluid is provided above the indirect evaporative section and wherein such collected fluid is re-sprayed onto the indirect evaporative section.  
         [0007]     The heat exchange system of the present invention utilizing the direct evaporative heat exchange section above the indirect evaporative heat exchange section is combined with a unique air inlet system between the direct heat exchange section and the indirect heat exchange section. Further, a central core exhaust is provided such that a duct is formed in the interior of the heat exchange unit to allow air drawn inwardly and downwardly across the indirect heat exchange section to exhaust into the air duct and upwardly and out of the heat exchange unit.  
         [0008]     Further, improved performance of the heat exchange unit of the present invention is provided with the utilization of a re-spray collection tray beneath the direct heat exchange section. The re-spray tray collects evaporative liquid that flows downwardly and through the direct heat exchange section. The re-spray tray then is configured to redistribute the evaporative liquid to a plurality of re-spray nozzles so as to provide a generally uniform spray of evaporative liquid downwardly onto and across the indirect heat exchange section. The provision of evaporative liquid from the re-spray nozzles provides a uniform and consistent supply of evaporative liquid across the indirect section and promotes more uniform circuit to circuit heat transfer within the entire indirect section.  
         [0009]     The indirect section itself is made up of a plurality of fluid filled coils that exchange heat in an indirect transfer to the liquid flowing across the outside of the coils. Further the plan area of the indirect heat exchange section can be optimally sized to maximize the capacity of the entire heat exchange apparatus. It is generally preferred that the plan area of the indirect heat exchange section would substantially equal the plan area of the direct heat exchange section.  
         [0010]     Further, the re-spray collection tray is located in a neutral area of the inlet plenum between the direct and indirect heat exchange sections and does not interfere with the natural streamlines of inlet air. Since the downward flow of sprayed evaporative liquid is eliminated in the region between the bottom of the re-spray tray and the top of the re-spray distribution branches, the air inlet pressure drop into the indirect section is further reduced. This dry area also permits easy inspection and maintenance of the re-spray nozzles during the operation of the heat exchange apparatus.  
         [0011]     The central exhaust duct, in addition to providing an upward pathway for the hot discharge air exiting the indirect heat exchange section, also provides a unique internal access to service the fan drive system and the evaporative spray distribution system for the direct heat exchange section. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0012]     Referring to the drawings,  
         [0013]      FIG. 1  is a side elevation view, in partial cross-section, of the heat exchange apparatus in accordance with the present invention;  
         [0014]      FIG. 2  is a front elevation view of a heat exchange apparatus in accordance with the present invention;  
         [0015]      FIG. 3  is a top plan view of a heat exchange apparatus in accordance with the present invention;  
         [0016]      FIG. 4  is a detail perspective view of the re-spray trough and branch system in accordance with the present invention;  
         [0017]      FIG. 5  is an end view, in cross section, of the re-spray trough and branch system in accordance with the present invention, and  
         [0018]      FIG. 6  is a side elevational view, in partial cross section, of a second embodiment of the heat exchanger apparatus of the present invention. 
     
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT  
       [0019]     Referring now to  FIGS. 1-5 , a heat exchanger in accordance with the present invention is shown generally at  10 . It should be known that such heat exchangers are usually comprised of sheet metal, with appropriate internal structural elements. Fan  12  is seen to be structurally mounted on supports at the top of heat exchanger  10 . Fan  12  is shown as a blade or propeller fan, and it should be understood that a plurality of smaller diameter fans could be located at the top of heat exchanger  10  in individual exhaust plenums. Fan motor  14  drives fan  12  by a belt or gear drive assembly. Typically, exhaust plenum  13  is made of formed fiberglass or shaped sheet metal. Evaporative liquid inlet  16  is shown as a tube, which is usually a polyvinyl chloride pipe. Evaporative liquid inlet  16  has a plurality of evaporative liquid upper spray branches  18  operatively connected thereto such that evaporative liquid is distributed throughout evaporative liquid upper spray branches  18 . A plurality of upper liquid spray nozzles  19  extend downwardly from each of evaporative upper spray branches  18  such that a spray of evaporative liquid is provided downwardly onto the top of direct evaporative section  20 .  
         [0020]     Direct evaporative section  20  is comprised of a plurality of fill sheets  22 . Each fill sheet is typically a thin sheet of polyvinyl chloride or other plastic either structurally supported or hung from appropriate structure. There are numerous such fill sheets  22  in a heat exchange apparatus  10 , with appropriate spacing to allow evaporative liquid to run downwardly across the fill sheets while air is drawn upwardly by fan  12  through direct evaporative section air inlet  32 .  
         [0021]     Direct evaporative section air inlet  32  is seen to extend across the front, as shown in  FIG. 2  and also across the back, not shown, faces of heat exchanger  10 . Direct evaporative section air inlet  32  is basically an open space to allow air to be drawn generally crossways into heat exchanger  10  and then generally upwardly through direct evaporative section  20 . It is seen that the airflow upwardly through direct evaporative section  20  is countercurrent to the downward flow of evaporative liquid from upper liquid spray nozzles  19 .  
         [0022]     Evaporative liquid falling downwardly and exiting direct evaporative section  20  is collected on re-spray tray  26 . Re-spray tray  26  is shown in detail in  FIGS. 4 and 5 , and is seen to comprise a generally flat, generally rectangular metallic structure or even structural plastic configuration or material. Re-spray tray  26  is seen to extend and block the entire structure below direct evaporative section  20  such that virtually all evaporative liquid exiting direct evaporative section  20  is collected on re-spray tray  26 .  
         [0023]     The collected evaporative liquid on re-spray tray  26  is seen to run due to the incline of re-spray tray  26  into re-spray trough  28 . Re-spray trough  28  is typically a structurally shaped metallic structure or is comprised of structural plastic. Re-spray branches  30  are seen to be operatively connected to re-spray trough  28  such that evaporative liquid may enter re-spray branch inlets  29  and be distributed across the entire length of re-spray branches  30 . This allows the liquid to be distributed to the plurality of re-spray nozzles  31  that protrude from each of re-spray branches  30 . Accordingly, there is a virtual dry zone between re-spray tray  26  and re-spray branches  30 .  
         [0024]     Evaporative liquid exiting re-spray nozzles  31  are seen to be evenly and uniformly distributed across the top of first indirect evaporative section  36 , as well as second indirect evaporative section  38 , considering the dual structure of heat exchange apparatus  10 . It is conceivable that only a single first direct evaporative section  20  and indirect evaporative section  36  could be utilized in a structure in accordance with the present invention.  
         [0025]     Indirect evaporative section air inlet  34  is seen to be an opening extending across the front, and, not shown, rear face of heat exchanger  10 . Accordingly, air is drawn into indirect evaporative section air inlet  34 , downwardly across indirect evaporative section  36  and out the bottom and part of the open side into center duct  24 . The structural sides of center duct  24  are seen to end at  27 , thereby allowing air drawn into indirect evaporative section air inlet  34  to proceed generally downwardly across first indirect evaporative section  36  and outwardly into and across into center duct  24 . Similarly, air is drawn through indirect evaporative section air inlet on the rear face of heat exchanger  10  downwardly and across second indirect evaporative section  38  and into center duct  24 . Similarly, the structural opening into center duct  24  from second indirect evaporative section  38  is shown at  33 .  
         [0026]     Indirect section process fluid inlet  47  is seen to be a pipe structure, typically comprised of a metal, usually steel, pipe, whereby process fluid is inlet into a header and into each indirect evaporative section  37  circuit tube of coil  36 . A similar arrangement is present at second indirect evaporative section  38 . Indirect section process fluid outlet  45  is seen to also be connected to a header arrangement whereby the end or top of each indirect section circuit tube  37  is extended to thereby provide an outlet for the cold process fluid. For operation as a condenser, the flow in the indirect section would be reversed, with a vapor entering the upper inlet and the condensed refrigerant leaving the bottom outlet.  
         [0027]     Evaporative liquid which exits first indirect evaporative section  36  and second indirect evaporative section  38  is seen to be collected in evaporative liquid collection pan  40 . Such collection pan is typically a metal structural arrangement at the bottom of heat exchanger  10 . Such evaporative liquid is allowed to accumulate in pump section  42 , whereby it is pumped through evaporative liquid outlet  44 , and back up to the evaporative liquid inlet  16 .  
         [0028]     Referring now to  FIG. 6 , a heat exchanger in accordance with a second embodiment of the present invention is shown generally at  110 . This embodiment is typically referred to as a crossflow arrangement, with a central fan  112  and two side areas of heat exchange elements. It should be known that such heat exchangers are usually comprised of sheet metal, with appropriate internal structural elements. Fan  112  is seen to be structurally mounted on supports at the top of the heat exchanger  110 . Fan  112  is shown as a blade or propeller fan, and it should be understood that a plurality of smaller diameter fans could be located at the top of heat exchanger  110  in individual exhaust plenums. A fan motor drives fan  112  by a belt or gear drive assembly. Typically, exhaust plenum  113  is made of formed fiberglass or shaped sheet metal. Evaporative liquid inlet  116  is shown. A redistribution box provides for a uniform level of evaporative liquid in the upper distribution pan. Gravity spray nozzles,  119 , located in the base of the upper pan distribute the evaporative liquid uniformly across the top of the direct evaporative section such that a spray of evaporative liquid is provided downwardly onto the top of direct evaporative section  120 .  
         [0029]     Direct evaporative section  120  is comprised of a plurality of fill sheets  122 . Each fill sheet is typically a thin sheet of PVC or other plastic either structurally supported or hung from appropriate structure. There are numerous such fill sheets  122  in a heat exchange apparatus  110 , with appropriate spacing to allow evaporative liquid to run downwardly across the fill sheets while air is drawn across by fan  112  through direct evaporative section air inlet  132 .  
         [0030]     Direct evaporative section air inlet  132  is seen to extend across the front with air inlet  133  extending across the back of heat exchanger  110 . Direct evaporative section air inlet  132  is basically an open face to allow air to be drawn generally crossways into heat exchanger  110  and then generally across direct evaporative section  120 . It is seen that the airflow across direct evaporative section  120  is crosscurrent to the downward flow of evaporative liquid from upper liquid spray nozzles  119 .  
         [0031]     Evaporative liquid falling downwardly from and exiting direct evaporative section  120  is collected on re-spray tray  126 . Re-spray tray  126  is seen to comprise a generally flat, generally rectangular metallic structure or is comprised of structural plastic. Re-spray tray  126  is seen to extend and block the entire structure below direct evaporative section  120  such that virtually all evaporative liquid exiting direct evaporative section  120  is collected on re-spray tray  126 .  
         [0032]     The collected evaporative liquid on re-spray tray  126  is seen to run due to the incline of re-spray tray  126  into re-spray trough  128 . Re-spray trough  128  is typically a structurally shaped metallic structure or is comprised of structural plastic. Re-spray branches  130  are seen to be operatively connected to re-spray trough  128  such that evaporative liquid may be distributed across the entire length of re-spray branches  130 . This allows the liquid to be distributed to the plurality of re-spray nozzles  131  that protrude from each of re-spray branches  130 . Accordingly, there is a virtual dry zone between re-spray tray  126  and re-spray branches  130 .  
         [0033]     Evaporative liquid exiting re-spray nozzles  131  are seen to be evenly and uniformly distributed across the top of indirect evaporative section  136 . It is conceivable that only a single first direct evaporative section  120  and indirect evaporative section  136  could be utilized in a structure in accordance with the present invention.  
         [0034]     Indirect evaporative section air inlet  134  is seen to be an opening extending across the front, with a similar opening at the rear face of exchanger  110 . Accordingly, air is drawn into indirect evaporative section air inlet  134 , downwardly across indirect evaporative section  136  and out the bottom and part of the open side into center section  124 . Similarly, air is drawn through indirect evaporative section air inlet on the rear face of heat exchanger  110  across second indirect evaporative section  138  and into center section  124 .  
         [0035]     Indirect section process fluid inlet  147  is seen to be a pipe structure, typically comprised of a metal, usually steel, pipe, whereby process fluid is inlet into a header and into each indirect evaporative section  137  circuit tube of coil  136 . A similar arrangement is present at second indirect evaporative section  138 . Indirect section process fluid outlet  145  is seen to also be connected to a header arrangement whereby the end or top of each indirect section circuit tube  137  is extended to thereby provide an outlet for the cold process fluid. For operation as a condenser, the flow in the indirect section would be reversed, with a vapor entering the upper inlet and the condensed refrigerant leaving the bottom outlet.  
         [0036]     Evaporative liquid which exits first indirect evaporative section  136  and second indirect evaporative section  138  is seen to be collected in evaporative liquid collection pan  140 . Such collection pan is typically a metal structural arrangement at the bottom of heat exchanger  110 . Such evaporative liquid is allowed to accumulate in a sump section, whereby it is pumped through an evaporative liquid outlet back up to the evaporative liquid inlet  116 .