Abstract:
A paint spray booth having a scrubbing solution flow control, wherein the spray booth includes a floor having an opening, a scrubber beneath the floor, at least one elongated distribution trough having an overflow weir, a source of scrubbing solution and a pump pumping scrubbing solution from the source to the distribution trough. The flow control includes a control valve between the pump and the distribution trough, a level sensor monitoring a level of solution in the trough and a controller receiving a signal from the level sensor operating the control valve to maintain a predetermined level of scrubbing solution in the distribution trough.

Description:
RELATED APPLICATIONS  
       [0001]     This application claims priority to Provisional Application Ser. No. 60/514,891 filed Oct. 24, 2003. 
     
    
     FIELD OF THE INVENTION  
       [0002]     This invention relates to a paint spray booth having a scrubbing solution flow control. More specifically, this invention relates to a control for maintaining a predetermined level of scrubbing solution in the distribution trough of a paint spray booth and thus controls the volumetric flow of scrubbing solution received in the paint spray booth.  
       BACKGROUND OF THE INVENTION  
       [0003]     Paint spray booths for high volume applications, such as used by the automotive industry to spray paint automotive bodies and by other industries to spray paint large substrates, are continuously ventilated to remove paint overspray. The exhaust air from the paint spray booth carries the paint overspray, which are typically fine droplets of paint 1-10 micrometers, out of the paint spray booth. The overspray paint must then be removed from the exhaust air before it can be vented to atmosphere. The removal of the paint overspray is typically done by a wet scrubber system.  
         [0004]     U.S. Pat. Nos. 5,100,442 and 6,228,154, both assigned to the assignee of this application, disclose wet scrubber systems. As will be understood by those skilled in this art, a wet scrubber system functions by contacting the exhaust air from the paint spray booth with a water based scrubbing solution, which is typically water with a surfactant to detacify the paint droplets, and other additives. The scrubbing system breaks the scrubbing solution into droplets (50 micrometers in diameter or larger) and contacts the fine paint particles entrained in air. The larger droplets of scrubbing solution are intermixed with air received from the paint spray booth having entrained smaller overspray paint droplets in turbulent flow through the scrubber, such that the small paint overspray droplets are adsorbed into the much larger scrubbing solution droplets. The scrubbing solution, now laden with paint overspray is then removed from the exhaust air from the paint spray booth by a separator and sent to a sludge tank for treatment. The exhaust air can then be vented to atmosphere or another destination.  
         [0005]     A paint spray booth is typically an enclosed structure having a conveyor which transfers the articles to be painted through the paint spray booth and paint application equipment, such as rotary paint atomizers, are spaced along the length of the paint spray booth on one or both sides of the conveyor as required by the application. For example, a paint spray booth for automotive applications may be 18 feet wide and 200 to 300 feet long. The cross-section of a typical conventional paint spray booth is shown schematically in  FIG. 1 .  
         [0006]      FIG. 1  illustrates a typical paint spray booth  20  for painting large articles, such as vehicle bodies  22  which are conventionally conveyed through the paint spray booth  20  on a conveyor  24 . As will be understood by those skilled in this art, a conventional paint spray booth includes an air supply house  26  where ambient air is filtered and environmentally treated by dehumidifiers, humidifiers, heaters, coolers, etc. (not shown) prior to receipt in the painting area  32 . The treated air is received from the air supply house  26  to the painting area  32  through a porous ceiling  28  as shown by arrows  30 . Ventilation air is typically supplied to the painting area  32  from the air supply house  26  at a velocity in the range of 40 to 120 feet per minute. The downward flow of air in the painting area  32  is important as it prevents paint overspray from settling onto the previously painted areas of the article  22 .  
         [0007]     The enclosed painting area  32  includes a floor  34  having a slot or opening  36  in the floor and a scrubber or scrubbers  38  are supported in the floor  34  beneath the floor aligned with the opening  36  through the floor. As described in the above-referenced U.S. patents, the scrubber  38  may comprise an elongated scrubber or an aligned spaced series of scrubbers which receive air from the painting area  32  laden with droplets of paint overspray and scrubbing solution, wherein the larger droplets of scrubbing solution adsorb the smaller paint droplets or particles and separates the paint droplets from the incoming air. A pump  40  pumps clean scrubbing solution into troughs  42 , typically located on the floor  34  of the painting area  32  on opposed sides of the painting area  32  through supply lines  44 . The scrubbing solution  48  in the troughs  42  then flows over the overflow weirs  46  onto the floor  34  of the painting area  32  and into the scrubber or scrubbers  38  through opening  36  where the paint droplets are separated from the incoming air. The scrubbing solution with the separated paint droplets are then received in a tank  50  and conveyed through line  52  to a scrubbing solution tank  54 . The cleaned air is then received through an exhaust  56  where it may be exhausted to atmosphere or another destination, as described above. The paint droplets are then removed either by a settling system or more typically a floating system, wherein the agglomerated paint droplets are skimmed from the scrubbing solution tank  54 . A flocculant is typically added to the scrubbing solution in the scrubbing solution tank  54 .  
         [0008]     The efficiency of the scrubber  38  is strongly dependent upon the correct volumetric flow rate of the scrubbing solution  48  to the troughs  42 . Either too much or too little scrubbing solution supplied to the troughs  42  by the pump  40  will be detrimental to the performance of the scrubber  38 . At present, the volumetric flow rate of scrubbing solution supplied to the troughs  42  is determined by the initial setting of the pump  40 . Because of the importance of the volumetric flow rate of the pump  40 , the initial flow volume by the pump  40  into the troughs  42  is carefully measured and set up to the correct value when the paint spray booth  20  is first started up.  
         [0009]     It is also important to distribute the flow volume properly over the length of the paint spray booth  20 . Distribution is typically accomplished a pipe and manifold (not shown) extending the entire length of the paint spray booth  20  with supplied drops spaced approximately 10 feet apart. The piping system is designed such that the pressure drop through the drops is much greater than the pressure drop in the manifold so that a nearly equal volume of scrubber solution flows through each drop in the manifold. In addition, the scrubbing solution drops into a trough  42  on each side of the paint spray booth  20  which subsequently feeds water to the booth floor  34  over the continuous overflow weir  46 , which in the disclosed embodiment, is an inner edge of the troughs  42 . The edge of the weir  46  is thus carefully leveled to provide equal flow distribution over the entire length of the paint spray booth  20 .  
         [0010]     One problem encountered with state of the art paint spray booths is that over time, the volumetric flow rate of the scrubbing solution changes. This change is primarily caused by fouling of the supply piping and pumps. Both of these factors cause the volumetric flow rate of scrubbing solution  48  to the troughs  42  to decrease over time. This decrease will have a number of detrimental effects. First, the decreased water flow rate will reduce the air flow resistance of the scrubber  38 . This will lead to an increase in exhaust air rate which causes a mismatch between the supply and exhaust airflow volumetric flow rates. This mismatch will result in air flowing parallel to the travel of the conveyor  24 , which can cause paint overspray to drift into and settle onto undesirable areas resulting in quality problems with the article being painted. Second, if the flow rate drops sufficiently, the scrubbing efficiency of the scrubber  38  will be reduced to the point where plant environmental permit conditions may be violated. This can also have legal consequences, including fines and possible plant shut down. Thus, there has been a longstanding problem with the control of scrubbing solution to the troughs  42 .  
         [0011]     The scrubbing solution flow control of this invention solves this problem by a simple and efficient means and maintains a predetermined level of scrubbing solution in the distribution troughs  42 .  
       SUMMARY OF THE INVENTION  
       [0012]     The paint spray booth of this invention may be generally conventional to permit retrofitting of existing paint spray booths with a scrubbing solution flow control. That is, the paint spray booth of this invention includes an enclosed paint spray booth receiving articles to be painted including a floor having an opening or a plurality of openings therethrough, a scrubber or scrubbers located beneath the floor opposite the opening or openings, an elongated distribution trough on at least one side of the floor having an overflow weir, a source of scrubbing solution and a pump pumping scrubbing solution from the source to the distribution trough, such that the scrubbing solution flows from the distribution trough over the overflow weir onto the floor and through the opening into the scrubber or scrubbers, removing paint droplets or particles from the air as described above.  
         [0013]     The scrubbing solution flow control of this invention includes a control valve located between the pump and the distribution trough, a level sensor monitoring the level of the scrubbing solution in the distribution trough and a controller receiving a signal from the level sensor operating the control valve to maintain a predetermined level of scrubbing solution in the distribution trough or troughs. In one preferred embodiment, the level of scrubbing solution in the distribution trough or troughs is measured by a pressure transducer. However, other level sensors may also be utilized, including ultrasonic, capacitive, conductance, thermal, optical or mechanical level sensing devices. The level sensor then sends an electrical signal corresponding to the level of scrubbing solution in the trough or troughs to a controller. Again, the controller may be any conventional type of digital or analog system that produces an output signal capable of changing the position of a control valve to control the signal from the transducer to maintain a specific set value or “setpoint.” The control valve may also be any type of conventional valve capable of responding to the output from the controller and varying the scrubbing solution flow rate of the pump to the paint spray booth distribution trough. Alternatively, the controller can vary the scrubbing solution flow rate by controlling the speed of the pump through the use of an electronic or mechanical variable speed drive.  
         [0014]     As will be understood, various modifications may be made to the paint spray booth having a scrubbing solution flow control of this invention within the purview of the appended claims and the disclosed embodiment of the scrubbing solution flow control of this invention in the following description of the preferred embodiments and drawings is for illustrative purposes only. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0015]      FIG. 1  is a schematic cross-sectional view of a conventional paint spray booth described above;  
         [0016]      FIG. 2  is a schematic cross-sectional view of a paint spray booth having a scrubbing solution flow control of this invention; and  
         [0017]      FIG. 3  is a graph of time versus level of scrubbing solution in the distribution troughs illustrating the efficiency of the scrubbing solution flow control of this invention. 
     
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0018]      FIG. 2  illustrates a scrubbing solution flow control of this invention adapted for a conventional paint spray booth described above with reference to  FIG. 1 . That is, the paint spray booth  120  may be utilized to paint large articles, such as a vehicle body  122 , which is conveyed through the paint spray booth on a conveyor  124 . The paint spray booth  120  may include an air supply house  126  which conditions ambient air and the air is received under pressure through a porous ceiling  128  as shown by arrows  130  in the painting area  132 . The painting area  132  includes a floor  134  having an opening or openings  136  and a scrubber or scrubbers  138  are located beneath the floor  134  aligned with the opening or openings  136 . The painting area  32  further includes distribution troughs  142  preferably on opposed sides of the painting area  132  extending the length of the painting area, each having an overflow weir  146  and scrubbing solution  148  is supplied to the distribution troughs  142  by a pump or pumps  140  through supply lines  144 . As described above, a tank  150  is located below the scrubber  138  which receives the scrubbing solution  148  with the adsorbed paint droplets and the scrubbing solution is then transferred through line  152  to the scrubbing solution tank  154 . An exhaust  156  is also provided to exhaust the cleaned air to atmosphere or another destination. That is, the scrubbing solution flow control of this invention may be utilized with a conventional paint spray booth  120  described above or any conventional paint spray booth.  
         [0019]     As now described, the scrubbing solution flow control of this invention controls the flow rate of scrubbing solution  148  to the distribution troughs  142  by measuring the level of scrubbing solution in the distribution troughs and controlling the flow rate of scrubbing solution from the pump  140  to the distribution troughs  142  to maintain a predetermined level of scrubbing solution in the distribution troughs. In the disclosed embodiment of the scrubbing solution flow control of this invention, a control valve  158  is located between the pump  140  and the distribution troughs  142  and the pump  140  pumps scrubbing solution through line  160  to the control valve  158 . The level of scrubbing solution  148  in the distribution troughs  142  is measured by a level sensor  162  which sends an electronic signal corresponding to the level of scrubbing solution in the distribution troughs  142  to a controller  164  through line  166 . The controller  164  then transmits a signal to the control valve  158  which adjusts the volumetric flow of scrubbing solution from the pump  140  to the distribution troughs  142 . That is, the control valve  158  may be adjusted by the controller  158  to provide a predetermined flow rate to the distribution troughs  142  through line  144  or the control valve  158  may control the volumetric flow rate by controlling the speed of the pump  140  through the use of an electronic or mechanical variable speed drive. The scrubbing solution flow control of this invention thus controls the flow rate of scrubbing solution from the pump  140  to the distribution troughs  142  to maintain a predetermined level of scrubbing solution in the distribution troughs, thereby eliminating the problem associated with the prior art paint spray booth described above with reference to  FIG. 1  and improving the efficiency of the scrubber  138  or scrubber system.  
         [0020]     As described above, the control valve  158  can be any type of conventional valve capable of responding to the output signal from the controller  164  and varying the flow rate of the scrubbing solution to the paint spray booth distribution troughs  142 . In a preferred embodiment, the control valve  158  is an electro-pneumatic valve which uses compressed air to open and close the valve with an electronic signal to control the compressed air, such as Model 4ACAADMFM2BC7 available from Tru-Tech Industries having a maximum pressure of 150 psi and a pipe diameter of four inches. However, as described above, the control valve  158  may be any type of control valve and a control valve to vary the speed of the pump  140  may also be utilized. A preferred embodiment of the level sensor  162  is a differential pressure transducer. The function of the pressure transducer is to measure the level of scrubbing solution  148  in the distribution troughs  142  and produce an electronic signal proportional to the level of scrubbing solution in the distribution troughs  142 . A suitable differential pressure transducer is available from Dwyer Instruments, Inc., Model 655-1. The high pressure connection to the differential pressure transducer may be piped to the distribution troughs  142  through a pipe (not shown) preferably located below the normal level of scrubbing solution in the troughs and the low pressure connection of the differential pressure transducer is preferably unconnected outside the booth so that the level of scrubbing solution is referenced to ambient pressure. As set forth above, however, other suitable level sensors may also be utilized, including ultrasonic, capacitive, conductance, thermal, optical or mechanical level sensors. The controller  158  may also be any type of digital or analog system that produces an output signal from the level sensor  162  capable of changing the position of the control valve  158  to cause the signal from the level sensor  162  to maintain a predetermined specific set value or setpoint. A suitable programmable logic controller or PLC is available from Allen Bradley Company, wherein the programmable logic controller is programmed to monitor the electronic signal from the level sensor  162  and adjust the control valve  158  to maintain a set point.  
         [0021]     Testing of the scrubbing solution flow control of this invention was conducted by first adjusting the control setpoint to the normal flow rate of 32 gpm per foot of booth. After the system was running, a bypass valve was opened and closed, simulating changes in the piping flow resistance or pump performance. The bypass valve allowed water from the pump discharge to return to the tank without passing through the scrubber  138 . The automated valve was able to adjust the flow through the spray booth to compensate for these changes. The programmable logic controller logging program was set up to run for a six hour period. During that time, the level setpoint was changed.  FIG. 3  is a graphical illustration of the level of scrubbing solution in the distribution troughs  142  as read by a pressure transducer level sensor  162  and the setpoint plotted versus time. As can be observed from  FIG. 3 , the scrubbing solution flow control of this invention was able to respond to changes in set point in approximately three minutes. Further, it can be observed that the system was able to maintain the level of scrubbing solution in the distribution troughs  142  within ±0.05 w.c. of the setpoint. The scrubbing solution flow control of this invention thus maintain a predetermined level of scrubbing solution in the distribution troughs  142  and solve the problems associated with the prior art, including improved efficiency of the scrubber  138 .  
         [0022]     As will be understood by those skilled in this art, various modifications may be made to the paint spray booth and the scrubbing solution flow control of this invention within the purview of the appended claims. As set forth above, the scrubbing solution flow control of this invention may be utilized with any conventional paint spray booth and thus is not limited to a specific design of paint spray booths. Further, as set forth above, other types of level sensors may be utilized in the scrubbing solution flow control of this invention, although a differential pressure transducer is preferred. The control valve  158  may also be any type of conventional valve capable of responding to the output signal from the controller  158  or the controller may vary the scrubbing solution flow rate by controlling the speed of the pump  140  by electronic or mechanical means. Finally, any type of digital or analog controller may be utilized that produces an output signal capable of adjusting the control valve. Having described a preferred embodiment of the paint spray booth having a scrubbing solution flow control, the invention is now claimed as follows.