Abstract:
An indicator system provides a visual indication of the cumulative level of an airborne contaminant. The indicator is a transparent sheath with support medium having an acidic surface and treated with a color pH indicator. As a basic contaminant is adsorbed by the medium, the medium color changes. As additional contaminant is adsorbed, the color front progresses the length of the indicator. The indicator system may be used with an adsorptive filter system to predict the life of adsorption bed assemblies. A sample flow is taken upstream of the adsorption beds and is passed through the indicator. Preferably, the flow rate is calibrated so that the rate of the color change in the indicator is proportional to the rate of depletion of the adsorption bed. By monitoring the indicator, an accurate prediction of adsorption bed life may be made.

Description:
The present application is a continuation-in-part of U.S. patent application having Ser. No. 08/893,876, filed Jul. 11, 1997, now U.S. Pat. No. 5,976,467, the entire disclosure of which is incorporated herein. 
    
    
     BACKGROUND 
     1. Field of the Invention 
     The present invention generally relates to a method and apparatus for indicating the presence of airborne contaminants and accumulation of airborne contaminants, such as organic and inorganic bases, and more particularly to predicting the life of filter systems for removing the airborne contaminants. 
     2. Prior Art 
     Gas adsorption beds are used in many industries to remove airborne contaminants, such as organic bases, to protect people, the environment and often, a critical manufacturing process for the products which are manufactured. A specific example of an application for gas adsorption beds is the semiconductor industry where products are manufactured in an ultra-clean environment, commonly known in the industry as a “clean room”. The manufacturing process typically requires the use of substances such as solvents to be used in the clean room environment. The use of these substances presents a problem when vapors are formed during the process which may contaminate the air and other processes in the room if they are not properly removed. In addition, many environments have several gases that may naturally occur in the ambient air that may contaminate the products and/or processes and are not removed by normal particulate filters. Typical contaminants that are produced by such processes are airborne bases, such as ammonia, organic amines and N-Methyl-2-pyrrolidone. 
     To eliminate the problem, contaminated air is often drawn through a granular adsorption bed assembly having a frame and adsorption medium, such as activated carbon retained within the frame. The adsorption medium adsorbs the gaseous contaminants from the air flow and allows clean air to be returned to the clean room and/or process. It can be appreciated that the removal efficiency of such beds is critical in order to protect the processes and the products that are involved. 
     It can further be appreciated that since the removal process involves passage of air through an activated carbon bed that adsorbs or chemically reacts with the airborne contaminants, there is no measurable pressure change as occurs when particulate filters are loaded. Therefore, it is difficult to directly monitor the status and deterioration of the activated carbon bed. Monitors placed downstream may detect performance, efficiency, or when a failure has occurred and that the adsorption beds are spent. However, presently available sensors may not be sensitive enough to work at the contaminant threshold levels which are critical in the semiconductor industry and are often quite costly. A problem with sensors having acceptable sensitivity is that they are often specific to a single contaminant. Although such sensors may detect a low level of one contaminant, others may accumulate to high levels and remain undetected. However, once there is an indication that an adsorption bed is spent, it is often too late and the process or products have often been ruined or damaged. 
     Other systems have been devised which can monitor adsorption bed life such as placing the beds in series. When adsorption beds are placed in series, a sensor may be placed in series intermediate the two adsorption beds. Therefore, as one adsorption bed becomes spent and the sensor indicates the presence of a contaminant, the second adsorption bed is still effective and failures are prevented. However, such detection systems have several drawbacks. When two adsorption beds are used, the pressure drop is doubled. This may be critical in some applications. In addition, once the first bed has been indicated as being spent, the adsorption beds are normally rotated in a sometimes complicated manner. Such rotation increases the maintenance and down time of such a system. At other times, both adsorption beds may be changed out, thereby decreasing labor, but also shortening the useful life of the downstream adsorption beds as they are removed prior to being fully spent. 
     Other systems utilize a sensor placed directly in the adsorption bed. However, in very thin adsorption beds, such a sensor may take up valuable space. In addition, an interface for detecting the presence of contaminants within an adsorption bed requires seals and can be complicated and expensive. 
     It can be appreciated that if the filtered air can be distributed in a balanced, even manner over the adsorption beds, a reliable prediction of the expected useful duration of each bed would enable a longer change out interval period without failure. It can be appreciated that achieving the greatest possible change out interval without failure would decrease filter materials cost and labor costs utilized in changing the adsorption bed filters. 
     It is desirable to have an indication of the actual amount of contaminant that the filter beds have been exposed to based on a known filter capacity and being able to accurately predict an optimal change out period for the adsorption beds. Such a process is more precise if the actual flow passing through the filters is known and the prediction based on a flow which is proportional to the actual flow through the adsorption beds. By sampling upstream of the adsorption beds, an accurate prediction of the amount of contaminants flowing to the adsorption beds can be made. 
     It can be seen then that an indicator system is needed that detects the cumulative levels of airborne contaminants. Such a system should be able to sample a proportional amount of airborne contaminants that are flowing past an adsorption bed device. Such an indication system should provide a clear visual indication of the bed usage and indicate when the adsorption beds should be changed. In addition, as contaminant concentrations may vary, the system should provide a real time indication of cumulative contamination levels for predicting the change out interval based on the actual contaminant flow past the adsorption bed. It can also be appreciated that such a system should provide for a variable safety factor to ensure that adsorption bed failures do not occur. Such a system should also be able to measure the presence and cumulative level of such contaminants in an environment and provide a visual indication. The present invention addresses these as well as other problems associated with indicating the presence of airborne contaminants. 
     SUMMARY OF THE INVENTION 
     The present invention is directed to a system and method for indicating the presence of airborne bases. Such a system may be used in clean rooms and other applications wherein air quality is critical. 
     The indicator system includes a valving arrangement to control flow into and out of the system. A probe is utilized to obtain a sample of air. In one embodiment, a sample taken is proportional to the actual flow rate of the air being sampled. Therefore, the system can be used as a predictor with greater accuracy as contamination levels vary. 
     The system uses a flow meter to monitor and calibrate the system. An indicator device includes a sheath, such as a tube, having an indicating medium therein. As contaminants enter the tube from a first end, the specially treated medium will change color to indicate the presence of contaminant. Because the tube is preferably substantially transparent, the color changing medium within the sheath is visually detectable. As additional contaminant passes through the medium, additional treated medium is affected and changes color to indicate the increasing levels. Thus, an advancing front of color is observed. A calibrated flow meter maintains the flow rate through the indicator proportional to the actual flow. A pump, for example an ejector type pump actuated by compressed air, maintains flow through the system. 
     As typical clean room processes emit ammonia, amines and other bases, it is desirable to provide a medium capable of detecting bases. Preferably, the medium is a silica gel or zirconium oxide having an acidic surface. This acidic surface is achieved by treating the medium with an acid, such as sulfuric acid. The acidic medium is then treated with a pH sensitive indicator, such as bromophenol blue, which changes color from yellow to blue upon an increase in pH. Therefore as airborne bases pass through the medium, a front of color, indicating spent medium, advances along the tube. 
     The adsorption type beds commonly used in clean room settings are designed to not clog in the same manner as normal filters. Unfortunately, because of this, the beds are difficult to monitor to determine their change interval. Although sensors may be utilized downstream of the filters to detect when the adsorption beds are spent, such sensors are expensive and often do not have sufficient sensitivity to monitor the low levels of airborne contaminants. In addition, control of the clean room processes may be so critical that by the time the expiration of the adsorption beds is detected, damage may have already occurred to the process. Therefore, it would be advantageous to be able to predict the interval when the adsorption beds will need to be changed. This will allow scheduling of the downtime required for the maintenance and will maximize the chance of changing of the filters prior to any failure. 
     To predict an interval, an indicator probe is placed upstream of the absorption beds to sample the air flow. Since the air passing through the indictor has the same composition as the air passing through the absorption beds, the advancement of the color change in the indicator probe can be an accurate indicator for the deterioration rate of the bed. As proportional flow is maintained, the color front on the indicator medium advances at a rate proportional to the deterioration of the adsorption bed. Thus, when the rate of the front advancement in the tube, the flow rate over the adsorption beds, and the bed deterioration rate, are known, the monitoring indicator can predict when the adsorption bed interval expires. 
     These features of novelty and various other advantages which characterize the invention described herein are pointed out with particularity in the claims. However, for a better understanding of the invention, its advantages, and the objects obtained by its use, reference should be made to the drawings which form a further part hereof, and to the accompanying descriptive matter, in which there is illustrated and described a preferred embodiment of the invention. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 shows a diagrammatic view of an indicator system according to the principles of the present invention; 
     FIG. 2 shows a side elevational view of a counterflow adsorption bed apparatus having the indicator system shown in FIG. 1; 
     FIG. 3 shows a side elevational view of an indicator device for the system shown in FIG. 1; 
     FIG. 4 shows a side elevational view of the indicator device shown in FIG. 2 with a portion of the indicator medium changed; 
     FIG. 5 shows a side elevational view of the indicator device shown in FIG. 2 with the indicator medium shown fully changed; and, 
     FIG. 6 shows a flow chart of a method of calibrating an indication system for predicting the interval of an adsorption device according to the principles of the present invention. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     Referring now to the drawings, and in particular FIG. 1, there is shown a system, generally designated  10 , for indicating the presence and level of an airborne contaminant. The system includes a probe  12  for sampling air from a duct or ambient air volume  100 . The indicator system  10  also includes a flow meter  16  with a control valve  14  between the probe  12  and the flow meter  16 . An indicator  18  is downstream of the flow meter  16 . A pump  40  maintains flow through the indicator system  10 . In a preferred embodiment, the pump is an ejector type pump such as Model No. LX-5, available from Piab Company, which is actuated by a compressed air supply  44 . A valve  42  acts as a safety valve between the compressed air supply  44  and the pump  40 . The valve  14  controls flow to the system  10  and is calibrated by flow meter  16 . In a preferred embodiment, the flow is proportional to the sampled flow. The pump  44  returns the sample air through a vent  46  to a duct and back to the sampled or ambient air  100 . It can be appreciated that for some monitoring applications, such as when sampling ambient air or flows, the pump  40 , flow meter  16  and valve  14  may not be used. Similarly, various other monitoring applications may not required the use of all the equipment described herein. 
     Referring now to FIG. 2, the indicator system  10  may be used with an adsorption filtering system  60 . Such a filtering system  60  typically includes a housing  62  with an access door  66 . The interior of the housing has an inlet plenum  64  extending vertically past a stack of adsorption beds  70 . Flow passes from the inlet plenum  64  in parallel through the adsorption beds  70  to an outlet plenum  72 . The treated air is circulated back to the ambient air. Another system is also shown in U.S. Pat. No. 5,290,345 to Osendorf and assigned to Donaldson Company, assignee of the present invention. 
     In the configuration shown in FIG. 2, the probe  12  is placed in the inlet plenum to intercept air flow prior to it being treated through the adsorption beds  70 . However, such systems may be used to measure contaminant levels in the ambient air or at different locations in a filter system, such as downstream of the adsorption beds  70 . In the configuration shown, the indicator system  10  can be used as a predictor of adsorption bed life. It can be appreciated that the adsorption beds  70  are positioned to receive a balanced air supply through the beds so that the deterioration rate of each adsorption bed  70  is equal. The indicator system  10  is used as a predictor of adsorption bed life and changeover interval, rather than as an indicator of when failure has already occurred, as in the prior art sensors. As explained hereinafter, the indicator  18  also gives an indication of the deterioration level of the adsorption beds  70 . 
     Referring now to FIGS. 3-5, there is shown greater detail of the indicator  18 . The indicator  18  includes a tube  20  which preferably has a transparency level sufficient to see a colormetric indicating medium  22  within the tube. In the preferred embodiment, the tube  20  is quite narrow, on the order of an ⅛ inch outside diameter, an inside diameter of 0.08 inches, and made from a suitable material such as Teflon™. Glass, plastic and sufficiently transparent ceramic materials may also be used. At each end of the tube  20  are porous plug elements  32  such as glass wool, which maintain the indicating medium  22  tightly packed within the tube  20 . The tube  20  also includes fittings  30  such as Swagelok™ elements or other suitable fittings for connecting to mating fittings. The fittings  30  provide for easy removal and replacement of the indicator  18  when the indicating medium  22  is spent, or when the associated adsorption beds are changed and it is desired to have a fresh indicator  18  associated with the fresh adsorption bed assemblies  70 . The overall length of the indicator  18  is about 7.5 inches, but may range from about 2 to 12 inches. 
     The medium  20  is typically a material having an acidic surface which is designed with advancement rate which coincides with the life of the filter bed system  70 . Preferably, the medium comprises silica gel or zirconium oxide (zirconia) that has been treated to provide an acidic surface. Examples of other suitable medium include glass beads or bubbles, porous polymers, alumina, and ceramic materials. It can be appreciated that by calibrating the flow rate, tube size, medium type, medium mesh size and surface area, the amount of surface acid, and/or the flow rate through the indicator  18 , it is possible to predict an optimum interval for changing a filter bed assembly  70  by monitoring the amount of spent indicator medium  24 . 
     In one preferred embodiment, the indicating medium  22  is a coated or impregnated silica gel. A specific mesh and surface area of the indication medium is chosen for the specific needs of each indicator system  10 . It can be appreciated that a smaller particle size will provide for a sharper divide in the color change of the advancing front of affected medium  24 , but will result in a higher pressure drop for the sampled air. An example of a typical medium for airborne bases is a 100/200 mesh silica gel or beads which has a specific surface area of approximately 500 square meters per gram. To prepare the gel, it is first immersed in a sulfuric acid solution for approximately two hours after which the excess acid is poured off and the silica gel is washed with distilled water several times. It can also be appreciated that other types of acid such as phosphoric acid, nitric acid, acetic acid, hydrochloric acid, trifluoromethane sulfonic acid, and trifluoroacetic acid may be used depending on the pH range which is desired, the medium being used and the indicator being used. The final solution of the silica gel is filtered and dried. Dried samples are wetted with an aqueous solution of isopropanol, to which is added a known amount of an appropriate indicator. An example of an indicator that has a color change at an appropriate pH is bromophenol blue. 
     In another preferred embodiment, the indication medium  22  is a coated or impregnated zirconia particle. Similar to the silica gel, a smaller particle size will provide a sharper divide in the color front. To acidify the surface of the zirconia particle, the zirconia is boiled in acid, for example, sulfuric acid, to provide a sulphate-zirconia (ZrO 2 /SO 4   −2 ) which is commonly referred to as a “superacid”. To prepare the zirconia, the particles are boiled in a sulfuric acid solution for approximately three hours after which the particles are washed with distilled water several times and dried. In general, zirconia is capable of being modified to be strongly acidic, more so than silica gel. A color change indicator is applied to the surface of the acidic particle from an aqueous isopropanol solution. 
     It can be appreciated that depending on the needs of the system and the type of contaminants that are being removed, that indicator mediums such as m-cresol purple, thymol blue, xylenol blue, cresol red, bromothymol blue, resolic red, phenolphthalein, thymolphtalein phenol red, and other colormetric indicators changing at a different pH may also be utilized. In addition, it can be appreciated that the concentration of the indicator that is used depends on the intensity of the color that is desired. It has been found that a 0.5% by weight bromophenol blue concentration works well, however concentrations of 0.1% to 5% by weight are useable. 
     To apply the color changing indicator to the particle, a solution of the acidified medium (for example silica gel or zirconia), a water/isopropanol mixture and the color indicating substance (for example bromophenol blue) is stirred for several minutes. After the mixture is allowed to stand, the medium is decanted and washed with isopropanol. The resulting medium is dried, for example in an oven at approximately sixty degrees (60°) Celsius. If silica gel medium is treated with bromophenol blue, the resulting silica gel is bright yellow, but upon exposure to a base such as ammonia, the color will change from yellow to blue. Higher concentrations of the bromophenol blue can yield an orange silica gel which changes to a blue/purple color upon exposure to a base. The indicating medium  22  is vacuum packed into the tube  20  and retained by the plugs of glass wool  32 . 
     Once a tube is prepared, it can be calibrated by exposing it to a controlled air flow that contains a known amount of the contaminant or airborne base. With the flow meter  16 , the amount of sample air passing through the system may be measured. The rate of the advancing color front  28  between spent medium  24  and fresh medium  26 , as shown in FIG. 4, is measured as a function of the amount of contaminant. A graphical curve can be derived to estimate the relation between the color change in the indicating medium  22  and the deterioration rate of the absorption bed. This curve can be compared to the known capacity of adsorption bed  70  and the expected breakthrough time or failure point of adsorption bed  70  can be predicted. The flow rate through the indicating system  10  (including indicator  18 ) can be increased or decreased as desired depending on the safety factor required by manipulating flow meter  16  and valve  14 . 
     Referring to FIGS. 3-5, it can be appreciated that when the indicator  18  is fresh and unexposed to any base, the indicating medium  22  shows an unchanged medium  26  of the initial color, typically yellow if the indicator used is bromophenol blue. As shown in FIG. 4, as more contaminant passes through the indicator  18 , the medium exposed to the contaminant base changes color and the spent medium  24  can be visually detected through the tube  20 . A front  28  provides a clearly visually perceptible line advancing along the tube  20 . As shown in FIG. 5, when the medium  22  is substantially entirely affected, all of the medium  22  has changed color to show the spent medium  24 , indicating that the adsorption bed filter  70  should be changed. 
     It can be appreciated that the above example is for a system for detecting airborne bases such as ammonia. However, it can be appreciated that other substances may be utilized for measuring the presence of other types of airborne compounds and used just as effectively. In addition, although the above described example provides for predicting the life of an adsorption bed assembly, the indicator system may be used to measure the presence of a contaminant and the cumulative concentration of such a contaminant over time and in ambient conditions that do not have a flow or an adsorption device. 
     Referring now to FIG. 6, there is shown a method for predicting the life of an adsorption filter using the indicator system  10  shown in FIG.  1 . When new adsorption medium, such as for example, activated carbon, is installed in an adsorption bed  70 , a fresh indicator tube  18  having fresh medium  24  is also installed. Using flow meter  16  and control valve  14 , the flow through the indicator  18  is calibrated to provide a relation between the rate of color change in the indicator  10  with the rate of deterioration of the adsorption bed  70 . Preferably, a time safety factor is included in the calibration. After the initial installation and calibration, further calibration is not necessary unless flow through the adsorption bed  70  changes. However, monitoring of the flow to ensure that the system is working properly can be done at any time with flow meter  16 . 
     After calibration, the flow meter  16  and valve  14  ensure that the proper flow is maintained so that the flow through the indicator  18  is proportional to and provides a relation to the flow through the absorption bed  70 . Over time, as the indicator tube  18  is exposed to more contaminants, the front  28  of spent medium  24  moves along the length of the tube  18  as shown in FIGS. 3-5. This moving color front provides a visually perceptible indication which can be monitored. Because the rate of advancement of the front  28  is comparable to the deterioration rate of the adsorption bed  70 , it can be appreciated that the usage and life of the adsorption beds  70  can be predicted by viewing the indicator  18 . When the indicator medium  22  is substantially spent by a total change in color, as shown in FIG. 5, system operators know that it is time to change the adsorption beds  70 . 
     Replacement of the indicator  18  is preferably done at the same time as the media for the adsorption bed  70  are changed. If the flow within the indicator system  10  has already been calibrated to the flow of the adsorption filtering system, it is not necessary to recalibrate, so that the indicator tube  18  is the only element replaced at the same time as the adsorption beds  70 . A new indicator tube  18  serves to predict the rate of usage of the new adsorption bed  70 . 
     Although the indicator  18  has been described in detail as a predictor for the life of adsorption bed  70 , it should be understood that it is also possible to place indicator  18  downstream of bed  70  to function as a detector or contaminants, rather than a predictor of bed life. In such a configuration, indicator  18  would change color if any contaminants are found to remain in the air flow stream after passing through the adsorption filtering system. As a downstream indicator, indicator  18  would typically be shorter (approximately one to 2 inches), because the goal is not to observe the color front moving the length of the indicator tube, but to see any color change. In some systems, for example, if the contaminant is on the order of about 100 parts per billion (ppb), the bed has already failed and the overall system may be adversely affected by the time indicator  18  shows any color change. For such an embodiment, indicator  18  may not be desirable in the downstream position. However, for some systems where the contaminant is about 5-10 ppb, color change in indicator  18  would warm that the bed is close to its failing point. The examples given above are examples used for the purpose of description only and should not be read as limiting values. 
     It is to be understood, however, that even though numerous characteristics and advantages of the present invention have been set forth in the foregoing description, together with details of the structure and function of the invention, the disclosure is illustrative only, and changes may be made in detail, especially in matters of shape, size and arrangement of parts within the principles of the invention to the full extent indicated by the broad general meaning of the terms in which the appended claims are expressed.