AIRBORNE CONTAMINANT INDICATOR

Abstract: An indicator system (18) provides a visual indication of the cumulative level of an airborne contaminant. The indicator is a transparent sheath (20) with support medium having an acidic surface and treated with a color pH indicator (20). 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: AIRBORNE CONTAMINANT INDICATOR 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 adsoφtion 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 adsoφtion bed life such as placing the beds in series. When adsoφtion beds are placed in series, a sensor may be placed in series intermediate the two adsoφtion beds. Therefore, as one adsoφtion bed becomes spent and the sensor indicates the presence of a contaminant, the second adsoφtion bed is still effective and failures are prevented. However, such detection systems have several drawbacks. When two adsoφtion 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 adsoφtion 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 adsoφtion beds may be changed out, thereby decreasing labor, but also shortening the useful life of the downstream adsoφtion beds as they are removed prior to being fully spent. Other systems utilize a sensor placed directly in the adsoφtion bed. However, in very thin adsoφtion beds, such a sensor may take up valuable space. In addition, an interface for detecting the presence of contaminants within an adsoφtion 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 adsoφtion 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 adsoφtion 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 adsoφtion 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 adsoφtion beds. By sampling upstream of the adsoφtion beds, an accurate prediction of the amount of contaminants flowing to the adsoφtion 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 adsoφtion bed device. Such an indication system should provide a clear visual indication of the bed usage and indicate when the adsoφtion 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 adsoφtion bed. It can also be appreciated that such a system should provide for a variable safety factor to ensure that adsoφtion 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 adsoφtion 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 adsoφtion 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 adsoφtion 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 adsoφtion 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 absoφtion beds to sample the air flow. Since the air passing through the indictor has the same composition as the air passing through the absoφtion 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 adsoφtion bed. Thus, when the rate of the front advancement in the tube, the flow rate over the adsoφtion beds, and the bed deterioration rate, are known, the monitoring indicator can predict when the adsoφtion 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 Figure 1 shows a diagrammatic view of an indicator system according to the principles of the present invention; Figure 2 shows a side elevational view of a counterflow adsoφtion bed apparatus having the indicator system shown in Figure 1 ; Figure 3 shows a side elevational view of an indicator device for the system shown in Figure 1 ; Figure 4 shows a side elevational view of the indicator device shown in Figure 2 with a portion of the indicator medium changed; Figure 5 shows a side elevational view of the indicator device shown in Figure 2 with the indicator medium shown fully changed; and, Figure 6 shows a flow chart of a method of calibrating an indication system for predicting the interval of an adsoφtion device according to the principles of the present invention. Detailed Description of the Preferred Embodiment Referring now to the drawings, and in particular Figure 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. X-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 Figure 2, the indicator system 10 may be used with an adsoφtion 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 adsoφtion beds 70. Flow passes from the inlet plenum 64 in parallel through the adsoφtion 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. Patent No. 5,290,345 to Osendorf and assigned to Donaldson Company, assignee of the present invention. In the configuration shown in Figure 2, the probe 12 is placed in the inlet plenum to intercept air flow prior to it being treated through the adsoφtion 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 adsoφtion beds 70. In the configuration shown, the indicator system 10 can be used as a predictor of adsoφtion bed life. It can be appreciated that the adsoφtion beds 70 are positioned to receive a balanced air supply through the beds so that the deterioration rate of each adsoφtion bed 70 is equal. The indicator system 10 is used as a predictor of adsoφtion 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 adsoφtion beds 70. Referring now to Figures 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 1/8 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 adsoφtion beds are changed and it is desired to have a fresh indicator 18 associated with the fresh adsoφtion 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 shaφer 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 shaφer 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 ) 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 puφle, 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/puφle 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 Figure 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 absoφtion bed. This curve can be compared to the known capacity of adsoφtion bed 70 and the expected breakthrough time or failure point of adsoφtion 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 Figures 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 Figure 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 Figure 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 adsoφtion 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 adsoφtion 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 adsoφtion device. Referring now to Figure 6, there is shown a method for predicting the life of an adsoφtion filter using the indicator system 10 shown in Figure 1. When new adsoφtion medium, such as for example, activated carbon, is installed in an adsoφtion 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 adsoφtion 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 adsoφtion 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 absoφtion 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 Figures 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 adsoφtion bed 70, it can be appreciated that the usage and life of the adsoφtion 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 Figure 5, system operators know that it is time to change the adsoφtion beds 70. Replacement of the indicator 18 is preferably done at the same time as the media for the adsoφtion bed 70 are changed. If the flow within the indicator system 10 has already been calibrated to the flow of the adsoφtion 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 adsoφtion beds 70. A new indicator tube 18 serves to predict the rate of usage of the new adsoφtion bed 70. Although the indicator 18 has been described in detail as a predictor for the life of adsoφtion 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 adsoφtion 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 warn that the bed is close to its failing point. The examples given above are examples used for the puφose 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.

Claims: WHAT IS CLAIMED IS: 1. An indicator system for indicating the level of an airborne contaminant, the system comprising: a pump for supplying a sample airflow from a main flow; a flow meter for measuring and calibrating the sample airflow; an indicator for providing a real time indication of increasing exposure to the airborne contaminant, the indicator comprising a hollow tube having a substantially transparent portion, wherein the tube is filled with a medium comprising zirconia having an acidic surface with a color-changing dye on the surface, wherein the surface of the medium changes color as the contaminant is accumulated. 2. An indicator system according to claim 1, wherein the color changing dye comprises a pH indicator. 3. An indicator system according to claim 2, wherein the sample airflow passes within the tube from a first end to a second end and wherein the color changing dye produces a visible changing color front along the tube as the dye is exposed to the airborne contaminant. 4. An indicator system according to claim 1, further comprising a control valve controlling the sample air flow. 5. An indicator system according to claim 4, wherein the control valve is calibrated to sample at a predetermined rate. 6. A method of indicating the deterioration level of an adsoφtive medium for removing airborne contaminants and having a first flow passing from upstream to downstream through the adsoφtive medium, comprising the steps of: obtaining a sample air flow proportional to the first flow; passing the proportional air sample through an indicator comprising indicating medium comprising zirconia and a coating having a first color, wherein on exposure to the airborne contaminant, the coating changes to a second color; calibrating the proportional air sample so that the indicating medium changes from the first color to the second color at the same rate that the absoφtive medium deteriorates. 7. A method according to claim 6, wherein the proportional air sample passes through the indicating medium from a first end to a second end, and wherein a front of second color passes through the indicating medium from the first end to the second end. 8. A method according to claim 7, comprising the further step of calibrating the sample flow through the indicating medium so that medium changes completely when the adsoφtion medium requires replacement. 9. A measuring system for measuring the life of an adsoφtive filter used to remove an airborne contaminant, the adsoφtive filter having flow passing from upstream to downstream through the filter, the measuring system comprising: an inlet for sampling air upstream of the adsoφtive filter at a rate proportional to the flow through the adsoφtive filter; a visual indicator receiving an air sample flow for indicating the presence of the airborne contaminant, the visual indicator comprising a length of substantially transparent tubing having a zirconia indicating medium with a color changing dye. 10. A measuring system according to claim 9, further comprising a calibrating device for calibrating the visual indicator relative to a deterioration rate of the adsoφtive filter. 11. A measuring system according to claim 9, further comprising a control valve for varying the sampling flow rate.

Inventors: DALLAS ANDREW J, GRAFE TIMOTHY H, GRAHAM KRISTINE M, DALLAS, ANDREW, J., GRAFE, TIMOTHY, H., GRAHAM, KRISTINE, M.
Applicants: DONALDSON CO INC, DONALDSON COMPANY, INC.
Application year: 1999
Main classification: G01N31
IPC codes: B01D53, G01N1, G01N21, G01N31, G01N33