Patent Publication Number: US-2007105494-A1

Title: Ductless fume hood with improved filter monitoring system and extended filter life

Description:
FIELD OF THE INVENTION  
      The present invention relates to an improved ductless fume hood with an improved system for monitoring filter life and an improved design for extending filter life.  
     BACKGROUND OF THE INVENTION  
      Ductless, or filtration, fume hoods are a specific type of fume hood that use a filtration system to remove contaminants from an air stream. Ductless hoods operate by simply forcing contaminated air from the hood enclosure through a filter to remove chemical vapors before returning the air to the laboratory environment.  
      Ductless hoods have several convenient advantages over laboratory installed, ducted hoods. They are mobile and portable and have minimal installation costs as they do not have to be connected to a duct system. They are environmentally friendly, as no toxic gases are released into the atmosphere. Ductless hoods also have very low operating costs, as no conditioned air is removed from the laboratory.  
      Because of the advantages listed above, ductless hoods are popular with academic laboratories, especially those with limited budgets. As ductless hoods are able to be operated anywhere in the laboratory and often are made transparent on all sides, they are ideal for teaching demonstrations, allowing students to surround the hood. Ductless hoods have also grown in popularity in industrial laboratories, where they can be used for specific projects with low costs.  
      The main drawback of ductless hoods is the potential release of toxic gases into the laboratory because of filter saturation and breakthrough. While most of the advantages of using ductless hoods are derived from the re-circulating of air from the hood back into the laboratory, this re-circulation means that the air exiting the hood must be filtered at all times. Most ductless hoods use an activated carbon filter as their filtration system. Although activated carbon is highly adsorbent, the activated carbon particles eventually become saturated. When the activated carbon becomes saturated through the thickness of the filter, chemical vapors are no longer adsorbed and begin to break through into the exhaust.  
      The primary inconvenience of operating a ductless hood is the need to monitor the hood filter to ensure the safety of those working in the laboratory. If the exhaust concentration of a specific compound exceeds the allowed limits set by the United States Occupational Health and Safety Administration (OSHA) or other local limits, then the filter must be changed. Prior designs have conventionally employed a timer that sounds an alarm every 60 hours of operation time to notify the user that it is time to check the condition of the filter. However, studies have shown that these arbitrary alarms rarely coincide with the actual timing of filter saturation. This is not surprising, as this arbitrary method of filter monitoring does not take into account the actual use of the hood while it is running.  
      Because of the unreliability of the 60 hour alarms, they are often ignored, leaving the user to test the condition of the filter whenever the user feels it might be necessary. In most cases, the user will wait until a detectable odor develops in the laboratory, which is often the point at which the concentration of the compound in the air has already exceeded the OSHA limits. In other cases where the compound being used is odorless, the user is forced to be very vigilant in checking filter efficiency, and often spends a great deal of time performing tests that are not necessary. Better methods of filter monitoring are needed to maintain the safety of the hood operator and others in the laboratory without imposing inconvenient requirements that the filter be checked more often than necessary.  
      Other ductless hood designs, such as that described in U.S. Pat. No. 4,946,480, which is hereby incorporated by reference herein, have attempted to solve this problem by installing a gas sensor downstream of the filter to detect the concentration of compounds in the filter exhaust. This effort has largely proven futile as it is not possible to monitor the hundreds of different compounds used in a laboratory with just one sensor. The sensors used in ductless hoods are typically broad range detectors without any specificity for particular compounds. Although control system read-out can be obtained for the exhaust gas concentration, it is difficult to correlate this read-out to an actual concentration of an actual gas, and report to the user if that concentration actually exceeds the OSHA exposure limits. Much of this phenomenon comes from the fact that the sensor has widely varied sensitivity to different gases. This varied sensitivity makes it very difficult to choose a level of detection for the sensor at which the alarm should be triggered, especially in a situation when multiple types of chemicals are to be used in the hood. An improved ductless hood filter monitoring system would greatly improve on the safety and ease of use of ductless hoods.  
      With both of the above filter monitoring methods, it is still necessary to perform air sampling tests to confirm that the filter is actually compromised. These tests usually involve use of a gas detection tube containing a color change reagent specific to the gas to be detected. Whenever a filter alarm sounds, the user must stop work and take the time to sample the exhaust air using a hand pump before deciding if a filter change is actually necessary. More convenient methods for determining filter life are necessary to simplify compliance with safety regulations.  
      Another current problem with ductless hood systems is uneven filter use. The filter of a ductless hood is usually about the same size as the ceiling of the enclosed hood area to allow for better filtration of the entire hood. However, most work is done in the center of the hood, meaning that the majority of chemical vapors come in contact with the center of the filter. This causes the center of the filter to quickly become saturated and allows for the breakthrough of chemical vapors even though the areas of the filter near the enclosure walls are largely unused. A device that would cause even loading of a ductless hood filter would greatly extend the lifetime of the filter, making ductless hoods even more convenient to use.  
      Therefore, there remains a need for a ductless hood that extends the life of the filter and simplifies compliance with safety regulations.  
     SUMMARY OF THE INVENTION  
      An object of the present invention is to provide a ductless fume hood with an improved system for monitoring filter life.  
      Another object of the present invention is to provide a system for measuring the filter efficiency of a ductless hood.  
      The system of the present invention provides a gas sensor upstream and a gas sensor downstream of the hood filter. The upstream sensor measures the concentration of gases entering the filter while the downstream sensor measures the concentration of gases exiting the filter. Data from the sensors is sent to a microprocessor, which calculates the efficiency of the filter based on the change in the concentration of gases between the inlet and exhaust air stream. When the efficiency of filtration falls below a specific, pre-determined percentage (for example 90%), the microprocessor sounds an alarm to warn the user that the filter is compromised.  
      The filter monitoring system of the present invention provides the distinct benefit of being able to accurately monitor efficiency of filtration regardless of the chemicals being used in the hood. Because sensors of the same type are used both upstream and downstream of the filter, they will exhibit the same response (i.e. voltage read-out to a microprocessor) regardless of the chemical vapors present in the air stream.  
      A further advantage of the upstream and downstream sensor system is that there are certain conditions under which filters that are reaching their maximum capacity begin to desorb. In such conditions, the concentration readings of the downstream sensor will be higher than the concentration readings of the upstream sensor. When such a condition occurs, the microprocessor can sound an alarm to let the user know that the filter is compromised before the expected end of its life. This is a clear advantage over the current systems with only a downstream sensor, which would not be able to detect such an event.  
      Another object of the filter monitoring system of the present invention is to allow a more accurate assessment of the actual exposure of the filter to chemical vapors by placing a sensor upstream of the filter. Instead of an alarm sounding after every 60 hours of use, the system of the present invention will allow for the prediction and warning of the end of filter life by detecting the approximate concentration of chemical vapors the filter has been exposed to and the rate of change of filter efficiency.  
      A further object of the present invention is to provide an improved fume hood comprising a novel diffuser that allows for even filter loading. The diffuser contains a plate of metal or other material with a series of holes that is positioned upstream of the filter. The diffuser causes the chemical vapors in the air stream to disperse over the entire air stream, regardless of the location of the source of the vapors in the hood. This effectively increases the volume of the filter that is exposed to the vapors which, in turn, extends filter life. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       FIG. 1  is a diagram of an embodiment of a ductless hood apparatus of the invention.  
       FIG. 2A  is a schematic front view of the embodiment of the invention shown in  FIG. 1 .  
       FIG. 2B  is a schematic cross-sectional view of the ductless hood apparatus of  FIG. 2A  taken along line II.  
       FIG. 3  is an electrical circuit diagram of a preferred embodiment for the control system of the invention.  
       FIG. 4  is a schematic view of an embodiment of the main board of the invention.  
       FIG. 5  is a flow diagram showing an embodiment of a microprocessor program of the invention.  
       FIG. 6A  is a schematic showing the air flow and filter wear patterns in a conventional ductless hood.  
       FIG. 6B  is a schematic showing the air flow and filter wear patterns in a ductless hood of the invention.  
       FIG. 6C  is a schematic of a preferred embodiment of the diffuser of the invention. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION AND DRAWINGS  
      The ductless fume hood apparatus of the present invention contains an improved system for monitoring filter life and a novel diffuser causing even filter loading. An aspect of the present invention is that the efficiency of the filter of the hood can be easily and reliably monitored. A further aspect of the present invention is that the diffuser causes chemical vapors from the hood to be diffused evenly across the surface of the filter, causing even filter loading and increasing filter life. Although containment of chemical vapors is referred to throughout the application, an embodiment of the present invention could also be used for the containment of biological vapors with appropriate filters and sensors.  
      Referring to the drawings, throughout this description, like elements are referred to by like numbers as shown in the drawings.  
      One embodiment of the apparatus of the present invention is a ductless hood  1  as shown in  FIG. 1 . A blower  3  is used to pull air from the laboratory into the hood enclosure  5 , where it will carry chemical vapors present in the hood up through the filter  7 . Filtered air is then pulled into the blower compartment  9 , through the blower  3 , and pushed out the top of the hood back into the laboratory environment.  
      A front view schematic of an embodiment of the present invention shown in  FIG. 1  is shown in  FIG. 2A . The ductless hood  1  comprises a hood enclosure  5  bounded on three sides, preferably by tempered glass windows,  11  and on the front by an adjustable sliding sash window  13 , also preferably made of tempered glass. The sliding sash window  13  adjusts to various heights by sliding up into the blower compartment  9  to allow the hood operator access to the hood enclosure  5 . The bottom surface of the hood enclosure  5  is a work surface  15 , preferably made of stainless steel, that allows for easy maintenance and clean up of spills. The internal back wall of the hood enclosure  5 , preferably has a number of electrical outlets  17  to allow for the use of laboratory equipment inside the hood enclosure  5 . The ceiling of the hood enclosure  5  is formed by the bottom surface of the blower compartment  9 . The microprocessor control panel  19  located on the blower compartment  9  is used for monitoring the filter sensors and adjusting the speed of the blower fan.  
      A side cross sectional view of  FIG. 2A  taken along line II is shown in  FIG. 2B . The hood enclosure  5  is as described above for  FIG. 2A . The windows  11  on the sides of the enclosure contain service fixture openings  21  that allow for attaching various laboratory equipment to the hood enclosure  5 . The hood enclosure  5  may be lit by fluorescent lights  23 . The blower  3  creates an air stream up from the hood enclosure  5  through the diffuser  24 , past the upstream gas sensor  25  and airflow sensor  27 . The air stream then passes through the optional pre-filter  29  and into the filter  7  where harmful vapors are removed. Finally, the air stream passes the downstream gas sensor  31 , through the blower  3  and out through the optional backup filter  33 . The electronics of the system are contained in the electronics panel  35 , which is preferably located on the front of the blower compartment  9  and are controlled by the microprocessor control panel  19 .  
      In one embodiment of the filter monitoring system, the gas sensors are volatile organic compound sensors, examples of which are described in U.S. Pat. Nos. 6,565,812, 6,499,335 and 6,128,945, which are hereby incorporated by reference herein. It will be obvious to one of skill in the art that various types of gas sensors may be used within the scope of the present invention. It is of primary importance that the upstream and downstream gas sensors are of the same type, because the readout of the two sensors will be compared by the microprocessor to determine the filter efficiency. However, the invention also contemplates many different sensor types, with the difference taken into account in monitoring filter efficiency.  
      The filters used in the ductless fume hood of the invention are preferably activated carbon filters. Activated carbon filters are suitable for use with many chemicals, but their use is dependant on a number of factors. In general, chemicals with a relative molecular weight over 30 g/mol and a boiling point higher than 60° C. (140° F.) can be adsorbed with relatively high efficiency by active carbon filtration. It is also generally true that larger molecules adsorb better than smaller molecules and that less soluble compounds adsorb better than more soluble compounds.  
      There are several factors that influence the adsorption of organic compounds to activated carbon filters. Branch chain organics are adsorbed more easily than straight chain organics, while unsaturated organics (those containing double or triple carbon bonds) are adsorbed more easily than saturated organics. Polarity is another factor, as less polar organic compounds are better adsorbed than highly polar compounds.  
      Environmental factors may also affect the adsorptive efficiency of the hood. The ambient temperature of the laboratory and the gas must be kept to a minimum, as higher temperatures lead to lower efficiency of adsorption. Relative humidity must also be kept to a minimum as high relative humidity can cause the activated carbon filter to adsorb water molecules in the place of chemical vapors.  
      The construction of the filter itself also affects the adsorption efficiency of the hood. A filter with a thicker media bed will have a longer contact time during which a larger amount of chemical vapor will be deposited in the filter bed by physical adsorption. In order to maximize this contact time, the air volume passing through the hood and the filter must be kept to a minimum. However, it is necessary that the inflow velocity of the hood (through the front opening) be maintained at a high enough rate to ensure proper containment of vapors in the hood.  
      In general, standard activated carbon filters work at an efficiency of typically higher than 95% across a broad spectrum of chemical compounds. In order to increase adsorptive efficiency for specific applications, specialty carbon filters impregnated with other compounds may also be used. This is commonly known as chemisorption. A non-limiting example of a specialty carbon filter is a filter in which the carbon media is impregnated with an oxidizing agent to oxidize formaldehyde and glutaraldehyde fumes. This type of filter is useful for specialty applications such as hospital pathology and endoscopy, which involve these compounds.  
      Because it is possible to use special filters for specific applications, the filter of the ductless fume hood of the present invention is readily interchangeable with other types of specialty filters. It should also be apparent that other, non-activated carbon, filters can be used in the ductless fume hood without departing from the scope of the invention.  
      A preferred embodiment of a control system  37  for a ductless hood of the invention is shown in the circuit diagram of  FIG. 3 . The main board  39  of the control system monitors input from the various switches and sensors and sends internal messages to the relay board  41  and output messages to the user for display on the LCD module  43 . Non-limiting examples of output messages may include feedback such as air flow rate and filter capacity remaining. The user can provide input through the membrane keypad  45 , which sends messages through the interface board  47  to the main board  39 .  
      The main board  39  receives signals from the system to monitor its performance. The upstream sensor  25  and downstream sensor  31  send their read-out through a printed circuit board (PCB)  49  to the main board  39  for calculation of filter efficiency. The main board also receives input on the air speed moving through the filter from the air flow sensor  27  and input on the ambient temperature of the system from the temperature sensor  51 . The system also optionally contains a magnetic switch  53 , which detects whether the sliding glass window is at a nominal height position.  
      The main board  39  also sends commands to the relay board  41  which controls various components such as the speed of the blower (PSC motor)  3 , the alarm system and fluorescent lights  23  for lighting the hood enclosure. The alarm system is integrated on the relay board  41  and will be triggered if any unsafe condition is detected by the main board. Non-limiting examples of alarm systems include buzzers, bells, sirens and steady or flashing lights.  
      The control system  37  diagrammed in  FIG. 3  is connected to a power supply through a power inlet  55 . The AC power source is then converted to DC through the switching mode power supply (SMPS)  57 .  
      The four male-female connectors  58  illustrate the connections between all of the components of the system shown in  FIG. 3 . Connections in the diagram are labeled to represent the connector and pin number through which the connection is made. For example, one of the connections between the SMPS  57  and the temperature sensor  51  and air flow sensor  27  is denoted D 10 F D 10 M, which is a connection between the female and male sides of connector D at pin  10 . It should be apparent that various connectors and types of connections can be used without departing from the scope of the invention.  
      A preferred embodiment of the main board  39  of the present invention is shown in  FIG. 4 . A microprocessor  59  on the main board  39  interacts with the other components of the control system through the connectors shown in  FIG. 4 , including: a relay board connector  61 , a LCD module connector  63 , analog and digital interface connectors  65 ,  67  for receiving data from the gas, air flow, and temperature sensors, and a membrane keypad connector  69  for user entry. The main board of  FIG. 4  also contains a SMPS input  71 , a hardware reset switch  73 , a battery backup power supply  75 , a JTAG interface connector for testing the circuit  77 , a JP 5  circuit for programming  78  and a DB 9  port  79 .  
      In a preferred embodiment, the microprocessor of the present invention is a microprocessor such as those described in U.S. Pat. Nos. 5,805,909, 5,877,641, and 6,828,869, which are hereby incorporated by reference herein. A non-limiting example of the microprocessor of the present invention is one of the Micro-Controller MSP430F series from Texas Instruments, Inc. (Dallas, Tex.). It should be apparent that microprocessors of various types could be used for controlling and monitoring the ductless hood system of the present invention.  
      An embodiment of a microprocessor program for monitoring filter efficiency is shown in the flow diagram of  FIG. 5 . The microprocessor receives input from the upstream and downstream sensors and computes the efficiency of filtration  80  by comparing the concentration of volatile organic compounds entering and exiting the filter. If the percent efficiency is found to be below a specific pre-determined number (for example 90%)  82 , then the alarm sounds  84 . The user is then given the choice to replace the filter  86 . If the user chooses to replace the filter the system can be reset and use can be restarted. If the user does not replace the filter, the user has the option of checking the exhaust of the hood  88  by manual air sampling. If the user chooses to test the exhaust  90  he/she may then determine whether a toxic level of gas in the exhaust was reached  92 . If a toxic level was not reached, the user may then adjust the alarm set point to prevent future false alarms  94 .  
      The microprocessor program diagrammed in  FIG. 5  allows the user to customize the alarm set point for specific applications. Once a reliable set point is established, the user will not be required to perform manual air sampling and will be able to rely on the alarm system to determine when the filter is no longer effective. This program allows for the added convenience of being able to rely on an alarm without compromising the user&#39;s safety.  
      It should be apparent that the percent efficiency for the alarm set point can be set at varying levels without departing from the scope of the invention. The percent efficiency for the alarm set point can be changed as needed for specific applications. By way of non-limiting example, it might be desirable to change the percent efficiency of the alarm set point when changing the compound or compounds being used in the hood. A further non-limiting example would be changing the percent efficiency of the alarm set point when a significantly greater or smaller amount of a compound is to be used in the hood. Most importantly, the percent efficiency of the alarm set point should be set at a level that warns the user when the filter is allowing harmful vapors to be exhausted from the hood at a concentration that is greater than OSHA or local standards.  
      It should also be apparent that other microprocessor programs could be used for monitoring filter efficiency within the scope of the present invention. Any program that is able to compare the input readings from the upstream and downstream sensors can be used in an embodiment of the filter monitoring system.  
      The microprocessor will constantly monitor the ductless hood system and provide feedback to the user. Non-limiting examples of feedback that the microprocessor will provide include: 
      a. Filter efficiency. This is calculated by examining the sensor outputs of the two sensors (one upstream and one downstream) by monitoring the reduction of concentration of chemical or other vapors by the filter. As the ductless hood will not be in continuous operation, there will be periods of time when the sensors will both read zero. The microprocessor programming will take this into account and display the “last known average value” of the filter efficiency.     b. Theoretical filter capacity utilization. When the hood is new, this starts at 100% (capacity remaining). As the filter efficiency deteriorates from the initial 100% value, to the minimum acceptance level (for example 90%), the filter capacity remaining will be pro-rated. For the non-limiting example shown in  FIG. 5 , when the filter efficiency is 100%, the capacity remaining is 100% as well. When the filter efficiency drops to 95%, the capacity remaining will be 50%.     c. In certain conditions, the carbon filter can start to desorb when close to maximum capacity, the filter will emit vapors such that the concentration on the effluent (downstream) side can be higher than on the influent (upstream) side. This will create a unique situation during which the efficiency cannot be calculated (due to a logic error that the outlet concentration is higher than the inlet concentration). In such a situation, the second sensor will register a reading while the first sensor will be zero. This will be an immediate indication to the control system to inform the user to change the filter.    

      In another embodiment of the present invention, the upstream sensor is used to approximate filter life. The upstream sensor provides continuous data to the microprocessor while the ductless hood is being operated. The data provided by the upstream sensor can be used to determine the approximate concentration of chemical vapors to which the filter has been exposed. This concentration can be integrated over time to determine when a filter has been exposed to an amount of chemical vapor that is close to its saturation point. This way, the user will have a method to predict when the filter needs to be changed, and may even change the filter in advance of the alarm sounding. This would be especially important to prevent the interruption of complicated laboratory procedures when the filter is near saturation.  
       FIG. 6  shows an embodiment of the diffuser system of the present invention.  FIG. 6A  is a schematic of the pattern of filter saturation in a conventional fume hood. As stated above, the center of the filter  7  is more highly exposed to chemical vapors due to the air flow of the hood and the location of the source of vapors in the center of the work surface.  FIG. 6B  is a schematic showing how the addition of a diffuser  24  to the ductless hood causes changes in air flow that lead to uniform exposure of the filter  7  to chemical vapors.  
       FIG. 6C  is a schematic of a preferred embodiment of the diffuser  24  of the present invention. The diffuser  24  in  FIG. 6C  is a perforated sheet with a specific pattern of holes that causes the air flow from the hood enclosure to be evenly spread out over the entire filter  7  surface. The diffuser  24  of the present invention can be made of metal, plastic or any other material that does not react with the chemical vapors that will be produced in the hood.  
      A preferred embodiment of the pattern of holes for the diffuser  24  is shown in View A of  FIG. 6C . In this preferred embodiment, the diffuser holes are circular holes 3 mm in diameter with a 5 mm pitch. It should be apparent that there are other patterns of holes that still fall within the scope of the diffuser of the present invention, including various shapes and sizes of holes in different arrangements with different spacings between them. It should be also apparent that a diffuser with any pattern of holes that allows for the air stream coming from the hood enclosure to be diffused over the entire surface of the filter falls within the scope of the present invention.  
      Specific embodiments of the apparatus of the present invention have been set forth above. It should be apparent to one of skill in the art that there are further variations that fall within the scope of the invention as set forth in the claims below. A non-limiting example of a variation that falls within the scope of the invention is a change in the size of the apparatus, such as a small desktop hood apparatus or an apparatus larger than a conventional fume hood. It should also be appreciated that the apparatus of the current invention can be used for protecting a user from chemical vapors in settings outside of a laboratory, such as in a manufacturing or commercial setting.