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Last Update: 05/01/00

Vacuum Pump Exhaust Filters

Part I - A Two Stage Vacuum Pump Exhaust Filter System

INTRODUCTION

The quality of indoor air is affected by many factors including construction materials, heating and ventilation systems and emissions of volatile organic compounds (VOC's) from building furnishings, equipment, and maintenance supplies.  In the laboratory, chemicals and instrumentation are additional contributors to the contamination of the indoor air.  Vacuum pumps are widely used for mass spectrometers, electron microscopes, lyophilizers, and other vacuum equipment.  These pumps can be a major source of indoor air contamination.  It is normally recommended that these vacuum pumps be vented outside the room or to a laboratory exhaust hood.  However this is not always practical when the instrumentation is located within the interior of a building. 

Indoor air contamination by vacuum pumps originates from the lubricating oils used in the pumps, and also from chemicals that have contaminated the pump oil.  When the vacuum pumps are first turned on, a large volume of air is pumped through the pump oil which causes a fine mist of oil fumes to be emitted from the pump exhaust and into the laboratory air.  When the vacuum pumps are used in instruments such as mass spectrometers, all the residual organic chemicals analyzed by the mass spectrometer are trapped in the vacuum pump oil.  Eventually these organics are purged out of the oil and into the laboratory air via the vacuum pump exhaust.  Often the organic chemicals analyzed by these vacuum system instruments can be quite toxic or carcinogenic.  This can present a serious environmental health problem in the laboratory.

This is the first of three articles reporting on the effectiveness of filters designed for use on the exhaust ports of vacuum pumps (1, 2 & 3).  This first article demonstrates the effectiveness of a two stage vacuum pump filtering system.  The first stage filter consists of an oil mist eliminator to trap the heavy oils from the pump oil mist and return them to the vacuum pump.  The second stage is a charcoal trap for the containment of volatile organics in the vacuum pump exhaust.  The second article in this series (2) describes the design of the oil mist eliminator and includes additional studies of the effectiveness of the oil mist eliminator.  The third article in this series (3) describes the efficiency of the charcoal trap in greater detail.  The combination of the two vacuum pump exhaust filters described in this article can provide for efficient and safe trapping of emissions from laboratory vacuum pumps and can improve the quality of air in the instrumentation laboratory. 

EXPERIMENTAL

Figure # 1 – Vacuum Pump Exhaust Filtering System Mounted on a Vacuum Pump

The two stage vacuum pump exhaust filtering system is shown above (Figure # 1 and 1a).  The two filters attach in series to the exhaust port of a vacuum pump.  For these studies, an Alcatel oil mist eliminator was used (Scientific Instrument Services part # 66827) as the stage 1 filter.  An NW25 vacuum pump exhaust fitting was first attached to the exhaust port of the vacuum pump, and the Oil Mist Eliminator was clamped to this fitting using a centering ring and clamp.  The Stage 2 charcoal trap used for this study is the Koby charcoal exhaust filter (Scientific Instrument Services Part # KA-1), which is fitted with an adaptor to mate it to the NW fitting on top of the oil mist eliminator.  The two filters are clamped together using a centering ring and clamp.

All studies were performed on a BOC Edwards model E2M1.5 rotary vacuum pump, which is the standard model used on the Agilent mass spectrometers.  The old oil in the pump was drained and the pump was charged with a volume of Inland 45 pump oil (Scientific Instrument Services).  Inland 45 pump oil is the vacuum pump oil of choice because it is a highly refined oil comprised of aliphatic hydrocarbons with chain lengths of between 20 and 40 carbon units. The pump was run for two hours and the oil drained.  The pump was again filled with a full charge of Inland 45 pump oil and run for 2 hours with the gas ballast valve open and then for an additional 2 hours with the gas ballast valve closed before any testing was done.  This cleaning and flushing procedure assured that the pump was free of contaminants from the previous use of the vacuum pump.  For this study a micro-needle valve was attached to the intake port of the vacuum pump to permit a calibrated flow of air into the vacuum pump.  The flow through the needle valve was adjusted to 100 ml/min (or 300 ml/min for some studies) and calibrated with a Gilibrator Air Flow Calibration System (Scientific Instrument Services Part # 800844-2).  

Figure # 2 – Analysis of Vacuum Pump Exhaust in the Two Stage Exhaust Filtering System

For these studies, air samples were collected at three levels in the exhaust stream from the vacuum pump (Figure # 2).  The first level (Stage A) is directly from the pump exhaust with no filtering.  Another air sample was collected after the oil mist eliminator (Stage 1) and the final air samples were collected after the charcoal trap (Stage 2) and right before the exhaust air would enter the laboratory air. 

Thermal Desorption tubes were packed with 100 mg of Tenax® TA and flow conditioned with pure nitrogen at 50 ml/min at 300 degrees C for 3 hours.  Air samples were collected from the three pump stages onto the Tenax adsorbent traps at a collection rate of 25 ml/min for 20 minutes (500 ml total) using a Gilian LFS-113 Air Sampling Pump (Scientific Instrument Services).  The collected samples were analyzed using the SIS Automated Short Path Thermal Desorption System (AutoDesorb Model 2000, Scientific Instrument Services) which is described in previous articles (4) and shown below (Figure # 3).  The desorption tubes were attached to the AutoDesorb system and first purged with 30 ml of helium. The desorption tubes were then desorbed from 100 to 250 degrees C at a rate of 60 degrees per minute and held at 250 degrees for a total desorption time of 6 minutes.  The desorbed analytes were trapped on a GC Cryo-Trap (Scientific Instrument Services), which was pre-cooled to –65 degrees C using liquid CO2.    

Figure # 3 – Schematic of the SIS AutoDesorb System on a GC/MS System

The AutoDesorb system was attached to an Agilent Technologies 6890 GC for the separation of the analytes and the Agilent 5973 MSD to detect and identify the analytes.  The GC column for these studies was an Agilent DB5-MS, 0.25 u film thickness, x 0.25 mm I.D. x 30 meters long.  A 6" long DB-5, 1.5 u film thickness x 0.53 mm I.D. guard column was used at the front of the GC column and inside the Cryo-Trap.  When the desorption was complete, the Cryo-Trap was rapidly heated to 250 degrees C.  The GC column was held at 40 degrees C for 5 minutes and then temperature programmed to 280 degrees C at 10 degrees per minute.  The total GC run time was 29 minutes.  The Agilent MSD was operated in the EI mode and scanned from 35 to 450 daltons.

For the studies of the ability of the filters to trap volatiles, 10 ul of gasoline was injected directly into the vacuum pump.  Gasoline was selected because it contains a wide range of volatile and semi-volatile organics and would accurately simulate the wide range of chemicals that might be injected into a mass spectrometer. 

DISCUSSION

Oil Mist Eliminator

Oil mist eliminators are designed to trap the heavy oil vapors or mists which escape from the vacuum pump during initial pump down, during extended operation or when using the gas ballast valve to purge or outgas the vacuum pump oil.  The oil mist eliminator contains a replaceable paper cartridge inside the filter housing.  The paper wick exposes the oil mist to a large surface area to adsorb the oil vapor and return it to the vacuum pump.  When a vacuum pump is first turned on, this oil mist is easily seen as a fine mist, which enters the laboratory environment.  See the second article in this series for a more thorough description of the oil mist eliminator (2).  

Figure # 4 -  Efficiency of the Oil Mist Eliminator at Removing Hydrocarbons (300 ml leak into pump)

The effectiveness of the oil mist eliminator is demonstrated in Figure # 4.  For this study, the purge leak valve on the vacuum pump inlet was opened and the leak flow was adjusted to 300 ml/minute.  At the vacuum pump exhaust port (stage A) of the pump, a high concentration of high molecular weight hydrocarbons is exhausted from the pump as shown in the top total ion chromatogram in Figure # 4.  A large number of straight and branched hydrocarbons were detected with carbon chains over 20 carbons in length.  The bottom total ion chromatogram in Figure # 4 shows the air sampled after the oil mist eliminator.  When the oil mist eliminator is used these high molecular weight organics are trapped by the filter and returned to the pump.  They do not exhaust out of the oil mist eliminator and are not present in the chromatogram.  This study demonstrates that the oil mist eliminators are quite effective at performing the job for which they have been designed, that is trapping the high molecular weight vacuum pump oils and returning them to the vacuum pump.  

Figure # 5 – Trapping Efficiency of Filter Stages (1.0 ul of gasoline injected into pump)  

However, the oil mist eliminators with their paper filters are not effective at trapping the lighter hydrocarbons and organics in the vacuum pump exhaust as shown in the chromatograms of the second study (Figure # 5).  In this study 1.0 ul of gasoline was injected directly into the vacuum pump and the exhaust air subsequently collected at the exhaust port of the vacuum pump and after the oil mist eliminator and analyzed as described above.  The exhaust at the top of the vacuum pump (top chromatogram - Stage A) as well as the exhaust after the oil mist eliminator (second chromatogram - Stage 1) exhibit high concentrations of the volatile aromatics and hydrocarbons from the gasoline. 

Additional studies described in the second article of this series (2), demonstrates that these volatiles are not purged out of the vacuum pump oil very quickly, but are in fact purged out over many hours or days of continuous pumping.  Therefore venting the vacuum pumps for a short period of time when hazardous materials are analyzed will not be effective in preventing these materials from entering the laboratory environment.  The vacuum pumps must be externally vented continuously or a better system such as the addition of a charcoal trap is needed to trap these more volatile organics. 

Charcoal Traps

Charcoal traps are ideally suited for the trapping of volatile and semi-volatile organics in air.  The Koby charcoal trap used for this study has been designed for the trapping of volatile and semi-volatile organics from the vacuum pump exhausts.  Each trap contains about 100 grams of carbon with a surface area of about 1500 sq. meters per gram.  This high surface area provides for very efficient air purification.  These charcoal filters have a high adsorption capacity and affinity for adsorbing organic contaminants.  Further testing described in the third article of this series (3), demonstrated that each charcoal trap could be saturated with 70 ml of gasoline with no volatile emissions being eluted from the filter.  In addition to trapping volatile and semi-volatile organics, these charcoal traps will adsorb water, oil mist dirt (to 0.5 microns) and bacteria (to 0.5 microns).  The Koby charcoal traps produce minimal backpressure to the vacuum pump, and can therefore operate safely on the exhaust port of the vacuum pump.   

Figure # 6 – Trapping Efficiency of Three Filter Stages (10 ul of gasoline injected into pump) 

In the next study, 10 ul of gasoline was injected directly into the vacuum pump and the exhaust air from all three locations or stages of the pump filters was collected and analyzed as described above (Figure # 6).  At the exhaust directly from the vacuum pump (top chromatogram - stage A) as well as the exhaust after the oil mist eliminator (middle chromatogram - Stage 1), high concentrations of the aromatics and hydrocarbons from the gasoline were detected.  However the charcoal trap was extremely efficient at removing all the volatile organics from the vacuum pump exhaust.  The exhaust from the top of the charcoal trap (bottom chromatogram – Stage 2) contained no peaks of any volatile or semi-volatile organics.  As shown above none of the light hydrocarbons including hexane and pentane were evident in the charcoal trap exhaust, nor was acetone or any of the aromatics common in gasoline present in the charcoal trap exhaust.    

Figure # 7 – Vacuum Pump Exhaust After the Koby Air Filter

An additional study (Figure # 7) demonstrates that the air exhausted by the vacuum pump through the Koby charcoal traps was cleaner than the normal indoor air in our testing facilities.  Note that the results shown in this graph are shown at 50X scale expansion from the data shown in Figure # 5 previously.  The small peaks shown in the graph are not from the gasoline, but are siloxanes from the GC septum. 

Assembly of system

Figure # 8 – Assembly of the Two Stage Vacuum Pump Exhaust Filtering System

The combination of the two exhaust filters are joined together, and to the vacuum pump, using standard NW (or QF) vacuum type connectors.  If the vacuum pump exhaust port is not equipped with an NW type fitting (either NW10, 16 or 25), then the appropriate adaptor for the pump must be first installed.  The oil mist eliminator is then attached to this fitting using a centering ring and ‘O' ring using a quick clamp.  The Koby filter comes with ¼" NPT male fittings on both ends of the trap.  Therefore a NW flange to ¼" female adaptor must be first attached to the Koby filter before it is attached to the top of the oil mist eliminator using another centering ring and quick clamp.  The oil mist eliminators, Koby charcoal traps as well as all the fittings necessary for the assembly of the dual exhaust filter system are available from Scientific Instrument Services (5).  In addition complete kits for the common pumps are also available (5).

CONCLUSION

This is Part I of a three part series of articles on the effectiveness of the vacuum pump exhaust traps.  Part I describes the two stage vacuum pump exhaust filter system for the complete treatment of vacuum pump exhaust (1).  Part II describes the operation and the effectiveness of the oil mist eliminator (2).  Part III of this series describes the charcoal trap and its effectiveness at removing volatiles and semi-volatiles from vacuum pump exhaust (3) 

The Two Stage Vacuum Pump Exhaust Filter System has been shown to be an effective system for removing volatile and semi-volatile organics from the exhaust of vacuum pumps and can help maintain an environmentally safe atmosphere in the instrumentation laboratory.  The oil mist eliminator (Stage 1) has been shown to effectively trap the high molecular weight hydrocarbons from the pump oil and return these oils back into the vacuum pump.  The charcoal trap (Stage 2) adsorbs and traps the volatile and semi-volatile organics from the pump exhaust and prevents these organics from entering the laboratory environment.  It is recommended that both filters be used.  The oil mist eliminator traps the large volume of heavy oils which would otherwise quickly saturate the charcoal trap during initial pump down or when using the gas ballast valve to purge the pump oil.  When both filters are used in series on a vacuum pump exhaust port, the exhaust from the final stage of filtering has been shown to be cleaner than the normal laboratory air.

References

(1) Vacuum Pump Exhaust Filters, Part I – Two Stage Vacuum Pump Exhaust Filter System, by John J. Manura, April 2000, SIS Application Note # 82 http://www.sisweb.com/referenc/applnote/app-82.htm 

(2) Vacuum Pump Exhaust Filters, Part II – Oil Mist Eliminators, by John J. Manura, April 2000, SIS Application Note # 83  http://www.sisweb.com/referenc/applnote/app-83.htm

(3) Vacuum Pump Exhaust Filters, Part III – Charcoal Exhaust Traps, by John J. Manura, April 2000, SIS Application Note # 84 http://www.sisweb.com/referenc/applnote/app-84.htm

(4)  Design, Development and Testing of a Microprocessor Controlled Automated

(5) AutoDesorb Short Path Thermal Desorption Apparatus, by John J. Manura, Vinod T. Das, Christopher Baker, Daniel Lieske, John C. Miller, John Manos, Roland Roadenbaugh, Thomas G. Hartman, SIS Application Note # 80  http://www.sisweb.com/referenc/applnote/app-80.htm

(6)  Two Stage Vacuum Pump Exhaust Filter Systems, Scientific Instrument Services, 1027 Old York Road, Ringoes, NJ  08551,  Phone: (908) 788-5550   http://www.sisweb.com/vacuum/sis/exfilter.htm

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