Abstract:
A penetration and removal efficiency calibration unit for a volatile particle remover in a solid particle counting system provides an accurate and efficient approach to calibration. The calibration unit includes an aerosol inlet, a flow meter downstream of the aerosol inlet, and a mixer. The flow meter receives the aerosol flow from the aerosol inlet and provides an output flow to the mixer. The mixer receives the output flow from the flow meter and also has a dilution gas inlet. The mixer provides a mixer output flow for reception by the volatile particle remover or particle counter. A first flow controller controls flow into the dilution gas inlet. The calibration unit also includes a bypass inlet. A second flow controller controls flow into the bypass inlet, and a control loop controls the bypass flow such that the aerosol flow tracks a reference value.

Description:
BACKGROUND OF THE INVENTION 
       [0001]    1. Field of the Invention 
         [0002]    The invention relates to measuring solid particle number concentrations from engine or vehicle exhausts in real-time, and to a penetration and removal efficiency calibration unit for a volatile particle remover (VPR) in a solid particle counting system (SPCS). 
         [0003]    2. Background Art 
         [0004]    European Particle Measurement Program (PMP) proposed a draft regulation for measuring solid particle number emission in exhaust from light-duty diesel vehicles. As shown in  FIG. 1 , the measurement system consists of a pre-classifier  10 , a hot particle diluter (PND 1 )  12 , an evaporation unit (EU)  14 , a cold particle diluter (PND 2 )  16 , and a condensation particle counter (CPC)  18 . The hot particle diluter (PND 1 )  12 , evaporation unit (EU)  14 , and cold particle diluter (PND 2 )  16  are referred to as the Volatile Particle Remover (VPR)  20 .  FIG. 1  shows a simplified schematic of the measurement system. 
         [0005]    The VPR  20  dilutes diesel aerosol in PND 1   12  and PND 2   16 . The EU  14  in the VPR  20  is operated at a high temperature (such as 300 to 400° C.) to evaporate volatile particles into gas phase. By following dilution from PND 2   16  with room temperature dilution air, the aerosol is cooled down, and the volatile material concentration is reduced to the level to avoid the formation of the volatile particles. Thus, volatile particles are removed, and solid particles only move into the CPC  18 . The concentration of the solid particles is measured in the CPC  18 . 
         [0006]    To have accurate measurement on solid particle concentration, PMP recommended that the solid particle penetration on the VPR  20  should be verified by mono-disperse solid particles at 30, 50, and 100 nm. The removal efficiency of the VPR  20  for volatile particles should be tested with mono-disperse C40 particles with 30 nm diameter. To measure penetrations for mono-disperse solid particle particles and removal efficiency for mono-disperse C40 particles on the VPR  20 , mono-disperse particles need to be sent into the VPR with a CPC for the diluted concentration, and be sent into a CPC for the raw concentration. Equations 1 and 2 show the calculation for the penetration and removal efficiency: 
         [0000]    
       
         
           
             
               
                 
                   P 
                   = 
                   
                     
                       
                         C 
                         Diluted 
                       
                       * 
                       
                         DR 
                         1 
                       
                       * 
                       
                         DR 
                         2 
                       
                     
                     
                       C 
                       Upstream 
                     
                   
                 
               
               
                 
                   ( 
                   1 
                   ) 
                 
               
             
             
               
                 
                   
                     E 
                     Removal 
                   
                   = 
                   
                     1 
                     - 
                     P 
                   
                 
               
               
                 
                   ( 
                   2 
                   ) 
                 
               
             
           
         
       
     
         [0000]    where, P is the penetration; E Removal  is the removal efficiency for C40 particles; C Diluted  is the diluted concentration for mono-disperse particles (solid or C40 particles), C Upstream  is the raw (upstream) concentration for mono-disperse particles; DR 1  is the dilution ratio on the PND 1 ; and, DR 2  is the dilution ratio on the PND 2 . While the single size aerosol is connected to point A in  FIG. 1 , C Diluted  is measured. While the single size aerosol is connected to point B in  FIG. 1 , C Upstream  is measured. 
         [0007]    The Differential Mobility Analyzer (DMA) is widely used to select the single size particles. The selected single size particle concentration is extremely sensitive to the inlet flow and the outlet flow from the DMA. With small change on those flows, large variation may be detected on the raw concentration particles. From equation 1, it is observed that the raw concentration C Upstream  is assumed no change and as the same as the measured raw concentration while the mono-disperse aerosol is sent into the VPR. Any variation on the raw concentration for the mono-disperse particle causes the error on the penetration and removal efficiency. Thus, the concentration of the mono-disperse particles should be stable and kept unchanged. 
         [0008]    Under most of circumstances, inlet flows for the VPR and a CPC are different. The system needs to be adjusted slightly to keep the outlet flow unchanged from the DMA. This flow needs to be monitored carefully to ensure the stable and constant concentration for the mono-disperse particles. Thus, the calibration test and experimental setups for solid particle penetration and removal efficiency with mono-disperse particles are time consuming, and require that the operator have good background and knowledge with aerosol science and particle instruments. This is unrealistic in the automobile industry since few operators have background for aerosol science and related technologies. 
       SUMMARY OF THE INVENTION 
       [0009]    It is an object of the invention to provide a penetration and removal efficiency calibration unit for a volatile particle remover (VPR) in a solid particle counting system (SPCS). 
         [0010]    In preferred embodiments of the invention, the flow in the DMA is controlled as constant while flows into the VPR or CPC are varied. Thus, the stable concentration particles for mono-disperse particles may be obtained. 
         [0011]    According to the invention, an accurate and more efficient approach is provided to calibrate the VPR in the SPCS for the penetration for single size solid particles and removal efficiency for single size volatile particles. 
         [0012]    In one particular implementation, single size particles are selected with a Differential Mobility Analyzer (DMA). An orifice flow meter is installed downstream of the DMA. The particle losses on the orifice flow meter can be ignored. The outlet flow from the DMA is measured by the orifice flow meter in real-time. 
         [0013]    In further detail, the aerosol flow is mixed downstream of the orifice flow meter with particle free dilution air in a mini-cyclone. The mini-cyclone mixes the aerosol with dilution air flow fast and without particle losses. In the meantime, the cyclone moves out particles larger than 2.5 μm. Thus, the cyclone protects the system from contamination. 
         [0014]    In this implementation, a mass flow controller or a proportional valve with a PID loop controls the by-pass flow. By adjusting the by-pass flow automatically, the outlet flow from the DMA is kept as constant while flow rates into the VPR in the SPCS and the CPC are different. As a result, the concentration from the DMA is kept as constant during the test. This ensures stable concentrations for the single size particles, and more accurate results are obtained. 
         [0015]    In one aspect of the invention, a penetration and removal efficiency calibration unit for a volatile particle remover in a solid particle counting system is provided. The calibration unit comprises an aerosol inlet, a flow meter downstream of the aerosol inlet, and a mixer. The flow meter receives the aerosol flow from the aerosol inlet and provides an output flow. The mixer receives the output flow from the flow meter, has a dilution gas inlet, and provides a mixer output flow. A first flow controller controls flow into the dilution gas inlet. The calibration unit further comprises a bypass inlet, and a second flow controller controlling flow into the bypass inlet. A control loop controls the bypass flow such that the aerosol flow tracks a reference value. 
         [0016]    At the more detailed level, the invention comprehends additional features. For example, the reference value may be a constant flow. The mixer may take the form of a mini-cyclone. The control loop may be implemented as a proportional, integral, derivative control loop. The first flow controller may comprise a mass flow controller. The second flow controller may comprise a mass flow controller or a proportional valve. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0017]      FIG. 1  illustrates a simplified schematic of an existing measurement system; 
           [0018]      FIG. 2  illustrates a schematic for a calibration unit in a preferred embodiment of the invention; 
           [0019]      FIG. 3  illustrates the connection between the calibration unit and the inlet of the VPR or CPC; 
           [0020]      FIG. 4  illustrates the operation procedure for the calibration unit. 
       
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
       [0021]      FIGS. 2-4  illustrate the preferred embodiment of the invention. 
         [0022]    As best shown in  FIG. 2 , the calibration unit consists of aerosol inlet  40 , a cyclone  42 , differential pressure transducer  44 , two mass flow controllers  46 ,  48  or a mass flow controller  46  and a proportional valve  48 , an orifice  50 , a PID control loop  60 , a ball valve  62  and needle valves  64 ,  66 , etc.  FIG. 2  shows the schematic for the calibration unit. 
         [0023]    Before operating the system, the system needs to be set up. The port  124  “To DMA” is connected to the inlet of a differential mobility analyzer (DMA), and the port  122  “From DMA” is connected to the outlet of the DMA. The DMA is not included in the system. To minimize particle losses, the tubing connecting the inlet and the outlet of the DMA should be as short as possible. 
         [0024]      FIG. 3  shows the connection between the calibration unit and the inlet of the VPR  20  or CPC  18 . There are three flexible tubings (from point  70  to point  74 , from point  74  to point  72 , and from point  74  to the inlet of the VPR  20  or CPC  18 ) and a Tee (at point  74 ) shown in  FIG. 3 . There are one inlet and two outlets for the Tee. The port  76  “To VPR or CPC” in  FIG. 2  is connected to point  70  in  FIG. 3 . Point  72  in  FIG. 3  is connected to port  80  “Inlet for the by-pass flow” in  FIG. 2 . The length of those three tubings should be minimized. The aerosol flow moves into the VPR  20  or CPC  18  through points  70 ,  74 , and the inlet of the VPR  20  or CPC  18 . The excess flow is vented through the other port of the Tee into the calibration unit. 
         [0025]    When the raw and diluted particle concentrations are measured, the aerosol flow rate in the tubing from  70  to  74  is kept as the constant. By varying the by-pass flow in the tubing from  74  to  72 , the right amount of flow moves into the VPR  20  or the CPC  18 . In the meantime, the inlet and outlet flows for the DMA are kept unchanged. The tubing length from the outlet of the Tee to the inlet of the VPR  20  or CPC  18  should be as short as possible. Under most of circumstances, it is much shorter than the length from  70  to  74 . Therefore, the difference of particle concentration can be ignored while the flow is different at the inlet of the VPR  20  and the CPC  18 . 
         [0026]    Poly-disperse solid particles or C40 particles are provided into the calibration unit from the port  40  of the aerosol inlet. By adjusting needle valves NV 1   64  and NV 2   66 , the excess aerosol flow is vented into the atmosphere through NV 1   64  and the HEPA  82  which is downstream of the NV 1   64 . Under some circumstance, the aerosol generator or C40 generator cannot provide enough flow for the DMA. The makeup air is needed, and moves into the DMA through the HEPA  82  and NV 1   64 . To have stable and constant concentration of single size particles, the size distribution and concentration from the aerosol generator or the C40 generator should be constant during the test. Most of commercially available particle generators can satisfy this requirement. 
         [0027]    As mentioned above, the single size particles are selected by the DMA. Aerosol flow rate into the DMA strongly influences the concentration and the selected size for particles. If aerosol flows into and out from the DMA are fluctuated, the concentration and size will not be stable. For most of DMA operation conditions, the aerosol inlet flow is the same as the outlet of aerosol flow on the DMA. The inlet flow to the DMA is measured but there may be no output signal on the DMA. An orifice flow meter  90 , which consists of differential pressure transducer  44  and flow orifice  50 , is installed downstream of the DMA. The flow on the orifice flow meter  90  is calibrated with an accurate flow meter, and it is a function of the pressure difference over the orifice  50 . This orifice flow meter  90  is used to measure the outlet flow from the DMA. Since the aerosol concentration downstream of the DMA is much lower than that of the upstream of the DMA, the chance for the orifice flow meter  90  getting plugged by particles is reduced by installing it downstream of the DMA. In an alternative arrangement, the flow meter could be installed upstream of the DMA. 
         [0028]    Particle free compressed air moves into the calibration unit through mass flow controller  1  (MFC 1 )  46  and ball valve (BV; which can be a manual valve or air actuated valve)  62  and mixes with aerosol from the DMA in the mini-cyclone  42  downstream of the orifice flow meter  90 . The flow rate for the dilution air is controlled by MFC 1   46 . The flow on the MFC 1   46  is set based on the inlet flow of the VPR  20  or CPC  18  and the concentration of the single size particles. Equations 3 and 4 show flow balance in the system while the calibration unit is connected to the inlet of the VPR  20  and the inlet of the CPC  18 , respectively: 
         [0000]        Q   total   =Q   DMA   +Q   MFC1   =Q   VPR   +Q   by-pass    (3) 
         [0000]        Q   total   =Q   DMA   +Q   MFC1   =Q   CPC   +Q   by pass    (4) 
         [0000]    where, Q DMA  is the outlet flow from DMA; Q MFC1  is the particle free air flow controlled by MFC 1   46 ; Q total  is the flow for the mixture of Q DMA  and Q MFC1 ; Q VPR  is the inlet flow to the VPR  20 ; Q CPC  is the inlet flow rate to the CPC  18 ; and, Q by-pass  is the by-pass flow controlled by MFC 2  or a proportional valve  48 . During the whole test, Q total  is kept as constant while Q VPR  and Q CPC  are changed. Thus, by varying Q by-pass , the total flow stays unchanged. 
         [0029]    The flow on MFC 1   46  is controlled based on the outlet flow from the DMA, the inlet flows to the VPR  20  and the CPC  18 , and the concentration of the mono-disperse particles. If the outlet flow on the DMA is larger than inlet flows on the VPR  20  and the CPC  18 , and the particle concentration (raw) from the DMA is lower than the upper limit of the CPC  18 , the flow rate on MFC 1   46  can be set at zero. Thus, no dilution air flow moves into the system through MFC 1   46 . To avoid leak on the MFC 1   46  to change particle concentrations, the ball valve  62  (BV) can be closed manually or automatically. If the concentration from the DMA is higher than the upper limit of the CPC  18  or a lower concentration is desired, the aerosol can be diluted to the desired concentration by adding dilution air flow from the MFC 1   46 . 
         [0030]    While one or both of flows for the VPR  20  inlet and CPC  18  inlet are larger than the outlet flow of the DMA, the flow on MFC 1   46  can be set to a value which the sum (Q total ) of Q DMA  and Q MFC1  is larger than the bigger flow between the inlet of the VPR  20  and the inlet of the CPC  18 . In the meantime, the concentration of the raw aerosol is at the desired and lower than the upper limit on the CPC  18 . Once the flow is set on MFC 1   46  and the desired concentration is obtained, the flow is kept constant during the test. Thus, Q total  is constant in the whole test. 
         [0031]    The outlet flow from port  76  “To VPR or CPC” flows into the VPR  20  or CPC  18  through points  70  and  74  in  FIG. 3  into the inlet of the VPR  20  or the CPC  18 . To ensure that the outlet flow from the DMA is constant during the test, the by-pass flow from point  74  to  72  into the calibration unit is controlled by mass flow controller  2  (MFC 2 ) or a proportional valve  48 . A vacuum source  90  draws the by-pass flow into MFC 2  or a proportional valve  48 . The by-pass flow moves through a HEPA filter  92  before it moves into the MFC 2  or the proportional valve  48 . The HEPA  92  protects the MFC 2  or the proportional valve  48  from contamination by particles. 
         [0032]    A proportional, integral, and derivative loop (PID)  60  is used to control MFC 2  or the proportional valve  48 . The reference flow  94  which is the desired flow rate for the DMA outlet flow is the set point. The flow measured by the orifice flow meter  90  is as the input for the PID loop  60 . By comparing the difference between the reference value  94  and the measured value  96  in the PID loop  60 , MFC 2  or the proportional valve  48  is adjusted to maintain the outlet flow on the DMA as a constant. As a result, the flow on the DMA can be kept as constant during the whole test. 
         [0033]    While the aerosol is connected to the CPC  18 , the raw (upstream) concentration is measured. By adjusting the by-pass flow automatically, the flow from the DMA is kept as constant. The variation of the single size particle concentration is minimized. After the concentration is stabilized, the data can be recorded manually or automatically. The diluted concentration downstream of the VPR  20  in the SPCS is measured by sending the aerosol into the VPR  20  in the SPCS. By adjusting the by-pass flow automatically with MFC 2  or the proportional valve  48 , the outlet flow on the DMA is the same as that for the flow into the CPC  18 . As a result, the concentration for the single size particle is not changed during the aerosol into either the VPR  20  or the CPC  18 . 
         [0034]      FIG. 4  summarizes the operation procedure for the calibration unit. At block  100 , the Differential Mobility Analyzer (DMA), particle counter (CPC), and aerosol generator are connected to the calibration unit. At block  102 , the system is warmed up. At block  104 , the flow for the particle free compressed air on MFC 1  ( 46 ,  FIG. 2 ) is set and the reference flow ( 94 ,  FIG. 2 ) is set. At block  106 , The needle valves ( 64 ,  66 ,  FIG. 2 ) are adjusted to supply the needed flow for the DMA. At block  108 , the single size for the particles from the DMA is selected. At block  110 , if the particle concentration provided to the CPC is higher than the CPC&#39;s upper limit, the flow at MFC 1  is adjusted as indicated at block  112 . Flow proceeds to block  114 , and after the system stabilizes, concentration is measured at the CPC. At block  116 , the single size aerosol is provided to the VPR, and at block  118 , data is recorded after the system stabilizes. Finally, at block  120 , penetration and/or removal efficiency are calculated. 
         [0035]    While embodiments of the invention have been illustrated and described, it is not intended that these embodiments illustrate and describe all possible forms of the invention. Rather, the words used in the specification are words of description rather than limitation, and it is understood that various changes may be made without departing from the spirit and scope of the invention.