ERROR RESEARCH ON A HEPA FILTER MEDIA TESTING SYSTEM OF MPPS(MOST PENETRATION PARTICLE SIZE) EFFICIENCY

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1 Proceedngs: Indoor Ar 2005 ERROR RESEARCH ON A HEPA FILTER MEDIA TESTING SYSTEM OF MPPS(MOST PENETRATION PARTICLE SIZE) EFFICIENCY S Lu, J Lu *, N Zhu School of Envronmental Scence and Technology, Tanjn Unversty Tanjn , Chna ABSTRACT Referrng to the Europe Standard En1822, we desgn a MPPS (most penetraton partcle sze) effcency testng system for HEPA flter meda. Ths Standard s based on partcle countng methods wth optcal partcle counter. It also allows ULPA (ultra low penetraton ar) flters meda to be tested, whch s not possble wth the prevous test methods because of ther nadequate senstvty.in our error analyss, we analyses the data from the testng result and study the possble reason to produce error n the procedure. Frst, we determne that the manly error sources le n the three part of testng procedure, vz. partcle countng, flow rate determnng and dluton system. Then, the stochastc errors effect s consdered to the testng system. Last, we draw the concluson that our testng equpment has good stablty and the testng data can repeat well. INDEX TERMS MPPS(Most Penetraton Partcle Sze), error, flter meda, fltraton effcency INTRODUCTION Wth the development of modern scence and technology, people have hgher control n quantty and sze of partcles n ar than before. Because the flter meda effcency s the most mportant ndex of aerosol controllng, a useful testng method s more and more mportant. The present Chna s standards have not been amended for many years and aren t adaptng to the development of flter meda. Accordng to t, Tanjn Unversty desgn a MPPS (most penetraton partcle sze) effcency testng system for HEPA flter meda, referrng to the Europe standard En1822(BSI 1998). Ths European Standard apples to hgh effcency partculate and ultra low penetraton ar flters (HEPA and ULPA) used n the feld of ventlaton and ar condtonng and for techncal processes, e.g. for clean room technology or applcatons n the nuclear and pharmaceutcal ndustry. It establshes a procedure for the determnaton of the effcency on the bass of a partcle countng method usng a lqud test aerosol, and allows a standardzed classfcaton of these flters n terms of ther effcency (R.Wepfer 1995). Specmens of the sheet flter medum are fxed n a test flter assembly and subjected to the test ar flow correspondng to the prescrbed flter medum velocty. Furthermore, the measurement of the pressure drop s made at the prescrbed flter medum velocty. In order to determne the effcency, partal flows of the test aerosol are sampled upstream and downstream of the flter medum. Usng a partcle countng nstrument, the number concentraton of the partcles contaned s determned from varous partcle szes. The results of these measurements are used to draw a graph of effcency aganst partcle sze for the flter medum, and to determne the partcle sze for whch the effcency s a mnmum. Ths partcle sze s known as the most penetratng partcle sze (MPPS). When measurng the partcles on the upstream sde of the flter medum t may be necessary to use a dluton system n order to reduce the concentraton of partcles down to the measurng range of the partcle counter used. * Correspondng author emal: jjlu@tju.edu.cn 3420

2 Proceedngs: Indoor Ar 2005 φ P Fgure1. A dagram of apparatus for testng the flter medum 1. Preflter 2. Fan 3. Needle valve 4. Secondary flter 5. Volume flow rate meter 6. Aerosol generator wth condtonng of supply ar and aerosol flow regulator 7. Dluton system (optonal) 8. Electromagnetc valve 9. Partcle counter 10. Fler meda mountng system 11. Atmospherc temperature and relatve humdty measurement 12. Pressure measurement PARTICLE COUNTING ERROR ANALYSIS In an optcal partcle counter, the partcles are led ndvdually through an ntensvely llumnated measurng volume. When passng through the measurng volume, the partcle scatters lght, whch s detected at a defned spatal angle by a photo detector and transformed nto an electrcal pulse and transformed nto an electrcal pulse. The level of ths pulse allows conclusons to be drawn about the sze of the partcle, and the number of pulses per unt tme s an ndcator of the partcle concentraton n the ar volume, whch s analyzed. Our testng system chooses Japanese RION KC-22A optcal partcle counter. Whch have fve channel: 0.1µm~0.15µm 0.15µm~0.2µm 0.2µm~0.3µm 0.3µm~0.5µm >0.5µm,samplng volume s 0.1 CFM. Accordng above, the calculate method of overlap loss s C C 0 V = e (1) C0 Where C s partcle concentraton measured by OPC; C 0 s actual partcle concentraton n the samplng ar ; V s ar volume through OPC Through the actually testng the overlap loss of OPC s showed n the fgure 2: Fgure 2. OPC overlap loss (Entres/ ltre) From the fgure 2, we can see that when the partcle concentraton exceeds Entres/ L, the overlap loss wll reach 5%. So t s an mportant factor. To the optcal partcle counter KC-22A, the measured concentraton should 3421

3 Proceedngs: Indoor Ar 2005 less than Entres/ ltre n order to ensure the veracty (Yongxang L 2004). If the partcle concentraton s too hgh so-called concdence errors (overlap loss) occur. In order to testng the HEPA and ULPA medum, the upstream concentraton s so hgh that we should adopt sutable dluton number to dlute upstream partcle concentraton, as a result keep the partcle number n samplng ar less than Entres/ L. DILUTION SYSTEMS ANALYSIS Dluton systems reduce the concentraton of an aerosol to a defned extent by the addton of a partcle-free gas (usually ar). The dluton number for the relevant partcle sze ranges ndependent of the partcle sze and shall be constant over tme. As s narrated above, when measurng the partcles on the upstream sde of the flter medum t may be necessary to use a dluton system n order to reduce the concentraton of partcles to the measurng range of the partcle counter used. Consequently, the dluton proporton becomes an mportant factor to the veracty of testng. The flter effcency s calculated by: Puprver D pdownrver E = Puprver D (2) Where E s fler effcency; P downrver s downrver partcle number (measured by OPC); and P uprver s uprver partcle number (measured by OPC); D: Dluton proporton In case the dluton proporton change, the flter effcency wll change a lot. To valdate the relatonshp between the proporton dfference and the testng error, we do the experment. The detal nformaton s showed n the table 1. Table1 Dluton proporton dfference result n flter effcency error Partcle sze Testng effcency Most error μm Normal Dff 20% Dff 30% Dff 40% Dff 50% % % % % % % % % % % % % % % % % % % % % % % % % Note:the mean of Dff20% s the true proporton less the nomnal proporton 20%,and so on. As the usage of the testng system, dluton proporton wll be changed. The partcle number measured by OPC multplyng the dluton proporton wll less than the actual number. The condton s same as the table 1. We can see from the table1 that Dff50% wll not nfluence the flter effcency. To draw the all-pervadng concluson, we do the theoretc analyss. Normal Dluton Proporton n Table 2 Basc condtons P uprver p downrver Flter Effcency A B E Wrong Dluton Proporton m When the dluton proporton changes, t results n the exchange of the proporton from n to m. Then the flter effcency E wll change to E : m A B m A n A (1 E ) E = = ' ' (3) m A m A Defnng the dluton proporton exchange rate s Y,namely m Y =,the formula change to: n 3422

4 Proceedngs: Indoor Ar 2005 E m A B Y n A n A (1 E ) Y (1 E ) ' = = = (4) m A Y n A Y ' When the dluton proporton change, the new flter effcency E relate wth the Y and orgnal effcency E. Accordng to the EN1822 s flter class, we can draw the concluson lsted n the table 3. EN1822 flter class Talbe3. The proporton dfference and the testng effcency ( Y = m / n ) Over value MPPS 100% 80% 60% 40% 20% (%) (%) (%) (%) (%) (%) H H H H H U U U Form the table3, we can know that the less of the flter medum effcency, the more nfluence of the proporton dfference. To the grade H13 or the hgher flter, 40% dluton proporton dfference wll not affect the flter proporton. TEST AIRFLOW RATE DETERMINING To the MPPS testng system, the flter element s tested at nomnal ar volume flow rate. In the testng procedure, the arflow rate should be mantaned regularly n accordance wth the manufacturer s nstructons. If arflow changes happen, the flter effcency wll alter. To see the change, we do the experment n our system. In ths experment, we assume the orgnal flow rate s 2.67cm/s. Then the flow determnng error happens: the actual flow reach to 6.99 cm/s 5.33cm/s 3.99 cm/s, namely change 261%, 200%, 150%, but the devce shows the flow rate s 2.67cm/s. The effcency curve s showed n fgure 3. Y Fgure 3. the flow rate error nducng the penetraton curve change 3423

5 Proceedngs: Indoor Ar 2005 We can see the relatonshp between the arflow rates and flter medum effcency. Namely, as the ncrease of the arflow rate, the effcency wll enhance. The concluson s accordance wth the Ar Flter ABC (Caje 2002).We can see form the fgure3, the flow rate change 261%, the flter penetraton varety to 0.12μm wll reach 355%.From the many experment we fnd that n order to ensure the testng accuracy the flow rate change should be remaned ±5%. STOCHASTIC ERRORS ANALYSIS To valdate the stochastc errors effect (Qanwe 2001) to the testng result, we test A~F seres flter at dfferent tme and test one seres flter more than 5 tmes. Although the testng result s not same completely, the flter effcency s consstent and the dfference s less than 5%. It showed our testng system have good stablty. Stochastc errors are not the manly factor to the machne. CONCLUSION Form the analyss above, we can fnd the manly error source may occurred n the three-part procedure n the testng: namely partcle countng, dluton system and flow rate determnng. To ensure the accurate of countng, the partcle concentraton should less than Entres/ L; to ensure the accurate of upstream partcle concentraton, the dluton proporton change should be remaned n 40%, otherwse exchange t; to ensure the testng accuracy, the flow rate change should be remaned ±5%. From many testng results, we draw the concluson that our testng equpment has good stablty and the testng data can repeat well. REFERENCES BSI, 1998, Brtsh Standard,The European Standard EN1822 Ca J. 2002,Ar Flter ABC,Bejng:Chna Archtecture&Buldng Press L YX.,2004, Study n Hgh Effcency Ar Flters Testng methods, Graduate Thess, Tanjn Unversty, 70 pages Qan W Weave Testng Error Analyss and Handle,Shangha Weave Technology,29(3 ) Wepfer R. Charactersaton of HEPA and ULPA Flter by Proposed New European Testng Methods, Fltraton and Separaton,1995:545~