INDUSTRIAL HYGIENE
SAMPLING STRATEGIES
PARTICLE SIZE DISTRIBUTION
PARTICLE SIZE DISTRIBUTION Purpose The purpose of this experiment is to demonstrate the utilization of cascade impactors for the determination of particle size distributions of airborne dust. Part I Two Impactors will be operated at the same time and for the same period of time. The stages of one impactor is coated with a thin layer of vacuum oil to reduce particle bounce. Arrange the apparatus as shown below to determine the setting of the vacuum pump that will provide an air flow rate of 28.3 l/minute using a rotameter. To determine the particle size distribution, use the following instructions and procedure: ROTAMETER--->CASCADE IMPACTOR--->FILTER--->PUMP Experimental setup for air flow calibration 1. Weigh the impaction stages & the final filter on the Mettler balance(+ 0.01 mg) and mark each stage as in the figure. Repeat the same for the second impactor. 2. Assemble the impactor as shown above and connect each one to a rotameter and a vacuum pump. 3. Place both impactors in the dust chamber and sample for 15 minutes. You do not need to sample for a very long period of time because too much sampling may cause an excessive build up of particles on the impaction stages. 4. Weigh each stage and the final filter and determine the weight of the dust collected on each stage in the two impactors. 5. Tabulate the data for each impactor in the following manner: COATED IMPACTOR Stage # ECD (mm) Net Wt. (mg) % Cum % Cum % < Than Collected Dust Size Range Stated Diam. 0 11 1 7 2 4.7 3 3.3 4 2.1 5 1.1 6 0.7 7 0.4 Filter <0.4 TOTAL UNCOATED IMPACTOR Stage # ECD (mm) Net Wt. (mg) % Cum % Cum % < Than Collected Dust Size Range Stated Diam. 0 11 1 7 2 4.7 3 3.3 4 2.1 5 1.1 6 0.7 7 0.4 Filter <0.4 TOTAL 6. Plot your size distributions on log-probability graph paper for the two impactors. Determine the MMD and GSD for the two distributions. DATA FOR COLLECTED DUST _______________________________________ Data for impactor with oil coating Stage # Dust Net Wt. (mg) 0 0.55 1 0.63 2 1.26 3 3.41 4 3.42 5 2.81 6 0.83 7 0.53 Filter 0.17 Data for impactor without oil coating 0 0.42 1 0.75 2 2.02 3 4.69 4 5.43 5 3.28 6 0.95 7 0.24 Filter 0.03 Part II 1) What are the inherent errors in the determination of the size distribution by impactors? 2) Did the stage coating prevent particle bounce? On what basis did you make that decision? 3) Determine the concentration of the respirable dust from the concentration size distribution. According to the British definition; < Penetration (P)= 1-[de2/50], where (de) is the aerodynamic particle diameter. The density of this dust is 2.6 gm/cc >. USE DATA FROM THE IMPACTOR WITHOUT OIL LAYER. Part III Given the following tabulated data for 2 (PVC-type) filters used in sampling for total and respirable dust. The entire sampling time for each filter was 64 minutes & 42 seconds. Both filters were sampling the same air. Filter # Filter Wt. Filter Wt. Flow Rate Int. Wt.(mg) After Sampling (mg) Q (l/min) Total DUST 1 11.86 13.40 1.70 ________________________________________________________________ RESP. DUST 2 12.55 13.27 1.70 ________________________________________________________________ 1) Determine the concentration in mg/m3 for both total and respirable dust. What percentage did the respirable dust constitute? 2) Assume that the respirable dust contains 13% free silica, calculate the concentration of silica in mg/m3, and compare your results with OSHA's standards. PEL for free silica = 10/(%+2)
SUPPORTIVE MATERIAL FOR SOLVING PARTS OF THIS LAB - ACCORDING TO THE BRITISH DEFINITION, RESPIRABLE DUST IS DEFINED AS PARTICLES WITH DIAMETER LESS THAN 7.07 mm. - PENETRATION BY WEIGHT = (% SIZE RANGE)(CONCENTRATION)(P).