William A. Heitbrink

Control of Paint Overspray in Autobody Repair Shops

Commercially available controls for reducing worker exposure to paint overspray were evaluated in six autobody shops and a spray-painting equipment manufacturers test facility. Engineering control measures included spray-painting booths, vehicle preparation stations, and spray-painting guns. The controls were evaluated by measuring particulate overspray concentrations in the workers breathing zone, visualizing the airflow in spray-painting booths and vehicle preparation stations, and measuring airflow volumes and velocities. In addition, respirator usage observations were collected at five of the autobody repair shops, and quantitative fit tests were conducted on existing respirators at three shops. Several conclusions were drawn from this study. Downdraft spray-painting booths provide lower particulate overspray concentrations measured on the worker than crossdraft and semidowndraft spray-painting booths. In the latter two booths, the spray-painting gun can disperse as much as half the paint overspray into the incoming fresh air, increasing worker overspray exposure. Vehicle preparation stations have no walls to contain the overspray and, commonly, a single exhaust fan removes air from the painting area. Airflow patterns suggest that these do not control the paint overspray. Switching from a conventional spray-painting gun to a high-volume low pressure spray-painting gun reduced the particulate overspray concentration by a factor of 2 at a manufacturers test facility. However, this change did not significantly affect solvent concentrations. Finally, respirator usage in five of the six shops studied was inappropriate. Respirators were poorly maintained and/or did not fit the workers, perhaps due to the absence of a formal respirator program.

AN APPROACH TO EVALUATING AND CORRECTING AERODYNAMIC PARTICLE SIZER MEASUREMENTS FOR PHANTOM PARTICLE COUNT CREATION

An aerodynamic particle sizer (APS) can be used to make real-time measurements of the aerodynamic particle size distribution over the range of 0.5 to 32 microns. This instrument is very useful in conducting health-related aerosol measurements involving aerosol generation, respirator efficiency, and particulate sampling efficiency. One of the two signal processors within the APS can create spurious or phantom particle counts that can significantly affect relative measurements and calculated mass distributions. In the APS, particle size measurement is based upon a particles transit time between two laser beams that are perpendicular to an accelerating airflow. The signal processors measure each particles transit from the time between the two pulses of scattered light that are generated as the particle passes through the two laser beams. When only a single pulse from a particle is detected, another pulse can cause the recording of a randomly sized phantom particle. The small particle processor (SPP), which measures particle transit from the times in digital increments of 4 nanoseconds, can create phantom particles; the large particle processor (LPP), which measures particle transit times in digital increments of 66.67 nanoseconds, is designed to prevent the creation of phantom particles. These two processors overlap in the range of 5.2 to 15.4 microns.(ABSTRACT TRUNCATED AT 250 WORDS)

Mist Control at a Machining Center, Part 1: Mist Characterization

At a machining center used to produce transmission parts, aerosol instrumentation was used to quantitatively study mist generation and to evaluate the performance of an air cleaner for controlling the mist. This machining center drilled and tapped holes at rotational speeds of 1000 to 3000 rpm. During most machining operations, the metal-working fluid (MWF) was flooded over the part. To facilitate metal chip removal during some operations, MWF was pumped through the orifices in some tools at a pressure of 800 psi. These machining operations were performed in a nearly complete enclosure that was exhausted to an air cleaner at a flow rate of 1.1 m3/sec (2400 ft3/m). Although the use of high-pressure MWF increased the mist concentration by about 200%, it did not affect the mist size distribution. The observed penetration through the air cleaner appeared to be mostly consistent with the manufacturers specifications on the air cleaners filters. During the testing, MWF was observed to accumulate in the bottom of the filter housing and may have been reentrained due to air motion or mechanical vibration.

Mist Control at a Machining Center, Part 2: Mist Control Following Installation of Air Cleaners

At a machining center used to produce transaxle and transmission parts, aerosol instrumentation was used to quantitatively evaluate size-dependent mist generation of a synthetic metalworking fluid (MWF) consisting primarily of water and triethanolamine (TEA). This information was used to select an air cleaner for controlling the mist. During most machining operations, the MWF was flooded over the part. These machining operations were performed in a nearly complete enclosure that was exhausted to an air cleaner consisting of three sections: a fall-out chamber, a trifilter section to capture metal chips and mist, and a 1.13 m3/sec (2400 ft3/min) blower. The partnering company requested that National Institute for Occupational Safety and Health (NIOSH) researchers perform an evaluation of the effectiveness of a commercially available air cleaner. After NIOSH researchers characterized mist generation at the machining centers and found that performance of a test air cleaner appeared to be suitable, the company installed more than 25 air cleaners on different machining centers in this plant and enclosed the corresponding fluid filtration unit. The facility also has implemented a maintenance program for the air cleaners that involves regularly scheduled filter changes; performance is ensured by monitoring static pressure. A NIOSH-conducted air sampling evaluation showed that area TEA concentrations were reduced from a geometric mean of 0.25 to 0.03 mg/m3. Personal total particulate concentrations were reduced from a geometric mean of 0.22 to 0.06 mg/m3. These results show the effectiveness of this combination of enclosure, ventilation, and filtration to greatly reduce the exposure to MWF mist generated in modern machining centers.

A Comparison of Conventional and High Volume-Low Pressure Spray-Painting Guns

The effect of spray-painting gun choice, high volume-low pressure (HVLP) or conventional, on solvent and particulate overspray concentrations was experimentally studied in a downdraft spray-painting booth. This experiment was conducted by repeatedly applying two coats of paint to a car body shell. The two spray-painting guns were a gravity-fed conventional and a gravity-fed HVLP gun. During each experimental run, particulate overspray concentrations, solvent vapor concentrations, film thickness on the autobody, and mass of paint were measured. The film thickness per mass of paint for the HVLP gun was 33% higher than that for the conventional spray-painting gun. This difference was statistically significant (p=0.0015). Apparently, the HVLP spray-painting gun had a much higher transfer efficiency than the conventional spray-painting gun. Also, the particulate overspray concentration per unit of film thickness for the conventional spray-painting gun was twice that of the HVLP gun. Again, this difference was ...

Evaluation of Leakage from a Metal Machining Center Using Tracer Gas Methods: A Case Study

To evaluate the efficacy of engineering controls in reducing worker exposure to metalworking fluids, an evaluation of an enclosure for a machining center during face milling was performed. The enclosure was built around a vertical metal machining center with an attached ventilation system consisting of a 25-cm diameter duct, a fan, and an air-cleaning filter. The evaluation method included using sulfur hexafluoride (SF6) tracer gas to determine the ventilation systems flow rate and capture efficiency, a respirable aerosol monitor (RAM) to identify aerosol leak locations around the enclosure, and smoke tubes and a velometer to evaluate air movement around the outside of the enclosure. Results of the tracer gas evaluation indicated that the control system was approximately 98% efficient at capturing tracer gas released near the spindle of the machining center. This result was not significantly different from 100% efficiency (p = 0.2). The measured SF6 concentration when released directly into the duct had a relative standard deviation of 2.2%; whereas, when releasing SF6 at the spindle, the concentration had a significantly higher relative standard deviation of 7.8% (p = 0.016). This increased variability could be due to a cyclic leakage at a small gap between the upper and lower portion of the enclosure or due to cyclic stagnation. Leakage also was observed with smoke tubes, a velometer, and an aerosol photometer. The tool and fluid motion combined to induce a periodic airflow in and out of the enclosure. These results suggest that tracer gas methods could be used to evaluate enclosure efficiency. However, smoke tubes and aerosol instrumentation such as optical particle counters or aerosol photometers also need to be used to locate leakage from enclosures.

Diffusion Effects Under Low Flow Conditions

Under certain conditions, diffusion was found to be a significant positive bias when sampling for gases using sorbent tubes and pumps at reduced flow rates. Based upon a theoretical analysis, this bias is predicted to be significant for inlet Peclet numbers smaller than 3. The inlet Peclet number is defined as vL1/D where: v=inlet velocity; L1=length to sorbent bed; D=diffusion coefficient. During experimental work, the analysis of sorbent tube samples collected at flow rates of 0.8 cm3/min resulted in measured concentrations as much as 75% higher than sorbent tube samples collected at flow rates of 20 cm3/min.