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 manufacturer's specifications on the air cleaner's 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.

Mist generation at a machining center.

Control of occupational exposure to metalworking fluid mist generally involves enclosing the machining center and exhausting to an air cleaner that returns cleaned air to the workplace. To select an appropriate air cleaner, particle size and generation rate of the mists need to be known. Mist particle size and concentration were measured as a function of tool speed, fluid flow rate, and cutting rate at an enclosed machining center. A vertical machining center was totally enclosed and the air from this enclosure was exhausted into a duct where mist concentration and size distribution were measured using a time-of-flight aerosol spectrometer and a cascade impactor. Mist generation during the face milling of a 30 x 31-cm piece of aluminum with a 10-cm diameter face mill was studied. Machining parameters were varied as a 2 x 2 x 3 factorial experiment with these variables: coolant flow rate (18 and 44 m/sec), tool rpm (1900 and 3800 rpm), and metal removal (no removal, two teeth on face mill, and six teeth on face mill). Mist concentration increased with increasing tool speed and fluid application velocity. Whether the tool was actually removing metal did not affect the mist generation. Thus, mist generation is a function of fluid and tool motion. During a second experiment, effect of tool speed and diameter on mist generation was studied. Mist concentrations measured with the aerosol spectrometer were proportional to the 2 and 3.5 powers of the tool speed for the face mill and end mill, respectively. In both experiments the shape of the size distribution was largely unaffected by the experimental variables.

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 system's 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.

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 manufacturer's 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 worker's 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 manufacturer's 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.

Evaluation of ventilated sanders in the autobody repair industry.

Orbital and reciprocating sanders were evaluated in autobody shops to obtain information on the ability of the sanders to control worker dust exposures. Air is exhausted from ventilated sanders at a rate of 0.25 to 1.0 m3/min. When unventilated sanders were used, short-term total dust exposures ranged between 2 and 170 mg/m3. When ventilated sanders were used, short-term total exposures ranged between 0.22 and 1.2 mg/m3. Aerosol photometer measurements showed that ventilation decreased dust exposure by a factor of 10 when body-filling compound was sanded. These results indicate that the use of sanders equipped with high-velocity, low-volume ventilation should be encouraged in the autobody repair industry.

An investigation of dust generation by free falling powders.

To identify the dust generation processes, aluminum oxide powder was dropped as a free falling slug in a test chamber. The effect of the slug's mass, diameter, and drop height upon the aerosol concentration and size distribution was measured with an aerodynamic particle sizer. To differentiate between aerosol generated during the free fall and at the end of the fall, the slug was dropped either onto a flat surface or into a container of water that suppressed dust generation associated with the impact at the end of the fall. Aerosol generation occurred during the slug's free fall as well as at the end of the fall. The falling solid induced an airflow that followed the falling solid to the end of the fall. This induced airflow contained the aerosol generated during the free fall. At the end of the free fall, the induced airflow, combined with air jets created on impact, dispersed the aerosol throughout the test chamber. Additional measurements were made by using "neutral buoyancy" helium-filled bubbles to visualize the airflow in the test chamber. The airflow and ensuing turbulence were sufficient to keep large, inspirable particles suspended throughout the test chamber for periods greater than 10 min. During experimental work, the effect of drop height, mass, and slug diameter upon aerosol generation by a single slug of powder was studied. The results indicated that the manner in which a powder is handled may be as important as material dustiness as measured by a dustiness tester. Aerosol generation can be reduced by minimizing the contact between the falling powder and the air.

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 particle's transit time between two laser beams that are perpendicular to an accelerating airflow. The signal processors measure each particle's 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)

Factors affecting the Heubach and MRI dustiness tests.

The effect of test parameters upon material dustiness measured by the Heubach dust measurement appliance and the MRI dustiness tester was studied. The users of these tests can alter test parameters such as flow rate, sampling time, mass of material tested, bulk density, and vibrator setting. The effect of these parameters upon the aerosol produced in the dustiness tester was experimentally studied. All of the parameters affected in a complicated manner, the amount of dust and the size distribution of the dust generated during these tests. Therefore, dustiness test results should not be adjusted for variations in test parameters. The users of dustiness tests need to carefully control dustiness test parameters in order to have reproducible dustiness tests.

The application of dustiness tests to the prediction of worker dust exposure.

Laboratory bench tests, known as dustiness tests, have been used to evaluate and compare the potential of various powders to cause occupational dust exposure. Dustiness tests are used to develop products with reduced dust emissions. The correlation between dustiness test results and dust exposures was evaluated at two bag dumping and bag filling operations. At one bag dumping and one bag filling operation, there was evidence of a relationship between dustiness test results and dust exposures. In one case, regression analysis showed that dust exposures could be predicted to within nearly one order of magnitude. The variability in this prediction was caused by the inherent variability in the occupational dust exposures. In the other case, there was evidence of a correlation after the data had been adjusted for the effect of varying drop height. At the remaining two operations, no correlation between dust exposures and dustiness test results were observed. These results indicate that the relevance of dustiness tests to occupational dust exposure needs to be evaluated at each site. Because a better option does not exist, manufacturers should continue to use empirical dustiness tests to develop better products in the laboratory. The conclusions reached in the laboratory need to be validated by dust exposure measurements in the field, however.

Control of air contaminants at mixers and mills used in tire manufacturing.

The National Institute for Occupational Safety and Health (NIOSH) has completed an evaluation of control techniques for airborne vapors and particulates in tire manufacturing. As part of this evaluation, Banbury mixers and mills were studied. This equipment is used to mix elastomers, carbon black, oil and other ingredients into a homogeneous mixture. These mixing and milling operations normally generate aerosols. Control of these air contaminants may be accomplished by using the following: local exhaust ventilation, equipment configuration and material substitution. Results of personal sampling for total aerosols at Banbury mixers showed the geometric means of mixer operators' exposures at five plants ranged from 0.08 to 1.54 mg/m3. Similar sampling at three plants for milling operations showed the geometric means of milling operators' exposures ranged from 0.20 to 1.22 mg/m3.

Application of industrial hygiene air sampling data to the evaluation of controls for air contaminants.

Air contaminant concentration measurements are an important part of the evaluation of a local exhaust system's effectiveness. Comparisons of concentration measurements have been used to assess the importance of work practices and emission sources in relation to the worker's exposure. These comparisons can be stated as a null hypothesis. Statistical tests can be used to evaluate the validity of the null hypothesis. Applicable tests include student t-test, analysis of variance and Duncan's Multiple Range Test.

A modified impinger for personal sampling.

The performance of a modified impinger is described. Laboratory and limited field studies found that the modification made the device relatively spill-proof. In addition, the collection efficiency of the spill-proof impinger was compared to the standard midget impinger. For particles larger than 0.8 micrometer equivalent aerodynamic diameter, the impaction efficiencies of the two devices were found to be identical. However, the capture efficiencies of the two devices were not always found to be equivalent. In three tests, the modified impinger collected proportionately more material than the midget impinger. In three other tests, the capture efficiencies of the two devices were found to be equivalent.