Rong F. Huang and Chin I. Chou
National Taiwan University of Science and Technology, Taipei, Taiwan, ROC
Abstract
Because the performance of conventional class II biological safety cabinets (BSC) is easily affected by the influences of dynamic motions such as when a person walks by, sash movements, and operator hands movements, this study applies an “air curtain” technique to BSC and tests its effectiveness in reducing the influences of dynamic motions. The containment performance was measured by using the tracer-gas leakage concentration method. When the cabinet was operated under the influences of dynamic situations, as are encountered mostly in the practical uses of BSC, the leakage levels of the cabinet without air curtain are significantly raised, while BSC with air curtain shows almost no leakage.
Key words: air-curtain BSC, occupational health, industrial ventilation.
1. Introduction
Biological safety cabinets (BSC) are used to provide personnel protection, product protection, and cross-contamination protection. There are three categories of safety cabinets which are designated classes I, II, and III. The class II cabinet has a downward flow of sterile air in the working area bathing the material being handled. The flow is drawn through two grills (or slots) which are respectively arranged near the doorsill and the lower back wall of the cabinet. The streams drawn through the grills pass through ducts to a system of fans and filters where most of the flow is recirculated through the working area and the rest is exhausted to the atmosphere through a filter. It is designated type B1 if 70% of the total flow is recirculated and type B2 when no flow is recirculated (NSF Joint Committee on Biosafety Cabinetry 2002). The class II type B2 biological safety cabinet is the subject of the present study.
Many research results have been published to discuss the static performance, test method, and containment improvement of the class II biological safety cabinet, e.g., Clark et al. (1990) and Osborne et al. (1999). However, studies on the dynamic effects induced by the crossdraft, people walking by, sash movements, and operator hands movements are relatively rare. Rake (1978) investigated the influence of the crossdrafts on the containment efficiency of the biological safety cabinet and found that BSC is very sensitive to the crossdrafts. When
the draft velocity is greater than about 0.4 m/s, the leakage increases logarithmically with the increase of draft velocity.
The purpose of this work is to develop a BSC with the application of an “air curtain” technique and to examine the aerodynamic characteristics and containment performance of the air-curtain BSC under the influence of dynamic motions by using the flow visualization technique, laser Doppler velocimetry, and tracer-gas detection method. The configuration of the air-curtain BSC proposed in this study is a modification of Huang et al (2007). It consisted of three parts - a sash (with a channel for the supply of push jet at blow velocity Vb), a suction slot (behind the doorsill for the exhaust of contaminants at suction velocity Vs), and a cabinet (with a HEPA filter installed atop so that the sterile air goes downwards at velocity Vc). The back suction slot commonly used in the conventional cabinet was discarded in this design in order not to create a counter force against the air curtain. The slot width was drastically smaller than those of the front and back grills in the conventional safety cabinets. The physical variables, Vs, Vb, and Vc can be independently adjusted. Inverters were used to control the flow speeds.
2. Experimental Methods
________________________________________________________________________________________________________________________
The sulfur hexafluoride tracer-gas concentration detection method was employed in this study to evaluate the performance of the biological safety cabinet. Ten percent sulfur hexafluoride (SF6) in N2
was used as the tracer gas and static test, walk-by test, sash movement test, and operator hands movement test were conducted. A Miran SapphIReTM Infrared Analyzer was used to measure the concentration of the sulfur hexafluoride gas. The lower and upper limits for the detection of SF6 with this instrument were 0.001 ppm and 100 ppm, respectively. The values lower than 0.001 ppm actually mean “undetectable” or “ignorable.” The inlet diameter and the effective flow rates of the detector probe were 1.3 cm and 14 L/min, respectively.
In the walk-by test, a flat rectangular plate of 190 cm in height, 40 cm in width and 2 cm in thickness, that was mounted on an electric motor-controlled traversing mechanism is shown.
The plate was installed upright and perpendicular to the aperture plane, 20 cm above the floor and 40 cm from the aperture plane. 60 cm to the left. Six traverses in total were done.
The separation time between two traverses was 30 s. The gas ejector is shown as a hollow cylinder with a diffusion plate made of sintered metal installed at the bottom. The diameter of the gas ejector is 3 cm. Nine gas ejectors were positioned on the plane inside the cabinet 20 cm inward from the front aperture plane which were placed horizontally with the exits facing the aperture plane in order to lead the gases to eject outwards. The nine ejectors were fixed in the non-uniform grids formed by the intersections and consisted of several horizontal and vertical lines. The ejection velocity of the tracer gas was about 0.1 m/s. Twenty sampling probes with an inner diameter of 1 cm were positioned at the grids on a rectangular area in the measurement plane. The inlets of the sampling probes were positioned on the plane 5 cm away from the sash plane. The suction velocity at the inlet of the sampling probe was approximately 0.15 m/s. The detector probe was inserted into the outlet of the the tracer gas was released into the cabinet (this instant was taken as the initial time). After 600 s, the sash was quickly opened to 25 cm or 50 cm within 1 s. The tracer gas concentration was continuously recorded until 780 s and the average concentration was calculated over the period of 600th s to 780th s.
In the operator hands movement test, two kinds of experiments were conducted: pipette and pouring motions by hands. The sash was set at 25 cm in height and two agar plates (10 × 1.5 cm) filled with water were placed on the work surface behind the suction slot. Each plate was separated 30 cm apart from the center. The operator reached his right hand into the cabinet and used the pipette to move water from the right agar plate to the left one. This hand motion took about 2 seconds. The tracer gas ejector with an inner diameter of 2.3 cm was placed at a height of 36 cm from the work surface, 8 cm away from the front aperture, and pointing outward. The hands into the cabinet. One of the beakers was filled with water, while the other one was empty. The operator poured the water from one beaker to the other alternatively over a period of about 2 seconds.
The data were recorded for 180 s and averages were taken over the final 120 s.
3. Results and Discussion
By seeding the smoke particles into the jet flow and applying the laser-light sheet across the vertical cross section of the air curtain in the symmetry plane, the flow patterns (oblique view) shown in Fig.
1 were observed. They are severely concave curtain, straight curtain, and slightly concave curtain.
Tables 1 and 2 show the leakage levels detected under the static test condition at the sash height H = 50 cm and 25 cm, respectively. The leakage levels of the straight curtain are the highest among the tested situations. The leakage levels of the severely concave curtain are lower than those of the straight
________________________________________________________________________________________________________________________
Figure 1 Typical smoke flow patterns of air curtain.
curtain and the no air curtain condition. At H = 25 cm, the leakages of the no air curtain condition and the severely concave curtain have relatively low values. Because the biological safety cabinets are mostly operated at low sash openings, the existing widely-used cabinets operated without air curtain configuration rarely display severe hazards under the static test conditions. However, the leakage velocity at the front aperture to set up the air curtain of the slightly concave curtain mode can improve the personnel protection performance of the cabinet.
Tables 3 and 4 show the leakage levels detected under the walk-by test condition at H = 50 cm and 25 cm, respectively. The sweep velocity of the plate is V = 1.0 m/s. At the no air curtain condition (V =
0), the leakages at Vs = 8 m/s and 10 m/s are all unexpectedly large, even when the sash height is 50 cm or 25 cm. It is apparent that the personnel protection would be drastically minimized as the walk-bys are applied in front of the aperture in the no air curtain condition. Operating the cabinet at a straight curtain mode will induce the largest cabinet becomes negative which makes the leakage level high. At the high sash H = 50 cm (Table 3), the leakage can be decreased to a negligibly low level at a suction velocity of Vs = 10 m/s. At the low sash height H = 25 cm (Table 4), which is about the aperture height recommended for use in most biological safety cabinets, the leakage levels are almost negligible when the cabinet is operated either at a severely concave curtain mode or slightly concave curtain mode. Since the severely concave curtain mode may minimize product protection and cross-contamination protection, operating the cabinet at a low suction velocity at the slightly concave curtain mode will be a reasonable way to optimize the considerations of leakage and energy consumption. In this case, operation condition (Vb, Vs, Vc) = (0.5, 8, 0.25) m/s for low sash H = 25 cm and (Vb, Vs, Vc) = (1, 10, 0.32) m/s for high sash H = 50 cm are recommended. The plate velocity Vp
varying from 0.5 m/s to 1.5 m/s was tested in the study. It was found that the above recommended operation conditions can still perform well.
However, the leakage concentration of SF6 at no air curtain condition increases significantly with the increase of the sweep velocity of plate. For instance, at Vp = 1.5 m/s, Cave attains 0.548 ppm and 0.381 ppm at (Vb, Vs, Vc) = (0, 8, 0.25) m/s for H = 50 cm and 25 cm, respectively. This result corresponds to the observation of Rake (1978) for the influence of crossdrafts on the leakage level of the conventional biological safety cabinet, that the leakage of containment increases drastically with the increase of draft velocity as the draft velocity is greater than about 0.4 m/s.
Tables 5 and 6 show the leakage levels detected under the sash-movement test condition at H = 50 cm and 25 cm, respectively. The straight curtain mode presents the largest leakage concentration among all cases and the slightly concave curtain measurements of static tests. H = 50 cm.
Table 2 Results of tracer gas concentration measurements of static tests. H = 25 cm.
________________________________________________________________________________________________________________________
concentration. In the no air curtain condition, the operator who opens the sash of the cabinet bears a much higher risk of containment leakage than in the slightly concave curtain mode.
The influences of operator hands movement, i.e., the pipette and pouring motions, on the leakage are
shown in Figs. 2 and 3. In Fig. 2(a) for the case of no air curtain condition at (Vb, Vs, Vc) = (0, 8, 0.25) m/s, the pipette motion causes the cabinet leakages to fluctuate with time evolution. However, the
average leakage concentration of SF6 is still low at 0.002 ppm. In the slightly concave curtain mode of the pipette test, as shown in Figs. 2(b) and 2(c), Cave are negligibly small. The pouring motion induces an appreciable leakage at about 0.006 ppm in the no air curtain condition, as shown in Fig. 3(a). The maximum leakage Cmax can attain is a high value of 0.025 ppm. While in the slightly concave curtain no air curtain condition. However, setting up the air curtain across the aperture at the slightly concave curtain mode is helpful to provide appreciably better protection for the operator.
4. Conclusions
The operator protection performance of the biological safety cabinet was improved by setting up an air curtain across the aperture of the cabinet.
Using the tracer-gas concentration method, the leakage levels for the dynamic tests (including the walk-bys, sash movement, and operator hands movement tests) was examined. The static test results indicate that the cabinet without the air curtain presents low leakage level, while the dynamic test results show significant deterioration in personnel protection. The leakage levels of the cabinet without air curtain became severe when the Modes Vb, Vs, Vc
________________________________________________________________________________________________________________________
operator hands movement would cause appreciable leakages, which are not as significant as those caused by walk-bys. The cabinet operated at the slightly concave curtain mode presents inappreciable leakage levels in both the static and dynamic tests at either low or high sash height.
References
NSF Joint Committee on Biosafety Cabinetry:
(1992) “Class II (Laminar Flow) Bioharzard Cabinetry”. National Safety Foundation No. 49.
USA: The National Safety Foundation.
Clark RP, Osborne RW, Pressey DC, Grover F, Keddie JR, and Thomas, C: (1990). “Open-fronted Safety Cabinets in Ventilated Laboratories”, J.
Applied Bacteriology 69, pp 338-345.
Osborne R, Durkin T, Shannon H, Dornan E, and Hughes C: (1999). “Performance of Open-fronted Microbiological Safety Cabinets: the value of operator protection tests during routine servicing”. J.
Applied Bacteriology 86, pp 962-970.
Rake BW (1978): “Influence of Crossflows on the Performance of a Biological Safety Cabinet”, Applied and Environmental Microbiology 36, pp 278-289.
Huang RF, Wu YD, Chen HD, Chen CC, Chen CW,
Chang CP, and Shih TS: (2007). “Development and Evaluation of Air Curtain Fume Cabinet with Its Considerations of Aerodynamics”, Ann.
Occupational Hygiene 51, pp. 189-204.
Figure 3 Results of pouring test. (a) no air curtain, (b) slightly concave curtain, (c) slightly concave curtain.