HEPA filters

Plutonium solutions that have a low activity (<3.7 x 10 Bq (1 mCi) or 10 mg of Pu) and that do not produce aerosols can be handled safely by a trained radiochemist in a laboratory fume hood with face velocity 125—150 linear feet per minute (38—45 m/min). Larger amounts of solutions, solutions that may produce aerosols, and plutonium compounds that are not air-sensitive are handled in glove boxes that ate maintained at a slight negative pressure, ca 0.1 kPa (0.001 atm, more precisely measured as 1.0—1.2 cm (0.35—0.50 in.) differential pressure on a water column) with respect to the surrounding laboratory pressure (176,179—181). This air is exhausted through high efficiency particulate (HEPA) filters. 

Beryllium oxides Baghouses or high-efficiency particulate air (HEPA) filters... 

Machining Beryllium dust Baghouses and HEPA filters... 

HEPA filter The efficiency of 99.97 percent used in removing monodispersed particles of 0.3 microns in diameter was considered HEPA. NIOSH no longer uses this term, but OSHA has retained... 

Answer Use the plant s PSA to determine the risk of accidents that include containment failure from overpressurization. Then make a preliminary design of a vented containment that has sufficiently low impedance to the gas at the pressure predicted for the most severe accident sequences such that the containment is not damaged. This containment bypass will include iodine and HEPA filters as well as scrubbers and a discharge through a stack. Estimate the dose that the population would get using this bypass for comparison with the PSA result for ruptured containment sequences. 

HEPA and ULPA filters have been developed. In the CEN EN 1822 1998 test method, the filter s efficiency is determined for the most penetrating particle size (MPPS). Depending on the filter s total level of separation and leakage, the filter is classified as HIO, Hll,.. ., H14 and U15, U16, or U17. HEPA filters are commonly used for inlet air in the pharmaceutical, optical, and food industries. 

The purpose of open unidirectional airflow benches is to protect products from particulate contaminants by creating a controlled environment. These benches are used, for example, in electronic, biological, pharmaceutical, and food industries. It should be mentioned that within pharmaceutical production, aseptic sterile processes must be carried out in a Class 100 environment (U.S. Federal Standard 209 E, Airborne Particulate Cleanliness Classes in Cleanrooms and Clean Zones). To avoid particle contamination in the bench, horizontal or vertical airflow with high-efficiency particulate air (HEPA)-filtered air is used. The air velocity is normally 0.4-0.5 ra s". Some examples of typical arrangements of open unidirectional airflow benches are shown in Fig. 10.51. 

In order to maintain a high level of air qualit), it is necessary to test airflow velocities and HEPA filters for integrity. These tests are described in a variety of standards and recommended practices depending on the use of the airflow bench. 

The bench should be supplied with HEPA-filtered unidirectional airflow, having a velocity sufficient to sweep particulate matter away from the working area. Normally a velocity of 0.45 m s plus or minus 20% is adequate. It is important to monitor the air velocity at suitable intervals because significant reduction in velocity or uniformity in velocity can increase the risk of contamination. 

In the unidirectional air flow of an open bench, a vortex street is easily created behind small obstacles. Such an obstacle can be as insignificant as a small lamp or a fixture connecting HEPA filters. Ljungqvist, et al. have, with the help of isothermal smoke, visually depicted the air movements behind such a horizontal 30 mm wide fixture at an air velocity of 0.45 m s The observed flow pattern is schematically shown in Fig. 10.55. 

Class I The Class I BSC provides personnel and environmental protection, but no product protection. It is similar in air movement to a chemical fume cupboard, but has a HEPA filter (see Chapter 9) in the exhaust system to protect the environment (Fig. 10.94). In the Class 1 BSC, unfiltered room air is drawn across the work surface. Personnel protection is provided by this inward air velocity as long as a minimum velocity of 0.37 m s" is maintained through the front opening (see the discussion on fume cupboards in Section 10.2.3.3). In many cases Class I BSCs are used specifically to enclose equipment. 

FIGURE 10.94 The Class I BSC ( Front opening. 6 Sash. C Exhaust HEPA filter. 0 Exhaust pie num). 

Some Class I BSCs are equipped with an integral exhaust blower the cab inet blower must be tiiterloclced with the building exhaust fan. In the event that the building exhaust fan fails, the cabinet exhaust blower must also turn off so that the exhaust ducts are not pressurized. If the ducts are pressurized and the HEPA filter develops a leak, contaminated air could be discharged into other parts of the building or the environment. 

Class U The Class U (Types A, Bl, B2, and biological safety cabinets provide personnel, environmental, and product protection. Airflow is drawn around the operator, through the hood opening and into the front grill of the cabinet, which provides personnel protection, in addition, the downward flow of HEPA-filtered air provides product protection by minimizing the chance of cross-contamination along the work surface of the cabinet. Because cabinet air has passed through the exhaust HEPA filter, it... 

Class HA in a Class IIA BSC, an internal blower (Fig. 10.9,St draws sui-ficient room air into the front grill to maintain a minimum calculated measured average velocity of at least 0.37 m s at the opening of the cabinet. The supply air flows through a HEPA filter and provides particulate-free air to the work surface. Laminar airflow reduces turbulence m the work zone and niim-mizes the potential for cross-contamination. 

FIGURE 10.95 The Class IIA BSC (A From opening. C Sash. C Exhaust HEPA filter. D Rear plenum E Supply HEPA fitter. F Blovyer). 

The air is then discharged through the rear plenum into the space benvcen the supply and exhaust filters hicated at the top of the cabinet. Due to the relative size of these two filters, approximately 30% of the air passes through the exhaust HEPA filter and 70% recirculates through the supply HEPA filter back into the work zone. Most Class IIA cabinets have dampers to modulate this 30%/70% division of airflow. 

CIlIss HBl In a Class IlBl cabinet, supply blowers draw room air (plus a portion of the cabinet s recirculated air) through the from grill and then through the supply HEPA filters located immediately below the work surface (Fig. 10,96). This particulate-free air flows upward through a plenum ai each side of the cabinet and then downward to the work area through a back-pressure plate. In some cabinets an additional supply HEPA filter removes particulate generated by the b [Pg.987]

FIGURE 10.96 The Class II BI BSC. classic design (A Front opening. 8 Sash. C Exhaust HEPA filter. 

D Supply HEPA filter. Negative pressure exhaust plenum, f Blower. C Additional HEPA filter for air ivppiyf Note The cabinet exhaust needs to be connected to the building exhaust. 

FIGURE 10.97 The Class II B2 BSC (A front opening, fl Sash. C Exhaust HEPA filter. D Suf ly HEPA fitter . Negative pressure exhaust plenum. Fz Supply blower. G Filter screen). Note Connection to building exhaust system is required. The carbon filter m the building exhaust Is not shown. 

Horizontal laminar flow clean air benches are not BSCs (Section 10.3.4). They discharge HEPA-filtered air across the work surface and toward the user. These devices only provide product protection. They can be used for certain dean activirie.s, such as the dust-free assembly of sterile equipment or electronic devices. These benches should never be used when handling potentially infectious materials. The worker can be exposed to materials on the clean bench. Horizontal clean air benches should never be used as a substitute for, i biological safety cabinet. 

By definition, a HEPA filter is a throw-away, extended-medium, dry-type filter having the following characteristics ... 

HEPA filters consist of a filter pack sealed into a case. The filter pack or core is made by pleating a continuous web of fiberglass paper back and forth over corrugated separators. The filter pack is sealed into a full-depth wood or steel casing using a elastomeric sealant. Gasketing is a critical item to ensure that the filter passes the air leakage tests. 

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