Filter pack

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. 

Filter pack A filter medium uniformly folded and interleaved with spacers. 

Masks are available with battery-powered filter packs which supply filtered air to a facepiece from a haversack filter unit. Another type comprises a protective helmet incorporating an electrically operated fan and filter unit complete with face vizor and provision for ear muffs. 

Reducing the intrinsic hazard of chemicals is the most effective and most fundamental of the risk reduction options available. Intrinsic risk reduction is based on the principle that the structure of a chemical drives hazard and molecular intentional, informed manipulation will result in the design of safer chemicals. Wastewater treatment plants employ large filters packed with granular activated carbon to remove polluted water. Synthetic chemists are... 

Filter pack. Filter pack is the term used to describe the materials placed in the annular space between the screen and the formation (Figure 1). Aller et al. and Driscoll provide a discussion of the purpose and selection of filter pack materials. The filter pack serves a number of important mechanical functions. During well construction, it helps to keep the well screen centered in the borehole and acts to block the sealant from entering the well screen. The filter pack also improves hydraulic conductivity between the well screen and the formation, prevents formation materials from migrating to the well screen by acting as a filter, and helps support the screen and borehole. ... 

The most common filter pack material is quartz (silica). Quartz is relatively inert, readily available, and workable therefore, it is preferred to replace formation materials removed from the borehole. The grain size of the filter material (i.e., sand or gravel) should be chosen based on the characteristics of the formation to be monitored and the slot size of the screen. Sand and gravel are available in various uniform sizes to accommodate different monitoring environments. 

Unconsolidated or weakly consolidated sediments sometimes collapse around the well screen before the filter pack can be installed. This phenomenon is called formation collapse . Formation collapse can occur as a result of the inherently unstable nature of certain sediments or the disruptive nature of the drilling process. Formation collapse is most common below the water table. Although steps can be taken to minimize the amount of collapse, it may not be entirely preventable. The groundwater monitoring plan may need to accept natural formation material as the filter pack for some or all of the screen section. Well development activities (see Section 2.1.6) can be designed to maximize the effectiveness of the formation collapse materials as a filter pack. 

If bentonite pellets or chips are to be used instead of grout, the bentonite pellets can be placed in shallow wells (within 15.0 m of ground surface) by pouring them through the annular space from above in much the same manner as the filter pack is placed. If this installation procedure is chosen, sufficient potable water must be added in conjunction with the bentonite to hydrate the individual pieces. Hydration causes the pieces to swell and form a highly impermeable seal. 

Pumping during well development performs two important functions. First, pumping removes the materials from the borehole left behind by drilling. Second, as the water in the well is removed, groundwater flow velocity from the surrounding formation increases when it reaches the higher permeability filter pack around the well... 

After the silica flour or soil mixture has set, add a layer of silica sand as a filter pack, then fill the remainder of the borehole with bentonite or a grout seal (Figure 5). Access lines may be run from the lysimeter reservoir through a protective PVC riser pipe to the land surface. A locking protective casing should be used to secure the abovegrade lysimeter components. If the lysimeter is used immediately after installation, it is necessary to purge any water used to prepare the siiica flour or soil slurry. Remove at least the water volume equivalent to approximately one third of that used to make slurry. 

In weakly consolidated, stratified sediments, the injection pressure must be controlled so that the surrounding formation is not fractured. If fracturing occurs, there is usually a severe loss in hydraulic conductivity because the bedding planes are disturbed. Pressures that will cause fracturing range from a low of 0.5 psi/ft of depth for poorly consolidated coastal plain sediments, to 1.2 psi/ft depth for crystalline rock. For most recharge wells in unconsolidated sediments, the injection pressure should be carefully controlled so that the positive head (in psi at the surface) does not exceed 0.2 x h, where h is the depth (in ft) from the ground surface to the top of the screen or filter pack. 

Remediation commenced with the interception of the recoverable gasoline. One 4-in.-diameter vapor extraction well and three 2-in. vapor monitor wells were installed. Each well was constructed with 4 ft of well screen placed and filter packed across the unsaturated portion of the shale. The annulus of the wells was sealed to the surface with bentonite and cement grout according to local standards. The monitor wells were located in a triangular pattern around the vapor extraction well at distances of 5.3, 10.6, and 25.4 ft. 

Air is drawn through a filter that consists of, or is coated with, a substance that takes up the species of interest. The filter is then extracted and the ions of interest are measured. For most species of interest, care must be taken to avoid interference from particles. For example, as discussed later, filter collection of HN03 will have a contribution from particulate nitrate if particles are not removed first. Figure 11.22 shows a typical filter pack used to measure gaseous HNO-, and NH-, (Anlauf et al., 1988). 

FIGURE 11.22 Schematic diagram of a typical filter pack used to measure gaseous HNO, particulate matter, and gaseous NH, (adapted from Anlauf et al., 1988). 

Filters. HN03 is efficiently trapped out on nylon filters. Typically, two or more filters are connected in series. A schematic of such a filter pack was shown in Fig. 11.22 (Anlauf et al., 1988). A Teflon filter first removes particles from the airstream and a nylon filter then removes gaseous HNO,. In this particular system, a third filter (Whatman 41 impregnated with an aqueous solution of glycerol and citric acid) was used to trap NH3. After sample collection, each of the filters is extracted separately and nitrate, ammonium, and additional particle components collected on the Teflon filter are measured by ion chromatography. The sensitivity of this method for nitric acid and the other species is determined in part by filter blank values (i.e., nitrate on unexposed filters) and by the total amount collected and hence the sampling time used. Times of... 

FIGURE 11.31 Comparison of gaseous HN03 measurements made simultaneously in Claremont, California, on two different days using FT-IR shown as with error bars and by a filter pack (FP), a difference denuder (DD), an annular denuder (AD), and tunable diode laser spectroscopy (TDLS) (adapted from Hering et al., 1988). 

Mass spectrometiy. Several instruments based on chemical ionization mass spectrometry (CIMS) have been developed and applied to ambient air (Huey et al., 1998 Mauldin et al., 1998). Figure 11.32 shows two such instruments that have undergone an informal intercomparison study between themselves and a filter pack (nylon) method (Fehsenfeld et al., 1998). 

An informal intercomparison of these two CIMS methods with a filter pack method shows generally excellent agreement between the mass spectrometric approaches and often, but not uniformly, good agreement with the filter pack method (Fehsenfeld et al., 1998). The latter was often high, which was attributed to interference from decomposition of ammonium nitrate to HNO-, + NH3 on the Teflon particle prefilter, followed by absorption of the HNO-, by the nylon filter. 

A brief informal intercomparison study of the mass spectrometry methods with a nylon filter pack method near Boulder, Colorado, gave average levels of 0.38-1.6 ppb when the wind carried air from the direction of the greater metropolitan Denver urban area and 0.14-0.56 ppb when the wind was downslope and westerly, where there are fewer emissions sources previous filter pack... 

NH3 A number of measurement techniques have been used for ammonia, including a spectroscopic method, denuder methods, and filter packs. 

Filter packs. As shown in Fig. 11.22, NH3 can be collected on impregnated filters in filter packs designed to collect particles and gas-phase nitric acid. Oxalic acid or citric acid on Whatman filters is often used to absorb the gaseous ammonia, which is then measured by extraction into aqueous solution and ion chromatography or by a colorimetric method (e.g., see Anlauf et al., 1988 and Williams et al., 1992). 

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