Micron retention

Over the years the performance standards of hydraulic equipment have risen. Whereas a pressure of about 1000 psi used to be adequate for industrial hydraulic systems, nowadays systems operating with pressures of 2000-3500psi are common. Pressures above 5000psi are to be found in applications such as large presses for which suitable high-pressure pumps have been developed. Additionally, systems have to provide increased power densities, more accurate response, better reliability and increased safety. Their use in numerically controlled machine tools and other advanced control systems creates the need for enhanced filtration. Full flow filters as fine as 1-10 micron retention capabilities are now to be found in many hydraulic systems. 

The particle size distribution of the material and the clarity required will dictate the micron retention of the medium. Fabrics tend to have a nominal micron retention range as opposed to an absolute micron retention rating. When using precoat on a machine that leaves a residual heel of solids, a more open cloth can be used. 

Two basic forms of twilled Dutch weave are produced by combinations of warp and weft wires of different diameters. The use of heavy warp wires (Dutch twill weave) permits production of fine grades of woven wire cloths which have very smooth surfaces on both sides, but these cloths have a relatively high flow resistance. With heavy weft wires (reverse Dutch twill weave) the flow resistance is less but there is a corresponding decrease in micron retention characteristics and both sides of the cloth have rougher surfaces. 

Even though fibres of the above synthetic materials can be woven multifilament or monofilament, there is a trend towards the use of monofilament filter fabrics. This is so mainly because in years past it was very difficult to weave monofilaments in the fine, low-micron retention areas. Today, monofilament filter fabrics can be woven twilled down to 6 pm and plain reverse Dutch down to 14 pm. Further finishing (shrinking and calendering) can bring the particle removal rating down to the 1 pm area. 

The filter elements should remove particles of five microns, must be water-resistant, have a high flow rate capability with low pressure drop, possess high dirt-retention capacity, and be rupture-resistant. The clean pressure drop should not exceed five psig at 100 °F (38 °C). The elements must have a minimum collapse differential pressure of 50 psig. Pleated-paper elements are preferred—provided they meet these requirements. Usually, the pleated-paper element will yield the five psig clean drop when used in a filter that was sized to use depth-type elements. This result is due to the greater surface area of the pleated element, more than twice the area of a conventional stacked disc-type or other depth-type elements. 

FIGURE 13.27 Urethane and some isocyanates on the same system, but including an infrared detector in series at 5.88 micron. The MDI and TDI have different retentions so they can be distinguished from each other in a urethane. 

Returning now to the subject of the chapter, in addition to appropriate retentive characteristics, a potential stationary phase must have other key physical characteristics before it can be considered suitable for use in LC. It is extremely important that the stationary phase is completely insoluble (or virtually so) in all solvents that are likely to be used as a mobile phase. Furthermore, it must be insensitive to changes in pH and be capable of assuming the range of interactive characteristics that are necessary for the retention of all types of solutes. In addition, the material must be available as solid particles a few microns in diameter, so that it can be packed into a column and at the same time be mechanically strong enough to sustain bed pressures of 6,000 p.s.i. or more. It is clear that the need for versatile interactive characteristics, virtually universal solvent insolubility together with other critical physical characteristics severely restricts the choice of materials suitable for LC stationary phases. 

Advantages Simplified regimen for patient Increased patient compliance at home Decreased labor Decreased costs Decreased risk of contamination (due to less manipulation) Minimize infusion-related reactions from intravenous lipid emulsions Decreased vein irritation (especially with PPN) Improved stability compared to TNA Increased number of compatible medications Decreased bacterial growth compared to TNA Easier visual inspection Can use 0.22-micron bacterial retention filter Cost savings if unused (i.e. not spiked) intravenous lipid emulsion can be reused... 

Disadvantages Decreased stability compared to 2-in-1 PN Cannot use 0.22-micron bacterial retention filter Increased bacterial growth compared to 2-in-1 PN Visual inspection is difficult Limited compatibility with medications Increased labor and costs (if intravenous lipid emulsion infused separately) Increased vein irritation, especially if PPN is not coinfused with intravenous lipid emulsion... 

The available data on retention in the respiratory tract, as summarized by Mitchell (M6) indicate that upwards of 20% of inhaled aerosols are retained in the respiratory tract, approaching 100% for particles over 5 microns in diameter. The order of retention follows reasonably well what would be expected from the physics of the various deposition mechanisms assuming that once deposited a particle is retained by the surface. This would,... 

The effect of solvent composition on the retention of a series of solutes, commonly used to measure column dead volumes, was also investigated by these authors. They employed mixtures of methanol and water as the mobile phase and measured the retention volume of the same salts together with a silica gel dispersion (containing particles 0.002 micron in diameter). They also measured the retention volume of the components of the mobile phase methanol, and water. The silica dispersion was chosen to simulate a solute of very large molecular size. The results they obtained are shown in figure (2). 

Horizontal separators. For sizing, it is necessary to specify a vessel diameter and a seam-to-scam vessel length. Gas capacity and retention time considerations establish certain acceptable combinations of diameter and length. The need to settle 500-micron water droplets from the oil establishes a maximum diameter... 

Vertical separators. As with vertical two-phase separators, a minimum diameter must be maintained to assure adequate gas capacity. Vertical three-phase separators also must maintain a minimum diameter to allow 500-micron water droplets to settle. Height of the three-phase separator is determined from retention time considerations. 

For samples with a broad size distribution in the micron range, it is important to avoid the transition region between the normal and the steric mode during the measurement. This can be achieved by proper adjustment of the channel thickness, channel flow and the strength of the applied field [69]. The transition region in Fig. 6 can be experimentally determined by plotting the retention ratio vs. the particle size, as illustrated in Fig. 7 for the example of flow-FFF. 

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