High efficiency filters

High-efficiency filters (HEF) are pressure filters designed to remove turbidity and SDI in water. They were originally applied to cooling towers, to reduce the concentration of suspended solids in those systems. 

Vortex filters use centrifugal force to swirl raw water above the surface of the media in a cross-flow manner. Large suspended solids are collected on the sidewall of the filter tank. The smaller solids drop down to the surface of the fine sand media and are filtered out through this media. When the pressure drop reaches about 15 psig, the vortex filters are backwashed. 

Vortex filters operate at high flow rates, typically 15 -20 gpm/ft2. Backwash flow rate is about 2.5 - 5 gpm/ft2 and requires only 4-8 minutes of backwash time. 

Vortex filters supplied by Sonitec, Inc., ( Vortisand filters— Vortisand is a registered trademark of Sonitec, Inc., Holyoke, MA) come complete with a chemical cleaning system. This system is used every 3-8 backwash sequences to reduce filter media fouling 

Top-over-bottom HEFs operate at higher flow rates, such as 10 -15 gpm/ft2. Backwash flow rate is about 10 gpm/ft2 for about 5 minutes per backwash. 

Top-over-bottom HEFs rely on tangential flow across the surface of the media to affect filtration. Raw water enters the filter tangentially to the surface of the media, thereby creating turbulence over the bed and a 

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Cold-generated DOP A cold-generated aerosol dioctylphthalate (DOP) test, used to measure the efficiencies of high-efficiency filters. A hot DOP test may also be used. 

Filtration is an efficient and inexpensive method for removing dust, particulates and bioaerosols from indoor air. High efficiency filters can remove up to 95 % of airborne particles as small as 0.3 microns. However, odor associated with gaseous VOCs cannot be removed by simple filtration and must be captured using adsorbents such as activated carbon and charcoal. Frequent replacement is needed since these adsorbents have finite capacity and cannot be regenerated. The aim of this project is to develop an effective remediation technology for common airborne VOCs found indoor. 

Airborne particles, bacteria, fungi spores and allergens will be removed by air filtration, which is often cheaper and more effective than electrostatic precipitators and ionizers. A disposable, high-efficiency filter capable of removing at least 95 percent of particles with size down to 0.3 micron will be installed. A washable coarse filter that removes fifty percent of particles larger than 10 microns will be used as a pre-filter to extend the life of the high efficiency filter by at least a third. An inexpensive sensor will be included to indicate when filter replacement is necessary. 

High efficiency filter Particulate Cadmium iodide on Chromosorb B Elemental... 

Figure 8.10 Top-over-bottom high efficiency filter.

Figure 8.11 Vortex high efficiency filter. Courtesy ofSonitec, Inc.

Table 8.3 Performance comparison of multimedia and high-efficiency filters.

High-efficiency filters are gaining in acceptance for RO pretreatment.3 These filters offer some advantages over conventional multimedia filters, the most important of which may be the ability to remove particles down to 0.25 microns in size for the top-over-bottom filters, and 0.45 microns for the vortex filters. 

The performance characteristics of wall-flow filter can be varied and managed. Its collection efficiency can be controlled to a large degree by the properties of the walls that form the channels. These include total pore volume, pore size distribution, and the thickness of the wall itself. The flow through the wall can be made more restrictive by adjusting the porosity of the wall. A smaller pore volume creates a highly efficient filter but at the same time restricts the flow and produces high back pressure. Conversely, with porosity adjusted in the opposite direction, low back pressure is achieved, but at the expense of reduction in collection efficiency. 

Another safety feature is a system of high-efficiency filters through which all air leaving the building must pass. These filters are designed to trap microscopic particles of radioactive materials that might otherwise be vented to the atmosphere. Other specialized devices and systems have also been developed for dealing with other kinds of accidents in various parts of the power plant. 

It should be noted that the use of dioctyl phthalate smoke to test high-efficiency filters is no longer used in field certification tests due to hazards. Poly alpha olefin is used instead. For more information,... 

Martin, S.B., B.R. Beamer, and E.S. Moyer. 2006. Evaluation of a high-efficiency, filter-bank system. Journal of Occupational and Environmental Hygiene 3 204-213. 

An evaluation used for high-efficiency filters, defined by ASHRAE Standard 52.1 (ASHRAE, 1992), that involves the use of unconditioned atmospheric air passed through the test material, and visualization of discoloration of the air downstream is compared to upstream (unfiltered) air. 

Decontamination of Equipment. In most cases of contamination of equipment and buildings, a mixture of normal household cleaning methods will remove the material. Prior to decontamination conduct a radiation survey of the equipment. Vacuum cleaners that can handle wet material and have high efficiency filters are particularly useful. Some surfaces may require... 

A high-efficiency filter means a filter that is at least 99.97% efficient against mono-disperesed particles of 0.3 pm (micrometers) in diameter or higher. ... 

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