Filters cleaning

The pressure drop through the filter is a function of two separate effects. The clean filter has some initial pressure drop. This is a function of filter material, depth of the filter, the superficial gas velocity, which is the gas velocity perpendicular to the filter face, and the viscosity of the gas. Added to the clean filter resistance is the resistance that occurs when the adhering particles form a cake on the filter surface. This cake increases in thickness as approximately a linear function of time, and the pressure difference necessary to cause the same gas flow also becomes a linear function with time. Usually, the pressure available at the filter is limited so that as the cake builds up the flow decreases. Filter cleaning can be based, therefore, on (1) increased pressure drop across the filter, (2) decreased volume of gas flow, or (3) time elapsed since the last cleaning. 

Manual operations such as filter cleaning, sampling, batch operations. 

Compressors will need a weekly oil level and receiver auto drain check and oil changes and filter cleaning at (typically) 500-hour intervals, although for compressors used only to operate ventilation this can be only an annual... 

Clean oil-pump strainer and replace filter Clean air-cleaner, replace as necessary... 

Hurly, P. 1987. New filters clean up in new markets. High Technology 21-24. 

The following general guidelines for filter cleaning frequency and duration are recommended. 

Single shaker unit, frequency-2-10 minutes between filter cleaning, 5-10 seconds for shaking. This may vary as the fine powders form granules and the frequency between the shakes or duration of shaking interval can be extended. In any case, the dead bed time should be kept at a minimum in a single shaker unit. 

Multiple-shaker unit Since this is a continuous process, the frequency of shaking for each section is approximately 15-30 seconds between filter cleanings and about 5 seconds for shaking the filters. If a low-pressure blow-back system is used for the bags, the frequency of cleaning is about 10-30 seconds. 

The cleaning frequency and cleaning duration is now offered as an automated system where instead of having base the cleaning frequency on time, the trigger point for filter cleaning is the buildup of a pressure drop across the filters. This automates the process and eliminates operator input. 

Viable alternatives to cartridge filters include bag filters and metal mesh filters. Metal mesh filters are available in very fine micron sizes and they can be cleaned and reused. Since it is very important to minimize all wastes, the issue of mesh filter cleaning waste reuse or recycling would need to be addressed before switching to these filters. 

Depth membrane filters are usually preferred for in-line filtration. As particles are trapped within the membrane, the permeability falls, and the pressure required to maintain a useful filtrate flow increases until, at some point, the membrane must be replaced. The useful life of the membrane is proportional to the particle loading of the feed solution. A typical application of in-line depth microfiltration membranes is final polishing of ultrapure water just prior to use. Screen membrane filters are preferred for the cross-flow microfiltration systems shown in Figure 7.1(b). Because screen filters collect the retained particles on the surface of the membrane, the recirculating fluid helps to keep the filter clean. 

GC/ECD has been used to measure tetryl in ocean floor sediment and fauna (Navy 1972). Preparation steps generally involved homogenization and extraction with benzene, filtering, clean-up with TLC, and concentration. The sensitivity for detecting tetryl in ocean floor fauna using this method was estimated to be in the ppt range. Precision and accuracy were not reported (Navy... 

Fig. 6.1. Twin-belt filter press (Bellmer) 1. Prior extraction of juice, 2. Wedge zone, 3. Low-pressure pressing zone, 4. High-pressure pressing zone, 5. Juice runoff, 6. Filter cleaning, 7. Filter cake removal.

Figure 4 Pressure drop in a hot gas filter cleaned by nitrogen pulsing...

Also calculate the efficiency at the start of the filtering (cleaning) cycle ... 

A wide variety of woven and felted fabrics are used in fabric filters. Clean felted fabrics are more efficient dust collectors than woven fabrics, but woven materials are capable of giving equal filtration efficiency after a dust layer accumulates on the surface. When a new woven fabric is placed in service, visible penetration of dust may occur until buildup of the cake or dust layer. This normally takes from a few hours to a few days for industrial applications, depending on dust loadings and the nature of the particles. 

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