Quantcast Filter Elements

Filter Rating Filters  are  rated  in  several  ways—absolute, mean,  and  nominal.  The  absolute  filtration  rating is  the  diameter  in  microns  of  the  largest  spherical particle that will pass through the filter under a certain test condition. This rating is an indication of the largest opening in the filter element. The mean filtration rating is the measurement of the average size of the openings in the filter element. The nominal filtration rating is usually interpreted to  mean  the  size  of  the  smallest  particles  of  which 90 percent will be trapped in the filter at each pass through  the  filter. Filter Elements Filter elements generally may be divided into two  classes—surface  and  depth.  Surface  filters  are made of closely woven fabric or treated paper with a  uniform  pore  size.  Fluid  flows  through  the  pores of  the  filter  material  and  contaminants  are stopped  on  the  filter’s  surface.  This  type  of element is designed to prevent the passage of a high percentage of solids of a specific size. Depth filters, on the other hand, are composed of layers of  fabric  or  fibers  which  provide  many  tortuous paths for the fluid to flow through. The pores or passages must be larger than the rated size of the filter if particles are to be retained in the depth of   the   media   rather   than   on   the   surface. Consequently,  there  is  a  statistical  probability that a rather large particle may pass through a depth-type  filter. Filter  elements  may  be  of  the  5-micron,  woven mesh, micronic, porous metal, or magnetic type. The  micronic  and  5-micron  elements  have noncleanable  filter  media  and  are  disposed  of when  they  are  removed.  Porous  metal,  woven mesh,  and  magnetic  filter  elements  are  usually designed  to  be  cleaned  and  reused. 5-MICRON   NONCLEANABLE   FILTER ELEMENTS.—  The  most  common  5-micron  filter medium  is  composed  of  organic  and  inorganic fibers integrally bonded by epoxy resin and faced with a metallic mesh upstream and downstream for  protection  and  added  mechanical  strength. Filters  of  this  type  are  not  to  be  cleaned  under any  circumstances  and  will  be  marked  Disposable or   Noncleanable. Another  5-micron  filter  medium  uses  layers of  very  fine  stainless-steel  fibers  drawn  into  a random  but  controlled  matrix.  Filter  elements Figure  9-14.—Cross-section  of  a  stainless  steel  hydraulic  filter element. of  this  material  may  be  either  cleanable  or noncleanable, depending upon their construction. WOVEN   WIRE-MESH   FILTER   ELE- MENTS.—   Filters  of  this  type  are  made  of stainless steel and are generally rated as 15 or 25 micron (absolute). Figure 9-14 shows a magnified cross  section  of  a  woven  wire-mesh  filter  element. This  type  of  filter  is  reusable. MICRONIC  HYDRAULIC  FILTER  ELE- MENT.— The term micronic is derived from the word  micron.  It  could  be  used  to  describe  any filter element; however, through usage, this term has become associated with a specific filter with a  filtering  element  made  of  a  specially  treated cellulose  paper  (fig.  9-15).  The  filter  shown  in figure 9-10 is a typical micronic hydraulic filter. This filter is designed to remove 99 percent of all particles 10 to 20 microns in diameter or larger. Figure 9-15.—Micronic filter element. 9-11 &


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