All gas phase filter tests are of the single-pass format using a particle challenge, but the methods differ widely, as do the particle formulations used in the challenge and the means of analysis to demonstrate performance. The materials used include natural sand/quartz mixtures, alumina dusts, methylene blue aerosols and di-octyl phthalate. A sand-quartz mix is […]
The test circuit used for fatigue tests subjects the filter to start-stop cycles (for evaluating fatigue behaviour under otherwise steady flow conditions), or to cycles of pressure fluctuation (for evaluation of fatigue under pulsating flow conditions). A bubble point test applied to the filter element before the fatigue test, and again after it, will demonstrate […]
A range of methods exists for testing various aspects of a filter ’ s performance. These will be briefly discussed here, although a more complete review of test methods and standards is given in Handbook of Filter Media (Derek B. Purchas and Ken Sutherland, 2002, 2nd Edition, Elsevier Advanced Technology, Oxford). Bead challenge test The […]
A steady flow of fluid through a filter will cause correspondingly steady accumulation of solids and rise in pressure drop. The effect of pulsating flow is to loosen the finer particles held in the filter cake, and so to allow them to pass through the filter and on into the filtrate. This effect is illustrated […]
The permeability is the reciprocal of the resistance to flow offered by the filter – thus, high permeability represents a low resistance and vice versa. Permeability is usually expressed in terms of a permeability coefficient, which is directly proportional to the product of flow rate, fluid viscosity and filter medium thickness, and inversely proportional to […]
One especially important case of purification concerns the removal of microbial contaminants so as to produce a sterile fluid. The use of membranes of 0.2 um rating is generally regarded in critical industries as a satisfactory means of achieving sterility, demonstrated by a bacterial challenge test (NB this is referring to sterility from bacteria, not from […]
The nominal rating of a filter can be expressed by means of this efficiency figure. Given as a percentage, it can be calculated from the beta ratio, or directly from the particle number count: specific to a particular particle size. It applies over the whole particle size range, down to the absolute cut-off value, at […]
The beta ratio is a rating system introduced with the object of giving both filter manufacturer and filter user an accurate and representative comparison among various filter media. It is the ratio between the number of particles per unit volume above a specific size in the suspension upstream of the filter to the same parameter […]
A mean filter rating is a measurement of the mean pore size of a filter element. It is far more meaningful than a nominal filter rating, and, in the case of filter elements with varying pore sizes, more realistic than an absolute rating. It establishes the particle size above which the filter starts to be […]
A nominal rating is an arbitrary value for the performance of a filter, determined by the filter manufacturer, and expressed in terms of percentage retention of a specified contaminant (usually spherical glass beads) of a given size. It also represents a nominal efficiency figure for the filter. Figures typically quoted are at the level of […]
