n = a filter life factor that can have values between 1 and 2.

For illustration, if Q₁ is the flow rate at which a given filter currently operates and equals 10 gpm, and L₁, the life of the given filter at that flow rate, equals 8 days, then at new flow rate Q₂ of 5 gpm, the expected life L₂ of the same filter for an n of 1.3, which is a reasonable value for real-life systems, is calculated from Eq. 1 to be:

L₂ = (8)(10/5)(1.3) = 19.7 days

This is about 2.5 times the life at the 10 gpm flow rate. The same result would be reached if the original number of filters in parallel were doubled, while maintaining the original process flow rate of 10 gpm, thus halving the flux through individual filters.

Operating range.

Disposable cartridge depth filters are convenient to use but, at some point, they must be changed. As the fluid contaminant load increases, the frequency of changeouts necessarily rises. For very dirty fluids, the volume available in a filter housing for deposited solids can become a limiting factor. A practical rule of thumb is that cartridge filtration is best suited for conditions where the fluid contaminant level is lower than about 1,000 mg/L (or ppm), which equals 0.1 wt.% suspended solids.

Filter performance. Two parameters are critical. One is filtration efficiency or degree of filtration, and the other is filter life. A filter that is too tight will deliver good quality filtrate but will have an uneconomically short life. In contrast, a filter with pores too open will have long life but the filtrate will be poor quality and cause problems for the downstream customer -- either the next processing stage within the plant or the final customer outside the plant. Both affect process economics and the true total filtration cost.

 

 

Important considerations

Let's look at key factors in choosing and installing a cartridge filter.

Filter ratings.

Degree of filtration reflects the cleanliness of the filtrate. It has to do with the size and number of particles in the filtrate. Comparing these values with the size and number of particles in the feed also indicates how efficient the filter is in removing particles larger than a specific size. The micron rating of a filter is related to the degree of filtration. Typically, filters are given nominal or absolute ratings. Both ratings assign small numbers to tight filters and large numbers to open filters. Nominal ratings also may be used for filters whose basic structures or manufacturing processes are not amenable to stricter characterization. For example, some filters are made by winding a synthetic yarn around a core. Typically, the yarn has a smooth, slippery surface. While in service, increasing differential pressure across these filters makes the yarn shift position and change the apparent pore size. A filter with changing pore size cannot be assigned an absolute rating.

Absolute ratings come closer to representing the true size cutoff of particles that can be kept from passing downstream. A typical absolute rating system cites the Beta Ratio or Beta Value of a filter. The Beta Ratio [3] is:

_x= (cumulative number of particles in feed larger than x)/
(cumulative number of particles in filtrate larger than x)                (2)

Equation 3 defines the relationship between Beta Ratio and particle removal efficiency (PRE) expressed as %:

PRE

= [( - 1) / ] x 100                                                                   (3)

Table 1 details how various Beta Values relate to PREs. The steepest increase in particle removal efficiency occurs at low Beta Values, while at high Beta Values particle removal efficiency changes relatively little.

Filter ratings depend upon the test protocol employed. Important variables include: choice of fluid and contaminant, flow rate, temperature, method of introducing contaminants to the fluid, piping system configuration and method of counting particles. ASTM F795 [4] is a commonly used basis for such testing. Practical considerations lead individual filter manufacturers to modify this procedure in various ways. Because of such differences in test methods, it is not possible to accurately compare the filter ratings from different manufacturers. Neither nominal nor absolute ratings can predict exactly what will happen in a specific practical filtration system. However, filter ratings are useful as guidelines in seeking the right filter for a particular job. Generally, absolute ratings more closely approximate actual performance.

Surface versus depth filters.

For a given pore-size rating, pleated surface filters deliver higher initial flow rates than depth filters. This can be attractive because of the potential for minimizing the housing size. However, many other factors affect filter performance in a real system. Chief among these is the nature of the contaminant. Deformable gelatinous contaminants are often best removed by depth filters operated at low fluid flux. Pleated filters can be effective for fluids with high total suspended solids. However, if there is too little space between the pleats for the application, the filter will blind off and form a circumferential cake at the outside diameter of the filter cartridge. This cake prevents dirty fluid from entering between the pleats and the fluid is unable to access the large pleated surface area. Depth filters, especially those with gradient pore size from outside to inside, combat this problem by trapping larger contaminants in the upstream part of the filter and smaller contaminants deeper within the cartridge. This maximizes the filter's contaminant load capacity. In real systems, the contaminant type, size distribution and load (mg/L), coupled with the fluid viscosity, etc., will determine whether surface or depth filters should be used.

Compatibility.