| When Does
Flow Rate Matter?
Criteria for Testing Optimal Filter Flow Rate
Flow Rate Matter?
Because total throughput is not the focus, filters for filling applications do not contain the protective layers common in throughput-enhanced filter designs (8, 9). Instead, high-flow-rate filters have a specific membrane design and single-layer construction.
Differential pressure. One way to increase a filter’s flow rate is to raise the differential pressure over the membrane. Before doing this, however, maximum operating pressures must be considered. If the operating pressures are too high—specifically if the filtration process stops frequently (i.e., in pulsating applications)—the filter membrane can be damaged. Figure 1 shows such damage on the outer pleat-edge of a membrane that had instabilities due to its membrane design and pleatability (8).
Filtration area. Another way to achieve an optimal flow rate is to increase the filtration area. Doing so may increase the cost per liter of filtered volume, however, and increasing the pleat density of a filter often does not achieve the required filter area. For example, it is generally not possible to double the flow rate by increasing the filtration area.
Membrane configuration. A membrane’s design determines its porosity, thickness, and pore structure (6). There is limited flexibility in membrane configuration, however. For example, flow rate can be enhanced by altering membrane thickness; the thinner the membrane, the lower the flow resistance and thus, the higher the flow rate. However, membrane thickness affects the filter’s retentivity; a membrane that is too thin might allow organism penetration.
Most flow-enhanced filters are single-membrane designs. Homogenous double-layer designs have a higher total membrane thickness and as a result, the flow rate of such filters is often insufficient (Figure 2) (10).
The membrane configuration is a key element to achieve optimal flow rate conditions. Figure 2 shows that a single-layer membrane configuration will achieve higher flow rates than a membrane double-layer combination, especially of homogenous (e.g., 0.2/0.2 µm) design.
The flow resistivity of a homogenous double-layer filter can be so high that a single-layer membrane filter of a smaller pore size (for instance, a 0.1-µm rated membrane) might reach a comparable flow rate (Figure 4). Furthermore, the support fleece and pleat densities must be well balanced to avoid a too-small effective filtration area or uneven flow distribution in the membrane pleat pack.
47-mm Discs Useful?
It is insufficient to test only the membrane, as is commonly done with 47-mm discs. Such tests will not be able to demonstrate the performance of a filter element used in the production process. We have conducted tests that show that initial flow rate comparison trials with 47-mm discs are meaningless (Figure 5).
The 47-mm filter discs
used in a comparison test all have the same theoretical filtration area
of 17.4 cm². When these filters are scaled up, however, the effective
filtration area of the resulting 10-in. filter cartridges varies widely,
from 4,500 to 7,500 cm². Also, the flow rates of these filters at
14.5 psi differential pressure can vary from 2,700 to 8,000 L/h. Testing
just the 47-mm discs does not capture these variations; if it did, the
results would show that one 47-mm disc filter has a flow rate threefold
higher than another 47-mm filter. In addition, the inlet and outlet connectors
of the test device might not be able to cope with such flow.
Large pleated devices must be used to evaluate and compare the real flow rates of the filters used in production. If the ultimate filter is a capsule device, the flow-rate tests should be conducted with a capsule; similarly, if the filter is a 10-in. filter element, the tests should be conducted in such a setup (7). If multiple filters are compared, water can be used in a side-by-side trial with comparable elements.
Such testing takes into account the entire design of the filter, the membrane design, the effective filtration area, flow distribution resulting from pleat densities, and the fleece thickness. The tests should be performed under the required or specified process conditions: a set inlet pressure should be established so that the time to filter a fixed fluid volume can be measured.
In such testing, it is very important to keep the process parameters constant. If the flow rate tests are performed to size a filter system, the actual fluid should be used, under the process conditions found in the production process. Using the actual fluid also accommodates for specific fluid properties (such as viscosity) that must be checked. A pleated device should be used, because only such a device can be scaled to the process size.
To be able to evaluate the effects of real pressure conditions, possible variations, temperature influence, and other variables, the filtration conditions during the trial must mirror production conditions. Only trials conducted in the described scope will result in appropriate sizing of the necessary filter element. As Figure 5 shows, the 47-mm flow rate results differ greatly from the 10-in. element flows, and we may conclude that 47-mm test discs are inappropriate to find an appropriate filter type and scale. Such tests are not only time consuming, but have no true value.