Filter Cake

The filter cake grows in the course of filtration, becomes “thicker” as particulate matter is being retained.

Many filter users look at their cartridge filters, see that they are dirty, and immediately change the filters.  Although on the surface this logic appears to be good, it is probably costing you money.

Cartridge filters are designed to remove particulate matter in the body of the filter as well as on the surface.  The particulate that is deposited on the surface of the filter (commonly known as Filter Cake) acts as a filter in its own right and as the depth of the filter cake increases the filter becomes more efficient.  In effect the dirt is filtering out the dirt.

By allowing filter cake to build up on the filter the life of the cartridge filters will also be extended, thus reducing the filtration cost for each liter filtered.

Pressure Gauges

The easiest way to tell if your filters are doing the job is by having a Pressure Gauge in the inlet and outlet sides of the filter vessel. It will tell you, 1) If the filter is bypassing, 2) When the filter needs to be changed, and 3) About how much life before change out is necessary.


You can take advantage of this life extension by installing a pressure gauge both before and after your Filter Housing and measuring the pressure of the liquid before it goes through the filter element as well as after the liquid has been filtered.  The difference between the readings on both gauges is referred to as differential pressure.

You will note that the longer the filter element is in the housing the greater the differential pressure will become because greater force is needed to move the liquid through the cartridge.  When the differential pressure reaches 20-30 psi you should change the filters.

If you allow the pressure to exceed the 20-30 psi differential you will run the risk of channeling particulate through the body of the filter thus bypassing the filter and negating the purpose of the filter.  When channeling occurs the differential pressure actually decreases.

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Pleated Microfiltration is an Elite Distributor for GE Nominally and Absolute Rated Membrane Filters. Shop over 800+ products at

Most particulate Filters are referred to as either Nominal, or Absolute. In the case of both of these filters the results achieved are very application sensitive. It is important to remember that lots of very creative claims are made by some manufacturers about how their filter will perform.

Nominal Ratings

Nominally Rated Filters capture most of the particulate of the assigned micron rating, or higher. In practice, the flow
rate has a great impact on performance. In most cases, the slower the flow, the greater the efficiency of the filter.

Pleated Microfiltration Nominal Filters available online:

Absolute Ratings

Absolute Rated Filters capture a much higher percentage of the particulate matter. Some manufacturers consider 95% as absolute, while others consider 99%, 99.9%, and 99.999%. Different test protocols are used to determine exactly what is absolute for a specific application. Although it is tempting to say that you need an absolute rated filter keep in mind that it is usually much higher priced.

Pleated Microfiltration Absolute Filters available online:

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Shop over 1500+ GE Depth Cartridges at

Shop over 1500+ GE Depth Cartridges at

Similarities and differences between Hytrex, Purtrex and Ro.Save.Z GE Depth Cartridges:


  • 100% Polypropolene
  • Nominally rated micron
  • Depth Cartridges
  • Graded Density; The media catches larger particales from the outside and smaller particles as it reaches the core
  • No lubricants or anti-static agents
  • Front Facial Velocity; The speed that liquid enters the filter and goes to the core.
  • Filters from the outside in
  • Low Media Migration
  • Low pre-flush requirements
  • Meet Federal Standard Requirements
  • NSF Standard 42 certified
  • Consistant in quality
  • Thermoplastic
  • FDA Compliant


  • Hytrex contains more media than Purtrex for better efficiency.
  • Purtrex has a wider window of acceptability
  • RoSave Has a greater void volume to catch more particles
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Slow the Flow for Efficiency

With in-depth cartridge filters, the slower the flow, the more efficient the cartridge is and the longer the user can go between change-outs. At, we typically size housings to start out with a clean differential pressure of two pounds or less. You will find that many people in the filter business will quote housings that are undersized for an application so that they quote the lowest capital equipment cost.

The Pressure Drop

The differential pressure (pressure drop) across both the cartridge and housing must be considered cumulatively. The pressure drop across the housing differs from housing to housing, but in most cases, it can be obtained from the housing manufacturer.

Flow Rates

Assuming a cartridge vessel is designed for cartridges with a one-inch inside diameter, keep in mind that the flow through the bottom of each filter should not exceed 15-25 gallons per minute (for membrane pre-filters, try not to exceed 15 gpm). These flow rates should not be exceeded because turbulent flow is created on the interior core of the filter, which frequently cause unloading of contaminant from the filter media.


Always consider the viscosity of the material to be filtered when sizing filters or vessels. Also keep in mind that the viscosity of most materials varies depending on temperature. If you have an application where the customer does not want to go over a certain differential pressure and the temperature of the product can go through a wide swing, be careful to find out what the viscosity of the liquid is at both extremes of temperature.

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