Cartridge filter - Bag filter
Comparison between cartridge filters and bag filters
In many filtering applications, a choice between the use cartridge filter or a bag filter has to be made. Both are sediments filters, that is to say they reduce the amount of sediments transported by the fluid trough filtration.
- The choice of cartridge filter depends on the application. Cartridge filters are preferable for systems with contaminations lower than 100 ppm, that is to say with contamination levels lower than 0.01% in weight.
Cartridge filter can be surface or depth-type filter: depth-type filters capture particles and contaminant through the total thickness of the medium, while in surface filters (that are usually made of thin materials like papers, woven wire, cloths) particles are blocked on the surface of the filter.
It can be generally stated that if the size of filter surface is increased, higher flows are possible, the filter last longer, and the dirt holding capacity increases. Cartridge filters are normally designed disposable: this means that they have to be replaced when the filter is clogged.
- Bag filters are in general frequently used for dust removal in industrial applications. The flow can be from the outside to the inside of the filter (that means, the separation of particles happens on the external surface of the filter) or the other way around, depending on the application. The particles are normally captured on the internal surface of the bag filter.
Bag filters are in general not designed for replacement when they are clogged, but some bag filters for gaseous applications like dust removal can be cleaned, for example by mechanical shaking or by backwashing with compressed air (so called reverse-flow bag filters).
Bag filters are mostly surface-type filters.
- A rule of thumb is that for concentrations higher than 5 mg/m3 a surface filter is favored, while for concentrations lower than 0.5 mg/m3 a depth-type filter is preferred. In general surface filters can by backwashed and cleaned more easily, while depth-type filters normally have to be disposed when clocked.
In the following table the compatibility of polypropylene bag filter/cartridge filters at room temperature is listed. Polypropylene is often used as filtering material.
Source: Filters and Filtration Handbook, 3rd edition, Christopher Dickenson, Elsevier Advanced Technology
- Filtration rate:
Bag filters are in general designed for applications with a desired filtration rate from 1 to 1000 micron.
Cartridge filters have a filtration rate from 0.1 up to 500 micron.
In both cases distinction between absolute and nominal filtration rate should be made:
The absolute filtration rate indicates the maximum size of a particle that can pass through the filtration unit.
The nominal filtration rate indicates that a certain percentage of material, bigger than the nominal filter rating will be able to pass through the filter. The % efficiency rating (e.g. 98 %) of the nominal filtration rate indicates the amount of larger particles that will be blocked by the filter.
The beta ratio is a mathematical expression, that indicates the ration between the number of particles of a given size entering and leaving the filtration unit.
The Beta Ratio is defined as follows:
Beta (x) = Number of particles > size (x) upstream / Number of particles > size (x) downstream
The beta ratio indicates how good a filter works: if one out of every three of the particles (>xµm) in the fluid pass through the filter, the filter's Beta ratio at xµm is "3." If only one out of every 300 of the particles (>xµm) pass through the filter, the Beta ratio at xµm is "300." Therefore, filters with a higher Beta ratio provide better particulate control and hence better system protection.
- Pressure drop over filter:
When a flow passes through a filter, a certain pressure drop occurs. This pressure drop depends on the filter media, the filter housing and the flow.
An increasing pressure drop over the filter indicates that the filter has to be replaced: When a filter is nearly clocked, pressure drop higher than in a new, clean filter.
This replacement pressure drop differs from system to system, and is dependent on the filter media.