The useful life of filtering media is a major concern in the design of any industrial dust collection system. Since the cost of filters replacement is a recurring expense, companies want to extend their use as long as possible, which makes perfect sense!
In an initial article on this issue, we established that the initial design of the unit greatly influences the useful life of the filters and that a system that is too small for the needs increases the risk of occultation. Therefore, it is wise to have a slightly over-sized dust collection system to withstand an increase in the dust load without creating problems.
This second article on the subject will more specifically cover the types of particles and how they interact with the filtering medium.
Understanding The Usefulness Of The “Dust Cake”
You may be surprised to learn that what first filters the air in your plant is not the filtering medium as such, but rather the collected particulate on it. In fact, the medium actually acts as a backing for building up a certain amount of particulates, thereby creating what is called the “cake”. So before it forms, a new filter is less effective. This is why it is coated in advance with Perlite, a product similar to flour, but intended to create a primer layer of dust on the medium.
Obviously, the more cake that builds up, the more resistance the airflow encounters when going through the filtering medium, with the result that the airflow rate exiting the filter is lower than when entering it, which is called the differential pressure. The reason for this is because the dust cake thickens and becomes denser over time, which is even more true in the presence of very fine particles.
Declogging With Pulsation
The pressure loss in filtration is measured using a Magnehelic or Photohelic pressure gauge connected to the control panel (PLC). Below a certain threshold (2 inches of water column), the situation is under control. However, when the differential pressure exceeds that point, the PLC triggers a pulsation declogging process. Those pulsations will occur at an established frequency that will increase to a maximum of twice a minute. The theory suggests not exceeding a critical threshold of 6 inches of water column, beyond which the filters should be changed. Extending that process would hardly be effective because it would result in overuse of the air compressor and thereby a waste of electricity, to say nothing of premature wear of various mechanical components such as the valve diaphragms.
Clogging Evolution of Filtering Medium
Note that, to ensure good declogging, the compressor must be equipped with a dessicant-type drier because the condensation caused by the moisture in the air can cause the particles to agglomerate to each other and to the filtering medium.
Choose A Filtering Medium Based On Your Operating Conditions
Filters must be selected with care, taking all of the following factors into consideration:
- Average air temperature
- Peak air temperature
- Humidity level
- Exposure to chemicals
- Type of particles
- Density (weight based on volume)
- Hydrophilicity (moisture absorption, agglomeration)
- Format (fibrous, round, etc.)
- Chemical stability (interaction with other particles)
- Electrostatic charge
Resistances table according to fiber type
max temp.°C (°F)
|Nomex||230 (450) ||••||•••••||••••|
|Polyester||150 (300) ||••••||•••||••••|
|Acrylic||140 (285) ||•••••||••||•••|
|Glass||340 (650) ||•••••||••||••|
|Teflon||260 (500) ||•••••||•••••||••|
|PVC||70 (165) ||•••||•••••||•••|
A Trump Card In Your Hand: The PTFE Membrane
The polytetrafluoroethylene membrane can be used to your advantage, but only under certain conditions. It acts as a protective layer on the filter to facilitate declogging. With its smooth surface, it prevents the build-up of particles and, thereby, increases the effectiveness and useful life of the filtering medium. However, you need to bear in mind that the membrane will increase resistance and, therefore, the differential pressure of the dust collection system.
Optimizing Costs With An In-Depth Analysis
For short-, medium- and long-term savings, dust-collection specialists take a number of aspects into consideration. A number of alternative solutions can be developed to reduce the overall costs of a filtration system. Filtering media are part of the equation that must include multiple aspects.
Our specialists will find places in your filtration system where savings are hiding. Consult us!