Our dust collection specialists are frequently asked to answer the following question: what is the normal useful life of a filtering medium?
In reality, operating parameters vary so widely from one process to another that it is nearly impossible to establish a standard scale. A filter can perform flawlessly after seven years, but it can just as easily come to the end of its useful life after two. It all depends on the context and environment to which it is exposed.
Let’s instead shift our focus by rewording the question like so: what factors determine optimal use of a dust collector?
Initial Unit Design
The engineers at Airex Industries ask themselves the same question when initially designing a dust collection system. The goal for dust collection specialists is to determine the balance between system efficiency and the costs incurred to support such a design. To achieve this goal, the design must incorporate four factors in order to make the system run optimally.
1- Air-To-Cloth Ratio
This ratio represents the lateral velocity (in ft/min) at which air flows through the filtering medium inside a dust collector. To determine this value, it is imperative to know the total airflow (in cubic feet per minute – CFM) and the total filtration area (in ft2). Once these data are known, the air-to-cloth ratio can be calculated using the following formula:
= AIR-TO-CLOTH RATIO
An air-to-cloth ratio is typically between 4 and 10 ft/min, depending on the type of application. A low value means that the filtering media are in less demand and that their replacement frequency remains low. However, to maintain such a ratio, the dust collection system must have a sufficient quantity of filtering media inside in order to maintain a high total filtration surface relative to airflow. This therefore requires a higher initial investment when purchasing the dust collector. Conversely, a high air-to-cloth ratio is more demanding on the filters and, at the same time, reduces their useful life. The owner of a smaller dust collector is therefore forced to replace its filtering media more frequently for the same airflow.
2- Upward Velocity
The goal of this variable (in ft/min) is to determine the force in opposition to gravity so that the particles can fall naturally to the bottom of the dust collector. Again, the total airflow (in CFM) and the free surface between filtering media (in ft2) must be known. Once these data are obtained, the upward velocity can be calculated using the following formula:
FREE SURFACE BETWEEN FILTERING MEDIA
|= UPWARD VELOCITY|
Standard upward velocity is maintained between 50 and 250 ft/min. Moreover, this velocity is bound by the configuration and weight of the particles sucked into the dust collector. Very light particles, such as cotton and feathers, are limited to an upward velocity of about 50 ft/min. Very heavy, large particles, such as rock or cement, can easily reach an upward velocity of 250 ft/min.
If the upward velocity is too high, the particles adhered to the filtering media will be incapable of dislodging to fall into the hopper because, even after the self‑cleaning system is pulsed, the particles will re-adhere to the same filtering medium or to the medium beside it. Completely blocked filters might therefore have to be replaced prematurely. To remedy this, the system needs to be entirely reconfigured.
3- Dust Load
Although essential, dust load (in lb/hr) is, in most cases, a complex piece of data to quantify (particularly when a new process has to be evaluated). There is no mathematical formula for accurately determining the dust load generated because there are too many variables to consider. By referring to internal databases on completed projects plus relying on their experience and empirical knowledge, the dust collection specialists at Airex Industries are able to estimate the dust load with more than adequate precision. This piece of data ensures that the filter surface is sufficient for the system to accept the load. That is why the design of the dust collector is directly influenced by this correlation.
4- Cleaning System
A cleaning system enables continuous operation and, at the same time, reduces the filtering media replacement cycles. In order to properly calibrate the cleaning system, the results of the air-to-cloth ratio and dust load discussed earlier are needed. Once these data are known, the programmed pulse frequency and the compressed air pressure can be determined. As explained earlier for the dust load, the configuration of the cleaning system is also calibrated based on an accumulation of past knowledge, not using a series of strict calculations.
In more than 90% of cases, frequency control is fixed. Programming too high a frequency makes the system pulse more often than necessary. That ends up overworking the air compressor, and the company must deal with an electricity bill that is higher than its actual need.
Conversely, an air pulse frequency that is too low is likely to create an accumulation of dust on the filtering media. Since it is unable to dislodge all the particles embedded in the filters during cleaning processes, the differential pressure between the dirty section and the clean air plenum will continue to increase. This results in a complete clogging of the filtering media in the dust collector. In this type of situation, reprogramming is strongly advised in order to reduce the differential pressure.
Interestingly, there are sufficiently sophisticated programmable logic controllers that can self‑adjust automatically based on the differential pressure of the dust collector in order to adapt their self-cleaning sequence. However, their price greatly exceeds the price of a conventional fixed-sequence system.
If a dust collector becomes clogged without spare filters on hand, there is manual cleaning equipment for individual unclogging. This solution, which involves injecting high-pressure air, offers the option of extending the useful life of filtering media for a short amount of time. However, this type of operation requires access through a clean air plenum, which is not the case for all dust collection systems.
Avoid Restricting The Capacity Of A Dust Collector
There is a golden rule for preventing any complications due to a conceptual error: it is best to oversize a dust collection system. In addition to providing a favourable operating environment, the dust collector will be able to accept an unexpected increase in dust load without the risk of the filtering media becoming clogged. This happens more often than people think and may be due, for instance, to higher productivity, a process producing more dust or the addition of new equipment on the dust collection network.
A system that is too small for the company’s needs will be perpetually faulty and will simply not run under optimal conditions. A full system analysis will be required to re‑assess the four aspects described earlier.
As noted earlier, two out of the four analysis factors refer to expertise based on tacit knowledge. That is why it is highly recommended, when designing or modifying a dust collection system, that you seek the expertise of a firm such as Airex Industries, which has proudly built up a solid reputation and experience in dust collection over the past 40 years.