Since we’ve been in the dust collection business for close to 150 years, it’s not surprising that we hear questions covering all aspects of this field. For example, we are often asked about cleaning filter bags in dry dust collectors. Is cleaning your filter bags a more cost-effective solution than simply replacing the old bags? Spoiler alert; the answer is a resounding “No!” (See also 3 Reasons to Replace Your Dust Collector Filter Bags).
How Many Filter Bags Are We Talking About?
Some people think that the decision concerning cleaning or replacing filter bags will be a function of the number of bags involved. Although this is not, in fact, the case, it’s worth noting that there are almost as many dust collection architectural scenarios as the companies that are using them.
For example, a small “mom-and-pop” operation may have only one dust collector with 20 filter bags, while a large enterprise might have 100+ dust collectors, each with 1,000 bags.
The importance of this is that the entire dust collector must be shut down to clean or replace even a single bag, so—whatever filter bag option you choose (clean or replace)—it’s a good idea to do it to all of the bags in a particular machine at the same time.
Taking the Pulse
When the dust collector is running, dust particles build up on the outside of the filter bag. Some of these particles work their way into the fabric of the bag. In the case of today’s pulse jet (also known as pressure-jet) cleaning, a high-pressure blast of air is used to remove dust from the bag.
The time between pulses is dependent on the application. In some cases, this time may be as short as 10 seconds. In other cases, a pulse may occur every minute or two. Pulsing the bag stretches and weakens its fabric, albeit by a small amount, so minimizing the number of pulses if they are not required is beneficial (it also saves on compressed air). For this reason, some systems only pulse on demand when the pressure drop across the system reaches a specified value.
Feeling the Pressure?
If your cleaning system is working correctly with pulsing occurring at the regular rate, but you are running with a high-pressure drop that’s substantially above the historical average, then this indicates the bags are clogged up. You will need to clean or replace them. A side effect of this high-pressure drop scenario is that the air volume transporting the dust is lowered, which means you will not obtain sufficient ventilation at your pickup points.
An alternative scenario is that you aren’t experiencing a high-pressure drop, but you see dust emissions coming out of the stack, signifying that you have one or more tears in your bags, which—in this case—dictates that you need to replace them (there’s no point in cleaning a bag that has holes in it).
Cleaning Isn’t All It’s Cracked Up to Be
The idea of cleaning filter bags may, at first, appear to be an attractive option because it would seem to offer a way to extend the lifetime of the bags. But then you have to consider how the bags are to be cleaned.
First, you will have to pulse the bags as hard as possible, stretching and stressing them. When it comes to the actual cleaning, you could beat the bags, but—once again—this will stress them while also releasing clouds of dust into the air. Alternatively, you can pass them through industrial washing machines, which will cost money (especially if you have to send the bags to another company to be cleaned). In addition to stressing the bags, the washing process can cause shrinkage, which will lead to additional stress when the bags are stretched once again as part of being returned to service.
How Important is Dust Collection to Your Operation?
The most significant deciding factor in all of this is considering the importance of dust collection to your operation. If you need to run 24/7 for most of the year, with some downtime once or twice a year for scheduled maintenance, for example, then it makes sense to service your filter bags as part of this maintenance cycle. (See also, Cleaning Up with Cartridge Filters.)
One way to think about this is that it costs you the same amount of money to take the bags out, whether you clean or replace them. Similarly, it costs you the same amount of money to put the bags back in, whether you clean or replace them. So, the only differentiating factor here is the cost of cleaning old bags versus purchasing new bags.
A more important differentiating factor is the cost of having to shut your operation down unexpectedly. Knowing that cleaning filter bags stresses them, thereby increasing the chances for a catastrophic failure that may shut down your operation, then replacing the bags is the obvious way to go.
Of course, this doesn’t mean you have to replace all of your bags every time your plant shuts down. Bags typically need to be replaced every one to three years, depending on the application and the environment (some companies only need to replace their bags once every five years, while others find that even one year is too long).
If you have two machines and your bags need to be changed once each year, it may be a good idea to stagger things, replacing the bags in the first machine one year and the second machine the following year, thereby spreading the costs. In certain extreme cases where it’s deemed mission-critical for the plant to run 24/7/365, one solution is to have an extra dust collector in the system, thereby allowing different units to be isolated and powered down for cleaning while the other collectors maintain operation.
The main thing is to stay ahead of the curve. If your bags are lasting 18 months on average, it may be good to schedule replacement every 12 months to be on the safe side.
Although the cleaning option may be a little cheaper in the short term, it can be significantly more expensive if things go wrong in the long term. Thus, taking all of the considerations and discussions presented above into account, our position and recommendation are that the safest option is always to replace filter bags rather than clean them.
Selecting the best dust collection system—including the most appropriate filter media and filter finish (or finishes)—is a multi-dimensional problem. The optimal answer depends on many factors. The best solution is to ask the experts at Sly to evaluate your unique dust collection situation and offer their expert advice.