Selection & maintenance

How long should a filter bag last in a medical waste incinerator?

Short answer: two to four years in a well-run baghouse, six to twelve months in a badly run one. The gap is not about which brand of felt you bought. It is about what the bag is being asked to survive.

Medical waste incinerator flue gas is one of the harder duties for a filter bag. It carries HCl and SO₂ from plastics and PVC, moisture from the waste itself, fine fly ash, and a temperature that swings every time a new batch is charged. Each of those alone is manageable. Together they eliminate most of the cheap options.

What normally limits bag life

1. Acid and moisture — hydrolysis

This is the dominant failure mode in incinerator baghouses, and it is why polyester is the wrong choice even at 120 °C. Below the acid dew point, moisture condenses onto the felt and the combination of water and acid attacks the fibre. The bag loses tensile strength, then tears at the seam or the cage contact points. If your bags are failing at 9–15 months with no visible holes until they suddenly split, suspect this first.

2. Oxidation

PPS — the usual workhorse for this duty — is sensitive to oxygen. Above roughly 8% continuous O₂, or in the presence of NO₂ and other oxidising species, the fibre embrittles and the bag turns from supple felt into something closer to cardboard. It then cracks along the fold lines. This is why a PPS bag can be the right answer in one incinerator and the wrong answer in the unit next door.

3. Mechanical damage

Cage burrs, misaligned venturis, over-tensioned snap bands, and pulse pressure set too high all abrade the felt. Abrasion failures are usually obvious: a hole or a worn band at a specific height, repeated across many bags. If five bags in the same row fail at the same point, it is the hardware, not the media.

Reading a failed bag

Before you throw the failed bag away, note these four things. They identify the cause better than any laboratory test you can afford on a single bag.

What actually extends life

• Keep the gas above the acid dew point at the baghouse inlet. This single change often doubles bag life, and costs nothing but a heat exchanger setpoint review.
• Use a PTFE membrane on fine incinerator ash. Surface filtration keeps the dust cake on the outside, so cleaning is gentler and residual dust inside the felt is lower.
• Dry and clean the pulse air. Wet compressed air is the second most common killer of both bags and diaphragms, and it is free to fix.
• Do not over-pulse. Shorter cleaning intervals than necessary wear the fabric and increase emissions. Tune on pressure drop, not on a timer.
• Pre-coat new bags where the ash is fine and sticky, and bring the unit up to temperature before the first dust load arrives.

Rule of thumb: if the bags failed in under 18 months, do not simply reorder the same specification at a lower price. Find out which of the three failure modes killed them, or you will buy the same failure twice.

Frequently asked

Is two years realistic for PPS in an incinerator?

Yes, where the gas is kept dry, oxygen is controlled and the air-to-cloth ratio is at the low end. Where the unit runs wet and pulses hard, 12–18 months is more typical.

Should I replace all bags at once or in rows?

In a single-compartment collector, replace all at once — mixing new and old bags gives uneven flow and shortens the life of the new ones. In a multi-compartment unit, one compartment at a time is normal.

Do I need to send a failed bag for analysis?

Usually not. A photo of the failure, the installation date, and the operating temperature and O₂ record answers 90% of the question.