A demister in a knockout pot catches droplets before they reach downstream equipment, and the choice is mainly between polypropylene and stainless steel mesh, supplied whole or in sections. For a 700 mm diameter, 200 mm deep demister quoted for a leachate ammonia stripping plant, the supplier offered both materials and split the unit into three sections to get it into the pot.
The challenge: a wet gas stream and a small opening
A knockout pot sits in a gas line and lets droplets fall out. A mesh demister fitted inside it makes small droplets join together on the wire or fibre and drain away, so the gas leaving is drier. On a stripping plant, droplets that pass through can carry water into equipment that should see dry gas.
Replacing a demister raises two practical questions. What should it be made of? And how will it get through the access opening?
The approach: options on material, and sections for access
Two sites, two designs
At one plant the replacement was a single-piece demister, 700 mm in diameter and 200 mm tall, in 316 stainless steel mesh and grids. At another plant the same dimensions were quoted as three sections, with a choice of materials:
- Polypropylene mesh and frame.
- 304 stainless steel mesh and frame.
An earlier version of that quotation also offered polypropylene mesh with a 304 stainless steel frame. It was dropped from the final offer.
Why sections?
A 700 mm disc will not pass through a small manway in one piece, and it is difficult to handle inside a vessel. Three sections can be carried in and assembled in place, which is why the supplier chose this form.
Why the choice of material
- Polypropylene is light, resists many chemicals and is easy to cut and handle, but it has a lower temperature limit than steel.
- 304 stainless steel tolerates higher temperature and is rigid.
- 316 stainless steel adds corrosion resistance, which suits a more aggressive condensate.
The right answer depends on gas temperature and the chemistry of the condensate, which the quotation did not state, so the supplier gave options rather than a single recommendation.
Why it works
Offering options puts the decision with the party that knows the operating conditions. The supplier knows what the pot can accept: its diameter and depth. The operator knows what passes through it. Delivery was around four to six weeks, which is short for plant spares.
What to consider for your own project
- Know the maximum gas temperature before choosing polypropylene.
- Check condensate chemistry. If it is aggressive, move up to 316.
- Measure the manway, and decide on sections early.
- Keep the old demister as a pattern for dimensions and support rings.
- Check for fouling. A demister that clogs should be inspected on a schedule.
For a related component, see our post on packing support plates for ammonia stripping columns.
FAQ
What does a demister do?
It makes fine droplets join into larger ones that drain out of the gas stream.
Is polypropylene suitable for a demister?
Often, if the gas temperature is within its limit and the chemistry suits it. Check both.
Why supply a demister in three sections?
So it can pass through an access opening and be assembled inside the pot.
How long is delivery?
About four weeks in this case.
Replacing a demister?
Send us the pot dimensions and gas conditions and we will recommend a material.