Standard open-air high-pressure showers can create significant mist when impacting a fabric. When shower pressures or fabric speed increases, the result is even more mist. This mist is laden with filler and fiber. It collects on the machine structure and cross machine members leading to both runnability and safety issues, particularly with top formers and tissue machines. Safety issues attributed to slip hazards can be minimized with mist elimination. Runnability issues that mist elimination can address include:
- Drop-offs: Elimination of drop offs that cause holes or sheet breaks
- Fabric cleanliness: Containing mist allows the mill to run the showers at higher pressure improving overall fabric cleanliness. This typically results in:
- Improved fabric life
- Less chemical cleaning, and
- Fewer wash ups
- Contaminants: Outside and inside high-pressure showers remove surface components and interstitial contaminants
- Surface fiber: Vacuum aids in removal of loose surface fiber improving machine performance
- Poor sheet characteristics: Dewatering the fabric can result in better sheet characteristics and improved sheet solids
What is an engineered mist elimination system? The main physical components are on-machine vacuum structures with embedded high-pressure showers, off-machine separators (pre and main), and a vacuum source (exhauster). Factors to consider when establishing airflow are machine speed, fabric caliper, and fabric permeability. Placement and piping design are critical to insure the vacuum is delivered as designed for containment of the mist.
A properly designed mist elimination system allows the in-rush of air into the vacuum structures to capture the mist from the showers to prevent the mist from “escaping” with the fabric boundary layer. The result is a mist-free environment. Mist elimination systems offer a dramatic improvement to wet end hygiene, can make a significant and positive impact on runnability, and can improve sheet properties in many installations.