Multi-cyclone

A multi-cyclone, or multiclone, is a parallel array of many small cyclone separators inside a common casing. Splitting the gas flow into many small cyclones improves collection of coarse particles compared with one large cyclone handling the same total flow, because cyclone efficiency generally improves as diameter decreases.

Multi-cyclones are used as pre-cleaners on biomass boilers, coal units, industrial furnaces, dryers, mills and some process exhaust systems. They reduce dust loading to downstream equipment such as ESPs, baghouses or scrubbers, but they are not usually sufficient as the final particulate-control device for tight emission limits.

How it works

Each cyclone tube accelerates gas into a swirl. Particle inertia drives dust toward the wall, where it falls through a dipleg or outlet into a common hopper. Cleaned gas exits through a central vortex finder. The bank shares inlet distribution, casing, hopper and sometimes outlet plenum hardware.

Performance depends on even gas distribution. If a few tubes carry too much flow, they lose efficiency and wear faster. If some tubes plug, neighbouring tubes take more flow and the whole bank becomes less effective.

Operational issues

• Tube-to-tube flow imbalance from poor inlet distribution or partial pluggage.
• Individual cyclone pluggage at the inlet, vortex finder or dust outlet.
• Abrasive wear in high-velocity inlet scrolls and tube walls.
• Common hopper bridging below the array.
• Air leakage through ash valves that disturbs dipleg flow.
• Re-entrainment when hopper level gets too high or discharge is erratic.

The visible symptom may be rising downstream dust load, higher pressure drop, hopper alarms or uneven wear found during inspection.

Cleaning and maintenance

Maintenance focuses on inlet distribution plates, tube wear, vortex-finder condition, dipleg clearance, refractory or liner condition and hopper discharge. Blocked tubes should be cleared rather than ignored, because a small number of failed elements can shift flow across the bank.

Sonic horns can be installed on the common casing or hopper to discourage dry dust build-up and bridging. They are most useful when the dust is friable and the casing provides a path for acoustic energy across the array. They are less effective against tarry deposits, wet biomass ash, large tramp material or a mechanically blocked dipleg.

Design and operating detail

A multiclone is sensitive to flow distribution. If the inlet plenum feeds one side harder than the other, some tubes overload while others contribute little. Tube diameter, inlet slot geometry, vortex-finder condition, dipleg sealing, hopper pressure balance and dust abrasiveness all affect performance. The unit may still show normal overall pressure drop while individual elements are worn through or blocked.

Common failure modes include erosion at tube inlets, cracked liners, plugged diplegs, hopper air leakage, dust re-entrainment, bypass through damaged tube sheets and corrosion during low-temperature operation. Maintenance should include inspection by zone rather than only looking through the main access door. A few failed tubes can let coarse particles pass downstream and create problems in fans, air heaters or secondary collectors.

Acoustic cleaning is most useful on dry dust build-up in the common casing, hoppers and dipleg region. It cannot restore separation efficiency lost to worn vortex finders or holes in the tube sheet. When used, horn timing should be coordinated with hopper discharge so loosened dust leaves the multiclone instead of being swept back into the cleaned gas path.

Related terms

• Cyclone separator
• Cyclone dipleg