Can velocity

Can velocity (also upward can velocity or interstitial velocity) is the upward gas velocity in the space between filter bags inside a baghouse compartment. It is calculated as the gas flow into the compartment divided by the open cross-sectional area between bags (compartment area minus bag-and-cage area).

Why it matters

Cake released from a bag during cleaning falls vertically into the hopper. If the upward can velocity is too high, the falling cake is re-entrained back up onto adjacent bags, defeating the cleaning cycle and raising differential pressure. Typical design limits:

Cleaning system	Max can velocity
Pulse-jet	1.5-2.5 m/s
Reverse-air	0.6-1.0 m/s (compartment offline during cleaning, so the limit applies only between cleans)

Relationship to A/C ratio

Can velocity rises with air-to-cloth ratio and falls with bag spacing. Designers tune both together: a high A/C only works if bag spacing is wide enough to keep can velocity in range.

How it is assessed

Can velocity is not the same as duct velocity or face velocity. It is based on the clear area available for upward flow through the bag bundle. A compartment with the same air-to-cloth ratio can have a much higher can velocity if the bags are long, closely spaced, or packed into a small casing. For this reason, retrofit bag-length increases can quietly create cleaning problems even when the total cloth area looks generous on paper.

The useful calculation uses actual operating gas volume at baghouse temperature, not standard flow. Hot gas occupies more volume, so a baghouse handling the same dry standard flow may see a higher internal velocity during high-temperature operation. Designers also account for compartments taken offline, because the remaining compartments must carry extra flow and their can velocity rises immediately.

Symptoms of excessive can velocity

High can velocity causes re-entrainment, hopper turbulence and local overloading of the lower bag rows. Dust released by a pulse falls only briefly before being lifted back to the fabric. The pressure-drop trend shows short-lived recovery after cleaning and a fast return to the high set point. Operators may respond by increasing pulse frequency, but that can shorten bag life and still fail to remove the root cause.

Other clues include dust boiling in hopper sight glasses, uneven cake thickness between upper and lower bag sections, abrasion on cages and bags near the inlet, and hopper discharge that cannot keep up with the apparent dust load. In reverse-air and shaker units, the offline cleaning step reduces re-entrainment during cleaning, but high interstitial velocity between cleanings still increases cake compaction.

Design and cleaning implications

Corrections include reducing gas flow, adding compartments, widening bag spacing, improving inlet baffling, changing bag length, or correcting hopper evacuation. Sonic horns can help keep released dust and early cake from consolidating, but they cannot overcome a fundamentally overloaded cross-section. In a troubleshooting sequence, can velocity should be checked before blaming filter media or pulse-valve performance.

Field checks

Can velocity is usually validated indirectly. A plant rarely measures the upward velocity between bags directly, so engineers infer it from operating gas volume, active compartment count, bag geometry and actual isolation state. The calculation should be repeated for upset cases: one compartment offline, high gas temperature, higher fan speed, or a temporary production increase. A design that looks acceptable with all compartments online can exceed the practical limit when one bay is isolated for maintenance.

The operating clue is a baghouse that cleans but does not stay clean. Pulses produce a short drop in delta P, followed by quick recovery as released dust is lifted back into the bag bundle. Hoppers may also show light dust flow even though the bags are heavily loaded, because the dust never settles. Acoustic cleaners can reduce early agglomeration and help dust detach more evenly, but they cannot overcome a casing that is too small for the gas volume. In that case the durable correction is more active cloth, lower flow, wider spacing or revised compartment logic.