Filters and Dust Control

Filters and Dust Control

IAOM Joint District Meeting Spring Technical Conference March 7, 2014 Darin Teetes

What is “Dust”? • Dust is a fine, dry powder consisting of tiny particles that are released or

transported into the air through processing or handling of materials.

Why is “Dust” a Problem? • Product degradation or loss • Health issue for workers • Safety issue for assets and facilities • Sanitation and infestation control • Maintenance

Where is “Dust” a Problem?

• Anywhere in a facility – product transfer, dump pits, load-out, tunnels, head house, even in the offices.

Dry-Type Collectors • Cyclone Collector • Baghouse Filter

Baghouse Filter • Utilizes fabric socks/media • Very efficient, up to 99.99% on particles in

airstream that are submicron size depending on media used

• Filter bags and “dust” cake on surface of media act to separate particles from incoming dirty airstream.

• Reverse air cleaning system knocks dust into the filter hopper to be discharged.

Baghouse Filters

Round Square Modular “Big Round”

Bag Cleaning Systems:

• High Pressure Low Volume: – 80 to 100 psig air at low volume – Cleaning air supplied by “house” compressed air

system – Oil and moisture contamination from “house”

compressed air system are common. – Uses venturi system to induce air and whip

material from the bags

Reverse-pulse (pulse-wave or pulse-jet)

Bag Cleaning Systems: Reverse-pulse (pulse-wave or pulse-jet)

• Medium Pressure – 10-15 psig at moderate to high air volume – Use of positive displacement blower to supply air. – Air supply is local to inlet of PD blower; oil and moisture contamination

not typically a problem.

Bag Cleaning Systems: Reverse-pulse (pulse-wave or pulse-jet)

• Low Pressure – 20-100” w.c. – Use of fan to deliver high volume of

air at low pressure. – Physically forces dust off the bags.

Baghouse Filters Air Inlet Configuration

• Properly designed and applied air inlet serves many functions: – Allow airstream with particles to enter baghouse

without damage to bags. – Allow adequate area for stock to enter baghouse

without choking. – Allows for particles to drop from airstream to filter

discharge.

Baghouse Filter Type of Air Inlet

• High Air Inlet(s) – for medium and light dust

loading. – Include deflectors to reduce bag

wear from incoming particles in airstream to filter.

– Used in general dust collection or handling air from air outlets of cyclone collectors.


• Cyclonic/Material handling inlet(s) – Ideal for Heavy-Loading or material conveying. – Inlet has tangential entry into section below filter bag section. – Typical to a heavy loaded negative lift; ie. Hammermill lift or positive pressure conveying system


• Low air inlet(s) – Airstream(s) introduced into filter below bags – Not recommended due to potential for air turbulence in filter upon dirty air entry. – Baffle to deflect airstream downward may aid

in reducing this potential.

Filter Bag Media • Selection of proper media plays a major role in filter

performance. • Must consider several factors of application:

– Operating temperatures – Humidity and moisture levels – Type of particulate being handled – Resistance to abrasion, chemicals, moisture, etc. – Filter Air to Cloth Ratio (bag length)

Filter Bag Media • Media for challenging applications such as where warm and moist air from

pneumatic conveying systems is handled: – Polyester bags can be glazed or coated with finishes known as Eggshell

or Defender coats – Polypropylene can be used in hi-moisture/temperature application – Consider media utilizing “surface filtration”

Baghouse Filter Performance

• Measure Pressure Differential across Baghouse filter bags/media. – The difference in static pressure

upstream (dirty air side) and downstream (clean air side) of the bags = pressure drop.

• U-tube manometer or Magnehelic gauge mounted to filter, visible to operator.

Air to Cloth Ratio

• Establishes design capacity of a baghouse • Term refers to ratio of air quantity (cubic feet per minute or cfm)

to the amount of media surface area within filter (square ft.)

• Air Volume (CFM) ÷ Cloth Area (SQ.FT.) = Air to Cloth Ratio • Example: 15,000 cfm ÷ 1,555 sqft = 9.65:1 Air to Cloth Ratio

Air to Cloth Ratio Rule of Thumb for design A/C Ratio

• 9-10:1 Ratio: Secondary Collector, Elevator, Cleaning House, General Suction

• 7:1 Ratio: Primary Collector, High

Humidity Air Systems,

• 5:1 Ratio: Filter / Receiver

Can-Velocity of a baghouse

• Can Velocity is measured in feet per minute (fpm).

• Determined by dividing the total air flow entering the baghouse chamber by the cross sectional area of the filter housing (less bag area)

Example: 15,000 cfm ÷ (77.2 sq ft - 20.63 sq ft) = 265 fpm

Can-Velocity of a baghouse

• “Rule of Thumb” for Can-velocity • Varies with application, but can generally assume a can-velocity at 300 fpm

or below to be acceptable.

How to Size Ducting

Minimum Duct Design Velocities Nature of Contaminant

Examples Minimum Design Velocity, fpm

Vapors, gases, smoke

All vapors, gases, smoke 1000 - 2000

Fumes Welding 2000 - 2500

Very fine light dust

Fine rubber dust, cotton dust, molding powder dust, jute lint, cotton dust, shavings (light), soap dust, leather shavings, agricultural dust

3000 - 4000

How to Size to Size Ducting

• Add the CFM of both incoming lines and select the size with appropriate velocity

• Add area of both incoming lines and select the size closest Warning: Common mistake is to add diameters of the incoming lines. However, if the velocity is the identical and acceptable. You can use the following equation. D1^2+ D2^2 = SQRT(D3) As a rule of thumb.

• Dust is in suspension because of correct velocities.

Good Velocity, Good Volume, Good Design

• Dust is dropping out (in locations); • System is balanced (at end of system).

Bad Velocity, Good Volume, Bad Design

• Dust is NOT in suspension because the system is not balanced. • Air taking path of least resistance.

Additions without Rebalancing

• Dust is in suspension (but at higher velocity); System is not balanced.

Deductions without Rebalancing

Thank you for your time Presented by: Darin Teetes

Documents

Filters and Dust Control