Filter Cleaning and Collected-Dust Conveying

Filter cleaning and collected-dust conveying are separate duties, so each requires its own airflow, pressure and control basis. A roots blower may supply this air where the cleaning system requires an approximately constant volume against low or moderate resistance.

The preliminary reverse-air requirement is:

This application must be distinguished from conventional pulse-jet cleaning. Pulse-jet baghouses normally use short bursts of high-pressure air supplied by a compressor and receiver. A roots blower is not automatically a replacement for that compressed-air system.

For extremely low-pressure and high-volume reverse-air duties, a centrifugal fan may also be more appropriate.

Collected-Dust Conveying

Dust separated by electrostatic precipitators and fabric filters must be removed from the collection hoppers. A roots blower can provide pressure or vacuum for an enclosed pneumatic transfer system.

Collected dust may include fly ash, sorbent residue, activated carbon, coal dust or biomass fines. Its abrasiveness, moisture sensitivity, temperature and combustible content must be evaluated.

In vacuum systems, the receiving filter and a secondary protection filter prevent material from reaching the blower. In positive-pressure systems, non-return and isolation devices prevent dust or hot gas from moving backward toward the machine.

Other Low-Pressure Process-Air Duties

roots blowers may also be considered for localized process-air services requiring relatively high volume and low or moderate pressure. These duties should remain part of the overall power-generation application assessment rather than being treated as a single independent application category.

Potential duties may include:

• Aeration of dry material hoppers
• Fluidization of ash or sorbent
• Low-pressure purge air
• Selected low-pressure filter-cleaning systems
• Air supply to small oxidation processes
• Pneumatic transport of process additives
• Air assistance for material discharge

Each duty must be evaluated independently. A general statement that a blower supplies “process air” is not sufficient for selection.

roots blowers are not normally used as a substitute for:

• Instrument-air compressors
• Plant compressed-air systems
• Boiler forced-draft fans
• Boiler induced-draft fans
• Large primary-air fans
• High-pressure dense-phase conveying compressors
• High-pressure pulse-jet filter compressors

These systems operate at different combinations of pressure, flow, air quality and control requirements.

Blower Sizing at Actual Inlet Flow

Roots blower capacity must be evaluated at the actual inlet pressure and temperature. Normalized or standard flow cannot be used directly as the blower inlet displacement.

The relationship between gas mass flow and actual volume is:

In vacuum operation, the reduced inlet pressure increases the actual volumetric flow required to transport a given gas mass. Blower selection must therefore use suction conditions, not atmospheric reference conditions.

Pressure Budget

The total differential pressure should include all components between the blower inlet and final discharge point:

The pressure ratio must be calculated using absolute pressure:

Gauge pressure must not be used in a compression-ratio calculation.

Preliminary Power Requirement

For a preliminary low-pressure assessment:

If flow is expressed in m³/s and pressure in pascals, the resulting power is in watts.

This equation is only an initial estimate. Final power and discharge temperature should be obtained from blower performance calculations using the specified gas, inlet conditions, speed and differential pressure.

Temperature and Thermal Protection

Discharge temperature increases with pressure ratio, inlet temperature and internal leakage. High discharge temperature can affect:

• Rotor clearances
• Bearing and lubricant life
• Shaft seals
• Gas-compatible elastomers
• Silencers and flexible connections
• Downstream piping
• Process safety

The blower should be checked at the highest ambient or inlet temperature and maximum continuous differential pressure. Continuous recycle operation can raise the inlet temperature if hot discharge gas returns directly to the suction.

Temperature monitoring should be included where overheating could damage the machine or create a process hazard.

Control and Energy Efficiency

Variable-speed control is often the most effective way to match blower output to changing process demand. Throttling a positive-displacement blower does not reduce displaced volume in the same manner as slowing the machine and may increase power consumption or suction restriction.

Potential control signals include:

• Conveying-gas flow
• Pipeline pressure
• Silo or hopper level
• FGD oxidation demand
• Gas-holder level
• Fuel-gas header pressure
• Filter differential pressure
• Power-generation load

Blower efficiency should be assessed over the complete operating profile rather than at one nominal point. Multiple smaller blowers may provide better turndown and redundancy where process demand varies substantially.

The lowest purchase price does not necessarily produce the lowest lifecycle cost. Pressure margin, pipeline diameter, control method, operating hours and maintenance accessibility all influence total energy and ownership cost.

Safety and Reliability

The required protection depends on whether the blower handles clean air, inert gas, combustible fuel gas or gas associated with combustible dust.

Typical protective measures include:

• Pressure-relief devices
• High-pressure shutdown
• High discharge-temperature shutdown
• Non-return valves
• Inlet filtration
• Secondary filtration in vacuum systems
• Gas and liquid separation
• Vibration monitoring
• Bearing-temperature monitoring
• Motor-current monitoring
• Gas-leak detection
• Correct vent routing
• Equipotential bonding and earthing
• Explosion isolation where required
• Emergency shutdown integration

Relief outlets handling combustible, toxic or process gas must be routed to an approved safe system. They should not be discharged casually into an enclosed blower room.

Information Required for Blower Selection

A reliable technical specification should define:

• Application and process function
• Minimum, normal and maximum flow
• Flow reference conditions
• Inlet pressure and temperature
• Required discharge pressure
• Maximum differential pressure
• Gas composition
• Moisture and contaminant content
• Normal and upset operating conditions
• Required turndown
• Continuous or intermittent duty
• Hazardous-area classification
• Material and seal requirements
• Leakage limitations
• Noise requirements
• Control philosophy
• Redundancy requirements
• Applicable certification and safety standards

The blower should be selected as part of the complete process system, not as an isolated source of airflow. Feeders, pipelines, filters, separators, valves, silencers, instrumentation and protective devices all influence the actual operating point.

Glossary

Reverse-air cleaning: Bag-filter cleaning method that flexes filter bags with reverse-flow air.

Filter cake: Layer of captured dust accumulated on filter media.

Air-to-cloth ratio: Gas volume flow divided by the effective filter-media area.

Dust hopper: Vessel beneath a collector that temporarily stores captured particulate.

Collected-dust conveying: Pneumatic transfer of dust from a collector hopper to storage.

Pulse-jet cleaning: Filter-cleaning method using short compressed-air pulses inside filter bags.

Filter-cleaning cycle: Sequence and interval used to remove captured dust from filter media.

Dust-hopper discharge rate: Rate at which collected dust is removed from a collector hopper.