Collected-dust conveying in power plants removes ash and process dust from filter hoppers and transfers it to storage or reuse. Efficient dust collection depends not only on the separator itself but also on two supporting functions: keeping the filtration surface operational and continuously removing the collected material from the hoppers.
Roots blowers can perform important duties in both areas. A properly selected rotary-lobe blower may supply low-pressure reverse air to certain fabric-filter cleaning systems or provide the conveying gas required to transfer collected dust from filter and electrostatic-precipitator hoppers to storage silos.
These applications must be defined precisely. A roots blower is not the induced-draft fan that moves the main flue-gas stream through the boiler and air-pollution-control equipment. It is also not automatically a replacement for the high-pressure compressed-air system used by a conventional pulse-jet baghouse. Its role is limited to duties whose required flow, differential pressure and operating characteristics match those of a positive-displacement rotary-lobe machine.
Dust-Handling Duties in Power Plants
Dust collected in a power plant may originate from:
- Boiler fly ash
- Economizer and air-heater hoppers
- Electrostatic precipitators
- Fabric-filter compartments
- Coal and petroleum-coke preparation systems
- Biomass receiving and processing equipment
- Fuel-transfer dust collectors
- Dry sorbent injection systems
- Activated-carbon injection systems
- Flue-gas treatment processes
- Ash-conditioning and storage equipment
- Material-transfer and silo-vent filters
The collected material may be fine, abrasive, cohesive, hygroscopic, combustible or chemically reactive. Temperature and composition may also change with boiler load, fuel type and pollution-control operation.
A blower and pneumatic conveying system should therefore be selected from measured or representative dust properties rather than from the generic term “fly ash” or “filter dust.”
The Two Distinct Blower Functions
The blower applications associated with dust collectors can be divided into two separate functions.
The first is filter cleaning. In certain compartmentalized reverse-air systems, a controlled flow of relatively low-pressure air is passed through the filter media in the opposite direction to normal filtration. This action releases the accumulated dust cake from the bags.
The second is collected-dust conveying. Once the dust falls into a hopper, it must be removed before the material level becomes excessive. A roots blower may supply conveying air for a positive-pressure system or create the suction required by a vacuum conveying system.
These functions have different airflow, pressure, filtration and control requirements. They should not be combined into one duty calculation merely because they serve the same dust-collection equipment.
How Low-Pressure Reverse-Air Cleaning Works
During normal filtration, dust-laden gas flows through the fabric, leaving a dust cake on the filter surface. This cake can improve fine-particle collection, but an excessively thick cake increases pressure drop and restricts gas flow.
In a compartmentalized reverse-air baghouse, one section is isolated from the main gas stream. Clean air is then introduced in the reverse direction. The reverse flow partially collapses or flexes the bags and separates the dust cake from the fabric. The released dust falls into the hopper below.
After cleaning, the compartment is returned to normal service and the next compartment can be isolated. This sequence allows a large filter to continue operating while individual sections are cleaned.
A roots blower may be suitable where the system requires a relatively consistent reverse-air volume against a low or moderate resistance. For very low-pressure, high-volume duties, a centrifugal fan may be more efficient. Equipment selection must therefore be based on the actual cleaning-air requirement rather than on the general filter type.
Reverse-Air Flow Requirement
A preliminary reverse-air flow may be estimated from the effective filter area being cleaned and the specified reverse face velocity:
Reverse-Air Flow
- Qreverse = actual reverse-air volume flow
- Acleaning = effective fabric area of the compartment under cleaning
- vreverse = specified reverse airflow velocity through the fabric
The required velocity should be determined from the filter design and media supplier’s recommendations. Excessive reverse flow can damage bags, increase mechanical stress and remove too much of the beneficial residual dust layer. Insufficient flow may leave a heavy cake on the fabric and cause progressively increasing filter resistance.
The blower capacity should include air leakage through isolation dampers, ducts and compartment seals:
Total Blower Flow
Both quantities must be stated at the same pressure and temperature conditions.
Pressure Requirement
The reverse-air blower must overcome the resistance of the complete cleaning-air circuit:
Cleaning Differential Pressure
- ΔPduct = loss through the reverse-air ductwork
- ΔPdamper = loss across dampers and control devices
- ΔPfabric = resistance of the filter medium
- ΔPdust_cake = additional resistance produced by the deposited dust
- ΔPoutlet = resistance on the cleaned-air discharge path
Maximum expected dust-cake resistance should be considered. Selecting the blower only from the clean-filter pressure drop may leave insufficient cleaning flow under actual operating conditions.
A roots blower supplies an approximately fixed displacement per revolution, but the actual delivered flow decreases as differential pressure increases because of internal leakage. Final capacity and power should be obtained from performance data at the specified inlet conditions and maximum expected cleaning pressure.
Reverse-Air Cleaning Is Not Pulse-Jet Cleaning
A conventional pulse-jet filter uses short bursts of stored compressed air released through blowpipes and nozzles. These systems normally require a compressor, receiver and pulse valves capable of delivering high-pressure pulses.
A low-pressure roots blower should not be presented as a direct substitute for that compressor. The two cleaning methods operate with different pressure levels, flow patterns and energy-storage principles.
Roots blowers are relevant to:
- Compartmentalized reverse-air cleaning
- Certain purpose-designed low-pressure cleaning systems
- Continuous or sequenced low-pressure air supply
- Applications specifically engineered around rotary-lobe blower characteristics
The filter manufacturer’s cleaning method and pressure requirements must always govern equipment selection.
Cleaning Sequence and Control
Reverse-air cleaning can be initiated by:
- Filter differential pressure
- A fixed time interval
- Boiler operating load
- Compartment operating history
- A combined pressure-and-time strategy
Differential-pressure-based cleaning can reduce unnecessary blower operation, but the control system must avoid allowing a heavily loaded compartment to reach an unstable condition.
Glossary
Baghouse: Fabric-filter collector that separates particulate from a gas stream.
Cleaning sequence: Ordered control steps that isolate and clean filter sections.
Collected-dust transfer: Movement of filter dust from the hopper to a silo or other receiver.
Dust hopper: Collector section that stores separated dust before discharge.
Dust-discharge rate: Mass rate at which collected dust is removed from the hopper or receiver.
Reverse-air cleaning: Cleaning cycle that reverses gas flow through filter bags.
Reverse-air duty cycle: Timing pattern of reverse-air flow used to clean filter bags.
Rotary airlock: Rotating feeder that discharges solids while limiting air leakage.