Selecting roots blowers for pneumatic conveying in power plants begins with the material, route, feeder leakage and receiver-filter resistance. Large quantities of dry bulk material must be transferred between unloading stations, storage silos, process equipment, pollution-control units and disposal or recycling facilities. When the material is sufficiently dry and its physical properties permit pneumatic transport, an enclosed pipeline can provide a clean, flexible and highly automated alternative to mechanical conveyors.
Roots blowers are widely applied as air or gas movers in low- and moderate-pressure pneumatic conveying systems. Their positive-displacement operating principle enables them to deliver a predictable inlet volume for each revolution. This characteristic is particularly useful where pipeline resistance changes during operation but the conveying system still requires a reasonably stable gas flow.
A roots blower does not physically handle the solids. It supplies the conveying air, while the material is introduced into the pipeline through a feeder, airlock, pressure vessel or another suitable feeding device. Proper system performance therefore depends on the complete relationship between the blower, feeder, pipeline, bulk material, separator and receiving filter.
Materials Pneumatically Conveyed in Power Plants
Power-generation facilities handle materials with substantially different particle characteristics. Depending on the plant design, roots blowers may supply conveying air for:
- Dry fly ash
- Electrostatic-precipitator ash
- Fabric-filter and baghouse ash
- Fine, dry bottom-ash fractions
- Coal fines
- Pulverized coal in specially designed systems
- Limestone powder
- Hydrated lime
- Pulverized activated carbon
- Dry sorbents used for emission control
- Biomass powders and selected granulated fuels
- Wood dust and fine biomass residues
- Combustion additives
- Process minerals
- Recovered filter dust
The inclusion of a material in this list does not mean that it can automatically be conveyed by the same system. Fly ash, pulverized coal, limestone and biomass have very different bulk densities, permeability, cohesiveness, explosibility and moisture sensitivity. Each material requires an individual conveying assessment.
Wet, sticky or highly cohesive materials may form deposits and block the pipeline. Coarse bottom ash containing large, irregular or partially fused particles may require crushing, cooling or classification before pneumatic transport. In many power plants, coarse bottom ash is handled mechanically or hydraulically, while pneumatic conveying is reserved for dry fly ash and fine combustion residues.
Why Roots Blowers Are Used
A roots blower traps gas between its rotating lobes and casing and transfers it from the inlet to the discharge side. The blower delivers an approximately fixed inlet volume per revolution, although actual capacity is reduced by internal leakage and is affected by speed, pressure difference, gas density and temperature.
The principal advantages in power-plant pneumatic conveying include:
- Predictable volumetric airflow
- Continuous-duty capability
- Relatively simple mechanical construction
- Suitability for both pressure and vacuum service
- Oil-free compression chamber
- Stable operation against changing pipeline resistance
- Wide capacity control through rotational-speed adjustment
- Capability to serve multiple conveying cycles when correctly controlled
These characteristics are valuable in ash-handling systems because filter condition, ash loading, feeder operation and pipeline condition can cause pressure resistance to vary. A centrifugal fan’s flow can decrease significantly as system pressure rises. A positive-displacement roots blower continues to transfer nearly the same inlet volume until its speed, power, differential-pressure or discharge-temperature limit is reached.
This does not mean that the blower can overcome unlimited pressure. If pipeline resistance continues to increase, discharge pressure, absorbed power and gas temperature rise. A blocked pipeline can therefore overload or damage the machine unless the system is protected by pressure-relief, temperature-monitoring and shutdown devices.
Positive-Pressure Conveying
In a positive-pressure system, the roots blower is installed near the beginning of the conveying line. It draws filtered ambient air and delivers it into the pipeline. Material is introduced downstream of the blower by equipment such as:
- Rotary airlock valve
- Screw feeder
- Ejector
- Blow-through rotary valve
- Pressure vessel
- Dome-valve transporter
- Dense-phase blow tank
The air-material mixture travels toward a receiving silo or process vessel. At the destination, a cyclone, filter receiver or silo-top filter separates the solids from the conveying air.
Positive-pressure systems are suitable when material must be transported from one source to one or more destinations. They are commonly used for transferring ash to storage silos or injecting dry sorbents into a process.
A roots blower is particularly relevant to dilute-phase and selected low-pressure conveying duties. High-pressure dense-phase systems may require a screw compressor, reciprocating compressor or dedicated plant-air system. The required equipment must be selected from the calculated conveying pressure, not merely from the description “pneumatic conveying.”
Vacuum Conveying
In a vacuum system, the blower is positioned downstream of the receiving separator and filter. It creates a pressure lower than atmospheric pressure in the conveying pipeline, drawing air and material from one or more pickup points.
Vacuum conveying provides effective dust containment because leakage tends to move ambient air into the system rather than allowing ash or powder to escape. This can be advantageous when collecting fly ash from multiple electrostatic-precipitator or baghouse hoppers.
A typical vacuum system includes:
- Material pickup points
- Conveying pipeline
- Vacuum receiver
- Primary material separator
- Final protection filter
- Roots Blower or exhauster
- Discharge silencer
- Material discharge valve
The final protection filter is critical. The roots blower should normally handle clean gas rather than ash-laden air. Fine ash entering the blower can erode the lobes and casing, damage rotor clearances, contaminate lubrication systems and cause mechanical failure.
The system must also prevent condensate from reaching the blower. Hot ash can release moisture as it cools, while ambient air entering the system can condense if pipe surfaces fall below the local dew point.
Dilute-Phase and Dense-Phase Operation
Dilute-phase conveying keeps most particles suspended in a relatively high-velocity gas stream. The solids loading ratio is generally lower than in dense phase, and the material travels as a dispersed suspension.
roots blowers are frequently used in dilute-phase systems because they can provide the required airflow at moderate pressure. However, high conveying velocity can cause:
- Pipeline erosion
- Bend wear
- Particle attrition
- Increased filter loading
- Higher power consumption
Glossary
Blow tank: Pressure vessel that batches and discharges solids into a conveying line.
Feeder leakage: Gas escaping through a solids feeder and affecting conveying performance.
Pickup point: Location where bulk solids enter the conveying gas stream.
Dilute-phase conveying: Transport mode using higher gas velocity and lower solids concentration.
Dense-phase conveying: Transport mode using lower gas velocity and higher solids concentration.
Solids loading ratio: Mass flow of conveyed solids relative to the conveying-gas mass flow.
Blow-tank cycle: Batch sequence for filling, pressurizing, conveying and depressurizing a blow tank.
Conveying-line pressure drop: Pressure reduction caused by gas flow, solids, bends and fittings in the route.