Contents
Impellers
An impeller is a rotating, bladed component in a machine. It transfers energy from a motor to a fluid. Unlike a propeller, which works in open fluid to generate thrust with little pressure, an impeller operates within a housing to raise pressure.
What Is an Impeller in Compressors?
An impeller is a wheel with a series of curved blades within a centrifugal compressor that transfers energy to the air. As it spins at high speed, it draws air into its center or eye and accelerates it radially outward.
This fast acceleration imparts considerable kinetic energy to the air. The high-velocity air flows into the diffuser, a stationary component that slows it down, converting kinetic energy into increased pressure.
Impellers can be shrouded or unshrouded and made of different materials. Asking about the impeller’s design when shopping provides information on the compressor’s energy efficiency potential, performance and long-term reliability.
FAQs
An impeller’s diameter, blade shape and speed significantly impact a compressor’s flow rate, efficiency and pressure ratio. Manufacturers engineer an impeller that meets a specific performance target with the highest possible efficiency.
- Diameter: A larger diameter could result in increased pressure and flow rate when matched correctly to the rest of the compressor design to be efficient. For the same rotational speed, large-diameter impellers have a higher tip speed because their edges travel a greater distance in the same amount of time. Tip speed is the actual velocity of the outer edge of the impeller’s vanes.
- Blade shape: This includes the blade curvature, angle and number of blades. Most modern compressors use a backward-leaning blade design, which provides a smooth, stable delivery of air and results in higher efficiency. The blades in this design curve away from the direction of rotation and exit at less than 90 degrees. The ideal number of blades depends on the compressor’s specific performance.
- Rotational speed: This is the speed at which the motor or turbine spins the impeller. Increasing the revolutions per minute (RPM) boosts the impeller tip’s speed, raising the pressure and flow rate. It is best to run the impeller at its best efficiency point (BEP), where it accelerates with the least amount of wasted energy. Running it too fast results in choke, while running it too slow causes a surge.
Surge is an unstable condition that occurs at low flow rates, in which the air pressure at the discharge is too high for the impeller to overcome, causing a flow reversal. It results in vibrations and potential damage to the compressor.
The main observable signs of a damaged impeller include increased vibration, unusual noises and a noticeable pressure and flow decline. During a visual inspection, physical signs of damage include cracks, pitting, blade wear and chips.
An impeller can fail due to damage from loose nuts and bolts and fragments of a failed air filter. Other causes include corrosion caused by chemical reactions with environmental factors, imbalance resulting from uneven wear and material fatigue due to repeated loading and unloading cycles.
Routine maintenance involves inspecting the impeller for damage and cleanliness. The technician should carefully remove any buildup or debris to maintain balance and performance. Servicing should also include rebalancing or replacement if the impeller is damaged.
Minor damage, such as small nicks, might be repairable. However, cracks and major wear typically require impeller replacement to ensure safe and efficient compressor operation.
The optimal inspection frequency of a compressor impeller depends on OEM recommendations, the operating environment and the asset’s operational criticality.
Generally, daily monitoring is recommended, as it allows you to check for vibrations or unusual noises. A hands-on annual or biannual inspection using a borescope is also important, while a major comprehensive inspection that involves compressor disassembly is often performed every three to five years.
Additional Resources
At Quincy, our goal is to support you and true support means connecting you to resources that best fit your needs.
