Centrifugal Compressors MCQ Quiz - Objective Question with Answer for Centrifugal Compressors - Download Free PDF

Explanation:

Turbocharger Compressor Drive

• A turbocharger is a device used to increase the power output and efficiency of an internal combustion engine by forcing more air into the combustion chamber. It consists of two main components: a turbine and a compressor. The turbine is driven by the exhaust gases from the engine, and the compressor is coupled to the turbine to compress the incoming air and deliver it to the engine at higher pressure.
• In a turbocharger, exhaust gases from the engine pass through the turbine, causing it to spin. This rotational energy is transferred to the compressor via a shaft. The compressor then draws in ambient air, compresses it, and delivers it to the engine's intake manifold at increased pressure. This process allows more air (and therefore more oxygen) to enter the engine, enabling the combustion of more fuel and resulting in increased power output.

Advantages of Turbochargers:

• Increased engine power output by allowing more air and fuel to combust.
• Improved fuel efficiency due to better utilization of exhaust energy.
• Reduction in engine size for the same power output, leading to lighter and more compact designs.
• Lower emissions as a result of more efficient combustion.

Disadvantages of Turbochargers:

• Turbo lag, which is the delay in power delivery as the turbine spools up.
• Increased complexity and cost of the engine system.
• Higher maintenance requirements due to additional components.

Explanation:

Surging in Compressors

• Surging is a phenomenon observed in compressors, particularly centrifugal and axial flow compressors, characterized by instability in their operation. It occurs due to a mismatch between the impeller and diffuser performance at low flow rates. Surging causes fluctuations in flow and pressure, leading to a complete reversal of airflow through the compressor, which can result in damage to the machinery and reduced efficiency.

Working Principle:

• Compressors are designed to operate within a specific range of flow rates. At low flow rates, the velocity and pressure conditions within the compressor may deviate significantly from the designed parameters, causing a mismatch between the impeller (which accelerates the air) and the diffuser (which slows down and increases the pressure of the air). This mismatch creates instability, leading to a rapid reversal of airflow (surge). Surging occurs cyclically as the compressor tries to recover from the instability but fails to maintain a stable operation.

Causes of Surging:

• Low Flow Rates: The primary cause of surging is operating the compressor at flow rates lower than its designed range. At these low flow rates, the diffuser and impeller cannot maintain a stable pressure rise, resulting in instability.
• Excessive resistance in the downstream system can reduce flow rates, pushing the compressor into the surge region.
• Poor design of the compressor components, such as the impeller and diffuser, can lead to mismatched performance, exacerbating the likelihood of surging.
• Sudden Changes: Abrupt changes in load demand or operating conditions can destabilize the compressor, leading to surging.

Effects of Surging:

• Damage to Components: Surging can cause mechanical stress and vibrations, potentially damaging the impeller, diffuser, bearings, and other components.
• Reduced Efficiency: The instability during surging reduces the compressor's ability to maintain consistent pressure and flow, resulting in decreased efficiency.
• Noise and Vibrations: Surging generates loud noises and vibrations, which can affect the surrounding equipment and personnel.
• Operational Downtime: Frequent surging can lead to interruptions in operation and increased maintenance requirements.

Explanation:

Surging Phenomenon in Centrifugal Compressors

• Surging in a centrifugal compressor is a dynamic instability that occurs when the flow through the compressor falls below a critical level.
• This phenomenon is characterized by oscillations in flow and pressure, which can lead to significant mechanical stress and potential damage to the compressor.
• Centrifugal compressors operate by converting the kinetic energy of a fluid into pressure energy through the action of a rotating impeller.
• The flow of the fluid is continuous and stable under normal operating conditions.
• However, when the flow rate drops below a certain threshold, the compressor can no longer maintain stable operation, leading to surging.

Causes of Surging:

• Decrease in Refrigeration Load: When the refrigeration load decreases, the demand for compressed fluid reduces. If the compressor continues to operate at the same speed, the flow rate through the compressor drops, potentially causing surging.
• Improper Control Systems: Inadequate or malfunctioning control systems may fail to adjust the compressor speed or guide vanes appropriately in response to changing load conditions.
• Blockages or Restrictions: Obstructions in the flow path, such as dirty filters or partially closed valves, can reduce the effective flow rate, leading to surging.

Consequences of Surging:

• Mechanical Damage: The oscillations in flow and pressure can induce vibrations and mechanical stresses, potentially damaging the compressor components.
• Reduced Efficiency: Surging disrupts the stable operation of the compressor, leading to a drop in efficiency and increased energy consumption.
• Operational Instability: Frequent surging can cause instability in the entire refrigeration or process system, affecting overall performance.

Preventing Surging:

• Proper Sizing and Selection: Ensuring that the compressor is appropriately sized for the expected range of operating conditions can help prevent surging.
• Effective Control Systems: Implementing advanced control systems that can adjust the compressor speed, guide vanes, and other parameters in real-time can help maintain stable operation even under varying load conditions.
• Regular Maintenance: Routine inspection and maintenance of the compressor and associated components can prevent blockages and other issues that may contribute to surging.

Explanation:

In centrifugal compressors, the design and shape of the impeller blades play a crucial role in determining the performance characteristics, such as pressure rise, efficiency, and stability. Among the different types of impeller blades, namely forward-curved, radial (straight), and backward-curved blades, each has its unique effects on the compressor's operation.

Forward-Curved Blades:

• Forward-curved blades can provide a higher pressure rise compared to other impeller blade designs.
• This is because they tend to impart a higher velocity to the air or gas, increasing the kinetic energy, which is subsequently converted into a higher pressure in the diffuser section.
• They are commonly used in applications where space is constrained, and high-pressure ratios are required.
• However, compressors with forward-curved blades typically have lower efficiency and can be more prone to instability at high flow rates compared to those with backward-curved blades.

Radial (Straight) Blades:

• Radial blades offer a compromise between pressure rise and stability.
• They tend to provide moderate pressure increases and are relatively easy to manufacture.
• Compressors with radial blades have stable performance over a wide range of operating conditions but do not achieve as high pressure rises or efficiencies as backward-curved blades.

Backward-Curved Blades:

• Backward-curved blades generally yield the highest efficiencies and are more stable across a broader range of operating conditions.
• They tend to impart a lower velocity to the fluid for a given rotational speed, resulting in lower kinetic energy and pressure rise compared to forward-curved blades.
• However, because of their high efficiency, they are widely used in industrial applications where energy savings are critical.

Backward Curved Blades: vane exit angle is less than 90°

Forward Curved Blades: vane exit angle is more than 90°

Radial Blades: Vane exit angle is 90°

Backward curved blades are slightly better in efficiency and are stable over a wide range of flow. While forward-curved blades are used for higher pressure ratio.

Concept:

In a single stationary blade type rotary compressor, it is crucial for the blade to maintain constant contact with the roller to ensure proper sealing and efficient compression. This contact ensures that the working fluid is effectively compressed within the cylinder.

Several mechanisms can be employed to maintain this contact, each relying on different principles of physics and engineering design.

1. Gravity: Gravity alone is insufficient to maintain the required contact between the blade and the roller, especially in varying orientations and during high-speed rotations.

2. Centrifugal force: While centrifugal force can help maintain contact during rotation, it is not reliable at lower speeds or when the compressor is stationary.

3. Cam and follower: A cam and follower mechanism could be used to maintain contact, but it would add complexity to the design and may not be as effective in ensuring continuous contact.

4. Spring: A spring mechanism provides a constant force that pushes the blade against the roller, ensuring continuous contact regardless of the compressor's orientation or speed. This method is simple, reliable, and effective for maintaining the necessary seal.

Conclusion:

The most effective and reliable mechanism for maintaining constant contact between the blade and the roller in a single stationary blade type rotary compressor is:

4) Spring

Thus, in a single stationary blade type rotary compressor, the blade is set into the slot of a cylinder in such a manner that it always maintains contact with the roller by means of a spring.

Concept:

Centrifugal compressor

• Air is drawn into the centre of a rotating impeller with radial blades and is pushed toward the centre by centrifugal force.
• This radial movement of air results in a pressure rise.
• The airflow rate is more than a reciprocating compressor. but the maximum pressure value is less than the reciprocating compressor.
• Centrifugal compressors were used in jet engines and smaller gas turbine engines.
• For larger engines, axial compressors need a lesser frontal area and are more efficient.

Pressure ratio for centrifugal compressor = 4 : 1

Pressure ratio for axial flow compressor = 1.2 : 1

Explanation:

Compressors are classified in two categories based on working principle:

Positive displacement type: In positive displacement type compressors, compression is achieved by trapping a refrigerant vapour into an enclosed space and then reducing its volume. Since a fixed amount of refrigerant is trapped each time, its pressure rises as its volume is reduced. When the pressure rises to a level that is slightly higher than the condensing pressure, then it is expelled from the enclosed space and a fresh charge of low-pressure refrigerant is drawn in and the cycle continues. Example:

• Reciprocating type
• Rotary type with sliding vanes (rolling piston type or multiple vane type)
• Rotary screw type (single screw or twin-screw type)
• Orbital compressors
• Acoustic compressors

Roto-dynamic type (Non-positive displacement type): The elevation of the pressure of the refrigerant vapour is by centrifugal force. In roto-dynamic compressors, the pressure rise of refrigerant is achieved by imparting kinetic energy to a steadily flowing stream of refrigerant by a rotating mechanical element and then converting into pressure as the refrigerant flows through a diverging passage. Example:

• Radial flow type (Centrifugal Compressor)
• Axial flow type

Explanation:

Centrifugal Compressor :

• In these compressors, the required pressure rise takes place due to the continuous conversion of angular momentum imparted to the gas by a high-speed impeller into static pressure.
• It can work reasonably well in a contaminated atmosphere.
• It is able to operate efficiently over a wide range of mass flow rate at any particular speed.
• A pressure ratio of 5:1 may be obtained in a single-stage centrifugal compressor. By employing multi-stage staging (up to 8 stages) the delivery up to 400 atm. may be obtained.
• A multistage centrifugal compressor is used for high compression ratio so the width of blades reduces progressively in the direction of flow so as to get constant mass flow rate.
• The isentropic efficiency of the centrifugal compressor is around 75%.
• Centrifugal compressors accelerate the velocity of the gases (increases kinetic energy) which is then converted into pressure as the airflow leaves the volute and enters the discharge pipe.​

Concept:

Centrifugal compressor

Operating Principle:

A centrifugal compressor works by converting kinetic energy into pressure energy. The gas enters the impeller, where it gains kinetic energy through the impeller's high-speed rotation. This kinetic energy is then converted into pressure energy in the diffuser.

Performance Parameters:

Pressure Ratio:

• Centrifugal compressors typically achieve a low to moderate pressure ratio (ranging from 1.1 to 4) per stage.
• This makes them suitable for applications where the required pressure increase is not very high.

High Mass Flow:

• Due to their radial flow design, centrifugal compressors can handle high volumetric flow rates effectively.
• They are often used in situations requiring high mass flow rates at moderate pressures.

Applications:

They are commonly used in applications where moderate pressure ratios and lower mass flow rates are required. Examples include refrigeration systems, small gas turbines, turbochargers, and HVAC systems.
Advantages:

• Compact and robust design.
• Less sensitive to variations in flow.
• Capable of handling a wide range of gases and operating conditions.
• Simpler and more economical maintenance compared to axial compressors.

Pressure ratio for centrifugal compressor = 4 : 1

Pressure ratio for axial flow compressor = 1.2 : 1

Explanation:

Compressor:

• It is the device which draws air from the atmosphere and compressed to the required pressure.

Compressors are of two types:

1. Positive displacement type Compressor:
   • Positive displacement type like reciprocating compressors, Root's blower, and vane - sealed machines.
   • For air at very high pressure, we require the reciprocating compressors.
   • The only disadvantage with reciprocating compressor is a low supply of air.
2. Rotary type Compressor:
   • Rotary type like centrifugal and axial flow compressor.
   • Rotary compressors are best suited for a large quantity of air at low pressure.

Explanation:

• Surging is a momentary backflow through the compressor from the discharge to the suction. This can occur when the mass flow of gas to the compressor falls below a critical level with a high-pressure difference. Surging is the complete breakdown of steady flow in the compressor which occurs at a low flow rate.
• Choking is the condition which occurs in the compressor in which it operates at a very high mass flow rate and flows through the compressor can’t be further increased as mach number at some part of the compressor reach to unity i.e. to sonic velocity and the flow is said to be choked.

Explanation:

Surging:

• It is unsteady, periodic and reversed flow condition in the dynamic compressor.
• Surging is a complete breakdown of steady flow through the compressor due to periodic reversal.

• This reversal of flow occurs during the closing of the valve in the operating range has a mass flow rate less than corresponding to the maximum pressure ratio.
• This reversal of flow causes the abnormal sound, vibration, a decrease in efficiency, an increase in temperature and if the intensity is more, it may lead to mechanical damage.

Stalling: 

• It is another instability in the flow of compressor that occurs nominally at the stable operating range.
• The stall may lead to noisy vibration and fatigue failure of the compressor.

Chocking: 

• The chocking occurs in the compressor which operates at low discharge pressure and maximum flow rate.

Explanation:

Compressor:

• The air compressor used in gas turbines is of rotary type mainly axial flow turbines. 
• It draws air from the atmosphere and compressed to the required pressure.

Compressors can be of two types:

1. Positive displacement type Compressor:
   • Positive displacement type like reciprocating compressors, Root's blower, and vane - sealed machines.
2. Rotary type Compressor:
   • Rotary type like centrifugal and axial flow compressor.

Losses in centrifugal compressor:

• Intel losses : 
  • At the inlet of the compressor, major losses occur due to the formation of eddy's, turbulence, and friction.
  • Since the entry of fluid in a centrifugal compressor is axially then suddenly fluid is being directed to radially towards blades and due to this off-design condition incidence losses occur.
• Impeller losses:
  • Recirculating losses: these losses occur due to flow reversal of fluid at the impeller exit, this backflow of fluid is a function of the vane exit angle.
  • Wake-mixing losses: this loss occurs when fluid flow through the passage of blades, and it causes awake in the vanless space of the rotor.
  • Slip losses: these losses occur due to the inertia effect of fluid between blade passage, and a component of velocity lost which is called slip in whirl component.
• Diffusor losses :
  • When fluid flows through diffusor frictional losses occur in the diffused area due to skin friction coefficient and since the area is decreasing in the direction of flow hence positive pressure gradient is created so back flow losses can also occur in diffusor.

Explanation:

Centrifugal compressor: A centrifugal compressor consists of the following four components:

• Inlet casing: To accelerate the fluid to the impeller inlet.
• Impeller: Impeller transfers the energy of the fluid in the form of increased static pressure (enthalpy) and kinetic energy.
• Diffuser: A diffuser converts the kinetic energy at the impeller outlet into enthalpy resulting in pressure rise.
• Volute casing: It collects the fluid and converts the remaining kinetic energy into the enthalpy resulting in further pressure rise.

The efficiency of the centrifugal compressor is given by

\({\eta _p} = \frac{{{\rm{\Delta }}{h_{o_i}}}}{{{\rm{\Delta }}{h_o}}}\)

where Δh_{o is} is the isentropic work required to raise the pressure in the compressor and Δh_o is the actual work required by the compressor.

The flow coefficient is given by \(\phi = \frac{{{C_r}}}{u}\)

The flow through the compressor is directly proportional to the radial component of velocity C_r therefore with the increase in velocity u C_r will increase which will increase the flow through the compressor.

The net work done upon the compressor \({\rm{\Delta }}{h_o} = u_2^2 - {u_2}{C_{r2}}\cot {\beta _2}\)

where β₂ is the outlet blade angle.

Therefore with the increase in velocity the work input to the compressor will increase which will reduce the efficiency of the compressor.