5.6 Pumps and compressors

Pumps and compressors are vital in chemical engineering, moving fluids and gases through systems. They come in various types, each suited for specific applications. Understanding their classification helps engineers choose the right equipment for different processes.

Power requirements and efficiency are crucial factors in pump and compressor operation. Calculating these values helps optimize energy use and performance. Additionally, concepts like Net Positive Suction Head (NPSH) and performance curves guide proper equipment selection and prevent issues like cavitation.

Pump and Compressor Classification

Types of Pumps

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• Pumps are mechanical devices used to transfer fluids by increasing their pressure or velocity
  • Centrifugal pumps use an impeller to create a centrifugal force that increases the velocity and pressure of the fluid
    • Suitable for high flow rates and low to moderate pressure applications (water supply, irrigation systems)
  • Positive displacement pumps work by trapping a fixed volume of fluid and forcing it into the discharge pipe
    • Further classified into reciprocating (piston, plunger, diaphragm) and rotary (gear, lobe, screw) pumps
    • Suitable for high-pressure and low-flow applications (metering, dosing)
  • Special-purpose pumps include jet pumps, air-lift pumps, and electromagnetic pumps
    • Used for specific applications (deep well pumping, solid-liquid mixtures, corrosive fluids)

Types of Compressors

• Compressors are mechanical devices used to increase the pressure of a gas by reducing its volume
  • Positive displacement compressors work by trapping a fixed volume of gas and reducing its volume to increase the pressure
    • Further classified into reciprocating (piston) and rotary (screw, scroll, vane) compressors
    • Suitable for high-pressure and low-flow applications (refrigeration, air compression)
  • Dynamic compressors use a rotating impeller or blades to impart kinetic energy to the gas, which is then converted into pressure energy
    • Further classified into centrifugal and axial compressors
    • Suitable for high-flow and low to moderate pressure applications (gas turbines, industrial processes)

Power Requirements for Pumps and Compressors

Pump Power and Efficiency

• The power required by a pump depends on the flow rate, pressure head, and the fluid properties (density, viscosity)
  • Calculated using the formula: $Power = (Flow rate × Pressure head × Density × Acceleration due to gravity) / ([Pump efficiency](https://www.fiveableKeyTerm:pump_efficiency))$
• Pump efficiency is the ratio of the fluid power output to the mechanical power input
  • Accounts for losses due to friction, leakage, and other factors
  • Calculated using the formula: $Efficiency = (Fluid power output) / (Mechanical power input) × 100$

Compressor Power and Efficiency

• The power required by a compressor depends on the flow rate, pressure ratio, and the gas properties (specific heat ratio, compressibility factor)
  • Calculated using the formula: $Power = (Mass flow rate × Specific heat at constant pressure × Temperature rise) / ([Compressor efficiency](https://www.fiveableKeyTerm:compressor_efficiency))$
• Compressor efficiency is the ratio of the ideal work required for the compression process to the actual work input
  • Accounts for losses due to friction, heat transfer, and other factors
  • Calculated using the formula: $Efficiency = (Ideal work) / (Actual work) × 100$
• The ideal work for a compressor can be calculated using the isentropic compression process
  • Assumes no heat transfer and reversible operation
  • The actual work is always greater than the ideal work due to irreversibilities in the compression process

Net Positive Suction Head (NPSH)

Definition and Importance

• Net positive suction head (NPSH) is the total suction head at the pump inlet minus the vapor pressure of the liquid being pumped
  • Represents the minimum pressure required at the pump inlet to prevent cavitation
• NPSH is expressed in terms of head (meters or feet) and is a critical parameter in pump selection and operation
  • Two types of NPSH: NPSH available (NPSHa) and NPSH required (NPSHr)

NPSH Available and Required

• NPSHa is the actual suction head available at the pump inlet, determined by the system design and operating conditions
  • Depends on factors such as the liquid level in the suction tank, suction piping losses, and atmospheric pressure
• NPSHr is the minimum suction head required by the pump to operate without cavitation, specified by the pump manufacturer
  • Depends on factors such as the pump design, flow rate, and impeller speed
• For proper pump operation, the NPSHa must be greater than the NPSHr by a sufficient margin (usually 0.5 to 1 meter) to account for uncertainties and provide a safety factor

Cavitation and Prevention

• Cavitation occurs when the local pressure in the pump drops below the vapor pressure of the liquid, causing the formation of vapor bubbles
  • When these bubbles collapse, they create high-pressure shock waves that can damage the pump impeller and reduce its performance and efficiency
• Ensuring adequate NPSH is essential for preventing cavitation, maintaining pump performance, and extending the pump's service life
  • Achieved by proper system design (providing sufficient suction head, minimizing suction piping losses, selecting a pump with a suitable NPSHr)

Performance Characteristics of Pumps and Compressors

Pump Curves

• Pump curves are graphical representations of the performance characteristics of pumps
  • Used to select the appropriate equipment for a given application and to predict their performance under different operating conditions
• A pump curve is a plot of the pump head (pressure) versus the flow rate at a constant speed and impeller diameter
  • Also includes curves for pump efficiency and power consumption
• The pump head curve shows the relationship between the head developed by the pump and the flow rate
  • As the flow rate increases, the head decreases due to friction losses and other factors
• The pump efficiency curve shows the relationship between the pump efficiency and the flow rate
  • The efficiency reaches a maximum at a specific flow rate, called the best efficiency point (BEP), and decreases on either side of the BEP
• The power consumption curve shows the relationship between the power required by the pump and the flow rate
  • The power increases with the flow rate, but the rate of increase depends on the pump design and operating conditions

Compressor Maps

• Compressor maps are graphical representations of the performance characteristics of compressors
  • Used to select the appropriate equipment for a given application and to predict their performance under different operating conditions
• A compressor map is a plot of the compressor pressure ratio versus the mass flow rate at constant speed lines and efficiency contours
  • Also includes surge and choke limits
• The pressure ratio is the ratio of the discharge pressure to the suction pressure, and the mass flow rate is the amount of gas flowing through the compressor per unit time
• The constant speed lines represent the performance of the compressor at different rotational speeds
  • As the speed increases, the pressure ratio and mass flow rate increase
• The efficiency contours represent the lines of constant compressor efficiency
  • The highest efficiency occurs near the center of the map, and the efficiency decreases towards the surge and choke limits
• The surge limit represents the minimum mass flow rate at a given pressure ratio, below which the compressor becomes unstable and can experience flow reversal and vibrations
• The choke limit represents the maximum mass flow rate at a given pressure ratio, above which the compressor performance deteriorates due to excessive Mach numbers and shock waves