5.6 Pumps and compressors
5 min read•august 14, 2024
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.
and efficiency are crucial factors in pump and compressor operation. Calculating these values helps optimize energy use and performance. Additionally, concepts like and performance curves guide proper equipment selection and prevent issues like .
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 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)
- Centrifugal pumps use an to create a centrifugal force that increases the velocity and pressure of the fluid
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)
- Positive displacement compressors work by trapping a fixed volume of gas and reducing its volume to increase the pressure
Power Requirements for Pumps and Compressors
Pump Power and Efficiency
- The power required by a pump depends on the , , and the fluid properties (density, viscosity)
- Calculated using the formula:
- 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:
Compressor Power and Efficiency
- The power required by a compressor depends on the flow rate, pressure ratio, and the gas properties (specific heat ratio, factor)
- Calculated using the formula:
- Compressor efficiency is the ratio of the ideal work required for the compression process to the actual
- Accounts for losses due to friction, heat transfer, and other factors
- Calculated using the formula:
- 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: and
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
Key Terms to Review (30)
Air-lift pump: An air-lift pump is a type of pump that utilizes compressed air to lift liquid from a lower elevation to a higher elevation, relying on the principle of buoyancy. This method is particularly useful in applications where the pumping of abrasive or corrosive fluids is required, as the pump can minimize wear and tear on mechanical parts. By injecting air into a column of liquid, the air bubbles reduce the density of the liquid, allowing it to rise through the pipe system.
API Specifications: API specifications are formal documents that describe how software components interact through application programming interfaces (APIs). They provide a blueprint detailing the endpoints, request/response formats, authentication methods, and protocols used for communication between systems, particularly in the context of integrating pumps and compressors within chemical engineering processes.
ASME Standards: ASME standards refer to the technical standards and guidelines established by the American Society of Mechanical Engineers to ensure safety, performance, and efficiency in engineering practices. These standards are vital in various engineering fields, including those involving pressure vessels, piping systems, and machinery, ensuring that components function reliably and safely in different applications.
Axial Compressor: An axial compressor is a type of mechanical device that increases the pressure of a gas by converting kinetic energy into pressure energy through a series of rotating and stationary blades. This design allows for continuous flow of gas along the axis of the compressor, making it efficient for applications requiring high pressure and high flow rates, such as in jet engines and gas turbines.
Cavitation: Cavitation is the formation and collapse of vapor-filled cavities or bubbles in a liquid, typically occurring in pumps and compressors when the pressure drops below the vapor pressure of the liquid. This phenomenon can lead to significant damage to equipment due to the high energy release from the implosion of these bubbles, which generates shock waves and can erode surfaces. Understanding cavitation is crucial for ensuring efficient operation and longevity of pumps and compressors.
Centrifugal compressor: A centrifugal compressor is a type of dynamic compressor that uses a rotating impeller to increase the pressure of a gas. This device works by converting kinetic energy, gained from the high-speed rotation of the impeller, into potential energy in the form of increased pressure as the gas is discharged. Centrifugal compressors are commonly used in various industrial applications, including gas pipelines, refrigeration systems, and air conditioning units.
Centrifugal pump: A centrifugal pump is a mechanical device that converts rotational kinetic energy into hydrodynamic energy by transferring fluid through a rotating impeller. This type of pump is widely used for moving liquids and is essential in various applications such as water supply, sewage treatment, and chemical processing. Its design allows for efficient flow rates and is favored for its simplicity and reliability.
Compressibility: Compressibility is a measure of how much a substance can be compressed under pressure, indicating the change in volume in response to an applied force. In fluids, this property is essential for understanding behaviors in various applications, such as flow dynamics and equipment performance. It helps engineers assess how gases and liquids will behave in systems involving changes in pressure and temperature.
Compressor efficiency: Compressor efficiency refers to the effectiveness of a compressor in converting input power into useful work for compressing gas. It is an essential measure that indicates how well the compressor is performing in terms of energy usage, revealing losses that may occur during the compression process due to factors like mechanical friction, heat generation, and leakage.
Dynamic compressor: A dynamic compressor is a type of mechanical device that increases the pressure of a gas through the kinetic energy imparted by rotating blades or impellers. It is distinct from positive displacement compressors as it relies on continuous flow rather than trapping and compressing gas in a fixed volume. This type of compressor is commonly used in applications requiring large volumes of gas at relatively low pressures, making it essential for various industrial processes.
Electromagnetic pump: An electromagnetic pump is a type of pump that uses electromagnetic forces to move electrically conductive fluids. This technology operates based on the principle of Lorentz force, where a magnetic field interacts with an electric current to produce motion. Electromagnetic pumps are particularly useful for pumping liquid metals or other conductive fluids, eliminating the need for mechanical moving parts, which can enhance reliability and reduce maintenance.
Flow Rate: Flow rate is a measure of the volume of fluid that passes through a given surface per unit of time, often expressed in units like liters per second or gallons per minute. It is crucial in various applications, including determining the efficiency of chemical processes, understanding pipe flow dynamics, and optimizing the operation of pumps and compressors. By converting flow rates into different units, engineers can standardize measurements for better communication and comparison.
Impeller: An impeller is a rotating component found in pumps and compressors that transfers energy to a fluid, typically by converting rotational energy into kinetic energy. Impellers play a crucial role in increasing the velocity of the fluid, which helps in its movement through piping systems or other equipment. By facilitating this fluid flow, impellers contribute significantly to the efficiency and performance of various industrial processes.
Jet pump: A jet pump is a type of pump that uses the principle of momentum transfer to move fluids, typically employing a high-speed jet of fluid to create a low-pressure area that draws in additional fluid. This technology is commonly used in applications such as water supply, irrigation, and in various industrial processes, showcasing its versatility and efficiency in transferring liquids.
Laminar Flow: Laminar flow is a type of fluid motion characterized by smooth, orderly layers or streamlines that move parallel to each other, minimizing turbulence and allowing for predictable behavior. This flow regime is crucial in various applications, as it influences how fluids interact with surfaces and how heat is transferred within the system.
Net Positive Suction Head (NPSH): Net Positive Suction Head (NPSH) is a crucial measurement in pump operation that quantifies the pressure available at the pump's suction port to prevent cavitation. It ensures that the pump can maintain proper flow without forming vapor bubbles, which could lead to damaging hydraulic issues. Understanding NPSH is essential for selecting the right pump and ensuring its reliable operation in various fluid transport applications.
Npsh available (npsha): NPSH available (NPSHa) is a measure of the absolute pressure at the suction side of a pump, expressed in terms of the height of a liquid column. It represents the energy available to push the liquid into the pump and is crucial for ensuring that the pump operates efficiently without cavitation. Understanding NPSHa helps in determining if a pump can handle the required flow rate while maintaining adequate pressure.
NPSH Required (NPSHr): NPSH required (NPSHr) is the minimum amount of Net Positive Suction Head that a pump needs at its inlet to prevent cavitation and ensure proper operation. It is crucial for maintaining the pump's efficiency and performance, as insufficient NPSH can lead to vaporization of the liquid, resulting in cavitation, noise, and potential damage to the pump components.
Overpressure: Overpressure refers to the pressure in a system that exceeds the normal or atmospheric pressure, often caused by rapid changes in volume or energy input. This phenomenon is critical in the design and operation of pumps and compressors, where maintaining safe pressure levels is essential to prevent equipment failure or hazardous situations. Understanding overpressure helps in assessing system performance and ensuring safe operational practices.
Positive Displacement Compressor: A positive displacement compressor is a type of mechanical device that increases the pressure of a gas by trapping a fixed volume of gas and forcing it into a smaller space. These compressors operate on the principle of creating a pressure difference to move the gas, making them essential for various industrial applications where consistent and reliable gas compression is required.
Positive Displacement Pump: A positive displacement pump is a type of pump that moves fluid by trapping a fixed amount of it and forcing that volume into the discharge pipe. This mechanism allows the pump to maintain a consistent flow rate regardless of changes in pressure, making it essential for applications where precise dosing or handling of viscous fluids is required. Positive displacement pumps can be classified into two main categories: rotary and reciprocating, both of which have unique operational characteristics.
Power requirements: Power requirements refer to the amount of energy needed to operate pumps and compressors efficiently and effectively within a system. This concept is crucial for ensuring that these devices perform their intended function while maintaining optimal performance and energy consumption, which is vital in various engineering applications.
Pressure Differential: Pressure differential refers to the difference in pressure between two points in a fluid system. This concept is crucial in understanding how fluids move through pumps and compressors, as it directly influences the flow rate and efficiency of these systems. A higher pressure differential often results in an increased flow rate, which is essential for effective fluid transport in various engineering applications.
Pressure head: Pressure head refers to the height of a fluid column that produces a specific pressure at a given point in a fluid system. It is an important concept in understanding how pumps and compressors operate, as it directly relates to the ability of these devices to move fluids against gravity and other opposing forces. Pressure head can be expressed in units of length, typically meters or feet, and is a crucial factor in calculating the efficiency and performance of fluid transport systems.
Pump efficiency: Pump efficiency refers to the ratio of the useful hydraulic energy output of a pump to the energy input required to operate it. This measure is crucial for evaluating how effectively a pump converts mechanical energy into fluid flow and pressure, which impacts system performance and energy consumption.
Reciprocating pump: A reciprocating pump is a type of positive displacement pump that uses a back-and-forth motion to move fluid through a cylinder and discharge it through a valve. This pump works by utilizing a piston or plunger that creates a vacuum during the suction stroke, drawing fluid in, and then forces it out during the discharge stroke. Reciprocating pumps are known for their efficiency in handling high-pressure applications and can be found in various industries including oil and gas, water treatment, and chemical processing.
Rotary pump: A rotary pump is a type of positive displacement pump that moves fluids through the rotational motion of its components. This mechanism can efficiently handle various fluids, including viscous and shear-sensitive substances, making it ideal for many industrial applications. The design and function of rotary pumps enable them to provide a consistent flow rate regardless of pressure changes in the system.
Turbulent flow: Turbulent flow is a type of fluid motion characterized by chaotic changes in pressure and flow velocity. It typically occurs at high velocities or in situations with obstacles, resulting in eddies and vortices that disrupt the smooth layering of fluid particles. This phenomenon is crucial for understanding various physical processes like heat transfer, momentum balance, and energy losses in pipelines.
Valve: A valve is a mechanical device used to control the flow and pressure of fluids in various systems. It regulates the movement of liquids, gases, and slurries by opening, closing, or partially obstructing pathways in piping systems. Valves are essential in maintaining operational efficiency and safety across many applications, especially in relation to pumps and compressors as well as in process flowsheets and piping diagrams.
Work input: Work input refers to the energy supplied to a system to perform a task or achieve a certain function, particularly in the context of mechanical devices like pumps and compressors. It is a critical measure as it directly relates to the efficiency and performance of these devices, determining how much energy is converted into useful work in moving fluids or gases. Understanding work input is essential for optimizing equipment operation and energy consumption in various applications.