Pumps, Fans, Blowers, Compressors, and Positive-Displacement Machines

Introduction

Fluid machines transfer energy between a mechanical device and a fluid. A pump raises the pressure or elevation of a liquid; a fan, blower, or compressor performs a similar function for gases. These machines are central to water supply, irrigation, chemical processing, refrigeration, power generation, mining, manufacturing, and building services.

The principal distinction is between dynamic machines, which continuously increase fluid momentum and convert it into pressure, and positive-displacement machines, which trap a definite volume of fluid and force it into the delivery system. Correct selection requires more than matching a rated flow and pressure. The engineer must consider viscosity, density, temperature, speed, system resistance, suction conditions, transient operation, efficiency, noise, maintenance, and safety.

Learning Objectives

By the end of this chapter, the reader should be able to:

1. Explain the operating principles of centrifugal, axial-flow, reciprocating, rotary, fan, blower, and compressor machines.
2. Apply energy, affinity-law, and positive-displacement relationships to practical calculations.
3. Interpret pump and compressor performance curves and identify a stable operating point.
4. Explain priming, net positive suction head (NPSH), cavitation, staging, surging, and choking.
5. Select operating and control methods while recognizing machine limitations, efficiency losses, and safety requirements.

1. Energy Transfer in Fluid Machines

For steady flow between inlet and outlet, the mechanical-energy equation may be written as

where H_m is the head added by a pump or removed from a turbine, p is pressure, ρ is fluid density, V is mean velocity, z is elevation, g is gravitational acceleration, and h_L is head loss. Pressures are absolute or gauge consistently; pressure differences may be expressed using either system. The equation assumes steady, single-phase flow and uses mean-section velocities.

The hydraulic power delivered to a liquid is

where Q is volume flow rate and H is pump head. If shaft power is P_s, the overall efficiency is

Efficiency includes hydraulic, volumetric, and mechanical losses. For a gas, compression power depends strongly on the thermodynamic path and cannot generally be calculated from ρgQH without care.

Figure: A fluid machine adds energy between suction and delivery systems; piping and fittings consume part of the available head.

2. Centrifugal Pumps

A centrifugal pump is a dynamic pump. Liquid enters near the impeller eye, receives angular momentum from rotating blades, and leaves at the impeller periphery. The casing or diffuser reduces velocity and converts kinetic energy into static pressure.

The ideal Euler head for a turbomachine is

where u is blade peripheral speed and V_w is the whirl component of absolute velocity. Subscripts 1 and 2 denote impeller inlet and outlet. For radial-entry pumps, V_w1 is often approximately zero, giving H_E ≈ u₂V_w2/g. Slip, friction, recirculation, leakage, and nonuniform flow make actual head lower than the Euler value.

Important components include the impeller, casing, shaft, bearings, wear rings, mechanical seal or packing, coupling, and motor. Impellers may be open, semi-open, or closed. Closed impellers generally provide high efficiency for clean liquids, whereas open impellers tolerate some solids.

Pump classifications

Priming

A conventional centrifugal pump cannot pump air effectively because its impeller develops pressure through liquid momentum. Priming removes air from the casing and suction line and replaces it with liquid. Methods include filling through a priming port, using a foot valve, an ejector, a vacuum pump, or a self-priming casing.

A foot valve must have low resistance and reliable sealing. Leaks in the suction line can admit air even when no liquid visibly leaks out. Operating an unprimed pump may cause overheating, seal damage, and rapid wear.

Performance curves and operating point

Manufacturers commonly provide head–flow, efficiency–flow, power–flow, and NPSH-required curves at a specified speed and impeller diameter. The system curve is often represented by

where H_static is the elevation and pressure difference independent of flow, and KQ² approximates friction and fitting losses in turbulent flow. The actual operating point is the intersection of the pump head curve and system curve. Changing a valve moves the system curve; changing speed or impeller diameter changes the pump curve.

The best-efficiency point (BEP) is the flow at which hydraulic and mechanical losses are minimized. Continuous operation far from BEP can produce vibration, radial thrust, recirculation, seal failure, and shortened bearing life.

Affinity laws

For geometrically similar operation with the same fluid and approximately unchanged efficiency,

where N is rotational speed and D is impeller diameter. For the same impeller diameter, these become

These relations are approximations, not substitutes for manufacturer curves. They are most reliable for moderate changes and similar flow regimes.

3. NPSH and Cavitation

Cavitation occurs when local absolute pressure falls to or below the liquid vapor pressure. Vapor bubbles form and then collapse in regions of higher pressure, causing noise, vibration, erosion, loss of head, and reduced flow.

The available net positive suction head is

where p_s,abs is absolute pressure at the pump suction reference, V_s is suction velocity, and p_v is vapor pressure at operating temperature. An equivalent reservoir-based expression must include atmospheric pressure, liquid level, suction losses, and vapor-pressure head.

The manufacturer supplies NPSH_r, the required NPSH, usually determined by a specified test criterion. Safe operation requires

with an appropriate engineering margin. Increasing liquid temperature raises vapor pressure and reduces NPSH available. Remedies include lowering the pump, raising the suction liquid level, reducing suction-line losses, increasing pipe diameter, reducing speed, or reducing liquid temperature. Throttling the suction valve is generally hazardous because it reduces suction pressure.

4. Axial-Flow and Mixed-Flow Pumps

In an axial-flow pump, fluid enters and leaves approximately parallel to the shaft. Propeller-like blades impart momentum primarily in the axial direction. These pumps deliver very large flow rates at low heads and are used in drainage, cooling-water circulation, and flood-control stations.

Blade pitch may be fixed or adjustable. Adjustable-pitch designs provide useful control but require robust hubs and actuators. Axial pumps are sensitive to inlet swirl, nonuniform approach flow, and insufficient submergence. Vortices at the intake can entrain air and reduce performance. Mixed-flow pumps occupy the intermediate region between radial and axial machines.

5. Positive-Displacement Pumps

A positive-displacement pump transfers a nearly definite volume per cycle. Its theoretical flow is

where V_d is displacement per cycle and n is cycles per unit time. Actual flow is reduced by slip and leakage:

where η_v is volumetric efficiency. Unlike a centrifugal pump, a positive-displacement pump can generate very high pressure if discharge flow is blocked. A relief valve, bypass, or other overpressure protection is therefore essential.

Reciprocating pumps

A piston or plunger alternately draws fluid into and expels it from a cylinder through check valves. For a single-acting pump with piston area A, stroke L, and crank speed N revolutions per minute,

For a double-acting pump, the ideal displacement is approximately twice this value, with allowance for the piston rod and end clearances. Air vessels reduce pulsation and power fluctuations by storing and releasing fluid during the cycle. Reciprocating pumps are appropriate for high pressure and low flow, metering, and viscous liquids, but they require valves, produce pulsation, and are sensitive to entrained gas.

Rotary pumps

Gear, screw, vane, lobe, and progressive-cavity pumps carry fluid in spaces between rotating elements and the casing. They provide smooth flow, compact construction, and good performance with viscous fluids. Excessive differential pressure increases slip, heating, and mechanical loading. Clearances, material compatibility, lubrication, and solids content must be considered.

6. Fans and Blowers

Fans and blowers handle gases at relatively low pressure rise. Terminology varies by industry, but fans generally operate at lower pressure ratios, while blowers operate at higher pressure rises. A compressor is designed for still greater pressure ratio and typically includes significant temperature rise.

Fan pressure may be expressed as total pressure or static pressure. Confusing these quantities leads to incorrect selection. Fan laws for geometrically similar machines are

where Δp is pressure rise. At constant density and diameter, doubling speed ideally doubles flow, quadruples pressure, and increases power eightfold. Real changes may be limited by motor capacity, sound, vibration, blade stress, and stall.

Centrifugal fans are suitable for higher pressure and duct resistance. Axial fans provide high flow at low pressure and are common in ventilation and cooling. Fan system resistance is approximately proportional to flow squared in turbulent duct systems. Dampers alter the system curve; variable-speed drives alter fan performance more efficiently than throttling in many applications.

7. Compressors

Compressors increase gas pressure, usually with an associated temperature increase. They are classified as dynamic or positive-displacement machines.

Reciprocating compressors

A piston compressor draws gas into a cylinder, compresses it, and discharges it through valves. Clearance volume leaves some compressed gas in the cylinder after discharge. This gas re-expands during the suction stroke and reduces volumetric efficiency. Reciprocating compressors provide high pressure and relatively low flow, but they generate pulsation, vibration, and significant maintenance demands.

Rotary compressors

Screw, vane, scroll, and lobe compressors provide continuous or nearly continuous delivery. Oil-injected screw compressors use oil for sealing, cooling, and lubrication; oil-free machines require tighter clearances or specialized coatings and control systems. Scroll compressors are common in refrigeration and air-conditioning because of low vibration and compactness.

Dynamic compressors

Centrifugal compressors convert impeller velocity into pressure through a diffuser and collecting casing. Axial compressors use successive blade rows and are capable of very large flow rates and high overall pressure ratios. Dynamic compressors are efficient near their design point but sensitive to flow instability.

For ideal-gas compression with constant specific heats, the isentropic relation is

where T is absolute temperature, p is absolute pressure, and γ = c_p/c_v. The isentropic compressor work per unit mass is

where R is the gas constant. This equation assumes an ideal gas, constant γ, reversible adiabatic compression, and negligible changes in elevation and velocity. Actual compressor work is greater. Isentropic efficiency for a compressor is

Intercooling between stages lowers the work required and reduces delivery temperature. For an ideal two-stage compressor with perfect intercooling and equal pressure ratios, the optimum intermediate absolute pressure is approximately

where p₁ and p₃ are inlet and final absolute pressures. Real machines require allowance for pressure losses, nonideal intercooling, and unequal stage efficiencies.

8. Surging, Choking, and Operating Limits

Surge is a system-level instability in a dynamic compressor at excessively low flow. Flow may oscillate, reverse, and produce severe vibration and thrust changes. Anti-surge control maintains a minimum stable flow, often by opening a recycle valve.

Rotating stall is a localized aerodynamic separation pattern that travels around the annulus. It can precede surge. Choking occurs at excessive flow when local gas velocity reaches sonic conditions; further reduction in downstream pressure produces little additional flow. Choking limits capacity and can generate noise and vibration.

Pumps also have operating limits. Low-flow operation may cause internal recirculation and heating; high-flow operation may exceed motor power, cause cavitation, or produce excessive vibration. The permissible operating range should be taken from the manufacturer’s allowable operating region, not inferred solely from the shutoff or maximum-flow point.

9. Worked Example: Pump Selection and NPSH Check

A water system requires Q = 0.040 m³/s at a total pump head of H = 32 m. The pump efficiency at this duty is 0.78. Determine hydraulic power and shaft power. If the calculated available NPSH is 6.8 m and the manufacturer specifies required NPSH of 4.9 m, comment on suitability.

Using water density ρ = 1000 kg/m³ and g = 9.81 m/s²,

Therefore,

A motor larger than 16.1 kW is selected after considering service factor, starting conditions, voltage, and local standards. The NPSH margin is

The pump passes the stated NPSH comparison, but the engineer should confirm that the margin is adequate for temperature variation, transient conditions, measurement uncertainty, and the site’s commissioning standard.

10. Applied Case: Cooling-Water Pump Instability

Background. An illustrative manufacturing plant uses two parallel centrifugal pumps to circulate cooling water. Operators report fluctuating discharge pressure, seal leakage, and intermittent rattling at one pump.

Problem. The pumps were selected for a duty substantially below their BEP. A partially closed suction isolation valve and a clogged intake screen further reduced suction pressure. The resulting NPSH available approached the required value, while parallel operation moved the system toward a low-flow region.

Analysis. Field measurements showed that the suction valve produced an unnecessary head loss and that the intake screen had not been cleaned according to the maintenance schedule. The pump curves demonstrated that the actual operating point was left of the recommended operating range. Vibration and acoustic observations were consistent with cavitation and internal recirculation, although vibration analysis was required for confirmation.

Approach and solution. The plant cleaned the screen, fully opened the suction valve, balanced the parallel pumps, and installed a variable-speed control strategy. A minimum-flow bypass was added to protect the pump during low-demand periods. Operators were trained to monitor suction pressure, discharge pressure, vibration, motor current, and bearing temperature.

Outcome. The pump operated closer to BEP, rattling disappeared, seal failures declined, and energy consumption fell because flow was controlled by speed rather than continuous discharge throttling.

Learning points. Pump problems frequently arise from system conditions rather than defective impellers. Curve interpretation, suction design, instrumentation, and operating discipline must be treated as one engineering problem.

11. Selection, Control, and Maintenance Best Practices

Selection should begin with a duty envelope rather than a single nominal point. Specify normal, minimum, and maximum flow; pressure or head; fluid properties; temperature; solids; suction conditions; materials; speed; available power; and control philosophy.

Use the following practices:

• Compare the system curve with the complete manufacturer performance map.
• Keep normal operation near BEP unless the machine is specifically designed for another region.
• Calculate NPSH using absolute pressure and the actual fluid vapor pressure.
• Install pressure, temperature, flow, vibration, and motor-current measurement where their diagnostic value justifies the cost.
• Protect positive-displacement pumps with relief systems and protect compressors with correctly set pressure, temperature, and overspeed safeguards.
• Inspect alignment, couplings, seals, bearings, lubrication, valves, strainers, and foundations.
• Treat unusual noise, rising vibration, falling flow, and increasing power as early warning signs.
• Follow lockout/tagout, pressure isolation, guarding, electrical, and pressure-vessel regulations.

Variable-speed drives can improve efficiency, but they introduce harmonics, cooling concerns at low speed, electromagnetic compatibility issues, and control-system dependencies. Digital monitoring and machine-learning fault detection can identify trends in vibration, current, and pressure; however, sensor quality, calibration, cybersecurity, explainability, and competent human diagnosis remain essential. An algorithm should support—not replace—engineering judgment and safe shutdown procedures.

12. Challenges and Limitations

Ideal equations are useful for understanding trends but do not capture every installation effect. Affinity laws assume similarity and may be inaccurate for large speed changes, viscous fluids, or strongly altered Reynolds numbers. System curves may change because of fouling, valve position, fluid temperature, or parallel equipment. Manufacturer curves are test-based and depend on tolerances and test standards.

Gas-machine analysis is especially sensitive to absolute pressure, temperature, gas composition, compressibility, cooling, and leakage. A compressor selected using gauge pressure ratios or an inappropriate gas constant may be seriously undersized or oversized. Finally, efficiency alone is not a sufficient selection criterion: reliability, maintainability, life-cycle energy, noise, emissions, and safety may dominate the economic decision.

Chapter Summary

Pumps, fans, blowers, compressors, and positive-displacement machines transfer mechanical energy to fluids through dynamic momentum exchange or trapped-volume displacement. Centrifugal pumps are selected by matching a pump curve to a system curve, with operation preferably near the best-efficiency point. Priming is essential because centrifugal pumps cannot normally evacuate air from a dry suction line. NPSH available must exceed NPSH required with an appropriate margin to prevent cavitation.

Axial and mixed-flow pumps serve high-flow, low- to moderate-head duties. Reciprocating and rotary positive-displacement machines provide nearly definite displacement and are suitable for high pressure, viscous fluids, or metering, but they require overpressure protection. Fans and blowers handle gases at relatively low pressure rise, while compressors produce larger pressure ratios and require thermodynamic analysis.

Compressor staging and intercooling reduce work and temperature. Surge, rotating stall, choking, cavitation, excessive low-flow recirculation, motor overload, and vibration define important operating limits. Sound engineering practice combines performance curves, system analysis, instrumentation, maintenance, safety controls, and life-cycle evaluation.

Key Terms

Pump: Machine that adds energy to a liquid.

Centrifugal pump: Dynamic pump using a rotating impeller and pressure-recovering casing.

Axial-flow pump: Pump in which flow moves mainly parallel to the shaft.

Positive-displacement pump: Machine that traps and displaces a definite fluid volume.

Priming: Filling a pump casing and suction line with liquid while removing air.

Best-efficiency point: Operating condition at which machine efficiency is maximum.

System curve: Relationship between required system head and flow rate.

NPSH available: Suction energy head above liquid vapor-pressure head.

NPSH required: Minimum suction head specified for satisfactory pump operation.

Cavitation: Vapor formation and collapse caused by local pressure falling below vapor pressure.

Slip: Difference between theoretical and actual displacement flow.

Fan: Gas-moving machine generally producing a relatively low pressure rise.

Blower: Gas machine operating at a greater pressure rise than a typical fan.

Compressor: Machine that raises gas pressure, usually with temperature increase.

Intercooling: Cooling gas between compressor stages.

Surge: Unstable oscillatory flow condition in a dynamic compressor.

Choking: Flow limitation associated with sonic velocity in a gas passage.

Affinity laws: Approximate relationships linking speed, size, flow, head, pressure, and power.

Volumetric efficiency: Ratio of actual displacement flow to theoretical displacement flow.

Review Questions

Short Answer Questions

1. Why must a conventional centrifugal pump be primed?
2. Define NPSH available and NPSH required.
3. What is the best-efficiency point of a pump?
4. State one essential safety device for a positive-displacement pump.
5. Distinguish surge from choking in a compressor.

Long Answer Questions

1. Explain the construction and operation of a centrifugal pump and discuss the causes of head and efficiency losses.
2. Derive and interpret the pump affinity laws, including their assumptions and limitations.
3. Compare reciprocating, gear, screw, and centrifugal pumps for industrial service.
4. Explain cavitation, its symptoms, its effects, and practical methods of prevention.
5. Describe multistage compression, intercooling, and the significance of optimum intermediate pressure.

Analytical / Application Questions

1. A pump delivers 0.025 m³/s against 45 m head at 80% efficiency. Calculate the hydraulic and shaft power using water properties.
2. A centrifugal pump operates at 1450 rpm and is changed to 1740 rpm with the same impeller. Estimate the new flow, head, and power ratios using the affinity laws, and discuss why the actual results may differ.
3. A compressor repeatedly trips during low-demand operation. Propose a diagnostic sequence using performance curves, pressure and flow measurements, recycle control, vibration data, and inspection of inlet and discharge systems.

Multiple Choice Questions with Answers

Question 1

Which condition represents the normal operating point of a centrifugal pump?

A. Intersection of the pump curve and system curve

B. The shutoff-head point only

C. The point of zero system resistance

D. The maximum-flow point regardless of system demand

Correct Answer: A. Intersection of the pump curve and system curve

Question 2

A pump delivers 0.040 m³/s against 25 m of water head. Using ρ = 1000 kg/m³ and g = 9.81 m/s², its hydraulic power is approximately:

A. 4.9 kW

B. 9.8 kW

C. 12.3 kW

D. 24.5 kW

Correct Answer: C. 12.3 kW

Question 3

Which action generally increases NPSH available?

A. Increasing liquid temperature

B. Increasing suction-line losses

C. Lowering the pump relative to the supply tank

D. Partially closing the suction valve

Correct Answer: C. Lowering the pump relative to the supply tank

Question 4

What is the principal danger of blocking the discharge of a positive-displacement pump?

A. The pump immediately loses all displacement

B. Pressure may rise to a destructive value

C. NPSH required becomes zero

D. The fluid becomes incompressible

Correct Answer: B. Pressure may rise to a destructive value

Question 5

For the same centrifugal-pump impeller, if speed is doubled, the ideal affinity-law prediction for head is:

A. Half the original head

B. The same head

C. Twice the original head

D. Four times the original head

Correct Answer: D. Four times the original head

Question 6

Which machine is most appropriate for very high flow at low head?

A. Axial-flow pump

B. Reciprocating plunger pump

C. High-pressure screw pump

D. Multistage boiler-feed pump

Correct Answer: A. Axial-flow pump

Question 7

Cavitation begins when local liquid pressure:

A. Exceeds atmospheric pressure

B. Falls to or below vapor pressure

C. Equals discharge pressure

D. Becomes independent of temperature

Correct Answer: B. Falls to or below vapor pressure

Question 8

Why is intercooling used between compressor stages?

A. To increase gas viscosity permanently

B. To reduce compression work and delivery temperature

C. To eliminate the need for valves

D. To cause compressor surge

Correct Answer: B. To reduce compression work and delivery temperature

Question 9

Compressor surge is best described as:

A. Stable operation at the design flow

B. Localized liquid vaporization in a pump

C. Oscillatory flow that may include reversal at low flow

D. A condition in which all gas passages are sonic

Correct Answer: C. Oscillatory flow that may include reversal at low flow

Question 10

For a reciprocating positive-displacement pump, actual flow is generally less than theoretical flow because of:

A. Slip and leakage

B. The absence of displacement

C. Infinite volumetric efficiency

D. The pump curve intersection alone

Correct Answer: A. Slip and leakage