Pumps and compressors are machines that add energy to a moving fluid so it can flow through pipes, vessels, heat exchangers, filters, and reactors. Pumps usually handle liquids, while compressors handle gases and raise their pressure by reducing volume or increasing velocity. In process engineering, choosing the right machine affects safety, energy use, reliability, and product quality.
A good selection starts by understanding the required flow rate, pressure rise, fluid properties, and how the system resistance changes.
Key Facts
- Pump hydraulic power: P_h = rho g Q H
- Compressor or pump shaft power: P_shaft = P_fluid / eta
- Pressure head relation for liquids: H = Delta P / (rho g)
- Centrifugal machines add energy mainly by increasing fluid velocity, then converting velocity to pressure.
- Positive-displacement machines move a fixed volume per cycle, so flow is strongly linked to speed.
- System curve for turbulent pipe flow is often approximated by H_system = H_static + kQ^2
Vocabulary
- Centrifugal machine
- A pump or compressor that uses a rotating impeller to give the fluid kinetic energy and then converts part of that energy into pressure.
- Positive-displacement machine
- A pump or compressor that traps a fixed volume of fluid and forces it from inlet to outlet during each cycle.
- Head
- Head is the energy added to a liquid per unit weight, often expressed as an equivalent height of fluid.
- Flow rate
- Flow rate is the volume or mass of fluid passing a point per unit time, such as m3/s or kg/s.
- System curve
- A system curve shows how much head or pressure rise a piping system requires at different flow rates.
Common Mistakes to Avoid
- Treating pump head and pressure as identical is wrong because pressure depends on fluid density, while head expresses energy per unit weight.
- Selecting a centrifugal pump from only the desired flow rate is wrong because the pump must also meet the required head at the operating point.
- Throttling the discharge of a positive-displacement pump without relief protection is dangerous because pressure can rise rapidly and damage equipment.
- Ignoring gas density changes in compressor calculations is wrong because gases are compressible and their temperature, pressure, and volume change during compression.
Practice Questions
- 1 A pump moves water at Q = 0.040 m3/s and adds H = 25 m of head. Using rho = 1000 kg/m3 and g = 9.81 m/s2, calculate the hydraulic power.
- 2 A liquid process requires a pressure rise of 300 kPa. If the liquid density is 850 kg/m3, calculate the required head using H = Delta P / (rho g).
- 3 A process stream is viscous and must be delivered at a nearly constant flow rate even when discharge pressure changes. Explain whether a centrifugal or positive-displacement pump is usually the better choice and why.