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.
Understanding Engineering: Pumps and Compressors in Process
A pump does not create flow on its own. It creates a pressure difference, and the pipe system decides how much liquid actually moves. Friction in straight pipe, bends, valves, filters, and equipment all resist flow.
As flow rises, this resistance usually rises rapidly. The operating point is where the machine capability matches the resistance of the complete system. Closing a valve increases resistance and reduces flow.
Opening a parallel line reduces resistance and can increase flow. This is why plant changes that seem small can move a pump far from its intended operating condition.
Centrifugal pumps use a spinning impeller. Liquid enters near the centre, gains speed between the blades, then slows in a casing called a volute or diffuser. That slowing converts much of the motion into pressure.
They work best near a designed flow range. Running far below that range can cause internal recirculation, vibration, heating, and seal damage.
Running too far above it can overload the motor or leave too little pressure for the downstream process. Variable speed drives are often more efficient than throttling with a valve because reducing speed reduces the energy added to the liquid.
A major pump risk is cavitation. This occurs when local pressure falls below the liquid vapour pressure, so small vapour bubbles form. When those bubbles reach a higher pressure region, they collapse violently.
The result can sound like gravel passing through the pump. Over time, cavitation erodes metal surfaces and reduces performance. Engineers prevent it by keeping enough pressure at the pump inlet.
A flooded suction tank helps. So do short, wide suction pipes with few restrictions.
Hot liquids need extra care because heating raises vapour pressure. Students should distinguish inlet pressure from discharge pressure, since a high discharge pressure does not guarantee safe inlet conditions.
Compressors need further attention because gases change volume greatly as pressure changes. Compressing a gas heats it, sometimes enough to damage lubricating oil, seals, or the product. Multi stage compressors reduce this problem by compressing in steps and cooling the gas between stages.
Centrifugal compressors can become unstable at very low flow. This condition is called surge. Flow can reverse briefly, creating strong pressure pulses and possible damage.
Control systems may recycle some discharge gas back to the inlet to maintain stable flow. Positive displacement compressors need relief protection because blocking their outlet can raise pressure extremely quickly.
Real plants use instruments to show suction pressure, discharge pressure, flow, temperature, vibration, and motor current. These readings tell a story when compared over time. A falling flow rate with rising pressure drop across a filter may mean the filter is fouling.
Higher vibration can point to misalignment, worn bearings, imbalance, or cavitation. A seal leak may show that the seal faces lack cooling or lubrication. When solving problems, start with measured conditions rather than assumptions.
Check units carefully. For liquids, pressure can be expressed as head, which depends on density. Power calculations must include efficiency because the motor must supply more power than the fluid receives.
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.