- Practice · 4 marks
Turbomachines versus positive displacement machines
Short answer
Turbomachines use dynamic blade action for continuous, steady flow; they are compact, high-speed, and need no valves. Positive displacement machines trap a fixed volume, causing intermittent flow; they are bulky, low-speed, require valves, and can generate very high pressure but need relief valves.
Differentiate between turbomachines and positive displacement machines.
Both transfer energy between a machine and a fluid, but the mechanism is different. In a turbomachine the energy exchange is due to the dynamic action of the fluid on rotating blades, whereas in a positive displacement machine a fixed volume of fluid is trapped, moved and pushed out by a moving boundary.
| Point | Turbomachine | Positive displacement machine |
|---|---|---|
| Principle | Change of angular momentum (Euler's equation) | Change of volume of a closed space |
| Flow | Continuous, steady | Intermittent, pulsating |
| Moving part | Rotor with blades, pure rotation | Piston, gear, vane, screw (often reciprocating) |
| Capacity | Large discharge, moderate head | Small discharge, very high head possible |
| Speed | High speed, direct drive from motor | Low speed, often gear or crank drive |
| Size and weight | Compact, less weight per unit power | Bulky, heavy |
| Balancing, vibration | Easily balanced, smooth | Unbalanced forces, more vibration |
| Valves | Not needed | Suction and delivery valves needed (reciprocating type) |
| Maintenance | Low | High (wear of valves, seals) |
| Examples | Centrifugal pump, turbines | Reciprocating pump, gear pump, piston engine |
In a turbomachine the delivered head depends on speed and discharge, so closing the delivery valve gives a limited head. A positive displacement pump will build up pressure until it fails, so it needs a relief valve.
✓ Answer checked by Claude Sonnet 5.5
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