Definition
Pressure drop is the reduction in fluid pressure that occurs as a fluid flows through a conduit, component, or system due to frictional resistance, changes in flow direction, obstacles, or variations in cross‑sectional area. It is expressed as the difference between the upstream (higher) pressure and the downstream (lower) pressure, typically measured in pascals (Pa), kilopascals (kPa), or pounds per square inch (psi).
Overview
In fluid dynamics and engineering, pressure drop is a fundamental parameter used to design and evaluate piping networks, hydraulic systems, ventilation ducts, heat exchangers, filters, and many other devices where fluids (liquids or gases) are conveyed. The magnitude of the pressure drop determines the required pumping power, influences flow rates, and affects overall system efficiency. Calculations of pressure drop often employ empirical correlations (e.g., Darcy–Weisbach, Hazen‑Williamson) or analytical equations that account for fluid properties (density, viscosity), flow regime (laminar or turbulent), pipe roughness, length, diameter, and the presence of fittings or accessories.
Etymology/Origin
The term combines “pressure,” derived from the Latin pressura meaning “a pressing,” and “drop,” from Old English drǣpan meaning “to fall.” The compound phrase began to appear in technical literature in the late 19th to early 20th centuries alongside the development of modern fluid‑mechanics theory and industrial piping practice.
Characteristics
| Characteristic | Description |
|---|---|
| Dependence on Flow Regime | In laminar flow (Re < 2000), pressure drop varies linearly with flow velocity (Hagen–Poiseuille law). In turbulent flow (Re > 4000), the relationship is non‑linear and often approximated by a power law (e.g., Darcy–Weisbach equation). |
| Influencing Factors | • Fluid viscosity and density • Pipe length and diameter • Surface roughness of conduit walls • Presence of fittings, valves, bends, and expansions/contractions • Flow rate (or velocity) |
| Units | Commonly expressed in pascals (Pa), kilopascals (kPa), bar, or pounds per square inch (psi). In engineering calculations, pressure drop per unit length (Pa / m) may also be reported. |
| Measurement | Measured using differential pressure gauges, manometers, or electronic pressure transducers positioned at two points along the flow path. |
| Impact on System Design | Determines required pump or fan head, influences pipe sizing, and affects energy consumption and operational cost. Excessive pressure drop can lead to flow restriction, cavitation, or system failure. |
Related Topics
- Darcy–Weisbach equation – a fundamental relation for calculating pressure loss due to friction in pipes.
- Hagen–Poiseuille law – describes pressure drop in laminar flow through circular tubes.
- Reynolds number – a dimensionless quantity used to characterize flow regime (laminar vs. turbulent).
- Friction factor – a coefficient that quantifies pipe wall resistance in pressure‑drop calculations.
- Pump head – the energy per unit weight that a pump must supply to overcome pressure drop.
- Venturi effect – a phenomenon where pressure drops as fluid velocity increases through a constricted section.
- Flowmeter – devices that often rely on measured pressure drop to infer flow rate (e.g., differential pressure flowmeters).