Fluid Machineries Terms

Pressure Ratio

The ratio of discharge pressure to suction pressure that determines the classification and compression level of a gas machine.

Fan

A fluid machine designed to move large volumes of air with minimal pressure increase.

Blower

A device that produces moderate pressure rise, where compressibility begins to have an effect.

Compressor

A machine that significantly increases gas pressure, causing a substantial increase in density and temperature.

Static Pressure

The useful pressure acting equally in all directions, responsible for overcoming system resistance.

Total Pressure

The sum of static pressure and velocity pressure; represents total energy per unit volume of the fluid.

Velocity Pressure

The pressure associated with the kinetic energy of a moving fluid.

Gauge Pressure

Pressure measured relative to atmospheric pressure.

Atmospheric Pressure

The pressure exerted by the surrounding air, serving as a reference baseline.

Discharge (Flow Rate)

The volume of fluid delivered per unit time (e.g., m³/s).

Mass Flow Rate

The mass of fluid flowing per unit time (kg/s).

Head

The energy per unit weight of fluid, usually expressed in meters.

Specific Energy

Energy per unit mass of fluid, combining pressure, velocity, and elevation effects.

Efficiency

The ratio of useful output to input energy, indicating machine performance.

Fluid Power

The rate at which energy is imparted to the fluid.

Shaft Power

The mechanical power supplied to the machine shaft.

Density

Mass per unit volume; increases significantly in compressors.

Viscosity

A measure of a fluid’s resistance to flow or deformation.

Reynolds Number

A dimensionless parameter indicating flow regime (laminar or turbulent).

Mach Number

The ratio of fluid velocity to speed of sound; indicates compressibility effects.

Inlet Temperature

Temperature of the fluid entering the machine.

Suction Pressure

Pressure at the inlet side of the machine.

Discharge Pressure

Pressure at the outlet side.

Velocity

Speed of fluid flow.

Elevation Head

Energy due to the height of fluid relative to a reference level.

Axial Compressor

A compressor where air flows parallel to the axis, suitable for high flow rates.

Centrifugal Machine

A machine where fluid flows radially outward due to centrifugal force.

Reciprocating Compressor

Uses a piston-cylinder arrangement for compression.

Positive Displacement Machine

Traps a fixed volume and compresses it mechanically.

Dynamic Machine

Adds energy continuously via rotating elements.

Roots Blower

Uses intermeshing lobes to transport air without internal compression.

Vane Blower

Uses rotating vanes in a casing to move air.

Scroll Compressor

Uses spiral scrolls to compress gas smoothly.

Cooling Tower Fan

Provides airflow for heat rejection systems.

Furnace Blower

Supplies combustion air.

Supercharger

A compressor used to increase engine intake pressure.

Refrigeration Compressor

Circulates refrigerant in cooling cycles.

Best Efficiency Point (BEP)

The operating point where efficiency is maximum and losses are minimized.

Characteristic Curve

Graph showing the relationship between pressure and flow rate.

Efficiency Curve

Shows efficiency variation with flow rate.

Power Curve

Shows power consumption vs flow rate.

Stall Point

Condition where flow separation begins, reducing performance.

Surge Region

Unstable operating zone with oscillating flow.

Choke

Maximum flow condition where further increase is impossible.

Stall Limit

Minimum stable flow rate before separation occurs.

Surge

Reversal or oscillation of flow due to instability.

Friction Loss

Energy loss due to viscous effects.

Minor Loss

Losses from fittings, bends, and expansions.

Leakage Loss

Loss due to fluid escaping through gaps.

Shock Loss

Loss from improper flow alignment at blades.

Specific Speed

Dimensionless parameter used for machine classification and similarity.

Similarity Parameter

Used to compare performance of geometrically similar machines.

Rotational Speed

Speed of rotating component (rpm).

Peripheral Velocity

Tangential velocity at the impeller tip.

Hydraulic Efficiency

Ratio of useful fluid energy to energy supplied.

Isentropic Efficiency

Ratio of ideal to actual compression work.

Volumetric Efficiency

Ratio of actual intake volume to theoretical volume.

Isothermal Process

Compression at constant temperature.

Adiabatic Process

Compression without heat transfer.

Polytropic Process

Real process involving partial heat transfer.

Polytropic Compression

More realistic compression model than ideal cases.

Ideal Gas Law

Relationship between pressure, volume, and temperature.

Specific Heat Ratio (k)

Ratio of Cp to Cv; affects compression behavior.

Compressibility Factor

Indicates deviation from ideal gas behavior.

Internal Energy

Energy stored in gas molecules.

Enthalpy

Total energy including flow work.

Entropy

Measure of disorder or irreversibility.

Discharge Temperature

Temperature after compression.

Discharge Pressure

Pressure after compression.

Impeller

Rotating element that imparts energy to fluid.

Diffuser

Converts velocity into pressure.

Inlet Blade Angle

Angle of fluid entry relative to blade.

Outlet Blade Angle

Angle at which fluid exits blade.

Blade Passage

Flow channel between blades.

Casing

Enclosure guiding fluid flow.

Shaft

Transmits mechanical power.

Seal

Prevents leakage.

Bearing

Supports rotating shaft.

Intercooler

Removes heat between compression stages.

Stall

Flow separation on blades due to adverse pressure gradient.

Vibration

Oscillation caused by imbalance or instability.

Thermal Stress

Stress due to temperature gradients.

Mechanical Loss

Energy loss due to friction in moving parts.

Noise

Acoustic energy from turbulent flow and vibration.

Off-design Loss

Efficiency reduction when operating away from BEP.

Sonic Condition (Mach 1)

Condition where flow velocity equals the speed of sound.