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Glossary of Terms

Definitions of key terms used in jet pump and eductor technology — from basic fluid dynamics to material specifications.

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A
Absolute Pressure
Pressure measured relative to a perfect vacuum (zero pressure). Equal to gauge pressure plus atmospheric pressure (14.7 PSIA at sea level). Abbreviated PSIA.
Atmospheric Pressure
The pressure exerted by the weight of the atmosphere at sea level — approximately 14.7 PSI or 29.92 inches of mercury. Decreases with altitude.
C
Cavitation
The formation and collapse of vapor bubbles in a liquid when local pressure drops below the vapor pressure of the liquid. Can cause noise, vibration, and erosion damage in pumps. Jet pumps are less susceptible to cavitation than centrifugal pumps.
Condensate
Liquid water formed when steam cools and condenses. In steam jet heaters and heating eductors, steam condenses directly into the liquid being heated, transferring its latent heat.
D
Diffuser
The diverging (expanding) section of a jet pump downstream of the throat. As the fluid slows in the diffuser, velocity converts back to pressure. The diffuser is responsible for the pressure recovery that makes the jet pump useful.
Discharge Head
The total pressure (expressed in feet of fluid) that a pump must overcome to deliver fluid to the discharge point. Includes static elevation, friction losses, and back pressure.
E
Eductor
A device that uses a high-velocity motive fluid to entrain and pump a secondary fluid, operating on the Venturi principle. Synonymous with jet pump. The term "eductor" is commonly used for tank-mounted mixing and heating devices; "jet pump" for inline pipeline applications.
Entrainment
The process by which the high-velocity motive fluid draws in (entrains) the secondary fluid in a jet pump. The ratio of entrained fluid to motive fluid is the entrainment ratio.
Entrainment Ratio
The ratio of secondary (entrained) fluid flow to motive fluid flow in a jet pump or eductor. For tank eductors at 20–70 PSI, the entrainment ratio is approximately 4:1 — four gallons of tank contents are mixed for every gallon of motive fluid.
G
Gauge Pressure
Pressure measured relative to atmospheric pressure. A gauge pressure of 0 PSIG equals atmospheric pressure. Abbreviated PSIG. Most operating pressures in industrial applications are expressed as gauge pressure.
H
Head
A measure of pressure expressed as the height of a column of fluid. One foot of water head equals approximately 0.433 PSI. Used to express suction lift, discharge pressure, and pump performance.
J
Jet Pump
A pump that uses a high-velocity jet of motive fluid to entrain and pump a secondary fluid, with no moving parts. Also called an eductor, Venturi pump, or ejector. The motive fluid can be liquid, steam, or gas.
L
Latent Heat
The heat energy absorbed or released during a phase change (e.g., steam condensing to water) without a change in temperature. Steam releases approximately 1,000 BTU/lb of latent heat when it condenses, which is what makes steam jet heaters and heating eductors so efficient.
M
Motive Fluid
The high-pressure fluid that drives a jet pump or eductor. Common motive fluids include water, process liquids, steam, compressed air, and other gases. The motive fluid must be available at sufficient pressure to drive the eductor.
MNPT
Male National Pipe Thread. A standard tapered thread used for pipe connections in the United States. Tank eductors are typically supplied with MNPT connections for easy installation into tank walls or manifolds.
N
NPSH
Net Positive Suction Head. The pressure available at the pump suction above the vapor pressure of the liquid. Insufficient NPSH causes cavitation. Jet pumps require a minimum NPSH of approximately 3 feet.
Nozzle
The converging (narrowing) section at the inlet of a jet pump that accelerates the motive fluid to high velocity. The nozzle converts pressure energy to velocity energy, creating the low-pressure zone that draws in the secondary fluid.
P
PPL
Glass Reinforced Polypropylene. A thermoplastic material used for plastic tank eductors. Offers excellent resistance to dilute and concentrated acids, alcohols, bases, and mineral oils. Maximum operating temperature: 220°F.
PSIG
Pounds per Square Inch Gauge. Pressure measured relative to atmospheric pressure. The standard unit for expressing operating pressures in industrial applications. See also: Gauge Pressure.
PSIA
Pounds per Square Inch Absolute. Pressure measured relative to a perfect vacuum. PSIA = PSIG + 14.7 (at sea level). Used when calculating vacuum levels and absolute pressures.
PVDF
Polyvinylidene Fluoride. A high-performance thermoplastic polymer sold under the trade name KYNAR®. Offers excellent resistance to oxidizing agents, halogens, most acids and bases, and chlorinated solvents. Maximum operating temperature: 250°F. Used for plastic tank eductors in aggressive chemical service.
S
Secondary Fluid
The fluid that is drawn in (entrained) and pumped by the jet pump. Also called the suction fluid or entrained fluid. The secondary fluid enters through the suction port and mixes with the motive fluid in the throat.
Stratification
The separation of a fluid into layers of different concentration, temperature, or density. Common in storage tanks when contents are not agitated. Tank eductors prevent stratification by continuously circulating and mixing the tank contents.
Suction Lift
The vertical distance below the jet pump inlet from which fluid must be lifted. Standard liquid jet pumps can achieve suction lifts up to 27 feet (approximately 11.7 PSI vacuum). Suction lift is limited by the vapor pressure of the liquid and atmospheric pressure.
T
Throat
The narrow section of a jet pump between the nozzle and the diffuser where the motive and secondary fluids mix. The throat is where momentum transfer occurs — the high-velocity motive fluid imparts energy to the slower secondary fluid.
Turnover Rate
In tank mixing applications, the number of times per hour the entire tank volume is circulated through the eductor system. A higher turnover rate means more thorough and rapid mixing. Typical recommendations are 3–6 turnovers per hour for most applications.
V
Vacuum
Pressure below atmospheric pressure. Expressed as inches of mercury (in Hg) vacuum, or as absolute pressure (PSIA). Jet pumps using steam or air as motive fluid can achieve vacuum levels down to 5 inches Hg Absolute.
Venturi Effect
The reduction in fluid pressure that occurs when a fluid flows through a constriction (narrowing). Named after Italian physicist Giovanni Venturi. The Venturi effect is the fundamental principle behind jet pumps and eductors — the constriction (nozzle) creates a low-pressure zone that draws in the secondary fluid.
Venturi Pump
Another name for a jet pump or eductor. Refers to the Venturi effect that drives the pumping action.

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