Jet Pump (Eductor)

How a jet pump works

A jet pump has no impeller, no piston, no rotor — nothing inside moves. Yet it pumps. The trick is to spend a little high-pressure fluid to move a lot of low-pressure fluid, using momentum exchange instead of mechanical displacement.

Trace the path of the working ("motive") fluid through the four sections that make up every eductor:

• Converging nozzle. Pressurized motive fluid — well water at 200–400 kPa, plant steam, or shop air — enters and is squeezed through a narrowing nozzle. By Bernoulli's principle, the falling pressure buys rising speed: a liquid nozzle commonly exits at 15–30 m/s, while a steam nozzle (a converging–diverging de Laval shape) can go supersonic, past 1,000 m/s.
• Suction chamber. The jet bursts out of the nozzle into a chamber connected to the fluid you actually want to move. The fast jet's static pressure here is low — often below atmospheric — so the suction fluid is pushed in by whatever pressure is behind it (atmosphere, a flooded sump, a tank head).
• Mixing throat. Jet and suction fluid travel together down a constant-area tube. Turbulent shear between the fast core and the slow surroundings drags the suction fluid up to speed. Momentum is conserved: the jet slows down, the entrained fluid speeds up, and the two blend into one stream at an intermediate velocity.
• Diverging diffuser. The mixed stream enters a gently widening cone, slows down, and — Bernoulli in reverse — converts its velocity back into static pressure. The mixture leaves the discharge at a pressure higher than the suction fluid had on the way in. That pressure rise is the pumping.

The whole device is a momentum machine. Nothing is "sucked" in the magical sense — atmospheric or static pressure pushes the suction fluid into a region the jet has made low-pressure, and the jet then hands over its momentum in the throat.

The governing engineering

The two numbers that define a jet pump are the entrainment (mass) ratio and the area ratio. Start with the conserved quantities.

Symbols
  ṁ_m  motive mass flow          ṁ_s  suction (entrained) mass flow
  V_m  motive jet velocity       V_s  suction velocity (~0)
  A_n  nozzle exit area          A_t  throat area
  P_s  suction pressure          P_d  discharge pressure   P_m  motive pressure

Entrainment (mass) ratio
  M = ṁ_s / ṁ_m              liquid-liquid: ~1 to 3
                             steam vacuum:  often < 1

Momentum balance in the throat (idealized, 1-D, incompressible)
  ṁ_m·V_m  =  (ṁ_m + ṁ_s)·V_mix
  ⇒ V_mix = V_m / (1 + M)     the jet always loses speed

Diffuser pressure recovery (Bernoulli, with recovery factor η_d ≈ 0.8)
  P_d − P_s = η_d · ½ ρ V_mix²

Nozzle exit velocity from motive head (incompressible)
  V_m = √( 2 (P_m − P_s) / ρ )

Efficiency is where reality bites. The eductor's useful output is the pressure energy it adds to the suction stream; its input is the pressure energy spent in the motive stream. A clean way to write it:

η = [ ṁ_s · (P_d − P_s) ] / [ ṁ_m · (P_m − P_d) ]
  = M · (P_d − P_s) / (P_m − P_d)

Single-stage liquid eductor:  η_max ≈ 25 to 35%
Steam-jet ejector (vacuum):   η often < 10%

Why so low? Because dragging slow fluid into a fast jet is intrinsically irreversible — the velocity mismatch in the throat dumps kinetic energy into turbulence and heat. There is no way to mix two streams at very different speeds without a large mixing loss. The classic Keenan–Neumann constant-area mixing analysis (1942) shows the best mass ratio and best efficiency for a given area ratio fall on a tradeoff curve: pick a small throat for high suction lift, a big throat for high entrained flow — you cannot maximize both.

Worked example: a tank-cleaning water eductor

Take a stainless eductor cleaning a tank, driven by plant water:

Given
  Motive pressure   P_m = 400 kPa (gauge)   above suction
  Suction pressure  P_s ≈ 0 kPa  (atmospheric reference)
  Fluid             water, ρ = 1000 kg/m³
  Recovery factor   η_d = 0.8
  Mass ratio        M = 1.5  (typical for this duty)

Nozzle exit velocity
  V_m = √(2 · 400 000 / 1000) = √800 = 28.3 m/s

Mixed velocity (momentum)
  V_mix = 28.3 / (1 + 1.5) = 11.3 m/s

Discharge pressure rise
  P_d − P_s = 0.8 · ½ · 1000 · 11.3² = 51 kPa

So: spend 400 kPa of motive head to deliver 51 kPa of lift to a
flow 1.5× the motive flow. That ~5 m of suction lift is plenty to
draw sludge off a tank floor with a device that never seizes on grit.

Notice the bargain: only about an eighth of the motive pressure shows up as discharge pressure, but you move 1.5 times the motive flow rate and the hardware is a hollow casting with no seals to fail. For a deep-well jet pump the same arithmetic explains why surface suction tops out near 8 m — beyond that the inlet pressure reaches water's vapor pressure and the column cavitates — so the nozzle has to go down the well.

Jet pump vs other pumps

	Jet pump / eductor	Centrifugal pump	Positive-displacement	Steam ejector
Moving parts	None	Impeller + shaft seal	Piston / gears / lobes	None
Efficiency	10 to 35%	60 to 85%	80 to 95%	< 10%
Self-priming / runs dry	Yes, indefinitely	No (cavitates dry)	Some types	Yes
Handles solids / slurry	Excellent	Limited (special trim)	Poor (wears)	Vapor only
Handles two-phase / gas	Yes	No (gas-locks)	No	Designed for it
Max practical lift	~8 m surface; deeper downhole	NPSH-limited	High	Vacuum (mbar)
Needs a motive supply	Yes (water/steam/air)	Motor only	Motor only	Boiler steam
Capital + maintenance	Very low	Moderate	Moderate to high	Low device, costly steam
Typical home	Wells, sumps, dredging, vacuum	Water supply, HVAC	Metering, hydraulics, food	Distillation, condensers

Real-world systems and examples

Application	Motive fluid	What it pumps	Notes
Shallow-well jet pump (single-pipe)	Recirculated well water	Well water	Jet at the pump; wells to ~7.5 m
Deep-well jet pump (two-pipe)	Pressurized drive water	Well water	Jet assembly down the well; 25 m+
Marine / bilge eductor	Firemain or ballast water	Bilge water, oily mix	No electrics in the bilge; intrinsically safe
Dredge / sand eductor	High-pressure water	Sand and gravel slurry	Hardened or ceramic-lined throat
Steam-jet ejector (1–6 stage)	Boiler steam	Air / vapor (vacuum)	Distillation, turbine condensers; with intercondensers
Lab water aspirator	Tap water	Air (filtration vacuum)	Vacuum limited to water vapor pressure (~23 mbar)
Aircraft fuel-tank scavenge ejector	Motive fuel from a boost pump	Fuel from tank low points	No electrics near fuel; feeds the collector tank
Tank-mixing / blending eductor	Recirculated tank liquid	Same tank liquid	One nozzle entrains ~3–5× its own flow to stir the tank

Two facts about that last row surprise people. A single recirculation eductor pointed into a tank entrains roughly three to five times its own flow, so a 100 L/min pump can turn over 400–600 L/min of tank contents — that is why eductors mix tanks far better than a bare return line. And the aircraft scavenge ejector exists precisely because nobody wants a powered pump and its electrical connections sitting in a fuel tank; a fuel-driven jet pump with no motor and no wiring removes a whole class of ignition risk.

When to use a jet pump

• You already have a pressurized fluid going to waste — firemain pressure, plant steam, a boost-pump discharge, mains water. The eductor turns that existing energy into a second pumping job for the price of a casting.
• The fluid would destroy a mechanical pump — slurry, grit, fibers, chemicals, hot condensate, or anything two-phase. No impeller or seal means nothing to abrade or chew up.
• The pump might run dry or sit idle for years — sump, bilge, and emergency-drain duties. An eductor priming itself from empty and running dry without harm is normal operation.
• You must pull or hold a vacuum on dirty vapor — vacuum distillation, condenser air removal, freeze-drying. Multi-stage steam ejectors are the workhorse here, reaching below 0.1 mbar with nothing that can seize.
• You need a spark-free, electrics-free pump — fuel tanks, flammable-atmosphere sumps, the bilge of a ship. A jet pump is intrinsically safe because it has no motor.

Reach for something else when efficiency is the priority (a centrifugal or positive-displacement pump will be two to eight times more efficient), when you have no convenient motive supply, or when you need precise metering (use a positive-displacement pump). The jet pump trades efficiency for ruggedness — pick it only when ruggedness is what you are buying.

Common misconceptions, failure modes, and pitfalls

• "It sucks fluid in." No device sucks. The jet lowers the static pressure in the suction chamber; ambient or static pressure then pushes the suction fluid in. That is why surface suction lift caps near 8 m — once the inlet pressure falls to the fluid's vapor pressure, the column boils and breaks rather than lifting further.
• "Higher motive pressure always pumps more." Past the design point, extra motive pressure mostly speeds up the jet without proportionally improving entrainment, and can drive the throat into cavitation (in liquids) or choked flow (in steam) — at which point suction flow stops responding to discharge changes entirely. Each eductor has an optimum motive pressure stamped on its curve.
• Nozzle and throat erosion. The motive jet is fast and the entrained stream is often gritty; the nozzle lip and throat see the highest velocity in the device and erode first. A worn nozzle enlarges, the jet slows and spreads, entrainment collapses. Dredge and slurry eductors use tungsten-carbide or ceramic-lined nozzles and throats and treat them as wear parts.
• Cavitation damage. If suction pressure drops too low, the liquid flashes to vapor in the low-pressure throat and the bubbles collapse violently downstream, pitting the metal (the same mechanism that destroys ship propellers). Cure: raise suction pressure, lower motive pressure to the design point, or move the unit to reduce static lift.
• Backflow when motive stops. Kill the motive supply and discharge pressure can drive flow backward through the suction line. Eductors used for vacuum or sump service need a check valve on the suction leg, or the system back-floods.
• Off-design operation. An eductor is sized for one motive pressure, one suction pressure, and one discharge pressure. Run it far off that point and entrainment falls off a cliff — there is no impeller speed to trim, so the only knob you have is the motive pressure. Re-rating means re-sizing the nozzle and throat, not turning a dial.