An automatic water ejector combines a jet-pumping device with level control. The illustration shows motive water entraining liquid from a sump, with a float controlling the motive supply. The calculator uses an entered pumped flow and run time to find swept sump volume and the area needed for a selected level change. It does not calculate the ejector’s pumping capacity.
Automatic Water Ejector Interactive Calculator
Vary ejector flow, pump time, and float hysteresis gap to size the sump volume swept between ON and OFF levels.
Equation Used
The volume calculation assumes steady sump outflow and no incoming water during pump-down. Dividing that volume by the selected high-to-low level gap gives the area of a straight-sided sump. Actual switching levels depend on the equipment.
- Pumped flow is steady while the ejector is ON.
- The sump has straight sides so volume equals plan area times level change.
- The level gap is a selected geometric input; no universal switching gap is specified.
3600 divided by pump-down time excludes refill time and is not a complete cycle rate. The calculator sizes swept volume and sump area, not ejector hydraulics. The float gap is a selected input, not a universal minimum.
Jet pumping with level control
The motive nozzle produces a fast water jet. Sump water enters through the suction connection and mixes with the motive stream; the combined flow leaves through the discharge passage. The float and valve start and stop this process between selected liquid levels.
The basic eductor has no rotating impeller, but the automatic assembly still has moving control parts. The drawing simplifies their operation into high and low thresholds rather than showing a complete snap-valve mechanism.
Reading the calculation
Pumped Flow refers to water removed from the sump, not motive-water consumption or the combined discharge. The entered value must be established for the real equipment and operating conditions. If motive water is supplied from outside the sump, its volume is additional at the discharge.
The animated refill provides a repeating visual sequence. No inflow rate is entered, so the tool cannot calculate refill time or how often a complete pumping cycle occurs.
Relating the drawing to real equipment
Schutte & Koerting’s Fig. 2645 automatic eductor is a documented example of a sump-draining jet device with float and snap-valve control. Its manufacturer provides separate operating and capacity data. The drawing here is representative, not a dimensional model of that product.
For an actual installation, the fluid properties, motive supply, suction conditions and discharge conditions must be considered together. A fixed flow entered in this volume calculator is not an entrainment-ratio prediction or a guarantee against overflow.
Changing the motive medium requires equipment intended for that medium; steam, compressed air and water do not share a single universal capacity curve. Supply protection, discharge handling and access arrangements also need assessment for the particular installation.
Swept volume and sump area
Let Qp be pumped sump flow in liters/hour, t be pump-down time in seconds, and h be the high-to-low level gap in millimeters.
- Swept volume V = Qp × t/3600 liters.
- Sump area A = (V/1000)/(h/1000) m².
- Pump-only rate = 3600/t per hour.
The two factors of 1000 convert liters to cubic meters and millimeters to meters. The gap output simply repeats the selected input. Pump-only rate counts how many run-time intervals fit in an hour; it excludes refill and switching delays.
The equations assume no incoming water while the level falls. If constant inflow Qin continues, net removal is Qp − Qin and actual level change follows that difference. Qin is not an input in this tool, so it must be accounted for separately. If inflow equals or exceeds outflow, this simple pump-down assumption fails.
For a sump with changing cross-section, volume must be integrated over the level range; one constant plan area is insufficient. The tool does not size the nozzle, calculate suction lift or predict motive-flow consumption.
Worked example: an 8-second pump-down
Choose Qp = 1500 L/h, t = 8 seconds and h = 75 mm. The swept volume is 1500 × 8/3600 = 3.3333 L. For a straight-sided sump, A = 0.0033333/0.075 = 0.044444 m².
The pump-only rate is 3600/8 = 450 per hour. This is not 450 complete cycles per hour: the sump must refill and the control must switch between runs.
At t = 16 seconds, volume doubles to 6.6667 L. Keeping the 75 mm gap doubles the calculated area to 0.088889 m² and halves pump-only rate to 225 per hour.
With the original 8-second interval but h = 150 mm, swept volume stays 3.3333 L and area halves to 0.022222 m². This compares sump geometry; it does not recommend either gap as a valve setting.
If 500 L/h also enters during the original pump-down, only 1000 L/h is removed net. Over 8 seconds the level-change volume would then be 2.2222 L, showing why the no-inflow assumption matters.
What this sizing step does and does not answer
| Question | Needed information |
|---|---|
| How much sump water leaves during a run? | Actual outflow and run time. |
| How far does the level fall? | Net outflow after inflow and sump cross-section. |
| How often does the unit restart? | Refill rate, switch behavior and high-to-low volume. |
| Can the ejector achieve the entered flow? | Manufacturer performance data for the selected supply, suction and discharge conditions. |
| What leaves the discharge? | The suction flow plus motive fluid and any relevant phase or mixing effects. |
Service life, efficiency, solids tolerance and maintenance cannot be inferred from a three-input volume calculation. Comparisons with centrifugal or diaphragm pumps require the same duty conditions and appropriate product data.
Automatic-ejector questions
No. Pumped flow is an input. The output describes swept volume and sump area for that assumed flow.
No. It is an example input. The actual high and low switching levels must suit the specific control and installation.
It means water removed from the sump. Motive water supplied separately also leaves in the mixed discharge.
Then use net removal to evaluate level change. This calculator has no inflow input, so its swept-volume result assumes no simultaneous inflow.
It includes only the selected pump-down interval. A full cycle also includes refill and control timing.
Only equipment designed and specified for the intended medium should be used. This model cannot establish compatibility or predict a different medium’s performance.
No. Supply, suction and discharge conditions or equipment faults can affect performance. The calculator cannot diagnose a cause or supply repair tolerances.
No. It represents switching at two liquid levels. Detailed latch and valve geometry is not modeled.
Manufacturer references
- Schutte & Koerting, Bulletin 2M: Water Jet Eductors, describes construction and the Fig. 2645 automatic assembly.
- Schutte & Koerting, Bulletin 2M Supplement, supplies product capacity data to use with the equipment documentation.
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