The Edwards wet air pump uses a solid, conical-bottomed bucket and radial inlet ports instead of foot and bucket valves. Near the bottom of the downstroke, the ports admit the condenser charge above the bucket. The rising bucket closes those ports and sends the trapped charge through delivery valves at the head.
Edwards Air Pump Interactive Calculator
Estimate gross bore sweep and compare a factor-adjusted flow with demand. Count complete pumping cycles and use the same volume conditions for capacity and demand.
Equation Used
- One delivery event per complete pumping cycle.
- Gross circular bore sweep.
- Supplied effective volume factor represents the intended capacity basis.
- Demand and estimated capacity use matching pressure, temperature and volume definitions.
- No pressure, two-phase or valve-dynamic calculation.
Gross volumetric estimate, not a pressure, vacuum or two-phase flow solver. The effective factor is supplied by the user.
Follow the bucket, ports and delivery valves
The condenser inlet joins the lower casing and annular space around the barrel. As the solid bucket descends with the delivery valves shut, the space above it expands. Near the bottom, the bucket uncovers the side ports and the charge enters above its flat upper face.
The conical underside displaces liquid from the base through the surrounding passages. During the following upstroke, the bucket closes the ports and traps the charge above it. Discharge then takes place through the head valves into the upper chamber and hotwell connection.
The drawing separates the lower inlet from the upper delivery path. The bucket is solid; admission takes place through the side ports rather than a foot valve. The extended rod gland and the upper delivery discs are shown separately.
The external drive is omitted. The bucket and its constant-length rod reciprocate together using an illustrative sinusoidal motion. Head-valve opening late in the upstroke is a timing illustration, not a calculation of the pressure needed to open it. Fluid shading identifies spaces rather than predicting a water level.
Estimate displacement from bore and travel
Enter barrel bore, bucket travel and the number of complete pumping cycles each minute. One cycle includes a downstroke and an upstroke, with one delivery event. Do not enter the sum of both one-way strokes as the cycle count.
The calculator reports the gross bore-swept volume and gross displacement rate. A supplied effective volume factor converts that gross rate to an estimate for comparison with the entered demand.
The factor must suit the real pump and the chosen volume basis. It is not a measured efficiency inferred from the drawing. Rod displacement, port timing, clearance and gas re-expansion can make useful capacity different from gross bore sweep.
Gross swept volume and flow comparison
For bore D and travel S in inches, gross volume per complete cycle is V=πD²S/(4×1728) cubic feet. Gross displacement rate is Qg=VN, where N is complete cycles per minute.
With supplied effective volume factor ηv expressed as a fraction, the adjusted estimate is Q=ηvQg. The relative margin against a positive demand Qload is 100(Q−Qload)/Qload percent. Negative values indicate that this estimate falls below the entered demand.
This calculation uses gross bore-swept volume. It does not explicitly subtract rod area or solve the period for which the side ports remain open. The supplied factor must account appropriately for the distinction between gross sweep and the capacity being estimated; no generic percentage is guaranteed.
Compare Q and Qload at the same pressure, temperature and fluid-volume definition. A gas volume at condenser suction conditions is not interchangeable with the same numerical volume at atmospheric or standard conditions. The model does not solve mixed air, vapour and liquid behavior or condenser vacuum.
Default displacement check
A 24 inch bore and 18 inch travel give a gross sweep of about 4.71239 ft³ per cycle. At 45 complete cycles per minute, gross displacement is about 212.058 ft³/min.
Using the supplied 80% factor gives an adjusted estimate of about 169.646 ft³/min. Compared with 150 ft³/min on the same volume basis, the arithmetic margin is about 13.10%.
Doubling complete cycles per minute doubles both flow estimates without changing swept volume per cycle. Changing the demand changes the margin, not the pump geometry or its estimated output. At zero cycles per minute, flow is zero and the margin is −100% against any positive demand.
These results are a volumetric comparison. They do not establish a vacuum reading, determine leakage or provide a validated selection for a condenser installation.
Model limits
The animation links port exposure to bucket position, but does not calculate pressures, valve inertia, two-phase flow, leakage or heat transfer. Its port heights and valve timing must not be read as manufacturing dimensions.
Bore and travel are drawn to a common relative scale so their changes are visible. Other components retain schematic proportions. Adjusting the effective volume factor changes the flow estimate, not the illustrated metal dimensions.
Vacuum performance and maintenance requirements depend on the actual pump and condenser installation. A positive calculated margin does not prove that a real pump will achieve a particular condenser vacuum.
Edwards pump questions
Why is there no valve in the bucket?
The Edwards arrangement uses a solid bucket and side ports. The illustrated charge enters above the bucket through those ports.
Are the side ports moving valves?
No. They are openings in the barrel. The bucket covers or uncovers them as it travels.
Where does the charge leave?
Through the head delivery valves into the upper chamber and hotwell connection.
Does 45 cycles per minute mean 45 strokes?
Here it means 45 complete down-and-up cycles, or 45 delivery strokes. Counting both one-way strokes would give 90.
Why does changing demand not move the bucket?
Demand is a comparison value. Bore, travel and cycle rate define the displacement estimate; demand changes the margin.
Does the calculator predict condenser vacuum?
No. It only compares a factor-adjusted volumetric estimate with a demand on the same volume basis.
Historical references and model basis
Science Museum Group: Edwards air pump section model, 1904–13 identifies the solid conical bucket, radial ports, head valves and pumping sequence.
William Rogers, Pumps and Hydraulics, Part Two, figures 358–360, provides historical section drawings of the component arrangement. Historical performance claims are not adopted as ratings for this calculator.
The numerical model is the explicitly defined gross cylinder sweep, cycle-rate conversion and supplied effective volume factor. The drawing is an explanatory section, not a manufacturing drawing.
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