The documented Sellers self-acting injector uses two separate steam jets: a lifting set maintains vacuum over the feedwater inlet, and a forcing set delivers water toward the boiler. A transfer/overflow arrangement lets the injector establish and re-establish flow without treating a guessed pressure multiplier as measured performance.
Sellers Two-Jet Injector Geometry
Compare selected lifting- and forcing-steam throat geometry and the static boiler-check threshold.
Equation Used
- Selected throats are circular.
- Pressures are gauge values using one consistent reference.
- The animation is a qualitative staged schematic of the documented two-jet topology.
- Fluid dynamics, heat transfer, performance and safety are excluded.
Throat diameters and check crack pressure are selected inputs, not validated dimensions for a particular injector.
Documented two-stage topology
The Smithsonian describes one steam jet maintaining vacuum over the feedwater tube and a second jet forcing water into the boiler. The 1882 patent assigned to William Sellers likewise describes one apparatus drawing and regularly delivering water to a second forcing apparatus, with an overflow available during preliminary action.
The animation stages those connected paths qualitatively. It does not solve compressible or two-phase flow.
What the retained calculator determines
Select circular lifting- and forcing-steam throat diameters, boiler gauge pressure and an assumed boiler-check crack increment. The calculator reports both areas, their ratio and the necessary static delivery threshold.
The former user-selected recovery multiplier, delivery-pressure claim and pressure margin were removed because they did not predict injector performance.
Geometry and static check threshold
For each selected circular throat, A = πd²/4. The forcing-to-lifting area ratio is Af/Al = (df/dl)². The boiler check can begin opening only when delivery gauge pressure exceeds Pboiler + Pcrack.
Selected-input arithmetic
For 4.0 mm lifting and 5.6 mm forcing throats, areas are about 12.57 and 24.63 mm² and their area ratio is 1.960. At 150 psig boiler pressure with a selected 5 psi crack increment, the static threshold is 155 psig.
Unresolved injector performance
Selected throat areas and a static check threshold do not establish delivery pressure, capacity or restart behavior. Steam state and quality, feedwater temperature and lift, all nozzle profiles and clearances, condensation, choking, two-phase momentum transfer, diffuser recovery, losses, moving-tube or valve dynamics and manufacturer calibration are absent.
Capacity, steam consumption, pressure margin, restart limits and safe operating range therefore remain unresolved. This candidate is intentionally partial.
Questions
Does the area ratio determine performance?
No. It is selected geometry only.
Does reaching the static threshold guarantee delivery?
No. The model does not show that the injector can generate or sustain that pressure.
Can this calculator size a boiler injector?
No.
Motion-control checks for Sellers' Self-adjusting Injector Mechanism
The practical actuator question in sellers' self-adjusting injector mechanism is where the force is applied and how the moment arm changes. That is why a simple load value should be checked against the installed bracket positions before choosing stroke or force.
Where springs or counterbalances are involved, calculate the actuator force over the full travel. The highest load may occur at a different position than the most obvious one.
The actuator stroke should also be checked against the useful motion, not just against the visible travel. End clearance, hard stops, bracket rotation, and wire routing can all reduce the usable stroke in the final build.
- Calculate actuator force at the worst bracket angle, not just at mid-stroke.
- Check stroke, retracted length, and extended length against the mechanism envelope.
- Confirm the structure can accept the higher loads created by a stronger actuator.
For a FIRGELLI actuator application, run the FIRGELLI actuator force calculator, then compare available stroke, force, and mounting envelope in the FIRGELLI linear actuator range. If the goal is to replace an existing unit, use the linear actuator replacement finder before assuming the new actuator is a direct fit.
References & Further Reading
- Wikipedia contributors. Injector. Wikipedia
Building or designing a mechanism like this?
Explore the precision-engineered motion control hardware used by mechanical engineers, makers, and product designers.
