The Steam Injector: Feeding a Boiler With Its Own Steam

Giffard's US drawing shows the whole injector in section, and every part has one job: - **The steam cone** — steam from the boiler enters and leaves through a converging nozzle as a fast jet. Its tip can be moved on guides by the handle, to regulate how much steam goes through. - **The combining cone** — the jet passes through, surrounded by feed water that has been drawn up from the tank. Here the steam condenses into the water. - **The gap and the overflow** — between the combining and delivery cones is an open gap. Giffard: *"H in the drawing represents the overflow pipe"*, and *"openings or windows K may be formed"*, covered with glass, to watch the state of the jet. While starting, water and steam spill out of the overflow; when the injector 'picks up', it falls silent. - **The delivery cone** — a diverging passage in which the fast water slows down and its speed turns into pressure. - **The check valve** — Giffard's *"small valve interposed between the boiler and the jet pipes"*, so the boiler cannot empty back when the injector is shut off. Match each item to the lettered parts on the drawing. The same four passages — nozzle, mixing throat, diffuser, check valve — are in every jet pump, steam or not.

The water-jet aspirator is the injector with cold water as the driving jet instead of steam. There is no condensation, so it cannot pump into a pressure higher than its own supply — but it has the same nozzle, the same throat and the same diffuser, and it makes a real vacuum. Print it in PETG as one piece with three connections: - a **hose inlet** leading to a converging nozzle with a 2 mm bore at its tip; - a **side port** into the chamber around the nozzle tip — the suction; - a **throat** of 3 mm bore starting 1–2 mm beyond the nozzle tip, 15 mm long, opening into a **diffuser** that widens at a shallow angle (about 7 degrees total) to 10 mm. Print it vertically with the nozzle at the top, at a fine layer height, and ream the nozzle and throat with a drill bit held in the fingers. The throat must be smooth and concentric with the nozzle — a jet that strikes the throat wall off-centre makes no vacuum at all. Seal the threads with PTFE tape.

Connect the aspirator to a garden hose on a tap, with its diffuser discharging into a sink or bucket. Connect the side port to a vacuum gauge through a short length of clear vinyl tube, clamped at both ends. Turn the tap fully on and read the gauge as it settles. Now close the tap part way and read again: the vacuum falls with the jet speed, the way the injector's delivery pressure does. Then the important test. Run it once on cold water and once on warm water from the hot tap, with the thermometer in the discharge. The warm run cannot reach the same vacuum, however fast the jet. Near the limit the water in the suction line boils at room temperature: you will see bubbles form in the clear tube. **No water jet can pull a vacuum below the vapour pressure of the water it is made of** — about 2.3 kPa absolute at 20 °C, over 7 kPa at 40 °C. Finally, use it as a pump: put the suction tube into a bucket set half a metre below the aspirator and watch it lift water. This is a jet pump, the injector's exact job, with a different driving fluid.

**US 27,979, 'Feed Water Apparatus for Steam Boilers', Henri Giffard of Paris, patented 24 April 1860.** Giffard was an engineer who had flown a steam-powered airship in 1852. The injector is widely dated to his French patent of 1858; the US patent that followed is the one verified here, and it prints its own operating limit — the sentence about water condensing steam only below 100 °C. William Sellers & Co. of Philadelphia made injectors under licence in the United States from 1860, and it became standard equipment on locomotives: a fitting with no moving parts that fed the boiler whether the engine was running or not. The same nozzle-throat-diffuser layout is the ejector used to pull vacuum on steam condensers and chemical plant, the jet pump down a water well, and the laboratory aspirator built in step 3. **Honest limits.** Hot feed water defeats it, as the notebook shows. It is fussy to start and must be adjusted to the boiler pressure. As a pump it is inefficient — but on a boiler that does not matter, because the heat it seems to waste goes into the feed water, and so back into the boiler. **Do not build a steam injector.** It works only with a pressurised boiler, which is a vessel that can kill; boilers are built, tested and inspected under regulation for that reason.