How the Injector Works

The injector is a device for delivering feed water into the boiler. It was invented by Henri Giffard, a French engineer in 1858 and patented in the UK by Messrs Sharp Stewart & Co. of Glasgow.

There are two types of Injector:

1) Lifting Injector - This is normally fitted to the boiler backplate and creates a vacuum to lift water up the feed pipe. This type of injector can be found on older engines. The Gresham & Craven Combination Injector is a typical example.

2) Flooding Injector - This is normally located below the running plate. Water is gravity fed along the feed pipe and floods the injector.

The Flooding (or non lifting) Injector has several advantages over the lifting type:

i) Its location below water level allows gravity to flood the injector when the water valve is opened.
ii) It is not exposed to conducted or radiated heat.
iii) A maximum head of water reaches the Injector.
iv) Less risk of freezing if the tank valve is used.
v) Just as accessible as lifting type. Easier to maintain.

Principle of Operation

In its simplest form, the injector consists of three cones; the steam cone, the combining cone and the delivery cone.

Steam enters the converging steam cone where partial condensation occurs, a partial vacuum is created and pressure energy is converted into velocity energy.

High velocity steam enters the converging combining cone where complete condensation occurs as the steam comes into contact with cool water and a high vacuum is created. The combining cone greatly increases the velocity sufficient for the feed water to jump the overflow gap into the delivery cone.

Here the diverging shape of the delivery cone converts the velocity energy into pressure energy which is slightly higher than boiler pressure and allows the clack (non-return) valve to open to allow the feed water into the boiler.

How does the condensation of steam produce a vacuum? The volume of steam is 1600 times greater than the volume of water from which it was produced. When condensation occurs, steam turns back to water resulting in the vacuum.