Boilers

Type:.........................Cornish (developed in Cornwall)

Maker:.......................Galloways, c. 1899

Size:...........................Length: 21ft; Diameter: 5ft 6ins Furnace tube diameter: 2ft 9ins

Capacity:...................Full: approx. 2,830 gallons (12,940 litres) Working level (mid glass) approx 1,800 gallons (8,200 litres)

Evaporation Rate:....1200 Ibs /hr (it can boil 1,200 Ibs of water each hour) 

Boiler Feed:................Original - Engine driven pump or Injector. Present - Small Duplex steam pump or Injector

Firing:..........................Original - hand firing, 1940's - Converted to mechanical stoker, 1990's - One boiler altered back to hand firing 

Flues:...........................Brick, leading to ornate chimney

Draught control:.........Vertical sliding dampers in side flues - manually operated

Explanatory notes 

Cornish Boiler:

This is a Shell type boiler, consisting of a large horizontal cylindrical shell with a cylindrical furnace tube running from end to end in the lower half. The coal fire is at the front of the furnace tube; the hot gasses pass through the tube to the back of the boiler and return under the boiler shell to the front, where they are again turned to pass along the sides of the shell, through the dampers to the chimney. This means that the water is heated as the gas flows through the furnace tube and also through about 70% of the boiler shell, making it fairly efficient, although it does not produce a lot of steam for its size. Because it has a large quantity of water inside, it steams at a fairly steady rate with no large fluctuations in pressure.

Safety Valves:

The boiler is fitted with two safety valves (originally one safety valve and a high/low water level alarm) and two water level gauges, which allow the stoker to see the water level in the boiler as this level must be kept well above the top of the furnace tube.

The original safety valve can be seen at the front top of the boiler. It is called a Deadweight safety valve as it is held closed by a large weight; when the boiler pressure reaches 90 psi, the pressure overcomes the weight and the valve opens releasing the excess steam. This is similar to the valves fitted to a pressure cooker (but a touch larger!)

A safety valve prevents the boiler exceeding its working pressure, which could occur through human error or sudden stopping of the engines whilst a full load. A second (spring-loaded) safety valve, replacing the high/low level alarm, was fitted to comply with later safety requirements.

Mechanical Stoker:

This fed coal to the furnace and reduced the manpower required to run the pumping station. There is a hopper for the coal supply, at the bottom of which is an electrically driven slow turning screw to carry the coal into a special type of grate. The electric motor also drives a fan which blows air (forced draught) into the grate to give a good rate of combustion without smoke. The boiler now in use was converted back to hand firing as this is simpler for Shrewsbury Steam Trust and much more interesting to watch - and good exercise for the stoker!

Boiler Feed Pumps:

These pump water into the boiler to replace the water that has been boiled and used as steam.

Engine driven pump:

A plunger pump driven from the engine camshaft (under the engine room floor), to pump water into the boiler whenever the engine was running (not operational at the present time.)

Injector:

Uses steam pressure passing through special nozzles to force water into the boiler. It can be used when the engine is stationary and it still works and is used regularly. The working principles of an injector can be found here.

Duplex feed pump:

This is not an original fitting, it was fitted to allow us to comply with current regulations which require two methods of supplying feed water to the boiler. The pump is steam driven and has two sets of cylinders (duplex) each of which has one steam and one pump cylinder in line with the steam and water pistons on the same rod. The piston rod of one set of cylinders controls the valves for the other.

Condenser:

By condensing the steam as it leaves the engine, a vacuum is created that allows the engine to do more work for the same amount of steam and so become more efficient. The condensed steam (which is warm distilled water) could then be pumped as feed water to the boiler, reducing scaling inside. The condenser is a cylindrical shell with small tubes running from end to end. Cold river water would be pumped through the shell round the tubes and the steam from the engine would be passed through the tubes, condensing as it cooled. It would then be extracted from the condenser by a special pump and pumped back into the boiler via the feed pump. This condenser is very unusual as it operates in the opposite way to normal practice, which is to pass the steam into the shell of the condenser, over the outside of the tubes and to pump the cooling water through the tubes.