The temperature of steam in boilers depends on the pressure but whilst it is in contact with the water it never exceeds the boiling point for that pressure. This is referred to as the “saturated steam” temperature. So long as the pressure and temperature remain the same the steam will not contain water and is referred to as “dry saturated steam”. As the steam travels from the boiler to the engine, however, despite insulation of the pipework etc, it will experience a drop in temperature and condensation will immediately result. Condensation will also occur when the steam comes into contact with the cooler cylinder walls and, as energy is extracted from the steam in the cylinders by performing work on the piston, a further drop in temperature will occur resulting in further condensation. Apart from the resulting loss of efficiency, the presence of incompressible liquid water in the cylinder is potentially disastrous. If it were trapped between the piston and the cylinder end-cap it could result in the end being blown off.
Whilst steam-dryers and steam-traps can remove condensed water from the pipes, and steam-jacketing can reduce cylinder condensation they do not eliminate the problem. It was realised in the 19th century that once the steam was removed from proximity with water in the boiler it could receive additional heat and its temperature could be raised above the boiling point. This is referred to as “superheating”. With sufficient additional heat, the rise in the temperature of the steam would more than compensate for the fall in temperature as it passed through the pipe-work and engine and condensation could be eliminated.
This was achieved by passing the steam through a device called a “superheater” before it reached the engines. Unfortunately, superheated steam could not be used with the organic lubricants used at the time and, since it was also found to cause damage to the type of valve seats used, after initial trials superheating was abandoned for many years. When high-flash point mineral oils and improved valve seat materials were later developed, superheating was rapidly deployed. It’s not clear when they were first used at Pleasley, but when the boiler ranges were consolidated in 1920, the new boilers were fitted with “Unit” superheaters manufactured by the Unit Superheater and Pipe Company Ltd. of Swansea, Wales and raised the temperature of the steam by 90 deg F.
The superheater consisted of a series of high-pressure steel tubes located in the hot gases leaving the flue tubes at the rear of the boiler. The superheater tubes were connected to two header pipes contained in a cast-iron box positioned above the rear down-flue. The top pipe was connected to the boiler and the bottom one to the steam main. Both headers had small drain valves to remove condensed water when the boiler was first fired after shutdown. A pressure relief valve was often fitted to the end of the inlet header.
Unlike many superheaters at that time, which had the pipes swaged into a single steam-chest, Unit superheater tubes had spigoted ends held into the inlet and outlet pipes by clamp bolts, making the design flexible and the renewal of any component a (relatively) easy job. The post 1922 boilers at Pleasley used twin units on each boiler in order to handle the large volumes of steam drawn when winding was in operation.