Field-tube boiler
A Field-tube boiler (also known as a bayonet tube) is a form of water-tube boiler where the water tubes are single-ended. The tubes are closed at one end, and they contain a concentric inner tube. Flow is thus separated into the colder inner flow down the tube and the heated flow upwards through the outer sleeve. As Field tubes are thus dependent on thermo-syphon flow within the tube, they must thus always have some vertical height to encourage the flow. In most designs they are mounted near-vertically, to encourage this. They are named after Edward Field, their inventor, and were originally developed for steam fire-engines where speed of raising steam was important. More recently, Field-tube boilers have been most widely used by French manufacturers, either for small vertical boilers, or as a marine boiler. The use of this type of Field tube has also been researched for the transfer of heat from nuclear reactors, vacuum condensers and heat exchangers. The thimble-tube boiler has some similarities, in that its water-tubes are also single-ended. The simpler thimble tube does not have the inner tube of the field-tube, therefore its flow is mixed, turbulent and dependent on random boiling. For this reason, thimble