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.