{{commonscat, Express boilers
Express boilers were
water-tube boiler
A high pressure watertube boiler (also spelled water-tube and water tube) is a type of boiler in which water circulates in tubes heated externally by the fire. Fuel is burned inside the furnace, creating hot gas which boils water in the steam-gene ...
s with small diameter tubes (typically 2" or less), so as to have the greatest ratio of heating area to water volume.
Other "large tube" boilers, such as the
Stirling boiler, had tubes of 3" and over.
Water-tube boilers
Steam boilers
Boilers