To supply steam with higher pressures the Cornish boiler was designed. This boiler had no sharp bends and a round shape so that pressures up till 12 bar were easily handled. The boiler is based on a design Trevithick made at the end of the 18th century.
In the middle is a large tube with the fire on a cast-iron grate. The smoke will pass along the outside of the boiler as well, so that as much as possible heat is transferred from the hot smoke to the water in the boiler. Since this firetube is horizontal, the smoke will not flow automatically, a large chimney is needed to maintain sufficient draught over and through the fire.
The boiler is totally enclosed in masonry.
The boiler is very easy to handle since it has a large warmth accumulation. Therefore changes in the fire are easily accepted without large changes in pressure.
The capacity of this boiler is relatively small, so that most of the time several boilers were needed. This is a disadvantage, but the Lancashire boiler did partially solve this problem with its larger capacity.
The boiler was a great success and even today many of them are still working.