The earliest form of fire-tube boiler was Richard Trevithick's "high-pressure" Cornish boiler. This is a long horizontal cylinder with a single large flue containing the fire. The fire itself was on an iron grating placed across this flue , with a shallow ashpan beneath to collect the non-combustible residue. Although considered as low-pressure (perhaps 25psi) today , the use of a cylindrical boiler shell permitted a higher pressure than the earlier "haystack" boilers of Newcomen's day. As the furnace relied on natural draught (air flow) , a tall chimney was required at the far end of the flue to encourage a good supply of air (oxygen) to the fire.
For efficiency , the boiler was commonly encased beneath by a brick-built chamber.Flue gases were routed through this , outside the iron boiler shell , after passing through the fire-tube and so to a chimney that was now placed at the front face of the boiler.