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.
Water tube boilers may be either straight tube boilers or bent tube boilers. The tubes of most straight tube boilers are connected into headers, which in turn are connected to the boiler drums.
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