Based on use classification, fire tube boiler is categorized as for process or heating boiler. It has range pressure not exceed 450 psi (small scale or medium scale). Design fire tube is regulated in ASME BPV Section I subpart PFT. Fire tube consists of tube and tube sheet.
|Figure 1. Fire Tube Boiler|
Fire tube design use tubes to flow hot gases and distribute it to the safe point of discharge through stack. Water at the outside of tubes is heated by hot tubes, so there is heat transfer between hot gases inside tubes and water outside tubes. Water is converted to steam for process purpose.
As long as combustion process there are some passes usually used. At the first pass, the hot gases in distributed in fire tube boiler through furnace. In Figure 1, the rear head of lower section had sealed the hot gas from first pass and direct it to the tubes at the second pass to front head. The hot gases from front head is redirected to the third pass then directed again to the fourth pass and the end the hot gas is discharged through stack.