Differences Between Direct and Indirect Fire Heaters

February 14, 2019

Available in two forms, furnace vessels are manufactured as either direct fired or indirect fired heaters. Now, everyone can recognize a direct fired unit when it's seen in use. It's the model that comes attached to a burner assembly. A burner-equipped boiler fits tidily under this label, one that's sometimes abbreviated. Indeed, expect to see this vessel type described as a "fired heater." As for how they differ from indirect fire technology, we need to talk about heat exchangers and how they add a third stage to their system architecture.

Heat Exchanger Fueled

Correct, indirect fire vessels are not placed in front of a naked flame. There's no soot-covered burner chamber and no air-to-fuel ratio to bear in mind. A heat exchanger replaces the fired burner chamber, so there's an additional nest of tubes to maintain. True, a burner stage is likely working away somewhere else, perhaps in an entirely different room, but the energy produced by the fuel and flame is transmitted through the heat exchanging tube stacks and shell. Advantageously, the thermal energy generated in an unfired system is pollution free. The heat exchanger translates and transmits the energy, and the burner combustibles are left elsewhere, perhaps back at some dirty equipment plant room.

Direct Fired Pros and Cons

More efficient and smaller in size, a burner unit sits right next to a fired heater. Gas or liquid fuel, electric elements or coal, the fuel source sits nearby. Gas-to-air ratios are adjusted while burner nozzles and ignition electrics regulate a naked flame. There are exhaust fumes to dispose of and flame retarding materials to incorporate. Of obvious importance here, the energy created by the fire is immediately converted into vessel heating power. From a vessel designer's perspective, direct fired heaters experience larger energy transients. The flame blackens a heat resistant steel, causes material expansion and contraction episodes, and there's that nasty exhaust to channel away from the vessel.

Expect a more responsive thermal load when a fired heater aims its burning fuel straight at a containment vessel. The energy responsively regulates complex chemical catalyzing processes, so expect to see them used in oil refineries. Additionally, primary boilers, those rugged plant room behemoths tend to defer to direct fire solutions. Switching to indirect fired appliances, their heat exchangers function more uniformly, probably because they use a wide-area heat exchanger stack, not a focused flame. Seen in smaller heating systems and places where combustible emissions cannot be tolerated, the efficiently channeled heat exchanger output warms homes, supplies thermal energy for food and pharmaceutical manufacturing lines, and keeps its burner located far away while it carries out its duties.

Contact Details

Fusion - Weld Engineering Pty Ltd
ABN 98 068 987619

1865 Frankston Flinders Road,
Hastings, VIC 3915

Ph: (03) 5909 8218

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