Industrial furnace combustion system with air preheat and burner safety sequence

01

Value the combustion-air preheat at your exhaust temperature

DOE's Process Heating Tip Sheet #1 (2007) tabulates percent fuel savings from preheated combustion air for natural gas at 10% excess air, quoting the IHEA Combustion Technology Manual. It describes recuperators as gas-to-gas exchangers on the stack and regenerators as paired heat-storage beds that alternate between flue gas and air, needing at least two regenerators and their burners for uninterrupted firing. [1]

The tip sheet treats processes above 1,600 °F as good candidates for preheat and those below 1,000 °F as hard to justify, while noting that justification rests on energy saved, not temperature, and that dirty or corrosive exhaust needs an exchanger chosen for it. [1]

Editorial calculation: a furnace burning 10 MMBtu/h with a 1,600 °F exhaust and 800 °F preheated air saves 22%, or 2.2 MMBtu/h; over 6,000 operating hours that is 13,200 MMBtu a year. Firing rate and hours are assumed examples. [1]

Give the supplier the furnace type and temperature, heated zones and their firing rates, fuel and its supply pressure, exhaust temperature and cleanliness, required turndown, emission limits, available space for preheaters and fans, and the control interface (PLC or fieldbus).

02

Match the burner type to the furnace

NPA's combustion-system page lists recuperative burners for W- and U-shaped radiant tubes that preheat air to over 650 °C, with a stated energy saving of 20–30% and NOx below 150 mg/Nm³; self-preheating burners with turndown up to 1:15 and flame speed of 60–150 m/s; open-flame burners from 15 to 5,000 kW with ionisation or UV flame detection; and regenerative burners of 400–1,500 kW for application temperatures up to 1,350 °C, with a stated thermal efficiency above 90%. [2]

The same page describes a burner controller with PROFIBUS communication, integrated ignition transformer and flame detector, pulse-fired or flow-regulated combustion, lockout alarm on flame failure, remote and local ignition and reset, and over-temperature and pressure interlocks intended to extend radiant-tube life. [2]

Editorial calculation: the 20–30% saving NPA states for its recuperative burners sits inside the 17–30% DOE tabulates for 600–1,200 °F preheated air on a 1,600 °F exhaust; 650 °C is about 1,200 °F. [2] [1]

03

Check the start-up sequence, purge and safety times

Siemens' data sheet for its LMO39 burner controls (type-tested to DIN EN 298) defines the start-up timings: waiting time, prepurge time t1, preignition time, safety time at start-up TSA, interval to the second fuel valve and postpurge. Listed versions carry a TSA requirement of 5 s maximum and prepurge requirements of 20 or 30 s minimum; because parameterised times carry ±3% tolerance, maximum times must be set at least 3% lower and minimum times at least 3% higher. [3]

Siemens' worked example sets a 5 s safety time by parameterising no more than 4.85 s (the next lower step is 4.704 s). The controls also supervise the air pressure switch with a function check during start-up and limit restarts after loss of flame. [3]

US Coast Guard rules for automatic auxiliary boilers in 46 CFR part 63 require a low-fire interlock for low-fire start and forbid automatic increase of air flow after an emergency safety trip (§ 63.20-1), and for small units of 400,000 Btu/h (117 kW) or less require a prepurge of at least four air changes of the combustion chamber and stack and not less than 15 seconds (§ 63.25-1). [4] Editorial calculation: if a furnace and flue hold 40 m³ and the purge fan delivers 8,000 m³/h, four air changes take 160 m³ ÷ 8,000 m³/h = 72 s, so the volume rule, not the 15 s floor, sets the purge. Volume and fan rate are assumed examples. [4]

04

Recuperative, self-preheating, open-flame and regenerative burners with heat exchangers and controllers

The listing reviewed covers recuperative, self-preheating, open-flame and regenerative burners, high-temperature heat exchangers and burner controllers for metallurgical furnaces.

05

Burners, air preheater, combustion-air fan, fuel train, burner controller and flame detector

06

Burner rating per zone, preheat savings, NOx guarantee and purge timings for a furnace combustion system

Ask for the basis of each efficiency and NOx figure: the DOE savings table assumes natural gas at 10% excess air, and NPA's NOx value is stated for its recuperative burners. [1] [2]

07

Send the furnace, fuel and control data together

Paste it into your enquiry below or into an email to a maker, then replace every bracket.

Furnace: [type; temperature; zones; firing per zone]
Fuel: [type; supply pressure]
Exhaust: [temperature; cleanliness]
Burners: [type; turndown; NOx limit]
Preheat: [recuperator or regenerator; air temperature]
Control: [purge; safety time; detection; interface]

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