Fundamentals

Chapter from the Fired Heaters Design Guide.

FIRED HEATER FUNDAMENTALS

Heater Section Overview

Section	Function	Heat Transfer Mode	Typical Duty Split	Temperature Range	Key Design Parameter	API Reference
Radiant Section	Primary heat absorption	Radiation (dominant) + convection	60-75% of total duty	Flame temp: 2,500-3,500°F	Average radiant flux (BTU/hr·ft²)	API 560 Section 5
Convection Section	Secondary heat absorption from flue gas	Convection (dominant) + some radiation	20-35% of total duty	Flue gas: 1,200-1,800°F inlet	Extended surface area, fin type	API 560 Section 5
Shield Section	First 2-3 convection rows (see radiant flux)	Radiation + convection	Included in convection	Highest flue gas temperature	Bare tubes (no fins), radiant shielding	API 560
Stack	Draft generation, flue gas disposal	N/A	Heat loss to atmosphere	300-700°F typical stack temp	Stack height and draft	API 560 Section 5

Key Heater Design Parameters

Parameter	Definition	Typical Range	Impact of Exceeding	How to Check
Average Radiant Flux	Total radiant duty / total radiant surface	8,000-12,000 BTU/hr·ft²	Hot spots, coking, tube failure	Duty / surface area from data sheet
Maximum Radiant Flux	Local peak flux (typically 1.5-2× average)	Design limit on data sheet	Localized overheating, short tube life	Estimated by flux factor
Bridgewall Temperature	Flue gas temp leaving radiant section	1,400-1,800°F typical	High stack temp, convection damage	Measure or calculate from O2/duty
Excess Air	Air above stoichiometric for complete combustion	15-25% (3-5% O2 dry basis)	Low excess: CO, soot; High: heat loss	Stack O2 analyzer
Stack Temperature	Flue gas temp at stack exit	300-700°F	Low: acid dew point corrosion; High: energy loss	Stack thermocouple
Draft	Negative pressure driving flue gas flow	-0.1 to -0.5 inWC (natural draft)	Low draft: poor combustion; High: excess air	Draft gauge at various locations
Tube Metal Temperature	Maximum tube wall temperature	Per metallurgy limit	Creep, oxidation, failure	Skin thermocouples or IR

Source: Fired_Heaters_Design_Guide_v1.xlsx · sheet “Fundamentals”

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