A General Guide to Fired Heater Design and Operation

1. Introduction

This guide is intended to assist process engineers in the selection, design, specification, and troubleshooting of fired heaters (also known as furnaces or direct-fired heaters) used in the refining and petrochemical industries.

It covers basic process heaters but excludes specialized units like ethylene crackers (pyrolysis furnaces), thermal oxidizers, and incinerators.

2. Heater Types and Configurations

Fired heaters are classified by their radiant tube orientation (horizontal or vertical) and their overall configuration. Common types include:

2.1 Radiant vs. Convection Sections

• Radiant Only: Lower capital cost but lower thermal efficiency (50-70%).
• Radiant + Convection: Higher capital cost but much higher thermal efficiency (70-95%) by recovering heat from exhaust gases.
• Convection Only: Specialized heaters with no radiant section. Lower flux, less risk of hot spots, but requires larger surface area and often uses forced draft fans.

3. Combustion and Draft

3.1 Fuels

• Fuel Gas: Refinery fuel gas is a mixture of H₂, C₁-C₄ hydrocarbons, and potentially heavier components. Hydrogen content can vary from 0% to >90%, which can cause flashback issues in pre-mix burners. Impurities like sulfur, chloride, and sodium must be minimized to prevent corrosion and refractory attack.
• Liquid Fuels: Must be atomized (using steam, air, or high pressure). Viscosity should be maintained between 28-38 cP (150-200 SSU). Vanadium and sulfur content are critical concerns due to high-temperature corrosion from vanadium slag and acid dew point corrosion from SO₃.

3.2 Draft Systems

• Natural Draft: Most common. Relies on stack height to create draft. Simple and reliable but susceptible to ambient weather changes affecting air-to-fuel ratio.
• Forced Draft (FD): Uses a fan to supply combustion air. Allows for better control, smaller burners, and is required for pre-heated air systems.
• Induced Draft (ID): Uses a fan to pull flue gas out of the heater. Typically used when complex convection sections create high pressure drop that a reasonable stack height cannot overcome.
• Balanced Draft: Uses both FD and ID fans. Offers the best control of box pressure and air/fuel ratio.

4. Heater Tube Design

4.1 Material Selection

Tube material is primarily selected based on the maximum tube wall temperature (per API 530). Special process conditions like hydrogen service, sour (H₂S) service, or carburization risk may dictate higher-grade alloys regardless of temperature.

4.2 Fluid Velocity and Pressure Drop

Maintaining high mass velocity inside the tubes is critical to:

• Maximize the inside heat transfer coefficient (cooling the tube wall).
• Minimize the fluid film temperature (reducing coking/fouling risk).

Recommended Mass Velocity: 1,220 – 1,710 kg/s·m² (250 – 350 lb/s·ft²) for liquid or vaporizing services.

Turndown: At turndown (e.g., 60% flow), mass velocity should be maintained above 730 kg/s·m² (150 lb/s·ft²). This may require pass recycling or steam injection.

4.3 Heat Flux (Radiant Rate)

Average radiant heat flux is a key design parameter balancing capital cost (higher flux = smaller heater) against tube life (higher flux = hotter tubes = shorter life).

• General Guideline: ~37,800 W/m² (12,000 Btu/hr·ft²) for single-fired heaters.
• Double-Fired: Can be higher, up to ~56,800 W/m² (18,000 Btu/hr·ft²), due to more uniform heating.

5. Operation and Control

5.1 Basic Process Control

• Firing Rate: Controlled by the process fluid outlet temperature.
• Draft: Controlled by the stack damper, typically to maintain -0.1 inches water column (-2.5 mmWC) at the arch (bridgewall).
• Excess Air: Monitored via O₂ analyzer at the arch. Typically targeted at 2-3% O₂ (equivalent to ~10-15% excess air) for gas firing.

5.2 Burner Management System (BMS)

The BMS is a safety system, completely separate from the process control system. It must NOT be used to modulate firing rate. Its sole purpose is to safely start up, monitor, and shut down the heater. Key mandatory trips include:

• Low-low fuel gas pressure
• High-high fuel gas pressure
• Loss of flame (if scanners are provided)
• Loss of combustion air (FD/ID fan trip)
• Low-low instrument air pressure

6. Troubleshooting Common Problems

6.1 Hot Tubes (Hot Spots)

• Internal Fouling (Coke): Coke acts as an insulator on the inside of the tube. To maintain the same process outlet temperature, the firing rate must increase, raising the tube metal temperature (TMT). If hot spots are widespread, fouling is likely.
• Flame Impingement: If hot spots are localized, check for flames actually touching the tubes. This is caused by misaligned burners, plugged burner tips, or insufficient draft pulling flames into the tube bank.
• Flow Imbalance: In multi-pass heaters, one pass may have less flow than others due to fouling or poor manifold design. It will run hotter while others run cooler.

6.2 Draft Problems (Positive Pressure)

A heater should always run with negative pressure (draft) in the firebox. Positive pressure is dangerous as it can push hot flue gas out through inspection ports, damaging equipment and posing a severe personnel hazard.

• Insufficient Draft: Usually caused by a fouled convection section, a stack damper that has failed closed (or fallen off its shaft), or simple over-firing beyond the heater's design capacity.
• Air In-leakage (Tramp Air): If the stack O₂ is high but the arch O₂ is low, air is leaking into the convection section. This "steals" stack draft capacity without helping combustion in the firebox. Look for open header boxes, damaged casing, or open peep doors.

6.3 High CO or NOx Emissions

• High CO: Indicates incomplete combustion.
  • Cold Firebox: Common during startup.
  • Lack of O₂: Burners are starved of air (fuel-rich). Check O₂ at the *arch*, not just the stack, to ensure the air is getting to the flames.
• High NOx: Indicates high flame temperatures.
  • Excess Air: Too much O₂ can actually increase NOx in some burners.
  • Flame Interaction: Flames from adjacent burners merging into one large, hot "fireball."
  • High Air Preheat: If an air preheater is used, excessively hot combustion air will increase thermal NOx.