Introduction to Fired Heaters
When starting something new, it is good practice to lay down a solid foundation to build upon and with our first post that is what we are going to do. Over the next several weeks, Scelerin will release a series of articles providing a high-level introduction to fired heaters. In this post, we define the term fired heater, identify various heater configurations and review key concepts & terminology.
Fired Heater Defined
The term “fired heater” gets used in many ways and what it actually means can get ambiguous. We define a fired heater as a direct-fired heat exchanger that uses hot combustion gases (flue gases) to raise the temperature of a process fluid flowing through the coils arranged inside the heater. This definition is consistent with that found in API Standard 560 (Fired Heaters for General Refinery Service), which states ”a fired heater is an exchanger that transfers heat from the combustion of fuel to fluids contained in tubular coils within an internally insulated enclosure.” For the purposes of this discussion direct fired heater, process heater, and furnace are considered synonymous and will be used interchangeably. Note that other types of fired equipment like fire tube, water bath, and indirect heaters do not fit within this definition.
Radiation and convection are the two primary means of heat transfer that occur inside a fired heater. They each occur in different areas of the heater appropriately named the “radiant section” and the “convection section.” Each section is internally insulated with brick, castable, or ceramic fiber refractory and houses its own a coil which carries the fluid being heated.
Types of Fired Heaters
As critical pieces of equipment in industrial process facilities, fired heaters are designed in a wide variety of configurations to meet the unique requirements of each application. Each configuration has its own technical and commercial advantages which we will explore in future postings. For now, we are simply going to identify and briefly describe different types of fired heaters.
Radiant Section Shape, Coil Orientation, and Burner Arrangement
Fired heaters can be classified based on the structural shape of the radiant section and the orientation of the radiant coil. Radiant sections are typically designed as cylinders, cabins, or boxes. Coils can run vertically up and down, horizontally across, or helically through the radiant section. The radiant coil can also be in the shape of an arbor. The arbor configuration is sometimes called a wicket. The diagrams below represent some of the more common heater configurations.
Burners are devices installed in a heater that mix the proper ratio of air and fuel to provide stable combustion with appropriately directed flames. The installation location and orientation of burners is known as the “burner arrangement.” Burners can be installed in the floor, walls, or roof of the radiant section and can be positioned to be upfired, sidefired, or downfired. Horizontally fired, wall mounted burners are further described as being end wall or side wall fired. End wall burners are fired horizontally across the length of the radiant section. Side wall fired burners direct flames across the radiant section’s width.
Classification by Draft Design
API Standard 560 defines draft as “the negative pressure or vacuum of the air and/or flue gas at any point in the heater.” There are 4 common approaches designers use to generate draft and ensure the proper amount of air for stable combustion.
Natural Draft Heaters
These heaters use the suction generated by the “stack effect” to create negative pressure inside the heater, pulling ambient air through the burner(s) and into the combustion zone.
Forced Draft Heaters
Forced Draft Heaters use a fan known as an “FD fan” to push combustion air through the burner(s) and into the combustion zone.
Induced Draft Heaters
Induced Draft Heaters use a fan known as an “ID fan” to remove flue gas from the heater and pull air through the burner(s) and into the combustion zone.
Balanced Draft Heaters
Balanced Draft Heaters use both FD and ID fans to push and pull combustion air through the burner(s) and into the combustion zone.
Basic Heater Terminology & Concepts
To conclude this post, let’s touch on a few key terms and concepts that will come up regularly when discussing fired heater design and operation.
Absorbed Duty: The total heat absorbed by the process fluid. Typically expressed in MMBTU/hr.
Approach Temperature: The difference between the process inlet temperature and the temperature of the flue gas leaving the convection section.
Average Heat Flux: Calculated by dividing the heat absorbed by the surface area of the coil. Typically expressed in BTU/hr-ft^2. Use the bare surface area of the tube when calculating flux on tubes with extended surfaces.
Bridgewall Temperature: The temperature of the flue gas measured at the arch.
Coil Pressure Drop: The difference between the pressure at the coil inlet and the coil outlet, excluding the effect of static head.
Excess Air: The air left after combustion has occurred. Presented as the percentage above the stoichiometric air requirement.
Floor Heat Flux: Calculated by dividing the total heat release by the radiant floor surface area contained within the bounds of the radiant process coils. Typically expressed in BTU/hr-ft^2.
Heat Release: The energy released from the specified fuel using the lower heating value (LHV). Also called “fired duty” and typically expressed in MMBTU/hr.
Pass: A continuous circuit of tubes from inlet to outlet. Each heater section may have multiple passes for a given process fluid. Each heater pass is designed to have equivalent process flow for equal heating and pressure drop.
Stoichiometric Combustion: The theoretical ideal where the exact amount of air required to provide complete combustion is uniformly mixed with the fuel resulting in 100% of the oxygen and fuel being converted to CO2 and H2O.
Thermal Efficiency: The percentage of energy absorbed by the process relative to the total energy input.
Volumetric Heat Release: Calculated by dividing the total heat release by the net volume of the radiant section contained within the bounds of the radiant process coils. Typically expressed in BTU/hr-ft^3.
That wraps up the first installment in our Fired Heaters 101 series. In our next post we will review key heater components, review their function, and discuss their design. In the meantime, send us your specific questions regarding fired heaters in the comments below or via our contact us page. Giving us a call at 918 499 2700 works too!
Until next time stay safe out there!