Introduction to Fired Heaters Pt 2: Components
Welcome to the second installment of Scelerin’s Fired Heaters 101 series. In part 1, we reviewed key heater concepts and classifications of heater configurations commonly found in process facilities. In this post, we will identify and describe specific heater components. That said, let’s get started…
The diagram on the left identifies the sections and major components in a typical vertical cylindrical fired heater.
Burners are critical devices installed in a heater that generate the energy absorbed by the fluid moving through the radiant and convection coils. They do this by mixing the proper ratio of air and fuel, providing stable combustion, and appropriately directing flames. As discussed in our first post, burners can be up, side, or down fired and mounted in the radiant section’s floor, wall, or roof. With so many burner arrangements, it should be no surprise that burners themselves come in a variety of designs. We will explore basic combustion theory and burner design in next month’s post.
The radiant coil carries the process fluid through the radiant section. As radiant heat is the primary energy transfer mechanism in this heater section, these coils have bare tubes (i.e. no extended surfaces are attached).
The radiant coil can be serpentine, helical, or arbor shaped and designed as a single pass or with multiple passes. In the context of fired heaters, a pass is defined as a continuous circuit of tubes from inlet to outlet. A “single pass radiant coil” has one inlet and outlet. One hundred percent of the process flow moves through that one pass. A “multiple pass radiant coil” divides the process flow equally among two or more sets of inlets and outlets. Each of these passes is designed for equivalent process flow to ensure equal heating and pressure drop across each pass.
The convection coil carries the fluid(s) being heated through the convection section. A key difference between convection and radiant coils is that most tubes that make up the convection coil have extended surfaces. The additional surface area improves convective heat transfer and increases thermal efficiency. Examples of common extended surfaces used include continuous fins, serrated fins or studs.
Note that not all tubes in the convection section have extended surfaces. The first two rows of convection tubes that the flue gas passes over are called the “shock tubes” or “shield rows.” As these rows are closest to the radiant section, and likely exposed to radiant energy, they are bare. Another common difference between the radiant and convection coil is that there can be multiple banks of convection coils carrying different process fluids. It is not uncommon for one bank of convection coils to pre-heat the main process fluid and another bank to be in a completely different secondary service.
Refractory is the insulating material that lines the interior of the heater. Its purpose is to reduce energy loss, while providing lower casing temperatures to protect operators. Refractory material in fired heaters is typically brick, castable, or ceramic fiber. Per API STD 560 (Fired Heaters for General Refinery Service), refractory should be designed to keep the outside skin temperature of the heater casing below 180°F assuming an ambient air temperature of 80°F and no wind.
The diagram and definitions below include several other important heater components. Red text indicates links to associated photos that can provide more detail:
Stack: The stack collects the flue gas and discharges it to the atmosphere.
Sample Ports: Nozzles near the stack exit where sampling probes or other sensors are inserted to measure flue gas composition, pressure, and/or temperature.
Stack Damper: A “valve” located inside the stack used to control flow of flue gas and adjust draft.
Breeching: The section of duct after the last row of convection tubes where flue gases are collected for transmission to the stack or exhaust ducts.
Corbels: Protrusions from the refractory lining the convection section. These protrusions redirect flue gases travelling along the walls of the convection section back toward convection coil.
Header Box: A refractory lined box covering the return bends and separated from the flue gas flow.
Return Bends: U-shaped fittings connecting tubes in the convection coil.
Tube Sheet: Support sheet for convection tubes passing through it. There are both end tube sheets and intermediate tube sheets.
Expansion Rows: Space left in the convection section for the future addition of rows. Tube sheets are also designed to accommodate these rows with the associated holes plugged to prevent flue gas from leaking into the header boxes.
Crossover Piping: Interconnecting piping between the convection coil and radiant coil.
Bridgewall: The bridgewall (sometimes call the arch) is the area where flue gas leaves the radiant section. It is not so much a component as a point of critical importance. Many of the operational adjustments made to optimize heater operation are based on measurements of draft, temperature, and oxygen content taken at this point in the heater.
Coil Supports: Brackets installed in the radiant and convection section to keep the associated coil in the proper position while allowing for thermal growth.
Observation Doors: Visual inspection ports that allow operators to view what is happening inside the furnace.
Tube Skin Thermocouples (TSTCs): Sensors installed directly on the heater coils to monitor the coil’s skin temperature. These critical sensors are used to ensure that the allowable tube metal temperature is not exceeded.
This wraps up the second installment in our Fired Heaters 101 series. Keep an eye out in early March for our next post covering Basic Combustion Theory and Burner Technology. In the meantime, post your questions regarding fired heaters in the comments section below or send us an email via our contact us page. You can also give us a call at 918 499 2700
Until next time stay safe out there!