How ethylene cracking furnaces work
An ethylene cracking furnace heats a hydrocarbon feed, ethane, propane, naphtha or gas oil, mixed with dilution steam in radiant coils to about 800 to 860°C for a fraction of a second, cracking it to ethylene and propylene. The cracked gas is quenched immediately in transfer line exchangers that raise high-pressure steam. Firing is with floor and wall burners burning methane-rich fuel gas from the plant's own cold box. Flue gas leaves the radiant box at about 1,050 to 1,150°C and passes a tall convection section with feed preheat, dilution steam superheat, high-pressure steam superheat and boiler feed water coils, leaving at only 100 to 140°C. Most cracking furnaces have natural draft burners and an ID fan; air preheat with an FD fan is less common.
Where the ID fan sits
- Radiant firebox
- Convection section coils
- Optional SCR
- ID fan
- Stack
The fan usually sits at the top of the convection section or at grade beside it, before a short stack. It supplies all the draft for the firebox, because the burners are natural draft burners that need a steady negative pressure to draw air in. Suction is typically 100 to 200 mmWC, covering the convection coils and any SCR. A plant often has many identical furnaces side by side, each with its own fan.
Gas at the ID fan
| Gas temperature, normal | 100 to 140°C at the convection outlet |
| Maximum or upset | 250 to 350°C during decoking or with convection coils fouled |
| Dust at the fan | Negligible; coke fines during decoking with air |
| Moisture and dew point | Water dew point 55 to 65°C; methane-rich fuel gives high water vapour |
| Corrosive species | Little sulphur in the fuel gas |
| Density at the design temperature | About 0.88 kg/m³ at 130°C, 73% of standard air. Pressure developed and power drawn scale with density, so the fan is selected at this condition and the motor is checked at the coldest operating or start-up condition. |
ID Fan selection for ethylene cracking furnaces
| Blade form | Backward curved or aerofoil, for efficiency on a clean flow |
| Material of construction | Mild steel, rated for the decoke and fouled-coil upset |
| Drive and arrangement | Direct drive or overhung on top of the convection section, arrangement 8 or 4 |
| Bearings and cooling | Standard bearings; oil-lubricated on large fans |
| Flow control | Variable speed drive or inlet vanes on firebox draft |
| Also specified | Structural design for mounting on the convection section and API 673 or equivalent |
What this ID fan has to cope with
Draft swings at natural draft burners
Natural draft burners draw air according to the firebox draft. If the draft swings, air to every burner swings with it, and the flames, excess oxygen and coil temperatures change. Fine draft control from the ID fan, with a fast loop, keeps the cracking conditions steady.
Decoking cycle changes the duty
Every few weeks to a few months the furnace is taken off feed and the coils are decoked with steam and air. Firing drops, the gas flow and temperature change, and coke fines can appear in the flue gas. The fan is checked at the decoke condition as well as the run condition.
Low stack temperature and condensation
With the flue gas leaving at 100 to 140°C, the fan and stack run close to the water dew point. During a cold start or at low firing, water can condense in the fan casing. A drain and insulated casing prevent standing water and corrosion.
Energy and control
A cracking furnace runs at near-constant firing for its run length, then drops during decoking. A variable speed drive matches draft to firing efficiently and handles the decoke condition without heavy throttling. With many furnaces in a plant, the combined ID fan load is significant, and efficient impellers and drives pay back across the fleet.
ID fan and FD fan in ethylene cracking furnaces
| ID fan | FD fan | |
|---|---|---|
| Position | At the convection section outlet | Before the air preheater, where fitted |
| What it handles | Clean flue gas after deep heat recovery | Ambient air |
| Temperature at the fan | 100 to 140°C | Ambient |
| Main design concern | Steady draft for natural draft burners | Floor and wall burner split |
| Control | Firebox draft | Air flow with oxygen trim |
See also the Forced Draft (FD) Fan for ethylene cracking furnaces.
Questions buyers ask
Why do ethylene cracking furnaces need an ID fan?
The tall convection section has many coil banks that recover heat down to 100 to 140°C, and their resistance is more than a short stack can overcome. The ID fan provides the draft for the natural draft burners and pulls the flue gas through the convection section.
What happens to the ID fan during furnace decoking?
During decoking the furnace is off feed and firing is reduced, while steam and air burn off the coke in the coils. The flue gas flow and temperature change and some coke fines may appear. The fan is controlled to hold draft at the lower firing rate.
Is there an FD fan on an ethylene cracker?
Most ethylene cracking furnaces use natural draft floor and wall burners with only an ID fan, which provides the firebox draft that draws the air in. An FD fan is used where the furnace has combustion air preheat, sometimes using gas turbine exhaust, low-pressure steam or flue gas heat to warm the air before the burners.
Information needed to quote an ID fan for ethylene cracking furnaces
- Duty point. Gas volume in m³/h or Am³/h at the fan inlet and the static pressure the fan must develop, with the margin you want on each.
- Gas condition. Normal and maximum gas temperature at the fan, dust loading after the collector, moisture and any acid gases such as SO₂ or HCl.
- System. Boiler or furnace capacity and fuel, collector type and its pressure drop, and the existing fan curve or nameplate if this is a replacement.
- Mechanical. Drive arrangement, flow control (damper, inlet vanes or variable speed drive), rotation and discharge orientation, and a layout drawing or site photograph.
- Run and decoke cases. Flue gas flow and temperature in normal run, end of run and decoking.
- Mounting. Whether the fan is mounted on top of the convection section or at grade, and the structural limits.
Built and tested in Chennai
Adroit Fantech Engineers has designed and manufactured industrial fans at Ayanambakkam, Chennai since 2004. Each impeller, casing and shaft is built for the gas and temperature it will see, performance tested as per IS 4894 or BS 848 Part I, and balanced to grade G 6.3 or G 2.5 as per ISO 21940-11, with the records supplied with the fan. We also replace rotors and retrofit existing fans of any make.