How Mannheim furnaces for potassium sulphate work
A Mannheim furnace makes potassium sulphate by reacting potassium chloride with sulphuric acid in a cast iron or refractory muffle at 500 to 600°C, with rotating rakes mixing the charge. The reaction releases hydrogen chloride gas. The muffle is heated from outside by a combustion chamber below and around it, fired with natural gas or fuel oil, with flame temperatures near 1,000°C. The two gas streams are kept separate. HCl from the muffle is drawn through coolers and absorption towers to make 30 to 33% hydrochloric acid, with a tail gas scrubber and fan after the absorbers. The combustion flue gas leaves the heating chamber at 450 to 600°C, usually passes an air preheater, and is drawn by the flue gas ID fan to the stack.
Where the FD fan sits
- Inlet filter
- FD fan
- Air preheater
- Hot air main
- Burners
- Combustion chamber
The fan stands beside the furnace and delivers ambient air into the air preheater, where it is heated to about 200 to 350°C, then through an insulated main to the burners. Outlet pressure covers the preheater, main and burner, typically 400 to 700 mmWC. Locating the fan before the preheater keeps it cold and small. One fan usually serves all the burners of a furnace.
Air at the FD fan
| Air temperature at the fan | Ambient, 25 to 45°C |
| Air after the preheater | 200 to 350°C, downstream of the fan |
| Dust | Filtered air; plant air can carry chloride and sulphate dust |
| Pressure at the fan outlet | Typically 400 to 700 mmWC |
| Corrosive atmosphere | HCl fume around the plant attacks external surfaces |
| Density at the design temperature | About 1.13 kg/m³ at 40°C, 94% 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. |
FD Fan selection for Mannheim furnaces for potassium sulphate
| Blade form | Backward curved, for efficiency |
| Material of construction | Mild steel with an acid-resistant external coating |
| Drive and arrangement | Direct coupled, arrangement 4 or 8 |
| Bearings and cooling | Standard bearings; no cooling needed |
| Flow control | Variable speed drive on firing rate with air to fuel ratio |
| Also specified | Inlet filter, coated casing and weatherproof motor for the acid atmosphere |
What this FD fan has to cope with
External corrosion
The air near a Mannheim furnace often carries HCl fume. Uncoated casings, bolts and motor frames rust fast. An acid-resistant coating, stainless fasteners and a suitable motor enclosure extend life.
Air preheater leakage
If the preheater leaks, combustion air passes to the flue gas side without reaching the burners. The fan then runs at higher flow and the ID fan load rises. Measuring air at the burners shows the leak.
Air to fuel ratio
Muffle temperature uniformity depends on burner combustion. A fan that cannot hold steady pressure across the burner range gives uneven heating and cracks the muffle. Speed control and a well sized fan keep the ratio steady.
Energy and control
The combustion air fan runs continuously at a fairly steady load. Speed control matched to firing, with oxygen trim, avoids excess air that cools the heating chamber, reduces muffle heat transfer and raises fuel use, which is a larger cost than the fan power itself. A standby fan is common because a stop chills the muffle and the charge.
ID fan and FD fan in Mannheim furnaces for potassium sulphate
| ID fan | FD fan | |
|---|---|---|
| Position | After the air preheater, before the stack | Before the air preheater, feeding the burners |
| What it handles | Combustion flue gas, possibly with HCl | Ambient air |
| Temperature at the fan | 250 to 350°C | Ambient |
| Main design concern | HCl leakage and acid dew point | Steady air to fuel ratio |
| Control | Combustion chamber pressure balance | Firing rate |
See also the Induced Draft (ID) Fan for Mannheim furnaces for potassium sulphate.
Questions buyers ask
Is the FD fan on a Mannheim furnace a combustion air fan?
Yes. It supplies combustion air to the burners heating the muffle, usually through an air preheater. It does not handle the HCl process gas. The HCl stream has its own exhaust fan after the absorbers.
Why is an air preheater used on a Mannheim furnace?
The flue gas leaves the heating chamber at 450 to 600°C. Using it to preheat combustion air saves a large share of fuel and lowers the temperature at the ID fan.
What protects a Mannheim combustion air fan from corrosion?
The fan handles clean air, but HCl fume around the furnace attacks the outside. Acid-resistant coatings, stainless fasteners and a suitable motor enclosure are used. The inlet should also be taken from a clean, fume-free area where possible.
Information needed to quote an FD fan for Mannheim furnaces for potassium sulphate
- Duty point. Combustion air quantity in m³/h or kg/h and the static pressure at the fan outlet, with the margins you want.
- Air condition. Site ambient temperature range and altitude, and whether the fan takes ambient air or air already heated.
- System. Boiler or furnace capacity and fuel, resistance of the air heater, windbox, grate or burner, and the existing fan curve or nameplate if this is a replacement.
- Mechanical. Drive arrangement, flow control (damper, inlet vanes or variable speed drive), noise limit, rotation and discharge orientation.
- Burners. Number of burners, fuel and air pressure needed at full firing.
- Air preheater. Air side pressure drop and hot air temperature.
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.