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 ID fan sits
- Combustion chamber
- Muffle heating flues
- Air preheater
- ID fan
- Stack
The fan sits after the air preheater, which takes the gas from 450 to 600°C down to about 250 to 350°C. The HCl side has its own fan after the absorption towers. The two fans together set the pressure difference across the muffle wall: most plants hold the combustion side at slightly less suction than the muffle, so that any leak carries flue gas into the HCl stream rather than HCl into the flue gas.
Gas at the ID fan
| Gas temperature, normal | 250 to 350°C after the air preheater |
| Maximum or upset | Up to 550°C if the air preheater is bypassed |
| Dust at the fan | Low; refractory and salt dust |
| Moisture and dew point | Normal combustion moisture; HCl and SO₂ raise the acid dew point |
| Corrosive species | HCl from muffle leaks, SO₂ with oil firing |
| Density at the design temperature | About 0.62 kg/m³ at 300°C, 52% 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 Mannheim furnaces for potassium sulphate
| Blade form | Backward curved, on clean hot gas |
| Material of construction | Mild steel or boiler plate with insulation; stainless or heat-resisting steel above about 400°C |
| Drive and arrangement | Direct coupled, arrangement 8, with casing on sliding supports |
| Bearings and cooling | Cooling disc and heat shield |
| Flow control | Variable speed drive on combustion chamber pressure |
| Also specified | Insulated casing and inlet box, casing drain, and a shaft seal to keep acid gas in |
What this ID fan has to cope with
HCl crossing into the flue gas
Cracked muffles and worn joints let HCl into the combustion side when its suction is stronger than the muffle's. The HCl condenses with water on cool parts of the fan, causing fast chloride attack. Pressure balance control and muffle maintenance limit the leak; insulation keeps the fan above the dew point.
Overheating if the preheater is bypassed
Bypassing a fouled or leaking air preheater sends gas at 500°C or more to the fan. The impeller must be rated for this, or a dilution damper must limit the temperature.
Cold end corrosion at low firing
At low firing the gas reaches the fan cooler, and with HCl present the acid dew point is high. Holding a minimum fan inlet temperature through the air preheater bypass avoids condensation.
Energy and control
Mannheim furnaces run continuously at a steady rate set by acid and chloride feed. The flue gas fan duty is fairly constant, but the combustion side suction must track the HCl side to hold the pressure balance. Speed control does this more smoothly than dampers and avoids excess suction that pulls in air and HCl.
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 Forced Draft (FD) Fan for Mannheim furnaces for potassium sulphate.
Questions buyers ask
How many fans does a Mannheim furnace have?
Usually three: a combustion air fan for the burners, a flue gas ID fan for the heating chamber, and an HCl tail gas fan after the absorption towers. The flue gas and HCl fans together set the pressure across the muffle.
Why does HCl corrode the Mannheim furnace flue gas fan?
If the muffle leaks and the combustion side is under stronger suction, HCl enters the flue gas. It condenses with water as hydrochloric acid on cool surfaces, attacking steel quickly.
What material is used for a Mannheim HCl tail gas fan?
The HCl tail gas after absorption is wet and acidic, so that fan is usually FRP, polypropylene or rubber-lined steel. The flue gas ID fan can be steel, provided it is kept above the dew point.
Information needed to quote an ID fan for Mannheim furnaces for potassium sulphate
- 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.
- Pressure balance. Muffle and combustion chamber pressure set points, and how the plant controls the difference.
- HCl presence. Any history of HCl in the flue gas and the fuel sulphur content.
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.