How sewage and ETP sludge dryers work
Dewatered sewage or ETP sludge leaves the belt press, screw press or centrifuge as a cake of about 15 to 25% dry solids and is dried to 70 to 90% dry solids for disposal, co-processing in cement kilns or use as fuel. Direct rotary drum dryers mix the cake with recycled dry product to pass through the sticky phase, then dry it in hot gas from a burner or furnace at typically 400 to 550°C at the inlet. The gas leaves at about 100 to 130°C, laden with water vapour, fines, ammonia, hydrogen sulphide and odorous organics. It passes a cyclone and often a bag filter, the ID fan, then a condenser or wet scrubber, and sometimes a biofilter, before the stack. Low temperature belt dryers use indirect hot air instead and run cooler.
Where the ID fan sits
- Hot gas generator
- Rotary drum or belt dryer
- Cyclone
- Bag filter (if fitted)
- ID fan
- Condenser or scrubber
- Biofilter or stack
The fan sits after the cyclone, or after the bag filter where one is fitted, and before the condenser or scrubber. Placing it ahead of the scrubber keeps the gas above its dew point at the fan and away from the saturated, acidic scrubber outlet. The fan works against the drum, the feed and discharge seals, the cyclone and filter on its suction side, and pushes through the condenser or scrubber, with total pressure typically 250 to 450 mmWC. It also sets the drum suction that keeps odour inside the plant.
Gas at the ID fan
| Gas temperature, normal | 100 to 130°C after the cyclone or filter |
| Maximum or upset | Up to 200°C on loss of feed with the burner still firing |
| Dust at the fan | Fine organic dried sludge, sticky when damp; low after a bag filter, high after a cyclone alone |
| Moisture and dew point | Water vapour often 30 to 50% by volume, dew point about 70 to 85°C |
| Corrosive species | Ammonia, H₂S, chlorides and organic acids, aggressive wherever condensate forms |
| Density at the design temperature | About 0.91 kg/m³ at 115°C, 76% 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 sewage and ETP sludge dryers
| Blade form | Backward curved with open, self-cleaning passages, or radial where the fan follows a cyclone only |
| Material of construction | SS 316 for the impeller and wetted casing, or mild steel with a chemical-resistant lining on smaller plants |
| Drive and arrangement | Direct coupled or belt driven, arrangement 8 or 9, with a variable speed drive |
| Bearings and cooling | Bearings outside the gas stream with a cooling disc and a gas-tight shaft seal |
| Flow control | Variable speed drive holding drum inlet suction |
| Also specified | Wash nozzles in the casing, a drain at the lowest point, insulation and a large inspection door |
What this ID fan has to cope with
Sticky deposits and unbalance
Fine dried sludge that passes the cyclone picks up moisture in the humid gas and bakes onto the blades in uneven layers. Vibration then rises within weeks. A bag filter ahead of the fan, smooth blade surfaces, casing wash nozzles and a planned wash at each stop keep the rotor in balance.
Condensation and acid attack
When the burner trips or the plant stops, the casing cools below the 70 to 85°C dew point and the condensate dissolves ammonia, H₂S and chlorides. Mild steel pits quickly. Stainless wetted parts, insulation, a casing drain and running the fan on warm air until the system is dry prevent most of the damage.
Odour leakage
If the fan loses margin, for example as the filter loads, the drum goes to positive pressure and odorous gas escapes at the seals. The fan is sized on the dirty filter condition and the drive keeps the drum suction set point. Operators then notice it first at the feed chute.
Fire in dried sludge
Dried sewage sludge is combustible and can self-heat in deposits. The fan is earthed, rubbing contact is designed out, and the casing deposits are removed rather than left to smoulder between runs. A temperature sensor at the fan inlet, interlocked with the burner, gives early warning of a fire travelling downstream from the drum or filter.
Energy and control
Sludge dryers run continuously while the dewatering plant runs, but the cake dry solids vary with the polymer dose and the press performance, so the evaporation load and the gas volume swing widely. A variable speed drive on drum suction follows those swings. Feeding drier cake from better dewatering cuts both fuel and fan power, and recirculating part of the exhaust back to the furnace reduces the volume the ID fan must send to the scrubber.
ID fan and FD fan in sewage and ETP sludge dryers
| ID fan | FD fan | |
|---|---|---|
| Position | After the cyclone or bag filter, before the scrubber | At the burner or hot gas generator |
| What it handles | Humid exhaust with sludge fines, NH₃ and H₂S | Clean ambient combustion air |
| Temperature at the fan | 100 to 130°C, dew point 70 to 85°C | Ambient |
| Main design concern | Deposits, condensation and corrosion | Burner turndown and air-fuel ratio |
| Control | Drum suction | Burner firing rate |
See also the Forced Draft (FD) Fan for sewage and ETP sludge dryers.
Questions buyers ask
What material should a sludge dryer ID fan be made of?
For a sewage or ETP sludge dryer, SS 316 for the impeller and casing parts that see condensate is the usual choice, because the exhaust carries ammonia, H₂S and chlorides. On small plants with a reliable warm shutdown routine, mild steel with a chemical-resistant lining can serve, but it needs regular inspection.
Should the ID fan be before or after the scrubber on a sludge dryer?
Usually before. Ahead of the scrubber the gas is above its dew point and has been dedusted by the cyclone or bag filter. After the scrubber the gas is saturated and acidic, which needs FRP or heavier stainless construction and still risks droplet carry-over. The dry position also lets the fan run on a lighter construction.
Why does the sludge dryer fan vibrate after a few weeks?
Fine sludge dust sticks to the blades in the humid gas and builds up unevenly. Improving the collector ahead of the fan, washing the fan casing at each stop and choosing an open, smooth blade form reduce build-up and keep vibration within limits. Checking the bag filter for torn bags is the first step.
Information needed to quote an ID fan for sewage and ETP sludge dryers
- 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.
- Sludge and cake. Source of the sludge (sewage, textile, tannery, chemical), cake dry solids and target dry solids.
- Odour treatment. Whether a condenser, scrubber or biofilter follows the fan, and its pressure drop.
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.