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 FD fan sits
- Inlet filter
- FD (combustion air) fan
- Burner windbox
- Hot gas generator
- Dilution air mixing
- Rotary drum dryer
The fan sits beside the hot gas generator and supplies primary combustion air to the burner, which may fire diesel, natural gas, biogas from the digester or, on some plants, part of the dried sludge itself. A share of air, either from the same fan or a separate dilution fan, is mixed into the hot gas to bring it down to the drum inlet temperature. The fan pressure covers the burner, the combustion chamber and the drum inlet, and is usually modest.
Air at the FD fan
| Air temperature at the fan | Ambient, 25 to 45°C |
| Air at the burner | Ambient, unless preheated with heat recovered from the exhaust |
| Dust | Clean air; the intake should not draw from the sludge reception or cake bunker area |
| Odour and corrosion | Air drawn from inside the dryer building can carry ammonia and H₂S; an outside intake avoids this |
| Pressure at the fan outlet | Typically 150 to 400 mmWC, set by the burner make and the hot gas generator |
| 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 sewage and ETP sludge dryers
| Blade form | Backward curved for efficiency on clean air; radial or high-pressure wheel for burners needing over 400 mmWC |
| Material of construction | Mild steel with a corrosion-resistant paint system for the plant atmosphere |
| Drive and arrangement | Direct mounted on the motor shaft or direct coupled |
| Bearings and cooling | Standard bearings; no cooling needed |
| Flow control | Variable speed drive or inlet damper on the burner air-fuel ratio control |
| Also specified | Outdoor inlet with filter and silencer; flow measurement for burner management |
What this FD fan has to cope with
Air-fuel ratio at low load
Cake moisture changes the heat demand, and the burner modulates over a wide range. A fan with poor turndown on a damper gives unstable flame and CO at low fire. Speed control and a burner-matched curve keep the ratio steady. The fan is matched to the burner maker's air curve.
Corrosive intake air
Sludge buildings smell of ammonia and H₂S, and drawing that air into the fan corrodes the impeller and motor. Taking air from outside, or through a filter in a clean area, protects the fan. The motor enclosure is also chosen for a damp, corrosive building, since the outside of the fan suffers as much as the inside.
Using biogas or dried sludge as fuel
Plants that fire biogas or dried sludge need different air volumes from a diesel burner. The FD duty is checked against each fuel the plant intends to use, including the start-up fuel. Dried sludge firing in particular needs more air per unit of heat and a higher pressure where it is burnt in a fluid bed or on a grate.
Energy and control
The combustion air fan follows the burner, so its energy tracks the evaporation load. Drier cake from the dewatering stage is the largest saving. Recovering heat from the dryer exhaust condenser to preheat combustion air cuts fuel, though it adds a small pressure drop that must be included in the FD fan duty.
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 Induced Draft (ID) Fan for sewage and ETP sludge dryers.
Questions buyers ask
What does the FD fan do on a sludge dryer?
On a sludge dryer the FD fan is the combustion air fan. It supplies air to the burner or hot gas generator that heats the drum, and sometimes the dilution air that cools the hot gas to the drum inlet temperature. It handles clean ambient air, so it is a simpler, smaller fan than the ID fan.
Can the combustion air fan take air from inside the sludge building?
It is better not to. The air inside contains ammonia, H₂S and moisture that corrode the fan and motor. An outside intake with a filter is the normal arrangement, and some plants deliberately use building air in the furnace for odour destruction with a suitably protected fan.
Is a separate dilution air fan needed?
It depends on the hot gas generator design. Some use one fan with a split duct, others a separate dilution fan for better control of the drum inlet temperature. The choice is made with the burner supplier. A separate fan adds a motor but gives the operator an independent handle on drum inlet temperature.
Information needed to quote an FD fan for sewage and ETP sludge dryers
- 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.
- Burner and fuel. Burner make, fuels to be fired and the required air pressure at the burner.
- Dilution air. Whether dilution air is taken from the same fan, and the drum inlet 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.