How lime hydration plants work
Quicklime is ground and fed with a controlled amount of water to a single or multi-stage hydrator, where the slaking reaction releases a large amount of heat. Excess water boils off, and the hydrator vents a stream of steam and air at about 90 to 105°C carrying hydrated lime dust. This vent is drawn off by the exhaust fan through a wet scrubber, often one that returns the lime-laden water to the hydrator, or through an insulated, heated bag filter. The hydrated lime then passes to a separator or air classifier, which has its own fan and dust collector. There is no burner and no combustion gas, so the plant has an exhaust (ID) fan but no FD fan.
Where the ID fan sits
- Hydrator
- Vent duct
- Wet scrubber or heated bag filter
- Mist eliminator
- Vent (ID) fan
- Stack
The fan sits after the scrubber or bag filter, at the end of a short vent duct from the hydrator. Its suction holds the hydrator slightly negative so steam and lime dust do not leak at seals and the feed point. Resistance is typically 150 to 300 mmWC for a venturi or packed scrubber. The fan must handle saturated vapour with droplets, and any lime that passes the scrubber reacts with carbon dioxide and dries into hard scale.
Gas at the ID fan
| Vapour temperature, normal | 60 to 85°C after a scrubber; about 95°C behind a bag filter |
| Maximum or upset | Up to 105°C if scrubber water is lost or the hydrator overheats |
| Dust at the fan | Fine hydrated lime in droplets; low in normal running, heavy if the scrubber is starved |
| Moisture and dew point | Saturated or near-saturated steam and air; steam can be half or more of the volume, lowering gas density |
| Corrosive species | Alkaline lime slurry, not acidic; the problem is scaling rather than corrosion |
| Density at the design temperature | About 1.00 kg/m³ at 80°C, 83% 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 lime hydration plants
| Blade form | Radial or backward inclined flat blades, open and easy to clean |
| Material of construction | Mild steel with a corrosion allowance, or SS 304 for the wheel where scale removal is frequent |
| Drive and arrangement | Belt driven, arrangement 9, so speed can be adjusted for the actual duty |
| Bearings and cooling | Bearings outside the gas stream with a good shaft seal against wet vapour |
| Flow control | Variable speed drive on hydrator pressure |
| Also specified | Wash-down nozzles in the casing, a large drain, a split casing or quick-opening door for descaling |
What this ID fan has to cope with
Lime scale on the impeller
Hydrated lime that reaches the fan as slurry dries on the blades and reacts with carbon dioxide to form a hard carbonate scale. It builds unevenly and causes vibration within weeks. Open blades, wash-down nozzles used on a schedule, and a door large enough to chip scale off the wheel are the practical answer.
Steam surge after feed changes
A short rise in quicklime feed or a change in reactivity releases more steam than expected. If the fan is sized for average steam, the hydrator goes positive and dust escapes at the seals. Sizing for peak steam generation, with a speed loop on hydrator pressure, keeps the vessel contained.
Wet duct and fan drains blocking
Condensate with lime solids settles in low points of the duct and the casing drain, then sets solid. Drains sized generously, sloped ducts and a flushing water connection prevent the casing flooding. A sight glass or level switch on the drain line shows when it has blocked, before the wheel starts running in water.
Bag filter blinding
When a bag filter is used instead of a scrubber, any cooling below the dew point wets the bags and lime sets on the cloth. The filter and fan casing are insulated and heated, and the fan is not stopped until the system has been purged warm.
Energy and control
Hydration plants often run one or two shifts, and steam generation varies with lime reactivity and feed. A variable speed drive on hydrator pressure saves much of the power that a throttled fan wastes, and it lets the operator compensate for scale on the wheel. Keeping the scrubber and mist eliminator clean holds pressure drop, and fan energy, at the design value.
Questions buyers ask
Why does the hydrator vent fan of a lime hydration plant keep going out of balance?
Lime slurry that reaches the fan dries on the blades and turns into calcium carbonate scale. It builds unevenly and breaks off in pieces. Regular washing of the wheel, an open blade design and good mist elimination ahead of the fan reduce how often it needs cleaning and rebalancing.
Does a lime hydration plant need an FD fan?
No. Hydration is an exothermic reaction with no burner, so there is no combustion air. The heat of slaking boils off excess water without any fuel. The plant has a vent or exhaust fan on the hydrator and a separate fan in the classification and dust collection system.
What temperature should a hydrator exhaust fan be designed for?
Usually 60 to 85°C after a wet scrubber, or about 95 to 105°C after a heated bag filter. Because the gas is largely steam, the density is lower than air at the same temperature, and the fan duty is calculated on the actual gas composition.
Information needed to quote an ID fan for lime hydration plants
- 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.
- Hydrator output. Quicklime feed rate, water addition and the expected steam generation at peak reactivity.
- Dust control. Whether the vent is cleaned by a wet scrubber or a bag filter, and the water recirculation arrangement.
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.