How vertical shaft kiln mini cement plants work
A vertical shaft kiln makes clinker in a refractory-lined shaft, typically 2 to 3 m in diameter and 8 to 10 m high. Raw meal is ground with fine coal or coke breeze, the black meal process, and formed into moist nodules with 12 to 14% water. The nodules are fed at the top onto a shallow burning zone just below the surface, where the fuel inside them burns at about 1,300 to 1,400°C. Combustion air is blown in at the bottom by a high pressure blower and passes up through the clinker, cooling it and preheating itself. The gas leaves the top of the bed at 100 to 250°C, laden with moisture and dust. Older VSKs used only a stack; current plants add a dust collector and an ID fan to meet emission limits.
Where the FD fan sits
- Inlet filter
- FD fan (air blower)
- Air duct
- Kiln bottom and discharge gate
- Clinker bed
The blower stands at ground level beside the kiln and delivers air into the bottom of the shaft through the discharge gate area. It must push the air through the whole clinker column, the cooling zone and the burning zone to the kiln top, a resistance that depends on nodule size, clinker sintering and bed height. Pressure at the blower outlet is typically 1,000 to 2,500 mmWC. The discharge gate must be sealed so the air goes up the shaft rather than out through the clinker outlet.
Air at the FD fan
| Air temperature at the fan | Ambient, 25 to 45°C |
| Air after the bed | Heated by the clinker to burning zone temperature, downstream of the fan |
| Dust | Clinker dust around the discharge area; inlet filter advisable |
| Pressure at the fan outlet | Typically 1,000 to 2,500 mmWC |
| Variation | Bed resistance changes with nodule quality and sintering |
| 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 vertical shaft kiln mini cement plants
| Blade form | Backward curved or radial at high tip speed; multi-stage or positive displacement blower for the highest pressures |
| Material of construction | Mild steel; high-tensile impeller at high tip speed |
| Drive and arrangement | Direct coupled or belt driven |
| Bearings and cooling | Standard bearings; no cooling needed |
| Flow control | Variable speed drive or inlet damper on air flow |
| Also specified | Inlet filter, silencer, non-return damper and air flow measurement |
What this FD fan has to cope with
Changing bed resistance
When nodules break down or the burning zone sinters into a solid mass, the bed resistance rises sharply. A fan with a flat curve loses air flow and the kiln goes out of balance. A steep pressure curve keeps the air flow steadier as resistance rises.
Surge at high resistance
A centrifugal blower pushed to high pressure at low flow can reach its stall region, giving pulsations through the bed. Selection with margin from the stall point, and flow measurement to warn the operator, avoid it.
Air leakage at the discharge
Air escaping through the discharge gate and seals is lost to the kiln and carries clinker dust into the shed. Good seals reduce the blower size and power needed.
Energy and control
The air blower is the largest power consumer on a VSK and runs continuously at high pressure. Matching it to the actual bed resistance, rather than oversizing and throttling, gives the biggest saving. Better nodule quality lowers bed resistance and blower power at the same time, so raw meal preparation and blower energy are linked.
ID fan and FD fan in vertical shaft kiln mini cement plants
| ID fan | FD fan | |
|---|---|---|
| Position | After the cyclone and bag filter or scrubber | At the kiln bottom, below the clinker bed |
| What it handles | Cool, humid kiln top gas | Ambient air |
| Temperature at the fan | 80 to 150°C | Ambient |
| Main design concern | Condensation and wet dust deposits | High pressure against a changing bed |
| Control | Kiln top suction | Air flow through the bed |
See also the Induced Draft (ID) Fan for vertical shaft kiln mini cement plants.
Questions buyers ask
What pressure does a VSK air blower need?
Typically 1,000 to 2,500 mmWC, depending on shaft height, nodule size and how strongly the clinker sinters. The blower must have margin above the normal pressure to cope with bed resistance changes.
Is a centrifugal fan or a positive displacement blower used on a VSK?
Both are used. A high pressure centrifugal fan suits most mini cement kilns and is easier to control. Positive displacement blowers are used where the bed resistance is very high or a constant air volume is wanted regardless of pressure.
Why does the VSK air flow drop during operation?
As the bed resistance rises with poor nodules or sintering, a fan with a flat curve delivers less air. The kiln then loses burning intensity. A blower with a steeper curve, and checking the bed condition, keeps the air flow up.
Information needed to quote an FD fan for vertical shaft kiln mini cement plants
- 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.
- Bed and kiln. Shaft diameter, bed height and nodule size.
- Air requirement. Air flow per tonne of clinker and the pressure at the kiln bottom under normal and worst bed conditions.
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.