How match splint dryers work
Match splints are cut from peeled veneer of soft woods such as poplar, aspen or local softwoods, then soaked in an ammonium phosphate solution so the stick does not glow after the flame goes out. The wet splints, at 50% moisture or more, are dried to about 6 to 10% in rotary drum dryers. Hot air at typically 80 to 130°C comes from a wood-waste or husk-fired furnace through an indirect heat exchanger, or from steam coils. The drum tumbles the splints in the air, and the exhaust leaves at 45 to 70°C, humid and carrying wood fines and phosphate dust, through a cyclone to atmosphere. Dried splints are polished in drums before the match machine. Many small units still sun dry.
Where the ID fan sits
- Rotary drum dryer
- Exhaust hood
- Cyclone
- ID fan
- Exhaust stack
The fan sits after the cyclone, which removes broken splints and coarse fines, so it handles fine wood dust and phosphate. It works against the drum, exhaust hood, cyclone and ducting, typically 80 to 180 mmWC. Small dryers often have only a supply blower, but an exhaust fan is needed to keep dust out of the work area and to control drum pressure when several dryers share a heater.
Gas at the ID fan
| Gas temperature, normal | 45 to 70°C, humid exhaust air |
| Maximum or upset | Up to 100 to 120°C when the drum runs empty |
| Dust at the fan | Wood fines and ammonium phosphate dust after the cyclone |
| Moisture and dew point | Humid early in each batch; condensation in cold ducts |
| Corrosive species | Ammonium phosphate, hygroscopic and mildly corrosive when damp |
| Density at the design temperature | About 1.06 kg/m³ at 60°C, 88% 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 match splint dryers
| Blade form | Radial tipped or open backward curved, which shed fibrous dust |
| Material of construction | Mild steel with an epoxy coating, or stainless steel where damp phosphate deposits are persistent |
| Drive and arrangement | Belt driven, arrangement 9 |
| Bearings and cooling | Standard pedestal bearings |
| Flow control | Inlet damper, or variable speed drive where several drums share a fan |
| Also specified | Inspection door, casing drain, earthing against static, and spark detection on larger systems |
What this ID fan has to cope with
Phosphate and wood dust build-up
Ammonium phosphate is hygroscopic. In humid exhaust it absorbs water and cements wood fines onto the impeller and scroll. Deposits harden, unbalance the wheel and cut flow. Self-cleaning blades, an inspection door and washing at regular stops prevent this. Good cyclone performance and dry splints at discharge reduce the load.
Corrosion of mild steel
Damp phosphate deposits slowly corrode uncoated mild steel, especially at the casing bottom. A coating, stainless parts where wetting persists and a casing drain extend life. Fasteners and the inspection door frame, where deposits collect and water sits, are worth making in stainless steel from the start, since they are hard to replace once corroded.
Fire in the dryer exhaust
Dry wood fines are combustible, and sparks from a wood-fired heater can enter through a leaking heat exchanger. Indirect heating, a spark screen on the hot air duct, a tight exchanger and earthing of the fan reduce the risk. Keep the cyclone hopper emptied and closed.
Energy and control
Splint dryers work in batches, with moisture removal high at the start and low at the end. An exhaust fan sized for the wettest moment runs oversized for most of the batch. Reducing exhaust flow late in the batch, by damper or speed, saves fan power and wood fuel. Small plants that run seasonally gain most from a correctly sized fan rather than a large one throttled.
ID fan and FD fan in match splint dryers
| ID fan | FD fan | |
|---|---|---|
| Position | After the cyclone, before the exhaust stack | Before the heat exchanger, feeding the drum |
| What it handles | Humid air with wood and phosphate dust | Clean ambient air |
| Temperature at the fan | 45 to 70°C | Ambient |
| Main design concern | Build-up, corrosion and fire | Spark-free air and even flow |
| Control | Drum suction | Drying air flow |
See also the Forced Draft (FD) Fan for match splint dryers.
Questions buyers ask
What fan is used for a match splint dryer exhaust?
A radial tipped or open backward curved centrifugal fan after a cyclone is usual. It handles warm humid air with wood fines and ammonium phosphate dust, so it must shed deposits and resist mild corrosion. Belt drive allows speed to be matched to the drum, and an inspection door allows regular cleaning.
Why does dust stick to my splint dryer fan?
The ammonium phosphate used to treat the splints absorbs moisture and binds wood fines together on the blades. Washing the fan at regular stops and using a self-cleaning blade form reduces build-up. Improving the cyclone and keeping the duct warm so that the phosphate does not pick up moisture also helps.
Does a match splint dryer need an ID fan?
Small dryers can run with only a supply blower, but an induced draft fan keeps the drum under suction, contains dust and gives better control of drying, especially in larger units. Where several drums share one heater, an exhaust fan also helps balance the air between them.
Information needed to quote an ID fan for match splint 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.
- Dryer data. Drum size, splints dried per batch and moisture removed.
- Heat source. Heater type and whether the air is heated directly or through an exchanger.
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.