How coil type instant steam generators work
A coil type instant steam generator is a once-through boiler: feed water is pumped into a helically wound tube coil, heated by a burner firing down the centre of the coil, and leaves as steam from the coil outlet, often with a separator to remove residual water. The water content is small, so the unit reaches pressure within a few minutes of firing, which suits laundries, food processing lines and plants with intermittent steam demand. Fuel is usually diesel, LDO, natural gas or LPG. The combustion gases turn at the base and pass back between the coil layers at high velocity before leaving through the stack at about 200 to 280°C. The burner's forced draft fan supplies the combustion air and pressurises the gas path.
Where the FD fan sits
- Inlet guard
- FD fan
- Air damper
- Burner head
- Combustion chamber
- Coil passes
- Stack
The fan is mounted on top of or beside the generator casing, usually as part of the burner assembly. It pushes ambient air through the burner and into a combustion chamber enclosed by the inner coil, after which the gas flows between the coil layers and out through the stack. The fan pressure must cover burner resistance, the coil passes and the stack. Narrow coil gaps make this pressure high, often 150 to 350 mmWC on compact units, and it rises further as soot or scale builds on the tubes.
Air at the FD fan
| Air temperature at the fan | Ambient plant air, 25 to 45°C, sometimes warmer in a cramped boiler corner |
| Pressure at the fan outlet | Often 150 to 350 mmWC through the coil passes |
| Operating pattern | Frequent starts and stops, often many cycles a day |
| Fouling | Soot on the coil with oil firing raises gas side resistance over time |
| Space | Compact skid; the fan has little room for long inlet ducts |
| 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 coil type instant steam generators
| Blade form | Backward curved at high tip speed, or a compact high pressure wheel |
| Material of construction | Mild steel, balanced to a fine grade for two-pole speeds |
| Drive and arrangement | Direct mounted on a two-pole motor shaft |
| Bearings and cooling | Motor bearings; no cooling needed |
| Flow control | Servo-driven air damper or variable speed drive following the burner firing rate |
| Also specified | Inlet silencer, air pressure switch, and a margin for coil fouling in the pressure duty |
What this FD fan has to cope with
Pressure margin lost to coil fouling
As soot builds on the coil, the gas side resistance rises and the fan delivers less air at the same damper setting. The flame turns smoky and fouling accelerates. Sizing the fan with margin for the fouled coil and cleaning the coil at fixed intervals keeps combustion clean.
Thermal and mechanical cycling
A steam generator that starts many times a day puts the fan motor through many direct-on-line starts. A drive with soft starting, a fan wheel of low inertia and a motor rated for the start frequency avoid overheating and early bearing wear. The motor rating is checked against the starts per hour, not only the running power.
Noise in a small boiler room
A high pressure fan running at two-pole speed is loud at its inlet, and coil type units are often installed close to the process. An inlet silencer and an acoustic hood on the burner bring the noise down without restricting the air. The silencer pressure drop is included in the fan duty from the start.
Energy and control
Instant steam generators earn their place by firing only when steam is needed, so the fan runs in short bursts. Pre-purge and post-purge air at every start cools the coil and costs fuel; keeping purge times to what the burner standard requires, and modulating rather than cycling on-off where the load allows, saves both fuel and fan energy. A variable speed drive pays back where the generator runs long hours at part fire.
Questions buyers ask
Why does a coil type steam generator need a high pressure burner fan?
The flue gas passes between closely spaced coil layers at high velocity to transfer heat in a small volume. That gives high gas side resistance, and the burner fan must overcome it while still delivering the full air for combustion. Typical fan pressures are higher than on a fire tube boiler of the same steam output.
Can the FD fan on an instant steam generator be replaced with another make?
Yes, if the replacement matches the air flow and pressure at the burner, including a margin for coil fouling, and suits the mounting and the burner controls. We measure the existing duty or work from the burner data and supply a matched fan. The motor frame, shaft and inlet arrangement are matched so the burner fits without changes.
What causes black smoke from a coil type steam generator?
Usually too little combustion air, from a fouled coil, a dirty fan inlet, a slipping damper linkage or a fan that has lost pressure. Cleaning the coil and checking the fan delivery against design restores the air-fuel ratio. If the smoke persists after cleaning, the burner nozzle and fuel pressure should be checked next, since a worn nozzle also upsets the mixture.
Information needed to quote an FD fan for coil type instant steam generators
- 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.
- Generator rating. Steam output, fuel, and the burner make and model if the fan is a replacement.
- Start frequency. Number of starts per day and whether the burner modulates or runs on-off.
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.