How diesel engine exhaust waste heat boilers work
Diesel engine exhaust waste heat boilers recover heat from the exhaust of generating sets and propulsion engines running on HSD, LDO or heavy fuel oil. Exhaust leaves the turbocharger at about 300 to 450°C and flows through a fire tube or finned water tube boiler, then a silencer and stack. A diverter or bypass damper lets the engine run when the boiler is off. The engine itself pushes the gas through the boiler, and its maker sets a maximum exhaust back pressure, typically a few hundred mmWC including silencer and stack. Within that limit the boiler needs no fan. HFO exhaust carries soot, unburnt oil and sulphur, and it fouls the tubes and raises the dew point, which sets how low the exit gas may be cooled.
Where the ID fan sits
- Engine turbocharger outlet
- Diverter damper
- Waste heat boiler
- Silencer
- ID (booster) fan
- Stack
Where fitted, the fan sits after the boiler and silencer, handling exhaust cooled to about 170 to 220°C. Its job is to take away part of the boiler and duct resistance so the turbocharger outlet stays within the maker's back pressure limit. It is controlled from the pressure measured at the engine outlet, held slightly positive, not from a fixed speed, because pulling the outlet into suction can disturb the engine and draws air in through the duct joints.
Gas at the ID fan
| Gas temperature, normal | 170 to 220°C after the boiler |
| Maximum or upset | Engine exhaust temperature, 350 to 450°C, if the boiler is bypassed through the fan route or water is lost |
| Dust at the fan | Soot and oily carbon, heavier with HFO and at low engine load |
| Moisture and dew point | About 6 to 8% water vapour; acid dew point rises with fuel sulphur, notable with HFO |
| Back pressure | The engine maker's allowance, typically a few hundred mmWC, shared by boiler, silencer, ducts and stack |
| Density at the design temperature | About 0.75 kg/m³ at 200°C, 63% 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 diesel engine exhaust waste heat boilers
| Blade form | Backward curved with a self-cleaning profile, sized for a low pressure rise |
| Material of construction | Mild steel up to normal duty; heat-resisting steel impeller where bypassed exhaust can reach the fan |
| Drive and arrangement | Direct coupled, arrangement 8, with an insulated casing |
| Bearings and cooling | Pedestal bearings with a shaft cooling disc |
| Flow control | Variable speed drive on engine outlet pressure |
| Also specified | Interlock with the diverter damper so a fan trip sends exhaust to the bypass stack; casing drain |
What this ID fan has to cope with
Back pressure is the engine maker's limit
Exceeding the allowed back pressure raises exhaust temperature, reduces turbocharger performance and can affect engine warranty. The boiler is designed first to stay inside the limit without a fan. Only when that is not possible is a booster fan added, and its control keeps the engine outlet pressure in the allowed band.
Soot and oil fouling
At low engine load and with HFO, the exhaust carries oily soot that coats the boiler tubes and the fan blades. The coating can catch fire in the boiler. Soot blowing, a fan blade shape that sheds deposits and access for washing reduce both the unbalance and the fire risk.
Fan trip with the engine running
If a booster fan stops while the engine runs, the boiler path suddenly becomes a restriction. The diverter damper must switch the exhaust to the bypass stack at once, interlocked with the fan, so the engine never sees a dead-ended exhaust. The changeover is tested at commissioning with the engine running at load.
Energy and control
Generating sets often run at varying load, and exhaust flow and temperature follow it. Where a booster fan is installed, speed control from engine outlet pressure lets it do only the work needed, often very little at part load. The first energy measure, though, is boiler and duct design with low resistance, which avoids the fan altogether and leaves the full exhaust energy for steam.
Questions buyers ask
Does a DG set waste heat boiler need an ID fan?
Usually not. The engine's exhaust pressure drives the gas through the boiler, silencer and stack, as long as the total stays within the maker's back pressure limit. A fan is added only where several engines feed one boiler, ducts are long, or extra heat recovery surface pushes resistance past the limit.
What is the allowable back pressure on a diesel engine exhaust boiler?
It is set by the engine maker and varies by engine, typically a few hundred mmWC for the whole exhaust system. The boiler, silencer, ducts and stack must share it. The engine documentation should be checked before the boiler is sized. Silencers often take a large share, so their pressure drop should be known before any fan is considered.
Can a booster fan damage the engine?
If it pulls the turbocharger outlet into suction, it can disturb engine operation and draw air into the ducts. That is why a booster fan is controlled from engine outlet pressure and held at a small positive value, with an interlocked bypass damper for fan trips.
Information needed to quote an ID fan for diesel engine exhaust waste heat boilers
- 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.
- Engine data. Engine make, rating, fuel, exhaust flow and temperature, and the maker's back pressure limit.
- Exhaust system. Number of engines on the boiler, duct lengths, silencer and stack resistance.
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.