How gas fired crematoria work
A gas fired crematorium uses LPG, natural gas or piped gas burners in both the primary and secondary chambers. The primary burner preheats the hearth to about 650°C and starts the cremation; once the body is burning, its own fat and tissue release heat and the burner turns down. The secondary burner holds the afterburner at about 850 to 1,000°C. A cycle takes about 60 to 90 minutes, and two or three cremations can be done back to back without cooling the chamber. Flue gas leaving the afterburner is cooled with dilution air or a water quench, passed through a scrubber or bag filter, and sent to a chimney by an ID fan. Burner blowers and a separate air fan supply combustion and secondary air.
Where the ID fan sits
- Primary chamber
- Afterburner
- Dilution air or quench
- Wet scrubber or bag filter
- ID fan
- Chimney
The fan is after the gas cleaning. If the plant uses dilution air to cool the gas to 150 to 200°C ahead of a bag filter, the fan is a dry, warm duty and the dilution air increases its volume two to three times. If it uses a water quench and scrubber, the fan sees saturated gas at 50 to 75°C. Suction is typically 120 to 250 mmWC, covering the chambers, quench and scrubber.
Gas at the ID fan
| Gas temperature, normal | 50 to 75°C wet; 150 to 200°C with dilution and bag filter |
| Maximum or upset | 300 to 400°C if dilution or quench fails |
| Dust at the fan | Very low; fine ash and calcium phosphate particles |
| Moisture and dew point | Gas firing and the body give 15 to 20% water vapour before any quench |
| Corrosive species | HCl from coffin furnishings and plastics; mercury vapour |
| Density at the design temperature | About 1.03 kg/m³ at 70°C, 86% 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 gas fired crematoria
| Blade form | Backward curved, on low dust gas |
| Material of construction | SS 316 for wet duty; mild steel for dry gas kept above dew point |
| Drive and arrangement | Direct drive, arrangement 4 or 8 |
| Bearings and cooling | Standard bearings; cooling disc for the dry duty |
| Flow control | Variable speed drive on primary chamber pressure |
| Also specified | Casing drain, high-temperature trip and a door switch signal to the drive |
What this ID fan has to cope with
Dilution air doubles the fan
Cooling with ambient air is simple but adds two or three times the flue gas volume, so the fan and bag filter become much larger. Comparing a quench or a heat exchanger against dilution at the design stage often gives a smaller fan with lower power.
Rapid volume swing in the main burn
Shortly after charging, the body releases volatiles quickly and the gas volume and temperature jump within a minute or two. If draft is held by a slow damper, the primary chamber goes positive and smoke leaks from the door seals. A variable speed drive responding to chamber pressure within a few seconds holds suction through the peak.
Mercury and acid in wet duty
Mercury from dental amalgam and HCl from coffin linings end up in the scrubber liquor and in the droplets that are carried to the fan. Stainless steel, usually SS 316, resists the acid, a mist eliminator keeps liquor off the impeller, and a casing drain stops liquid pooling at the scroll bottom between cremations.
Energy and control
Gas fired crematoria burn fuel mainly to heat air, so every cubic metre of excess air drawn through costs gas. Draft control on a variable speed drive, holding a small fixed suction rather than a large one, reduces air ingress at doors and peepholes. Between cremations the fan can run at minimum speed, which keeps the chamber warm and cuts both fuel and power.
ID fan and FD fan in gas fired crematoria
| ID fan | FD fan | |
|---|---|---|
| Position | After quench or dilution and gas cleaning | Beside the cremator |
| What it handles | Cooled flue gas, wet or dry | Ambient air |
| Temperature at the fan | 50 to 75°C wet, 150 to 200°C dry | Ambient |
| Main design concern | Volume swing and door-open draft | Air split through the cycle |
| Control | Chamber pressure | Chamber oxygen or temperature |
See also the Forced Draft (FD) Fan for gas fired crematoria.
Questions buyers ask
What size ID fan does a gas fired crematorium need?
It depends on the gas flow from the chambers and on the cooling method. With dilution air the flow at the fan is two to three times the flue gas from the chambers; with a quench or heat exchanger it is much smaller. Draft at the chamber and the gas cleaning resistance set the pressure.
Should the ID fan on an LPG crematorium be stainless steel?
If a wet scrubber is used, yes, because the gas leaving it is saturated and acidic, and an LPG crematorium often stands idle overnight with the casing cold. If the gas is cooled with dilution air and kept dry through a bag filter, mild steel is usually adequate, provided the fan is purged warm before shutdown.
Why does smoke come out when the crematorium door opens?
The ID fan cannot add suction fast enough when cold air rushes in through the open door, so the chamber briefly goes positive. A variable speed drive that receives a door-open signal and ramps up immediately, together with correct fan sizing for the door-open flow, keeps the chamber negative and the hall free of smoke.
Information needed to quote an ID fan for gas fired crematoria
- 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.
- Cooling method. Dilution air, water quench or heat exchanger, and the gas temperature after it.
- Burner ratings. Primary and secondary burner capacity and the fuel.
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.