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 FD fan sits
- Inlet
- FD fan
- Air header
- Hearth air ports
- Afterburner air nozzles
The fan is mounted next to the cremator and feeds a header with branches to the primary chamber air ports, the afterburner nozzles and, on some designs, the door cooling jacket. Branch dampers or separate fans set each flow. The pressure needed is modest, about 100 to 250 mmWC, enough to give nozzle velocities that mix the air into the gas. It draws ambient air through a filtered inlet away from the chimney.
Air at the FD fan
| Air temperature at the fan | Ambient, 25 to 45°C |
| Dust | Clean air |
| Pressure at the fan outlet | Typically 100 to 250 mmWC |
| Air demand | Highest in the main burn, low in calcination |
| Burner air | Supplied separately by packaged burner blowers in most designs |
| 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 gas fired crematoria
| Blade form | Backward curved, quiet at moderate pressure |
| Material of construction | Mild steel |
| Drive and arrangement | Direct drive |
| Bearings and cooling | Standard bearings |
| Flow control | Variable speed drive with motorised branch dampers |
| Also specified | Inlet silencer and low-noise motor |
What this FD fan has to cope with
Too little air during the main burn
Volatile release peaks within minutes of charging. An undersized air fan leaves the afterburner short of oxygen during this peak, and the chimney smokes even though the burners are firing correctly. Sizing the fan for the peak secondary air demand, with speed control for the rest of the cycle, avoids visible smoke without wasting air later.
Air nozzles that do not mix
Low nozzle velocity lets the secondary air run along the chamber wall without mixing into the gas, leaving unburnt pockets that pass to the chimney. Selecting the fan pressure for proper nozzle velocity at the peak flow, rather than just the flow itself, gives good mixing and complete burnout in the afterburner.
Fuel wasted by cold air
Excess air cools the chambers, and the gas burners then fire harder to hold the afterburner temperature, so fuel use climbs. Controlling the air flow from the chamber oxygen or temperature, rather than leaving the fan at full flow through the whole cycle, keeps LPG or natural gas consumption down per cremation.
Energy and control
Air demand drops sharply during calcination, the last half of the cycle. A variable speed drive tied to the cremator controller follows the phases, saving power and avoiding excess air that the burners must heat. The fuel saving from correct air control is usually larger than the fan power saving.
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 Induced Draft (ID) Fan for gas fired crematoria.
Questions buyers ask
What does the combustion air fan do on an LPG crematorium?
On an LPG crematorium the combustion air fan supplies air to the hearth ports and the afterburner, separate from the burner blowers. It changes the split through the cremation cycle, giving more secondary air during the main burn and less during calcination, to keep combustion complete and the chimney clear without wasting fuel on excess air.
How much pressure does a crematorium air fan need?
Usually 100 to 250 mmWC. The fan must push air through the header, the branch dampers and the nozzles, and give the nozzles enough velocity to mix the air into the gas in the chambers. The exact figure depends on the nozzle design and the length of the air piping around the cremator.
Can one fan supply both primary and secondary air?
Yes. One fan with a header and motorised branch dampers can supply both primary and secondary air on a gas fired crematorium. Some designs use two smaller fans instead, one for each chamber, which gives fully independent control and a degree of redundancy if one fan is out of service.
Information needed to quote an FD fan for gas fired crematoria
- 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.
- Air branches. Number of air ports and nozzles in each chamber and the flow to each.
- Control. Whether air is controlled from chamber temperature, oxygen or a fixed programme.
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.