How crematoria work
A modern crematorium has a primary chamber, where the body is placed and burns at about 650 to 850°C, and a secondary chamber or afterburner, held at about 850 to 1,000°C, where smoke, odour and partly burnt organics are destroyed. Heat comes from LPG, natural gas or diesel burners, from electric elements, or from wood on a grate, depending on the type. A cremation takes roughly 60 to 120 minutes. The flue gas is cooled by dilution air, a water quench or a heat exchanger and cleaned in a cyclone, wet scrubber or bag filter before an ID fan sends it to a chimney. Combustion and secondary air come from an FD fan or burner blowers, except in electric units where the ID fan alone draws air in.
Where the FD fan sits
- Inlet filter
- FD fan
- Air header
- Primary chamber air ports
- Secondary chamber air
- Afterburner
The fan sits beside the cremator and delivers into a header with branches to the primary chamber air ports, the secondary chamber and, on some designs, a cooling air jacket around the door. Each branch has its own damper so that the operator or controller can change the split through the cycle. Required pressure is moderate, typically 100 to 300 mmWC.
Air at the FD fan
| Air temperature at the fan | Ambient, 25 to 45°C |
| Dust | Clean ambient air |
| Air split | Varies through the cycle between primary, secondary and door cooling air |
| Pressure at the fan outlet | Typically 100 to 300 mmWC |
| Operating pattern | Cyclic, with high demand during the main burn and low demand at the end |
| 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 crematoria
| Blade form | Backward curved, for quiet running near the hall |
| Material of construction | Mild steel, painted |
| Drive and arrangement | Direct drive |
| Bearings and cooling | Standard bearings |
| Flow control | Variable speed drive, with branch dampers for the primary and secondary air split |
| Also specified | Inlet silencer and acoustic casing where the fan is near the mourning hall |
What this FD fan has to cope with
Visible smoke from too little secondary air
The main burn releases volatiles quickly, and the secondary chamber needs extra air to finish burning them. A fan sized only for the burners leaves the afterburner short of oxygen during this peak, and dark smoke appears at the chimney. Sizing the fan for the peak secondary air need, with speed control for the rest of the cycle, cures it.
Overheating the hearth
Too much primary air during the main burn drives the hearth temperature up, damaging the refractory and making the burn hard to control, especially on wood fired units. Separate dampers or separate fans for primary and secondary air let the operator or the controller hold the primary chamber in range while the afterburner still gets the air it needs.
Noise near the hall
Crematoria are places of ceremony, and a combustion air fan running close to the hall is intrusive, especially a high-speed fan with an open inlet. A low-speed backward curved fan, an inlet silencer, anti-vibration mounts and an acoustic enclosure keep noise down to a level that does not disturb the families and staff nearby.
Energy and control
Air demand follows the cremation cycle closely. Speed control from the chamber oxygen or temperature lets the fan deliver more air during the main burn and much less during calcination and cool-down. Too much air cools the chambers and burns more fuel, so tighter air control saves fuel as well as fan power.
ID fan and FD fan in crematoria
| ID fan | FD fan | |
|---|---|---|
| Position | After gas cleaning, at the chimney | Beside the cremator, feeding air ports |
| What it handles | Flue gas, often saturated after a scrubber | Clean ambient air |
| Temperature at the fan | 50 to 80°C wet, 150 to 200°C dry | Ambient |
| Main design concern | Door-open suction and wet corrosion | Primary and secondary air split |
| Control | Chamber draft | Chamber temperature or oxygen |
See also the Induced Draft (ID) Fan for crematoria.
Questions buyers ask
Does a gas fired crematorium need a separate combustion air fan?
Packaged gas burners usually have their own blowers, so they do not depend on a central fan. A separate fan is still common on gas fired crematoria to supply secondary air to the afterburner and air to the primary chamber ports, because these flows must be controlled independently through the cycle to keep the chimney clear.
What is secondary air in a crematorium?
Secondary air is supplied to the secondary chamber, or afterburner, to burn the smoke, odour and volatiles leaving the primary chamber. It is essential for a clear chimney during the main burn, when volatile release peaks, and it is delivered through nozzles at enough velocity to mix with the gas at about 850 to 1,000°C.
Does an electric crematorium have an FD fan?
Usually not. With no burner, an electric crematorium needs only the air for burning the body itself, and the ID fan draws this in through controlled air inlets on the chamber. Some designs add a small blower for secondary air into the afterburner chamber, but there is no combustion air duty in the usual sense.
Information needed to quote an FD fan for 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 distribution. Number of air branches and the required split between primary, secondary and door cooling air.
- Burners. Whether burners have their own blowers and the fuel used.
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.