How regenerative thermal oxidisers work
Regenerative thermal oxidisers destroy VOCs in exhaust air from coating lines, printing, pharma and chemical processes. The air passes through a bed of ceramic media that has been heated by previously treated gas, reaches a combustion chamber at about 800 to 850°C where the VOCs oxidise, and leaves through a second bed that absorbs its heat. Valves switch the flow direction every one to three minutes. Thermal efficiency is typically 95% or more, so outlet gas is only about 30 to 60°C warmer than inlet air. A small burner supplies the heat not provided by the VOCs. The main system fan either pushes process air into the RTO or, as an ID fan, pulls clean gas out of it to the stack.
Where the ID fan sits
- Process exhaust hoods
- Collection duct
- Inlet ceramic bed
- Combustion chamber
- Outlet ceramic bed
- ID fan
- Stack
The fan sits after the RTO outlet, at the stack. It works against the collection duct, the switching valves, two ceramic beds and the combustion chamber, typically 400 to 700 mmWC. Placing it after the RTO keeps the ducts and RTO under suction, so VOC-laden air cannot leak out, and keeps condensable VOCs, sticky resins and dust off the impeller. Push-through systems put the fan before the RTO instead.
Gas at the ID fan
| Gas temperature, normal | 60 to 120°C at the RTO outlet |
| Maximum or upset | 200 to 400°C with hot side bypass at high VOC load |
| Dust at the fan | Clean; fine ceramic dust from media wear |
| Moisture and dew point | Process humidity plus water from VOC oxidation; condensation possible at low outlet temperature |
| Pressure pulses | Short pressure swings each time the valves switch |
| Density at the design temperature | About 0.95 kg/m³ at 100°C, 79% 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 regenerative thermal oxidisers
| Blade form | Backward curved for efficiency on clean air |
| Material of construction | Mild steel; stainless steel where halogenated or sulphur VOCs make outlet gas acidic |
| Drive and arrangement | Direct coupled, arrangement 8 for larger units |
| Bearings and cooling | Pedestal bearings; cooling disc where hot bypass raises temperature |
| Flow control | Variable speed drive on inlet duct pressure |
| Also specified | Fresh air dilution damper, casing drain, and inspection door |
What this ID fan has to cope with
Pressure pulses at valve switching
Each valve change briefly disturbs flow and pressure in the system. If the fan and its control react too strongly, suction at the source hoods swings and VOC can escape. A fan with a stable curve and a drive tuned with suitable damping avoid hunting.
Hot side bypass temperature
At high VOC concentrations, hot gas from the combustion chamber is bypassed around the outlet bed to avoid overheating. This raises the gas temperature at the fan sharply. The fan is selected for the bypass temperature, with bearings and impeller rated to suit.
Acidic condensate
Chlorinated or sulphur-bearing VOCs produce HCl or SO₂. With low outlet temperatures, these condense in the fan and stack. Stainless steel construction, or a scrubber and an outlet temperature kept above dew point, protect the fan.
Energy and control
The RTO fan runs whenever the process runs and is often the largest motor on the unit. Its power is set by flow and the pressure drop of the beds. A variable speed drive on source suction follows process demand, such as lines stopping or hoods closing. Keeping media beds clean of particulate and condensed resins holds the pressure drop at design.
Questions buyers ask
Should the RTO fan be before or after the oxidiser?
After, as an ID fan, keeps the system under suction and the fan clean, but the fan sees warmer gas and hot bypass temperatures. Before, as a push fan, handles cooler process air but is exposed to VOC condensation and dust, and leaks go outwards.
What pressure does an RTO system fan need?
Typically 400 to 700 mmWC, covering ducts, switching valves, two ceramic beds and the combustion chamber. Beds that are partly blocked by particulate or condensed resin increase this, so the fan selection should include margin for bed fouling between cleanings.
What temperature does the ID fan on a regenerative thermal oxidiser see?
Usually 60 to 120°C, only slightly above the inlet air temperature, because the ceramic beds recover most of the heat. With hot side bypass at high VOC loads it can rise to 200 to 400°C.
Information needed to quote an ID fan for regenerative thermal oxidisers
- 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.
- VOC type. Main VOCs, concentration range, and any halogenated or sulphur compounds.
- Bypass. Whether the RTO has hot side bypass and the maximum bypass temperature.
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.