When an exhaust gas fan on an ammonia stripping plant is replaced, the key choices are direct drive, protection from heat, the material of the casing and how the fan is mounted. A quotation for a landfill plant specified a 75 kW direct-drive fan with a radiation shield and cooling impeller, a painted carbon steel casing, and an optional 304L stainless steel casing.
The challenge: a fan handling hot exhaust
The exhaust gas fan moves hot gas out of the process, so it sees high temperature and corrosive conditions all day. Heat travels along the shaft to the bearings and the motor. If the casing corrodes or the impeller wears, the fan loses performance and vibration rises. When that happens, a complete replacement may be the practical choice.
The approach: a direct replacement for a known serial number
The fan
The supplier quoted a fan to replace one of a given serial number, so the connections, flow and pressure match. The main features are:
- Direct drive through a flexible coupling to a 75 kW motor, a high-efficiency (IE4) design with IP55 protection.
- A radiation shield and a cooling impeller, which keep heat from the fan away from the bearing and motor.
- A flanged inlet to suit the existing duct.
- An impeller in a specified steel grade, chosen for the stresses of a large fan.
- A carbon steel casing and pedestal painted with a black high-temperature paint.
The stainless steel option
An alternative offered the casing and pedestal in 304L stainless steel. It costs more but resists corrosion better where condensate or ammonia is present. Carbon steel is the standard choice if the gas is hot and dry enough that corrosion is not a problem. The quotation gave both and left the choice to the operator.
Anti-vibration mounts
A later revision added a set of eight anti-vibration mounts. A fan of this size creates vibration that can crack pipework or damage the foundation, so mounts are an important part of the installation, not an extra.
Lead time
The fan took eight working weeks to make in the UK, followed by five to seven weeks by sea. That is roughly 13 to 15 weeks from order to arrival, so a plant without a spare could be down for months.
Why it works
Quoting against the old fan’s serial number lets the manufacturer reproduce the duty exactly. Direct drive removes the belts, which are one less item to maintain. A shield and cooling impeller are the standard answer to heat conducting along the shaft. Offering a stainless option separates the decision on cost from the decision on life.
Where fans move flammable or explosive gas, extra requirements apply. For the UK, see the HSE’s guidance on ATEX and explosive atmospheres.
What to consider for your own project
- Quote the existing serial number. It saves guesswork.
- Look at gas temperature and moisture before choosing carbon or stainless steel.
- Include anti-vibration mounts in the first order.
- Plan for lead time, or hold the impeller and shaft as spares.
- Check the motor. An efficient, well-protected motor reduces running cost and trips.
For the wider approach to planning replacement parts, see our post on replacing obsolete instrumentation and controls.
FAQ
What is a radiation shield on a fan?
A plate that blocks heat radiating from the hot casing towards the bearing and motor.
Why is there a cooling impeller?
It moves air over the shaft to remove heat before it reaches the bearing.
Should I choose a stainless casing?
If the gas is wet or corrosive, yes. If it is hot and dry, carbon steel with high-temperature paint may do.
How long does a replacement fan take?
About eight weeks to make and five to seven weeks of sea freight in this case.
Replacing a fan?
Send us the serial number of your fan and we will quote a like-for-like replacement.