High Temperature Induced Draft Fan for 400 C Service

Overview

400 C sits near the limit of plain carbon steel. The practical answer is to increase the strength margin with Q345B, protect the surface with a silicone heat resistant paint and thicken the thermal barrier around the bearing assembly.

Material Selection at 400 C

Strength margin becomes important at this level, while coating choice still dominates oxidation protection.

Location	Material / process	Basis
Impeller and casing	Q235B or Q345B (GB/T 1591)	Greater strength margin than plain carbon steel
Surface treatment	Silicone heat resistant paint rated to 400 C	Resists thermal oxidation and light attack
Bearing assembly	External housing with thermal break pads	Interrupts the heat path
Insulation	80 mm aluminium silicate fibre	Cuts shell temperature and heat loss
Coupling	Expansion gap sized for thermal growth	Prevents the shaft binding when hot

Structural Measures at 400 C

Location	Material / process	Basis
Gap at the coupling	Sized from the calculated thermal elongation	Avoids binding in the hot state
Blades thickened by 2 mm	Extra allowance at wear prone areas	Combines temperature and wear resistance
Sliding guide support	Casing slides on the base frame	Absorbs radial expansion
High temperature lubrication	Grease rated for the duty or oil circulation	Maintains bearing life under radiant heat
Thermal break at the base	Insulating pad between casing and frame	Reduces heat soak into the frame

Applications

Typical duties include asphalt mixing plants, dewatering kilns, textile stenters, gypsum board drying lines and chemical reaction off gas.

Moisture and mild acidity begin to appear at this level, and coating selection generally pays off better than a material upgrade.

Where the medium carries moisture, a water trap or demister ahead of the fan reduces deposit build up on the blades.

Acceptance and Verification

Standard	Title	Purpose
GB/T 1236	Industrial fans - Performance testing using standardized airways	Air flow, pressure and efficiency testing
GB/T 9239.1 / ISO 21940-11	Mechanical vibration - Balance quality requirements for rigid rotors	Impeller balancing grade
JB/T 10563	General purpose centrifugal fans - Technical specifications	General technical conditions and works acceptance
GB/T 2888	Methods of noise measurement for fans and blowers	Noise measurement and acceptance
GB/T 12777	General specification for metal bellows expansion joints	Expansion compensation for hot ducts and casing

Please provide medium temperature, composition, moisture content and air flow. Fan size and motor power are calculated for the actual duty.

FAQ

Why not use stainless steel at 400 C?

Q345B already provides adequate strength at this temperature and the corrosion resistance of stainless steel is not needed. Stainless becomes worthwhile only when the medium contains sulphur, chlorides or significant moisture.

How long does silicone paint last at 400 C?

Silicone heat resistant paints are typically rated to about 400 C, which suits this band. Above 500 C an inorganic zinc silicate system, or a material that does not rely on a coating, should be used.

Why does noise increase during hot operation?

It is usually caused by the clearance between impeller and inlet changing with thermal expansion. Clearances should be set for the calculated growth and the rotor re-balanced.

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