High Temperature Induced Draft Fan for 500 C Service

Overview

500 C marks the boundary between carbon steel and heat resistant steel. The impeller and casing move to 16Mng or 15CrMo, the coating becomes an inorganic zinc silicate system, and low temperature dew point corrosion has to be addressed in the same design.

Material Selection at 500 C

Creep resistance replaces simple strength as the governing property at this level.

Location	Material / process	Basis
Impeller and casing	16Mng or 15CrMo (GB/T 713, GB/T 5310)	Better creep resistance than carbon steel
Surface treatment	Inorganic zinc silicate primer with heat resistant topcoat	Handles heat and flue gas attack together
Bearing assembly	Insulated housing with extended shaft	Reduces the temperature at the bearings
Insulation	100 mm aluminium silicate fibre	Controls shell temperature and heat loss
Sealing	Labyrinth seal combined with graphite packing	Limits hot gas leakage

Structural Measures at 500 C

Location	Material / process	Basis
Thermal isolation of the drive	Insulating plate between housing and casing	Blocks radiation and conduction paths
Sliding feet	Fixed foot paired with a sliding foot	Releases axial growth at 500 C
Thicker welds at wear zones	Wider beads and added allowance	Combines hot strength with wear resistance
Dew point control	Operation kept above the acid dew point	Prevents low temperature corrosion
Expansion joints	Bellows joints in hot ducts (GB/T 12777)	Absorbs duct movement

Applications

Applications include coal fired boiler flue gas, hot air furnaces, ceramic roller kilns and waste incineration heat recovery.

Boiler flue gas usually contains sulphur dioxide and moisture, so corrosion below the dew point must be considered together with temperature.

If the gas temperature stays below 150 C for long periods, dew point corrosion should be re-assessed and the material upgraded or the inlet air preheated.

Acceptance and Verification

Standard	Title	Purpose
GB/T 713	Steel plates for boilers and pressure vessels	Heat resistant plate acceptance
GB/T 1236	Industrial fans - Performance testing using standardized airways	Air flow, pressure and efficiency testing
GB/T 12777	General specification for metal bellows expansion joints	Expansion compensation for hot ducts and casing
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

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

FAQ

Why does a 500 C fan also need corrosion protection?

Boiler and kiln flue gas contains sulphur dioxide and water vapour. Below the acid dew point sulphurous acid forms and corrosion accelerates sharply, so material and insulation must address temperature and dew point together.

How do I choose between 15CrMo and 16Mng?

Temperature and moisture content decide. Where creep risk dominates at higher temperature, 15CrMo is preferred; where flue gas corrosion dominates and temperature stays under 500 C, 16Mng with a coating is more economical.

Does thickening the blades reduce air flow?

Adding thickness changes the flow passage and the rotor mass, so air flow, pressure and balancing all have to be recalculated rather than substituted directly.

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