High temperature induced draft fan

High Temperature Induced Draft Fan for 700 C Service

Product description: Induced draft fan for 700 C service: material selection, insulation and water-cooled bearing design, applicable processes and acceptance standards.
INQUIRY

Overview

At 700 C the material has to move to austenitic stainless steel. Grade 304 or 321 protects itself with a dense oxide film and no longer depends on a coating, while a thermal sleeve on the shaft and water cooling on the bearing housing carry the heat load.

Material Selection at 700 C

Austenitic stainless steels resist scaling far better than pearlitic grades at this temperature.

Location	Material / process	Basis
Impeller and casing	06Cr19Ni10 or 06Cr18Ni11Ti (GB/T 4237)	Better scaling resistance than pearlitic steel at 700 C
Welding consumables	Stainless electrodes (GB/T 983)	Matches strength and corrosion resistance
Bearing assembly	Water cooled housing with extended shaft	Isolates radiant heat
Insulation	Ceramic fibre lining with external insulation	Lowers wall temperature and heat loss
Fasteners	Heat resistant stainless steel	Prevents seizure from oxidation

Structural Measures at 700 C

Location	Material / process	Basis
All stainless impeller	Fabricated to a stainless welding procedure	Avoids dissimilar metal thermal stress
Thermal sleeve on the shaft	Insulation between impeller and bearing	Reduces heat flow to the bearings
Sliding guide support	One fixed and one sliding leg	Releases large thermal growth
Re-balancing after fabrication	G6.3 or tighter as required	Controls vibration at temperature
Internal lining anchors	Stainless anchors hold the fibre	Prevents lining collapse

Applications

Applications include metallurgical reheat and annealing furnaces, forging furnace off gas and heat recovery boilers.

Austenitic stainless steel relies on a dense oxide film. Where the medium contains sulphur, the stability of that film has to be assessed.

Where the fan is started and stopped frequently, the heating rate should be controlled to avoid thermal shock cracking of the lining and impeller.

Acceptance and Verification

Standard	Title	Purpose
GB/T 4237	Hot rolled stainless steel plate, sheet and strip	Stainless impeller and casing material
GB/T 983	Stainless steel covered electrodes	Weld strength and corrosion resistance
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
GB/T 3003	Refractory fibres and products	High temperature lining material and acceptance

Please provide gas composition, temperature, dust load and air flow. Fan size and motor power are calculated for the actual duty.

FAQ

Should I choose 304 or 321?

Grade 321 contains titanium, which gives better intergranular corrosion resistance and high temperature stability for continuous duty above 600 C. Grade 304 costs less and suits duties where temperature fluctuates little. Neither is suitable for high sulphur gas.

Does stainless steel still need corrosion protection?

It has to be assessed. Stainless steel corrodes in chloride bearing or reducing sulphur atmospheres, and such duties call for 316L, duplex grades or an internal lining.

How long does a 700 C impeller last?

Life depends on dust load, temperature fluctuation and the number of starts. Wall thickness measurement and vibration monitoring give a better replacement interval than a fixed service life.