Air preheater tube

Tubular air preheater is designed for boiler, kiln, furnace to improve combustion air temperature by heat transfer from flue gas. APH is key equipment to improve thermal efficiency and reduce fuel consumption. So, it get widely used in sectors like power generation, cement, paper, glass, petrochemical, metallurgy, etc. But in normal operation, air preheater tubes must be replaced every several years due to fly ash erosion and dew point corrosion. It is important to select proper APH tube material to extend service life for economic benefits of plant.

 

Fly ash erosion factors

 

Fly ash velocity: erosion rate is proportional to cube of fly ash speed. If flue gas velocity doubles, erosion will increase by 7 times. But lower velocity of flue gas means lower coefficient of convective heat transfer, a larger heating surface is required to transfer certain amount of heat. And reduced flue gas velocity would result increased soot deposit on boiler tubes.

Fly ash concentration: higher concentration means more collisions between fly ash particles and boiler tubes.

Fly ash property: the particles are bigger and harder, the erosion is worse.

Fly ash collision ratio: generally speaking, fly ash with bigger size, higher density and velocity has higher chance to collide with boiler tubes.

 

Dew point corrosion

 

After combustion, sulfur in boiler fuel will be converted into SO2, some of which will be oxidized into SO3, which will turn into acid vapor after reaction with water vapor in flue gas. Acid dew point temperature is much higher than that of water, it could be up to 140~160℃. Acid vapor is no harm to tube metal, but when tube wall temperature is lower than dew point, acid vapor would condensate on tube surface and cause series corrosion.

 

Enamel tube for air preheater

 

Enamel tube is produced by coating high temperature & corrosion & erosion resistant enamel material onto surface of carbon steel tube, and then sintered under high temperature environment.

 

Benefits of APH enamel tube

 

1. Corrosion resistant. Its resistance to corrosion is hundred times of Q235 carbon steel tube. Air preheater made of enamel tube could allow exhaust temperature lower than 140℃. Thermal efficiency could be improved by 2%.
2. Lower pressure drop. Absolute roughness of enamel tube surface is only 1/40 of Q235 tube. Its smooth surface results in less soot deposit and lower pressure drop of flue gas.
3. Erosion resistant. Hardness of enamel layer is 6 times of steel tube. Service life is longer.

 

Corten steel tube for APH

 

Corten tube is low alloy high strength weathering tube. It is produced by adding some anti-corrosion elements like Cu, Ni into carbon steel. Its weathering ability is 2~8 times of carbon steel. So, it could serve a longer time in poor condition. Great ability in dew point corrosion makes it widely used to fabricate air preheater. Popular material is ASTM A847, Q355GNHL, 09CuPCrNi-A, SPH-A, etc.

 

ND steel tube for APH

 

ND steel, also called 09CrCuSb in China, now is the best material to withstand dewpoint corrosion of flue gas. Moreover, it has good resistance to corrosion caused by chloride ion. So, ND steel tube is used for fabrication of air preheater popularly.

 

Below is a comparison of corrosion rate between different steel grades.

 

Steel grade	ND	CRIR	1Cr18Ni9	Corten	20G
Corrosion rate	7.30	13.40	21.70	63.00	103.50

Physical properties

 

Steel grade	Tube wall thickness (mm)	Tensile strength (MPa)	Yield strength (MPa)	Elongation (%)
09CrCuSb	≤8	390~550	≥245	≥25

Chemical composition

 

	C	Si	Mn	Cr	Cu	Sb	P	S
09CrCuSb	≤0.12	0.20~0.40	0.35~0.65	0.70~1.10	0.25~0.45	0.04~0.10	≤0.030	≤0.020