APEX® Technology

Protection against Corrosion

A-CORREX® Technology

The Expanding Need to Avoid Corrosion

Nowadays, the limitation in the reduction of the exhaust flue gas temperature is usually defined by the need to avoid acid condensation. Especially in the case of fired heaters, boilers or process furnaces, the heat exchangers in the air preheat systems are usually designed to have a minimum metal temperature close to, but above the acid dew point (ADP) of the flue gas (e.g. 10-15 °C above ADP) to avoid condensation of H2SO4 and, as a result, to avoid corrosion of the heat transfer surface.

Current impulses to further decrease the exhaust flue gas temperature below the acid dew point (ADP), but above the water dew point (WDP), motivated APEX Group to develop an innovative heat exchanger capable of working in a corrosive environment in heavy duty applications.

Process
Fuels (mainly gas, oil, coal or biomass), usually contain a certain amount of Sulfur components. During the combustion process, SOx is generated (mainly as SO2). Downstream of the flue gas, part of SO2 is oxidized to SO3 (conversion rate for gas/liquid fuels: 2% to 8%; conversion rate for coal: 0.05% to 1%). Then, SO3 combines with H2O to form H2SO4 gaseous (g) defined by its ADP. If the metal temperature falls below the ADP, variable concentrations of H2SO4 liquid (l) will condense on the surface.

Protection against Cold-end Corrosion
Standard metallic materials, like carbon steel or stainless steel, are not resistant against acid corrosion.

APEX Group’s solution to this common problem is the development of the A-CORREX Technology: a range of heat exchangers handling highly corrosive fluids and suitable for service in region B:

Materials of Construction

To manufacture a heat exchanger capable to resist corrosive environments, APEX Group uses the following materials of construction for the heat transfer elements:

PPS-Gr composite
Metallic substrates enameled with borosilicate glass
Metallic substrates with polymer lining
Steel alloys

Key Benefits

Resistant to chemical attacks below the ADP and WDP;
Extended lifetime of the heat exchanger;
Downstream equipment is not exposed to the acidic condensate liquid entrained with the flue gas stream

Acid resistant heat exchanger constructed from polymer composite: PPS Gr

Resistant to virtually any chemical attack below ADP and WDP.
Light weight.
Operating temperature up to 200 °C (short term peaks: 220 °C).
Good resistance to heat cycles.
High thermal conductivity > 3 W / m-1 K-1.

Applicable to the following APEX Group Products: V-FLEX, CORTEX and C-PEX Heat Exchangers.

Typical applications: DeNOx and/or DeSOx gas/gas heater, flue gas cold-end air preheater, CCS flue gas cooler.

Acid resistant heat exchanger constructed from metallic substrate enamelled with borosilicate glass

Borosilicate glass enamel ensures protection of metallic substrates against attack of sulfuric acid condensate.
Operating temperature up to 400 °C.
Operating pressure up to 5 barg for non-pressure vessel, virtually no limit for pressure vessel.
High thermal conductivity of metallic substrate.
Resistant to thermal shocks and heat cycles; they can resist temperature variations up to 400 °C.
Remarkable resistance against high differential pressure during operation.
Excellent mechanical strength.

Applicable to the following APEX Group Products: CORPEX, CORTEX and C-PEX Heat Exchangers.

Typical applications: flue gas cold-end air preheater.

Acid resistant heat exchanger constructed from metallic substrates with polymer lining

Polymer lining, typically PFA, ensures excellent protection of metallic substrate against virtually any chemical attack below ADP (but above WDP);
Preserves the mechanical strength of the metallic substrate.
Operating temperature up to 260 °C constant, 350 °C intermittent
Operating pressure up to 5 barg for non-pressure vessel, virtually no limit for pressure vessel.
Resistant to thermal shocks and heat cycles.

Applicable to the following APEX Group Products: CORTEX and C-PEX.

Typical applications: flue gas cold-end air preheater, flue gas economizers, flue gas coolers.

Acid resistant heat exchanger constructed from steel alloys

Satisfactory to excellent resistance to corrosion by selected acids or alkalis at specific concentration and temperature.
Excellent mechanical strength and integrity.
Operating temperature up to 600 °C.
Operating pressure up to 5 barg for non-pressure vessel, virtually no limit for pressure vessel.
Resistant to thermal shocks and heat cycles.

Applicable to the following APEX Group Products: CORPEX, CORTEX and C-PEX.

Typical applications: flue gas cold-end air preheater, flue gas economizers, flue gas coolers.