Industrial Applications: Heat Exchangers

Rotary heat exchangers and enamel

Rotary heat exchangers are located in heat recuperation and fume treatment systems. They use small sheets of metal that are subjected to significant acidic and thermal shocks. Enamel lengthens their working life.

The question has been asked “What are heat exchangers and what relationships do they have with vitreous enamel?” We can answer that question immediately, with the assistance of companies which for more than 25 years, have been enamelling the exchange elements (small metal sheets) used in the rotary heat exchangers (Ljungström) installed in fuel-burning power stations, applying the experience and know-how necessary for enamelling in compliance with all the international standards and specifications.
Rotary heat exchangers (RA) are located at the exit of the boilers, so as to recuperate the heat of exhaust fumes and transfer it to the combustion air, and in fume treatment facilities (GGH), to exchange heat between the fumes before and after the purification treatment.

The heat exchange matrix comprises shaped sheets installed in the rotor, “packaged” inside containers (baskets), to ensure that they do not move or vibrate during rotation or when subjected to the action of the steam blowers or washing.
During the rotary movement, the heat given off by the fumes is absorbed with each turn of the rotor by the exchange surface as its passes through the waste fumes area and is then yielded up to the combustion air when it passes through the inlet air area.
The exchange elements are subject to deterioration, especially as a result of the aggressive acidic nature of the fumes at low temperatures.
Their working life depends on the type of fuel being used, the temperature of the fumes and the air and the humidity of the coumbustion air.

For this reason, if the surfaces are exposed to severe operating conditions of corrosion, enamelling can increase the working life of the elements quite considerably.
A vitreous enamel coating is therefore eminently suitable for preserving the sheets against the aggressive actions of acidic fumes and abrasion by particles suspended within the fumes.
One of the enamelling techniques used these days which guarantees a very high quality of the coating is achieved by applying the vitreous enamel in the liquid state “by automatic dipping followed by the electrostatic coating of the sheet edges”.
Before this process, the sheets are pre-treated with phases of degreasing, pickling and nickel-plating.
Another possibility is the electrostatic powder enamelling.
When completed, the surface of the enamel coat must be perfectly.

Functional Characteristics of enamel coatings
Characteristic Reference Requirements References
Support suitable for enamel coating Decarburised steel DC03-04ED EN 10209
Thickness of enamel Min. 140 μm - Max. 230 μm ISO 2178
Blistering Total blister surface should be less than 30%
of total surface of coating
Metallographic examination
Porosity - high voltage Examination with spark meter - voltage
of 5 KV. Max. defects 2 pores per dmq
ISO 2746
Porosity - low voltage Examination with voltage of 9 V. Max. defects
20 pores per m2
ISO 8289
Resistance to thermal shock from 250° C to 450° C, at 50° C increments
- five cycles of tests in water
D.E.Z - MB 7.12.1
Chemical Resistance    
Resistance to boiling sulphuric acid H2SO4 - 30% concentration boiled for six
hours at 105° C
Weight loss in six hours - max. 0.8 gr/m2
Weight loss in 168 hours - max. 2.5 gr/m2
ISO 2743
ISO 2733
Resistance to cold citric acid Class AA ISO 2722
Mechanical resistance    
Adherence The enamel coating must adhere to the steel
support Level 2
D.E.Z – MB 7.10.3
EN 10209 annex D
Shock resistance No damage with a diameter of more than 2
mm after 24 hours, with a shock of 20 N
ISO 4532



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