PROBLEMS AND ANSWERS CONCERNING ENAMELLED AND PACKED HEAT EXCHANGERS FOR POWER PLANTS
Anton W.C.M. van Dijk
Ferro Techniek BV, Bremstraat 1, NL-7011 AT, Gaanderen, The Netherlands

Introduction
In this paper we will inform you about corrosion problems, caused by flue gasses in power plants.
We will talk about the use of an enamel coating with his corrosion braking effect on heat exchangers. Then you will be informed about as well as the big advantage as the restrictions of the enamel properties.
During the operation of the regenerative heat exchanger, the heat exchange of the enamelled and packed heat exchanger is slowly but permanent reduced, caused by erosion, corrosion and fouling.
So a lot of enamel parameters and influences of the packing of the elements will affect the lifetime.
You will be informed about the influences of steel, enamel, enamel application and enamel properties on the lifetime of the heat exchanger. In this case the packing has his restrictions too.
As well as pros and contras of the basket as the elements designs are discussed.At the end a computer program will give you the expected lifetime.

Surface requirements of heat exchangers

Heat exchangers used to regulate temperature in e.g. desulphurizing plants are in direct contact with the flue-gasses. To fulfil their function - the transfer of heat for many years - the surface of heat exchangers must meet following requirements:
:: High transfer of heat
:: Low flow resistance
:: Easy-to-clean surface
:: Resistance to corrosion, filth and erosion
:: Long lifetime in aggressive environments
:: Favourable surface properties
:: Resistance to temperature-shocks
:: Fire-resistance
:: Fast and easy to replace

Corrosion behaviour
In defects (open connections with the steel element, as pores, cracks, chipping, no edge covering and damages) the actual corrosion will start by the attack of (Fig.)

Defects (ISO 8289)

water and vapour on the steel according to the reaction:

Fe + 2H2O → Fe(OH)2 + H2

At first, the iron hydroxide will give the steel passivity and therefore a corrosion resistant coat. But caused by the more or less concentrated minerals in the condenses of the flue-gasses, this passivity coat is dissolved in the following step, according to the reaction:

Fe(OH)2 + 2H+X- → Fe++(X-)2 + 2H2O

Important here is the acid ion H3O+, coming by dissociation of the mineral acids in a water solution.
If the condense contents more surplus of H3O+ ions, the faster the dissolving of the metal will take place. Due to continued emission of acid ions and fast discharge of the iron salts, the dissolving of iron is linear.
Unfortunately, several tests proved that not only condense of the untreated flue-gasses, but also the condensed clean-gasses after the desulphurisation and DENOX create critical corrosive environments. Clean-gasses turned out to be extremely aggressive, as the formed H2SO4 in connection with water. Even the formed NH4HSO4 in the DENOX will cause a very low pH-value with water.
Besides the electrochemical attack of the different condenses we have also to look for the mechanical erosive attack, by the flue-ashes. Dust in flue-gas causes the erosion of the enamel surface and opens the bubble structure, through which pores can come into existence. In this case the enamel partly will lose his corrosion protection.
Further an obstruction of dust in the heat exchanger will change the thermal heat. So a periodical cleaning of the heat exchanger is important. That’s why enamel is preferable in his property of a few susceptible against fouling.

Corrosion protection by enamel
After all, we have to consider the corrosion of the enamel surface. Normally seen the enamel layer of 0.15 – 0.20 mm is used to prevent corrosive attack of the metal core. Provided that the metal core is completely covered with enamel, the corrosion protection is determined by the enamel properties.
The normally used heat exchanger enamels exist of quartz, titanium oxide, boric acid network formers. Alkali and earth alkali oxides, but also fluorine compounds, provide the necessarily physical and chemical properties, CoO and NiO are used to get an optimal adherence.
At a temperature of over 1250 °C this mix is melted as homogeneous glass.
The enamel used for heat exchangers is a compromise between chemical resistance and workability for the needed one coat system.
The present used heat exchanger enamel is a closed almost ideal corrosion resistant layer. Acids and liquid condense cause, even in temperatures around 100 °C, only a slight attack on the enamel layer.
Measurements of weight loss after one hour in boiling 30 % H2SO4 on heat exchanger enamel will give a result of <0,02 g/m2, against 6000 g/m2 of uncoated steel.

A negative exception is hydrofluoric acid, which can also be found in the liquid condense of fluegasses.
The attack of the enamel layer by the hydrofluoric acid is a result of dissolving of the main component of the enamel: silicon dioxide [3].

(SiO2)Email + 6HF → H2SiF6 + 2H2O

The higher the fluoric concentrations and the lower the pH value is, the quicker the dissolving of silicon dioxide is going. A redundancy of acid in the corrosion product (H2SiF6) forms hydrofluoric acid again.

H2SiF6 + 2H2O → SiO2.aq- + 6HF

As a result of already a minor concentration of HF in the condense, surfaces of enamel can be dissolved, and the big bubbles from the bubble structure will be opened. Now the acid liquid will start to attack the metal core.
A lot depends on the possibility of condensation of HF. In coal-fired installations, most of it can be bound with fly ash, what is separated in E-filter and partly the fluorine is eliminated too in the fluegas desulphurisation plant.
It turned out that at normal operation conditions (we are talking of a max. of 10 ppm HF in untreated flue gasses) there have been hardly any damages of enamelled heat exchanger elements caused by hydrofluoric acid [6].

Importance function on the quality by:
In this quality circle you can see the influence of the different parameters on the packed heat exchanger. (Fig. 1) We are talking about: Design of the steel, the elements and the baskets, pretreatment, enamel, milling process, application of the enamel, firing and packing.

Steel
Low carbon steels were used in the beginning of the 60’s.
Enamel layers in which a lot of defects, conform ISO 8289 method B (till 500/m2), were created by the still consisting carbon and the big range of reactivity in the steel.
As more heat exchangers were installed in desulphurisation plants, it was necessary to decrease the number of defects.
By using decarburised steel (Fig. 2) the number of defects has decreased to 150 m2 in the 80’s.
Improvements of the decarburised steel, as a smaller range of reactivity (=weight loss conform EN 10209), resulted in a reduction till about 75 defects m2 in the beginning of the 90’s [4, 5].
Ferro Techniek here requires a pickling speed (weight loss) of 15 – 31 g/m2 or 19 – 35 g/m2.
Depending of the grade of the steel and a steel quality with a hydrogen permeability, which gives the guaranty that there is no fish scaling after enamelling.

Enamelling
Also a better pre-treatment of the steel and using better enamels have played their part in the development of the corrosion protection of the enamel layer.

The 'European Enamel Authority“ (EEA 2001) requires heat exchangers with less than 50 defects a m2.
An EN draft standard (2003) for regenerative enamelled elements is less than 50 defects a m2 for air/gas and less than 20 defects a m2 for gas/gas heaters
Nowadays Ferro Techniek will be able to produce enamelled heat exchanger elements with a maximum of only 5 defects m2. By Fig. 3 it is made clear that there is a stated conformity in the measured defects, in accordance with ISO 8289 and the bubble structure of the enamel. It also means that there will be formed a higher amount of corrosions spots in practice, than the measured quantity of defects before the installation. Research done together with KEMA [1] has proved that the increase of defects in industrial circumstances was owing to the opened bubbles in the enamel layer.
Fig. 4 shows the influence of the enamel adherence to the steel on the loss of the base material, which affects the lifetime of the heat exchanger.
The explanation is the hydrogen formation, during the reaction between steel and sulphuric acid.
This phenomenon pushed the little spalls of the enamel easier aside, when the enamel has a bad adherence.
A nickel flash given during the pre-treatment of the steel gives here the optimal adherence.
Also the method of enamelling is decisive for the quality of the corrosion layer. A research at KEMA [2] proved that enamel, which is applied by wet electrostatic spraying, gives a better protection against cracking, than the dry powder electrostatic spraying (Fig. 5). The powder sprayed elements give a more brittle enamel layer than wet sprayed ones. So the wet enamel system gives a major benefit by packing the elements in the baskets, because the with powder applicator ones provide more defects.
Also a draft European test Standard, to measure the edge covering in 2003 is made (Fig. 6). An optimal edge covering, measured accordance this Standard, by powder enamelling (together with the application by dipping in an enamel suspension) is misery enough not realizable. For gas/gas heat exchangers an edge covering in flue gas direction of more than 98 % is seen as an optimal.
Elements sprayed wet electrostatic against that reach nearly 100 %.
Finally Ferro Techniek has chosen for this application after a lot of experience in wet and powder spraying. That’s why a guaranty is given of a nearly covered edge of 98 % and more.
Improvements in covering the edges of the metal core will extend the lifetime of heat exchangers enormously. Laboratory tests proved that there is a corrosion slowing down from the edges, when the edge covering is more than 95 %. The corrosion speed from the edge is linear. Edge covering percentages more than 95 % decrease the corrosion attack particularly in the first part of the life time, which result in a longer life time. In (Fig. 7) is seen the influence on the lifetime, by different covering percentages of edge covering. Not before 95 % there is a slowing down of the corrosion speed.

Packing
The following and one of the most important steps in the production of heat exchangers is the packing of the enamelled elements in baskets. Research shows a minimum of worsening of edge covering and forming defects (as cracks) by the optimal pressure. But all the perfect enamelled elements can be destroyed now with a bad packing method. Important is the element plates are laying flat on each other. In this way the force of pressure is distributed over all the contact points (Fig. 8), formed by the corrugations of the plates.
(Fig. 9) shows the importance of the correct force of pressure. There is an increase of defects in case there is a too high force of pressure. The result shows (Fig. 10). Formation of cracks on the contact points of the plates makes open connections from steel to the air.
Also the loss of edge covering (Fig. 11) is the result of a too high force of pressure.
Not only the force of pressure, but also the method of pressure is responsible for the increase of defects. In comparable examination is acknowledged, that a periodical increasing pressure build up is recommended above a normal pressure build up.

Design of the steel elements
Also the design of the elements has an important part in the total quality.
Very significant is: the undulated plate of the element has always the bigger part of the measured defects (could be go up till 80% of the defects in a pair of elements).
The profile height of the undulated plate is the cause of a worse spreading of lines of force against the corrugated plate, that’s why a bigger bending-through on the undulated plate forms.
enamel on the contact points. The enamel in (Fig. 12) of the undulated plate with the lowest profile height (0.5 mm) will be loaded stronger than the 2 mm high profile.
Besides profile height, number of contact points a m2, tensile strength, yield point and E-modulus of the steel and the enamel, the steel thickness is of consequence. It is clearly made here, that the defects, which are caused by packing, are reduced with a higher sheet steel thickness of the plate.
See in (Fig. 13) the many defects in the 0.6 mm plate against the 1.25 mm plate and measured under the same conditions, this plate shows only defect. (Fig. 14). Also calculations have proved that the profile height and the sheet steel thickness are the most important parameters in the design of the steel elements, to get the optimal effect in the total quality. So it is meaningful to increase the steel thickness of the undulated plate against the corrugated one.

Design of the baskets
By designing the baskets you have to realize, that the cover plates of the basket have the function of holding the packing pressure in the heat exchanger pack. An example of a design (Fig. 15), of which there is already the possibility of a loss of pressure in the pack during the transport to the power station. Some alternatives (Fig. 16). Hereby an example (Fig. 17), in which the proportions of length, width and height of the basket is so unfavourable that you can’t have a stable package.
On this (Fig. 18) you see the design of the convex cover plate of the baskets for the outer ring of the rotor. That is the reason of using filling elements in the basket. It is a disadvantage that the force of pressure is not homogeneous distributed over the contact points. As a result of this a lot of defects (cracks) will be formed (See Fig. 19).

Calculation of the lifetime
Thanks to an experience of years, Ferro Techniek is able to calculate the lifetime of the enamelled heat exchanger. The ‘LIFETIME EXTENSION’ computer program of FERRO Techniek calculates the expected lifetime with help of the different quality parameters, dimensions of the elements and process circumstances in the power plants (Fig. 20). The lifetime warranty of Ferro Techniek is based on mentioned calculations.

Financial consequence
With help of the Ferro Techniek ‘MARKET VALUE’ Computer program and available information of the necessary costs in the power station, the new installation or the replacement can be calculated. Also alternatives can be compared.

Summary
Enamelled regenerative heat exchangers have been proved their ability as corrosion resistant heat exchanger system for the last 30 years. It’s a high-tech system now, which shows his advantages especially under critical circumstances. Elements, who still show a shining gloss after using in desulphuring plants for years, establish this on an impressive way.
Only in a good co-operation with the designer of the installation, the steel and enamel supplier, the operating management of the installation and Ferro Techniek, you will have the optimal result.

References
[1]. W. Podesta. Emaillierte Wärmetauscher für den REA-Einsatz. Mitt. Verein Deutscher Emailfachl. 42(1994).
[2]. J.G.J.van der Gun. Onderzoek naar de scheurgevoeligheid van nat en poeder email d.m.v. 3- puntsbuigtest. KEMA 10026-WMK-1.
[3]. L.Lorentz. Korrosion von Chemieemail durch fluorhaltige Säuren. Werst. u. Korr. 33 (1982).
[4]. J.G.J. van der Gun. Onderzoek naar de kwaliteit van de emaillering opgeprofileerde staalplaten, uitgevoerd in de PKI. KEMA 60063-WMK-16.
[5]. J.G.J. van der Gun. Onderzoek naar de kwaliteit van emaillering op staalplaten, beproeving in poederkoolgestookte proefketel bij KEMA. KEMA 13685-KIM-1.
[6]. W. Podesta. Emaillierte Wärmetauscher für den REA-Einsatz. Wärmeaustauscher 2.Ausgabe (303-308) Vulkan-Verlag Essen.



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