Technical email
Technical enamels are used in technical applications, for example in the construction of highly acid-resistant pressure vessels that are used in chemical and pharmaceutical process engineering. Typical apparatuses are stirred tanks , storage tanks and receivers (e.g. tanks for collecting a distillate or tanks for storing media that are kept available for a process engineering process), columns , pipelines and pipeline parts as well as a large number of enamelled components that are used to complete these Apparatus are required.
With these devices, which are manufactured up to a volume of 120,000 liters, the entire inner surface that comes into contact with the product is coated in several firings with an enamel layer thickness of 1.4 to 2 millimeters. Quality features of technical enamels are standardized in ISO 28721-1 (formerly DIN EN 15159-01: 2006, previously DIN 28063). The devices are usually designed to be "pore-free", which means that the inner surface of the container, which is several square meters in size, should be completely free of defects in the enamel layer. "Technical enamels" are high-quality glasses that are optimized for a wide pH spectrum in terms of thermal shock resistance, impact resistance and, above all, corrosion resistance. Like all glasses, technical enamels are extremely resistant in the acidic range, but not against hydrofluoric acid . Due to the chemical properties of the silicates, they are less resistant in the alkaline range.
Enamelled pressure vessels and process engineering apparatus
Technical enamels are used in technical applications, for example in the construction of highly acid-resistant pressure vessels that are used in chemical and pharmaceutical process engineering. Typical devices are stirred vessels, storage tanks and receivers (containers for collecting a distillate), columns, pipelines and pipeline parts as well as a large number of enamelled components that are required to complete these apparatuses.
With these devices, which are manufactured up to a volume of 120,000 liters, the entire inner surface that comes into contact with the product is coated in several firings with an enamel layer thickness of 1.4 to 2 millimeters. Quality features of technical enamels are standardized in ISO 28721-1 (formerly DIN EN 15159-01: 2006, previously DIN 28063). The devices are usually designed to be "pore-free", which means that the inner surface of the container, which is several square meters in size, should be completely free of defects in the enamel layer. "Technical enamels" are high-quality glasses that are optimized for a wide pH spectrum in terms of thermal shock resistance, impact resistance and, above all, corrosion resistance. Like all glasses, technical enamels are extremely resistant in the acidic range. Due to the chemical properties of the silicates, they are less resistant in the alkaline range.
Technical enamelling always consists of one to two layers of base enamel and up to five layers of top enamel. The latter give the enamel its chemical resistance and the required operating properties. Fine-grain structural steel (such as P275 NH, P265 GH), which is limited in terms of carbon and manganese content , is used as the base material. For economic reasons, the required quality, layer thickness and freedom from defects of the enamel layer should be achieved with a small number of top enamel layers. There are two well-known manufacturers of technically enamelled apparatus in Germany. The company Pfaudler Werke GmbH in Schwetzingen is based in Germany since 1907 enameled apparatuses. The company THALETEC GmbH in Thale am Harz , which emerged from the former Thale ironworks (EHW Thale), has over 180 years of enamelling tradition .
It is now possible to enamel components that have been generatively manufactured with the help of “ Selective Laser Sintering ” or “Selective Laser Melting” .
Standardization and interchangeability
Glass-lined process engineering equipment is defined and standardized in a large number of DIN , EN and ISO standards. This enables the user to fall back on a large number of suppliers for spare parts, re-enamelling and for the delivery of accessories such as seals, etc., regardless of the original manufacturer of the device. Individual manufacturers can ensure that they can be exchanged with existing components without restrictions.
Accessories for enamelled apparatus
These include, for example:
- Baffle
- Heat exchanger
- Dip tubes
- Stirrer
- Manhole cover
- Manhole protection rings
- Bottom outlet valves with a dynamic sealing of the valve stem by corrugated or bellows
- Enamelled pumps
Enamelled storage tanks
Large-volume tanks for the storage of liquid manure, water and other solid and liquid substances are built from individual panels (enamelled sheets with dimensions of up to 2 × 2 m). These containers are not pressure vessels.
Enamelled pipelines
Enamelled pipes and pipelines are used in the chemical and pharmaceutical industries and in drinking water supply. In the chemical and pharmaceutical industry, pipelines up to a nominal diameter of 200 mm are coated with multiple layers of enamel inside in order to obtain a sufficient and chemically resistant corrosion protection layer. The material of the pipes is usually a carbon steel such as P 235 G1 TH (formerly St35.8). For the drinking water supply, pipes made of cast iron are used, which are covered with only one layer of enamel on the inside.
Re-enamelling
Re-enamelling is the renewal of an email order.
Special technical emails
Emails for processes in the pharmaceutical industry
Emails for processes in the pharmaceutical industry are usually white or light blue in color, as this makes it easier to observe the processes during the reaction and to inspect the cleaned apparatus (see illustration in the article for examples ).
Antibacterial enamels
A special development are enamels with antibacterial properties that contain small amounts of silver particles. The effect of silver as a bacteria-killing substance is available as a high-quality surface enamel coating for special applications in biotechnology and other areas where the risk of contamination by viruses and microorganisms is to be reduced.
Abrasion-resistant enamels
Abrasive wear ( hydroabrasion ) can occur, particularly in stirred vessels in which solid suspensions are stirred . The enamel wears out due to sliding or impact wear processes in which it is progressively removed. Special enamel recipes can in part significantly improve the wear resistance compared to conventional enamels:
- With "partially crystalline enamels", crystallizing precipitates in the glass matrix improve the wear resistance compared to conventional enamels.
- Enamels that are fired with nanoscale carbide-like hard materials in an oxygen-free atmosphere achieve a drastic improvement in wear behavior because the embedded hard materials (e.g. carbides) offer significantly higher resistance to wear than the amorphous glass matrix. The glass matrix only serves as a carrier for the embedded substances. The abrasion resistance of such an enamelling is particularly high when the size of the abrasive particles is greater than the layer thickness of the (softer) amorphous glass matrix.
Electrically conductive emails
Enamel is an electrical insulator, but it can be made electrically conductive by adding metals. For this purpose, chemically resistant metals such as platinum, silver or gold are used, which are distributed in the glass matrix.
Semiconductor materials are another way of ensuring the conductivity of the otherwise non-electrically conductive enamel layer. The semiconductor materials are distributed in the glass matrix and are arranged in network-like structures in the glass matrix with the help of a specially tailored firing process. As a result, enamels produced in this way usually have better chemical resistance to corrosive attack than those with embedded metallic particles.
Emails for polymerisation processes
Since technical enamel is particularly easy to clean and also has non-stick properties, it has been used for decades in the production of polymers such as polyvinyl chloride (PVC). Corresponding stirred tanks are coated with a special “poly enamel”. Since polymerizations are usually exothermic , it is particularly important in such polymerization apparatus that the exothermic heat of reaction can be dissipated effectively. Therefore, the enamelling of such devices is usually "thin", i.e. H. in the range of a thickness of max. 1.1 mm. This minimizes the thermal resistance of the container wall due to the enamelling. Since polymerizations are rarely corrosive, the reduced layer thickness has no effect on the service life of the device.
In order to ensure heat transfer, especially when the polymers are cooled (exothermic reaction), glass-lined heat exchangers are often used, which are built into the glass-lined apparatus as baffles and can improve the heat transfer capacity by a factor of 3 and more compared to conventional jacket cooling.
Emails for biocorrosive processes
Special enamels are particularly resistant to biocorrosive attacks such as those caused by fungi, sponges or bacteria. The phenomenon known as " biofouling " is harmless compared to enamelling on surfaces that are enamelled with special biocorrosion-resistant enamels. Corresponding enamels for biocorrosive processes can also be composed in such a way that they also have an antibacterial and germicidal effect (see above under "Antibacterial enamels").
Enamels for flue gas heat exchangers or air preheaters
Corrosive exhaust gases occur in the exhaust gas flows of conventional fossil power plants (see economiser ), in coal-fired power plants or in waste incineration plants. These exhaust gases are cooled down in glass-lined heat exchangers (below the dew point ) to such an extent that corrosive liquids such as sulfuric acid are produced. Special enamels are modified so that they are particularly resistant to sulfuric acid stress. Compared to conventional chemical enamels, such enamels specially designed for sulfuric acid applications are up to 30-40% more resistant, ie. H. they have a correspondingly lower corrosion rate. Similar enamels are also available for the construction of glass-lined apparatus and are used for plants for the processing of sulfuric acid.
Other technical applications
Architectural enamelling
Enamelled panels are used in architecture to clad walls. Enamelled panels are easy to clean and have a high-gloss finish. As a rule, architectural enamelling is used on sheet metal with a thickness of 1 to 2 millimeters or on float glass. The enamel can be applied by screen printing, digital printing, roller coating or other methods. Another possibility is to apply the enamel slip using a spray gun or brush. The component is then fired. One application is the cladding of road tunnel walls with enamelled panels, whereby the ease of cleaning and the incombustibility of the material are the benefits.
lighting
Gas lamps with a mantle mounted on top usually have a ceiling plate made of white enamelled sheet metal to reflect light emitted upwards to illuminate the street or a room. The high temperature resistance is necessary in the constant flow of exhaust gas. Outside the glass cylinder (cone), another reflector ring can be attached, similar to that used in early street lamps based on incandescent lamps . The housings of these lamps are typically dark enamelled to hide the dirt. Reflector lights for angled illumination of a football field or construction site had a cylindrical reflection pot with white enamel on the inside and a curved rear wall and a socket that protruded radially downwards for an incandescent lamp with a power of around 300–500 W. The company Austria Email enamelled since 1939, lamps manufactured and is now one of Europe's major manufacturer of enameled water heaters .
Enamelling of sanitary objects
Bathtubs, shower trays and washbasins made of steel enamel are considered hygienic, easy-to-clean and robust sanitary objects and as an alternative to heavy cast bathtubs and the plastic bathtubs developed later. When enamelling steel bathtubs and shower trays, industrial systems are used that consist of two large production units: forming technology and enamelling technology. In the first one, titanium steel is shaped with large presses, in the second the steel is provided with an enamel coating. It is a melt of glass, quartz, borax, soda and water. Any colored coating is sprayed on and then fired in an oven.
After rectangular cisterns made of reinforced concrete hung upside down, enamelled cisterns that were covered for the first time - white outside, gray inside - appeared around 1960. Its control lever protruded from the front at chest height, and the water inlet and outlet were fitted with pressed rubber seals.
"Brown" and "White goods"
"Brown goods" are essentially understood to mean ovens for heating purposes, while "white goods" mean the household appliances stove, washing machine, dishwasher and refrigerator. Furnaces that were manufactured industrially from cast iron, sheet iron and chamotte could be rubbed with graphite paste and heated up, blackened and alloyed and gradually protected from rust by moisture , just like the still common cast iron electric hotplates . As email as corrosion protection came up the blinds were furnaces for space heating in dark tones coated (brown, black, ocher, green, blue), table cookers , electric stoves , gas stoves enamelled and other equipment for the kitchen and laundry in accordance with the hygiene claim white. Enamel is very smooth and so hard that it can be cleaned with not too hard knife blades and abrasives, even if food is burnt on it. Until around 1975 the troughs (cover plates) of electric stoves - even in fitted kitchens - were white enamelled, only later, but then mostly built into a worktop made of NiRo steel; In both cases, the round cast iron heating plates are fitted with a chrome steel sheet ring. The troughs of gas stoves, whose grids were formerly made of black enameled cast iron and later of nickel-plated or chrome-plated, welded steel rods, took an analogous development.
Stovetops, both by fire and electrically heated, will be hotter than enamel will tolerate. A cast iron electric hotplate that is operated in idle will get so hot over time that enamel from an empty pot on the plate can permanently melt. Oven pans (and steel baking trays) are typically enamelled dark gray to hide charred splashes of food. Oven cleaners or special coatings can help burn such charring through atmospheric oxygen through catalysis at high temperatures. In old table stoves, enamel sheets are combined with nickel-plated iron elements.
Particularly high corrosion resistance promise inside - typical dark blue - enamelled hot water heater, the boiler wall addition of steel by a sacrificial anode made of magnesium is protected.
The material enamel is used to coat surfaces from a hygienic, tactile and optical point of view and because it is easy to clean. One accepts the brittleness that can result in cracks and flaking when bumped.
Gas instant water heaters and gas boilers (for hot water and heating) are often white enameled to this day, as combustion gases can produce tarry condensate. Gas stoves often have a For these devices, a classification into brown or white goods can hardly be decided.
Enamelling of vessels and devices for the kitchen, household and laboratory
The enamelled pots, pans and other items of everyday use in the household are known from “Grandma's Kitchen”. Such items were used almost everywhere in the past due to the lack of other corrosion-resistant materials. It was only with the advent of stainless steel and especially plastics that enamelled household items became rarer. Since high-quality enamelling of pots and pans is well suited for allergy sufferers, enamelled cookware is now returning to the modern kitchen. Cast-iron pots, roasting pans and pans were traditionally only protected against corrosion with a thin, black matt enamel coating, later multicolored enamelling came up: inside light gray, sometimes speckled, on the outside outside in a decorative color (beige, brown, green ...) and black on the underside. Small milk cans for private households - with rings soldered to the side of the neck for the carrying handle - were often enamelled white or at least lighter on the inside than on the outside, but later replaced by polyethylene or aluminum.
Conical, enamelled buckets with a standing rim - typically with a galvanized wire bracket with a wooden handle - were more corrosion-resistant than cheaper galvanized ones. For food, delicate cleaning tasks, as well as for the permanent collection of damp or chemically aggressive waste, with a lid in the dormitory of the children's home also for the uncomplicated nightly "go to the bucket" instead of the distant, cold toilet. Chamber pot with lid under the bed or in the toilet chair , spittoon with ring-shaped lid, soap dish, dinnerware for prisoners that do not produce dangerous splinters, replaced by (white) melamine since 1970.
Gradually, in addition to lacquered cardboard, gutta-percha and clear glass, bowls for the wet-chemical development of photo paper were also made of enamelled sheet metal, but were later replaced entirely by light, somewhat flexible, break-proof thermoformed thermoplastics.
Chipping on enamelled household items - harmless
Enamelled tableware can suffer damage, so-called flaking, in the course of use. This flaking can occur, for example, when a blow or impact has acted on the enamelled part. With thin sheet enamelling, as is the case with dishes, the enamel can flake off down to the steel, so that the steel, which is now not protected against corrosion, is exposed. Since enamelled crockery is usually made of a non-corrosion-resistant carbon steel, the crockery item can rust at the damaged area. However, due to the bond between the enamel and steel created during enamel firing, rust can not infiltrate the intact enamel layer next to the damage and thus cannot lead to progressive flaking of the enamel. Flaking on the outside of a pot or pan is completely harmless, but visually unattractive. With modern high-quality enamelling, chipping is basically only possible when subjected to rough force. Nowadays the enamel layer is much thinner and more flexible than in the last century. In individual cases there is also spontaneous flaking, which looks similar in shape to fish scales. Such damage is caused by the outgassing of hydrogen from the metal when the structure of the metal does not meet the requirements of the enamelling process. These fish scales are also known as hydrogen defects and are therefore causally attributable to the steel. The fatal thing about these errors is that they usually do not occur immediately during or after production, but with delays of up to several months. They can be avoided through careful selection of the sheet steel grades used, in that only sheet steel grades that conform to standards and guaranteed to be enamelled are used.
Web links
- Technical email (PDF; 1.9 MB) DEV
Individual evidence
- ↑ see video on YouTube
- ↑ Austria Emal: At a glance austria-email.at, accessed April 7, 2019.
- ↑ New enamelling plant for more individuality