Porcelain Enamel application

After preparation of the inner surface Porcelain Enamel can be applied either in one coat (direct-on) or two-coat. The two-coat (normally both by wet application requiring drying and firing twice) is very expensive and has limited application for particular boiler types or parts. The one-coat system is widely preferred as today direct-on Porcelain Enamels meeting the most severe requirements for resistance to hot water (less than 3,5 g/m2 loss according to the German Industrial Standard DIN 4753 and acid resistance Class A) are widely available. For this stage of processing various Porcelain Enamel application systems and equipments are used whose choice is based mainly upon the construction of the boiler: - Flow coating - Spraying wet - Electrostatic dry - Filling by pump or vacuum The flow-coating system is the most used because of versatility and possibility of full automatic operation. The enamel slip is distributed evenly on the inner surface of the tank by means of rotational movement and gravity at different angles of inclination, taking into account the setting time of the enamel slip and the eventual differing sizes of boilers. The excess slip is reclaimed and recirculated as new slip from the mill area is supplied into the system. After drainage of excess slip tanks move through a drying system where water has to be thoroughly removed from the wet layer of enamel. This operation, whose importance in the enamelling process is often underestimated, is accomplished in a drying tunnel blowing warm air into the boiler inside through the bottom head opening. After drying some finishing operations are required before firing, such as brushing the tank openings. The firing process consists of heating the tanks coated with a dry layer of enamel slip until enamel melts in order to obtain the desired continuous glassy coating bonded to the steel. For appropriate and convenient firing special furnace are necessary.

Complete enamelling plant with pretreatment for boilers designed by Schmitz & Apelt ITALIA

1 - loading	3 - transfer to the enamelling line	5 - dryer	7 - unloading
2 - pretreatment	4 - enamel application	6 - furnace