Basically two fabrication methods are used:

1 - tank body is let open on one side and bottom head is welded after enamelling

2 - tank body is entirely welded before enamelling

The first method is usual in the U.S.A. and allows easier operations for pretreatment, enamelling and control of the surface after coating but two or more parts have to be separately enamelled, then assembled and welded. Pretreatment by wheel-blasting and porcelain enamel application by spraying are preferred techniques.

The second method, normally used for tanks to be galvanized, is traditionally adopted in Europe. Pickling and flow-coating application are suitable processing techniques for closed tanks with small bottom opening. For tanks without coils or pipes inside, with adequate bottom head slope and opening, blasting by compressed air nozzles and electrostatic Powder Enamel application are practicable alternatives, even if not cheap because of the higher equipment and operating costs of these techniques. Porcelain Enamel powder is considerably more expensive compared with frit.

In order to allow a proper automatic "Flowcoating" applica-tion of Porcelain Enamel tanks must have possibly a cylindrical shape, free spacing for the rollers on the outer cylinder body, external fixtures if strictly necessary as small as possible and an adequate central bottom head opening, whose diameter obviously determines both qua-lity and time of application and drying. It is essential to recognize the importance of a proper design for a satisfactory production of Porcelain Enamelled boilers. Improper design is responsible for more difficulties in the enamelling process than faulty processing itself. Selection of the proper gage of steel is very important as well as the quality of steel, the absence of harmful impurities, structural defects, inclusions, etc.

As boiler tanks cannot be formed from one piece of steel, welding is necessary to obtain the complete shape. Different types of welding are acceptable for porcelain enamelling. Welding equipment manufactures should be consulted for choice of proper procedure, which has to be carefully tested for compatibility with Porcelain Enamel coating before adoption.

Gage of lugs and clips which are spot welded to the outside of boilers should be thinner than the boiler sheet and the double thickness area of contact should be as small as possible. This will reduce danger of underfiring at the points where such attach-ments are welded.