Boiler Coverings. Much the same remarks may be made with regard to boiler covering as have been made with regard to pipe covering, except that the covering put on boilers is usually somewhat less efficient and is applied in greater thickness. Probably one of the best coverings for a marine boiler—or, in fact, for any internallyfired boiler—is a layer of air-cell asbestos board, covered with a coating perhaps two inches thick of magnesia or asbestos. This comes in powder form, and when mixed with water can be readily applied with a trowel. Coverings on boilers are best placed directly against the shell without an air-space, so that any leak in a joint or rivet will reveal the spot by moistening the covering; otherwise the escaping water may run down through the air-space and appear at some remote point, the leak thus being difficult to locate.
An efficient covering for boilers is made of either magnesia or asbestos in the form of blocks of the proper curvature, which can lie directly against the boiler; but this form of covering is rather more expensive than the asbestos or magnesia cement. To secure an extra hard finish a coating of plaster of Paris may be put on outside the magnesia or asbestos. No boiler or pipe covering should contain sulphate of lime, as this is liable to cause corrosion.
If an internally-fired boiler is properly lagged, there is little danger that any large amount of heat will be lost, as the heat of the fire must pass through the water before radiating. This is not true with an externally-fired boiler, where a considerable amount of heat may radiate through the brick setting of the boiler without coming in contact with the boiler at all. The setting of such a boiler should be arranged with properly confined air-spaces; and an efficient protection from the radiation of heat at the top of the boiler may be had by allowing a slight space between the boiler and the top covering for the circulation of the hot gases of combustion. These are on their way to the chimney; and as they are necessarily hotter than the water in the boiler, they prevent radiation at this point.