In modern practice, under certain conditions of mill operation, and with bagasse of certain physical properties, the bagasse available from the cane ground will meet the total steam requirements of the plant as a whole; such conditions prevail, as described, in Java. In the United States, Cuba, Porto Rico and like countries, however, auxiliary fuel is almost universally a necessity. The amount will vary, depending to a great extent upon the proportion of fiber in the cane, which varies widely with the locality and with the age at which it is cut, and to a lesser extent upon the degree of purity of the manufactured sugar, the use of the maceration water and the efficiency of the mill apparatus as a whole. [Illustration: Fig. 27. Babcock & Wilcox Boiler Set with Green Bagasse Furnace] Experience has shown that this fuel may be burned with the best results in large quantities. A given amount of bagasse burned in one furnace between two boilers will give better results than the same quantity burned in a number of smaller furnaces. An objection has been raised against such practice on the grounds that the necessity of shutting down two boiler units when it is necessary for any reason to take off a furnace, requires a larger combined boiler capacity to insure continuity of service. As a matter of fact, several small furnaces will cost considerably more than one large furnace, and the saving in original furnace cost by such an installation, taken in conjunction with the added efficiency of the larger furnace over the small, will probably more than offset the cost of additional boiler units for spares. The essential features in furnace design for this class of fuel are ample combustion space and a length of gas travel sufficient to enable the gases to be completely burned before the boiler heating surfaces are encountered. Experience has shown that better results are secured where the fuel is burned on a hearth rather than on grates, the objection to the latter method being that the air for combustion enters largely around the edges, where the fuel pile is thinnest. When burned on a hearth the air for combustion is introduced into the furnace through several rows of tuyeres placed above and symmetrically around the hearth. An arrangement of such tuyeres over a grate, and a proper manipulation of the ashpit doors, will overcome largely the objection to grates and at the same time enable other fuel to be burned in the furnace when necessary. This arrangement of grates and tuyeres is probably the better from a commercially efficient standpoint. Where the air is admitted through tuyeres over the grate or hearth line, it impinges on the fuel pile as a whole and causes a uniform combustion. Such tuyeres connect with an annular space in which, where a blast is used, the air pressure is controlled by a blower.