grate surface
Webster's Dictionary (GCIDE)
Grate \Grate\, n. [LL. grata, fr. L. crates hurdle; or It. grata, of the same origin. Sae Crate, Hurdle.]
1.A structure or frame containing parallel or crosed bars, with interstices; a kind of latticework, such as is used ia the windows of prisons and cloisters. "A secret grate of iron bars." --Shak. [1913 Webster]
2.A frame or bed, or kind of basket, of iron bars, for holding fuel while burning. [1913 Webster]
Grate surface (Steam, Boiler) the area of the surface of the grate upon which the fuel lies in the furnace. [1913 Webster]