tube block 16 and the tubes 12 of tube assembly 13 suitable mortar compositions are generally based upon silicon carbide and furthermore contain a bonding agent that will adhere strongly to the tube block and metal tube wall assembly as the tube block 16 is placed against the tube assembly 13 it is attached to the tube assembly 13 at the membrane 14 or other suitable place on the tube assembly by means which can bring the tube assembly into intimate contact with the tubes such as a threaded stud as the threaded stud is tightened the tube block 16 is forced against the tube assembly with the mortar filling the grooves 36 between the ribs 32 thereby eliminating air spaces the ribs 32 are forced into direct contact with the walls of tubes 12 resulting in excellent heat transfer between the tube assembly 13 and the tube block 16 the mortar also acts to hold the tube block 16 in contact with the tube assembly 13 should the attachment means i e threaded stud and bolt corrode during prolonged use although not required during assembly the tube block 16 may also be hammered using for example a leather faced mallet against the tube assembly to assure maximum direct contact of the ribs 32 and the elimination of all air gaps although not shown additional tube blocks can be placed on adjacent portions of the tube assembly depending upon the size of the boiler tube blocks will normally be placed above below and on both sides of each other to cover.