A compact endothermic reaction apparatus employing metallic reaction tubes (28; 152) in a close-pack arrangement using offset nozzle tubes (90; 167) and an air distribution plate (68; 160) for introducing fuel and air into a combustion chamber to produce long and thin flames thereby to avoid excessive localized heating of the reaction tubes and provide high reaction tube life expectancy. Also, excessive localized heating of the reaction tubes at the inlet ends of exhaust tubes (94; 172) is eliminated and provision is made for preventing buckling of individual reaction tubes thatr may be subjected to higher than average reaction tube temperatures.