An apparatus for conducting endothermic reactions able to meet the heat balance with a null or limited amount of export steam, through a flexible heat recovery, comprising: a combustion chamber having a closed bottom end, an open top end, two opposite end walls, and two opposite side walls; an upper convection chamber with a reduced section area flue gas side, in communication with said combustion chamber through said open top end, allowing the flow of flue gas through a plurality of convection channels in flow communication with a top plenum chamber connected to an outlet flue gas channel; a plurality of catalytic vessels arranged at a distance and disposed in line or staggered along a centerline of the combustion and convection chamber in such a way that a bottom portion is immersed in the combustion chamber and a remaining top portion is immersed in the convection chamber and is provided with an extended surface selected from longitudinal fins or studs, wherein the hot flue gases generated, 3a and 3b, in the combustion chamber 100 and flowing through the convective channels 201 to the top exit 3 exchange their heat content through the outer surface of the catalytic tubes 20, flowing in countercurrent with the reacting gas inside the tubes. In the tube portion immersed in the combustion chamber 100 the prevailing heat transfer mechanism by flue gas is radiation, while in the portion immersed in the convection chamber 200 the prevailing heat transfer mechanism is convection. The exposed tube length of the bottom portion is in the range of 50% to 75% of the overall exposed tube length, and it is selected in order to limit the flue gas temperature at inlet of the convection channels 201 at a level compatible with the extended surface material; the heat distribution along the endothermic process in the radiant chamber is realized by means of two or more firing levels, each one being configured to be regulated by firing control, with the burners symmetrically located along the side walls at an adequate distance from the tubes to avoid flame impingement; in this way the heat firing can be released according to the required reaction heat absorption pattern. The gradual heat release along the catalytic vessels involves that no heat distributor is needed around the same vessels, resulting in a less complex and more reliable design and in the same time obtaining the desired and controlled heat flux profile, also limiting the maximum catalyst tube wall temperature; and wherein each catalytic vessel has a closed bottom end and includes an inner heat transfer tube having both ends open, mechanically joined on the top end and free to move axially inside the catalytic vessel in order to obtain an additional heat recovery from process syngas. a combustion chamber having a closed bottom end, an open top end, two opposite end walls, and two opposite side walls; an upper convection chamber with a reduced section area flue gas side, in communication with said combustion chamber through said open top end, allowing the flow of flue gas through a plurality of convection channels in flow communication with a top plenum chamber connected to an outlet flue gas channel; a plurality of catalytic vessels arranged at a distance and disposed in line or staggered along a centerline of the combustion and convection chamber in such a way that a bottom portion is immersed in the combustion chamber and a remaining top portion is immersed in the convection chamber and is provided with an extended surface selected from longitudinal fins or studs, wherein the hot flue gases generated, 3a and 3b, in the combustion chamber 100 and flowing through the convective channels 201 to the top exit 3 exchange their heat content through the outer surface of the catalytic tubes 20, flowing in countercurrent with the reacting gas inside the tubes. In the tube portion immersed in the combustion chamber 100 the prevailing heat transfer mechanism by flue gas is radiation, while in the portion immersed in the convection chamber 200 the prevailing heat transfer mechanism is convection. The exposed tube length of the bottom portion is in the range of 50% to 75% of the overall exposed tube length, and it is selected in order to limit the flue gas temperature at inlet of the convection channels 201 at a level compatible with the extended surface material; the heat distribution along the endothermic process in the radiant chamber is realized by means of two or more firing levels, each one being configured to be regulated by firing control, with the burners symmetrically located along the side walls at an adequate distance from the tubes to avoid flame impingement; in this way the heat firing can be released according to the required reaction heat absorption pattern. The gradual heat release along the catalytic vessels involves that no heat distributor is needed around the same vessels, resulting in a less complex and more reliable design and in the same time obtaining the desired and controlled heat flux profile, also limiting the maximum catalyst tube wall temperature; and wherein each catalytic vessel has a closed bottom end and includes an inner heat transfer tube having both ends open, mechanically joined on the top end and free to move axially inside the catalytic vessel in order to obtain an additional heat recovery from process syngas.