Indirect cooling (hot-wall retort furnaces)

• Ambient air is blown onto the outer retorte surface to cool it down. The waste heat is removed via the exhaust air outlet of the furnace.
• The charge is cooled indirectly, which means that the atmosphere in the retort is not affected by the cooling
• The charge cannot be quenched with the cooling system

Direct cooling (cold-wall and hot-wall retort furnaces) 

• Rapid gas cooling in the retort. For this purpose, the furnace atmosphere is circulated through a heat exchanger.
• The system pressure is not increased by the cooling; there is no gas quenching at high pressure
• Not available for processes with flammable furnace atmospheres