A simplified hybrid-dimensional unsteady heat transfer model for the ascension-pipe heat exchanger with a helically coiled tube embedded in the jacket and supported by high conductivity particles is established based on the non-thermal equilibrium theory of porous medium. This idea can also be used in the design of other heat exchange devices involving the helically coiled tube. According to calculations, the wall temperature of the ascension-pipe heat exchanger periodically varies due to the periodic variations of flow rate and temperature of raw coke oven gas. A great thermal stress will be produced due to this reason. The coal tar precipitation mainly occurs in the late stage of the coking period due to the continuous cooling of molten salt. Based on the detailed analysis, thermal conductivity of packing layer, heat capacity per unit volume of packing layer, volume of packing layer, flow rate of molten salt and inlet temperature of molten salt are important factors that should be scientifically designed in the design of this type of heat exchanger to reduce thermal stress and coal tar precipitation. Cutting off molten salt in the late stage of the coking period is an effective measure to control the wall temperature of the ascension pipe above the dew point of coal tar vapor.