TECHNOLOGY FOCUS - CHEMICAL ENGINEERING - Preferred Conditions: FT1 is preferably 20-79, and especially 25-64. Ratio R is at least 560 and up to 1400, preferably 670-1300, and parameter S1 is 1-8.5, preferably 1.5-7. Parameter S2 modifying the parameter S1 is defined by formula S2 = (FT1)2/Rxf(CONV)xg(TMT), where f(CONV) = 1 + 0.4 square root ((CONV-77)/100); where CONV is the percentage of conversion of ethane present in charge, and g(TMT) = 5.07x109e(-28000/(TMT+273)), where TMT is the maximum temperature of the pipes at the beginning of cycle, in the part of radiation zone corresponding to residence time at least 110 minutes. Parameter S2 is 1-20, especially 1.5-17, and it is sufficiently low to attain at least 77% conversion of ethane in charge, and maintain the same level of conversion during the cycle of at least 8 days. Preferably, the ratio R is selected and the thermal flow FT is adjusted in such way that the parameter S3, calculated from formula S3 = Delta T/(FT1xf(CONV)xg(TMT)) is 0.25-5, and especially 0.5-4 (where Delta T = T1-TMT is at least 30 degrees C, TL is the critical temperature of use for end part of pipe) and is sufficiently high to attain at least 77% conversion of ethane in charge, and maintain the same level of conversion during the cycle of at least 8 days. The length of pipe L is equal at least KxQ Delta H70/FT SL (where Q is a wt. rate of charge enriched with steam (kg/s), Delta H70 is transferred heat corresponding to 70% conversion of ethane (kJ/kg) and SL is average linear external surface of pipe in heating zone (m2/m) and K is 1.05-2, preferably 1.1-1.6). The period of residence of charge is preferably 250-1500 minutes, especially 350-1200 minutes. Partial pressure of hydrocarbons and hydrogen (ppHC) in gaseous effluents of steam cracker is 0.2-2.8 bar (abs.), especially 0.4-1.6 bar. Temperature at the exit of steam cracker (COT) is preferably at least 876 degrees C, and especially it is less than or equal to 980 degrees C and up to (inclusive) 858 + 10 (f(CONV) + ppHC). The effluents from radiation zone pass through adiabatic zone, and their time of residence in the latter T1 is between 1 and 220 minutes, especially 8 and 80 minutes, before they enter the tempering zone. Maximum temperature of the pipes at the beginning of cycle (TMT) is preferably 975-1060 degrees C, and especially 990-1044 degrees C (inclusive). Dimension parameters of steam cracker are chosen in such way as to ensure that TMT - COT equals 50-130 degrees C, especially 80-106 degrees C. The end part of main pipe comprises at least one tubular element of group formed by pyrolysis pipes, with internal anti-coking coating, or with helicoidal internal rim.