In an application to large diameter Czochralski (CZ) silicon (Si) single crystal growing the influence on crystal temperature field of various thermal shield assemblies located near to its surface is discussed. By means of mathematical modeling the computer model of thermal processes in an application to a hot zone of "Redmet-90" puller , intended for 200 and 300 mm diameter Si single crystal growth is developed. The role of the ring shield and the shield assembly, consisting of two shields (an internal cone and an external one is repeating the crucible shape) and being as a basis of some patents, is investigated. On the basis of the carried out calculations the new thermal shield assembly for "Redmet-90" puller was offered. Its influence on formation of the characteristic thermal zones in growing single crystal, corresponding to defect formation processes in dislocation-free Si crystals (the recombination of intrinsic point defect – IPD, and the formation of their agglomerates) is discussed. The influence of a melt flow on the liquid/solid interface (LSI) shape and thermal stability of crystal growing process is analyzed.