The information given in Table 1 may be used as a guide only. It shall be considered with which degree of accuracy the expected performance data has been established, often requiring agreement between purchaser and supplier when selecting a suitable grade. Uncommon installation conditions — for example, distorted inlet flow — or a system with components situated in the inflow, such as protective screens or structural supports, shall be allowed for. Special agreements shall be made for fans whose drive causes marked fluctuations in rotational speed.such as protective screens or structural supports