Abstract: During long tunnel construction, convenient and effective analytical methods for formulating fan operation adjustment strategies in pressed-in ventilation systems are lacking. To address this issue, fan efficiency is proposed as the criterion for evaluating the appropriateness of the series spacing between two fans, replacing the traditional empirical approach. Based on a theoretical model for evaluating the performance of pressed-in ventilation systems, a method for analyzing fan operation strategies throughout the entire construction period is developed. The method determines the effective air supply range of a single fan at different rotational speeds, as well as the minimum and maximum series spacing and the effective air supply distance for two fans of different models connected in series, thereby establishing a fan operation scheme that can be dynamically adjusted according to the tunnel excavation distance. In addition, a quantitative method for determining the appropriate series spacing is proposed based on an evaluation of the effect of series spacing on dual-fan efficiency and is demonstrated through a case study of the Caojiadi inclined-shaft tunnel project. The results indicate the following: (1) As the leakage rate per 100 m increases from 1% to 4%, the maximum air supply distance decreases by up to 1 270 m. During the initial stage of tunnel excavation, staged adjustment of the fan rotational speed is required. (2) Based on the criterion that the efficiencies of both fans remain at or above 60%, appropriate series spacing intervals are quantitatively determined for various leakage rates and air supply distances. (3) When the leakage rate per 100 m is 1%, the recommended series spacing ranges from 1 050 m to 1 130 m, extending the effective air supply distance from 2 580 m to 3 910 m.

Key words: long-distance tunnel, fan operation strategy, series spacing between two fans, pressed-in ventilation, fan operating parameters, entire construction period