Some results concerning the NEB air fan replacement and speed optimization

On 18 November the "forward-blades" fan of NEB AHU has been replaced with a "reverse-blades" type of fan. The driver motor was replaced as well but with a similar model (1450 rpm nominal speed). Figures 1 and 2 are pictures of the old fan, while Figures 3, 4 and 5 show the new fan.  When dismounting the old fan Roberto observed that its shaft was not well fixed and jittered significantly inside its housing.

It then followed a long work of optimization of the fan's speed (thanks to Roberto!) which consisted in replacing the fan pulley with a larger diameter one (150 mm --> 250 mm) and the motor pulley with smalled diameter (150 mm --> 118 mm) thus increasing the gear ratio Dia_fan_pulley/Dia_motor_pulley, and then progressively reducing the speed (of the inverter driving the motor (see previous comments). The larger gear ratio allows to increase the range of operation of the inverter towards lower speed values without overheating the motor (f_fan = Dia_motor_pulley / Dia_fan_pulley * f_motor (Hz) * f_inverter (Hz) / 50 Hz).

Figure 6 shows some trend data since the end of October, where the slow down actions are indicated. We notice that as the speed of the outlet air progressively reduced, the hall temperature stability is preserved. The hall overpressure has reduced from about 10 Pa to about 5 Pa. The latest inverter setting (35 Hz) seems to cause some instability of the TE and overpressure. The dust contamination trend should be checked as well. The acoustic noise in the hall (bottom plot of Figure 6) also progressively reduced. Figure 7 is a zoom over the last two weeks. We can notice how the hall acoustic noise RMS (20-40Hz) shows more and more clearly the influence of anthropic noise (see elog 58345).

Figure 8 shows the achieved reduction in seismic and acoustic noise inside the experimental hall and their coherence. The blue curves (November 4) correspond to the old fan with the inverter set at 35 Hz, the red curves (January 14th) correspond to the NEW fan with the inverter set as well at 35 Hz. Using the pulley values reported above we derive that the nominal fan rotation speed is similar for the two configurations, i.e. ~9Hz for the old fan (Dia_fan_pulley = 280mm, Dia_motor_pulley = 150mm) and ~8 Hz for the new fan. Nevertheless, the produced noise is much reduced with the new fan. The acoustic noise in the hall reduces by a factor 3 to 10 from 1Hz up to 50Hz at least. Also seismic noise reduces, in particular in correspondence of the nasty 20Hz bump which is almost disappeared. The coherence between Acoustic and seismic noise in the hall is reduced almost at the level of when the AHU is switched OFF (black curve). It is interesting to notice that among all mitigation actions this action is the first achieving a substantial reduction of the low frequency noise (1-10 Hz) produced by the AHU. The selected times are not affected by wind noise (low wind).