HVAC problem

A welding process fresh-air vent system in a non-
heated/non-air-conditioned plant consists of ten vent drops (one at each of ten welding
stations). The vent system consists of a large metal duct system with one large fan.
The fan system requires one 200 HP 3-phase induction motor at the fan drive shaft.
The system was originally designed for a single speed, with on/off switching only.
On the first 8-hour shift from 7 AM until 3 PM all of the welding stations are working.
On the second shift 50% of the stations are working. And on the third shift only three
stations are working. This is a five-days-per-week, 52 weeks-per-year plant, where the
welding vent system is shut down on the weekends. The vent system, at the present
time, is either on or off. The vent system is currently operational for all three shifts.
A possible energy savings project involves installing a VFD to drop the cfm of the
system by 50% on the second shift and by 75% on the third shift. The proposed cfm
reductions assume that unused drops will be automatically blocked off so that the vents
will pull out the welding fumes in an effective manner.
The cost of electricity is $0.06 per kWh at all times. Demand cost is $9 per kW from
May 1 through September 30, between 12 noon and 7 PM; and $3 per kW otherwise.
Determine the theoretical annual utility-related savings possible from this project.