Variable Frequency Drives (1)

1. Variable Torque Loads

Variable loads offer a tremendous opportunity for energy savings with AFD’s. The areas of greatest opportunity are fans and pumps with variable loads.

Fan and pump applications are the best opportunities for direct energy savings with AFD’s. Few applications require 100% of pump and fan flow continuously. For the most part, these systems are designed for worst case loads. Therefore, by using AFD’s, fluid affinity laws can be used to reduce the energy requirements of the system (Fig. 1).

Fig.1 Pump and Fan Affinity Laws
Eq. 1: N1 / N2 = Flow1 / Flow2
Eq. 2: (N1 / N2)^2 = Head1 / Head2
Eq. 3: (N1 / N2)^2 = T1 / T2
Eq. 4: (N1 / N2)^3 = HP1 / HP2
By using the affinity laws, you can determine the approximate energy savings:
Ex. 1: 250hp Fan Operating 160 hrs / Week
hp1 / hp2 : (N1 / N2)^3
100% spd = 40 hrs = 100% ld = 250hp
75% spd = 80 hrs = 42% ld = 105hp
50% spd = 40 hrs = 13% ld = 31hp
kWh / wk = (hp) x (0.746) x (hrs / eff)
250 x 0.746 x (160 / 0.95) = 31,411kWh/wk
Assuming no loss of efficiency at reduced speeds:
(250 x 0.746 x (40/0.95)) + (105 x 0.746 x (80/0.95)) + (31 x 0.746 x (40/0.95)) = 15,422 kWh
By using an AFD the approximate kWh savings per year would look like:
(31,411 – 15,422) x 50 = 800,000 kWh/yr

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