tips you should know to save energy with variable frequency inverters (AC drives) (part 4)
Continued from the previous article: tips you should know to save energy with variable frequency inverters (AC drives) (part 3).
Centrifugal pumps.
Conventional systems use a control valve at the discharge of the pump. The pump runs at full speed and the additional head generated is wastefully dissipated at the valve during lower flowrates. A motor powered through a variable frequency inverter (VFI) allows the pump to run at the suitable speed, avoiding such dissipation.
Let's calculate the energy savings of a pump that uses 100 kW at full throughput, but is run at only 70% for 12 h a day, i. e., 4,000 h/yr. Table 1 shows the energy savings for the cases, and a short payback for the second case. In this and the following table, P = power, N = speed, H = head at 100% flow, and Q = flowrate.
The first case is at 70% of maximum flowrate with no variable-system resistance (VSR), and the second, also at 70% of flow, has a VSR = 50 m. The energy savings are substantial in both cases. In addition to the savings shown in Table 1, there will be reduced wear and maintenance of the motor owing to low-speed running, minimizing the purchase cost and recurring maintenance of control valves and other pieces of equipment.
Positive-displacement pumps.
Consider the top column pressure-control system of a 250,000 bbl/d crude distillation column that uses a screw compressor driven by a motor with the following specifications: 6.6 kV, 1 MW, 3,000 rpm, and 50 Hz power supply (the unit is not in the U.S.).
A variable frequency inverter (VFI) powers the motor. The compressor runs at 1,800-2,100 rpm for most of the time to maintain the column pressure. Assume that the variable- and fixed-system resistances are 5 and 30 bar, respectively, at 100% flow. The energy savings are substantial (Table 2) too. The cost of the inverter would be almost the same as that for the control system of a constant-speed motor. Thus, the plant enjoys a recurring annual savings of lower power costs and reduced motor maintenance due to low-speed runs, with little or no additional capital cost. (to be continued)
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