Assessing Annual Energy Costs

Nov. 2, 2008
Annual energy costs are determined via:  $/year = (BHP)(kW/BHP)(days/year)(hours/day)($/kWh) making it easy to compare the economics of the airflow-control options.Consider that Company A wants to adjust its air movement system to operate at 20 inches of static pressure (SP) at a volume of 105,000 cubic feet per minute (CFM). It is deciding between using a fixed-speed motor running at 1,180 RPM with dampers or a VFD-controlled motor.
Figure 2 -- Damper control: Inlet damper requires much less power than outlet damper for the same performance. Basis: motor running at a fixed 1,180 rpm.
Outlet Dampers. Partially throttled outlet dampers can achieve the desired static pressure and volume at 1,000 BHP (Figure 2). The annual operating cost, therefore, is:  (1,000 BHP)(0.746 kW/BHP)(350 days/year)(24 hours/day)($0.05 /kWh) = $313,320.Inlet Dampers. Partially throttled radial inlet dampers can reach the same static pressure and volume at only 700 BHP (Figure 2), cutting the annual cost to:  (700 BHP)(0.746 kW/BHP)(350 days/year)(24 hours/day) ($0.05 /kWh) = $219,324.Variable Frequency Drives. Employing a VFD allows achieving the same operating pressure and volume at a fan speed of 840 RPM (Figure 3). The figure shows that this reduction in speed drops the required horsepower to only 450 BHP, which generates an annual operating cost of:  (521 BHP)(0.746 kW/BHP)(350 days/year)(24 hours/day)($0.05 /kWh) = $140,994.
Figure 3 -- VFD control: For the same performance as in Figure 2, VFD cuts power requirement substantially. Basis: motor control at 840 rpm.
So, Company A will save 55% of the energy costs by choosing a VFD over outlet dampers and nearly 36% over inlet dampers. In addition, it may realize additional savings from the reduced equipment wear and tear that a VFD provides compared to dampers.

Sponsored Recommendations

Connect with an Expert!

Our measurement instrumentation experts are available for real-time conversations.

Heat Recovery: Turning Air Compressors into an Energy Source

More than just providing plant air, they're also a useful source of heat, energy savings, and sustainable operations.

Controls for Industrial Compressed Air Systems

Master controllers leverage the advantages of each type of compressor control and take air system operations and efficiency to new heights.

Discover Your Savings Potential with the Kaeser Toolbox

Discover your compressed air station savings potential today with our toolbox full of calculators that will help you determine how you can optimize your system!