Calculating Electrical Operating Cost of Air Compressors

Delivering compressed air to a manufacturing facility is an expensive operation. Delivery requires costly equipment that needs significant amounts of electricity and frequent maintenance. Many facilities have no idea how much their compressed-air systems cost on an annual basis. Often, they could save money by improving the performance of these systems. Electricity costs are by far the largest expense of owning and operating a compressed air system. Annual costs can often be more than the price of the system, depending on the manufacturer and design. This article presents a simple calculation to estimate the annual electricity costs of two compressors: one controlled by modulation and one by a variable speed drive.

Types of Controls –
To calculate electrical consumption, first choose the type of control used to regulate the air volume delivered by the compressor. The Performance Comparison Chart at the left shows the power efficiency curves for the four basic types of regulation controls at varying loads. At 100% capacity, all control types are equally efficient and consume 100% of full power input. However, as capacity decreases, efficiency varies based on control type. For example, a modulation control compressor will require 88% brake horsepower (BHP) at 60% capacity utilization. A variable speed drive control compressor will require about 61% BHP at 60% capacity utilization.

Data Needed For Cost Calculation – The following data is needed to calculate the annual electricity costs for a compressor:

(1) Compressor Full Package Rated BHP (full load compressor package BHP)
(2) 0.746 Conversion Factor which converts BHP to kW
(3) % Brake Horsepower (BHP) for the type of regulation control – Obtain this from the Performance Comparison Chart by choosing the compressor regulation control type. Then, based on the capacity load, read the % brake horsepower Input for the regulation control type.
(4) Motor Nameplate Efficiency (or an estimate of efficiency)
(5) Annual Hours of Operation (hrs/year)
(6) Cost of Electricity ($/kWh). Annual electricity costs can be calculated by inserting this information into the following equation:

Annual Electricity Operating Costs Calculation
Full Package Rated BHP x .746 x % BHP x Annual Hours of Operation x Cost of Electricity $/kWh ÷ % Motor Efficiency

Example 1: A 100 HP modulation controlledcompressor running at 60% capacity with a 90% efficient motor operating 8,760 hours per year, with a $0.05/kWh cost of electricity would cost $31,948.00/year to operate (100 HP x .746 x .88 BHP x 8,760 Hours x $0.05/kWh ÷ .90 Motor Efficiency).

Example 2: A 100 HP variable speed drive (VSD) compressor running at 60% capacity with a 95% efficient motor operation 8,760 hours per year, with a $0.05/kWh cost of electricity would cost $20,980.00/year to operate (100 HP x .746 x .61 BHP x 8,760 Hours x $0.05/kWh ÷ .95 Motor Efficiency).

Conclusion – Due to the relatively low initial cost of the compressor when compared to lifetime electricity expenses, users should utilize life cycle cost analysis when making decisions about compressed air systems. Overall system efficiency is the key to maximum cost savings. As shown in the operating cost examples above, the nearly $10,000.00 annual cost savings of the more efficient VSD control can be significant enough to pay for its additional cost within a short period. Too often users are only concerned with initial cost and accept the lowest bid on a compressed air system, ignoring system efficiency. Too many decisions regarding compressed air systems are made on a first-cost basis, or with an “if it ain’t broke, don’t fix it” attitude. To achieve optimum compressed air system economics, compressed air system users hould select equipment based on life-cycle economics. Users should also properly size the components, turn off unneeded compressors, use appropriate sequence control and storage strategies, and operate and maintain the equipment for peak performance.

Air Technologies^® specializes in custom engineered compressed air systems that maximize efficiency. Contact us at a location nearest you or email us for a professional consultation on your compressed air system.

[ back to top ]