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  • The Hidden Costs of Air Compressor Operation Sue Benes, Polaris+ Product Marketing Manager, FS-Elliott Co., LLC

  • The Hidden Costs of Air Compressor Operation by Sue Benes, FS-Elliott Co., LLC Page 2

    With all capital equipment purchases, the initial cost is not the only expense to consider. Often there are associated costs that are not always apparent at the time of purchase and can significantly impact operational budgets.

    When purchasing an air compressor, it is critical to understand the life-cycle costs associated with the unit. The following article will review how to quickly identify issues such as energy and maintenance costs, in addition to not so obvious items like product warranty impacts. It will also examine how one company used this process to save money while providing a reliable, quality air source.

    Doing Your Homework – Air System Auditing Although it can be a daunting process, reviewing the entire life-cycle cost of a compressor is essential to understanding the true cost of your purchase. This includes all the costs the compressor will require to operate. An air audit will provide a baseline of how your current system is operating, and can be done with little to no impact on the production process. An air audit can provide the building blocks to identifying those hidden life-cycle costs in your system and eliminating them. An air system audit can encompass the following:

    • Energy usage data logging – Log the energy air compressors are consuming.

    • Air measurement – Identify a factory’s true compressed air demand and operating schedule.

    • Air leak detection and control – Identify how much air is being lost due to leaks.

    • Air quality – Measure the amount of particulates in the air system, including water and oil which may have an impact on the efficiency of the machines.

    • Maintenance review – Assess the effectiveness of the current maintenance plan.

    • Monitoring and control program – Make recommendations to maintain optimum efficiency.

    Companies that provide air compressor system audits focus primarily on the largest operating cost, energy, but they can also help review maintenance costs as well. The individuals that conduct these audits are certified by the Department of Energy AirMaster+ Program or have gone through the Compressed Air Challenge training courses for compressed air systems. The return on investment of an air system audit can be immediate based on what is uncovered during the process and the final recommendations. To learn more about compressed air system auditing visit www.compressedairchallenge.org.

    Understanding Compressed Air Life-Cycle Costs Compressor life-cycle costs are generally organized into three groups, capital investment or purchase price, maintenance, and energy usage. Organizations are becoming more aware that the main cost in the life-cycle of an air compressor is energy usage, followed by maintenance of the compressor, with the initial capital investment being the lowest of the three costs. Air audits are a great way to quickly identify these costs, but it is important to understand how each of these impacts your bottom line and what steps can be taken to minimize the overall life-cycle cost.

    Energy Costs The most common energy source in an industrial setting is electrical energy. For compressed air systems, an electric motor is often the power supply. An auditor can log the energy usage data and identify the air demand required by your facility to uncover hidden electrical energy costs. Since the goal of all facilities is to deliver the largest volume of

  • The Hidden Costs of Air Compressor Operation by Sue Benes, FS-Elliott Co., LLC Page 3

    RATED POWER

    MOTOR EFFICIENCY ANNUAL

    ENERGY COST

    SERVICE FACTOR

    RUNNING HOURS

    ENERGY RATEx x

    = x

    MOTOR EFFICIENCY

    Operating efficiency of the motor; found on the motor nameplate.

    RATED POWER

    Horsepower rating of the motor powering the air compressor; found on the motor nameplate.

    SERVICE FACTOR How far above the rated horsepower a motor can operate without damaging the motor; found on the motor nameplate.

    RUNNING HOURS Number of hours the air compressor operates on a yearly basis.

    ENERGY RATE Current cost of energy from your local power supplier.

    Figure 1 –Air Compressor Energy Cost Calculation

    air using the lowest amount of energy possible, it is important to understand what the specific demands are for air capacity and how your existing installations are managing those demands. There are five key elements to calculating the energy requirements of a compressor; these include rated power, motor efficiency, service factor, running hours, and energy rate. (See Figure 1 – Air Compressor Energy Cost Calculation.)

    1. Power and Motor Efficiency

    Every electrical motor has an efficiency rating, which is the percentage of electrical input power that the motor uses to power the air compressor. The National Electrical Manufacturers Association (NEMA) has released guidelines for motor efficiency.

    Although there is a wide range of efficiencies, a good rule of thumb is the older and smaller the compressor motor is, the less efficient it is. As an example, a 100 HP premium efficiency motor typically has a motor efficiency rating at around 96%. To calculate this for your air compressor, locate the motor efficiency in the motor manufacturer’s data sheet and the horsepower from the motor nameplate.

    2. Service Factor

    Service factor is a rating applied to the motor that determines how far a motor can operate over the rated horsepower without damaging the motor. For example, a motor with a service factor of 1.15 means that the motor can operate at 15% over its rated horsepower. It is not a good practice to size motors to operate continuously above the rated load in the

  • The Hidden Costs of Air Compressor Operation by Sue Benes, FS-Elliott Co., LLC Page 4

    service factor area. When reviewing compressor capacities, bear in mind some companies advertise a full load that utilizes a portion of the service factor.

    3. Running Hours and Energy Rate

    The running hours are the number of hours the air compressor is operating at the given power value and is typically identified on a per year basis. The running hours are usually grouped by labor shifts due to air demand changes between shifts. The energy rate for your area can be obtained from your local power company.

    Full Load vs. Partial Load Energy Use In an operating environment, there are usually demand cycles when a facility will require more or less compressed air during certain times of the day or week. The majority of air compressors are designed with sophisticated control systems that modulate along an operating range to meet demand. The demand could result in the air compressor running at full load consistently, partial load or modulating between the two. It is important to consider this demand when looking at purchasing an air compressor or energy costs could escalate after compressor installation.

    Most compressors are designed to be at their highest efficiency when they are operating at full capacity. With the need to meet a varying demand there is always at least one compressor that will have to operate at partial capacity. The compressor type and the control mode utilized will determine how efficient a compressor is at partial capacity. Air compressor efficiency is measured in specific power. Specific power is usually reflected in kW/100 cfm or HP/100 cfm and has been referred to as the gas mileage for an air compressor. Figure 2 shows a graph of the specific power for a variable speed drive compressor. Notice that the lower the flow delivered, the higher the specific power. The higher the specific power, the less efficient the compressor operates.

    It is important to remember system demand will vary since a compressor may be operating at a less efficient point in certain demand situations. Because of this, it is best to identify when your compressors are the most efficient and balance the compressor load for the most efficient operation to meet system demand. The most efficient operating points can be identified by the compressor manufacturer or an auditor.

    When estimating the energy consumption of an air compressor, many auditors reference data sheets located on www.CAGI.org, which features the full load power consumption of most rotary screw air compressors. For all other types, such as centrifugal or reciprocating, it is best to ask the manufacturer for a data sheet with the full load energy consumption and partial load efficiencies of the machine.

    System Over Pressurizing One common finding during an air audit is that the facility is operating their compressors well over the required system pressure. For example, the compressed air application may require 100 psi; however, the compressors deliver air at 120 psi to overcome losses in the compressed air delivery system. Losses may be due to filtration after compressor discharge, poor piping design, or leaks in the air system. The greater the discharge pressure of the air compressor the more energy required to compress the air. As a rule of thumb, 0.5% additional electrical energy is required for a discharge pressure increase of 1 psi. Therefore, a 20 psi increase in discharge pressure would require 10% more energy. Plugging this into our equation from Figure 1 would result in an additional $5,258 per year in energy costs due to system over-pressurizing. A properly trained air auditor can p