The Hidden Costs of Air Compressor OperationSue 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 AuditingAlthough 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 CostsCompressor 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 CostsThe 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 EFFICIENCYANNUAL

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 UseIn 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 PressurizingOne 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 provide

Figure 2 – Specific Power vs. Capacity of Variable Speed Drive Compessor – Source: CAGI.org

Specific Power Curve30

25

20

15

10251 410 568 665 722 806

Spec

ific

Pow

er (k

W/1

00 A

CFM

)

Capacity (ACFM)

InefficientOperating Range Efficient

Operating Range

The Hidden Costs of Air Compressor Operation by Sue Benes, FS-Elliott Co., LLC Page 5

several solutions to reduce pressure drop in an air system.

Maintenance CostsEvery piece of machinery requires some form of maintenance to keep operating efficiently. A maintenance review done during an audit can uncover hidden costs a facility may not realize is a result of their compressed air system.

ConsumablesAll air compressors rely on consumable products, such as filters, separators, and oil, which need to be replaced on a regular basis. The amount of consumable products varies greatly based on whether the compressor is an oil-flooded or oil-free machine. Oil-free compressors do not require oil filters after the compressor discharge because the oil is never injected into the compression process. There can be as many as three or four post-compressor filters and an air/oil separator to maintain in an oil-flooded air compressor. The cost of keeping the filters maintained to prevent pressure drops and maximize efficiency for an oil-flooded machine would be non-existent in an oil-free machine. Figure 3 shows a comparison of the compressed air systems of an oil-free and oil-lubricated compressor after discharge.

In addition to the filtration required to keep the air clean, there is the cost of oil itself. It is recommended to change the oil every 6-12 months in an oil-lubricated compressor. To operate one oil-lubricated compressor, many facilities pay between $3,000 and $6,000 per year for oil. Oil-free compressors still require oil, but since the oil never goes into the compression process, they only require oil changes every 2-3 years. Depending on the operating location, there may also be disposal fees for properly removing oil from the site.

Condensate RemovalCondensate is generated from both oil-flooded and oil-free compressors. The difference is that for oil-flooded machines the oil mixes with the condensate creating water/oil mixture that is regulated at the state and local level. This condensate must either be properly disposed of or require another filtration system to filter the oil out of the water. Both alternatives can be a significant burden on operational expenses, with proper condensate disposal costing upwards of $50,000 per year. For more information, Plant Engineering magazine has several articles on condensate disposal guidelines.

Airend ReplacementIn most cases, an air auditor or manufacturer may recommend replacing or overhauling your entire airend to improve performance. Airend replacement can cost up to 70% of the original equipment price. The lifetime of an airend varies depending on the type of compressor (rotary screw, reciprocating, centrifugal) and whether it is oil-lubricated or oil-free. Be sure to ask the manufacturer or auditor the expected lifetime of the airend and how this is addressed in the warranty of the machine.

Finished Product ImpactsOften it is overlooked that when an oil-flooded compressed air source is used, trace oil could transfer and affect the finished product. For some, this may not be an issue, but for companies that produce textiles, food products, or even medicines, clean, dry air is essential to the manufacturing process. Once the oil is injected into the air system,

Oil Lubricated Compressor

Oil-Free Compressor

Air/Oil Separator

Air Dryer

Air Dryer

Air Receiver

Air Receiver

Oil Filters1, 2, 3

Figure 3 – Oil-lubricated and oil-free compressor system comparison

The Hidden Costs of Air Compressor Operation by Sue Benes, FS-Elliott Co., LLC Page 6

as in an oil-flooded machine, it is impossible to produce 100% oil-free air. ISO 8573-1 establishes guidelines for purity classes of compressed air. Figure 4 shows the allowable oil concentration in each class. It is critical for any manufacturer to ask themselves what level of oil can be tolerated in their production paths.

Best Practices – A Simple Air Audit Saves ThousandsRising energy costs forced an automotive component manufacturer located in North Carolina to reevaluate where energy was being utilized in their current plant operations. Knowing that they already had an issue with their five oil-flooded rotary screw compressors forcing a shutdown in production from lack of air supply, they thought they would start by looking at their compressed air system. The facilities manager reached out to a local compressor distributor to conduct an audit of their compressed air system. The findings of the air audit were:

1. The lack of air supply was due to insufficiently sized air compressors.

2. The facility was using extra energy to operate their air compressors 10 psi over the required system pressure.

3. Consumable spending reached upwards of $50,000 per year to operate all five oil-flooded rotary screw compressors.

The recommendations from the air audit were:

1. Replace three oil-flooded rotary screw compressors with a more efficient oil-free centrifugal compressor and keep the remaining two rotary screw compressors running.

2. Remove filtration that was required by the three oil-flooded rotary screw compressors.

3. Minimize energy consumption by operating all compressors at the pressure demanded by the applications.

The benefits of the new air system were:

1. Due to higher efficiencies and lower discharge pressures, the facility was able to meet their air demand requirement while saving $100,000 per year in electrical energy costs.

2. Due to the amount of energy saved by switching from their previous set up to the new set up, the manufacturer was also eligible for a rebate program through their local energy provider. The oil-free centrifugal compressor required fewer consumable maintenance items and the manufacturer was able to reduce their annual air compressor maintenance costs by $25,000.

Bottom line, this automotive parts manufacturer took the time to do their homework and educate themselves on how their compressed air supply was impacting their business. Using many of the discussion points addressed above they applied the results of the air audit to combat their rising energy costs and provide a more reliable air source for their vital operations.

ClassConcentration Total Oil (Mg/M3)

0 As specified, more stringent than Class 1

1 ≤ 0.01

2 ≤ 0.1

3 ≤ 1

4 ≤ 5

Figure 4 – ISO 8573‐1 Oil Concentration Table

The Hidden Costs of Air Compressor Operation by Sue Benes, FS-Elliott Co., LLC Page 7

About FS-Elliott Co., LLCFS-Elliott Co., LLC, is a leading manufacturer of centrifugal air and gas compressors with sales, service, and manufacturing locations around the world. First introduced to the market over 50 years ago their energy-efficient machines incorporate the latest aerodynamic and control system technologies to ensure optimum performance. For more information, visit www.fs-elliott.com.

© 2016 FS-Elliott Co., LLC