- A compressor audit has the potential to provide immediate energy savings.
- Audits typically include the interaction between supply and demand.
- Independent auditors should provide recommendations that are unbiased.
Air compressors are widely used throughout industry. Almost every manufacturing facility, from the smallest machine shop to the largest automotive plant, uses some form of compressed air. In many facilities, compressed air is a core function and the largest electricity user. Electricity typically represents 76 percent of total cost of ownership.
A compressor audit has the potential to provide immediate energy savings and a sizable impact on a company's profits. Energy savings can be as high as 30 percent and operating costs can be reduced by as much as 50 percent. Other benefits of an audit include:
- Improved system reliability
- Increased productivity
- Reduced unscheduled downtime
What is included in an audit?
Audits typically include an examination of both air supply and use, and the interaction between supply and demand. The output of a compressed air system, energy consumption in kilowatt-hours, and the annual cost of operating the system are calculated. The auditor also measures total air losses from leaks and locates those that are significant. All components of the compressed air system are inspected individually and problem areas identified. The findings are provided in a report with a recommended course of action.
However, before scheduling an audit, the plant manager should check for and repair the following: clogged filters, major leaks, damaged filter/regulator/lubricators (FRLs), unauthorized modifications and inoperable drains. By completing this checklist of maintenance items, an external air audit may no longer be needed. These issues must also be addressed before an auditor can perform a successful audit and achieve optimum efficiency.
Supply and demand
Improving and maintaining peak compressed air system performance requires addressing both the supply and demand sides of the system, by answering the following questions:
Supply side issues:
- Is the location, connection to cooling water and ventilation of the compressor adequate? What is the compressor's condition and efficiency?
- What is the pressure across air-inlet and lubricant filters? Should a maintenance schedule be recommended?
- What are the aftercooler and separator efficiencies, cooling effectiveness and condensate separation effectiveness in relation to the relative humidity?
- What is the dryer size, pressure drop and efficiency and is it suitable for the current application? Is a filter needed to prevent contamination of the dryer?
- Is the receiver tank adequate in terms of location and size? Is the receiver drain trap operating properly?
Demand side issues:
- Is the layout of the piping system properly designed? What is the pressure drop and efficiency? Is the condensate removal system adequate?
- What is the load profile (compressor demand over time) over a seven-day period based on pressure, ambient temperature, amps and voltage under different demand conditions? Will advanced control strategies or storage options be needed?
- Are the equipment and processes using the compressed air appropriately? Can pressures be lowered or existing air compressors replaced with another source of energy such as blowers, vacuums or air conditioning?
Addressing the system as a whole
An understanding of the interaction of components on both the supply and demand side is essential to a successful audit. System issues are addressed by answering the following questions:
- What is the appropriate level of air treatment for proper operation of the equipment? What are the air quality levels at critical points in the system?
- How large are the leaks and where are they located? What is the most appropriate leak management program?
- Is the system operating at excessive pressures? What is the lowest possible pressure level required to operate production equipment effectively?
- Is the existing control system appropriate for the system demand profile? Should the system operate in a different mode or alternative control strategy be used?
- What is the chemical analysis of the cooling water? Is a treatment for hardness needed to avoid fouling of cooling surfaces?
- What are the potential applications for heat recovered from the compressed air system? How much energy will this save?
More detailed analysis of the system may be recommended if a system is poorly designed, in unsatisfactory operating condition or in need of substantial retrofit. Some auditors also prepare a detailed systems flow diagram, as well as a financial evaluation of retrofit costs.
After a circuit board manufacturer completed a system audit, plant personnel were able to reduce the plant’s compressor use, lower its system pressure and thereby improve its efficiency. With a total project cost of $55,000 and annual compressed air energy savings of $63,000 and 742,000 kWh per year, which reduced annual energy costs by 5 percent, the plant achieved a simple payback of 10.5 months.
Outsourcing the audit
Plant managers should consider using an independent auditor who has worked at similarly sized facilities and can provide strong references documenting true energy savings. A variety of organizations also provide compressed air system audits, including utilities, equipment distributors and manufacturers, energy service companies and engineering firms. As expected, the quality and comprehensiveness of audits vary. An independent auditor should provide unbiased recommendations that do not focus on any particular manufacturer.
For further information
The Compressed Air Challenge offers a best practices manual for purchase, as well as free analysis software. The online library also has a wealth of useful information. Also see:
Bob Baker. Compressed Air Auditing 101. Atlas Copco Compressors. (Last accessed April 23, 2013.)