The 14 Rs of compressed air efficiency

Install, operate, and maintain a system that makes the best use of resources.

By David M. McCulloch, Frank Moskowitz, and Ron Marshall, Compressed Air Challenge

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Educational institutions have realized the need for emphasizing the basics, sometimes referred to as the three Rs — reading, ’riting and ’rithmetic. There are also several basic Rs to be kept in mind if you want to install, operate, and maintain an efficient compressed air system:

  1. reduce leakage losses
  2. reduce pressure at points of use
  3. reduce pressure at source (compressors)
  4. reduce system pressure fluctuations using adequately sized and located air receivers and controls
  5. reduce number of partially loaded compressors to only one
  6. remove inappropriate applications
  7. reduce system pressure drop losses with properly sized piping and valves
  8. remove moisture content of compressed air with the proper type and size of dryers
  9. remove condensate without loss of compressed air
  10. reduce downtime through preventive maintenance
  11. record system data and maintenance
  12. review air usage patterns regularly
  13. recover heat
  14. reduce energy costs (return on investment and cost of operation).

R #1 — Reduce leakage losses

Compressed Air Challenge

The Compressed Air Challenge is a voluntary collaboration of manufacturers, distributors and their associations; industrial users; facility operating personnel and their associations; consultants; state research and development agencies; energy efficiency organizations; and utilities. The mission of the CAC is to be the leading source of product-neutral compressed air system information and education, enabling end users to take a systems approach, leading to improved efficiency and production and increased net profits.

In a typical plant compressed air leaks amount to 20-30% of the total of all the compressed air produced. In worst case scenarios, where no detection and repair programs exist, leakage levels can be more than 50%.

A ¼-in. leak in a 100 psi system having a pressure of 100 psig, will allow more than 3 million ft3 of free air to escape in one month. At an average specific power of 18 kW/100 cfm, this amounts to 107,000 kWh of lost energy or $10,700 in energy cost per year at $0.10/kW. This problem is worsened in systems operating at even higher pressures.

Leakage rates drop with lower operating pressures. If the system pressure could be reduced to 80 psig, for example, the leakage flow, and energy use in a well controlled system, would drop by 17% not including additional savings due to compressing to a lower pressure, which could amount to additional savings of 10%.

Leaks can be both intentional and unintentional.

  • Intentional leaks include open condensate drain cocks and valves.
  • Unintentional leaks include leaking pipe joints and valves, damaged hoses, and inexpensive poor-fitting quick-disconnect couplings.
  • Equipment not in use may also be using some compressed air. Such equipment should be isolated from the distribution system by a valve.

One way to determine the leakage rate in a system is to do special testing when all of the production equipment in the plant is shut down. If the compressors can be run in load/unload mode, the time loaded as a percentage of total running time will represent the percentage of total capacity going to leaks. Alternatively, for compressors with other than load/unload controls, you can do a volume bleed down test. This method requires the use of a pressure gauge downstream of the receiver and an estimate of total system volume, including any downstream secondary air receivers, air mains, and piping (V, in cubic feet). The system is then started and brought to the normal operating pressure (P1), and the compressor is turned off. Measurements should then be taken of the time (T) it takes for the system to drop to a lower pressure (P2), which should be a point equal to about one-half of the operating pressure. Leakage can be calculated as follows:

Leakage (cfm free air) = [V x (P1-P2)/(T x 14.7)] x 1.25
Where:
V is volume in cubic feet
P1 and P2 are starting and ending pressures in psig
T is time in minutes

The 1.25 multiplier corrects leakage to normal system pressure, allowing for reduced leakage with falling system pressure to 50% of the initial reading. Again, leakage of greater than 10% indicates that the system can likely be improved. These tests should be carried out once a month as part of a regular leak detection and repair program. Click here to view a description of these tests.

Unfortunately, leaks are not a problem with a one-time cure. Maintaining a lower leak level requires ongoing vigilance and a mindset that will not allow leaks to be tolerated. Recognized leaks must be tagged and repaired as soon as possible.

Case study

At a large automotive manufacturing plant an energy team consisting of volunteers, mostly union labor from the shop floor, was led by an energy coordinator. The first step in initial mission was to reduce energy waste by targeting air leaks. Baseline data was gathered during normal production and during a Christmas shutdown. From this information, a leak reduction program was developed and approved by management, based upon estimated potential savings.

In the next step, leaks were identified and tagged for repair. The results of the efforts were published each weekend, and news of the success spread through the plant. Each team member was given a red “Energy Team” jacket. They developed a procedure for all employees to report leaks and be rewarded for their efforts. Bulletin "leak" boards were installed, and progress in fixing leaks was posted. Messages were displayed on TV monitors throughout the plant. Soon, everyone was aware and involved in the program, which produced a cultural change.

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