Compressed air: minimising pressure drop

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Compressed air is used widely in farm equipment and for operations in most primary and auxiliary agricultural activities, including erosion control, land drainage and irrigation, tilling, planting, insect and weed control, pruning, harvesting and threshing. Other applications include spraying trees, dusting insecticides and fungicides, feeding livestock in transit, disinfecting poultry houses, handling rice hulls, changing tractor tyres, cleaning eggs, picking raw cotton, and seeding and fertilising with compressed-air guns.

Excessive pressure drop (i.e. the drop in pressure between the compressor and end users) along the compressed-air distribution system increases the cost of running an air compressor. Causes of excessive pressure drop include unnecessary bends in compressed-air lines and unnecessary fittings (e.g. valves and pipes) that are too thin for the duty.

Small pipe sizes result in high air velocities, which cause excessive pressure drop due to friction inside the pipes. For example, if the system air pressure drops from 700 kPa at the compressor to 550 kPa at the equipment – a common occurrence – this results in a 36 percent loss of energy.

The distribution of air should be managed with a maximum pressure drop of about 50 kPa, including all fittings and connections. This should result in a compressed-air speed of no more than six to 10 metres per second. As a rule of thumb, an energy-efficient system results in a pressure drop of less than 10 percent of the compressor’s discharge pressure.

In terms of energy savings, a general rule says that a 100kPa reduction in the operating pressure can save approximately eight percent of the energy used.

Technical factors

Pipe pressure losses are proportional to:

  • the length of the pipe; and
  • (the compressed air velocity in the pipe)².

When designing a piping system, check that the pipe air velocity is between six and 10 metres per second. To select the optimal pipe diameter, use the following procedure:

1. Determine required free air flow rate for the end use (L/s)

2. Convert this figure to compressed-air use:

3. Select pipe diameter for a velocity of about six metres per second:

Note: free air use is at atmospheric pressure (101 kPa or 14.7 psi).

Other design aspects also affect the likelihood of pressure drops within compressed-air systems. The factors listed below also need to be considered.

  • Flexible piping creates more friction than solid pipes, so use them only when necessary.
  • Short pipe runs with few bends and corners have low pressure drops.
  • Install long pipes at a downward angle so water drains to a low point. Install drain points to trap water before it reaches end users. Use automatic condensate traps that open when water is present rather than manual traps.
  • Avoid U-shaped bends in pipes, which can trap water and may rust.

Regardless of how efficient your pipe design, some amount of friction is unavoidable. Filters, dryers and separators may be needed to clean and dry the air but these can cause pressure drop along the line. Typically, the higher the quality of air required, the greater the pressure drop you can expect from these components. Components should be selected to meet the minimum required air quality, as over-specifying them will result in higher equipment costs and increased energy use.

In the case of an existing installation, replacing existing pipes with larger-diameter pipes can reduce the air velocity, thus lowering losses through friction. A good opportunity to do this is in situations where air demand has increased following the installation of a distribution system.

Ensuring that air filters and drying equipment are well maintained will also help prevent pipe corrosion and reduce the friction losses it causes.

To assess the efficiency of your piping system and identify causes of major pressure drop, develop a pressure profile (such as that shown below) by taking measurements of compressed-air pressure at different points along the system.

Compressed air pressure profile
Figure 1: Compressed-air system pressure profile.
NSW Farmers

Evaluating quotes

When selecting a pipe size, the cost of pipe and the energy loss costs need to be balanced, as often energy costs are not considered.  Request that the supplier provides calculations addressing this matter as part of the quotation.

Further information

US Department of Energy, Energy Efficiency & Renewable Energy: Advance Manufacturing Office, Compressed Air Systems

References

Queensland Government, n.d. Eco-Efficiency for Queensland Manufacturers, Stage 3  Opportunities for Improvement, Foundry Factsheets. [Online]

Sustainability Victoria, 2009. Energy Efficiency Best Practice Guide: Compressed Air Systems 2009. [Online]

U.S. Department of Energy, n.d. Compair Webinar, 'Compressed Air Assessment Basics'. [Online]


This content was produced with assistance from Energetics
www.energetics.com.au

 

Facilities, Cropping, Energy

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