Energy Management in Plastics Processing - Part 11
A series of energy efficiency worksheets by Dr. Robin Kent for the Carbon Trust to help the plastics industry reduce costs through efficient
use of energy.
UK Government Environment and Energy Helpline 0800 585 794
Compressed air is a convenient and often essential utility, but it is very expensive to produce. In fact, most of the energy used to compress air is turned into heat and then lost. At the point of use, compressed air costs more than ten times the equivalent quantity of electrical power i.e., an equivalent cost of around 50p/kWh. At this price, it should never be wasted and only be used when necessary. Air also needs to be treated to remove moisture, oil and dirt and the higher the quality required, the greater the energy consumed by the treatment system.
The chart below shows the cost of compressor ownership over ten years. In a typical 24 hour day, five and half day week, a 100 kW motor will use energy worth around £30,000 per year, assuming the cost of electricity to be £0.045/unit. At these cost levels, an energy efficient system is highly cost effective, even if it costs slightly more to install.
The cost of compressed air makes it an expensive resource and the way to achieve the best savings is to minimise the demand and then to optimise the supply. Up to 30% savings can be made by inexpensive good housekeeping measures such as making end users aware of the cost of generating compressed air and enlisting their help in reporting leaks.
Compressed air is an expensive resource. Minimise the demand and then optimise the supply.
A significant amount of energy is wasted through leakage. Typically, leak rates are up to 40% - i.e. 40% of the generating power is wasted in feeding leaks. A 3 mm hole in a system at 7 bar will leak about 11 litres/sec and costs £1,000 per year. In a system with numerous leaks, this cost will multiply rapidly!
Simple leak surveys and maintenance can produce dramatic cost reductions and in some cases, leak reporting and repair has enabled companies to shut down some compressors for all or most of their operating time.
- Tip - Simple and repeated walk-round surveys, with leaks tagged and repaired as soon as possible, will significantly reduce leakage rates.
- Tip - Isolate redundant pipework, this is often a source of leakage.
- Tip - Measure losses due to leakage and target reductions.
Compressed air is often misused because everyone assumes itís cheap. Check every application to see whether it is essential or simply convenient.
- Tip - Stop the use of compressed air for ventilation or cooling - fans are cheaper and more effective.
- Tip - Fit high efficiency air nozzles - payback can be as short as four months.
- Tip - Consider the use of electric tools instead of compressed air tools.
- Tip - Do not use compressed air for conveying granules or products.
Reduce generation costs
The higher the compressed air pressure, the more expensive it is to provide the air. Twice the pressure means four times the energy cost. The real needs may be lower than you are supplying. In some cases the machine rating is for a 7 bar supply but pressure reducers are fitted inside the machine. What are your real needs?
- Tip - Check that compressed air is not being generated at a higher pressure than required.
- Tip - Switch off compressors during non-productive hours. They are often only feeding leaks or creating them.
- Tip - Check that compressors are not idling when not needed - they can draw up to 40% of full power when idling.
- Tip - Position air inlets outside if possible - it is easier to compress cold air.
- Tip - If there is a machine or area that requires compressed air longer than the rest, consider zoning or a dedicated compressor so that others can be switched off.
- Tip - Investigate electronic sequencing to minimise compressors going on and off-load.
- Tip - Maintain the system - missing a maintenance check increase costs.
The longer the compressed air pipeline, the greater the pressure loss over the pipeline and the greater the cost of the system.
- Tip - Make sure that pipework is not undersized, this causes resistance to air flow and pressure drops.
- Tip - Use a ring main arrangement in each building - air can converge from two directions. This reduces the pressure drop and makes changes to the system easier.
- Tip - Avoid sharp corners and elbows in pipework as these cause turbulence and hence pressure drops.
Reduce treatment costs
- Tip - Treat the bulk of air to the minimum quality necessary, e.g. 40-micron filters are usually sufficient. Specifying 5 micron will increase filter purchase cost, replacement frequency, and pressure drop.
- Tip - Test filters regularly to make sure pressure drop does not exceed 0.4 bar - if the pressure drop is higher than 0.4 bar, replace the filters, since the cost of power to overcome this drop is usually greater than the cost of a filter.
- Tip - Manual condensate traps are often left open and act as leaks. Consider fitting electronic traps to replace these.
Compressed air is not free and processors can save at least 30% of the costs of compressed air by simple management systems and maintenance. Start now by contacting the Environment and Energy Helpline for the full set of free information on how to reduce your costs.
The "Energy Management" series is designed to give plastics processors an insight into how to manage a valuable resource.
Download the complete series as an Adobe Acrobat file.
Last edited: 11/03/10
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