Compressed air is used by industry for a vast number of applications. As a result, 15 per cent of electricity consumed by industry is used to compress air.
For organisations looking to identify energy savings, compressed air systems are an ideal starting point, according to Quentin St Baker.
Simply put, compressed air systems consist of a supply side, where the air
is compressed and treated, and the demand side, where the air is consumed.
Opportunities for energy savings exist on both sides, and a good first step is to develop an understanding of both sides of your system.
The typical compressor cost of ownership consists of the initial purchase, maintenance and energy consumed during operation.
Over 10 years, the cost of energy consumption for a compressed air system can be 10 to 30 times the initial capital investment cost. In inefficient systems, this proportion may be even higher.
It takes roughly 10 times more energy to perform a task with compressed air compared to using electricity directly. This is because 90 per cent of the electrical energy consumed by a compressor is discharged as waste heat.
With so much heat energy available, why not find opportunities to use it? For example, pre-heat water to a boiler, or even use the heat to provide warmth to a factory or office space.
It is not uncommon to discover that 50 per cent of the air being generated by a compressor is not being used for productive activities.
Air leaks are the most widely known compressed air waste, but other waste includes system losses, artificial demands and inappropriate use.
So where do you start? Data is the key! Engaging a specialist to help you collect adequate and appropriate data on your system is recommended.
Once a performance and demand profile is developed you can start investigating and quantifying the opportunities for savings associated with various demand-side or supply-side improvements.
There are many technologies on the market today that can be employed to reduce energy consumption. Permanent magnet motor technology with variable speed control does not suffer from reduced efficiency at part-load and benefits from unlimited starts.
Two-stage compressors are around 15 per cent more efficient than traditional single-stage ones and offer the ultimate in efficiency when used in conjunction with variable speed control.
If more than one compressor is required to meet varying demand at different stages of a process, it is important that they are controlled effectively to ensure that optimum performance is maintained.
It is important that the operation and output of all the compressors is balanced. Also, running the same compressor at maximum capacity all the time can cause undue wear and tear.
A centralised control system linking all your compressors can be programmed to regularly change the unit that is the master or lead compressor so that wear and tear is spread equally over all the units.
Moisture in a compressed air system can increase operating and maintenance costs. It is therefore important to decide air quality requirements when planning a system and install the most efficient drying technology to minimise operational stress on the equipment.
Variable frequency drives (VFDs) provide the ability to exactly match a compressor’s output to the capacity required by varying the speed of the main drive motor.
By allowing continual operation at the desired pressure, the VFD can provide up to 35 per cent energy savings compared to conventional cycling.
In addition, the VFD is “soft starting”, which allows for controlled acceleration and deceleration. This reduces the stress on mechanical components and enhances system reliability, all of which extends the life of your compressor.
For oil-free systems, three-stage centrifugal compressors are highly efficient and should be considered alongside variable speed controlled oil-free screw compressors in systems with high demand variability.
Take care, however, as the technology that you choose may not be the best for your needs. Any number of secondary issues relating to environmental conditions, maintenance or power quality may affect the choice of technology.
It is possible to reduce energy consumption and potentially save thousands of dollars each year when you make informed decisions about your compressed air system.
The wrong advice or applying the wrong technologies can result in excessive ongoing costs for the life of your system.
You can avoid this by using the services of a professional air-energy auditing company with independent consulting services and sophisticated auditing tools and procedures to properly analyse your system.
For further information, see www.capsaust.com.au
About the Author: Quentin St Baker is the national manager for the energy efficiency services division at CAPS Australia. He has extensive experience in the compressed air industry and represents CAPS on the Energy Efficiency Council. St Baker is a mechanical engineer as well as a leading specialist in the science of compressed air.