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Analysing the energy efficiency of compressor technologies

On the road to compressed air energy efficiency

Part 2: Analysing the energy efficiency of compressor technologies

The electricity cost of running a compressor accounts for around three quarters of its lifetime costs. In view of ever increasing energy prices, it is therefore more important than ever to determine and implement the most efficient compressed air system concept possible. In part 2 of our ‘on the road to compressed air energy efficiency’ blog post series we look at how to analyse the energy efficiency of compressor technologies.

In part 1 of this series of blog posts, we looked at how you can start your journey on the road to compressed air energy efficiency, by initially undergoing a comprehensive compressed air audit to identify your existing compressed air demand and usage. We discussed how this can highlight simple improvements that could be made to an existing compressed air system such as fixing compressed air leaks. However, a compressed air audit may also demonstrate that significant energy savings can be achieved by replacing ageing or inefficient equipment. In this scenario it is important to investigate the energy efficiency of any proposed new equipment. The heart of a compressed air system is the air compressor itself and so in this blog post we will look at how you can analyse the energy efficiency of a compressor by considering the key components.

It has been estimated that energy savings of approximately 10% can be achieved through the use of efficient airends, 1:1 direct drives and efficiency optimised IE3 and IE4 energy saving electric motors;

The Airend
The airend, or compressor block, is critical to the compressor’s overall efficiency and therefore the compressor’s energy consumption and operating costs.
The specific power requirement of a rotary screw compressor is derived from the relationship between flow rate and power consumption, and it will reach its optimum specific performance only at a certain pressure and a certain speed. Current manufacturing technology for compressors larger than 18.5 kW and in the pressure range 5.5 to 15 bar, is to directly couple the airend and motor in order to utilise the most advantageous specific power requirement characteristic. This means the airend and motor turn at the same speed with no transmission loss.
Advanced compressor manufacturers are able to make low speed airends for every size range that work in their zone of optimal performance. These have the advantage over most of the compressors on the market using higher speed airends and gearbox or belt transmission. They are more efficient and are able to deliver more compressed air for the same drive power. As energy consumption represents 70 to 80% of overall air production costs, this will generate considerable savings.

Drive Motor
When it comes to the drive motor, efficiency and simplicity are important. Direct coupled 1:1 drives offer the best efficiency with no loss in transmission efficiency and require no maintenance. Belt drives require only simply maintenance and offer advantages such as flexibility in pressure selection. Automatic belt tensioning devices further ensure transmission efficiency and protect bearings from excess stress.

Motor Efficiency
Motor efficiency affects electrical consumption. The efficiency classes according to the International Efficiency (IE) Code for low-voltage AC motors, describe the efficiency of motors in converting electrical to mechanical energy. Previously, low-voltage AC motors were classified as EFF3, EFF2 and EFF1 in Europe and EPACT in the USA. The standard EN 60034-30:2009 globally defines the above efficiency classes for low-voltage AC asynchronous motors in the range from 0.75 kW to 375 kW and replaces national standards such as EPACT or EFF classes.

The standard defines the minimum requirements on the energy efficiency of asynchronous motors. These definitions are provided in three classes at this time and specified according to the international efficiency standard IEC 60034-30. In addition to the classes IE1 to IE3, the most efficient class of the standard is IE4 (‘Super premium efficiency’) which is – at this point in time – not yet legally binding.

From June 16, 2011, it was required that motors meet the energy efficiency class IE2. Since January 2015, the output class 7.5 to 375 kW were required to meet the energy efficiency class IE3, with 0.75 to 375 kW motors after January 2017.

In addition to the high degree of efficiency and lower energy consumption, the other benefits of optimised compressor drives are low operating temperatures and therefore a longer life.
Heat losses as a result of poor efficiency can be as high as 20% in small motors and 4-5% in larger motors from 160 kW upwards. With correct operation, energy saving motors according to IE3 need significantly less heating and so correspondingly exhibit less energy losses, which means that they have a higher temperature reserve at the same operating mode. Lower working temperatures mean less thermal stress on the motor, the motor bearings and the terminal box, resulting in an extended service life of the drive motor.

Let’s put this into practice and compare the energy costs of a 15 kW base load compressor where one has an IE1 motor and the other an IE3 motor. The performance efficiency of the compressor with the IE1 motor would be around 87%, with a power loss of around 1.95 kW. Whereas the compressor with the IE3 motor would have a performance efficiency of around 92% and only a 1.5 kW power loss. The energy savings of opting for the compressor with an IE3 motor would there be 937 W. Based on the compressor running 8.760 h per year and a 0.15 $/kWh this would equate to AUD $1,231.00 of energy cost savings per annum!

Other components of an air compressor that should be considered in evaluating its energy efficiency include;

Cooling system
Coolers and fans must be sized to provide low discharge temperatures in high ambient temperatures. In air cooled units, low noise radial fans generally provide better cooling while using less electricity than axial fans.

Control Panel
What features does the compressor control panel or operating interface have? The control panel should be reliable, user-friendly and run the compressor efficiently. It should indicate operational status as well as offer maintenance intervals reminders, diagnostic information and external communications capability for remote monitoring and control.

Vibration Isolation
Vibrations can loosen lubricant and air fittings as well as electrical connections. Some compressors mount the motor and compressor block on vibration isolators to eliminate this source of stress. Additional isolators under the compressor package offers another layer of vibration protection, and for most rotary screw compressors these isolators eliminate the need for special foundations.

Following the recommendations from a comprehensive compressed air audit will certainly assist you in optimising the energy efficiency of your compressed air system. However there are further steps that should be taken to ensure efficiency in the long run. Join us next month for part 3 of this blog post series where we will look at the importance of maintenance in operating an energy efficient compressed air system in the long term.

Click here if you would like to learn how energy efficient your compressed air system is!

References:
KAESER Compressors, Inc. (2011): Air Compressor Guide – Getting the most for your money