FEATURE COMPRESSED AIR
RE-ASSESSING THE LIFE CYCLE COST OF COMPRESSORS
Left: How do screw and rotary vane compressors differ?
Andy Jones, managing director at Mattei
WEAR AND TEAR In the case of screw compressors, efficiency deteriorates from the beginning of operation. This is essentially due to wear related to the fundamental design of this technology and leads to a degradation of performance over time. It is widely accepted that roller bearings and thrust bearings are subject to wear, and that the rate of this is contingent on speed and load. To counter this, screw compressor manufacturers advise the substitution of all major rolling and thrust elements after a certain number of operational hours – often between 40,000 and 50,000.
Buyers looking to invest in an air compressor often rely on zero hour data sheets verified by the Compressed Air and Gas Institute (CAGI) to predict lifetime performance. However, this commonly accepted method of calculating life cycle costs is inaccurate. Andy Jones, managing director at Mattei, explains why a re-evaluation of established methods of analysis is needed
E
nergy efficiency remains right at the top of the political agenda in the UK.
Government initiatives, such as the recently launched ‘Clean Growth Strategy’, have set out the actions it will take to reduce emissions, increase efficiency and lower the amount that consumers and businesses spend on energy. By putting clean growth at the centre
of a modern industrial strategy, it hopes to create new low carbon opportunities, whilst meeting national and international commitments to tackle climate change. It is estimated that the industrial sector accounts for more than 50 per cent of global electricity consumption. Of this, up to 20 per cent – equating to 1,335TWh/y – is due to air compression and delivery to final uses. With the current global rotary air
compressor market estimated to be worth $16 billion and expected to grow at a CAGR of 3.6 per cent over the next seven years, it is clear why energy saving or energy recovery in industrial compressed air systems is considered an important contributor to the clean growth cause. With the efficiency of compressors likely to come under even greater scrutiny in the context of wider energy saving initiatives, the lifetime performance of compressors is a focal point of interest. The problem is that the established
14 JUNE 2018 | FACTORY EQUIPMENT
means of calculating the lifetime energy consumption of an industrial air compressor is intrinsically flawed.
CHALLENGING CONVENTIONAL THINKING Throughout industrial manufacturing, life cycle cost (LCC) is a well-recognised method to simulate the full cost of ownership for capital equipment. It is usually calculated by taking into account capital equipment expenditure, ordinary maintenance costs and energetic consumption costs. However, a study by Mattei has found inaccuracies in how compressor efficiency is currently assessed, especially in respect to the difference between vane and screw technologies. At the centre of the issue is how the
LCC is arrived at, taking into account capital equipment expenditure (CAPEX), ordinary maintenance costs and energetic consumption costs. Generally based on independently verified CAGI datasheets, this information is combined to produce an LCC for a compressor. But this calculation assumes that air compressor Specific Energy stays constant over time – which is not the case for vane or screw compressors. So what is the real life cycle cost of an air compressor? And how do screw and rotary vane compressors differ?
EFFICIENCY GAINS The situation with rotary vane compressors is very much different, with efficiency actually improving over an initial running- in period. This is because the design of rotary vane compressors means that there are no roller and thrust bearings to experience wear within the unit. Furthermore, from the moment a rotary
vane compressor is turned on, to around the 1,000-hour mark, the blades undergo a polishing process. This results in less friction, and consequently, better operation and reduced energy requirement. Tests conducted by Mattei in 2016
found that after 1,000 hours of operation, the two Mattei units analysed (Maxima 75 Xtreme and Maxima 55) both presented remarkable improvements in Specific Energy levels, with an enhancement of up to five per cent.
CALCULATING COSTS The reality is that without factoring in the change in compressor efficiency across its lifespan, a true LCC cannot be reached. As a result, the current method of calculating lifetime costs may often present the screw as a more efficient and cost effective option, but the vane compressor is often a much better choice. Not only are buyers being misinformed about the real running costs of units, but inaccurate efficiency statistics could also have a negative impact on any future legislation. That’s why a true life cycle cost that correctly assesses the relative impact of screw and vane compressor efficiency is essential for real progress on energy consumption to be made.
Mattei
www.mattei.co.uk T: 01789 450577
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