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Compressors: Finding the true life cycle cost

24 July 2017

Those 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, as this is the only source of independently verified performance data on air compressors. However, Andy Jones, MD at Mattei, says efficiency levels change over the life cycle of a unit and whilst two compressors may have the same zero-hour performance,  the reality is that life cycle costs may vary considerably

The consultation period for the Government’s new Industrial Strategy Green Paper has just closed, and sector conversation around energy efficiency has never been more intense. In fact, a report produced by Engineering the Future, a collective representing 450,000 UK engineering professionals, cites energy efficiency as the single most important area for the Government to focus on when finalising this new strategy.

Meanwhile, the UK’s decision last year to leave the European Union means that national energy reduction targets are under review, and may be subject to change, leaving those in the industry more aware than ever of their energy use.

On a wider scale, an independent study conducted by the Intergovernmental Panel on Climate Change revealed that if the current rate of greenhouse gas emissions is maintained, a warming exceeding 4°C of the average global temperature by the year 2100 is a certainty. In view of this, energy efficiency is a responsibility for everyone – from individual usage to that of major industrial enterprises.

So how do compressors factor into this? According to a 2016 paper on the energy saving potential of existing air compressors, the electrical consumption of the industrial sector constitutes more than 50% of global usage, and up to 20% of this can be attributed to air compression and delivery to final uses. Considering that the global market for rotary air compressors is set to grow by 3.6% over the next seven years, the role of compressors in global energy use is only likely to grow. 

It is therefore safe to say that this is an area where energy efficiency is critical, and that compressors are a key part of lowering the overall usage of energy in industry. This is not lost on policy makers, and legislation is likely to come into play in the near future which removes inefficient compressors from the market.

However, it is at this point, where compressor efficiency is evaluated, that problems begin to arise and questions must be asked about how we analyse lifetime performance. 

A whitepaper launched by Mattei this year, Air Compressors – The real economic and environmental impact of using the current industry standard Life Cycle Cost analysis - has revealed the serious inaccuracies in how compressor efficiency is currently assessed, especially in respect to the difference between vane and screw technologies.

The core issue centres around the accuracy of Life Cycle Cost (LCC) calculation. At present, this considers three factors – 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, as addressed in Mattei’s whitepaper, this calculation assumes that air compressor Specific Energy stays constant over time. 

Specific Energy is generally based on the number of kW required to compress 1m3/min of air. This is then multiplied by the Free Air Delivery (FAD), operating hours, and local cost of electricity, to produce the complete running cost of the compressor. However, as referenced earlier, this can only form part of an accurate LCC if we believe that this level of Specific Energy will not change over time – and this is not correct for vane or screw compressors.

Screw compressors

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.

It is an accepted fact 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. 

This wear results in a degradation of performance over time, especially as the unit approaches the point where parts need to be replaced. Although there is a surprising lack of research around this topic, there are small studies available from energy audit companies which illustrate the phenomenon. For example, one such audit, which tested 27 refrigeration screw compressors varying in age up to ten years, found that the average deterioration level was 30%, with the worst compressor performing at 55% degradation level. 

This clearly has major implications for the LCC of a screw unit. As the standard calculation uses the zero-hour data provided by the manufacturer’s CAGI verified datasheet, it paints a very misleading picture, as it doesn’t take into account inevitable wear, and how this relates to the Specific Energy level of the unit over time. This poses a real problem, as it means that those looking to purchase a compressor, or policymakers building energy related regulations, are using an incorrect LCC, which is far removed from the real costs and efficiency of these units over their lifespan.

Vane compressors

Conversely, the efficiency of a rotary vane compressor actually improves over an initial running-in period. 

The design of rotary vane compressors means that there are no roller and thrust bearings to experience wear within the unit, which means that the technical datasheet remains accurate throughout the life of the compressor, regardless of running hours. This also means that there is no need for part substitution, so rotary vane manufacturers like Mattei can offer extended warranties without a cap on operational hours.

Beyond this, from the moment a rotary vane compressor is turned on, to around the 1000-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 1000 hours of operation, the two Mattei units analysed (Maxima 75 Xtreme, Maxima 55) both presented remarkable improvements in Specific Energy levels, with an enhancement of up to 5%.

Given that the Specific Energy level of a screw compressor will degenerate from the beginning of operation, whilst a rotary vane will improve, it is clear that the zero hour Specific Energy of these units should not be used as part of an authoritative LCC. Whilst the current method of calculating lifetime costs may often present the screw as a more efficient and cost effective option, the actual situation is very different, and the vane compressor is often a much better choice. 

This is no small matter – a screw compressor which sustains a degradation in performance of 10% over ten years can cost an owner 12% more, including part replacement costs, than a vane compressor which has an identical zero hour performance, amounting to an additional cost of almost £150,000. 

Without factoring in the change in compressor efficiency across its lifespan, a true LCC cannot be reached. This not only means that buyers are being misinformed on the true running costs of units, but that energy efficiency measures could be being built upon misleading information. Zero hour compressor performance data is currently being used to draft new legislation in an effort to tackle the global warming crisis, and important information like true life cycle costs must be accounted for.  If it is not, then the benefits of any new regulations around industrial air compression could be drastically limited.

* Compressed Air and Gas Institute, based in North America. This body only provides information for 60Hz air compressor models, but as there is no 50Hz published in the same format, these are often referred to as a guide.