Plant & Works Engineering
Searching for energy efficiency
Published:  08 July, 2015

To survive in a tough competitive environment, many manufacturers are searching for ways to achieve their energy efficiency and sustainability targets and improve the efficiency of machinery, in a bid to cut their production spend. Compressed air supply systems can be a source of wasted energy, especially if they have been in place for some years. Steve Sands, product manager at Festo, reviews how energy costs can be reduced by as much as 60% through optimisation at both the production and system level.

What are the costs of compressed air generation?

Compressed air costs (normally expressed in pence/Nm³) can be determined using the sum of fixed and variable costs and using the annual delivery output of the compressor station (fig.1). Annual fixed costs include the depreciation of the investment made, interest rate, space utilisation costs. Whereas, variable costs are made up of energy costs over the full-load and no-load periods of the compressor’s use, costs of consumables such as oil, coolant etc. per year and maintenance costs.

By far, energy costs account for the largest part of the costs, at approximately 75%. In order to generate 1 Nm³ of compressed air, modern compressor stations require between 100 and 120 Wh/Nm³. Compressed air costs can be immediately reduced by up to 30% through using a central waste heat recovery system (WHR) – the savings are based on the heating costs saved. For example, using a WHR with warm water, up to 72% of the compressor output can be utilised as heat or, in the case of air cooling only, the figure can be as much as 90%.

In practice, the costs of compressed air per Nm³ can be determined very accurately by measuring the electricity requirement and the actual delivery rate. However, this rate varies depending on the intake conditions and should therefore be measured at the same time as the energy consumption. The average cost for compressed air estimated at between 1.2 to 2.2 pence/Nm3. The average price of compressed air for a system pressure of 6 bar (rel) is 1.5 pence/Nm3, which is based on the following assumptions: generation costs at 120 Wh/Nm³, which is for 8 bar (rel) at the compressor, electricity costs of 9.5 pence/kWh and the ratio of energy costs to additional costs is 3:1.

To get a complete picture of consumption, possible leakage costs that may occur in non-productive operation should be considered and can be calculated in the same way. Leakage detection is, in fact, a fundamental factor in locating potential energy savings. As a rule of thumb, 20% of the detectable leakages in existing systems account for up to 80% of the avoidable costs. It is worth rectifying leakages quickly as every leakage fixed saves energy, and hence costs, straight away.

Where can potential energy savings be made?

As part of an integrated analysis, all four areas of a compressed air system must be considered: compressed air generation, air preparation, compressed air distribution and compressed air application. The average potential savings in the individual areas of the system vary greatly depending on the applicability of the energy-saving measure, how cost-effective it is and the savings produced.

There are numerous measures for realising potential savings in compressed air systems; these have different effects depending on the particular part of the compressed air system.

First, performing a compressed air audit, i.e. a precise analysis of the existing system with a focus on energy efficiency at each stage is recommended. As the wasted cost for air preparation is just one percent, we will concentrate on the other three areas.

In air generation, accurate energy analysis of the compressor station is necessary to create consumption profiles, gather information about the complete system with its power reserves and implement a leak detection and elimination process, as well as identify opportunities for savings and calculating the individual, plant-specific compressed air costs.

During air distribution, the most effective way of reducing costs is through leak detection analysis. Systematic checks and classification of leakages found according to volume can be made with all relevant data for leakage elimination, for example, necessary spare parts, estimated repair time, etc.

Knowing the compressed air consumption of the production hall – and hence of each machine – during the application stage is vital for optimally designing and configuring the compressed air supply and distribution at the system level. Furthermore, if the consumption of each individual system is known, then sizeable deviations from standard consumption act as early warning signs, e.g. of an existing or developing fault.

In addition, the following information must be collected so that the energy efficiency measures can be precisely evaluated and determined: compressed air components (in particular air nozzles, sealing air) and pneumatic drives used, sizing of components and connecting components, and technical requirements on the application (force, speed etc.).

Who should take which energy efficiency measures?

There is no one-size-fits-all solution and measures must be defined individually for each plant. Which measures depends on factors such as the state of the compressed air system and the extent of utilisation, etc. With machine energy efficiency analysis, these factors are directly dependent on the application. Moreover, the measures should always be reviewed in relation to the total consumption so that the economic efficiency of a particular optimisation measure can be determined.

An integral approach to optimising energy usage in a compressed air system has a range of benefits for the operator: reduction in energy costs and, as a result, in operating costs, reduction in costs for maintenance and servicing, increase in process security and reduction in unplanned production downtime and associated costs. Like any other technology, compressed air systems are efficient if they are used and maintained professionally.

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