Balancing act

Improving and maintaining peak compressed air system performance requires addressing both the supply and the demand sides of a system and understanding how the two interact. Chris Dee, executive director of the British Compressed Air Society, explains

The systems approach to energy and economic efficiency involves more than just fixing leaks. If you don't consider the rest of the system, then you will continue to waste a lot of money every hour you run your system.

Over the years BCAS has found that one of the biggest issues that prevents companies from maximising their compressed air system's efficiency is understanding that it is a system - not just compressors and some piping. In many cases when we ask to be shown a company's compressed air system we are taken only to the compressor room, an important part but just one part - the supply side.

Few are able to identify their largest or most critical end uses - the demand side.

Fewer still know the end uses' actual requirements - pressure, flow and air quality.

The BCAS Installation Guide introduces these concepts by providing some basic knowledge on components and then builds on them to get participants thinking not just about how they produce air, but how they actually use it.

The investigation of the demand side involves breaking the system down into end uses and assessing the characteristics of these elements. In this way, proper planning of efficiency and other system improvement measures can be done without negatively affecting the system. Too often companies buy a new compressor thinking that it will solve all their energy efficiency difficulties. If you step back and take a systems approach and start analysing the compressed air system as a whole, you start to see what is reasonable and efficient for the whole plant and not just one component or one use. Often a company will specify and spend more money on a compressor just because it is 1 to 2% more efficient at some rated full load condition. But that same plant may be adding up to 20% to their annual costs because of unnecessarily elevated pressures or wasting over 30% of their compressed air in leaks or driving equipment that should not be powered by compressed air.

How much air do you need? What is your current compressed air usage? Each industry is different and each plant within any given industry is unique. A new plant will have specific initial requirements; these will change over time and anticipated growth should be considered in the initial design. Identify and tabulate anticipated end uses and quantify rate of flow compressed air requirements.Machinery and processes requiring compressed air should be identified by type and the manufacturer's specified air flow requirements noted. General use plant air also should be identified and quantified.

What is your actual compressed air requirement? List each by type, identify alternatives to any potentially inappropriate uses, identify steady or intermittent demand and check number of shifts and resulting variations. Use equipment manufacturer's specifications for: Average flow rate requirement, maximum flow rate requirement, minimum pressure requirement (challenge the stated requirement), maximum pressure requirement (Is there a built-in pressure regulator?), and air quality requirement, (for example oil free, degree of dryness and filtration). Note the locations - groupings or scattered. Estimate potential future additions, estimate total air flow and best and worst case scenarios. (Include provision for leaks). Develop a Demand Profile Chart, shut off air to any application not in use. Determine the range of pressure requirements. Should some be segregated? Determine if oil free applications should be segregated or should all be oil free? Determine maintenance needs. Each end use should be compared with alternatives and justified.

Why are you using compressed air? Not all uses of compressed air are appropriate and in some cases BCAS may be able to suggest a better alternative.We prefer to be open and ensure that you are only using compressed air in the appropriate applications.

How to develop a Demand Profile Chart: Misapplication of compressed air at the end use is very common and results in the system operating inefficiently. A demand profile chart can assist in identifying applications that lead to poor system performance, excessive energy costs, and increasing maintenance expenses.Minimum, average and peak rates of flow, combined with on-off cycle times, are important criteria. The demand profile chart will provide the information for an overview of the system to determine the actual end-use requirements. For a new plant the chart is a compilation of the data required to select the total compressor capacity and the system operating pressure.

This data should be provided by the manufacturers of the end-use equipment.

For an existing plant, an analysis of data can assist in determining where inefficiencies exist or where there may be opportunities for improvement. For example, an end use with small air consumption can cause the entire system to operate at elevated pressure.

Determine if the application can be modified to operate at a lower pressure or if it should be segregated to allow the main system to operate at a lower pressure. For intermittent high volume use, additional storage may avoid the need to continually operate compressors inefficiently at partial load merely to meet an occasional peak requirement. End use equipment is often added or modified, so you should update the chart periodically so that it remains current.

With a systems approach the focus and effort must be balanced between the supply and demand sides. For peak efficiency, you must produce compressed air efficiently, but you must also conserve and use it efficiently.

Imagine your system as a child's see-saw and that you want to achieve a level balance; if you move the fulcrum or focus point too far to one side, it requires significantly more effort on that side to reach a balance. If you focus all your efforts on only one side (supply or demand), at first you may achieve reasonably good results, but it will become more and more difficult and will require expending more capital and human resources to keep improving. A smaller effort and investment on the other side may result in huge savings.