Serious air: How to get the right data when upgrading your plant's compressed air system

Plan your compressed air system for maximum efficiency and future flexibility.

By Ron Marshall, Certified Engineering Technologist

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Congrats! You’ve been given a project to upgrade your plant’s air system to properly supply a new plant expansion. Now the critical questions: How much compressed air are you using right now? What flow will your new system need? How do you size the system for maximum efficiency while providing enough flexibility for the future? You need to consider all of these when upgrading your compressed air system, but unfortunately, the answers often aren’t readily available or apparent. In addition, it’s prudent to plan for many circumstances instead of just one scenario. Here, we’ll explore some ways to do this and why you should.

Need for measurement

The first step in planning for the future is to develop a flow profile (Figure 1). But it’s tough to estimate the flow coming out of compressors just by looking at them or even by measuring the power consumption if you’re lucky enough to have submetering. Flow estimations depend on the compressor control mode, and the complexity of the control modes is enough to make your head spin.

Luckily, help is available. There are many compressed air companies offering system assessment services, also called a compressed air audit. Most major compressor suppliers have national audit teams, and there are often competent local personnel to perform basic measurements. These companies know enough about compressor control to be able to estimate accurately using compressor characteristics or to measure your flow profile with a flow meter.

When an assessment is done, temporary measurement power devices would be placed on all your running compressors to determine the electrical consumption; pressure loggers would measure critical system pressure points; and flow meters might also be installed to measure the air output. These meters would log system data for a significant period of time – long enough to establish a baseline reading of average, peak, and minimum levels. This baseline, say based on a few weeks of data, might then be used to project annual operating costs, assuming the same profile repeats itself over time. But the most important outcome would be the system flow profile.

Some of the most important parts of the flow profile:

  • Peak flows: This establishes the required flow capacity of your system. If the system can’t supply this flow, then low-pressure problems will occur.
  • Average flows: This is the flow level where the compressors will spend the most time. For an efficient system, the compressors must be able to supply this load without wasted power. To do this, the system must have efficient “turndown.”
  • Minimum flows: Minimum flows often show the level of system leakage. For shift-oriented plants with little production during nights and weekends, this is often a time where any running compressor is at its least efficient.
  • Lowest pressure: The plant will have a required minimum pressure than must be achieved during the peak flow or else production will be affected.
  • Highest pressure: The highest plant pressure often occurs during the lowest flows. Extreme variability in pressure is a sign of poor compressor control.

System sizing

Once the system peak is established, then if a plant expansion is planned, the new load must be added to the old. The new load profile will have a characteristic shape of its own and will depend on what machines or operations will be implemented in any new area, how much air they will use, and when and how often they will operate. There might be hundreds of compressed air loads in a new part of a plant operating almost at random. There are logical ways to calculate these loads if equipment flows are known ahead of time and production levels are accurately estimated, but even with the best of spreadsheets, someone usually needs to polish up his or her crystal ball and do some guesswork.

This new load profile will overlay on top of the existing profile to create a new overall system characteristic. Sometime the peaks of both profiles will add if they are coincident, causing a new significantly higher peak. Other times, there will be no coincident flow, and the peak will change very little. This can all be very confusing and risky when facing a project of significant value.

When it comes to sizing of compressed air systems, very few people have ever been fired for oversizing a system. But it’s awfully embarrassing, and sometimes career-ending, to purchase and install a system that is too small, especially for projects costing in the high six figures. To prevent undersizing, the designers usually apply safety factors to equipment sizing.

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