Industrial Utility Efficiency

Flow Metering Demand-Side Projects in Large Compressed Air Systems

As a reader of this journal, you are well aware that large compressed air systems often have significant wasted air — often from leaks — that represent tens of thousands of dollars of waste per year. However, it is our experience that the so-called “low-cost” measures identified often go un-repaired, while other more costly capital projects get funded. Why? With an ROI of a half year or less, they seem like IQ tests to many compressed air auditors.

The answer is that the low-cost items are really not low cost. They take the most expensive resource in the plant — key staff’s time. Even if staff is aligned, it can be poorly utilized. Staff might have no idea where to start, start in an area that has little opportunity, run out of time and drop the project, and have no idea what their results are.

Some audits (not ours) do not point out a prioritized implementation plan, just a brief description of the measures. They are made to seem “easy” when they really are not. Oversimplifying will not work, but neither will making it too complicated. As Oliver Wendell Holmes, Jr. wisely said:

“I would not give a fig for the simplicity this side of complexity, but I would give my life for the simplicity on the other side of complexity.”

How can a demand-side project be started and ended in a reasonable manner, while many other maintenance activities are happening? How can you get concrete feedback as to the results of that project? And how does management know if scarce resources (time) were spent on the right opportunities? How can you avoid having the project literally evaporate into thin air?

This article will show how flow metering, done properly and aligned with a practical project methodology, can help you focus on the right project, achieve results, and document savings. It will help you find the “simplicity on the other side of complexity.”



Before I launch into the article, let me define several key terms:

Supply Side: The equipment and controls in all running compressor rooms.

Demand Side: The distribution system and usages in a compressed air system.

Key Performance Indicator: Abbreviated KPI, this is a calculated value, based on real-time monitoring, that indicates the energy (or other) performance of a system.

Dead Load: This KPI is the uncontrolled airflow required to keep the plant systems pressurized during a non-production period. It includes leaks, but also includes 24/7 demands like dust-collector pulsing or boiler gas valve control air.

Live Load: This KPI is the airflow required to produce product. It can be cylinders, aerators, blow-offs, etc. It is controlled by automatic valves and varies with production.

System Efficiency Slope: This KPI is the ratio of power to flow in a compressed air system. In other words, how much compressor and dryer power drops as flow in the system drops. A “flat-lined” system (like one with a compressor in blow-off) has no power savings associated with demand-side projects.

Hot Tap: Installation of a coupling, ball valve and drilled hole in a pressurized line. This requires a specialized drill and contractor who is experienced in operating it. Most mechanical contractors can do this.


Recommended Project Methodology

In a nutshell, I recommend a “top-down” and “zone-based” process instead of a “bottom-up” one, with the right kind of metering. The recommended steps are as follows:

  1. Install metering and perform technical work.
  2. Plan first zone project.
  3. Implement first zone project.
  4. Measure project results and learn.
  5. Repeat in other zones.


1. Install Metering and Perform Technical Work

This is the part of the project that we recommend an outside specialist for, one who has significant auditing and metering experience. Since the purpose of this article is to show how flow metering facilitates the project, we will highlight the flow meter sections:

  • Meter Supply Side First

This can be done temporarily at first, with the ability of integrating into the plant’s SCADA system as the projects develop and budgets are justified. In some systems, this metering is already installed:

  • Current (or power) transmitters on all compressors and dryers.
  • Flow meter at the outlet of each compressor room. We currently recommend thermal mass flow meters like the VP FlowScope from VP Instruments (Figure 1, and Prime by Sage (Figure 2, They are relatively inexpensive, can be hot-tapped, and can work in any line size from 2 inches and up. Meter after the dryer, since they are only for dry air.

VP FlowScope from VP Instruments

Figure 1: The VP FlowScope from VP Instruments


Prime thermal mass flow meter by Sage

 Figure 2: The Prime thermal mass flow meter by Sage


  • A smart logging system that can be programmed to collect data and calculate KPIs. IT support is usually not available for exploratory projects, so a canned logging system is recommended. The logging interval needs to be set fast enough to capture system dynamics. We currently recommend VP Instruments VP Vision or Logic Beach IL80.
  • Develop a total flow, total power and system efficiency metric. See Figure 3 for an example.
  • The above instrumentation can be purchased and installed for about \$8,000 to \$15,000 for typical compressed air systems (200 to 800 hp). That is less than 10 percent of one year’s electric cost to operate the system — well worth it.


Typical Total Flow Profile

Figure 3: Typical Total Flow Profile
Click here to enlarge


  • Program Logger with KPIs
  • Either in the logger or a spreadsheet, develop a supply-side profile with several key performance indicators, particularly dead load, live load and system efficiency slope. KPIs should be visible to all. In the early phase of a project, this might have to be done via individual reports.
  •  NOTE: If the system slope is too “flat” (less than 14 kW per 100 scfm reduced), then the supply side needs to be addressed in a project before the demand side. That is outside the scope of this article.
  • Determine realistic magnitude of overall savings opportunity. Demand-side projects improve two KPIs:
  • Reduction in the dead load and associated kW (see Definitions).
  • Reduction of the live load and associated kW.


  • Find and Meter Zones of Potential Opportunity in Demand Side
  • Determine potential zones of opportunity via a walk-through.
  • Hot tap and temporarily flow meter the perimeter of all zones with potential opportunity. We recommend a thermal mass flow meter with built-in data logging. These don’t have to be done at the same time, so you only need a few flow meters. Ideally, log the flow for a couple days to see highs and lows, including a no-production period if possible (See Figure 4).


Typical 3-Zone System Diagram

 Figure 4: Typical 3-Zone System Diagram
Click here to enlarge


  • Hot taps cost about \$500 each when several are done at the same time. The cost for a portable flow metering system ranges from \$3,000 to \$7,000, which is less than 5 percent of one year’s electric cost of the system — well worth it.
  • Calculate the net consumption of each zone. Flow might be coming and going, so it is important to have the meters in the right direction. We know of only one bi-directional thermal mass flow meter on the market.
  • Prioritize zones by wasted air.
  • Identify “classes” of measures that are in each zone. For instance, leaky hoses or stuck open solenoid valves.


2. Plan First Zone Project

Once the flow metering has identified the areas with the most opportunity, we recommend starting with one zone as the first project. This might or might not be the zone with the most opportunity. Sometimes that one is the most complex. We have recommended that approach at one huge plant, and they installed more than 50 flow meters. The complexity of the projects that resulted were beyond their experience to implement, so they have delayed the start of the project. It might be best to start with a more modest project.

  • Establish Flow Reduction and Average kW Goal

This can be tricky. You might need to bring your technical resource back in for this. Just because the zone metering identified a large constant demand during a non-production period, and several large leaks are found, the dead load might be from something besides leaks. It might take some additional testing on a non-production period to determine what the source of dead load is. A recommended methodology is as follows:

  • During a non-production period, with flow meter(s) installed and logging, shut off sub-zones. The area that made the most difference is where to look first. It might be a leaky area, or it might be an air lance that is needed during production that was left on during non-production.
  • Determine flow change and hours per year opportunity. For the “non-interlocked” air bar, it might be a high flow for a small number of hours. For leaks, it might be a small flow for a large number of hours.
  • Estimate the total opportunity for the zone. Usually some percentage goal is used for leaks (usually around 50 percent of all leak load, or maybe all of found leaks), plus some goal for interlocked air demands.
  • Calculate average kW opportunity and \$/yr:
  • Savings = sum of each flow reduction x kW/scfm slope x hours x energy rate.
  • For instance, for 100 scfm leak reduction, 300 scfm off for 2 days per week, a 20 kW/100 scfm slope, and \$0.08/kWh rate:
  • Average kW savings goal = [(100 / 20) + (300 / 20) x 2/7] Average kW = 9.3 kW
  • Cost savings goal = 9.3 x 8760 x \$0.08 = \$6,507 / year
  • Establish One Project Manager/Planner for the Project

This person collects data and manages the project.

  • Identify a “Finder” and a “Fixer”

In some systems and plant organizational cultures, these two steps need to be done by separate departments. Aligning them closely will improve project efficiency and organizational learning. The perfect “Superman” would do everything from assessment to verification. In the real world, a plant needs to align available staff as well as possible. The project planning recommended above will integrate efforts done by separate parties. In some cases, these functions need to be a contracted out. The “finder” can be an air auditor, and the “fixer” can be a mechanical contractor.

  • Train “Finder” and “Fixer”

The Compressed Air Challenge has a great Fundamentals class. There are other resources. They also need to be trained on data collection and cost documentation.

  • Create One Work Order for the Zone

Without cost documentation, you won’t be able to justify the next project, so measure costs at the same time as energy reduction. The logistics need to be determined. Some areas can be only be shut down at specific times. Most demand-side projects require a shutdown period for detailed assessment and repair. Lock-out/tag-out, hot work, confined space, etc. plans are developed at this time.


3. Implement First Zone Project

If planning is done well and metrics are in place, this part can focus on the details, and the measurement will happen on its own.

  • Start with One “Class” of Measure

For instance, leaky hoses or stuck open solenoid valves. The finder/fixer would go through as many of those as possible to gain experience, refine methods, and see results. Then move on to the next class, like air bars.

  • If Finder and Fixer are Separate, Develop Detailed Action Items

This is where the “leak audit” can come in handy.

  • Combine Compressed Air Project with Other Projects in Same Area if Possible

It is hard to justify resources solely for reducing compressed air usage, and shutdown times are obviously used for mission-critical maintenance. So, combining air reduction efforts for the same time as other work might be needed. However, we recommend that you manage the compressed air part of the project separately.

  • Monitor Project Status and Conclude Project When Done

The project can be ended either when enough results have been obtained or when efforts are not resulting in progress.

  • Keep Work Order Open Until the “Zone” Project is Complete


4. Measure Project Results and Learn

The project metrics are simple — energy saved and costs incurred. The zone-based flow metering is the key item that shows what you really accomplished. The method above will make it easy to report results. Lessons learned from this first zone project need to be used in the next, and passed on if different staff are implementing it.


5. Repeat in Other Zones

Once the team has worked through one zone, it will be pretty clear what to do next. The numbers will be your guide, and the staff will take the logical next step, going after the next priority area. It will start to get simple. You will be on the simplicity side of complexity!


For more information, contact Tim Dugan, P.E., President, Compression Engineering Corporation, tel: (503) 520-0700.


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