Industrial Utility Efficiency    

Integration: Should Compressed Air Monitoring be Combined with Control?

In the first article in this two-part series, I discussed the people who would be internal customers of a compressed air control system and of a monitoring system. They are very different people. Maintenance is the customer of controls and energy engineering is the customer of monitoring. And I discussed potential problems that can occur when combining monitoring and control in the same system. In this article, I will get more specific about building practical systems that address both controls and monitoring.

Since people and infrastructure are the scarce resources, this article is organized around the following scenarios:

  1. Plant has an EMCS or SCADA system and a person to look at it.
  2. Plant has no EMCS or SCADA, system, but has a vendor with an available auditor.

I will not address OEM-specific approaches, or emerging cloud-based data-loggers. A future article will address these, as some new products are coming on the market at this time and haven’t been proven yet.  I will discuss monitoring done in a “vendor agnostic” way by a third party.


Plant With EMCS or SCADA System and Person to Look at It

Let’s take a look at an example project at a cement plant. This is a large plant with multiple compressor rooms and different brands of air compressors, supported marginally by different air compressor dealers. This adds complexity to the control and monitoring issues. And it shows how a controls integrator attempted to do monitoring.

A specialty compressed air integrator had done a good job managing control of six air compressors in two compressor rooms as shown in Figure 1. The integrator also embedded some monitoring functionality into the controller. 

Figure 1

Figure 1. A simplified diagram of a compressed air system with integrated control and monitoring. Click here to enlarge.

The control system, installed in 2012, was based on an Allen Bradley Control Logix PLC, a robust, powerful, industry-standard controller, with a touchscreen display. The PLC received analog inputs from sensors including motor current and pressure; calculated real-time values like power, flow and efficiency (calculated from Amps and speed); and it displayed the operator interface function, not trend-logging. The plant’s HMI software performed monitoring, pulling data from the PLC. In general, it was built like a typical large custom integration project with compressed air expertise. However, there was a fuzzy division between monitoring and control. 

Long-term operation of a system like this is difficult with changes in staff and type of air compressors. In 2018, the plant decided to replace four of the older air compressors with two new ones from another brand altogether. Fortunately, they did call the integrator back to reprogram and re-commission the system to operate the new units optimally. At that time, the integrator had to change the monitoring calculated values in the PLC.  Since the monitoring function was separate from the control panel, and owned by the customer, that data was still available. 

Unfortunately, monitoring data being sent to the HMI system was not accurate. Power was not calculated properly on the new Variable Frequency Drive (VFD) air compressor.  Additionally, the time stamp on the data was incorrect. It appeared to be off by a factor of about 60, with minutes being recorded as hours. Personally, I would have preferred the analog inputs going directly to the plant SCADA system with transparent calculations, if any. The plant did not trust the power and flow values coming from the controller/monitoring PLC either.

The strengths of the monitoring/control integration were as follows:

  1. Inputs were selected and installed properly. The right things were being measured.
  2. Inputs were protected by fuses so they couldn’t cause problems with the controller if they failed.
  3. There was no local database so the controller did not end up as an “island” of unused data.

The weaknesses in the monitoring system were as follows:

  1. Values were being calculated by a PLC. Those formulas were unknown to the user and to the energy engineer, thus not trusted.
  2. The customer couldn’t do it without outside expertise.
  3. Air compressor data was not available for two of the air compressors. There was a network interface problem between the German-made air compressors and the master PLC.

Recommendations for improvement are as follows:

  1. Separate the monitoring system from the control PLC. Wire current transmitters (CTs) and other monitoring-only transmitters to the plant’s analog input modules, and poll the data directly from the HMI. A redundant pressure transmitter needs to be installed since one is used for control.
  2. If the plant’s analog input modules are not available in the compressor room, patch the raw values directly through to the HMI, and do the scaling and calculations on the plant HMI.
  3. Train the plant engineer/technician responsible for HMI programming on compressed air.
  4. With the assistance of the air compressor dealer, another dashboard can be created for maintenance, that show maintenance KPIs like motor current, oil temperature and air outlet temperature. If the air compressor Modbus interface is already wired to the controller, tag the appropriate monitoring values in the PLC, and provide them to the plant HMI. Program some basic analytics into the HMI. It should know if the KPIs have drifted, and alarm at predetermined variances.


Plant With no EMCS or SCADA System, But Has Vendor Support and an Auditor

An example for this scenario is a mid-sized aerospace component manufacturer. They completely upgraded their system, installing new air compressors, storage, filtration, dryer, and master controls. Fortunately, an expert auditor designed the system so it had the correct configuration and controls for efficient and reliable operation, and the best available technology for controls and monitoring that was appropriate for the site as shown in Figure 2.

Figure 2

Figure 2: A diagram of a compressed air system with separate control and monitoring. Click here to enlarge.


The local support was good for the equipment and engineering services, but the customer did not have a large enough facility management staff to handle the data-collection side, nor did they have the staff to analyze the data. The local equipment supplier was able to provide a basic master controller, and after much wrangling, got it working. But they were definitely not capable of developing a monitoring system for the customer. 

The approach taken on this project was synergistic, matching the best available, appropriate technology to the plant, considering their infrastructure and staff. The elements of the project were as follows:

  1. This followed the design principles discussed in a previous article, which can be viewed at
  2. Wet-side storage.
  3. Mist eliminator.
  4. Regenerative (blower purge) dryer, tuned for energy management.
  5. Master controller, Allen Bradley PLC-based, target sequencer algorithm.
  6. Monitoring system:  flow, current, pressure and dewpoint trending.

The monitoring system, which was designed for long-term monitoring, became essential for commissioning the various aspects of the system. Since it was installed before the new equipment was started up, data was immediately available. Temporary data loggers did not have to be installed. This system commissioned and validated the following:

  1. Dryer dewpoint demand controls working properly, after cool-down purge turned off and demand controls turned on (The vendor fought this.)
  2. Air compressor unloading happening properly. Amps were initially too high when unloaded.
  3. Sequencer proper operation.

The monitoring system is simple enough so the staff can be trained to see if it is out of tune, and can contact the outside resources to help at that time. At the current time, the customer has not yet run the Ethernet from the compressor room to the plant Ethernet hub. After that is done, they merely have to hit the fixed IP address of the monitoring system on a web browser to see all the monitoring data and KPIs. They will also be able to download data.

Recommendations for improvement are as follows:

  1. Complete Ethernet connection.
  2. Train operators and engineering on use of data collection system.
  3. When future upgrades become available, connect the monitoring system data to a cloud-based server via a cellular modem.
  4. Provide the compressed air data to the air compressor dealer and auditor, via the cloud database. They can provide analytics and service remotely.

When plants don’t have an EMCS or SCADA system, some prefer the simplicity of having the controller and monitoring system integrated. This is typically a European approach, and requires a custom controller, not an open-architecture PLC.

In cases where the customer prefers this approach, you need to use a system with significant experience integrating monitoring and control. We are aware of several integrated control/monitoring systems on the market. Figure 3 is an example of a common system.

Figure 3

Figure 3

Neither of the ones we are aware of have cloud-based monitoring. They are local controller/monitors with the ability to communicate to the OEM for data collection.  However, the suppliers support them well remotely so these are a valid option for control plus monitoring.

Recommendations for improvement are as follows:

  1. If a plant has an EMCS or SCADA system and a person to look at it a master controller without embedded monitoring is recommended, with the plant EMCS or SCADA system doing the monitoring. Get technical support from a qualified compressed air specialist to set up monitoring properly.
  2. If a plant has no EMCS or SCADA system, but has a vendor with an available auditor, use a simple, separate monitoring system. If a standard PLC is not required an integrated monitoring/control system can be a good idea, as long as the firm supplying it has extensive experience.
  3. In either case, use suppliers who can integrate remote monitoring into their product, and get that data in front of the right person.


For more information, contact Tim Dugan, President of Compression Engineering Corporation, tel: (503) 520-0700, email:, or visit

To read the first part of this article, Reliability: Should Compressed Air Monitoring be Combined with Control?, click here.

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