Measurement Principles for Compressed Air System KPIs -Efficiency

It is a known fact that roughly 15% of the electrical energy absorbed by air compressors is converted into compressed air. The other 85% is lost as heat. Despite this low efficiency, compressed air is still a very popular utility in many industries. Compressed air system users are faced with challenges to get the best possible efficiency and to reduce the associated costs (energy, maintenance) as much as possible. Therefore, monitoring the right set of KPIs is nowadays a must-have for any energy manager or plant owner. In this article we clarify how the main KPIs can be monitored, and we will take a closer look at one of the main KPIs: efficiency. This article is part of the series where we dive into the most important KPIs in detail.

Key Performance Indicators (KPIs) are variables or measures by which the performance of organizations, machines and/or processes can be measured and analyzed in a targeted manner. For example, a KPI can be a certain production number per time unit, but also the amount of profit a company makes on a specific product.

A KPI dashboard is like a simplified cockpit for a pilot of an airplane. It provides the right information on time, on which you can rely when making decisions. For example: if overall system efficiency is off, you can take a closer look into the settings of your master controller. If consumption is significantly higher than normal or you see a pressure event, you need to look at your production lines or talk to operators in the plant. If dew point is off, fix/service the dryer or check if it is properly sized. For each issue there are many solutions and the KPIs will tell you soon if the implemented solution was right. Even more, KPIs can be used to calculate ROI on a future investment. In this way they will help to predict your future compressed air related energy costs.

For compressed air systems, some important?KPIs?are:

• Efficiency
• Dew Point
• Leakage rate
• Pressure loss
• Cost per product/part produced

Specific power (efficiency), leakage rate and pressure loss are a good starting point for any compressed air system, so this article will zoom in further on one of these KPIs, efficiency: what influences them, how you can measure them, and how to interpret them. We will continue discussing the other main KPIs in our upcoming articles.

KPI: Specific Compressor Power/Efficiency

The specific power of an air compressor is the ratio between compressor output (the amount of compressed air produced) and?power consumption?of the compressor. So, the KPI is calculated by input power divided by output flow, expressed in kW/100 cfm or kW/m3/min.

Specific power is important in the following situations:

• Asset management: when efficiency starts to deviate from initial situation, this can be seen as an early warning sign (e.g., need for a filter change on a centrifugal machine or internal fooling of heat exchangers).
• Air over the fence: In these contracts, efficiency is an important part of the deal. The compressor needs to deliver a pre-determined X amount of air for a Y amount of electric power.
• Compliance: Permanent monitoring is becoming increasingly important worldwide to achieve environmental and CO2 reduction targets. In this context governments also play a crucial role with legislation such as Title 24 in California and the Energy Efficiency Directive in Europe.
• Acceptance testing: when testing a compressor in a lab (ISO1217).
• Control system optimization: The efficiency a given set of compressors controlled by a master controller, should match industry benchmark average.

Total Compressor Station Efficiency vs. Individual Machines

When the budget for instrumentation is limited, you can use one flow meter to monitor all compressors. This involves measuring the power consumption per compressor and measuring the flow on the main pipeline, i.e., after the dryers and the buffer tank. The advantage of placing the flow meter on the main line is that the actual compressed air consumption of the plant is also monitored. This information can be used to determine leakage and to create a “fingerprint” of the air demand using a histogram function.

In this scenario, it is important to calculate the average consumption over sufficient time, and divide this by the average power consumption, as loading/ unloading will cause fluctuations in the KPI. Modern energy management platforms can refine this further, by splitting up “load” power consumption, “unload” power consumption and dividing this by the flow in a proper way. We recommend plotting efficiency as a function of air demand, to see if your system is running optimally.

To get a more granular insight needed to optimize compressor controls, flow should be measured per compressor. Pay special attention to flow meter selection and installation due to the harsh conditions in the discharge pipe of the compressor.

Total measurement uncertainty. Propagation of errors. Boring subject?

Total measurement uncertainty. Propagation of errors. For many of us these are subjects we leave to mathematicians. However, they are of extreme importance when looking at measurement data. Particularly when it comes to the large investment decisions for air compressors, this topic should not be underestimated, and great care must be taken when comparing efficiency numbers.

Therefore, when monitoring?efficiency, it is important to determine which accuracy level is acceptable. The required accuracy level depends on the application and on the size of the installation. The schematic below shows a (simplified) relational map between all parameters that are involved, just to get an idea about the complexity of efficiency measurement in volumetric (i.e., piston, screw, scroll) compressors. In the ISO1217 directive, one can read more on this topic and find other directives which tell you how to interpret results, confidence intervals, which type of flow meters must be used, and so on.

The total measurement uncertainty of any measurement is defined as the statistical variation of a measured quantity. For further reading on this matter, we refer to the “GUM” (Evaluation of measurement data – guide to the expression of uncertainty in measurement, 2008).

Systematic Errors and How They Propagate

Each measured signal has its own uncertainty. Now what is the effect of a systematic error on the measured efficiency? Let us look at a simple example. We want to check the efficiency of a 44 kW machine, running full load. We used a power meter (± 1% reading), and a flow meter (± 5% reading). What is the maximum error of the efficiency?

?As the table shows, taking systematic errors to the extremes, the observed efficiency can be 16.6, 17.6, or 18.7 kW/ 100 SCFM. This is +/- 6%. Now suppose the compressor manufacturer tested the 250 CFM compressor, the allowable deviation of efficiency is also 6% (See CAGI sheet). Worst case, you could have purchased a compressor which has a -6% deviation on output flow. When measuring this with an insertion probe (+/-5%) you should be careful not to jump to the conclusion that the compressor is wrong, or the flow meter is right and vice versa.

What is the effect of a wrong efficiency number on the perceived annual costs? In this example, at a cost of $0.06/kWh and 8760 annual running hours, the “money tolerance” is +/- 1400 USD annually for this compressor. This may have an impact on your decision making: Any (major) decision based on less than +/- 1400 USD energy savings could be based on systematic errors, so it might be a “wrong” decision. If this were an altitude meter of an airplane, and you were the pilot flying that plane… what would you do when the altitude meter is +/- 140 feet accurate? You probably want to stay at 280 feet minimum (2 x the error). And this also applies to making decisions for this compressor; if the projected costs due to poor efficiency will exceed 2800 USD/year it is time to do something.

How to Take Measurement Errors into Account

Most important is to be honest and transparent that there are always errors to be considered. Should you always invest in the most accurate measurement equipment? Well yes, if you need to verify the output according to ISO1217. It is even recommended to hire an external expert in such a case, as there are many more parameters to look at than just flow and power. When trending efficiency, a systematic measurement error is acceptable. You can see change of efficiency over time and monitor if a service job had the desired effect. In these cases, relative numbers are compared, not absolute numbers.

What always is important, is that the sensors are maintained properly and that they keep long term stability. The sensors should be stable for a time which exceeds the time with a factor of ten (rule of thumb) to be able to see the difference in efficiency caused by system degradation. So, when 5% degradation of your assets takes one year, the drift of the sensors should be less than 0.5% per year. Care should be taken when flow sensors are exposed to humid and dirty air, which may foul or clog the sensor. This can shorten the maintenance interval of the sensor.

Installation effects are also important to consider. Some technologies, like insertion probes and vortex meters, need specific pipe runs to be accurate. Other technologies can be prone to vibrations and pulsations in the flow. These things all combined can have a dramatic impact on the measurement uncertainty, which exceeds the stated accuracy of 5% and therefore, in the field, it is often challenging to meet the requirements of ISO1217. We have also seen cases where an orifice flow meter was wrongly programmed, resulting in a 30% error on the efficiency number. This was a simple human error making a dramatic impact on the relationship between manufacturer and end customer.

Watch out for our upcoming newsletter next month where we discuss the other important KPIs in detail. In the meanwhile, if you need advice on monitoring the right KPIs and compressed air monitoring systems, contact us here.