How to Measure Compressor Efficiency and FAD

How important is Compressor Efficiency?

The distribution of compressed air in industrial plants is prone to efficiency losses through pressure drops and leakage but the largest inefficiency is at the point of generation. Compressor units are most commonly run with electrical induction motors driving piston or screw compressors. Depending on the scale of the installation, the electrical consumption of these compressor plants can be a significant part of the overall plant electric bill as they are always responding to system demands to maintain pressure. The mechanical compression process is very inefficient with over 90% of the electrical energy lost as heat and noise. Cooling or heat reclamation systems can have an influence on the overall efficiency but it is still very costly to produce compressed air.

Compressors may be installed as individual units or more often, in groups with demand and standby control systems in place to respond to varying demand. This can be a centralised compressor plant or distributed compressors at key consumption points or a combination of both strategies. Either way, knowledge of the electrical consumption for all the compressors will give a clear idea of the scale of the operating costs just to create the air. More detailed analysis can follow from this to show the actual load cycles experienced by each compressor such that changing the control scheme could more effectively balance the loads across the compressor network. This would allow more effective use of the generating capability of the plant.

Monitoring the electrical efficiency in isolation will not give a true picture of the compressor efficiency without considering the pressure and flow at the outlet. Measuring the Free Air Delivery (FAD) in terms of flow and pressure allows direct comparisons between different compressors and the duty cycles in order to rationalise the overall efficiency and determine the accurate operating costs.

What about Downstream Efficiency?

Measuring air production at the compressor outlet can be difficult due to the very wet and possibly contaminated air present which can damage some sensors. Beyond that point, pressure losses start to build from piping, valves, filtration and drying plant and if measurements are taken downstream of the air preparation plant, they are not truly indicative of the efficiency of the compressor. If the compressor efficiency is measured directly at the outlet as FAD, additional measurements downstream of the preparation plant will not only give an accurate picture of the losses or efficiency of the plant but also the raw volume and pressure delivered to the distribution system.

Knowing both the generated and distributed air delivery figures for each compressor stage and plant section can highlight areas of high or low demand that can be used to balance the overall compressor load and reduce the operating costs.

Distribution losses through pressure drops and leakage further reduce the overall efficiency and optimisation of pressure reduction strategies combined with regular maintenance guided by leakage surveys can ensure that the compressed air system is as efficient as practically possible. Measuring leakage is much simpler when the generated and distributed figures are known as any demand when the plant is not in operation is purely replenishment of pressure lost through leakage. Measurement by section can isolate the largest leaks to target maintenance teams efforts.

Periodic audit or continuous monitoring?

While many plants use a degree of delivered air quality monitoring in accordance with ISO 8573 for dew point, oil and particles, extending the monitoring to include compressor and air preparation efficiency is straightforward.

Monitoring of air quality or compressor efficiency is often done through third party contractors on a periodic audit basis and this may be sufficient for a particular need but will always be limited by the length of time the survey gathered data from. For smaller users and plants where the air quality is not particularly critical, this may indeed be sufficient but may allow potential issues to be missed until the next audit.

Continuous monitoring of power consumption, FAD flow rate and pressure provides detailed data for longer term efficiency analysis than is possible over a short audit period. Some processes may not reach peak demands during an audit that can skew the subsequent analysis and affect any changes implemented based on the survey.

Air quality surveys have the same weakness and the potential dangers that poor compressed air can inflict on products and processes can be missed until it’s too late. Continuous monitoring of air quality can be achieved with modern sensors that can even be combined with the efficiency monitoring devices, such that a complete picture of the health and efficiency of the compressed air generation and distribution system can be evaluated for deviations from acceptable limits and managing operating costs.

Essential Tools for Efficiency & FAD Analysis

S110 power meter for measuring of three phase voltage, current, power and power factor to determine consumption in kWh

S551 portable data recorder and compressed air analyser. When used with S110 and S430 sensors, it will collate working data from the compressor for efficiency analysis

S430 pitot tube flow sensor for accurate measurement of wet air flow at the outlet of the compressor where thermal mass meters cannot be used

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