Monitoring Compressed Air : Week 1/1/2012

Last article was a concluding article on Scale removal. In this article, we are going to discuss one more new idea, ‘The Free Air Delivery (FAD) measurement for compressors’. The FAD metering has gained importance today in terms of Energy Conservation and of course, in terms of increasing the profitability of an organization

Is Compressed Air costly?

The air we breathe is free. But the air we compress and distribute around any plant is not.

Did you know, about 10% of any plant's electricity costs are contributed to the generation of compressed air?

In some cases this number can be as high as 30%. Further, the efficiency of a compressed air system can be as low as 10-15%, meaning compressed air is one of the most expensive sources of energy in your plant. In fact, Compressed air is more expensive to provide than steam.

The average facility loses an average of 30% of its compressed air to leaks; much of which cannot be detected by ear, touch or sight. Many plants, yet, take little or no action to fix this problem

Most facilities do not know their cost for producing compressed air. Thus it is important to determine the cost of compressed air.

As a compressed air user, one needs to know two things:

(a) How much air are you actually getting from your compressor and,
(b) How much are you losing along the way as expensive leaks

In this context, it would be good to know a little more about FAD.

What is FAD?

FAD, in simple terms, stands for Free Air Delivery. But by scientific definition, it is the amount of Atmospheric Air (Free Air) that can be sucked in by the Compressor at inlet conditions (Suction Side) with:

  • Atmospheric Pressure at 1 atmosphere.
  • Atmospheric Temperature at 20°C/15°C.
  • Relative Humidity at 0% (100% Dry Air) &
  • The motor RPM at 100% of its rated value

Issues with FAD

The FAD, by definition, can be very misleading when it comes to actually knowing what your plant is getting at the delivery end of the compressor.  Your compressor's Free Air Delivery (FAD) rating only tells you how much air it sucks in, not how much air your process actually gets. During compression and cooling, a typical 1500 CFM (ft3/ min) compressor removes up to 100 CFM of water vapour, leaving only 1400 CFM for your process. That missing 100 CFM (Cubic Feet per Minute) could be enough to cripple your process and drastically affect your product quality or cause starvation in the circuit.

Is there any way to measure the amount of air

In real sense, the FAD (Free Air Delivery) would be the actual quantity of compressed air at the discharge of the compressor. The units for FAD are CFM in the imperial system and l/min in the SI system. The units are measured according the ambient inlet standard conditions ISO 1217 - 1 bar abs and 20 DegC. It is difficult to measure this FAD, as the conditions at which compressors operate will not be the ideal conditions given in the definition of FAD.

This would call for a metering method that will compare the actual conditions with the ideal conditions & give the compensated output in line with the standards.

It would be interesting to know that companies often waste a lot of money in the compressed air without knowing about it. It’s time we all should know it !

In our next article, we will discuss on how can we measure the accurate FAD at the discharge point of the compressor. With this knowledge, one can work out what he is getting from the compressors. He can then, work further on distribution of this compressed air & the possible losses in this distribution circuit.