“Free” Compressed air? Really? – Part 1
(This is Part 1 of a 2-part article on the issues involved with using compressed air for load applications. Link to Part 2 of the article.)
Most people would not come out and say that compressed air is free. After all, nothing is really free. Besides that, this topic of compressed air not being free has been discussed at length in trade journals and publications. Douglas Waetjen’s article is but one example. So, who in their right mind would confess to having the notion that compressed air is free?
Yet, even today, one can witness wastage and misapplication of compressed air when walking through a manufacturing facility. Using compressed air without considering energy costs or weighing those costs against the benefits achieved from its use is more commonplace than one would imagine. Closet compressed-air-is-free beliefs and mindsets are alive and well! Not considering other lower cost, more energy efficient solutions is indeed a travesty.
Compressed air is not free! Moreover, using compressed air for load applications is, well, unconscionable!
Unconscionable? Travesty? These are strong words. I feel that they are justified when it comes to some applications of compressed air in manufacturing environments. Let me explain with data and facts. (This article has been written so that a reader with little or no technical knowledge can follow along and comprehend the logic. As a result, this may prove to be a bit too basic for those with more technical and engineering knowledge. )
The graphic below is based on data from the “Compressed Air Reference Guide”, published by Hydro One. It shows the energy costs involved in operating air compressors of various capacities in single/double and three-shift operations. Other references such as the one by the Compressed Air Challenge organization also provide good technical references on computing energy costs of compressed air.
Compressed air systems in a typical manufacturing environment are prone to be mismanaged, inadequately maintained, and, as a result, create a significant amount of wasted energy. Wasted energy has an associated cost. Hence, cash is literally being thrown away because of the misplaced perception that compressed air is free. The Hydro One publication referenced above goes on to report that 10 – 20% of operating energy costs of air compressors can be saved simply by following some good practices. There are studies in abundance that provide data and resources on how to minimize this waste.
This concept that compressed air has an associated cost or that there are significant savings opportunities in the elimination of the inherent waste in compressed air systems has been well covered in articles like the ones referenced above. This article will not reiterate those facts. The intent of this article is to focus on the gross misapplication of compressed air!
One of the most common misapplications is the use of compressed air for applying load. For example, air pressure is used in clamping, pressing, punching, forming, and a host of other applications. In the majority of these cases, a more cost effective and energy efficient solution exists. Not considering these alternatives is essentially throwing away money, and, more importantly detrimental to the environment.
A recent article in Production Machining reports, that by replacing hydraulic clamping or gripping systems with electrical systems, “in the course of a year and based on three shifts of operation, a shop could potentially save as much as 13,000 kWh……. on one machine tool”. For air clamping systems, the savings is significantly much greater! When taken over several machines, the energy savings can be staggering.
Basic principle of fluid power to apply/transfer load
In manufacturing facilities, involved in the fabrication of parts, a load is applied in order to cut, shape, deform, hold, join and so on. All this requires energy, whether it is mechanical, electrical, pneumatic (air), heat, kinetic, etc., in order to do the work of fabrication.
One popular method of generating the energy to perform the work of fabrication is to apply load on a contained fluid, air, oil, etc. (Fluid power). The applied load on the fluid creates pressure inside the fluid container. This pressure is then intensified (using Pascal’s Law) by several orders of magnitude to the levels required to do the appropriate work.
Let’s look at an everyday example of a hydraulic bottle jack to help illustrate this pressure intensification. A person can use simple muscle power to lift a car using a bottle jack. The pumping action on the handle of the jack pressurizes the oil in a small cylinder having a cross-sectional diameter (A1) to get pressurized to pressure P1 (as shown in schematic below). The pressurized oil acts on a much larger cross-sectional area (A2 in the schematic below). According to Pascal’s Law, for a confined incompressible fluid, Pressure P1 = Pressure P2. The Pressure is intensified by the number of times the Area (A2) is greater than Area A1. In the example shown in the schematic, the intensification is by a factor of 25. That is how a 120 lb person is able to lift one end (approx. half the car weight) of a car which may weigh 3,000 lbs.
Image adapted from Siyavula.
Compressibility and energy consumption
The crux of Pascal’s law for pressure transfer, without loss, depends on the fluid being incompressible. Oil is incompressible. Air is not. That is the fundamental reason as to why compressed air is so much more expensive compared to hydraulic systems, which as will be explained later, are not the most energy efficient either.
Let’s look at compressibility a bit more closely. Assume that oil completely fills a 1 cu ft vessel. If one were to try to inject more oil into that vessel (after the lid was closed, of course) it would not be possible to get any more oil into that vessel. That is because oil is incompressible and does not compress under pressure. The pressure applied in the efforts to inject additional oil into the closed can would simply transfer to the oil inside the vessel. If you apply more pressure in the hopes of “squeezing” more oil into the can, then the pressure in the can will increase and match the applied pressure. Ultimately the pressure increase would reach the strength limits of the material of the can and burst. The ability of oil to transfer pressure without being compressed is at the heart of the functioning of hydraulic oil machines.
Now consider that same 1 cu ft vessel filled with air. With the lid of the vessel open, there would be 1 cu ft of air at 1 atmosphere pressure (or 0 gage pressure, psig). Next we close the lid and try to introduce more air into that vessel. We find that we can actually squeeze more air into the vessel and the pressure also increases at the same time.
A rule of thumb, to increase the pressure inside a 1 cu-ft vessel by 10 psig, one will have to inject 1.68 cu-ft of free air (i.e. air existing under atmospheric conditions). Therefore, to achieve a pressure of 100 psi inside that vessel, we will need to inject 16.8 cu-ft of free air. So 16.8 cu-ft of free air is compressed to generate 1 cu-ft volume of pressurized air inside the vessel. Air is compressible!
For those wishing to do this calculation more often, the formula for calculating the volume of free air (VS) necessary to increase the gage pressure to a pressure, say P, inside a vessel of a certain volume (VC) is given by:
VS = VC x (P + 14.7) / 14.7
VS = Volume of air (free) at standard atmospheric conditions,
VC = Volume of compressed air,
VC = Volume of compressed air,
P = Gage pressure reading of the volume of compressed air.
Using the example above, we can see how the above equation works:
VS = 1 x (10 + 14.7) / 14.7 = 1.68 cu-ft of free air.
The Hydro One publication referenced earlier represents it in another way. It is worth sharing here. It states that “Powered by electricity, a typical air compressor takes approximately 7 volumes of air at atmospheric conditions and squeezes it into 1 volume at elevated pressure (about 100 psig, [7 bar]). The resulting high-pressure air is distributed to equipment or tools where it releases useful energy to the operating tool or equipment as it is expanded back to atmospheric pressure.”
This is represented in the schematic below. Note the heat of compression (in other words energy) is being lost into the surrounding air. That is wasted energy, i.e. money!
At this point, the reader should have an intuitive feel for why compressed air is more expensive than, say, hydraulic oil.
In Part 2 of this article, I illustrate the actual energy costs resulting from the compressibility of air. The difference in energy consumption can be staggering. Examples of misapplication of compressed air will also be discussed with illustrations and simple thought exercises.
About the author:
Rahul Sarkar is a registered professional engineer with 30+ years of experience in manufacturing environments. Through Phoenix Manufacturing Systems, Rahul works on his passion in improving profitability in manufacturing operations with the applications of common sense problem solving, as well as Lean Manufacturing and Quality Systems principles.
(Feel free to email me with any questions at firstname.lastname@example.org. I shall be delighted to help in whatever way I can.)