Most people would not come out and say that compressed air is free. After all, nothing is 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 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. This article intends 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.
The basic principle of fluid power to apply/transfer load
In manufacturing facilities, involved in the fabrication of parts, a load is applied to cut, shape, deform, hold, join and so on. All this requires energy, whether it is mechanical, electrical, pneumatic (air), heat, kinetic, etc., 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 the 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 can 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 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 the 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 of, 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.
Now let us do the same exercise, except this time we assume that we have somehow converted the hydraulic bottle jack to function using compressed air (i.e. pneumatic) instead of hydraulic oil.
An exercise to demonstrate improper use of compressed air: Pneumatic jack
Consider the converted bottle jack to be hooked up to a 100 psig compressed air pressure line. That pressure (see schematic below) is now acting on what used to be area A2 in the previous illustration of the hydraulic bottle jack.
Let us now calculate the cross-sectional area A2 required to lift 3,000 lbs of load (F2). Since force is equal to pressure time area, therefore, to lift 3,000 lbs we shall require 100 psig (pounds per sq in gage pressure) acting on 30 sq in of area. Now compare this 30 sq in area value in the pneumatic application to say 3 sq in of area required in a hydraulic bottle jack to do the same work. The cylinder diameter for the pneumatic application calculates out to be 6.2 inches vs approximately 1 inch in diameter for the hydraulic jack. That explains why we don’t see many pneumatic bottle jacks. They would be too heavy, bulky and unwieldy. Using compressed air in this application is unwise.
Incredibly, pneumatic jacks do exist. The photographs below should illustrate the point made above about the size difference between a hydraulic and a pneumatic jack.
From the example above one can intuitively understand why using compressed air for performing work through the application of load (i.e. pressure) is wasteful. Pressure has (theoretically) a one to one transfer through a hydraulic fluid because it does not compress while the air gets compressed and does not have a 1-to-1 transfer of load. But how does that translate to cold hard cash?
Attempting to show how energy consumption is calculated from first principles was turning out to be too lengthy and complicated for this article. For that reason, I decided to use some empirical facts and data to provide some simple rules of thumb to aid in estimating the energy costs for load transfer using hydraulics as well as air (pneumatics).
This excerpt below, from Bud Trinkel’s comments, sums up, quite well, how expensive it can be to operate an air motor.
“….cost of an air circuit may be less than a hydraulic circuit but operating cost can be five to ten times higher. Compressing atmospheric air to a nominal working pressure requires a lot of horsepower.
….. It takes approximately one horsepower (hp) to compress 4 CFM (cubic feet per minute) of atmospheric air to 100 psi.
A 1-hp air motor can take up to 60 CFM to operate, so the 1-hp air motor requires (60/4) or 15 compressor horsepower when it runs. Fortunately, an air motor does not have to run continuously but can be cycled as often as needed.”
Therefore, when using compressed air in an air motor or diaphragm pump, it takes about 10 to 15 units of electrical energy at the compressor (compressor hp) to produce about one unit (motor or pump hp) of actual mechanical output to the work.
So what does it cost, in dollars and cents, to generate one cubic foot (CF) of compressed air at a given pressure? Let’s illustrate how to calculate using an actual example.
We go back to the thought experiment illustrating the hypothetical pneumatic bottle jack. The size of the cylinder required for the pneumatic jack had to be six times larger to transfer air pressure at standard shop air (between 80 – 120 psi) to lift only 3,000 lbs.
Imagine how large a cylinder would have to be to move, say, 30,000 pounds (approx. 14 MT). For example, if we were, say, trying to punch steel sheets by simply using compressed air. A 62-inch diameter cylinder (theoretical without accounting for any losses), to be precise, going by the parameters used in the thought experiment.
We will use this example of a 62-inch diameter cylinder only to demonstrate how the calculations work. (In actuality there are other methods to intensify air pressure and one would not use a 62-inch diameter cylinder in a pneumatic press of 15 MT capacity). However, the energy consumption would be conceptually similar but calculated using different methods. Using this simple cylinder equivalent will help you get into the ballpark of energy costs for that kind of tonnage.
We are going to first calculate the air consumption of a 62″ bore cylinder with a 1-inch stroke under full load, operating 30 complete cycles (out and back) per minute at 100 psi inlet pressure. The calculations follow the method described in The Mead Pneumatic Handbook.
Step 1: Calculate the area of the piston by converting the bore diameter into square inches. (62 in. bore/2)2 x 3.1416 (π) = 3,019 sq. in.
Step 2: Determine air consumption per single stroke. (Volume of a cylinder) 3,019 sq. in. x 1 in. stroke = 3,019 cu. in.
Step 3: Determine consumption per complete cycle – to (Load application stroke) and fro (return stroke) (disregard the piston rod that will take up some of the volume because it is generally not significant). 3,019 cu. in. x 2 = 6,038 cu. in. of air at 100 psi, per cycle.
Step 4: Determine volume of 100 psi air that is consumed per minute. 6,038 cu. in. x 30 cycles/minute = 181,140cu. in./min. of 100 psi air
Step 5: Convert cu. in. to cu. ft.: 105 cu ft/min or CFM of 100 psi air
Step 6: Calculate compression ratio, which is how many times free air has to be compressed so as to get it to 100 psi. (This equation was discussed earlier) (100 psi + 14.7)/14.7 = 7.8
Step 7: Now determine cubic feet of free air used per minute (SCFM). 105 cu. ft. x 7.8 compression ratio = 819 cu. ft. of free air used per minute
Now we go on to calculate energy costs to run this pneumatic press of approximately 14 MT capacity. In other words, we will calculate how much energy is expended to compress 819 cu ft of air per minute.
We learned earlier, that it takes 1 hp to compress 4 SCFM of free air. So 819 SCFM would require 205 hp. Also, let’s assume that this pneumatic press runs for 2,000 hours per year.
Annual Electricity Costs = (horsepower required) x (0.746 kW/hp) x (Annual Hours of Operation) x (Electricity Cost in $/kWh)
= 205 x 0.746 x 2000 x 0.10
= $30,586 annually or $2,548 per month for a pneumatic press.
This cost can be compared to approximately $200 per month to operate a hydraulic press of the same capacity!!! (Reference)
I hope that this illustrates how much money is wasted in using compressed air for load applications.
Energy costs and sheer impracticality make compressed air an “unconscionable” choice for any applications that require the application of loads! Especially when there are less expensive alternatives.
Smarter Alternatives to Compressed Air
Having said all this about improper use of pneumatics for load applications, what then is the better alternative? Well, it depends….
In this article, I have been referring to hydraulic systems when comparing the ineffectiveness of compressed air systems. I don’t intend to make it appear that I am recommending hydraulic systems as a default alternative to pneumatic systems. Hydraulic systems are not without their inefficiencies and energy waste. Certainly not as much as pneumatic systems, yet enough to be a factor of concern for a lean and green environment.
John A. Neun, explained the inefficiencies of hydraulic systems quite clearly in his online article.
“The worst offenders of low efficiency are hydraulic systems using fixed-displacement pumps sized to meet peak flow and pressure demands. Constantly running at full speed, pumps in these circuits continuously produce maximum flows at maximum system pressure. But except for a few moments in an operating cycle, most machines only require a fraction of a system’s total pressure and flow capabilities. During standby, positioning, pick-and-place, or holding operations, the system’s pump consumes maximum energy while the hydraulic system is performing little or no work. As a result, hydraulic system operating efficiency suffers dramatically.”
Apart from hydraulic presses, mechanical and servo presses are also among some of the better-known alternatives. There is an informative chart (reproduced below) that summarizes the difference in costs between mechanical, hydraulic and servo systems.
As you can see, these three systems are not uniformly better or worse than the other in the different aspects that were measured.
Note that the data in the chart above were normalized to the values of a servo press in each of the categories being measured. So everything is relative to a servo press. Also, the graph does not give actual values which makes it less useful to someone seeking to justify one system or another by doing a payback analysis or ROI. Another aspect is one of omission of data, namely, the capital cost for the presses. That would have probably shown the servo press as the more expensive option. Regardless, this schematic is useful for making some broad comparative choices in choosing one system over another.
There is another source of useful data comparing servo presses to hydraulic presses. This data is quantitative and has been replotted for easier viewing.
(Caution when viewing the above data: Each of the two charts shown above has been provided by companies that are selling Servo presses. Hence they are not from unbiased sources, yet considered useful enough to make a point, to be included here.)
There is another type of system that combines the benefits of air (speed) and hydraulics (power) to yield a more energy-efficient system (compared to pneumatic or hydraulic systems) for load applications up to 60 or 100 MT. These are known as the hydropneumatic or air-over-oil systems.
For example, an air-over-hydraulic bottle jack can lift up to 20 MT. This jack is operated using compressed air which acts on a hydraulic system (very similar to what was shown in the hydraulic bottle jack example). The compressed air is not used to do the mechanical work. The compressed air works on the hydraulic fluid (through pressure intensification as was discussed above) and the hydraulic fluid, at the intensified pressure, is what does the ultimate work of load transmission.
Air over hydraulic systems are by far more energy-efficient than pneumatic and even hydraulic systems and depending on the application, much less expensive than servo systems.