Compressed air driven devices are always given a specification for the compressed air flow at a certain pressure. For example, an EXAIR model 1101 Super Air Nozzle has a specified flow of 14 SCFM at 80 PSIG. This means that when this nozzle is operated at 80 PSIG, it will require 14 SCFM of compressed air flow. But what if the force from the nozzle is too high when operated at 80 PSIG and a lower operating pressure is needed?
Thankfully, we can calculate the compressed air flow at a different pressure using the absolute pressure ratio. The absolute pressure ratio says that for any given change in absolute operating pressure, there will be a proportional change in the air consumption of a device. So, what is an absolute pressure?
Put simply, an absolute pressure is the value which you would measure on pressure gauge plus the atmospheric pressure (PSIA, or Pounds per Square Inch Atmospheric). So, our 80 PSIG operating pressure mentioned above is an absolute pressure of 94.5 PSI (80PSIG + 14.5 PSIA). Similarly, if we wanted to determine the compressed air flow at an operating pressure of 60 PSIG, our absolute pressure would be 74.5 PSI (60 PSIG + 14.5 PSIA).
The absolute pressure ratio is a ratio of the new absolute operating pressure (new PSIG + PSIA) compared to the known absolute operating pressure (known PSIG + PSIA). For example, when comparing an operating pressure of 60 PSIG to an operating pressure of 80 PSIG, we will end up with the following ratio:
This means that our absolute pressure ratio in this case is 0.7884. To determine the compressed air flow for the model 1101 Super Air Nozzle at 60 PSIG, we will take this ratio value and multiply it by the known flow value at 80 PSIG. This will yield the following:
Utilizing this formula allows us to truly compare a compressed air powered device at different operating pressures. If we did not use the absolute pressures when comparing compressed air devices at differing pressures, our values would be erroneously low, which could yield to improper compressed air system planning and performance. And, using the absolute pressure ratio allows anyone to make a true comparison of compressed air device performance. If specifications are given at different pressures, performance data can be misleading. But, by using the absolute pressure ratio we can make a more exact evaluation of device operation.
If you have a question about your compressed air device and/or how a change in pressure will impact compressed air flow, contact our Application Engineers. We’ll be happy to help.
On Friday my colleague, Russ, blogged about the Super Air Amplifier (see that BLOG here, including a video demo) In discussing the Air Amplifiers, the topic of amplification was mentioned. Today, I’d like to expand a bit further the amplification aspect of the Air Amplifier performance.
As the name of the device implies, the compressed air used by the Air Amplifier is added to, and thus ‘amplified’, the total output flow of the unit. Depending on the size and type of Air Amplifier, the amplification ratio starts at 12:1 and goes up to 25:1, with the ratio being the output flow to the compressed air usage.
EXAIR offers (2) types- the Super Air Amplifier and the Adjustable Air Amplifier. The Super Air Amplifier uses a patented shim technology to maintain a precise gap, which controls the compressed air flow and expansion through the unit. As the expanded air flows along the Coanda profile, a low pressure area is created at the center which induces a high volume flow of surrounding air into the primary air-stream. The combined flow of primary and surrounding air exhausts from the Air Amplifier in a high volume, high velocity flow. The larger diameter units have a greater cross sectional area with larger low pressure areas, resulting in greater amplification ratios.
The Below table shows the amplification ratios.
The Adjustable Air Amplifier does not use a shim, but rather has an infinitely adjustable gap, allowing for fine adjustment of performance. Force and flow is changed by turning the exhaust end to adjust the gap, and is then locked into place. The method of the amplification is the same as for the Super Air Amplifier, and the amplification ratios are similar and shown below.
The Super Air Amplifiers and Adjustable Air Amplifiers are ideal for use in applications and processes that require cooling, drying and/or cleaning of parts, or the ventilation of confined areas or weld smoke or the exhausting of tank fumes.
If you have questions regarding the Air Amplifier, or would like to talk about any EXAIR Intelligent Compressed Air® Product, feel free to contact EXAIR and myself or one of our Application Engineers can help you determine the best solution.
Air compressors have come a long way over the years. When sizing a new system, a few terms are commonly used: CFM, SCFM, ACFM, and ICFM. The term CFM, simply put, stands for Cubic Feet per Minute. This term can often be confusing and impossible to define for just one condition. One definition will not satisfy the conditions that will be experienced in many of your applications due to a number of variables (altitude, temperature, pressure, etc.). Air by nature is a compressible fluid. The properties of this fluid are constantly changing due to the ambient conditions of the surrounding environment.
This makes it difficult to describe the volumetric flow rate of the compressed air. Imagine you have a cubic foot of air, at standard conditions (14.696 psia, 60°F, 0% Relative Humidity), right in front of you. Then, you take that same cubic foot, pressurize it to 100 psig and place it inside of a pipe. You still have one cubic foot, but it is taking up significantly less volume. You have probably heard the terms SCFM, ACFM, and ICFM when used to define the total capacity of a compressor system. Understanding these terms, and using them correctly, will allow you to properly size your system and understand your total compressed air consumption.
SCFM is used as a reference to the standard conditions for flow rate. This term is used to create an “apples to apples” comparison when discussing compressed air volume as the conditions will change. EXAIR publishes the consumption of all products in SCFM for this reason. You will always notice that an inlet pressure is specified as well. This allows us to say that, at standard conditions and at a given inlet pressure, the product will consume a given amount of compressed air. It would be nearly impossible, not to mention impractical, to publish the ACFM of any product due to the wide range of environmental conditions possible.
ACFM stands for Actual Cubic Feet per Minute. If the conditions in the environment are “standard”, then the ACFM and SCFM will be the same. In most cases, however, that is not the case. The formula for converting SCFM to ACFM is as follows:
ACFM = SCFM [Pstd / (Pact – Psat Φ)](Tact / Tstd)
ACFM = Actual Cubic Feet per Minute
SCFM = Standard Cubic Feet per Minute
Pstd = standard absolute air pressure (psia)
Pact = absolute pressure at the actual level (psia)
Psat = saturation pressure at the actual temperature (psi)
Φ = Actual relative humidity
Tact = Actual ambient air temperature (oR)
Tstd = Standard temperature (oR)
Let’s run through an example of a compressor operating at a “non-standard” condition:
In this example, the actual flow is greater. To determine the total ACFM consumption of any of our products with your system, take the published total consumption of the product and plug in the values for your compressed air system along with the standard variables.
The last term that you’ll see floating around to describe compressed air flow is ICFM (Inlet Cubic Feet per Minute). This term describes the conditions at the inlet of the compressor, in front of the filter, dryer, blower, etc. Because several definitions for Standard Air exist, some compressor manufacturers have adopted this simpler unit of measure when sizing a compressor system. This volume is used to determine the impeller design, nozzle diameter, and casing size for the most efficient compressor system to be used. Because the ICFM is measured before the air has passed through the filter and other components, you must account for a pressure drop.
The inlet pressure is determined by taking the barometric pressure and subtracting a reasonable loss for the inlet air filter and piping. According to the Compressed Air Handbook by the Compressed Air and Gas Institute, a typical value for filter and piping loss is 0.3 psig. The need to determine inlet pressure becomes especially critical when considering applications in high-altitudes. A change in altitude of more than a few hundred feet can greatly reduce the overall capacity of the compressor. Because of this pressure loss, it is important to assess the consumption of your compressor system in ACFM. To convert ICFM to ACFM use the following formula:
ICFM = ACFM (Pact / Pf) (Tf / Tact)
ICFM = Inlet Cubic Feet Per Minute
Pf = Pressure after filter or inlet equipment (psia)
Tf = Temperature after filter or inlet equipment (°R)
For this example, let’s say that we’re in Denver, Colorado. The barometric pressure, as of today, is 14.85 psi with current ambient temperature at 71°F. The compressor system in this example does not have any blower or device installed before the inlet, so there will be no temperature differential after filter or inlet equipment. The ICFM rating for the system is 1,000 ICFM.
ACFM = 1,000 (14.85/14.55)(530.67/530.67)
ACFM = 1,020
In order to maintain the 1,000 ICFM rating of the system, the ACFM is 1,020, about a 2% increase.
If you’re looking into a new project utilizing EXAIR equipment and need help determining how much compressed air you’ll need, give us a call. An Application Engineer will be able to assess the application, determine the overall consumption, and help recommend a suitably sized air compressor.
Sometimes we get calls or emails from our customers experiencing a problem with their application already using EXAIR products. These calls can range from difficulties associated with installation angle, installation, or, in many cases, the compressed air plumbing to the product itself.
That was the case in the application of the photos above. The end user had been using our model 6084 Line Vac to move plastic pellets from the floor to the top of a machine hopper, and they needed to increase the flow. The problem, was that they weren’t getting the performance from the 6084 that they thought they should.
Given the bulk density of the plastic pellets in this application, the end user should’ve been able to move more than enough material in the time they desired for the application needs. But, instead, the Line Vac was moving little-to-no material and even “stalling” – a condition in which the conveyed material could enter into the Line Vac and then cease to convey.
What we found, after exchanging contact information and discussing the photos above, is that the compressed air line feeding the Line Vac is too small, creating a pressure drop and leading to an inadequate compressed air flow. This, in turn, leads to lower air velocity at the exhaust of the Line Vac, which simultaneously means lower vacuum and material flow at the inlet.
The end result is the condition described by the end user – a low flow, or no-flow, of the material being conveyed.
After our discussion the end user set out to make the required change to the supply line, providing proper flow to the Line Vac at the proper pressure, and moving the material as required.
It’s always rewarding to help an end user solve their problems. If you have an application problem and think EXAIR might be able to help, contact an EXAIR Application Engineer.
Yes, ALMOST. This week, the EXAIR Blog has featured some excellent explanations of the science behind the operation of compressed air products. On Tuesday, John Ball posted the best explanation of SCFM vs ACFM that I’ve come across, and I’ve been explaining this to callers for almost four years now. I’m using his blog to perfect my “elevator pitch” on this topic. It will still likely require a building with more than ten floors, but I think that’s OK.
Also on “Two Blog Tuesday,” (this week only; I’m not trying to start anything) Dave Woerner’s gem of a blog detailed the terminology associated with pressure measurement, and why we use “psig” (g = gauged) – in a nutshell, the compressed air inside the pipe doesn’t care what the pressure outside the pipe is. And, since he mentioned it, I might add that most of agree that we care even less about how a certain NFL team’s footballs were (or were not) properly inflated.
I mention these earlier blogs to get to the point of MY blog today…a bit of theory-to-practice, if you will. Once you’ve gotten a decent understanding of these principles (or have the above links bookmarked for quick reference,) we can apply it to what’s needed for the proper operation of a compressed air product itself.
With a working knowledge of air flow (SCFM) and compressed air supply pressure (psig,) we can more easily understand why certain pipe sizes are specified for use with particular products. For instance, the longer the Super Air Knife and/or the longer the run of piping to it, the larger the pipe that’s needed to supply it:
The reasons for this are two-fold: First, the pipe…longer runs of pipe will experience more line loss (a continuous reduction in pressure, due to friction with the pipe wall…and itself) – so, larger diameter pipe is needed for longer lengths. For another practical demonstration, consider how much faster you can drink a beverage through a normal drinking straw than you can through a coffee stirrer. Not as dramatic as the leaf blower & volleyball (you really want to try it now, don’t you?) but you get my point.
Second, the Air Knife…the longer the Air Knife, the more air it’s going to use. And, if it’s longer than 18”, you’ll want to feed it with air at both ends…line loss will occur in the plenum as well.
In closing, I want to leave with another video, shot right here at EXAIR, showing the actual reductions in pressure due to line loss through different lengths, and diameters, of compressed air supply line to a Super Air Knife.
If you ever have any questions about compressed air use, or how EXAIR products can help you use your compressed air more efficiently, safely, and quietly, please give us a call.