On any given day myself and my Application Engineering Brethren here at EXAIR have discussions with customers on air starvation of any given EXAIR Product. The calls generally start off the same, “The Line Vac is not performing like it should”. We at EXAIR absolutely want to help you get the most out of our products and we certainly want them to perform to your expectation. However they must be supplied with clean/dry compressed air at sufficient pressure and volume.
Just the other day I was discussing a performance issue with a customer on a 1″ Line Vac. The customer thought he needed a larger Line Vac. I asked the questions regarding the diameter of his Supply Line and if he was using Quick Connect or Push Lock connectors. He was attempting to feed this Line Vac with 1/4″ Poly Tubing through a elbow Push to Loc fitting.
This 1″ Line Vac was being severely starved for air and therefore not performing as expected. The 1″ Line Vac require’s 14.7 SCFM @ 80PSI to reach the rated performance of 42″ of water column.
Below is a table for Pipe/Hose sizing from the Line Vac installation manual that you can use as a reference guide. It is recommended that if using hose for the supply air to go up to the next size over the pipe recommendation.
Don’t forget that quick connects and Push Lock fittings are not recommended and could restrict the air flow which will have a negative impact on performance.
Flow rate is the quantity of material that is moved per unit of time. Generally, the quantity of material can be expressed as a mass or a volume. For example, mass flow rates are in units of pounds per minute or kilograms per hour. Volumetric flow rates are stated in cubic feet per minute or liters per hour. The trick begins when volumetric flow rates are used for a compressible gas. In this blog, I will go over the various acronyms and the reasons behind them.
What acronyms will be covered?
CFM – Cubic Feet per Minute
SCFM – Standard Cubic Feet per Minute
ACFM – Actual Cubic Feet per Minute
ICFM – Inlet Cubic Feet per Minute
The volumetric component of the flow rate is CFM or Cubic Feet per Minute. This term is commonly used for rating air compressors. From history of air compressors, they could calculate the volume of air being drawn into the air compressor by the size of cylinder. With the volume of the compression chamber and the rotations per minute of the motor, RPM, they could calculate the volumetric air flows. As conditions change like altitude, temperature, and relative humidity, the value of CFM changes. To better clarify these conditions, compressor manufacturers decided to add terms with definition. (For your information, air compressors still use CFM as a unit of air flow, but now this is defined at standard temperature and pressure).
The first letter in front of CFM above now defines the conditions in which the volumetric air flow is being measured. This is important for comparing pneumatic components or for properly sizing pneumatic systems. Volume is measured with three areas: temperature, pressure, and relative humidity. We can see this in the Ideal Gas Law: P * V = n * R * T or Equation 1:
V = n * R * T / P
V – Volume
n – Number of molecules of gas
R – Universal Gas Constant
T – Absolute Temperature
P – Absolute Pressure
The volume of air can change in reference to pressure, temperature, and the number of molecules. Where is the relative humidity? This would be referenced in the “n” term. The more water vapor, or higher RH value, the less molecules of air is in a given volume.
SCFM is the most commonly used term, and it can be the most confusing. The idea of this volumetric air flow is to set a reference point for comparisons. So, no matter the pressure, temperature, or relative humidity, the volumetric air flows can be compared to each other at that reference point. There have been many debates about an appropriate standard temperature and pressure, or STP. But as long as you use the same reference point, then you can still compare the results. In this blog, I will be using the Compressed Air and Gas Institute, CAGI, reference where the “Standard” condition is at 14.5 PSIA, 68 deg. F, and 0% RH. Since we have a reference point, we still need to know the actual conditions for comparison. It is like having a location of a restaurant as a reference, but if you do not know your current location, you cannot reach it. Similarly, we are “moving” the air from its actual condition to a reference or “Standard” condition. We will need to know where the air began in order to reach that reference point. We will talk more about this later in this blog.
ACFM is the volumetric air flow under actual conditions. This is actually the “true” flow rate. Even though this term is hardly used, there are reasons why we will need to know this value. We can size an air compressor that is not at “Standard” conditions, and we can use this value to calculate velocity and pressure drop in a system. We can correlate between SCFM and ACFM with Equation 2:
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)
ICFM is one of the newest terms in the history of air compressors. This is where devices are added to the inlet of an air compressor, affecting the flow conditions. If you have a blower on the inlet of an air compressor, the volumetric flow rate changes as the pressure and temperature rises at the “Inlet”. If a filter is used, then the pressure drop will decrease the incoming pressure at the “Inlet”. These devices that affect the volumetric flow rate for an air compressor should be considered. The equation to relate the ACFM to ICFM is with Equation 3:
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)
Examples of these different types of flow rates can be found here in this EXAIR blog by Tyler Daniel.
To expand on my explanation above about SCFM and ACFM, a technical question comes up about the pressure when using SCFM. The reference point of 14.5 PSIA is in the definition of SCFM. Remember, this is only a reference point. The starting location is actually required. This would be the ACFM value where the air values are true and actual. As an example, two air nozzles are rated for 60 SCFM. An EXAIR Super Air Nozzle, model 1106, is cataloged at 80 PSIG, and a competitor is cataloged at 60 PSIG. By comparison, they look like they use the same amount of compressed air, but actually they do not. To simplify Equation 2, we can compare the two nozzles at the same temperature and RH at 68 Deg. F and 0% RH respectively. This equation can be reduced to Equation 4:
Even though the SCFM is the same amount, you are actually using 21% more air with the competitive nozzle that was reported at 60 PSIG. So, when it comes to rating compressed air products or air compressors, always ask the conditions of pressure, temperature and RH. The more you know about volumetric flow rates, the better decision that you can make. If you need help, you can always contact our application engineers at EXAIR.
Everyday here at EXAIR we talk about pressure, specifically compressed air pressure. The other day I was looking up our model 9011, 1/4″ NPT Pressure Gauge , and it got me to wondering just how does this small piece of industrial equipment work. The best way to find out is to tear it apart.
Most mechanical gauges utilize a Bourdon-tube. The Bourdon-tube was invented in 1849 by a French watchmaker, Eugéne Bourdon. The movable end of the Bourdon-tube is connected via a pivot pin/link to the lever. The lever is an extension of the sector gear, and movement of the lever results in rotation of the sector gear. The sector gear meshes with a spur gear (not visible) on the indicator needle axle which passes through the gauge face and holds the indicator needle. Lastly, there is a small hair spring in place to put tension on the gear system to eliminate gear lash and hysteresis.
When the pressure inside the Bourdon-tube increases, the Bourdon-tube will straighten. The amount of straightening that occurs is proportional to the pressure inside the tube. As the tube straightens, the movement engages the link, lever and gear system that results in the indicator needle sweeping across the gauge.
The video below shows the application of air pressure to the Bourdon-tube and how it straightens, resulting in movement of the link/lever system, and rotation of the sector gear – resulting in the needle movement.
If you need a pressure gauge or any of the EXAIR Intelligent Compressed Air® Products, feel free to contact EXAIR and myself or one of our Application Engineers can help you determine the best solution.
Over the last few months, my EXAIR colleagues and I have blogged about several different types of air compressor types including single and double acting reciprocating,rotary screw and sliding vane air compressors. You can click on the links above to check those out. Today, I will review the basics of the rotary scroll-type compressor.
The rotary scroll type compressor falls under the positive displacement-type, the same as the other types previously discussed. A positive displacement type operates under the premise that a given quantity of air is taken in, trapped in a compression chamber and the physical space of the chamber is mechanically reduced. When a given amount of air occupies a smaller volume, the pressure of the air increases.
Each of the previous positive displacement type compressors use a different mechanism for the reduction in size of the compression chamber. The rotary scroll uses two inter-meshing scrolls, that are spiral in shape. One of the scrolls is fixed, and does not move (in red). The other scroll (in black) has an “orbit” type of motion, relative to the fixed scroll. In the below simulation, air would be drawn in from the left, and as it flows clockwise through the scroll, the area is reduced until the air is discharged at a high pressure at the center.
It is of note that the flow from start to finish is continuous, providing air delivery that is steady in pressure and flow, with little or no pulsation.
There is no metal to metal sliding contact, so lubrication is not needed. A drawback to an oil free operation is that oil lubrication tends to reduce the heat of compression and without it, the efficiency of scroll compressors is less than that of lubricated types.
The advantages of the rotary scroll type compressor include:
Comes as a complete package
Comparatively efficient operation
Can be lubricant-free
The main disadvantage:
A limited range of capacities is available, with low output flows
EXAIR recommends consulting with a reputable air compressor dealer in your area, to fully review all of the parameters associated with the selection and installation of a compressed air system.
If you would like to talk about compressed air or any of the EXAIR Intelligent Compressed Air® Products, feel free to contact EXAIR and myself or one of our Application Engineers can help you determine the best solution.
What is Air? Air is an invisible gas that supports life on earth. Dry air is made from a mixture of 78% Nitrogen, 21% Oxygen, and 1% of remaining gases like carbon dioxide and other inert gases. Ambient air contains an average of 1% water vapor, and it has a density of 0.0749 Lbs./cubic foot (1.22 Kg/cubic meter) at standard conditions. Air that surrounds us does not have a smell, color, or taste, but it is considered a fluid as it follows the rules of fluid dynamics. But unlike liquids, gases like air are compressible. Once we discovered the potential of compressing the surrounding air, we were able to advance many technologies.
Guess when the earliest air compressor was used? Believe it or not, it was when we started to breathe air. Our diaphragms are like compressors. It pulls and pushes the air in and out of our lungs. We can generate up to 1.2 PSI (80 mbar) of air pressure. During the iron age, hotter fires were required for smelting. Around 1500 B.C., a new type of air compressor was created, called a bellows. You probably seen them hanging by the fireplaces. It is a hand-held device with a flexible bag that you squeeze together to compress the air. The high stream of air was able to get higher temperature fires to melt metals.
Then we started to move into the industrial era. Air compressors were used in mining industries to move air into deep caverns and shafts. Then as the manufacturing technologies advanced, the requirements for higher air pressures were needed. The stored energy created by compressing the air allowed us to develop better pneumatic systems for manufacturing, automation, and construction. I do not know what the future holds in compressed air systems, but I am excited to find out.
Since air is a gas, it will follow the basic rules of the ideal gas law;
PV = nRT (Equation 1)
P – Pressure
V – Volume
n – Amount of gas in moles
R – Universal Gas Constant
T – Temperature
If we express the equation in an isothermal process (same temperature), we can see how the volume and pressure are related. The equation for two different states of a gas can be written as follows:
P1 * V1 = P2 * V2 (Equation 2)
P1 – Pressure at initial state 1
V1 – Volume at initial state 1
P2 – Pressure at changed state 2
V2 – Volume at changed state 2
If we solve for P2, we have:
P2 = (P1 * V1)/V2 (Equation 3)
In looking at Equation 3, if the volume, V2, gets smaller, the pressure, P2, gets higher. This is the idea behind how air compressors work. They decrease the volume inside a chamber to increase the pressure of the air. Most industrial compressors will compress the air to about 125 PSI (8.5 bar). A PSI is a pound of force over a square inch. For metric pressure, a bar is a kg of force over a square centimeter. So, at 125 PSI, there will be 125 pounds of force over a 1” X 1” square. This amount of potential energy is very useful to do work for pneumatic equipment. To simplify the system, the air gets compressed, stored as energy, released as work and is ready to be used again in the cycle.
Compressed air is a clean utility that is used in many different applications. It is much safer than electrical or hydraulic systems. Since air is all around us, it is an abundant commodity for air compressors to use. But because of the compressibility factor of air, much energy is required to create enough pressure in a typical system. It takes roughly 1 horsepower (746 watts) of power to compress 4 cubic feet of air (113L) to 125 PSI (8.5 bar) every minute. With almost every manufacturing plant in the world utilizing compressed air in one form or another, the amount of energy used to compress air is extraordinary. So, utilizing compressed air as efficiently as possible is mandatory. Air is free, but making compressed air is expensive
If you have questions about getting the most from your compressed air system, or would like to talk about any EXAIR Intelligent Compressed Air® Products, you can contact an Application Engineer at EXAIR.
An important part of operating and maintaining a compressed air system is taking accurate pressure measurements at various points in the compressed air distribution system, and establishing a baseline and monitoring with data logging. A Pressure Profile is a useful tool to understand and analyze the compressed air system and how it is functioning.
The profile is generated by taking pressure measurements at the various key locations in the system. The graph begins with the compressor and its range of operating pressures, and continues through the system down to the regulated points of use, such as Air Knives or Safety Air Guns. It is important to take the measurements simultaneously to get the most accurate data, and typically, the most valuable data is collected during peak usage periods.
By reviewing the Pressure Profile, the areas of greatest drop can be determined and the impact on any potential low pressure issues at the point of use. As the above example shows, to get a reliable 75 PSIG supply pressure for a device or tool, 105-115 PSIG must be generated, (30-40 PSIG above the required point of use pressure.) As a rule of thumb, for every 10 PSIG of compressed air generation increase the energy costs increase 5-7.5%
By developing a total understanding of the compressed air system, including the use of tools such as the Pressure Profile, steps to best maximize the performance while reducing costs can be performed.
If you have questions about getting the most from your compressed air system, 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.
Have you ever counted the amount of carbs that you eat? People typically do this to lose weight, to become healthier, or for medical reasons like diabetes. Personally, I like to eat cereal in the morning. I will pull a box of cereal down from the cupboard and look at the Total Carbs field. One morning, I looked at a box of gluten-free rice flakes and compared it to a peanut butter nugget cereal. I noticed that the carbs were very similar. The rice cereal had 23 grams of total carbs while the peanut butter nuggets had only 22 grams of total carbs. Then I looked at the serving size. The rice cereal had a serving size of 1 cup while the nuggets only had a serving size of ¾ cups. So, in comparison, for one cup of nugget cereal, the total amount of carbs was 27.5 grams. Initially, I thought that they were similar, but the peanut butter nugget was actually 20% higher in carbs. This same “misdirection” occurs in your compressed air system.
Here is what I mean. Some manufacturers like to use a lower pressure to rate their products. This lower pressure makes it seem like their products will use less compressed air in your system. But, like with the serving sizes, it can be deceiving. It is not a lie that they are telling, but it is a bit of misconception. To do an actual comparisons, we have to compare the flow rates at the same pressure (like comparing the carbohydrates at the same serving size). For example, MfgA likes to rate their nozzles at a pressure of 72.5 PSIG. EXAIR rates their nozzles at 80 PSIG as this is the most common pressure for point-of-use equipment. You can see where I am going with this.
To compare nozzles of the same size, MfgA nozzle has a flow rate of 34 SCFM at 72.5 PSIG, and EXAIR model 1104 Super Air Nozzle has a rating of 35 SCFM at 80 psig. From an initial observation, it looks like MfgA has a lower flow rating. To do the correct comparison, we have to adjust the flow rate to the same pressure. This is done by multiplying the flow of MfgA nozzle by the ratio of absolute pressures. (Absolute pressure is gage pressure plus 14.7 PSI). The ratio of absolute pressures is: (80PSIG + 14.7) / (72.5PSIG + 14.7) = 1.09. Therefore; the flow rate at 80 PSIG for MfgA nozzle is now 34 SCFM * 1.09 = 37 SCFM. Now we can compare the flow rates for each compressed air nozzle. Like adjusting the serving size to 1 cup of cereal, the MfgA will use 9% more compressed air in your system than the EXAIR model 1104 Super Air Nozzle. This may not seem like much, but over time it will add up. And, there is no need to waste additional compressed air.
The EXAIR Super Air Nozzles are designed to entrain more ambient air than compressed air needed. This will save you on your pneumatic system, which in turn will save you money. The other design features gives the EXAIR Super Air Nozzle more force, less noise, and still meet the OSHA compliance.
If you want to run a healthier compressed air system, it is important to evaluate the amount of compressed air that you are using. To do this correctly, you always want to compare the information at the same pressure. By using the EXAIR Super Air Nozzles in your compressed air system, you will only have to worry about your own weight, not your pneumatic system.