There are so many uses for compressed air in industry that it would be difficult to list every one of them as the list would be exhaustive. Some of the uses are the tools used in production lines, assembly & robotic cells, painting, chemical processing, hospitals, construction, woodworking and aerospace.
It is considered as important as water, electricity, petroleum based fuels and often referred to as the fourth utility in industry. The great advantage of compressed air is the high ratio of power to weight or power to volume. In comparison to an electric motor compressed air powered equipment is smoother. Also compressed air powered equipment generally requires less maintenance, is more reliable and economical than electric motor powered tools. In addition they are considered on the whole as safer than electric powered devices.
Even amusement parks have used compressed air in some capacity in the operation of thrill rides like roller coasters or to enhance the “wow factor” of certain attractions. Compressed air can be found in your dentist’s office where it is used to operate drills and other equipment. You will find compressed air in the tires on your car, motorcycle and bicycles. Essentially, if you think about it, compressed air is being used nearly everywhere.
Here at EXAIR, we manufacture Intelligent Compressed Air Products to help improve the efficiency in a wide variety of industrial operations. Whether you are looking to coat a surface with an atomized mist of liquid, conserve compressed air use and energy, cool an electrical enclosure, convey parts or bulk material from one location to another or clean a conveyor belt or web, chances are we have a product that will fit your specific need.
If you would like to discuss quiet, efficient compressed air products, I would enjoy hearing from you…give me a call.
If you’re an active reader of the EXAIR blog, you’ve seen several posts over the last year about the various different types of air compressors. From the positive-displacement style of compressors (Rotary Scroll, Rotary Screw, Single and Double Acting Reciprocating Compressors,) as well as a review of a dynamic style (Centrifugal Compressors). In this blog, I’ll be discussing another of the positive-displacement variety: The Sliding-Vane Compressor.
In positive-displacement type compressors, a given quantity of air or gas is trapped in a compression chamber. The volume of this air is then mechanically reduced, causing an increase in pressure. A sliding-vane compressor will consist of a circular stator that is housed in a cylindrical rotor. The rotor then has radially positioned slots where the vanes reside. While the rotor turns on its axis, the vanes will slide out and contact the bore of the stator wall. This creates compression in these “cells”. An inlet port is positioned to allow the air flow into each cell, allowing the cells to reach their maximum volume before reaching the discharge port. After passing by the inlet port, the size of the cell is reduced as rotation continues and each vane is then pushed back into its original slot in the rotor. Compression will continue until the cell reaches the discharge port. The most common form of sliding-vane compressor is the lubricant injected variety. In these compressors, a lubricant is injected into the compression chamber to act as a lubricant between the vanes and the stator wall, remove the heat of compression, as well as to provide a seal. Lubricant injected sliding-vane compressors are generally sold in the range of 10-200 HP, with capacities ranging from 40-800 acfm.
Advantages of a lubricant injected sliding-vane compressor include:
Relatively low purchase cost
Vibration-free operation does not require special foundations
Routine maintenance includes lubricant and filter changes
Some of the disadvantages that come with this type of compressor:
Less efficient than the rotary screw type
Lubricant carryover into the delivered air will require proper maintenance of an oil-removal filtration system
Will require periodic lubricant changes
With the host of different options in compressor types available on the market, EXAIR recommends talking to a reputable air compressor dealer in your area to help determine the most suitable setup based on your requirements. Once your system is up and running, be sure to contact an EXAIR Application Engineer to make sure you’re using that compressed air efficiently and intelligently!
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.
For properly designed compressed air systems, air compressors will use primary storage tanks, or receivers. They are necessary to accommodate for fluctuations in airflow demand and to help prevent rapid cycling of the air compressor. (Reference: Advanced Management of Compressed Air – Storage and Capacitance) There are two types of primary receivers, a wet receiver tank and a dry receiver tank. The wet receiver is located between the air compressor and the compressed air dryer where humid air and water will be stored. The dry receiver is located after the compressed air dryer. In this blog, I will be reviewing the wet receivers and their requirements as a storage tank.
Air compressors discharge hot humid air created by the internal compression. A byproduct of this compression is water. By placing a wet receiver on the discharge side of the air compressor, this will create a low velocity area to allow the excess water to fall out. It will also give the hot air time to cool, allowing the compressed air dryers to be more effective. With wet receivers, it will reduce cycle rates of your air compressors for less wear and store compressed air to accommodate for flow fluctuations in your pneumatic system.
But, there are some disadvantages with a wet receiver. For compressed air dryers, it is possible to exceed the specified flow ratings. If the demand side draws a large volume of air from the supply side, the efficiency of the compressed air dryers will be sacrificed, allowing moisture to go downstream. Another issue with the wet receiver is the amount of water that the air compressor is pumping into it. As an example, a 60 HP air compressor can produce as much as 17 gallons of water per day. As you can see, it would not take long to fill a wet receiver. So, a condensate drain is required to get rid of the excess water.
Condensate drains come in different types and styles. They are connected to a port at the bottom of the wet receiver where the water will collect. I will cover the most common condensate drains and explain the pros and cons of each one.
Manual Drain – A ball valve or twist drain are the least efficient and the least expensive of all the condensate drains. The idea of having personnel draining the receiver tanks periodically is not the most reliable. In some cases, people will “crack” the valve open to continuously drain the tank. This is very inefficient and costly as compressed air is being wasted.
Timer Drain Valves – These valves have an electric timer on a solenoid to open and close a two-way valve or a ball valve. The issue comes in trying to set the correct time for the open and close intervals. During seasonal changes, the amount of water going into the wet receiver will change. If the timer is not set frequent enough, water can build up inside the receiver. If too frequent, then compressed air is wasted. Compared to the manual valve, they are more reliable and efficient; but there is still potential for compressed air waste.
No-waste Drains – Just like the name, these drains are the most efficient. They are designed with a float inside to open and close a drain vent. What is unique about the float mechanism is that the drain vent is always under water. So, when the no-waste drain is operating, no compressed air is being lost or wasted; only water is being drained. The most common problem comes with rust, sludge, and debris that can plug the drain vent.
All wet receivers require a condensate drain to remove liquid water. But, the importance for removing water without wasting compressed air is significant for saving money and compressed air. EXAIR also has a line of Intelligent Compressed Air® products that can reduce your compressed air waste and save you money. You can contact an Application Engineer for more details.
It is important to know the cost of compressed air at your facility. Most people think that compressed air is free, but it is most certainly not. Because of the expense, compressed air is considered to be a fourth utility in manufacturing plants. In this blog, I will show you how to calculate the cost to make compressed air. Then you can use this information to determine the need for Intelligent Compressed Air® products.
There are two types of air compressors, positive displacement and dynamic. The core construction for both is an electric motor that spins a shaft. Positive displacement types use the energy from the motor and the shaft to change the volume in an area, like a piston in a reciprocating compressor or like rotors in a rotary compressor. The dynamic types use the energy from the motor and the shaft to create a velocity energy with an impeller. (You can read more about air compressors HERE). For electric motors, the power is described either in kilowatts (KW) or horsepower (hp). As a unit of conversion, there are 0.746 KW in 1 hp. The electric companies charge at a rate of kilowatt-hour (KWh). So, we can determine the energy cost to spin the electric motors. If your air compressor has a unit of horsepower, or hp, you can use Equation 1:
hp * 0.746 * hours * rate / (motor efficiency)
hp – horsepower of motor
0.746 – conversion to KW
hours – running time
rate – cost for electricity, KWh
motor efficiency – average for an electric motor is 95%.
If the air compressor motor is rated in kilowatts, or KW, then the above equation can become a little simpler, as seen in Equation 2:
KW * hours * rate / (motor efficiency)
KW – Kilowatts of motor
hours – running time
rate – cost for electricity, KWh
motor efficiency – average for an electric motor is 95%.
As an example, a manufacturing plant operates 250 day a year with 8-hour shifts. The cycle time for the air compressor is roughly 50% on and off. To calculate the hours of running time, we have 250 days at 8 hours/day with a 50% duty cycle, or 250 * 8 * 0.50 = 1,000 hours of running per year. The air compressor that they have is a 100 hp rotary screw. The electrical rate for this facility is at $0.08/KWh. With these factors, the annual cost can be calculated by Equation 1:
In both equations, you can substitute your information to see what you actually pay to make compressed air each year at your facility.
The type of air compressor can help in the amount of compressed air that can be produced by the electric motor. Generally, the production rate can be expressed in different ways, but I like to use cubic feet per minute per horsepower, or CFM/hp.
The positive displacement types have different values depending on how efficient the design. For a single-acting piston type air compressor, the amount of air is between 3.1 to 3.3 CFM/hp. So, if you have a 10 hp single-acting piston, you can produce between 31 to 33 CFM of compressed air. For a 10 hp double-acting piston type, it can produce roughly 4.7 to 5.0 CFM/hp. As you can see, the double-acting air compressor can produce more compressed air at the same horsepower.
The rotary screws are roughly 3.4 to 4.1 CFM/hp. While the dynamic type of air compressor is roughly 3.7 – 4.7 CFM/hr. If you know the type of air compressor that you have, you can calculate the amount of compressed air that you can produce per horsepower. As an average, EXAIR uses 4 CFM/hp of air compressor when speaking with customers who would like to know the general output of their compressor.
With this information, we can estimate the total cost to make compressed air as shown in Equation 3:
C = 1000 * Rate * 0.746 / (PR * 60)
C – Cost of compressed air ($ per 1000 cubic feet)
1000 – Scalar
Rate – cost of electricity (KWh)
0.746 – conversion hp to KW
PR – Production Rate (CFM/hp)
60 – conversion from minutes to hour
So, if we look at the average of 4 CFM/hp and an average electrical rate of $0.08/KWh, we can use Equation 3 to determine the average cost to make 1000 cubic feet of air.
Once you have established a cost for compressed air, then you can determine which areas to start saving money. One of the worst culprits for inefficient air use is open pipe blow-offs. This would include cheap air guns, drilled holes in pipes, and tubes. These are very inefficient for compressed air and can cost you a lot of money. I will share a comparison to a 1/8” NPT pipe to an EXAIR Mini Super Air Nozzle. (Reference below). As you can see, by just adding the EXAIR nozzle to the end of the pipe, the company was able to save $1,872 per year. That is some real savings.
Making compressed air is expensive, so why would you not use it as efficiently as you can. With the equations above, you can calculate how much you are paying. You can use this information to make informed decisions and to find the “low hanging fruit” for cost savings. As in the example above, targeting the blow-off systems in a facility is a fast and easy way to save money. If you need any help to try and find a way to be more efficient with your compressed air system, please contact an Application Engineer at EXAIR. We will be happy to assist you.
On the whole most of us are quite aware of the considerable savings that can be accomplished by wise use and recovery of energy. One way that a plant can save substantially is to capture the energy that an electric motor adds to the compressed air from the air compressor. As much as 80% to 93% of the electrical energy used by an industrial air compressor is converted to heat. A properly designed heat recovery system can capture anywhere between 50% to 90% of this energy and convert it to useful energy.
The heat recovered is sufficient in most cases to use in supplemental ways such as heating water and space heating, however generally there is not enough energy to produce steam directly.
Packaged air cooled rotary screw compressor lend themselves easily to heat recovery, supplemental heating or other hot air uses very well due to their enclosed design. Since ambient air is directed across the compressors aftercooler and lubricant cooler where the heat can be easily collected from both the compressed air and the lubricant.
Packaged coolers are normally enclosed cabinets that feature integral heat exchangers and fans. This type of system only needs ducting and an additional fan to minimize back pressure on the air compressors cooling fan. This arrangement can be controlled with a simple thermostat operated vent on a hinge and when the extra heat is not required it can be ducted outside the facility.
The recovered energy can be used for space heating, industrial drying, preheating aspirated air for oil burners or other applications requiring warm air. Typically there is approximately 50,000 Btu/Hr of energy available from each 100 SCFM of capacity (at full load). The temperature differential is somewhere between 30°F – 40°F above the air inlet temperature and the recovery efficiency is commonly found to be 80% – 90%.
We all know the old saying there is “no free lunch” and that principle applies here. If the supply air is not from outside the plant a drop in the static pressure could occur in the compressor cabinet thereby reducing the efficiency of the compressor. If you choose to use outside air for makeup, you might need some return air to keep the air above freezing to avoid compressor damage.
Heat recovery is generally not utilized with water cooled compressors since an extra stage of heat exchange is required and the efficiency of recovering that heat is normally in the 50% – 60% range.
To calculate annual energy savings:
Energy Savings (Btu/Yr) = 0.80 * compressor bhp * 2,545 Btu/bhp-hour * hours of operation.
Rarely does the compressed air demand match the supply of the compressor system. To keep the generation costs down and the system efficiency as high as possible Compressor Controls are utilized to maximize the system performance, taking into account system dynamics and storage. I will touch on several methods briefly, and leave the reader to delve deeper into any type of interest.
Start/Stop – Most basic control – to turn the compressor motor on and off, in response to a pressure signal (for reciprocating and rotary type compressors)
Load/Unload – Keeps the motor turning continuously, but unloads the compressor when a pressure level is achieved. When the pressure drops to a set level, the compressor reloads (for reciprocating, rotary screw, and centrifugal type)
Modulating – Restricts the air coming into the compressor, as a way to reduce the compressor output to a specified minimum, at which point the compressor is unloaded (for lubricant-injected rotary screw and centrifugal)
Dual/Auto Dual – Dual Control has the ability to select between Start/Stop and Load /Unload control modes. Automatic Dual Control adds the feature of an over-run timer, so that the motor is stopped after a certain period of time without a demand.
Variable Displacement (Slide Valve, Spiral Valve or Turn Valve) – Allows for gradual reduction of the compressor displacement while keeping the inlet pressure constant (for rotary screw)
Variable Displacement (Step Control Valves or Poppet Valves) – Similar effect as above, but instead of a gradual reduction, the change is step like (for lubricant injected rotary types)
Variable Speed – Use of a variable frequency AC drive or by switched reluctance DC drive to vary the speed of the motor turning the compressor. The speed at which the motor turns effects the output of the system.
In summary – the primary functions of the Compressor Controls are to match supply to demand, save energy, and protect the compressor (from overheating, over-pressure situations, and excessive amperage draw.) Other functions include safety (protecting the plant and personnel), and provide diagnostic information, related to maintenance and operation warnings.
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.