There are two main types of compressors, positive displacement and Dynamic.
Positive displacement air compressors raise air pressure by reducing the volume of air within a confined space. The scroll compressors use two inter-meshing scrolls, where one scroll is moving, and the other scroll is stationary (reference photo below). Ambient air will get trapped at the inlet side, and as the orbiting scroll moves, the spiral volume gets smaller and smaller. When volume decreases, the pressure will increase. Rotary Scroll air compressors are less common in the rotary family, as they are limited in capacity.
The dynamic type raises the air pressure by using kinetic energy and velocity with rotating impellers that continuously bring in airflow. In this blog, I will cover the centrifugal type of the dynamic branch.
As mentioned, the centrifugal compressor works by transforming kinetic energy and velocity into pressure. Ambient air passes through guide vanes into the center of a rotating Impeller with radial blades and is then pushed outward by a centrifugal force.
With the increase in pressure, you will get an increase in heat. It is a natural occurrence with air compressors. Heat from the centrifugal compressor is dissipated by heat exchangers before moving onto the next stage. Multiple stages are required to raise the pressure to a sufficient level for typical industrial plant requirements. The most common centrifugal air compressors have two to four stages to generate pressures up to 100 to 150 PSIG. Centrifugal compressors are near the middle of the road regarding efficiency. Their typical operating cost is 16 to 20 kW/100 CFM.
Advantages:
Up to 1500 HP systems are available
Price per horsepower drops as system size increases
Supplies lubricant-free air
Special installation pads are not required for installation
Disadvantages:
Costs more Initially
Requires specialized maintenance
No matter the type of air compressor that you use, they are very costly to operate. To help you use them efficiently and safely, EXAIR offers a range of products that can clean, cool, blow, conserve, and convey. This would include our Super Air Knives, Super Air Nozzles, Safety Air Guns, Cabinet Coolers, and much more. If you want to save energy, increase safety, and cut costs no matter what size air compressor you have, contact an Application Engineer at EXAIR. We will be happy to help you make your use of compressed air as efficient and safe as possible.
There are two main ways to compress air for supplying pneumatic systems; Positive Displacement and Dynamic. Positive Replacement reduces the volume of air within a confined space to generate pressure. The dynamic type raises the air pressure by using kinetic energy and velocity with rotating impellers that continuously brings in airflow. In this blog, I will cover the centrifugal type of the dynamic branch.
As mentioned, the centrifugal compressor works by transforming kinetic energy and velocity into pressure. Ambient air passes through guide vanes into the center of a rotating Impeller with radial blades and is then pushed outward by a centrifugal force. This radial velocity of air results in an increase in pressure due to kinetic energy. Let’s look at the equation for kinetic energy in Equation 1:
Equation 1:
K = ½ * m * V2
K – Kinetic Energy (J)
m – mass (Kg)
V – velocity (m/s)
As you can see, the energy increases with the square of the velocity. How do we increase the velocity? Let’s look at Equation 2:
Equation 2:
V = w * r
V – linear velocity (m/s)
w – angular velocity (rad/sec)
r – radius (m)
As you can see, as the air travels along the impeller towards the outside, the radius increases. Since the rotations per second are constant, the velocity will increase. In combination with Equation 1, you can see how the energy will increase, thus increasing the pressure.
With the increase in pressure, you will get an increase in heat. It is a natural occurrence with air compressors. Heat from the centrifugal compressor is dissipated with heat exchangers before moving onto the next stage. Multiple stages are required to raise the pressure to a sufficient level for typical industrial plant requirements. The most common centrifugal air compressors have two to four stages to generate pressures up to 100 to 150 PSIG. Centrifugal compressors are near the middle of the road regarding efficiency. Their typical operating cost is 16 to 20 kW/100 CFM.
Advantages:
Up to 1500 HP systems are available
Price per Horsepower drops as system size increases
Supplies lubricant-free air
Special installation pads are not required for installation
Disadvantages:
Costs more Initially
Requires specialized maintenance
No matter the type of air compressor that you use, they are very costly to operate. To help you use them efficiently and safely, EXAIR offers a range of products that can clean, cool, blow, conserve, and convey. This would include our Super Air Knives, Super Air Nozzles, Safety Air Guns, Cabinet Coolers, and much more. If you want to save energy, increase safety, and cut costs no matter what size air compressor you have; you can contact an Application Engineer at EXAIR. We will be happy to help you.
Centrifugal air compressors are one example of dynamic style air compressors. The dynamic type of compressors have a continuous flow of air that has its velocity increased in an impeller that is rotating at a higher speed. The kinetic energy of the air is increased due to the increase in velocity and then becomes transformed into pressure energy through the use of a volute chamber, or a diffuser. The volute chamber is a curved funnel that increases in surface are as it approaches the discharge port. This converts the kinetic energy into pressure by allowing the velocity to reduce while the pressure increases. Approximately 1/2 of the energy is developed in the impeller and the other half is developed in the volute chamber or diffuser.
1 – Basic Centrifugal Air Compressor
The most common centrifugal air comppressor has between two and four stages in order to generate pressures up to 150 psig. A water cooled inter-cooler and separator is placed between each stage in order to remove condensation and cool the air down prior to being passed on to the next stage. These compressors still have advantages and some disadvantages. The list below showcases just a few.
Advantages:
Lubricant-free air is generated
Complete packages up to 1,500 hp
Initial costs decrease with increase in compressor size
No special foundations or reinforcements needed
Disadvantages:
Specialized maintenance requirements
Higher initial investment
Unloading/waste of air required to drop system pressures
To determine which type of compressor may be best suited for your facility, we suggest to locate and contact a compressor sales company in your geographic area. When it comes to determining the volume of air required to operate the EXAIR products and even some other point of use compressed air applications, EXAIR’s Application Engineers can help you determine the volume you will need to ensure the compressor is sized appropriately. If you would like to discuss any other point of use application, please contact us.
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:
Equation 1:
hp * 0.746 * hours * rate / (motor efficiency)
where:
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:
Equation 2:
KW * hours * rate / (motor efficiency)
where:
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 $0.08/KWh. With these factors, the annual cost can be calculated by Equation 1:
In both equations, you can substitute your information to see what you actually pay to make compressed air each year at your facility.
The type of air compressor can help in the amount of compressed air that can be produced by the electric motor. Generally, the production rate can be expressed in different ways, but I like to use cubic feet per minute per horsepower, or CFM/hp.
The positive displacement types have different values depending on how efficient the design. For a single-acting piston type air compressor, the amount of air is between 3.1 to 3.3 CFM/hp. So, if you have a 10 hp single-acting piston, you can produce between 31 to 33 CFM of compressed air. For a 10 hp double-acting piston type, it can produce roughly 4.7 to 5.0 CFM/hp. As you can see, the double-acting air compressor can produce more compressed air at the same horsepower.
The rotary screws are roughly 3.4 to 4.1 CFM/hp. While the dynamic type of air compressor is roughly 3.7 – 4.7 CFM/hr. If you know the type of air compressor that you have, you can calculate the amount of compressed air that you can produce per horsepower. As an average, EXAIR uses 4 CFM/hp of air compressor when speaking with customers who would like to know the general output of their compressor.
With this information, we can estimate the total cost to make compressed air as shown in Equation 3:
Equation 3:
C = 1000 * Rate * 0.746 / (PR * 60)
where:
C – Cost of compressed air ($ 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.
Compressed Air 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.
