My colleague, Eric Kuhnash, wrote a blog “About Rotary Screw Air Compressors”, and I wanted to expand that conversation to a close cousin; Rotary Scroll type Air Compressors. As you see in the chart below, this type of compressor falls within the same family as positive displacement compressors.
Positive displacement air compressors raise air pressure by reducing the volume of air within a confined space. The scroll compressors use two intermeshing 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. The Rotary Scroll type of air compressors is less common in the rotary family, as they are limited in capacity.
What they lose in capacity, they make up for in simplicity. They are compact and can fit into small areas. They require very little maintenance; and the majority of them are oil-free. They were initially used in refrigeration systems because they were compact, inexpensive, and required little maintenance. Since they are quiet and oil-free, they work great in doctor’s offices and medical fields.
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.
What is an air compressor? This may seem like a simple question, but it is the heartbeat for most industries. So, let’s dive into the requirements, myths, and types of air compressors that are commonly used. Like the name states, air compressors are designed to compress air. Unlike liquid, air is compressible which means that it can be “squished” into a smaller volume by pressure. With this stored energy, it can do work for your pneumatic system.
There are two types of air compressors, positive displacement and dynamic. The core component for most air compressors is an electric motor that spins a shaft. Positive displacement uses the energy from the motor and the shaft to change volume in an area, like a piston in a reciprocating air compressor or like rotors in a rotary air 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 types of air compressors HERE).
Compressed air is a clean utility that is used in many different ways, and it is much safer than electrical or hydraulic systems. But most people think that compressed air is free, and it is most certainly not. Because of the expense, compressed air is considered to be a fourth utility in manufacturing plants. For an electrical motor to reduce a volume of air by compressing it. It takes roughly 1 horsepower (746 watts) of power to compress 4 cubic feet (113L) of air every minute to 125 PSI (8.5 bar). With almost every manufacturing plant in the world utilizing air compressors much larger than 1 horsepower, the amount of energy needed to compress air is extraordinary.
Let’s determine the energy cost to operate an air compressor to make compressed air by Equation 1:
motor efficiency – average for an electric motor is 95%.
As an example, a manufacturing plant operates a 100 HP air compressor in their facility. The cycle time for the air compressor is roughly 60%. To calculate the hours of running time per year, I used 250 days/year at 16 hours/day for shifts. So operating hours equal 250 * 16 * 0.60 = 2,400 hours per year. The electrical rate at this facility is $0.10/KWh. With these factors, the annual cost to operate the air compressor can be calculated by Equation 1:
Cost = 100hp * 0.746 KW/hp * 2,400hr * $0.10/KWh / 0.95 = $18,846 per year in just electrical costs.
So, what is an air compressor? The answer is an expensive system to compress air to operate pneumatic systems. So, efficiency in using compressed air is very important. EXAIR has been manufacturing Intelligent Compressed Air Products since 1983. If you need alternative ways to save money when you are using your air compressor, an Application Engineer at EXAIR will be happy to help you.
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.
Recently, EXAIR Application Engineers have written blogs about reciprocating type air compressors: Single Acting (by Lee Evans) and Dual Acting (by John Ball.) Today, I would like to introduce you, dear EXAIR blog reader, to another type: the Rotary Screw Air Compressor.
Like a reciprocating compressor, a rotary screw design uses a motor to turn a drive shaft. Where the reciprocating models use cams to move pistons back & forth to draw in air, compress it, and push it out under pressure, a rotary screw compressor’s drive shaft turns a screw (that looks an awful lot like a great big drill bit) whose threads are intermeshed with another counter-rotating screw. It draws air in at one end of the screw, and as it is forced through the decreasing spaces formed by the meshing threads, it’s compressed until it exits into the compressed air system.
Rotary Screw Air Compressor…how it works.
So…what are the pros & cons of rotary screw compressors?
Pros:
*Efficiency. With no “down-stroke,” all the energy of the shaft rotation is used to compress air.
*Quiet operation. Obviously, a simple shaft rotating makes a lot less noise than pistons going up & down inside cylinders.
*Higher volume, lower energy cost. Again, with no “down-stroke,” the moving parts are always compressing air instead of spending half their time returning to the position where they’re ready to compress more air
*Suitable for continuous operation. The process of compression is one smooth, continuous motion.
*Availability of most efficient control of output via a variable frequency drive motor.
*They operate on the exact same principle as a supercharger on a high performance sports car (not a “pro” strictly speaking from an operation sense, but pretty cool nonetheless.)
Cons:
*Purchase cost. They tend to run a little more expensive than a similarly rated reciprocating compressor. Or more than a little, depending on options that can lower operating costs. Actually, this is only a “con” if you ignore the fact that, if you shop right, you do indeed get what you pay for.
*Not ideal for intermittent loads. Stopping & starting a rotary screw compressor might be about the worst thing you can do to it. Except for slacking on maintenance. And speaking of which:
*Degree of maintenance. Most maintenance on a reciprocating compressor is fairly straightforward (think “put the new part in the same way the old one came out.”) Working on a rotary screw compressor often involves reassembly & alignment of internal parts to precision tolerances…something better suited to the professionals, and they don’t work cheap.
Like anything else, there are important factors to take under consideration when deciding which type of air compressor is most suitable for your needs. At EXAIR, we always recommend consulting a reputable air compressor dealer in your area, helping them fully understand your needs, and selecting the one that fits your operation and budget.
Russ Bowman
Application Engineer
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