Evaluating all of the different types of compressors and which is right for you can seem like a daunting task. Today, I’d like to take some time to talk about the Double-Acting Reciprocating type of air compressor.
Double-Acting Reciprocating compressors are a subset of the larger family of positive displacement compressor types. In positive displacement compressors, air is drawn into a chamber where the volume is then mechanically reduced. The energy used to displace the air volume is converted to an increase in air pressure. Dynamic compressors operate a little differently. They utilize an increase in air velocity to create the change in pressure. Air is accelerated to a high velocity through an impeller. The kinetic energy of the air is converted to an increase in potential (pressure) energy.
The Double-Acting Reciprocating compressor is a close relative to the Single-Acting Reciprocating compressor. In these types of compressors, an “automotive-type” piston driven by a crankshaft provides the compression. In a Double-Acting Reciprocating compressor, air is compressed as the piston moves in each direction. Hence the name, “double-acting”. In a Single-Acting Reciprocating compressor, air is only compressed on each full revolution of the piston. This makes the Double-Acting Reciprocating compressor much more efficient than its brethren.
Double-Acting Reciprocating compressors are also available in much larger sizes. While Single-Acting compressors can be found up to 150HP, generally they’re much less common any larger than 25HP. Whereas a Double-Acting compressor is available from 10HP-1,000HP, making it a better choice for larger plants that require a significantly greater volume of compressed air. While they’re a bit more expensive due to the added mechanisms to produce the double-action compression, this cost is quickly offset by the increase in efficiency. At a performance of 15-16 kW/100 cfm, they’re 32% more efficient than a single-acting reciprocating compressor.
If you’re in the market for a new compressor and are struggling to determine the most suitable compressor, talk with your local compressor sales representative. Once you’re up an running, EXAIR has a wide-range of products that’ll make sure you’re using your compressed air safely and efficiently!
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.
Of all the types of air compressors on the market, you can’t beat the single acting reciprocating air compressor for simplicity:
This simplicity is key to a couple of major advantages:
Price: they can cost 20-40% less than a similar rated (but more efficient) rotary screw model, up to about 5HP sizes. This makes them great choices for home hobbyists and small industrial or commercial settings.
High pressure: It’s common to see reciprocating compressors that are capable of generating up to 3,000 psig. Because the power is transmitted in the same direction as the fluid flow, they can handle the mechanical stresses necessary for this much better than other types of air compressors, which may need special modifications for that kind of performance.
Durability: out of necessity, their construction is very robust and rugged. A good regimen of preventive maintenance will keep them running for a good, long time. Speaking of which…
Maintenance (preventive): if you change your car’s oil and brake pads yourself, you have most of the know-how – and tools – to perform regular upkeep on a reciprocating air compressor. There’s really not that much to them:
Those advantages are buffered, though, by certain drawbacks:
Efficiency, part 1: The real work (compressing the air) only happens on the upstroke. They’re less efficient than their dual acting counterparts, which compress on the downstroke too.
Efficiency, part 2: As size increases, efficiency decreases. As stated above, smaller sizes usually cost appreciably less than more efficient (rotary screw, vane, centrifugal, etc.) types, but as you approach 25HP or higher, the cost difference just isn’t there, and the benefits of those other types start to weigh heavier in the decision.
Maintenance (corrective): Whereas they’re easy to maintain, if/when something does break, the parts (robust and rugged as they are) can get pretty pricey.
Noise: No way around it; these things are LOUD. Most of the time, you’ll find them in a remote area of the facility, and/or in their own (usually sound-insulated) room.
High temperature: When air is compressed, the temperature rises due to all the friction of those molecules getting shoved together…that’s going to happen with any air compressor. All the metal moving parts in constant contact with each other, in a reciprocating model, add even more heat.
Oil in the air: If you’re moving a piston back & forth in a cylinder, you have to keep it lubed properly, which means you have oil adjacent to the air chamber. Which means, no matter how well it’s built, you’re likely going to have oil IN the air chamber.
All that said, the benefits certainly do sell a good number of these compressors, quite often into situations where it just wouldn’t make sense to use any other type. If you’re in the market for an air compressor, you’ll want to find a local reputable air compressor dealer, and discuss your needs with them. If those needs entail the use of engineered compressed air products, though, please feel free to give me a call to discuss. We can make sure you’re going to ask your compressor folks the right questions.
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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.
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.
So…what are the pros & cons of rotary screw compressors?
*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.)
*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.
My colleague, Lee Evans, wrote a blog “About Single Acting Reciprocating Compressors”, and I wanted to extend that conversation to a more efficient relative, the double acting reciprocating compressor. As you see in the chart below, this type of compressor falls within the same family under the category of positive displacement compressors.
Positive displacement compressors increase air pressure by reducing air volume within a confined space. The reciprocating type of air compressor uses a motor that turns a crank which pushes a piston inside a cylinder; like the engine in your car. In a basic cycle, an intake valve opens to allow the ambient air into the cylinder, the gas gets trapped, and once it is compressed by the piston, the exhaust valve opens to discharge the compressed volume into a tank. This method of compression happens for both the single and double acting reciprocating compressors. With a single acting compressor, the air is compressed only on the up-stroke of the piston inside the cylinder. The double acting compressor compresses the air on both the up-stroke and the down-stroke of the piston, doubling the capacity of a given cylinder size. This “double” compression cycle is what makes this type of air compressor very efficient. A single acting compressor will have an operating efficiency between 22 – 24 kW/100 cfm of air while the double acting compressor has an operating efficiency between 15 – 16 kW/100 cfm. Therefore, electricity cost is less with a double-acting reciprocating air compressor to make the same amount of compressed air.
To explore the internals a bit closer, the mechanical linkage used to move the piston is slightly different as well as the additional intake and exhaust valves. Instead of the connecting rod being attached directly to the piston as seen inside a single acting compressor, a crosshead is added between the compression piston and the connecting rod (view picture below). The rod that connects the crosshead to the compression piston can be sealed to keep the cylinder completely encapsulated. For every rotation of the electric motor, the air is being compressed twice. With the added heat of compression, the double acting compressors are generally water-cooled. Also, with the added mechanism between the crank and the piston, the rotational speeds are typically less. Because of the larger size, water jackets, and added parts, the initial cost is more expensive than the single acting compressor, but the efficiency is much higher.
Double acting compressors are generally designed for rugged 100% continuous operations. Dubbed the work horse of the compressor family, they are also known for their long service life. They are commonly used in high pressure services in multistage styles and can come in lubricated and non-lubricated configurations. With the dual compression, slow speed and inter-cooling, it makes this type of air compressor very proficient in making compressed air.
A frequent question (and rightfully so) for compressed air products is “How much compressed air does it use?” Fortunately for EXAIR, we can provide these values with confidence, knowing the research and development, testing, and quality control that goes into the products we make.
For many applications, this question is cut and dry. For others, particularly those that do not currently have compressed air on site, the question leads to further conversation. One of the questions that is often asked, is “What type of compressor should we use with these products?”
While the end-use products (EXAIR products) will operate regardless of the compressor type, there are benefits and advantages to various compressor types in different applications.
For short-term or intermittent use, a reciprocating compressor can be an excellent choice. The size and weight, maintenance requirement (relatively low), and ease of procurement make them very suitable for small demand applications. They are also suitable for high pressures. Keep in mind that reciprocating compressors typically have higher noise levels and higher cost of compression when compared to screw compressors.
When the compressed air need is high volume, and the demand requires a continuous supply of compressed air, a rotary screw compressor can be a better choice. Rotary screw compressors are designed for more regular use in industrial applications, are (generally) more maintenance intensive, feature partial load capability allowing to align supply and demand, and can be found in a variety of sizes. You can expect to pay more for these models than the reciprocating compressors.
From an engineering standpoint, reciprocating compressors are dynamic devices, and screw compressors are positive displacement devices. Click here for a more in depth look at screw compressor operation.
EXAIR manufactures many, many compressed air driven devices with a concentration on solving problems, conserving compressed air and making it safe by meeting OSHA standards. And, although we do not supply or support any specific compressor manufacturer, our Application Engineers are well versed in compressed air generation and suited to discuss those needs with our customers.
If you have a compressed air related question, contact an EXAIR Application Engineer.