1.1 Preliminary understanding of several common types of positive displacement compressors
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1. Reciprocating compressor
(1)Reciprocating piston compressor (commonly known as the piston machine or,
Figure 3-2 of the compression element including piston and cylinder, through the piston in the cylinder for reciprocating motion to achieve gas compression.
Piston compressor is the most widely used
Figure 3-2 piston compressor
(2)Diaphragm compressor refers to the compressors that rely on the diaphragm deformation caused volume change in the cylinder.
Diaphragm compressor (see Figure 3-3) of the special structure, making its flow smaller,
but the compressed gas does not stain the gas lubrication, can ensure a high degree of purity of the gas,
suitable for compression of a small amount of gas does not allow
lubricants contamination of the gas, especially for precious and high purity of rare gases as well as extremely dangerous gases compression, transport.
Figure 3-3
The special structure of diaphragm compressor (see Figure 3-3) makes its flow smaller, but the compressed gas does not contact with any lubricant,
which can ensure the high purity of the gas, suitable for compression of small amounts of gases that are not allowed to be contaminated by the lubricant.
This kind of compressors are especially suitable for the compression and transport of precious and highly pure rare gases and extremely dangerous gases.
2.Rotary compressor
(1) Rotary screw compressor is divided into single screw compressor and twin screw compressor.
① single screw compressor, as the name suggests, single screw compressor only a screw (rotor), the screw at the same time
with a few star wheel engagement, screw type surface, star wheel end face, the screw ends of the cover plate together into a number of closed volume for the realization of the gas compression.
Figure 3-4 is commonly used in air compression of a screw, two star wheel structure of the single screw compressor.
Figure 3-4 single screw compressor
② twin-screw compressor (see Figure 3-5) is formed by the male and female rotor meshing vice, through the screw rotor teeth to achieve the purpose of compression of the volume change,
is the most widely used rotary compressor of a kind.
Figure 3-5 Twin screw compressor
(2) sliding vane compressor
(see Figure 3-6), the axial slide in the same cylindrical cylinder eccentric rotor for radial sliding, trapped in the air between the slide with
the rotation of the rotor and constantly be squeezed, so as to achieve compression.
3-6 Compression
(3) scroll compressor (see Figure 3-7), is a compressor consisting of a fixed involute scroll disc and an involute motion scroll disc that is eccentric gyratory flat.
Figure 3-7 Scroll compressor
2.1.2 Advantages and Disadvantages of Several Commonly Used Models
This section will focus on the application of air compression of positive displacement compressors several common models of the characteristics.
(1) Reciprocating piston air compressor
This is the earliest development of various types of compressors, the widest range of application of a class of compressors. Almost in a variety of uses in the compressor,
you can always see the figure of reciprocating piston compressor. Almost all the new structure of the compressor came out, mostly to replace the reciprocating piston compressor
in some areas of application for the purpose. So far, reciprocating piston compressors are still irreplaceable in some areas, such as ultra-high pressure.
the main advantages of reciprocating piston compressors are as follows.
① strong adaptability, regardless of the size of the flow rate, can achieve the required pressure. In general, a single-stage compression end pressure up to 0.3 ~ 1.0MPa,
multi-stage compression end pressure up to 300MPa or more!
② high adiabatic efficiency. Medium and large units adiabatic efficiency up to 0.7 ~ 0.85.
③ When adjusting the gas volume, the exhaust pressure is almost unchanged, the exhaust range is wider, and it can adapt to a wider pressure range.
pressure range.
(4) In the general pressure range, the material requirements are low, processing is easier, and the cost is cheaper.
(5) The characteristics of the gas have little influence on the performance of the compressor, and the same compressor can be used for different gases.
(6) The driving machine is relatively simple, most of them adopt electric motor, generally without speed regulation, and can be maintained.
(7) more mature technology, in the production and use of the accumulated experience.
The main disadvantages of reciprocating piston compressor are as follows.
(1) the speed is not high, if the exhaust volume is large, the machine volume is large.
② heavy parts, complex structure, more wearing parts, daily maintenance.
③ Poor power balance, machine running with vibration.
③ Poor power balance, vibration during operation.
④ Discontinuous exhaust, uneven airflow, easily caused by pipeline vibration.
⑤ Oil lubricated compressor, airflow with oil.
(2) Twin Screw Air Compressor
Compared with other types of compressors such as reciprocating piston compressors, the twin-screw compressor is relatively new. It has been widely valued by the industry
since its birth in the s and s, and has matured and improved in the s. After years of development, twin-screw compressor in the flow range of 1 ~ 60m'/min pressure range ≤ 2.0MPa
within the widely used, in the industrial field occupancy rate has been close to 80% (almost completely replace the reciprocating piston air compressor).
(1) The main advantages of twin-screw air compressor are as follows.
① High reliability, simple structure, few moving parts, long service life.
② Good power balance, the equipment can achieve smooth and high-speed operation without foundation.
③ Strong adaptability, with the characteristics of forced gas transmission, volumetric flow is almost independent of the influence of exhaust pressure, in a wide range can maintain high efficiency.
④ Multi-phase mixed transmission, can compress wet gas and gas containing liquid droplets.
⑤ Simple operation and maintenance, high degree of automation.
(2) The main disadvantages of twin-screw air compressor are as follows.
① Higher construction cost. Screw compressor rotor tooth surface is a space surface, need to use special cutting tools, in special equipment for processing, special equipment is expensive.
In addition, the screw compressor cylinder machining accuracy is also a high requirement, making the cost increase accordingly.
② Can not be used for high-pressure occasions. By the rotor stiffness and bearing life and other aspects of the limitations of the screw compressor is only suitable for low-pressure range,
exhaust pressure is generally not more than 3.0MPa.
(3) can not be made into a miniature. Screw compressors rely on gap sealing gas, only when the volume flow is greater than 0.2mmin, screw compressors have superior performance.
Single screw air compressor was invented in the s and s. A single screw compressor with CP-type is a type of rotary compressor with a typical structure consisting of the primary
screw rotor, two star-wheels, and housing, where it has efficiency and performance preferable to other types.Single screw air compressors usually require fluid injection to act as a seal, cool
the gas and lubricate the engaging pair.
(3)Single Screw Air Compressor
(1) The main advantages of the single screw air compressor are as follows
① Reasonable structure with ideal force balance. This is shown in all the structure of the compressor. Acting in the screw groove of the gas,
axial load is bound to offset, and then due to the symmetrical configuration of the star wheel, for particularly prominent. Used in the screw on the radial force also cancels each other,
so the screw is not subject to any axial and radial unbalanced force. Because of its good balance of forces, compared with the twin-screw compressor, can achieve higher pressures.
② Low noise and vibration. Because of the good force balance of the single screw air compressor, there is little vibration and no foundation is required.
The star wheel is made of composite material, plus the compression chamber is lubricated by spraying liquid, and there is no speed increasing gear, so the noise is also small.
Therefore, the noise is also small. The capacity of single machine is large, and there is no gap volume.
When the single screw air compressor is working, each screw groove on the screw is used twice in one revolution, so that the space of the screw groove can be fully utilised, therefore,
compared with other rotary compressors, the size of the single screw air compressor can be smaller.
(2) The main disadvantages of the single screw air compressor are as follows.
① More moving parts. Single-screw air compressor has three rotary axis, and screw and star wheel rigidity difference is large, the movement of the thermal expansion coefficient is different,
easy to deformation is not uniform, so it is difficult to ensure the accuracy of mutual cooperation.
② star wheel and screw mesh vice composition of the gap and positional accuracy between the high requirements of today's machining process efficiency is low,
looking forward to the development of high-speed milling and grinding means of processing.
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Should you buy a reciprocating air compressor or a rotary screw air compressor? If you’re shopping for a new air compressor, you may be wondering about the differences between rotary and reciprocating (piston) compressors. But while both types of compressors create compressed air, there are important differences in how they work and the applications they are best suited for.
Deciding which compressor type is right for you really depends on your air requirements: How much air (CFM) are you using? Is your air use intermittent or continuous? How clean does your air need to be? And how important are maintenance requirements and compressor efficiency in your decision? In this article, we’ll take a look at the advantages of rotary screw compressors vs. reciprocating compressors, the pros and cons of each compressor type, and which applications each is suited for. And we’ll try to answer the ultimate question: which is better, a piston or a screw compressor?
A reciprocating air compressor (also known as a piston-type air compressor) uses pistons driven by a crankshaft to compress the air. Pistons were first used to compress air in the mid-s. Today’s reciprocating air compressors are largely the same as compressors used in the late s during the industrial revolution.
A reciprocating air compressor works by using a piston to compress air inside a cylinder. The piston moves back and forth, drawing in air on one stroke and compressing it on the next. The basic design of a piston compressor consists of a cylinder, a piston, and a crankshaft. The cylinder is typically made of metal and is closed at one end, while the piston is a metal disc that fits snugly inside the cylinder. The crankshaft is connected to the piston and is used to drive the piston back and forth inside the cylinder.
When the piston moves downward, it creates a vacuum inside the cylinder, which draws air into the cylinder through an intake valve. As the piston moves upward, it compresses the air, which is then forced out of the cylinder through a discharge valve. This process repeats continuously as the piston moves back and forth inside the cylinder, creating an intermittent stream of compressed air.
Piston compressors can be either single-stage or two-stage. In a single-stage compressor, the air is compressed in a single stroke, while in a two-stage compressor, the air is compressed in two stages. In a two-stage compressor, the air is compressed to an intermediate pressure in the first stage, and then further compressed to a higher pressure in the second stage. This two-stage process allows for higher output and greater efficiency.
These machines are easy to maintain and excellent performers for many general-use applications. Reciprocating compressors are relatively simple in design, and many of their parts can be replaced or repaired by the user. They are best for applications that require intermittent use of small amounts of compressed air. They are often used in small shops, auto repair centers, and other settings where a high-CFM output is not required.
Rotary screw air compressors (or simply rotary air compressors) have been around since the mid-s. They use two meshing helical screws, known as rotors, to compress the air. As the interlocking spirals turn, air is forced through the chambers and compressed into a smaller space. With this process, air is continuously compressed as the rotors turn.
The basic design of a rotary screw compressor consists of two rotors, or screws, that rotate inside a cylinder. The rotors are usually made of high-strength steel and have a helical shape, with grooves cut into them to allow air to move through the compressor. As the rotors rotate, air is drawn in through an inlet valve and compressed as it moves through the compressor. The compressed air is then discharged through an outlet valve.
Because there are fewer moving parts, rotary screw air compressors are more reliable and tend to last longer compared to reciprocating compressors — as much as two to four times as long. While they tend to cost more up front, these advantages make a rotary compressor a good investment for many industrial users. They are preferred for applications requiring continuous operation and high airflow (CFM). One of the main advantages of rotary screw compressors is their high output capacity. They are capable of delivering continuous, high-pressure output, making them well-suited for industrial applications that require a large volume of compressed air. Another advantage of rotary screw compressors is their efficiency. They are designed to operate continuously, with minimal pulsation, which allows them to deliver a steady output of compressed air with minimal energy loss. Additionally, rotary compressors are known for their low vibration and noise levels, making them more comfortable to work around than other types of compressors.
For these reasons, rotary compressors are a popular choice for industrial applications that require high volumes of compressed air, such as manufacturing production lines, conveyor systems, process manufacturing, plastic molding, woodworking, and many more.
There are many differences between rotary and reciprocating air compressors, including operation, performance, efficiency, maintenance requirements, and CFM production. Understanding the key differences between reciprocating vs. rotary screw air compressors can help you make the best choice for your application.
One of the main differences between reciprocating and rotary screw compressors is how they compress air.
When comparing the efficiency of reciprocating vs. rotary screw air compressors, reciprocating compressors have a slight edge at lower horsepower and CFM ratings, especially in intermittent-use scenarios. However, for higher CFM applications and continuous operation, rotary compressors offer a clear advantage. Overall, rotary compressors offer higher CFM per horsepower than reciprocating compressors. There are a few reasons for this.
There are two considerations when it comes to air compressor maintenance: How much (and how often) is maintenance needed? And how much can you do yourself?
Cost is always an important consideration when choosing between rotary and reciprocating compressors. It is important to consider both the initial investment and the total cost of ownership over the lifetime of the compressor. In general, a reciprocating compressor is a better value at the low end of the industry (10 HP or less), but rotary compressors provide better overall value at higher HP ranges. The more CFM you require for your applications, the better the cost savings of a rotary compressor.
For applications requiring a highly reliable source of compressed air, rotary screw compressors once again come out on top.
Rotary screw compressors usually last much longer than reciprocating or piston compressors, up to twice as long on average.
Both rotary screw and reciprocating compressors may utilize air receiver tanks.
The operating environment can have a big impact on air compressor performance, reliability and maintenance requirements. Particulates in the air — such as pollen, dust, particulate from combustion engines, and other airborne contaminants — can gum up the works inside the air end, contaminate lubricants, and clog inline filters. In general, reciprocating compressors are less sensitive to particulates in the air supply.
Compressor noise can be an important consideration, especially if equipment will be operated in environments where people will also be working. Piston and rotary compressors differ in their noise level, with rotary compressors generally being quieter than piston compressors. Here are some factors that contribute to this difference:
Both types of compressors require ample free space and ventilation around the machine to prevent overheating. When looking at space requirements for compressor installation, be sure to take into account the need for ventilation space as well as room for the compressor and all its components.
One of the biggest differences between piston vs. screw compressors is in the duty cycle. The air compressor duty cycle is the amount of time it spends producing air vs. the amount of time it spends resting (also known as load/unload time or run/rest time).
Rotary screw and reciprocating compressors differ in their ability to maintain even pressure (PSI) in a plant. The pressure band is the difference between the cut-in pressure (the point at which the compressor will start making air) and the cut-out pressure (the point at which the compressor will stop making air). In other words, how much pressure is allowed to fluctuate.
When choosing a compressor, it is important to ensure that its maximum delivery pressure (PSI) meets your needs. Most industrial applications require pressure of 100 PSI. Both rotary screw and reciprocating compressors can easily meet this requirement. For high-pressure applications requiring PSI of over 120 PSI, a multi-stage compressor may be needed.
With proper filtration and air treatment, both piston and screw compressors can deliver clean, high-quality air. However, rotary compressors do have an advantage here.
Reciprocating and rotary screw compressors are both suitable for a variety of applications, but there are some differences in the types of applications for which each type of compressor is best suited.
Reciprocating
CFM @ 175 PSI
Rotary Screw
CFM @ 150 PSI
CFM @ 125 PSI
5HP
18
5HP
16
18
7.5HP
24
7.5HP
26
38
10HP
35
10HP
38
40
15HP
50
15HP
54
60
20HP
80
20HP
78
85
25HP
95
25HP
102
108
30HP
125
130
40HP
155
160
50HP
185
200
60HP
210
235
Which is better: a reciprocating or a rotary compressor? Choosing between a reciprocating and a rotary screw air compressor depends on how you use compressed air, how much compressed air you need, and the environment that the compressor will be operating in. This handy chart provides an overview of the pros and cons of reciprocating vs. rotary air compressors.
Which type of air compressor is right for you? It really depends on your application. Reciprocating compressors are more versatile and easier to maintain, while rotary screw compressors offer higher output and are better suited for large-scale industrial applications. The advantages of rotary screw air compressors are quite apparent when it comes to large-scale industrial applications requiring continuous airflow or high CFMs. On the other hand, if your compressed air needs are more intermittent, a piston-style compressor may just fit the bill.
Reciprocating or piston-type air compressors are best for applications when you are using short bursts of air intermittently. A reciprocating compressor is a great choice for homeowners and DIYers, small machine shops, construction work and other small businesses. A reciprocating air compressor can be used to run manual air-powered tools and for blow-off and cleaning, tire inflation (and other inflatables), airbrushing and sandblasting.
One advantage of a reciprocating air compressor is that it is not damaged by being run intermittently or below its maximum capacity. That means you can purchase a larger machine to “grow into” if you know your shop will require more air in the future. In fact, it is recommended that the reciprocating air compressors be sized at 50% higher than the required CFM to allow the compressor to cycle properly and avoid excessive heat generation and wear and tear on the motor.
Rotary air compressors are best for applications that require continuous air. These are the workhorses of the industry, used to power robotic manufacturing equipment and conveyor systems. They are designed to operate nonstop and produce a strong and consistent flow of air. Because the air produced by rotary air compressors is much cleaner than air produced by reciprocating compressors, they are the best choice for paint lines, food processing and packaging, and other applications where clean, dry air is essential.
Fixed-speed rotary screw compressors are not designed for intermittent use efficiently and may experience performance issues if they are not used close to their full capacity. If your compressed air demand varies, but you want the advantages of a rotary screw machine, you may want to consider a variable speed drive (VSD) compressor. While a fixed-speed compressor is always operating at the same RPM, a VSD motor can ramp up or down depending on demand.
Aire Tip: If your compressed air demand is variable, a VSD rotary screw compressor can reduce energy costs for your compressed air system by up to 70%.
What’s the best type of compressor? As you can see, it depends on your application and use case. There are many factors to consider when choosing between a rotary screw and a reciprocating air compressor.
If you need a little help, the experts at Fluid-Aire Dynamics can help you evaluate your needs and decide on the best compressor type for you. We carry a large stock of both reciprocating and rotary screw compressors in sizes from 5-10 HP to 400 HP and up, including VSD compressors. We are here to support you with compressor selection and compressed air system design, whether you need a small piston compressor for occasional use or a large rotary screw compressor for heavy-duty industrial applications. Contact our Aire Experts today!
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