How to Select Air Compressor? Rotary Compressor vs Reciprocating Compressor

How to select air compressor? Rotary screw vs reciprocating compressor? Let Kotech show you the advantages and disadvantages of rotary compressors and reciprocating compressors, and provide a comparative analysis to help you decide which one is more suitable for your business needs.

When choosing an industrial air compressor, you often see two main types: rotary screw and reciprocating compressor. They both have their advantages in industrial applications, but there are also some big differences. So, for users who need to choose an air compressor, which one is better, rotary screw vs reciprocating compressor?

Chaz, Sales Manager

How to Select Air Compressor Rotary Compressor vs Reciprocating Compressor

Working principles comparison

Working principle of rotary compressor:

Rotary air compressor pressurizes air through the mechanical movement of the master and slave screws. Its working principle can be summarized as the following main steps:

  1. Intake air: The air intake valve of the Rotary air compressor is opened, and the air pressure in the screw compression chamber is lower than the atmospheric pressure. The air is filtered out of impurities and pollutants by the filter device and then sucked into the Rotary air compressor chamber.
  2. Compression: When the two screws rotate, the air is restricted by the gap between the outer shell of the rotary air compressor chamber and the rotating screw. The interaction between the gear and the rack causes the gas between the rotating screws to be compressed, and the air pressure and temperature in the chamber gradually increase.
  3. Exhaust: After the gas is compressed to the pressure standard, it will be discharged through the outlet valve. During the exhaust process, the air will be cooled by the cooler, and the water and sediment will be separated by the separator.
  4. Circulation: During the operation of the rotary air compressor , the gas will circulate in the rotary air compressor chamber, and after being cooled by the air cooler, it will enter the rotary air compressor chamber again for compression.

Principle of KOTECH Rotary Air Compressor
The working principle of the reciprocating compressor:
By using the crank-connecting rod mechanism to convert the rotational motion of the prime mover into the linear reciprocating motion of the piston, and use the automatic opening and closing of the inlet and exhaust valves to inhale, compress and discharge the air.

Because the method used is relatively backward, the air will contain a large amount of oil. This kind of equipment is not suitable for industries with high air precision requirements. Secondly, there are also problems such as high noise, gas pulses, and easy damage to the main engine.

Duty cycle comparison

This is the biggest difference between the two compressors. The duty cycle refers to the ability of the machine to run continuously during a day’s work.

Take the reciprocating compressor as an example. During the process of compressing air, the temperature of the compressed air and the reciprocating compressor will continue to rise. If the reciprocating head and valve exceed the specified operating temperature, the machine must be shut down to allow the system to cool down, otherwise the system will crash. Usually, the duty cycle of a reciprocating compressor is 60% to 70%, that is, in order to cool the reciprocating compressor system to a temperature that can continue to work, it will be shut down for about 30% to 40% of the working time.

The kind of duty cycle makes the factory needs to choose a reciprocating compressor that can produce more pressured air than its actual pressured air consumption and a large enough air storage tank so that the production line can still use compressed air during the reciprocating compressor shutdown and cooling period, so as not to stop production with the reciprocating compressor.

But even if there is enough air storage, the downtime of the reciprocating compressor may still cause operational interruptions, especially when the company’s air demand often fluctuates greatly or the production line needs to be expanded, which will cause unnecessary trouble to factory managers.

kotech rotary screw and reciprocating compressor oil

Rotary compressors can work continuously at 100% of the duty cycle because rotary compressor technology uses continuous lubrication cooling. The air is mixed with the cooling medium (mineral oil or compound oil) and compressed by the continuous rotation of the two rotors, forcing the air and oil into a path with reduced volume and increased pressure.

The oil in the rotary compressor has four main functions:

  1. Cooling;
  2. Compression seal;
  3. Lubrication;
  4. Cleaning internal components.

Reciprocating compressor can only work 60% to 70% of the working time, so the design of reciprocating compressors will be enlarged to meet the needs of the actual system.

When purchasing an air compressor, if you need an air compressor with the same air flow, the reciprocating compressor will have a higher power than the rotary compressors (about +30%). Therefore, the reciprocating compressor has a higher energy cost! Compared with rotary compressor, reciprocating compressors also require larger air tanks because the air tanks also have a buffering effect when the pressured air consumption is large.

Efficiency and energy consumption comparison

Rotary compressor: Due to its design principle, rotary compressors usually have high efficiency and can automatically adjust operation under different flow and pressure to reduce energy waste. Rotary compressors achieve air compression through the rotation of the rotor. Since there is no need for a reciprocating piston, the size of the rotary air compressor is relatively smaller. For some occasions with limited space, rotary compressors may be more suitable.

In addition, the operation of rotary compressors is smooth. Due to the structural design of rotary compressors, its operation process is relatively stable and has little vibration. This may be more suitable for some occasions that require relatively high equipment operation stability.

However, rotary compressors also have some shortcomings. The starting performance of rotary compressors is relatively poor. Due to its working principle, its starting process is relatively slow and it takes more time to reach normal working state than reciprocating compressor. This may not be suitable for some occasions that require frequent starting and stopping.

KOTECH Reciprocating compressor

Reciprocating compressor:

Due to the special structural design of the reciprocating compressor, it can achieve a higher air compression ratio, therefore it is a good choice in industrial applications that require high-pressure air.

In addition, the starting performance of the reciprocating compressor is better. Due to its special working principle, it can achieve fast start-up in a shorter time, this is a very important factor for industrial production lines that need to start and stop frequently.

However, reciprocating compressors may be less efficient at low loads because they are usually designed to run at full load and may require manual adjustments to improve efficiency under different loads.

However, there are also some disadvantages of reciprocating compressors. The size of the reciprocating compressor is relatively large. Since the reciprocating compressor needs to accommodate large parts such as large gas tanks, its size is also large, which may not be suitable for some occasions with limited space.

Temperature and compressed air quality comparison

The internal operating temperature of a reciprocating compressor may be between 150 and 200 degrees, while that of a rotary compressor is much lower, around 75 to 95 degrees. Why is this important for air quality? Because the hotter the air, the more moisture it contains.

Also, high temperature can also cause air compressor overload tripping(Know more about air compressor overload tripping)

Piston machines have greater difficulties in cooling down the compressed air than screw systems. Therefore, during the cooling of the compressor, there is more moisture in the air in a piston machine than in a screw system. This is because the air has a greater water storage capacity as it increases in temperature. This means that for reciprocating compressors, it is much more difficult to deal with water and impurities in the compressed air than for rotary compressors, so dryers and air after-treatment systems are required.

Especially where high-purity compressed air is required (e.g. painting, printing…), even small flow rates must be taken into account. If you use compressed air for car body work (e.g. painting), high air quality is essential to achieve high-quality paint results. For example, compressed air with moisture in it can lead to small pits in the paint, which can lead to expensive rework requirements.

In addition, higher air volumes are usually required to achieve high air quality. It is not uncommon for auto repair shops to use compressors up to 22 kW. In this regard, rotary compressors have a clear energy efficiency advantage, delivering more air per unit of input energy than reciprocating compressors. Of course, the cost advantage of running an energy-efficient compressor can be seen in the electricity bill.

Another issue to consider when producing high-quality air is oil carryover. In reciprocating compressors, as the moving parts (such as pistons, cylinders, rings, etc.) wear, more lubricating oil is able to pass through the rings and enter the air line to the point of use. Like moisture, oil is another substance you don’t want in your compressed air, especially when you use the air for applications such as painting. The performance of rotary compressors does not change much over time because the rotors in the rotary compressor body do not contact each other or the rotor housing due to the action of the lubricating oil.

The compressor lubricating oil acts as a sealant and is captured, filtered, cooled and recirculated. This extends the life of the compressor pump, but it also means that the rotary compressor needs to be equipped with complex oil handling equipment, such as oil separators with better separation effects, oil coolers, etc. Thus, a typical reciprocating compressor may have more than 10 ppm (or parts per million) of oil residual, while a rotary compressor has only 1 to 7 ppm.

Are you looking for a reliable and efficient power source for your industry? Kotech dealers are experienced in finding the perfect air compressor for your needs.

Contact your local Kotech dealer today and discover how an air compressor can increase your productivity and safety.

Air compressor wear and tear comparison

Many parts of reciprocating compressors, such as reed valves and piston rings, wear due to friction. Especially the wear of piston rings, after long-term operation, this wear will lead to reduced efficiency of reciprocating compressors. In the case of poor cooling, wear will be accelerated.

In this case, these parts must be subjected to multiple special maintenance, which will be repeated as the operating time increases. The same is true for rotary compressors, because in order to maintain long-term stable efficiency, rotary compressors force oil spray on the rotor surface to form liquid channels to prevent metal-to-metal contact between the rotors. After long-term operation, the efficiency of rotary compressors remains almost unchanged.

Wear of scrapers and piston rings of reciprocating compressors may gradually evolve into oil leaks. The lubricating oil entering the crankcase needs to lubricate the internal components of the reciprocating compressor and no longer stays inside the system. Instead, it is sucked through the scrapers and piston rings and finally enters the compression chamber and eventually contaminates the outlet air of the air compressor.

The rotary compressor has no cooling problem because it adopts refrigeration lubrication design and provides a dedicated oil tank to separate the oil in the air and filter it through a specially designed separation filter. By regularly replacing the separation filter, the oil content can be kept constant over time. Because the separator is a spin-on type, the end user only needs to perform simple operations.

Noise and vibration comparison

A major drawback of reciprocating compressors is noise and vibration, and reciprocating compressor noise is usually more than 80dBA. This means that it must be placed in a suitable area outside of people’s working areas.

The noise of reciprocating compressors makes it difficult to use in work areas and annoying when used in urban environments where low noise is required.Comparison of noise and vibration between screw air compressor and reciprocating air compressor

Through the screw compression method, rotary compressors can keep vibrations at a low level, so that the noise level is kept below 80dBA.

If well designed, many rotary compressors can easily reach 65-70dBa (a level that will not interfere with normal conversations), so there is no problem using them in work sites and urban environments.

And reserving a separate room for noisy air compressors may indirectly increase costs and be inconvenient to manage. It is easy for operators to forget about the air compressor working in a corner of the workshop, so they do not perform routine inspections, leading to bigger problems.

If the air compressor is located in an environment completely isolated from the operating area, it is likely that the abnormality will not be discovered until the air compressors stop working.

 

Repair and maintenance comparison

Regarding maintenance, it is important to consider routine and non-routine maintenance, which will have different effects between the two types of machines.

Routine maintenance for reciprocating compressors mainly includes replacing the air intake filter, checking the belt, checking and replenishing the oil level and condensate discharge. As mentioned earlier, the compression area of the reciprocating compressor is not lubricated, and the high temperature and friction cause the sealing components (piston rings, reed valves, etc.) to wear and age easily, resulting in the air compressor efficiency decreasing over time.

Timely replacement of worn and aged parts is a non-routine maintenance cost, but fortunately, the structure of the reciprocating compressor is relatively simple, and it is relatively easy to repair and maintain. Moreover, the parts of the reciprocating compressor are relatively common, and it is also easier to find the corresponding accessories when repairing.

Due to the characteristics of the rotary compressor, it requires some additional routine maintenance, which includes not only replacing the air filter, but also replacing the oil filter, oil-gas separator and engine oil. Proper regular maintenance can effectively extend the service life of the rotary compressor. This is because the rotary compressor forces lubrication of all moving parts, so good maintenance can keep its efficiency unchanged for many years. Non-routine maintenance is only required in extreme cases.

Therefore, when choosing an air compressor, users also need to take into account the long-term maintenance costs of the two types of air compressors and conduct an economic evaluation, which will become an important basis for selection.

Conclusion

Through this article, we learned that it’s not a good idea to choose an air compressor based on the comparison of the initial purchase cost. It is important to evaluate those 7 points mentioned above. rotary compressors are highly praised for their high efficiency and stable operation. Compared with the intermittent gas production of piston type, its continuous gas production process is more suitable for occasions with large flow and long-term continuous operation.Comparison of screw air compressor and reciprocating air compressor parameters

In addition, rotary compressors have low noise and vibration during operation, and relatively simple maintenance, which meets the requirements of modern industry for quietness and energy saving.

However, reciprocating compressors also have their unique advantages. In small factories, workshops and other occasions, reciprocating compressors are favored because of their simple structure and easy maintenance. It is more flexible in responding to the demand for high pressure and high power, and its performance price is relatively moderate, which is suitable for small and medium-sized enterprises.

We do not unilaterally advocate that rotary compressors are superior, nor do we favor the price advantage of piston compressors. On the contrary, we hope to help each customer start from the actual needs and provide the most suitable options and suggestions. Our mission is to help you find the solution that best meets the actual production needs and economic benefits.

The kotech group offers you the most suitable screw compressors and reciprocating compressors

Kotech group have been providing professional industrial air compressor services for 10 years, manufacture and supply all kinds of high-quality and excellent-performance piston and rotary compressors, Whether you are considering which kind of air compressor is better, or are wondering where to find a reliable air compressor supplier, we can always help you. contact us for free from below:

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