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How do the oil-free rotary screw air compressors work

April 23, 2021

How do the oil-free rotary screw air compressors work


Many users would like to know the working principle of oil-free rotary screw air compressors, product features, what applications , how to select and purchase oil-free screw air compressors, today we are here to introduce the oil-free screw air compressor. I hope this article can help you and add some values when you buy an oil free screw air compressor.


As user requirements for the quality of compressed air sources increase, oil-free air compressors are more and more widely used in industries that require high quality of compressed air sources, such as biology, medicine, electronics, food, medical, etc. In the automotive and chemical industries, the output accounts for more than 90% of the total output of oil-free screw air compressors.


There are many types of oil-free screw air compressors, with various types, and the value ranges from a few hundred dollars to tens of thousands of dollars.


Commonly used oil-free compressors are dry-type oil-free screw air compressors, water-lubricated oil-free screw air compressors, oil-free scroll air compressors, and oil-free centrifugal air compressors.


How do the oil-free rotary screw air compressors work

How do the oil-free rotary screw air compressors work

Air end of water-lubricated oil-free screw air compressor.


How do the oil-free rotary screw air compressors work


How do the oil-free rotary screw air compressors work

Star Wheel for air end


How do the oil-free rotary screw air compressors work

Star Wheel for air end


How do the oil-free rotary screw air compressors work


Rotors of screw air compressor


How do the oil-free rotary screw air compressors work

Dry oil free air end assembly

Today we only talk about dry oil-free screw air compressors and water-lubricated oil-free screw air compressor.


1. The working principle of oil-free air compressor

1) The single-screw water-lubricated air compressor uses pure water for lubrication and cooling. The internal parts of the body are made of special materials, medical-grade stainless steel, and will never corrode.

Dry-type oil-free air compressors still rely on lubricating oil to cool, and the body will have oil odor.

2) The pure water of the water-lubricated air compressor system always circulates in the system. The system will automatically change the water at the set time to ensure the quality of the water, and the operating temperature of the body is not higher than 50 degrees, so the water inside the body is not affected And the problem of scale.

The operating temperature of dry-type oil-free machines is higher than 200 degrees, which directly affects the life of the cooling oil and bearings.

3) The single-screw water-lubricated oil-free machine adopts imported special bearings, so there is no problem with direct water lubrication, high efficiency, low head noise, and few bearing combinations.

Twin-screw air compressors generally use FAG or SKF bearings, which are low-temperature bearings with short life and high noise, and there are many combinations of double-head bearings.

4) The single-screw water-lubricated air compressor sprays pure water directly into the body. Because of the high specific heat value of water, the compression chamber is almost in the ideal state of isothermal compression, and the compression process with the lowest energy consumption.

The cooling oil of the dry-type oil-free machine is about 100 degrees, and the temperature of the compression chamber exceeds 200 degrees. The temperature difference is relatively large, so the energy consumption is relatively high.

5) Single-screw water-lubricated oil-free machine, the body and the motor are directly connected, the speed is as low as 3000 rpm, there is no speed-increasing gear, the bearing life is long, and the maintenance cost is low.

A speed-increasing gear must be added between the body of the dry-type oil-free machine and the motor. Because the speed is higher than 18,000 rpm, the speed-increasing gear is required to increase the speed, the bearing life is short, and the maintenance cost is high.

6) The water-lubricated air compressor uses water to cool and lubricate the seal, the air quality is guaranteed, and the speed is about 3000 rpm. The temperature is around 55 degrees, and the maintenance cost is low.

The dry twin-screw air compressor uses oil to cool the bearings. The air quality cannot be completely clean and environmentally friendly. The secondary compression speed reaches about 18,000 rpm, which consumes labor, and the temperature reaches about 200 degrees, which is unsafe and high maintenance costs. .


2. Working principle of oil-free water lubrication air compressor

Oil-free water-lubricated screw air compressors belong to a relatively common type of oil-free air compressor family. They are lubricated by pure water, so the compressed air is 100% free of any oil. The biggest feature is that the operating temperature is not high. The theoretical "isothermal compression" can be achieved, and the noise emitted during work is also very small, which will not cause any impact on the surrounding environment. Therefore, oil-free water lubricated screw air compressors have also attracted many customers' attention.


Water cooling is a heat dissipation method, in addition to air cooling; and water lubrication also has air cooling and water cooling heat dissipation methods, its advantage is that ordinary air compressors need to add air compressor oil, while water lubrication air compressors Only pure water is needed to replace lubricating oil.


Benefits of Oil-Free water lubricated Air Compressed

The following is a brief list of the advantages of oil-free water lubricated screw air compressors.

Features:

(1) Reasonable structure and ideal force balance.

The main characteristics of oil-free screw compressors are caused by the symmetrical structure of a star wheel on each side of the screw.

(2) High efficiency

1. The volume utilization rate of the screw groove is high (the screw groove is used twice for one rotation of the screw).

2. No clearance volume, large single machine capacity.

3. Ideal isothermal compression. Due to the good thermal conductivity of water, when lubricating water is injected into the compression chamber, the temperature will drop to within 45°C, which is close to the suction temperature.

(3) No oil

Water is used instead of oil to realize the four major functions of lubrication, cooling, sealing, and noise reduction, providing high-quality 100% oil-free air, pollution-free, and no special treatment of discharged water, which meets environmental protection requirements.

(4) Low noise and low vibration

The oil-free screw compressor has good force balance, so the vibration is small, and there is no special requirement on the foundation, especially the star wheel material is made of special customized polymer composite material, the working cavity is sprayed with lubricating water, and there is no speed-increasing gear, so it runs Low noise.

(5) Long life of the host

The symmetry of the structure and the possibility of setting the return hole make the radial and axial forces generated by the oil-free screw compressor work to be completely balanced. Therefore, the load of the rotor bearing is extremely small and can run for 50,000 hours; the star wheel made of resin material can be used for 25,000 to 30,000 hours, and the design life of the compressor is as high as 300,000 hours.


However, its shortcomings are also obvious:

That is, the ambient temperature cannot be lower than 0°C, otherwise the pure water used as a lubricating medium will freeze!


How to select and purchase an oil-free screw air compressor not only involves how to use funds rationally, but also has a direct relationship with the economy and reliability of normal operation in the future.

  

General-purpose oil-free screw air compressors are in accordance with national standards, and the exhaust pressure is 0.7MPa-1 Mpa. If it is higher or lower than this pressure, it is a non-standard special oil-free screw air compressor.


The basic criteria for choosing oil-free screw air compressors are economy, reliability and safety.

One is that the level of exhaust pressure and the size of the exhaust volume should be considered. The exhaust pressure of the oil-free screw air compressor for general-purpose aerodynamics is 0.7MPa, and the old standard is 0.8MPa. At present, there is an oil-free screw air compressor with an exhaust pressure of 0.5MPa, which is unreasonable from the perspective of use, because the pressure margin is too small for pneumatic tools, and the air delivery distance is a little longer. use.


In addition, from a design point of view, this oil-free screw air compressor is designed for one-stage compression. If the pressure ratio is too large, it will easily cause the exhaust temperature to be too high, causing the cylinder to deposit carbon and causing accidents. If the oil-free screw air compressor used by the user is larger than 1MPa, it should be specially manufactured, and the method of forced pressurization cannot be adopted to avoid accidents.


The air displacement is one of the main parameters of an oil-free screw air compressor. The air volume of an oil-free screw air compressor should be matched with the required air displacement, and a 10% margin should be left. If the air consumption is large but the displacement of the oil-free screw air compressor is small, when the pneumatic tool is started, the exhaust pressure of the oil-free screw air compressor will be greatly reduced, and the pneumatic tool cannot be driven. Of course, blindly pursuing a large displacement is also wrong, because the larger the displacement, the larger the motor equipped with the oil-free screw air compressor, not only the higher the price, but also a waste of purchase funds, as well as a waste of electricity when used.


In addition, when selecting the exhaust air volume, the peak volume, the usual volume and the low volume volume should also be considered. If the trough consumption is large, but the usual and peak consumption are not large, the usual method abroad is to use oil-free screw air compressors with smaller displacements in parallel to obtain a larger displacement, which will increase one by one as the air consumption increases. Turn on the machine, which is not only good for the grid, but also saves energy.


The second is to consider the occasion and conditions of air use. If the air space is narrow (for boats, vehicles), vertical type should be selected; if there is a long-distance change (more than 500 meters) in the air usage occasion, mobile type should be considered; if the occasion cannot be used for power supply, diesel engine-driven type should be selected ; If there is no tap water in the application, the air-cooled type must be selected.


In the two cooling methods of air cooling and water cooling, users often have the wrong understanding that water cooling is good, but it is not. The air-cooled type accounts for more than 90% of the small oil-free screw air compressors at home and abroad. This is because the air-cooling is simple in design and does not require a water source when in use.


The fatal shortcomings of water-cooled oil-free screw air compressors are four:

1. There must be a complete water supply and discharge system, and the investment is large;

2. Water-cooled cooler has a short life;

3. It is easy to freeze the cylinder in the northern winter;

4. In normal operation, a lot of water will be wasted.


The third is to consider the quality of compressed air. Generally, the compressed air produced by oil-free screw air compressors contains a certain amount of lubricating oil and a certain amount of water. In some cases, oil and water are prohibited. At this time, not only should pay attention to the selection of oil-free screw air compressors, it is necessary It is necessary to increase the air after-treatment device.


The solution: One is to use a non-lubricated and oil-free screw air compressor. This kind of oil-free screw air compressor basically does not contain oil in its cylinder, and its piston ring and packing are generally PTFE. But this kind of machine also has shortcomings, poor lubrication and high failure rate; PTFE is also a harmful substance, which cannot be used in the food and pharmaceutical industries; non-lubricated and oil-free screw air compressors can only deliver air without oil, and cannot be used. To no water.


The second and commonly used method is to add an oil-free screw air compressor (no matter which type) to a primary or secondary purification device or a dryer. This device can make the air of the oil-free screw air compressor contain neither oil nor water, so that the oily water content in the compressed air is below 3ppm, which can meet the process requirements.


We have some misunderstandings about oil-free air compressors and the compressed air they produce.

1. The compressed air produced by an oil-free air compressor is oil-free - Wrong

Although no oil is used in the compression chamber, the oil will not come into contact with the compressed air, but the oil vapor contained in the atmosphere itself cannot be removed, which makes the compressed air still contain a small amount of oil impurities. After measurement, there is no The oil content in the compressed air produced by the oil compressor has a very close relationship with the air quality, the air compressor room environment, and the cleanliness of the compressor.

2. Oil-free air compressors do not use oil - Wrong

Although oil is not used in the process of compressing air to come into contact with air, oil is still used to cool the compressor and lubricate the operating parts, so that the compressor can operate normally and stably.

3. The compressed air produced by the oil-free air compressor is pollution-free - Wrong

There are many pollutants in the atmosphere, including dust, water vapor, oil vapor, microorganisms, etc. The compressed air produced by the oil-free air compressor can only ensure that no oil is brought into the compressed air during the compression process, but it cannot remove the magazines in the air itself. This means that the compressed air produced by an oil-free compressor is not necessarily pollution-free, and the use of air treatment products can help reduce pollution to the required level.

4. Oil-free air compressor does not require air treatment products - Wrong

As described in point 3, the air quality of compressed air produced by oil-free air compressors is largely restricted by the environment. Therefore, users need to configure corresponding air treatment equipment according to their own requirements for compressed air quality to meet their own needs and demand for compressed air quality.

5. Low cost of oil-free screw air compressor- Wrong

In fact, because the compression ratio is much lower than that of oil-lubricated compressors, the energy consumption is higher. Oil-free compressors with the same power can produce more than 40% less air than energy-saving screw compressors. Although the compression chamber is not lubricated by oil, other The grease and air filter in the parts still need regular maintenance. The core rotor is generally treated with a special coating, has a service life, has a high requirement for water, and is easily damaged. The maintenance of the air end is a necessary option for an oil-free machine. The high cost can account for 50% of the equipment cost.


We believe that for most users, it is not necessary to really need 0 grade oil-free compressed air, but more to solve the problem of oil impurities. For refrigeration dryers, heat-less regeneration dryers, micro-heat regeneration dryers, etc. The pollution of equipment is a choice to prevent the efficiency of air treatment equipment from decreasing or even losing the water removal function. For users who need real grade 0 oil-free compressed air, it is difficult to obtain oil-free compressed air steadily and continuously by using oil-free air compressors alone, and they still need to be equipped with corresponding air treatment equipment.


Conclusion:

Oil-free screw air Compressors are widely used in textile, energy, transportation, medicine, food, petrochemical, and machinery manufacturing industries due to higher air quality requirements from customers.

Dry-type and water-injected oil-free have different working principles.


Dry-type oil-free screw air compressor - have bigger airflow capacity, More expensive, because more maintenance cost

Oil-free water-injected screw air compressor- Due to the use of cheap pure water for lubrication, there is no air compressor oil, oil fine separation core, oil filter, and its maintenance cost is relatively low.


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