Faced with various compressors with different functions in the market, many users can't have an exact understanding of the selection of compressors. Sometimes, because we can't fully understand the functions and performance of different compressors, we can't choose a reasonable type and a reliable, efficient and energy-saving compressor model.

According to the user's specific situation and actual process requirements, choose an air compressor suitable for production needs. It is not advisable to blindly choose high-quality and high-priced models, spend unnecessary expenses, and blindly choose inferior models with frequent failures in order to save money. After all, air compressor is an important power equipment in industrial production.

This paper briefly introduces the advantages and disadvantages of several commonly used compressor models and their application scope, hoping to provide reference for users when choosing compressors.

According to the different gas modes of compressors, compressors are usually divided into two categories, namely, positive displacement compressors and dynamic compressors (also known as rotary compressors). Due to different structural forms, positive displacement compressors and power compressors are classified as follows:

screw compressor

Screw air compressor is a rotary positive displacement compressor. Two rotors with spiral gears mesh with each other to compress and discharge gas.

Screw air compressors are divided into single screw and twin screw according to quantity; According to whether there is lubricating oil in the compression process, it can be divided into oil injection type and oil-free screw air compressor, and oil-free compressor can be divided into dry type and water injection type.

Generally speaking, the screw air compressor has the advantages of simple structure, few wearing parts, low exhaust temperature and large pressure ratio, especially not afraid of liquid and dust compression in gas. The appearance of oil-injection screw compressor makes the power technology and screw compressor for refrigeration (including screw air compressor and screw refrigerator) develop rapidly at home and abroad.

principle of operation

Screw air compressor uses the meshing of male and female screw rotors to reduce the volume between teeth and increase the gas pressure, thus continuously generating compressed air. Screw air compressor also belongs to positive displacement compressor, but due to the working principle of screw air compressor, compared with piston air compressor, screw air compressor has stable gas supply and generally does not need to be equipped with gas storage tank. The working process is shown in the figure below.

Main advantages

1, high reliability: the screw air compressor has fewer parts, less wearing parts, reliable operation and long service life.

2. Convenient operation and maintenance: the operator can run unattended without long-term professional training, and the operation is relatively simple, and the gas can be supplied according to the required exhaust volume.

3. Good power balance: the screw air compressor has no unbalanced inertia force, and the machine can work stably at high speed and can run without foundation. Especially suitable for mobile compressors, with small volume, light weight and small floor space.

4. Strong adaptability: the screw air compressor has the characteristics of forced gas transmission, the displacement is almost unaffected by the exhaust pressure, the operation is stable, the vibration is small, the exhaust is stable, and the high efficiency can be maintained in a large range.

5. Multiphase mixed transportation: In fact, there is a gap between the rotor tooth surfaces of the screw air compressor, so it can resist the impact of liquid and transport gas containing liquid, gas containing dust and gas easy to polymerize under pressure.

6. The volume per unit displacement is small, saving the floor space.

Although the screw compressor has the above advantages, it is necessary to make a detailed maintenance plan to keep the screw compressor unit working normally, safely and reliably and have a long service life. It is best to carry out personal operation, regular maintenance and regular inspection and maintenance to keep the compressor clean and free of oil and dirt. Only by fully mastering the common sense of maintenance and being familiar with the solutions of faults can we ensure the smooth operation of the compressor.

applied range

Screw compressor has the unique advantages of high reliability, convenient maintenance and strong adaptability. With the deepening of its research and the continuous improvement of design technology, the performance of screw compressor will be further improved and its application fields will be more and more extensive. In addition to traditional applications, the application of screw compressors in new fields such as fuel and batteries will expand rapidly. At the same time, due to the continuous improvement of working reliability of screw compressor, it has gradually replaced reciprocating compressor in the range of medium refrigeration capacity, and occupied part of the market of centrifugal compressor.

development trends

In the field of petrochemical industry, at present, domestic centrifugal compressors can not meet the needs of domestic high-tech and special products. In addition, there is still a gap with foreign countries in terms of technical level, quality and completeness. With the continuous expansion of petrochemical production scale in China, centrifugal compressors will face new problems in large-scale production, but China has no mature experience in designing and manufacturing these large gas compressors. Due to the challenge of single-screw compressor, a part of the twin-screw air compressor market will be squeezed by single-screw compressor. However, the domestic twin-screw process compressor has always relied on imports, so the twin-screw process compressor will be a development direction.

piston compressor

Piston compressor is the most common positive displacement compressor. It transforms the rotary motion of the driving machine into the reciprocating motion of the piston through the crank-connecting rod mechanism. The piston and cylinder together form the working chamber of the compressor. Depending on the reciprocating motion of the piston in the cylinder and the automatic opening and closing of the intake valve and exhaust valve, gas periodically enters the working chamber of the cylinder for compression and discharge.

Piston compressor is mainly composed of three parts; Moving mechanism (crankshaft, bearing, connecting rod, crosshead, pulley or coupling, etc. ), working mechanism (cylinder, piston, air valve, etc. ) and the fuselage. In addition, there are three auxiliary systems: lubrication system, cooling system and regulating system.

The motion mechanism is a crank-connecting rod mechanism, which converts the rotary motion of the crankshaft into the reciprocating motion of the crosshead. The fuselage is used to support and install the whole movement mechanism and working mechanism. Working mechanism is the main component to realize the working principle of compressor.

principle of operation

In pneumatic transmission, a positive displacement piston air compressor is usually used. The piston air compressor uses the crankshaft to drive the piston to reciprocate, so that the gas in the cylinder cavity is compressed and compressed air is continuously generated. Piston air compressor is a positive displacement compressor. Because of the working principle and characteristics of this kind of compressor, in order to provide a stable air source, piston air compressors are generally equipped with air storage tanks.

Main advantages

1, wide pressure range. Because its working principle is volume change, it can reach high working pressure regardless of its flow rate. At present, various compressors of low, medium, high and ultra-high pressure have been manufactured, among which the working pressure of industrial ultra-high pressure compressor can reach 350MPa(3500kgf/cm2).

2. The equipment has low price, less initial investment, convenient operation and long service life.

3. Because the compression process is a closed process, the thermal efficiency is high.

4, strong adaptability, large range of exhaust volume, and little influence by the change of exhaust pressure. When the medium changes greatly, its volume displacement and exhaust pressure change little.

Main disadvantages

1, the inertia force is large, and the rotation speed cannot be too high, so the machine is heavy, especially when the displacement is large.

2. Complex structure, many wearing parts, heavy maintenance workload and relatively high maintenance cost.

3, the exhaust is discontinuous, the air pressure fluctuates, and it is easy to produce air column vibration.

4. The vibration and noise during operation are high, which requires high equipment installation.

Because the piston machine can only intermittently intake and exhaust air, the cylinder volume is small, and the reciprocating speed of the piston can not be too fast, so the displacement and power of the piston machine are greatly limited.

application area

Piston compressor belongs to reciprocating compressor, and its pressure grade belongs to medium pressure, high pressure and ultra-high pressure, which is suitable for high pressure occasions. The flow range is medium and small, which is mainly suitable for medium and small displacement and high pressure occasions.

development trends

Piston compressor is the most widely used compressor in the traditional field, but with the rise of other rotary compressors and other products, its market in refrigeration and many other fields has gradually shrunk.

The key ethylene construction projects in domestic petrochemical field and the vigorous rectification in coal field in recent years will drive the development of piston compressor technology and its industry. Piston compressors mainly develop in the direction of large capacity, high pressure, low noise, high efficiency and high reliability. Continuously develop new air valves operating under variable working conditions to improve the service life of air valves; In product design, the performance of compressor under actual working conditions is predicted by comprehensive simulation using thermodynamics and dynamics theory. Strengthen the electromechanical integration of compressor, and adopt automatic control by calculation to realize optimized energy-saving operation and online operation.

In order to develop the advantages and overcome the disadvantages, the structural parameters tend to be high speed and short stroke, making the structure compact. At the same time, the life of vulnerable parts such as air valves and seals is prolonged and the operation rate is improved. With the development of optimization design theory and computer technology, it opens up new prospects for rational selection of design parameters and improvement of efficiency.

How to choose the type

1), and the type selection is as required.

Combining with the needs of customers, finding the best operation economy and expanding the scale in the future require a lot of decisions. Decision-making is based on the use or process of compressed air, and the key point is to calculate the demand, reserve and future expansion of air. Pressure is the decisive factor and has a great influence on energy consumption. It may sometimes be economical to use different compressors in different pressure ranges.

2), the calculation of working pressure

The equipment of compressed air determines the necessary working pressure, which depends on the compressor, equipment and pipeline, and the highest working pressure determines the necessary equipment pressure. The pressure reducing valve is used to meet the equipment requirements of gas-using places. In extreme cases, it is not economical to equip a separate air compressor.

Working pressure: end user+final filtration+piping system+dust particle filtration+dryer+compressor adjustment range.

The higher the pressure, the greater the power consumption, and the pressure drop caused by the size and length of the pipeline must be considered. List the working pressures of various models. If the operating pressure is too different, you must buy air compressors with different pressures, and you must not reduce the pressure and increase the cost.

3) Calculation of air consumption

Compressed air is a kind of clean power that converts electric energy into air potential energy and does work with the expansion of compressed air, but it also consumes a lot of electric energy. Generally speaking, it takes about 7kW to compress 1m3 air to 0.7MPa. According to statistics, the electricity consumption of air compressor stations accounts for about 20% of the total electricity consumption of the whole enterprise. This means saving compressed air and using it rationally will bring you new profit space!

Air demand: air consumption of all tools+mechanical equipment+related processes+leakage+wear+future gas+service factor (20% of the standard value).

4) Determine the number and specifications of compressors.

According to the required flexibility+control system+energy efficiency

(1), which machine should I choose, or which ones should I choose?

The shutdown cost in production, the utilization rate of electricity, the change of load (load), the cost of compressed air system, and the available floor space. Because of the cost, all the air in a device needs only one compressor to supply, so the system can be connected with a quick interface of a mobile compressor when it is used, and an old air compressor can be used as a cheap backup power supply to provide a backup air source.

(2) Stability (always a very important issue);

(3) Energy consumption expenditure

(1) pipeline leakage; ② The gas demand fluctuates all the time (this is the most easily overlooked and the most serious).

③ Single machine output efficiency (choose the best range of output efficiency model)

(4) Generalization of spare parts

The optimal combination of multiple 1 10KW models may be the best choice for the gas consumption range of 40- 160m3/min.

(5), operation analysis

It should be observed that more than 90% of the measured energy is recovered within one week. In a certain period of time, when the working pressure often drops, the control system can be modified with reference to the change of production, and another factor to improve the use of air compressor is to check whether there is air leakage.

In order to save electricity, we should pay attention to the energy consumption ratio: actual displacement/actual motor power consumption. The larger the value, the more power consumption.

2. Selection of freeze dryer

In order to remove the moisture in the air, your company used an adsorption dryer, which shows that gas equipment and tools have high requirements for air quality.

In general, the choice of freeze dryer only needs to choose a dryer with throughput equal to or slightly greater than that of the air compressor according to the flow rate of the air compressor.

For enterprises that have high requirements on moisture in the air of freeze-dryer and do not need to use adsorption dryer, freeze-dryer can be equipped with one or two dryers in parallel.

3. Selection of adsorption dryer

1), non-thermal regeneration (PSA)

After a period of use, the desiccant tends to be saturated and must be dried and regenerated. The simplest and most common method is to extract some dry air from the outlet of another tank, and then blow dry the wet desiccant after decompression and expansion. For dryers without thermal regeneration, a rated air flow rate of about 14% is required for complete regeneration.

Compressed air is suitable for small airflow regeneration process, and its gas consumption needs 15-20% compressed air at 7bar working pressure, and the pressure dew point is -40? C. The lower the pressure dew point of the dryer, the greater the gas consumption.

2), heating regeneration (TSA)

In addition, if some heating devices, such as heating rods, are added to the drying tank, and the temperature of the desiccant is raised to 200℃ during regeneration, regeneration air can be used less and a lot of energy consumption can be saved. Only 4% recycled air can achieve the purpose of complete regeneration, saving compressed air by about 10%. Generally speaking, heating regeneration is usually used in larger units or places with limited compressed air flow. Although its initial investment is high, it can save more costs under long-term use.

Heating regeneration is usually used for larger units or in the case of large compressed air flow.

1) micro-thermal regeneration

Micro-heat regenerative adsorption dryer is a kind of compressed air adsorption dryer with China characteristics. The original intention of the design is to reconcile the characteristics of the athermal dryer and the heated dryer, and to produce a dryer that consumes less regeneration gas than the athermal dryer and less heating power than the heated dryer.

Structurally, the micro-heating type uses its own dry air for desorption, and uses an external heat source to heat the desorption gas into micro-heating. It is said that the purpose of doing this is to save the consumption of re-anger. However, theoretical research shows that the actual situation is not so ideal; After a small amount of regenerated gas heated to a certain temperature enters the regeneration tower, the temperature is immediately absorbed by a large number of adsorbents. In other words, in order to make the regeneration exhaust temperature reach the required value, the adsorbent in the tower must reach this temperature first, which will consume a lot of regeneration gas.

Micro-thermal regeneration uses its own dry air to desorb the adsorbent after decompression. Because of the low water content, it has the ability to desorb adsorbent even without heating, just like non-thermal regeneration. By heating, the gas can carry more water vapor when it comes out, thus saving regeneration gas. The higher the temperature of regeneration waste gas, the less the regeneration gas consumption-this is the design idea of micro-heating.

The micro-thermal type, like the thermal type, not only has the problem of desorption temperature, but also has the problem of heat required for desorption process. Because when the auxiliary gas is heated, the metal cylinder and the adsorbent are heated together, the heat required by these auxiliary heating greatly exceeds the heat required by the desorption gas itself. If the heat required in the desorption stage is determined by the power of external electric heating equipment, then the heat entering the regeneration tower should be carried by desorption gas. That is, the compressed air from the dryer itself is not only used to desorb the adsorbent, but also undertakes the additional task of heating the adsorbent and the metal tower. As a result, gas consumption is greatly increased. The above steps are only the first step in the whole regeneration process, and the purging and cooling stages of adsorbent will consume roughly the same amount of gas. Therefore, generally speaking, micro-heating method does not necessarily save the amount of regeneration gas when it achieves the same effect as non-heating method. Micro-thermal desorption of adsorbent based on pressure swing adsorption principle. However, because the regenerated gas is heated, it must be blown cold in the later stage of adsorbent growth, so it is a long-time dryer (half working period is as long as 1~4 hours). Its adsorbent volume is smaller than that of athermal adsorbent. Therefore, the moisture absorbed by the adsorbent per unit mass is much more than that of the non-thermal adsorbent, which will have a negative impact on the dew point index.

In addition, the disadvantage of thermal type is also reflected in micro-thermal type. Whether the micro-thermal type is less than the thermal type in terms of regeneration energy consumption cannot be generalized. If it is not handled properly, it is entirely possible that the comprehensive energy consumption will be greater. Compared with non-thermal type, it is undoubted that the comprehensive energy consumption of micro-thermal type is greater to achieve the same therapeutic effect.

Therefore, unless the air compressor is seriously insufficient and the power supply in the factory is extremely rich, there is no outstanding reason to choose the micro-heat type.

Conclusion: No matter which of the above three kinds is selected, compressed air is needed, and the regenerated air needed by the dryer should be considered when selecting the air compressor.

4. Selection of pipeline filter

Generally, the selection of pipeline filter only needs to select the filter with processing capacity equal to or slightly greater than that of the air compressor according to the flow rate of the air compressor.

The accuracy of pipeline filters is different, and the choice of accuracy depends on the requirements of enterprises for air quality. At present, air compressors widely used in the market are mainly divided into screw air compressors and piston air compressors. Because the oil content of compressed air of piston air compressor is 25- 150PPM, it needs three-stage filtration. For screw air compressor, the oil content of compressed air is usually 2-3PPM, so the selection of filter can generally meet the customer's requirements for air quality through two-stage filtration. Of course, under special circumstances, we can also deal with it by adding activated carbon filter.

5. Selection of gas storage tank

Usually, the simple method is 15- 30% of the air volume of the air compressor (m3/min). If you want to calculate it, the following formula is available.

V = air barrel volume; Q = air compressor discharge (m3/min); 8 = constant (usually used at 7 bar);

△P = pressure difference (bar, usually set at least 0.6- 1 bar)

In addition, in the choice, we should also determine the work pressure according to the needs of customers, which not only meets the needs of customers, but also saves investment costs.