It is a common power equipment and a device that converts mechanical energy into gas pressure energy. Its function is mainly to provide the gas required for production. Air compressors are widely used in production, mainly in air separation units, biological fermentation, textile, metallurgy and mining, wind tunnel test, instrument control, military and other fields. The operation effect of air compressor largely depends on the early selection demonstration of air compressor and the agreement of technical parameters.
1、 Introduction to types of air compressors
According to the structural type of its rotor, air compressors are divided into screw air compressors, centrifugal air compressors, piston air compressors, axial-flow air compressors, mixed flow air compressors, rotary air compressors and sliding vane air compressors. According to its working principle, it is divided into screw air compressor, piston air compressor, rotary air compressor and sliding vane air compressor. They all belong to volume type and directly rely on changing the gas volume to improve the gas pressure; Centrifugal air compressor, axial-flow air compressor and mixed flow air compressor all belong to speed type (or power type). Relying on the high-speed rotating blades, the gas passing through them is accelerated, so as to convert the speed energy into pressure. Different types of air compressors have their own characteristics, and the use occasions are different. The screw air compressor is mainly used in industrial processing occasions and the preparation of instrument gas; Piston compressor is mainly used in places with high air pressure; Centrifugal air compressor and axial-flow air compressor are mainly used in air separation unit, metallurgy and chemical industry, biological fermentation and other fields because of their large exhaust capacity. In recent years, frequency conversion technology and permanent magnet technology have been applied to the field of air compressor, which greatly improves the stability and economy of air compressor operation.
2、 Selection principle of air compressor
As an important power and energy supply facility in industrial production, in order to ensure its safe, stable and efficient service production, the selection of air compressor should adhere to the principles of safety, reliability, economy and efficiency and low installation and maintenance cost. Firstly, according to the air pressure and air flow required by the user, select the air compressor with appropriate structure. The air compressor has good mechanical performance (low vibration and low noise) during operation, good adaptability under variable working conditions and can operate permanently and stably, which are the basis of air compressor selection; Secondly, the economy of air compressor system operation should be taken as an important index for air compressor selection, which includes comprehensive indexes such as unit power consumption or unit steam consumption of air compressor operation, quality and water consumption of desuperheating water required by air compressor, waste heat benefit of air compressor, etc; In addition, the selection of appropriate technical parameters (exhaust volume and exhaust pressure) of the air compressor is the premise of whether the air compressor can meet the production demand and run economically; Finally, the installation and maintenance cost of air compressor should be one of the indicators of air compressor selection, and the air compressor with simple installation and low maintenance cost should be selected as far as possible.
The selection of air compressor shall refer to the following procedures: (1) investigate the user's needs (air pressure, air flow, air temperature, air humidity, etc. required by the user)（ 2) Calculate the resistance of the air from the outlet of the air compressor to the user point（ 3) Determine the rated exhaust pressure of the air compressor (the rated exhaust pressure of the unit can be taken as 1.1 times of the actual calculated theoretical data), exhaust volume, exhaust temperature of the air compressor after passing through the post-treatment device, etc（ 4) Select appropriate electric control and automatic control system according to the automatic operation requirements of the unit（ 5) Draft the technical requirements of air compressor and prepare for procurement（ 6) Conduct field investigation on air compressor manufacturers, and users, find out the manufacturer's production level and production capacity, and deeply understand the real feedback of air compressor users（ 7) Carry out bidding procurement for air compressors, formulate reasonable scoring standards, and select air compressor units with high cost performance through bidding（ 8) After signing the equipment contract, conduct face-to-face technical document docking with the air compressor supplier to form an air compressor technical agreement as an annex to the contract.
3、 Common problems and suggestions on air compressor selection
1. For example, the lack of understanding of the structural performance of different types of air compressors will lead to unreasonable selection of air compressors and directly affect the subsequent economic operation of air compressors. In general, the unit power consumption of multi shaft centrifuge, axial flow machine, ordinary single shaft centrifuge, screw machine and piston air compressor increases in turn. For example, in the biological fermentation industry, the generally required air pressure (absolute pressure) is between 0.30mpa-0.40mpa. For the selection of air compressor above 1200nm3 / min, it is best to choose axial flow air compressor or multi axis centrifugal unit, which has good operation economy and low maintenance cost; For the axial flow air compressor with adjustable stationary blades, its advantage is that the working condition can be adjusted in a wide range, and its best operation area is a curved surface. The unit can ensure that the unit is always at the best economic operation point under different loads. For instrument air with small air demand, its air pressure (absolute pressure) is generally between 0.5-0.8mpa. Screw air compressor is usually selected instead of piston air compressor, because screw air compressor has the advantages of compact structure, less vulnerable parts, stable operation and good economy.
2. The unreasonable selection of air compressor parameters makes the air compressor unable to operate at the best working point, and the economy of unit operation decreases.
For the centrifugal air compressor, the pressure and flow marked on the nameplate are the operating point with the highest operating efficiency of the air compressor. If you deviate from this operating point, the operation of the air compressor is not economical. In practical work, due to the inaccurate grasp of the pressure at the air demand point and the estimation of air transmission resistance from the outlet of the air compressor to the user, for the sake of safety, the exhaust pressure and exhaust volume of the air compressor are often overestimated when drafting the bidding documents of the air compressor, resulting in a large deviation between the actual operation data and the design value of the unit. For example, in an enterprise, the rated exhaust pressure (absolute pressure) of the ordered air compressor is 0.4MPa, while in actual operation, the exhaust pressure of the air compressor is only about 0.31mpa, and the power consumption of the unit is high. Therefore, when determining the technical parameters of the new air compressor, it is necessary to find out the pressure at the air use point and the calculation of air flow resistance, so as to ensure that the design parameters of the air compressor are consistent with the actual operation. Only the air compressor selected in this way can give full play to its operation efficiency.
3. The common conditions of air compressor design are harsh, which affects the safe and economic operation of air compressor. For example, an enterprise purchased a foreign air compressor with a flow of 855m3 / min many years ago, and the exhaust pressure (absolute pressure) of the unit is 0.33mpa. The common conditions of the air compressor design require that the cooling water temperature of the interstage cooler is 5 ℃. In the actual operation, the cooling water temperature is often higher than this temperature, the secondary air inlet temperature of the air compressor is high, and the unit efficiency decreases. When using 5 ℃ water, the air compressor has high air supply cost due to the high cost of 5 ℃ cold water, and the unit cannot operate for a long time. To ensure the economical operation of the air compressor system, the user shall provide the public system data consistent with the site during the design of the air compressor.
4. The unreasonable design and installation of air compressor post-treatment device and the increase of air resistance cause the increase of air compressor exhaust pressure and unit power consumption.
Some users purchase the air compressor body and post-treatment device separately. If the design capacity of the post-treatment device manufacturer is insufficient and only considering the cooling effect and production cost of the air, the number of heat exchange tube fins will often be increased in the limited container space, resulting in air flow obstruction. At the same time, the air from the outlet of the air compressor is incorporated into the air main pipe. The increase of the number of elbows will increase the air flow resistance. Reducing the number of elbows can reduce the air resistance; In addition, for the post-treatment device of large flow air compressor, if the site space allows, two sets of devices can be operated in parallel, which can effectively reduce the air flow resistance. The author suggests that the post-treatment device and pipeline connection of the air compressor should be designed and provided by the air compressor manufacturer to ensure the subsequent economic and stable operation of the air compressor system.
As an important gas supply equipment in industrial and mining enterprises, air compressor has relatively large energy consumption and relatively long operation cycle. It is the bounden responsibility of equipment managers to choose a safe, efficient and economic air compressor. Combined with several problems in air compressor selection found in daily work, this paper gives the basic principles and methods of air compressor selection, hoping to help you