The most complete version｜Basic theoretical knowledge of compressed air (1)

The most complete version｜Basic theoretical knowledge of compressed air (1)

1. Compression purpose

　　The basic purpose of gas compression is to deliver gas at a pressure higher than the original pressure. The original pressure level may vary from a very low absolute pressure (a few thousandths of a kilogram) to several thousand kilograms; the pressure ranges from a few grams to several thousand kilograms; and the amount of air transmitted ranges from a few cubic meters per minute to hundreds of thousands. Cubic meters per minute.

The most complete version｜Basic theoretical knowledge of compressed air (1)

The specific purpose of 　　 compression has various:

　　 1. Transmit power in the compressed air system that drives the pneumatic tool;

　　 2. Provide air for combustion; (90% of practitioners pay attention to the public account of Compressor Network)

　　3. Transport and distribute gas in natural gas pipelines and city gas distribution systems;

　　 4. Make the gas circulate through a process or system;

　　 5. Create a more active condition for chemical reactions;

　　6. ​​To create and maintain a higher pressure level than the original for various purposes, the method is to exhaust the gas leaking into or flowing into the system or the originally existing impurities out of the system.

　　 2. Compression method

　　 There are 4 ways to compress gas: 2 is the discontinuous airflow method, and the other 2 is the continuous airflow method (this is a descriptive terminology, not classified by thermodynamics or functions). These methods should:

　　 1. A certain amount of continuous gas is trapped in a certain container, its volume is reduced to increase the pressure, and then the compressed gas is pushed out of the container.

　　2. A certain amount of continuous gas is trapped in a certain container, and the gas is brought to the exhaust port without changing its volume. The gas is compressed through the reverse flow of the exhaust system, and then the compressed air is pushed out of the container.

　　 3. The gas is compressed by the mechanical movement of the rapidly rotating rotor. The rotor transmits speed and pressure to the flowing gas (the speed is further converted into pressure on a fixed diffuser or baffle).

　　4. Feed the gas into a high-speed nozzle of the same or another gas (usually, but not necessarily steam), and convert the high velocity of the mixed gas into pressure on the diffuser.

Compressors using methods 1 and 2 belong to the type of intermittent airflow and are called variable-capacity compressors; those using method 3 are called speed compressors; those using method 4 are called jet compressors, and their intake pressure is generally lower than the atmosphere pressure.

　　 3. Types and characteristics of compressors

　　 Below are the definitions of various compressors. Compressors such as cam type, diaphragm type and diffusion pump are not included because of their special purpose and relatively small size.

　　 positive displacement compressor-is to restrict a certain amount of continuous air flow in a closed space to increase the pressure.

　　 reciprocating compressor-is a positive displacement compressor, the compression element is a piston, reciprocating in the cylinder.

　　 rotary compressor-is a positive displacement compressor, compression is achieved by the forced movement of rotating elements.

　　 sliding vane compressor-is a rotary variable displacement compressor, the axial sliding vane slides radially on a rotor eccentric to the cylinder block. The air trapped between the sliding plates is compressed and discharged.

　　 Liquid-piston compressor-is a rotary positive displacement compressor in which water or other liquid acts as a piston to compress gas and then discharge the gas.

　　 Roots twin-rotor compressor-a rotary positive displacement compressor, in which two Roots rotors mesh with each other to intercept the gas and send it from the inlet to the outlet. There is no internal compression.

　　 Screw compressor-is a rotary positive displacement compressor, in which two rotors with helical gears mesh with each other to compress and discharge gas.

　　Speed ​​type compressor-is a rotary continuous flow compressor in which high-speed rotating blades accelerate the gas passing through it, thereby converting speed energy into pressure. This conversion occurs partly on the rotating blades and partly on the fixed diffuser or reflux baffle.

　　 Centrifugal compressor-a speed compressor, in which there are one or more rotating impellers (blade usually on the side) to accelerate the gas. The main air flow is radial.

　　 Axial compressor-a speed compressor in which gas is accelerated by a rotor equipped with blades. The main air flow is axial.

　　 Mixed flow compressor-also a speed compressor, the shape of its rotor combines some of the characteristics of both centrifugal and axial flow.

　　 Jet compressor-use high-speed gas or steam jet to take away the inhaled gas, and then convert the speed of the mixed gas into pressure on the diffuser.

　　 Fourth, the classification and characteristics of air compressors

　　 Three basic types of air compressors include:

　　 reciprocating

　　 rotary

　　 Centrifugal

　　 The above three types of air compressors can be further divided into:

　　Bare machine and whole machine

　　 wind and water cooling

　　 oil injection and oil-free

　　 Let us briefly discuss the following three types of air compressors:

　　 reciprocating air compressor

　　 The size is 0.7MPa (G) - the range of 0.72Kw and 0.028m3/min to 932 Kw and 176.4m3/min reciprocating air compressor is a variable capacity compressor. This type of compressor sequentially compresses (shrinks its volume) the air enclosed in a confined space to increase its air pressure. The reciprocating air compressor uses a piston in the cylinder as the original part of the compression displacement to complete the above compression process.

　　 When the compression process is completed by only one side of the piston, the reciprocating type is called a single-acting air compressor, if it is completed by the two ends of the piston, it is called a double-acting. The reciprocating air compressor has many spring-loaded valves on each cylinder, and the valve opens only when the pressure difference between the two sides of the valve reaches a certain value.

　　 When the pressure in the cylinder is slightly lower than the intake pressure, the intake valve opens, and when the pressure in the cylinder is slightly higher than the exhaust pressure, the exhaust valve opens. (90% of practitioners pay attention to the public account of Compressor Network)

　　 If the compression process is completed by a cylinder or a group of single-stage cylinders, the air compressor is called a single-stage air compressor. Many actual operating conditions exceed the capacity of a single-stage air compressor. The compression ratio (exhaust/intake pressure) will cause overheating of the exhaust temperature or other design problems.

　　Many reciprocating air compressors with a power exceeding 75Kw are designed as multi-stage units. The compression process is composed of two or more stages. Generally, there is a cooling function between the stages to reduce the temperature entering the next stage.

　　 There are two types of reciprocating air compressors, oil-injected and oil-free, with a wide choice of pressure and air volume.

　　 rotary air compressor

0.85m3/min-85m3/min rotary air compressor is a variable capacity compressor. The most common rotary air compressor is a single-stage oil-injected screw air compressor. This type of compressor has two A rotor, through which the air is compressed, there is no valve inside. This air compressor is generally oil-cooled (the cooling medium is air or water), and this oil acts as a seal.

　　Because the cooling is carried out inside the air compressor, the parts will not have a high temperature. Therefore, the rotary air compressor is a continuous working system and can be designed as an air-cooled or water-cooled unit.

　　 Due to the simple structure and few wearing parts, the rotary screw air compressor is easy to maintain and operate, and has the characteristics of flexible installation. The rotary air compressor can be installed on any ground that can support weight.

　　The two-stage oil-injected rotary screw air compressor has two pairs of rotors in the main part, and the compression process is completed by the first and second stages of serial compression. The two-stage rotary air compressor has the characteristics of simple structure, flexibility and high efficiency. The two-stage rotary screw air compressor is air-cooled, water-cooled, and fully enclosed.

　　Oil-free rotary screw air compressor uses a specially designed main engine to compress without oil injection, thereby producing oil-free compressed air. The oil-free rotary screw air compressor has two types: air-cooled and water-cooled, and has the same flexibility as oil injection. As you can see, rotary screw air compressors are air-cooled, water-cooled, oil-injected, oil-free, single-stage and two-stage, and have wide applicability in pressure, air volume, and structure.

　　 Centrifugal air compressor

　　 11.2m3/min - 420m3/min centrifugal air compressor is a dynamic air compressor. It uses a rotating turbine to complete the energy conversion, and the rotor achieves the above conversion by changing the kinetic energy and pressure of the air. The static diffuser reduces the flow rate of air to achieve the conversion of kinetic energy to pressure.

　　 centrifugal air compressor is an oil-free air compressor, the lubricating oil of the moving gear is separated by the shaft seal and air.

　　 The centrifugal compressor is a continuous working condition compressor with few moving parts, which is especially suitable for the requirement of no oil in the atmosphere.

　　 Centrifugal air compressor is water-cooled, and a typical unit includes an aftercooler and all control devices.

　　 Determination of gas consumption

　　 The traditional method of determining the compressed air requirements of a new plant is to add up the air consumption (m3/min) of all air-consuming equipment, and then consider adding a safety, leakage and development factor.

　　In an existing factory, you only need to do some simple tests to know whether the compressed air supply is sufficient. If not, you can estimate how much more needs to be added.

　　The output pressure of the general industrial air compressor is 0.69MPa (G), and the pressure to the point of use of the equipment is at least 0.62MPa. This shows that the typical air compressor we use has an unloading pressure of 0.69MPa (G) and a cylinder loading pressure of 0.62MPa (G), or system pressure. With these numbers (or the unloading and loading values ​​of a certain system) we can determine.

　　 If the cylinder pressure is lower than the nominal loading point (0.62MPa(G)) or does not gradually rise to the unloading pressure (0.69MPa(G)), more air may be required. Of course, always check to make sure that there are no major leaks and that the compressor's unloading and control systems are operating normally.

If the compressor must work at a pressure higher than 0.69MPa(G) to provide a system pressure of 0.62MPa(G), check that the pipe size of the distribution system may be too small, or how much more air volume needs to be added to the air consumption at the blocking point , What is the impact of system leakage and how to determine the size of the gas storage tank to meet the intermittent peak demand of gas consumption.

　　 1. Test method-check the capacity of the existing air compressor

　　 Timing pumping test is a relatively easy and accurate method to check the air volume or output of existing air compressors, which will help determine that the shortage of compressed air is not caused by machine wear or failure.

　　 The following is the procedure for timing pumping test:

　　A. Volume of gas storage tank, cubic meter

　　 B. The volume of the pipeline between the compressor tanks, cubic meters

　　C. (A and B) total volume, cubic meters

　　D. Compressor running at full load

　　E. Close the air valve between the air storage tank and the factory air system

　　F. Vent the gas tank and reduce the pressure to 0.48MPa (G)

　　 G. Close the bleed valve soon

　　H. The time required for the gas storage tank to pump gas to 0.69MPa (G), seconds

　　 Now you have the data needed to determine the actual capacity of the existing compressor. The formula is:

　　The most complete version｜Basic theoretical knowledge of compressed air (1)

　　 In the formula:

　　C= Compressor air volume, m3/min

　　V=volume of gas tank and pipeline, m3 (item C)

　　P2=final unloading pressure, MPa (A) (H item + PA)

　　P1=Initial pressure, MPa (A) (F term + PA)

　　PA=Atmospheric pressure, MPa(A) (0.1MPa at sea level)

　　T= time, s

　　 If the calculated result of the test data is close to the rated air volume of your plant’s air compressor, you can be more certain that the load of your plant’s air system is too high and you need to increase the air supply.

　　 Two, estimation method

　　V=V air consumption of existing equipment + V air consumption of post-processing equipment + V leakage + V reserve

　　 Three, determine the required increase in compressed air

　　 According to the amount of air that increases the system pressure to the required pressure, the amount of compressed air supply that needs to be increased can be determined.

　　 Required m3/min= (existing m3/min) P1/P2

　　In the formula, the required m3/min=the required compressed air supply

　　 Existing m3/min = Existing compressed air supply

　　P2=Required system pressure, MPa (A)

　　P1=existing system pressure, MPa (A)

　　 Need to increase m3/min= Need m3/min－Existing m3/min

　　The result will tell you how much gas you need to increase to meet the existing gas demand. It is recommended to increase enough gas volume to not only meet the current gas demand, but also take future demand and leakage factors into consideration.

　　 Fourth, the impact of system leakage

　　 Insufficient air supply is often or definitely due to system leakage. Air system leakage is a continuous source of power loss, so it is best to make it as small as possible. Several small leakage points equivalent to 1/4 inch small holes may leak as much as 2.8M3 of compressed air at a pressure of 0.69MPa, which is equivalent to the loss of air volume of an 18.75Kw air compressor, with electricity per degree 0.4 yuan, 8000 hours of operation per year (three shifts) calculation, these leaked air cost you 60,000 yuan in vain.

　　 Most factories will provide maintenance personnel and parts to repair leaks. Damaged tool. Valves, packing, joints, droppers and hoses should be inspected and repaired in time. (90% of practitioners pay attention to the public account of Compressor Network)

　　The leakage of the entire plant system can be diagnosed by measuring the time required for the system pressure (on the upper side of the gas cylinder) to drop from 0.69MPa(G) to 0.62MPa(G) without air supply. Using the pump air test, we can calculate the leakage of the entire system:

　　The most complete version｜Basic theoretical knowledge of compressed air (1)

　　 If the air leakage rate exceeds 5 percent of the air volume of the entire system, a leak must be built.

　　 5. Select the compressor specifications

　　 Once you determine the air volume (m3/min) and pressure (MPa(G)) requirements of the factory, you can select the specifications of the air compressor. Factors you may consider when choosing include:

　　 What is the current gas consumption? What is the gas consumption requirement after the factory expansion? Generally speaking, the annual growth rate of gas consumption is 10%. Do you consider using special manufacturing processes and tools in the future?

　　The ideal approach is that the specifications of rotary screw compressors and centrifugal compressors should ensure normal operation within the modulation and adjustment control range.

The specifications determined by the single-acting air-cooled reciprocating air compressor should guarantee a 30-40% unloading time based on the constant speed control system.

　　 water-cooled reciprocating air compressor can work continuously, but it is best to consider a 20-25% buffer or unload time when selecting specifications.

　　 Research the performance characteristics of various types of air compressors to estimate power costs, so as to determine which one is the best choice to meet your plant's current and future requirements.

　　Is the factory leaking serious? Do you need to build a leak plan so that the load on the compressed air system can ultimately be reduced?

　　 Are you satisfied with the operation, maintenance, installation and performance characteristics of the selected air compressor?

　　 Have you considered the quality requirements of compressed air when selecting the air compressor and its additional equipment (such as dryers and filters)?

　　How does the additional equipment affect your choice of air compressor?

　　 Have you considered the spare air volume in case the main air compressor fails?

　　 Does each shift need to use the same amount of compressed air?

　　How does the selected air compressor work when the air consumption is low?

　　You may want to consider using a smaller air compressor to save energy and avoid excessive circulation and wear of the main air compressor.

　　 Does the factory have unusual intermittent peak demand loads that need to be considered?

　　……

　　-To be continued-