Air compressor
The compressor is a universal type of mechanical equipment used in industrial production. It is widely applied in various sectors such as petroleum, chemical industry, power, metallurgy, mining, construction, light industry, pharmaceuticals, civilian use and national defense, and holds an important position in the national economy. Therefore, it is of utmost importance to select the appropriate machine model that meets the production configuration requirements of the process. This can significantly reduce the purchase cost of the compressor, and is of great significance for saving energy, increasing production, reducing the burden on the enterprise, and increasing the enterprise's income.
The so-called rational selection of compressors means that comprehensive technical and economic indicators such as the investment and operation costs of the compressor unit and the compressor station should be taken into account. These indicators should conform to the principles of economy, safety and applicability. Specifically, there are the following aspects:
1.It is necessary to meet the flow and pressure requirements specified by the production process. That is to say, the operating condition point of the compressor (the intersection point of the device characteristic curve and the performance curve of the compressor) should be kept constantly within the efficient operating range. This not only saves power but also reduces the risk of component damage.
2. The selected compressor should not only be small in size, light in weight, and low in cost, but also possess excellent performance and high efficiency.
3. It has excellent anti-shudder performance, operates smoothly and has a long service life.
4. Simple structure, convenient operation, and easy availability of accessories.
5. The selected compressor station has a low initial investment and low operating costs.
The selection of an air compressor mainly depends on the working pressure and flow rate of the starting system. The working pressure of the air source should be about 20% higher than the maximum working pressure in the pneumatic system, as it needs to take into account the loss along the supply pipeline and local losses. If the working pressure requirements in some parts of the system are lower, a pressure reducing valve can be used to supply air. The rated exhaust pressure of the air compressor is divided into low pressure (0.7 - 1.0 Mpa), medium pressure (1.0 - 10 Mpa), high pressure (10 - 100 Mpa) and ultra-high pressure (above 100 Mpa), and can be selected according to actual needs. The common operating pressure is generally 0.7 - 1.25 Mpa.
Compressor performance characteristics:
① Air characteristics: Local atmospheric pressure, local air relative humidity, etc.
② The size and quantity of solid dust particles contained in the air.
③ Air temperature: (°C)
④ The required flow rate (Nm³/min or Nm³/h)
a. Flow rate, commonly referred to as discharge volume by domestic users. Flow rate refers to the amount of gas discharged by the air compressor within a unit of time under the required exhaust pressure. It is converted to the intake state for calculation.
b. The unit of flow is: m³/min (cubic meters/minute) or L/min (liters/minute), where 1 m³ (cubic meter) = 1000 L (liters).
c) Generally speaking, the commonly used unit for flow rate is: m³/min (cubic meters per minute).
d. Flow rate is also known as discharge volume or nameplate flow rate.
6. Pressure: Design pressure, Maximum working pressure, Working pressure.
There are many ways to represent pressure units. Here, we mainly introduce the commonly used pressure units for centrifugal air compressors.
① Work pressure, which is commonly referred to as exhaust pressure by domestic users, refers to the maximum pressure at which the air compressor discharges gas.
② The commonly used units for work pressure are: bar or MPa. 1 bar = 0.1 MPa.
③ Generally speaking, users usually refer to the pressure unit as: kg/㎡, and 1 bar = 1 kg/㎡.
7、Pipeline system data (including pipe diameter, length, types and quantities of pipeline accessories, etc.).
When designing and arranging the pipelines, the following points should be noted：
① Select the pipe diameter reasonably. A larger pipe diameter results in lower air velocity and less resistance loss under the same flow rate, but the price is higher. A smaller pipe diameter will lead to a significant increase in resistance loss, causing the power of the selected compressor to increase, and thus increasing the cost and operating expenses. Therefore, a comprehensive consideration should be made from both technical and economic perspectives.
②The gas supply pipeline and its pipe joints should be designed to withstand the maximum pressure they are capable of handling.
③ The pipeline layout should be designed to be as straight as possible. The number of fittings in the pipeline should be minimized and the length of the pipeline should be shortened as much as possible. When a turn is necessary, the bending radius of the elbow should be 3 to 5 times the diameter of the pipeline, and the angle should be as large as possible, preferably greater than 90 degrees.
④ At the elevated section of the compressor's pipeline, a supply valve and a check valve must be installed. The valve is used to control the operating condition of the compressor, while the check valve prevents the compressor from reversing when the air pressure suddenly increases and the pipeline pressure is higher than the compressor's outlet, thus preventing damage to the impeller and the reverse rotation of the motor.
When a user is about to purchase a compressor, the first step is to determine the working pressure required at the gas outlet, adding 1 to 2 bars as a margin. Then, select the pressure of the air compressor. (This margin takes into account the pressure loss due to the pipeline distance from the air compressor installation location to the actual gas outlet. The pressure margin should be appropriately considered within the range of 1 to 2 bars depending on the length of the pipeline.)
Of course, the size of the pipe diameter and the number of turning points are also factors that affect pressure loss. The larger the pipe diameter and the fewer the turning points, the smaller the pressure loss will be; conversely, the pressure loss will be greater.
Therefore, when the distance between the compressor and the gas-using pipelines is too large, the diameter of the main pipeline should be appropriately increased. If the environmental conditions meet the installation requirements of the air compressor and the working conditions permit, it can be installed nearby at the gas-using end.
① When choosing the air compressor flow rate, one should first understand the flow rates of all the gas-consuming equipment, and then multiply the total flow rate by 1.2 (i.e., increase by 20% as a safety margin).
A、If the minimum, normal and maximum flow rates have been specified in the production process, then the maximum flow rate should be considered.
B、If only the normal flow rate is provided in the production process, it is necessary to consider leaving a certain margin.
C、The unit of flow rate should be standardized. Currently, the flow rate units used in China are: Nm³/min or Nm³/h. Generally, for industrial compressors in the process flow, the leakage volume in the pipeline system can be ignored, but the impact of process changes on the flow rate must be considered.
② For the new project, the selection can be made based on the flow values provided by the design institute.
③ Inquire from the gas equipment supplier about the flow parameters of the gas equipment for equipment selection.
④ The renovation of the air compressor station can be carried out by referring to the original parameter values and making selections based on the actual gas usage situation.
In recent years, the compressor industry has developed many efficient and energy-saving products. However, in various fields of the national economy, due to improper selection, many compressors are operating in an inefficient manner, with low efficiency and waste of a large amount of energy. Some compressors, due to improper selection, cannot be used at all, or the maintenance and repair costs increase, resulting in low economic benefits. Therefore, appropriate selection can protect the equipment, extend the service life of the equipment, reduce the number of maintenance times, lower operating costs, save energy consumption, and avoid the number of unplanned shutdowns.
