Principles of screw air compressor selection
As an important power energy supply facility in industrial production, screw air compressors should be selected based on the principles of safety, reliability, economy, efficiency, and low installation and maintenance costs to ensure that they can serve production safely, stably, and efficiently.
First, according to the air pressure and air flow required by the user, select an screw air compressor with a suitable structure. Good mechanical performance (low vibration and low noise) when the screw air compressor is working, good adaptability under variable working conditions, and long-term stable operation are the basis for screw air compressor selection; secondly, the economic efficiency of the screw air compressor system operation should be an important indicator for screw air compressor selection, which includes comprehensive indicators such as the unit electricity consumption (kwh/km3) or unit steam consumption (t/km3) of the screw air compressor operation, the quality and water consumption of the cooling water required by the screw air compressor (t/km3), and the waste heat benefit of the screw air compressor; in addition, choosing the appropriate screw air compressor technical parameters (exhaust volume, exhaust pressure) is the premise of whether the screw air compressor can meet production needs and whether it can run economically; finally, the installation and maintenance costs of the screw air compressor should be one of the indicators for screw air compressor selection, and try to choose an screw air compressor with easy installation and low maintenance costs.
The selection of screw air compressors should refer to the following procedures:
(1) Investigate user needs (the air pressure, air flow, air temperature, air humidity, etc. required by the user end);
(2) Calculate the resistance between the air outlet of the screw air compressor and the user point;
(3) Determine the rated exhaust pressure of the screw air compressor (the rated exhaust pressure of the unit can be calculated according to 1.1 times the theoretical data), exhaust volume, exhaust temperature of the screw air compressor after the post-processing device, etc.;
(4) According to the requirements of the unit’s automated operation, select appropriate electronic control and automatic control systems;
(5) Draft the technical requirements of the screw air compressor to prepare for procurement;
(6) Conduct on-site inspections of screw air compressor manufacturers and users to understand the manufacturer’s production level and production capacity, and deeply understand the real feedback of screw air compressor users;
(7) Conduct bidding for screw air compressor procurement, formulate reasonable scoring standards, and select screw air compressor units with high cost performance through bidding;
(8) After the equipment contract is signed, conduct face-to-face technical document docking with the screw air compressor supplier to form an screw air compressor technical agreement as an annex to the contract.
3. Common problems and suggestions for screw air compressor selection
1. Lack of understanding of the structural performance of different types of screw air compressors will lead to unreasonable screw air compressor selection, which will directly affect the subsequent economic operation of the screw air compressor.
In general, the power consumption of multi-axis centrifugal machines, axial flow machines, ordinary single-axis centrifugal machines, screw machines, and plug-type screw air compressors increases in turn. For example, in the biological fermentation industry, the air pressure (absolute pressure) generally required is between 0.30MPa-0.40MPa. For screw air compressors above 1200Nm3/min, it is best to choose axial flow screw air compressors or multi-axis centrifugal units, which have better operating economy and low maintenance costs; for axial flow screw air compressors with adjustable stator blades, the advantage is that the adjustable range of working conditions is wide, and its optimal operating area is a curved surface. The unit can ensure that the unit is always at the best economic operating point under different loads. For instrument air with a small air demand, the air pressure (absolute pressure) is generally between 0.5-0.8MPa. Screw screw air compressors are usually selected instead of piston screw air compressors because screw screw air compressors have the advantages of compact structure, fewer wearing parts, stable operation, and good economy compared to piston screw air compressors.
2. Unreasonable selection of screw air compressor parameters causes the screw air compressor to be unable to operate at the optimal operating point, and the economic efficiency of the unit operation decreases.
For centrifugal screw air compressors, the pressure and flow marked on the nameplate are the operating points with the highest operating efficiency of the screw air compressor. Deviating from this operating point, the screw air compressor operation is uneconomical. In actual work, due to the inaccurate grasp of the pressure at the air demand point, coupled with the estimation of the air transmission resistance from the screw air compressor outlet to the user, for safety reasons, the exhaust pressure and exhaust volume of the screw air compressor are often estimated too high when drafting the screw air compressor bidding documents, resulting in a large deviation between the actual operating data and the design value of the unit. For example, a company ordered an screw air compressor with a rated exhaust pressure (absolute pressure) of 0.4MPa, but in actual operation, the exhaust pressure of the screw air compressor is only about 0.31MPa, and the power consumption of the unit is relatively high. Therefore, when determining the technical parameters of a new screw air compressor, it is necessary to find out the pressure at the air use point and the calculation of air flow resistance to ensure that the design parameters of the screw air compressor are consistent with the actual operation. Only in this way can the selected screw air compressor exert its operating efficiency.
3. The public conditions for the design of screw air compressors are harsh, which affects the safe and economical operation of screw air compressors.
For example, a company purchased a foreign screw air compressor with a flow rate of 855m3/min many years ago, and the exhaust pressure (absolute pressure) of the unit was 0.33MPa. The public conditions for the design of the screw air compressor require that the cooling water temperature of the interstage cooler be 5°C. In actual operation, the cooling water temperature is often higher than this temperature, resulting in a high secondary air intake temperature of the screw air compressor and a decrease in the unit efficiency. When 5°C water is used, the cost of 5°C cold water is high, resulting in a high air supply cost for the screw air compressor, and the unit cannot operate for a long time. To ensure the economic operation of the screw air compressor system, when designing the screw air compressor, users should provide public system data that is consistent with the site.
4. The design and installation of the screw air compressor post-processing device are unreasonable, the air resistance increases, the exhaust pressure of the screw air compressor increases, and the power consumption of the unit increases.
Some users purchase the screw air compressor body and the post-processing device separately. If the design capacity of the post-processing device manufacturer is insufficient, and only the cooling effect and production cost of the air are considered, 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, from the screw air compressor outlet to the air entering the air main pipe, the increase in the number of elbows will increase the air flow resistance, and reducing the number of elbows can reduce the air resistance; in addition, for large-flow screw air compressor post-processing devices, if the on-site space allows, two sets of devices can be used in parallel operation, which can effectively reduce the air flow resistance. The author recommends that the screw air compressor post-processing device and pipeline connections should be designed and provided by the screw air compressor manufacturer to ensure the subsequent economic and stable operation of the screw air compressor system.
Post time: Aug-08-2024
