The fundamental criteria for selecting a screw blower are cost-effectiveness, reliability, and safety.
First, consider the discharge pressure and air delivery volume. For general-purpose pneumatic applications, the standard discharge pressure is 0.7 MPa (the older standard was 0.8 MPa). Compressors with a discharge pressure of 0.5 MPa are currently available on the market, but they are impractical; the pressure margin is too narrow for pneumatic tools, rendering them unusable if the air transmission distance is even slightly long. Furthermore, from a design perspective, these units typically employ single-stage compression with an excessively high pressure ratio, which can lead to high discharge temperatures, carbon buildup in the cylinder, and potential accidents. If a discharge pressure exceeding 0.8 MPa is required, a custom-built unit is generally necessary; attempting to force a standard unit to operate at higher pressures is dangerous and can cause accidents.
Air delivery volume is a key parameter; the compressor's capacity should match the required output while allowing for a 10% margin. If the air demand exceeds the compressor's capacity, the discharge pressure will drop significantly as soon as pneumatic tools are engaged, rendering them inoperable. Conversely, blindly opting for excessive capacity is also a mistake; larger capacities require larger motors, resulting in higher purchase costs and wasted electricity during operation.
Additionally, selection should account for peak, normal, and off-peak usage levels. If off-peak demand is high while normal and peak demands are relatively low, a common international practice is to connect multiple smaller compressors in parallel. Units are activated sequentially as demand increases; this approach benefits the power grid and saves energy.
Second, consider the specific application environment and conditions. For instance, a vertical model is suitable for confined spaces (such as on ships or vehicles), while a mobile unit should be considered if the air supply needs to cover long distances (exceeding 500 meters). A diesel-driven model is required if mains electricity is unavailable, and an air-cooled model is necessary if a water supply is not accessible. Users often hold misconceptions regarding air-cooled versus water-cooled systems, mistakenly believing that water cooling is superior; in reality, this is not the case. Air-cooled models account for over 90% of small compressors both domestically and internationally, primarily because the design is simple and they do not require a water supply during operation.
Conversely, water-cooled compressors suffer from four critical drawbacks: they require a comprehensive water supply and drainage system, entailing high investment costs; the water-cooled heat exchangers have a short service life; the cylinders are prone to freeze damage during winters in northern regions; and significant amounts of water are wasted during normal operation.
Thirdly, one must consider the quality of the compressed air. Compressed air produced by standard compressors typically contains traces of lubricating oil and moisture. In applications where oil and water are strictly prohibited, careful compressor selection is essential, and the addition of auxiliary equipment may be necessary.
Solutions include: First, selecting an oil-free compressor. These units operate with virtually no oil in the cylinder, utilizing piston rings and packing made primarily of PTFE (polytetrafluoroethylene). However, they have downsides: poor lubrication leads to higher failure rates, and PTFE is considered a hazardous substance, making these compressors unsuitable for the food and pharmaceutical industries. Furthermore, while oil-free compressors ensure oil-free output, they do not eliminate moisture. A second, commonly used approach involves equipping the compressor (regardless of type) with one or two stages of purification or drying equipment. Such systems can remove both oil and moisture, reducing their content in the compressed air to below 5 ppm, thereby meeting process requirements.
Fourthly, operational safety must be prioritized. As compressors operate under pressure and generate heat, safety is paramount. National regulations mandate a standardized "dual-certificate" system for compressor manufacturing, requiring both a compressor production license and a pressure vessel (air receiver tank) production license. Therefore, when purchasing a compressor, it is crucial to rigorously verify these two certificates. Manufacturers holding these licenses typically maintain robust quality assurance systems, minimizing the risk of major defects; should issues arise, the manufacturer is responsible for providing the standard "three guarantees" (repair, replacement, and refund).