Piston Screw Compressor.doc

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Chapter 8 Piston Compressors and Screw Compressors Section 1 Overview 1. Characteristics of Piston Compressors Piston compressors belong to the category of positive-displacement compressors and are suitable for medium and low gas flow rates. Their discharge pressure can range from low to ultra-high levels. Compared with other types of compressors, they have a number of advantages: Their advantages include: (1) The desired pressure can be achieved regardless of the flow rate; the discharge pressure range is wide, with maximum pressures reaching 320 MPa in industrial applications and even 700 MPa in laboratory settings. (2) The capacity of a single unit can handle any flow rate below 500 m³/min. (3) Within normal pressure ranges, the requirements for materials are low, with ordinary steel materials being commonly used. (4) They have high thermal efficiency; the adiabatic efficiency of large and medium-sized units generally ranges from 0.7 to 0.85. (5) When adjusting the gas flow rate, the discharge volume is hardly affected by changes in discharge pressure. (6) The density and properties of the gas have little impact on the compressor’s performance, allowing the same compressor to be used for different gases. (7) The driving mechanism is relatively simple, usually involving electric motors, and speed control is not typically required. Due to these advantages, piston compressors are widely used in industry. However, they also have some disadvantages: (1) They have a complex and bulky structure, many wear-prone components, require a large amount of space, involve high investment costs, demand significant maintenance efforts, and have a shorter service life, although this can be extended to over 8,000 hours with proper care. (2) Their rotational speed is not high, the machines are large and heavy, and the discharge volume of a single unit is generally less than 500 m³/min. (3) Vibration occurs during operation. (4) The gas discharge is discontinuous, with pulsations in the airflow, which can cause vibrations in the pipelines. In severe cases, these pulsations and resonance can lead to damage to the piping system or components. (5) Flow rate adjustment is achieved using auxiliary volumes or bypass valves, which are simple, convenient, and reliable, but they result in high power losses and reduced efficiency under partial load conditions. (6) In compressors that use oil for lubrication, the oil present in the gas must be removed. (7) In large factories where multiple compressor units are used, this requires a large number of operators or leads to high workloads. 2. Types of Piston Compressors There are various types of piston compressors, which can generally be classified in the following ways: (1) Classification by discharge pressure: Low-pressure compressors: 0.2