Disassembly and analysis of core components of centrifugal compressors

Thread Content

The performance and reliability of centrifugal compressors depend heavily on the design and coordinated operation of their core components. The following is a detailed analysis of each core component, including the impeller, diffuser, volute, shafting, and sealing devices, with explanations based on their design principles, technical features, and applicable operating conditions: 1. Types and structural characteristics of the impeller: 1. Closed impeller: It has front and rear cover plates, with the blades enclosed within the flow channel ； Applicable operating conditions: high pressure ratio, high efficiency scenarios (such as chemical processes, large-scale air separation) ； Advantages: Low leakage loss, high efficiency ； Disadvantages: Complex to manufacture, heavy, and not resistant to solid particles. 2. Semi-open impeller: equipped only with a rear cover, with one side of the blades open ； Applicable operating conditions: medium to high pressure ratios, gases containing impurities or prone to scaling (such as natural gas compression, flue gas treatment) ； Advantages: Strong resistance to pollution, easy to clean ； Disadvantages: Slightly higher leakage loss and slightly lower efficiency. 3. Open impeller: No front or rear cover plates; the blades are directly fixed to the hub ； Applicable operating conditions: low pressure ratio, high flow rate, or gases with high solid content (such as in ventilators and sewage treatment) ； Advantages: Simple structure, low cost ； Disadvantages: High leakage loss and the lowest efficiency. Key design parameters A. Blade shape: 1. Backward-curved blades **(β₂90°): High pressure ratio, but prone to stall (requires design for specific operating conditions). B. Impeller material: high-strength alloy steel (such as 17-4 PH), titanium alloy (corrosion-resistant) or aluminum alloy (lightweight). II. Design principles and types of diffusers: 1. Vaneless diffuser: The free vortex flow of gas (𝑉𝜃𝑟=constant) undergoes natural expansion through an annular channel ； Advantages: Simple structure, adaptable to flow rate changes ； Disadvantages: Low diffuser efficiency and large size. 2. Bladed diffuser: Uses blades to guide the flow forcibly, control the angle of the airflow, and enhance the diffusion effect ； Advantages: High efficiency, compact size ； Disadvantage: Sensitive to conditions other than those designed for, prone to surge. 3. Wedge diffuser: The channel cross-section increases linearly, suitable for high Mach number flows ； Advantages: Suppresses shock waves, reduces losses ； Disadvantage: High requirements for processing precision. Performance impact 1. Diffuser angle (channel expansion angle): It is usually kept between 7° and 12°; too large an angle leads to flow separation, while too small an angle results in insufficient diffusion. 2. Mach number limit: The exit Mach number must be below 0.3~0.4 to avoid shock wave formation. III. Functions and Design Considerations of the Volute 1. Purpose: To collect the gas exiting the diffuser, further reduce its velocity and increase its pressure, and then direct the gas into the pipeline. 2. Profile design: A. Equal circulation design: The cross-sectional area of the volute decreases as the angle increases, to maintain conservation of the airflow’s angular momentum. B. Isobaric design: Optimize the cross-sectional area distribution to achieve uniform circumferential pressure and reduce flow losses. 3. Materials: cast iron (low cost), stainless steel (corrosion resistance) or welded steel (high-pressure applications). IV. Stability Design of the Shaft System 1. Critical Speed: The operating speed should be kept away from the first and second critical speeds; it is typically designed as a **rigid shaft** (with the operating speed being 1.3 times the first critical speed). 2. Bearing type: A. Sliding bearing (oil film bearing): Suitable for high loads and high speeds, requires a lubrication system. B. Rolling bearings: Used in small and medium-sized compressors, with easy maintenance. 3. Dynamic balancing: The impeller must be dynamically balanced in accordance with ISO, 1940, and G 2.5 standards to prevent excessive vibration. V. Types of Sealing Systems and Sealing Mechanisms 1. Labyrinth seal: Creates vortex resistance through multiple stages of tooth gaps to reduce leakage ； Advantages: contactless, heat-resistant ； Disadvantages: Allows for slight leakage; requires use in conjunction with a vacuum system. 2. Dry gas seal: Achieves non-contact sealing by utilizing the dynamic pressure effect of a micron-scale gas film (3–5 μm) ； Advantages: zero leakage, long lifespan ； Disadvantages: High cost, requires a clean gas source. 3. Carbon ring sealing: Sealing is achieved through the frictional contact between the carbon ring and the shaft ； Advantages: Simple structure ； Disadvantage: Wear requires regular replacement. |Selection reference 1: High-pressure conditions: Dry gas seals should be preferred (e.g., in ammonia synthesis compressors). 2. High-temperature dusty gases: labyrinth seal + nitrogen purging (e.g., gas turbine compressors). VI. Collaborative Work and System Optimization 1. Matching Design: The flow angle at the outlet of the impeller must match that at the inlet of the diffuser to avoid shock losses. 2. Surge control: Flow rate is adjusted through bypass valves or variable inlet guide vanes (VIGV) to avoid the surge zone. Efficiency improvement: A. Impeller: adopts a three-dimensional flow design (CFD-optimized blade profile). B. Diffuser: Adaptable adjustable blades (to accommodate changing operating conditions). VII. Conclusion: The performance of a centrifugal compressor depends on the precise design and coordinated operation of its various components: 1. Impeller: Determines the energy input and the ability to accelerate the gas ； 2. Efficiency of the diffuser and volute in converting kinetic energy into static pressure energy ； 3. The shafting and sealing systems ensure the stability and safety of long-term operation.