R&AC Lecture 21 - Lesson 21 Centrifugal Compressors...

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Lesson 21 Centrifugal Compressors Version 1 ME, IIT Kharagpur 1
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The specific objectives of this lesson are to: 1. Explain the working principle of a centrifugal compressor ( Section 21.1 ) 2. Present the analysis of centrifugal compressors ( Section 21.2 ) 3. Discuss the selection of impeller diameter and speed of a centrifugal compressor using velocity diagrams ( Section 21.3 ) 4. Discuss the effect of blade width on the capacity of centrifugal compressor ( Section 21.4 ) 5. Discuss the methods of capacity control of a centrifugal compressor ( Section 21.5 ) 6. Discuss the performance aspects and the phenomenon of surging in centrifugal compressors ( Section 21.6 ) 7. Compare the performance of a centrifugal compressor with a reciprocating compressor vis- á -vis condensing and evaporator temperatures and compressor speed ( Section 21.6 ) 8. Describe commercial refrigeration systems using centrifugal compressors ( Section 21.7 ) At the end of the lecture, the student should be able to: 1. Explain the working principle of a centrifugal compressor with suitable diagrams 2. Analyse the performance of a centrifugal compressor using steady flow energy equation and velocity diagrams 3. Calculate the required impeller diameter and/or speed of a centrifugal compressor 4. Explain the limitations on minimum refrigeration capacity of centrifugal compressors using velocity diagrams 5. Explain the methods of capacity control of centrifugal compressor 6. Explain the phenomenon of surging 7. Compare the performance aspects of centrifugal and reciprocating compressors 21.1. Introduction: Centrifugal compressors; also known as turbo-compressors belong to the roto-dynamic type of compressors. In these compressors the required pressure rise takes place due to the continuous conversion of angular momentum imparted to the refrigerant vapour by a high-speed impeller into static pressure. Unlike reciprocating compressors, centrifugal compressors are steady-flow devices hence they are subjected to less vibration and noise. Figure 21.1 shows the working principle of a centrifugal compressor. As shown in the figure, low-pressure refrigerant enters the compressor through the eye of the impeller (1). The impeller (2) consists of a number of blades, which Version 1 ME, IIT Kharagpur 2
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form flow passages (3) for refrigerant. From the eye, the refrigerant enters the flow passages formed by the impeller blades, which rotate at very high speed. As the refrigerant flows through the blade passages towards the tip of the impeller, it gains momentum and its static pressure also increases. From the tip of the impeller, the refrigerant flows into a stationary diffuser (4). In the diffuser, the refrigerant is decelerated and as a result the dynamic pressure drop is converted into static pressure rise, thus increasing the static pressure further. The vapour from the diffuser enters the volute casing (5) where further conversion of velocity into static pressure takes place due to the divergent shape of the volute. Finally, the pressurized refrigerant leaves the compressor from the volute casing (6).
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This note was uploaded on 02/29/2012 for the course CHEMICAL 302 taught by Professor Nptel during the Spring '12 term at Birla Institute of Technology & Science.

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R&AC Lecture 21 - Lesson 21 Centrifugal Compressors...

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