R&AC Lecture 19 - Lesson 19 Performance Of...

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Unformatted text preview: Lesson 19 Performance Of Reciprocating Compressors Version 1 ME, IIT Kharagpur 1 The specific objectives of this lecture are to: 1. Discuss the performance aspects of ideal reciprocating compressors with clearance, specifically: a) Effect of evaporator temperature on system performance at a fixed condenser temperature ( Section 19.1.1 ) b) Effect of condenser temperature on system performance at a fixed evaporator temperature ( Section 19.1.1 ) c) Effects of pressure ratio and type of refrigerant on compressor discharge temperature ( Section 19.1.3 ) 2. Discuss the performance aspects of actual compressor processes by considering: a) Effect of heat transfer in the suction line and compressor ( Section 19.2.1 ) b) Effects of pressure drops in the suction and discharge lines and across suction and discharge valves of compressor ( Section 19.2.2 ) c) Effect of refrigerant leakage ( Section 19.2.3 ) 3. Describe various methods of capacity control ( Section 19.3 ) 4. Discuss methods of compressor lubrication ( Section 19.4 ) At the end of the lesson, the student should be able to: 1. Describe qualitatively the effects of evaporator and condenser temperatures on performance of reciprocating compressors 2. Discuss the effects of heat transfer, pressure drops and refrigerant leakage on performance of actual compressors 3. Explain various methods of regulating the capacity of reciprocating compressors, and 4. Discuss aspects of compressor lubrication Version 1 ME, IIT Kharagpur 2 19.1. Ideal compressor with clearance: 19.1.1. Effect of evaporator temperature: The effect of evaporator temperature on performance of the system is obtained by keeping the condenser temperature (pressure) and compressor displacement rate and clearance ratio fixed. To simplify the discussions, it is further assumed that the refrigeration cycle is an SSS cycle. a) On Volumetric efficiency and refrigerant mass flow rate: The volumetric of the compressor with clearance is given by: [ ] (19.1) 1 r 1 P P 1 n / 1 p n / 1 e c cl , V − ε − = ⎟ ⎟ ⎠ ⎞ ⎜ ⎜ ⎝ ⎛ ε − ε + = η For a given condensing temperature (or pressure), the pressure ratio r p increases as the evaporator temperature (or evaporator pressure) decreases. Hence, from the expression for clearance volumetric efficiency, it is obvious that the volumetric efficiency decreases as evaporator temperature decreases. This is also explained with the help of Fig.19.1, which shows the P-V diagram for different evaporator pressures. As shown, as the evaporator pressure decreases, the volume of refrigerant compressed decreases significantly, since the compressor displacement remains same the clearance volumetric efficiency decreases as evaporator temperature decreases. In fact, as explained in the earlier lecture, at a limiting pressure ratio, the volumetric efficiency becomes zero....
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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 19 - Lesson 19 Performance Of...

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