RAC Lecture 12 - Lesson 12 Multi-Stage Vapour Compression...

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Lesson 12 Multi-Stage Vapour Compression Refrigeration Systems Version 1 ME, IIT Kharagpur 1
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The objectives of this lesson are to: 1. Discuss limitations of single stage vapour compression refrigeration systems ( Section 12.1 ) 2. Classify multi-stage systems ( Section 12.1 ) 3. Discuss the concept of flash gas removal using flash tank ( Section 12.2 ) 4. Discuss the concept of intercooling in multi-stage vapour compression refrigeration systems ( Section 12.3 ) 5. Discuss multi-stage vapour compression refrigeration systems with flash gas removal and intercooling ( Section 12.4 ) 6. Discuss the use of flash tank for flash gas removal only ( Section 12.5 ) 7. Discuss the use of flash tank for intercooling only ( Section 12.6 ) At the end of the lesson, the student should be able to: 1. Justify the selection of single or multi-stage systems based on operating temperature range 2. Classify multi-stage systems 3. Applying mass and energy balance equations, evaluate the performance of multi-stage vapour compression refrigeration systems with: a) Flash gas removal b) Intercooling c) Flash gas removal using flash tank and intercooling using flash tank and/or external intercooler d) Flash tank for flash gas removal only e) Flash tank for intercooling only, and f) A combination of any of the above 12.1. Introduction A single stage vapour compression refrigeration system has one low side pressure (evaporator pressure) and one high side pressure (condenser pressure). The performance of single stage systems shows that these systems are adequate as long as the temperature difference between evaporator and condenser ( temperature lift ) is small. However, there are many applications where the temperature lift can be quite high. The temperature lift can become large either due to the requirement of very low evaporator temperatures and/or due to the requirement of very high condensing temperatures. For example, in frozen food industries the required evaporator can be as low as –40 o C, while in chemical industries temperatures as low as –150 o C may be required for liquefaction of gases. On the high temperature side the required condensing temperatures can be very high if the refrigeration system is used as a heat pump for heating applications such as process heating, drying etc. However, as the temperature lift increases the single stage systems become inefficient and impractical. For example, Fig. 12.1 shows the effect of decreasing evaporator temperatures on T s and P h diagrams. It can be seen from the T s diagrams that for a given condenser temperature, as evaporator temperature decreases: Version 1 ME, IIT Kharagpur 2
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i. Throttling losses increase ii. Superheat losses increase iii. Compressor discharge temperature increases iv. Quality of the vapour at the inlet to the evaporator increases v. Specific volume at the inlet to the compressor increases As a result of this, the refrigeration effect decreases and work of compression increases as shown in the P h diagram. The volumic refrigeration effect also decreases rapidly as the specific volume increases with decreasing evaporator temperature. Similar effects will occur,
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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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RAC Lecture 12 - Lesson 12 Multi-Stage Vapour Compression...

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