RAC Lecture 13 - Lesson 13 Multi-Evaporator And Cascade...

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Version 1 ME, IIT Kharagpur 1 Lesson 13 Multi-Evaporator And Cascade Systems
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Version 1 ME, IIT Kharagpur 2 The objectives of this lesson are to: 1. Discuss the advantages and applications of multi-evaporator systems compared to single stage systems ( Section 13.1 ) 2. Describe multi-evaporator systems using single compressor and a pressure reducing valve with: a) Individual expansion valves ( Section 13.2.1 ) b) Multiple expansion valves ( Section 13.2.2 ) 3. Describe multi-evaporator systems with multi-compression, intercooling and flash gas removal ( Section 13.3 ) 4. Describe multi-evaporator systems with individual compressors and multiple expansion valves ( Section 13.4 ) 5. Discuss limitations of multi-stage systems ( Section 13.5 ) 6. Describe briefly cascade systems (Section 13.6) 7. Describe briefly the working principle of auto-cascade cycle ( Section 13.7 ) At the end of the lecture, the student should be able to: 1. Explain the need for multi-evaporator systems 2. Evaluate the performance of: a) Multi-evaporator systems with single compressor and individual expansion valves b) Multi-evaporator systems with single compressor and multiple expansion valves 3. Evaluate the performance of multi-evaporator systems with multi- compression, intercooling and flash gas removal 4. Evaluate the performance of multi-evaporator systems with individual compressors and multiple or individual expansion valves 5. Evaluate the performance of cascade systems 6. Describe the working principle of auto-cascade systems 13.1. Introduction As mentioned in Chapter 12, there are many applications where refrigeration is required at different temperatures. For example, in a typical food processing plant, cold air may be required at –30 o C for freezing and at +7 o C for cooling of food products or space cooling. One simple alternative is to use different refrigeration systems to cater to these different loads. However, this may not be economically viable due to the high total initial cost. Another alternative is to use a single refrigeration system with one compressor and two evaporators both operating at 30 o C. The schematic of such a system and corresponding operating cycle on P-h diagram are shown in Figs. 13.1(a) and (b). As shown in the figure the system consists of a single compressor and a single condenser but two evaporators. Both evaporators-I and II operate at same evaporator temperature (-30 o C) one evaporator (say Evaporator-I) caters to freezing while the other (Evaporator-II) caters to product cooling/space conditioning at 7 o C. It can be seen that operating the evaporator at – 30 o C when refrigeration is required at +7 o C is thermodynamically inefficient as the system irreversibilities increase with increasing temperature difference for heat transfer. The COP of this simple system is given by:
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) h h ( ) h h ( W Q Q COP 1 2 4 1 c II , e I , e = + = (13.1) In addition to this there will also be other difficulties such as: evaporator catering to space cooling (7 o C) may collect frost leading to blockage of air-flow
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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 13 - Lesson 13 Multi-Evaporator And Cascade...

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