## Lesson 20. Working and mathematical analysis of above systems

Module 5. Multi-evaporator and compressor systems

Lesson 20
WORKING AND MATHEMATICAL ANALYSIS OF ABOVE SYSTEM

20.1 Introduction

Multi-evaporator vapour compression refrigeration systems are used where it is necessary to maintain different temperatures for the storage of products. For example, ice-bank evaporator operates at about -10 oC while evaporator of ice-cream cold storage is operated at -35 oC. Therefore, it is a basic requirement to maintain different temperature in dairy plants. Multi-compression (mostly two-stage compression) is usually required when compression ratio is above 7. It is obvious that multi-evaporator and multi-compressor system is employed to achieve different temperature requirement and higher compression ratio. In addition to this, sub cooling and inter-cooling of refrigerant become essential to improve the performance of the refrigeration plant. The actual mathematical analysis of the entire plant is based on the actual refrigeration cycle involved in the plant.

20.2 System with Multiple Expansion and Individual Compressor

The power requirement can be reduced by the use of an individual compressor for each evaporator and by the multiple expansion valves. A system consisting of two evaporators with individual compressor is shown in Fig. 20.1. Discharge gas from these compressors is combined and is condensed in the condenser of the plant. The liquid refrigerant is throttled to a pressure equivalent to evaporating temperature of E2 evaporator. The quantity of refrigerant throttled by this expansion valve is sum of the refrigerant flow rate needed for E2 and E1. This is known as multiple throttling of refrigerant. The corresponding P-H diagram is shown in Fig. 20.2.

Fig. 20.1 Multi evaporator and individual compressor system

Fig. 20.2 Multi evaporator and individual compressor system
on P-H diagram

System with multi-evaporator and multi compression

A system consisting of two evaporators and multi-compressor are shown in Fig. 20.3. The refrigerant from the receiver is supplied to E2 which is operating at higher temperature as compared to E1 (say -5 oC). At the same operating pressure, flash chamber is installed to inter-cool the vapour refrigerant after first stage of compression. The corresponding P-H diagram is shown in Fig. 20.4.

Fig. 20.3 Multi evaporator and individual compressor system on P-H diagram

Fig. 20.4 Multi evaporator and compressor system on P-H diagram

The mathematical analysis is just similar to previous case. The condition of refrigerant leaving E2 and the vapour coming from inter cooler is the same.