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## Lesson 18. Dual compression and individual compressors systems, compound compression

*Module 5. Multi-evaporator and compressor systems*

**Lesson 18DUAL COMPRESSION AND INDIVIDUAL COMPRESSORS SYSTEM, COMPOUND COMPRESSION**

**18.1 Introduction **

It is a compressor having a low pressure inlet valve, medium pressure inlet ports and high pressure discharge valve. Thus, the purpose of such compressor is to take vapour from two different evaporators which are at different pressures. The discharge of the compressed refrigerant is at some common discharge pressure. The dual compressor is used to replace two compressors which are required for two sources of refrigerant vapour. A simple form of the dual compressor is shown in **Fig. 18.1**. The compressor consists of low pressure suction valve, medium pressure suction ports and high pressure delivery valve.

**Fig. 18.1 Multi-evaporator and expansion with individual compressor system **

**Fig. 18.2** showing the expansion of liquid refrigerant from P_{3 }to P_{2 }(3-4) and vapour produced in the expansion process is directly passed to dual compressor. The corresponding P-H diagram is shown in **Fig. 18.3**.

**Fig. 18.2 Dual compressor with multi-expansion valve **

**Fig. 18.3 P-H diagram of dual compressor with multi-expansion valve **

A dual compressor with multi loads at different temperatures is shown in **Fig.** **18.4**. Evaporators are provided with individual expansion valves. Dual compressor pumps the vapour from E_{1 }and E_{2} evaporators and delivers to a condensing pressure. The corresponding P-H diagram is shown in **Fig. 18.5**. The analysis of such system can be done by calculating the mass flow rate in E_{1 }and E_{2} based on the capacity of these evaporators. The vapour is compressed from point **h _{m }**to h

_{2}and rise in enthalpy during compression gives the work of compression.

**Fig. 18.4 Dual compressor with multi-loads **

**Fig. 18.5 P-H diagram of dual compressor with multi-loads **

**18.2 Individual Compressor System**

Dual compressor pumps vapour refrigerant from two separate evaporators without affecting the performance of the system while individual compressor requires two compressors. A vapour compression refrigeration system consisting of multi-evaporators with multiple expansion valves having individual compressor is shown in **Fig. 18.6**.

**Fig. 18.6 Multi-evaporator and expansion with individual compressor system **

**Fig. 18.7 P-H diagram of Multi-evaporator and expansion with individual compressor system **

The corresponding P-H diagram is shown in **Fig. 18.7**. Assuming, T_{1} and T_{2 }are the refrigeration load on E_{1 }and E_{2}.

Power required for C_{1 }= m_{1 }(h_{3}-h_{2}) kW

Mass of refrigerant handled by C_{2 }compressor is given as under.

mC_{2} = m_{1} + m_{2 }+ m_{v}

Where, m_{v }= mass of refrigerant vapour carried with (m_{1} + m_{2})

Power required for C_{2 }= mC_{2} (h_{4 }– h_{8}) kW

Total power required for the system = (C_{1} + C_{2}) kW

_{1}+T

_{2}))/(m

_{1}(h

_{3}-h

_{2})+mC

_{2}(h

_{4}-h

_{8}) )