## Lesson 17. Importance of multiple evaporator and compressor systems, multi evaporator and one compressor systems.

Module 5. Multi-evaporator and compressor systems

Lesson 17
IMPORTANCE OF MULTIPLE EVAPORATOR AND COMPRESSOR SYSTEMS, MULTI EVAPORATOR AND ONE COMPRESSOR SYSTEMS

17.1 Introduction

There are many applications in dairy and food plants where refrigeration is required at different temperatures depending on the type of products to be stored. As instance, milk is normally stored at 3-4 o C while ice-cream storage requires –30 o C temperature. One simple alternative is to use different refrigeration systems to meet different requirement of temperatures for different cold storages. But, this may not be economically choice due to the high total initial cost of different systems. Another alternative is to use a single refrigeration system with one compressor and multi- evaporators operating at different temperatures as per the requirement of the products. It is also necessary to use multi-evaporator and two stage compression system if the compression ratio is more (>7).

17.2 Multi-Evaporator Systems With Single Compressor

17.2.1 Multi- evaporators operating at the same temperature

A vapour compression system having two evaporators operating at the same temperature with single compressor is shown in the Fig.17.1 and the corresponding cycle is presented on P-H diagram in Fig. 17.2. The refrigerant from receiver is supplied to evaporators through individual expansion valves and the vapour produced in evaporators is pumped by the compressor. The saturated suction vapour having h1 enthalpy is compressed to condensing pressure and the enthalpy the discharged vapour is h2.

C.O.P. of the system = 210 X (T1+T2),kJ/min / work of compression,kJ/min

Where, T1 = Capacity of E1 in ton

T2 = Capacity of E2 in ton

Mass flow rate of refrigerant (m1) in E1= 210 x T1 /h1- h4 kg/min

Mass flow rate of refrigerant (m2) in E2= 210 x T2 /h1- h4 kg/min

Thus, C.O.P. = 210 x (T1 + T2) / (m1 + m2) (h2 - h1)

Fig. 17.1 Two Evaporator with single compressor system

Fig. 17.2 Two Evaporator with single compressor system on P-H diagram

17.2.2 Multi- evaporators operating at different temperatures

This is a practical requirement to have different temperatures for storage of product in dairy and food factories. Ice bank evaporators operate at about -8 oC to -10 oC, while evaporator of ice-cream hardening room may operate at -35 oC. Therefore, it is very common under field conditions to use multi-evaporators maintained at different temperatures.

A block diagram of vapour compression refrigeration consisting of two evaporators operating at different temperature and one compressor system is shown in Fig. 17.3. The working cycle of the system is represented on P-H diagram in Fig. 17.4.

Fig. 17.3 Two Evaporators operating different temperature and one compressor system

Fig. 17.4 Working Cycle of two evaporators operating two different temperature and one

The refrigerant is supplied to two evaporators operating at different temperature through individual expansion valves. The saturated vapour leaving the lower temperature evaporator at point 7 is combined with the vapour coming from higher temperature evaporator. A pressure reducing valve is installed to reduce the pressure of the vapour corresponding to the pressure of lower temperature evaporator. The enthalpy of suction vapour can be obtained by taking heat and mass balance of the refrigerant vapour leaving the evaporators.

C.O.P. of the system = 210 x (T1+T2),kJ/min / work of compression,kJ/min

Where, T1 = Capacity of E1 in ton

T2 = Capacity of E2 in ton

Mass flow rate of refrigerant (m1) in E1= 210 x T1 /h6- h4 kg/min

Mass flow rate of refrigerant (m2) in E2= 210 x T2 /h7- h5 kg/min

Thus, C.O.P. = 210 x (T1 + T2) / (m1 + m2) (h2 - h1)

The enthalpy of the suction gas is determined as under.

m1 · h6 + m2· h7 = (m1 + m2) h1

h1 = (m1 + m2) / m1 · h6 + m2· h7

For calculating theoretical C.O.P. of such systems, it is necessary to read the values of enthalpy from refrigerant chart and P-H diagram. By calculating the enthalpy at point 1and locating that point on P-H diagram, the value of enthalpy at point 2 can be obtained from the refrigerant chart. The above analysis may deviate, if there is any superheating of vapour or sub-cooling of liquid refrigerant.

17.2.3 Multi-evaporator with a single compressor and multiple expansion valves

Fig. 17.5 and Fig. 17.6 show system schematic diagram and P-H diagram of a multi-evaporator with a single compressor and multiple expansion valves. It can be seen from the P-H diagram that the advantage of this system compared to the system with individual expansion valves is that the refrigeration effect of the low temperature evaporator increases as saturated liquid enters the low stage expansion valve. This is possible as the flash gas is removed at state 4.

Fig. 17.5 System schematic diagram of a multi- evaporator with a single compressor and multiple expansion valves

Fig. 17.6 System P-H diagram of a multi- evaporator with a single compressor and multiple expansion valves

C.O.P. of the system = 210 x (T1+T2),kJ/min / work of compression,kJ/min

Where, T1 = Capacity of E1 in ton

T2 = Capacity of E2 in ton

Mass flow rate of refrigerant (m1) in E1= 210 x T1 /h7- h4 kg/min

Mass flow rate of refrigerant (m2) in E2= 210 x T2 /h8- h6 kg/min

Thus, C.O.P. = 210 x (T1 + T2) / (m1 + m2) (h2 - h1)

The enthalpy of the suction gas is determined as under.

m1 · h6 + m2· h7 = (m1 + m2) h1

h1 = (m1 + m2) / m1 · h6 + m2· h7