Refrigeration & Air-Conditioning

Module 5. Multi-evaporator and compressor systems

Lesson 19
COMPARISON OF COMPOUND COMPRESSION WITH SINGLE COMPRESSOR SYSTEM

19.1 Introduction

It becomes necessary to adopt compound compression when compression ratio is high in order to reduce input power for the refrigeration system. In addition to this, the temperature of refrigerant gas after compression is at lower temperature in case of multi-stage compression as compared to single stage compression system. Normally, single stage compressor is used for compression ratios of around 7.0 to 9.0 depending upon the capacity of the plant.

19.2 Removal of Flash Gas

The saturated liquid refrigerant is throttled through expansion valve from condensing pressure to evaporating pressure. In the process, 5-10 percent of liquid refrigerant becomes vapour depending on the extent of expansion of the refrigerant. This flash gas is removed by installing a tank which is known as flash chamber after the expansion valve as shown in Fig. 19.1. The separated flash gas is directly by-passed to the compressor which reduces the size of evaporator required in the system as pure liquid refrigerant enters in the evaporator. The arrangement has no effect on the thermodynamic cycle.

Fig. 19.1 Flash chamber

19.3 Inter-Cooling

It is necessary to cool the refrigerant between two stages of compression to get full advantage of multi-compression system. This process is called inter-cooling which is done by using refrigerant or combination of water and refrigerant. A two stage compressor with inter cooling using refrigerant is shown in Fig. 19.2. The corresponding cycle is indicated on P-H diagram in Fig. 19.3. The saving in work done by adopting two-stage compression with inter-cooling is shown in Fig. 19.4.

Fig. 19.2 Inter cooler in two stage compressor system

Fig. 19.3 Inter cooling on P-H diagram

Fig. 19.4 L P-H diagram of two stage compression with inter cooling

Multi evaporator and single compressor system

A refrigeration system having three evaporators operating at different temperatures and single compressor is shown in Fig. 19.5

Fig. 19.5 Multi evaporator and single compressor system

Three evaporators, E₁, E₂ and E₃ having T₁, T₂ and T₃ ton cooling capacity respectively are provided with individual expansion valve as operating temperatures are different to achieve storage conditions in cold storages. E₂ and E₃ are provided with back pressure reducing valve in order to reducing the pressure of the gas to the operating pressure of E₁ as it is operating at lower pressure. The corresponding P-H diagram is shown in Fig. 19.6.

Fig. 19.6 Multi evaporator and single compressor system on P-H diagram

Mass flow rate in E₁, E₂ and E₃ is calculated as under.

Mass Flow rate in E₁,m₁ = (3.5T₁)/(h₁₁- h₁₀ ) kg/s

Mass Flow rate in E₂,m₂ = (3.5T₂)/(h₈- h₇ ) kg/s

Mass Flow rate in E₃,m₃ = (3.5T₃)/(h₅- h₄ ) kg/s

By taking the heat and mass balance of the refrigerant vapour leaving each evaporator, the value of h₁ can be calculated as under.

m₁ h₁₁ + m₂ h₉ + m₃ h₆ = (m₁ + m₂ + m₃) h₁

∴ h₁ = m₁ h₁₁ + m₂ h₉ + m₃ h₆ )/ m₁ + m₂ + m₃

Work done of compressor and C.O.P. of the plant can be calculated as under.

kW = (m₁+m₂+m₃) (h₂ - h₁)

And

C.O.P.= 3.5 (T₁+ T₂+ T₃))/ (m₁ + m₂ + m₃) (h₂- h₁))

It is very essential to find enthalpy values from P-H diagram or refrigerant properties tables corresponding to the operating conditions of the plant to carry out energy analysis of the system. If each evaporator is provided with individual compressor, then determination of kW and C.O.P. is in the line with simple vapour compression refrigeration system. Power requirement of E₁, E₂ and E₃ can be calculated separately taking corresponding work of compression per kg of refrigerant and individual mass flow rate of refrigerant of each evaporator.