Refrigeration & Air-Conditioning

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

Module 5. Multi-evaporator and compressor systems

Lesson 18
DUAL 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 P₃ to P₂ (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 E₁ and E₂ 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 E₁ and E₂ based on the capacity of these evaporators. The vapour is compressed from point h_m to h₂ 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, T₁ and T₂ are the refrigeration load on E₁ and E₂.

Power required for C₁ = m₁ (h₃-h₂) kW

Mass of refrigerant handled by C₂ compressor is given as under.

mC₂ = m₁ + m₂ + m_v

Where, m_v = mass of refrigerant vapour carried with (m₁ + m₂)

Power required for C₂ = mC₂ (h₄ – h₈) kW

Total power required for the system = (C₁ + C₂) kW

COP of the system= (3.5 (T₁+T₂))/(m₁ (h₃-h₂ )+mC₂ (h₄-h₈ ) )