ABOUT EJECTOR REFRIGERATION TECHNOLOGIES
The earliest ejector refrigerators dating from the early 1900s used water as their working fluid. Because they have few moving parts ejector systems can be very reliable. In the early systems high temperature sources were needed to power them and the ejector used in some cases were physically large, with a scale similar to ejector used today in chemical plant. The need for physically large ejector in the past confined their application to air-conditioning large buildings and ships. Therefore, after their first wave of popularity in the 1930s, steam ejector refrigeration units were supplanted by more compact electricity powered vapour compression machines.
The main drawbacks of steam ejector refrigeration systems are:
- Low coefficient of performance (COP).
- The high temperature source needed to power them.
- Deep vacuum in evaporators and condensers, necessity to have additional system to evacuate air from condenser of ejector machine.
- The large dimensions and weight of the ejector refrigeration machine (ERM) and ejectors used.
- Unable to generate refrigeration below 0 °C
Many attempts have been made to make ejector refrigeration systems viable by using more suitable low-boiling (halocarbon compound) refrigerants. Although the ejector cycle, using halocarbon compounds have several important advantages over steam-jet refrigeration systems, most halocarbon refrigerants damage the ozone layer. Therefore, one important area of research at this time aims to find the most suitable environmentally friendly (preferably natural) low boiling refrigerants for ejector refrigeration machine in terms of thermal efficiency, economic viability and environmental safety.
The basic components of an ejector refrigeration machine include an ejector, a generator, an evaporator, a condenser, a thermo-expansion valve and a feed pump. Figure 1 shows the arrangement of these components.
Figure 1. Schematic diagram of ERM
The working principles are as follows: Liquid refrigerant is heated in the generator by low-grade heat energy Qg to produce vapour at relatively high pressure Pg. This vapour with a mass flow rate mp accelerates through the primary convergent-divergent nozzle of the ejector so that at its exit it creates low-pressure region due to its supersonic velocity. This low pressure region causes vapour at low pressure Pe and with a mass flow rate ms is drawn from the evaporator into the ejector. The primary and secondary fluids combine within the mixing section from where they undergo a pressure recovery process in the diffuser. The combined stream flows to the condenser where it condenses at an intermediate pressure Pc. The heat of condensation Qc is rejected to the environment. Some of the condensate is pumped back to the generator via the feed-pump whilst the remainder returns to the evaporator via an expansion valve. The liquid that returns to the evaporator where it absorbs heat from a low temperature source to produce the necessary cooling effect Qe by generating low pressure vapour, which flows to the ejector.
a) Constant area ejector
b) Constant pressure ejector
Figure 2. Schematic diagram of supersonic ejector
The ejector is the key component in the ejector refrigeration machine. Ejector is a jet compressor, using gas-dynamic compression effect, intended to suck the refrigerant vapor from the evaporator and then to compress it and to force up to the condenser. The ejector executes simultaneously the turbine and the compressor functions, but being much simpler in structure and having no moving parts.
Figure 2 shows classical structures of supersonic ejector: constant area ejector (a) and constant pressure ejector (b). The ejector assembly can be divided into four parts: a supersonic nozzle (de Laval nozzle), suction chamber for the secondary fluid flow, a constant area or constant pressure mixing chamber and a sub-sonic diffuser. Supersonic ejectors are simple mechanical device in which mechanical energy transfer from higher to lower level of two fluid streams to perform the thermal compression action within the jet-pump refrigeration system. The fluid with higher energy is the primary stream that flows through the primary nozzle to reach supersonic velocity at the nozzle exit. A secondary stream is drawn into the mixing chamber by an entrainment effect; it is accelerated to sonic velocity and then is mixed with the primary stream in the mixing chamber. The mixing process ended by a normal shock system at the end of mixing chamber, where the mixed stream pressure increases and velocity decreases to subsonic value. Within the diffuser more pressure is recovered to reach the condenser pressure.