In steam jet refrigeration systems, water can be used as the refrigerant. Like air, it is perfectly safe. These systems were applied successfully to refrigeration in the early years of this century. At low temperatures the saturation pressures are low (0.008129 bar at 4°C) and the specific volumes are high (157.3 m3/kg at 4°C). The temperatures that can be attained using water as a refrigerant are not low enough for most refrigeration applications but are in the range which may satisfy air conditioning, cooling, or chilling requirements. Also, these systems are used in some chemical industries for several processes, e.g. the removal of paraffin wax from lubricating oils. Note that steam jet refrigeration systems are not used when temperatures below 5°C are required. The main advantages of this system are the utilization of mostly low-grade energy and relatively small amounts of shaft work.
Steam jet refrigeration systems use steam ejectors to reduce the pressure in a tank containing the return water from a chilled water system. The steam jet ejector utilizes the energy of a fast-moving jet of steam to capture the flash tank vapor and compress it. Flashing a portion of the water in the tank reduces the liquid temperature. Figure 3.66 presents a schematic arrangement of a steam jet refrigeration system for water cooling. In the system shown, high-pressure steam expands while flowing through the nozzle 1. The expansion causes a drop in pressure and an enormous increase in velocity. Due to the high velocity, flash vapor from the tank 2 is drawn into the swiftly moving steam and the mixture enters the diffuser 3. The velocity is gradually reduced in the diffuser but the pressure of the steam at the condenser 4 is increased 5-10 times more than that at the entrance of the diffuser (e.g. from 0.01 bar to 0.07 bar).
This pressure value corresponds to the condensing temperature of 40°C. This means that the mixture of high-pressure steam and the flash vapor may be liquefied in the condenser. The latent heat of condensation is transferred to the condenser water, which may be at 25 °C. The condensate 5 is pumped back to the boiler, from which it may again be vaporized at a high pressure. The evaporation of a relatively small amount of water in the flash tank (or flash cooler) reduces the temperature of the main body of water. The cooled water is then pumped as the refrigeration carrier to the cooling-load heat exchanger.
An ejector was invented by Sir Charles Parsons around 1901 for removing air from steam engine condensers. In about 1910, the ejector was used by Maurice Leblanc in the steam ejector refrigeration system It experienced a wave of popularity during the early 1930s for air conditioning large buildings. Steam ejector refrigeration cycles were later supplanted by systems using mechanical compressors. Since that time, development and refinement of ejector refrigeration systems have been almost at a standstill as most efforts have been concentrated on improving vapor compression cycles (Aphornratana et al., 2001).
Furthermore, another typical gas-driven ejector is shown schematically in Figure 3.67a. High-pressure primary fluid (P) enters the primary nozzle, through which it expands to produce a low-pressure region at the exit plane (1). The high-velocity primary stream draws and entrains the secondary fluid (S) into the mixing chamber. The combined streams are assumed to be completely mixed at the end of the mixing chamber (2) and the flow speed is supersonic. A normal shock wave is then produced within the mixing chamber’s throat (3), creating a compression effect, and the flow speed is reduced to a subsonic value. Further compression of the fluid is achieved as the mixed stream flows through the subsonic diffuser section (b).
Figure 3.67b shows a schematic diagram of an ejector refrigeration cycle. It can be seen that a boiler, an ejector and a pump are used to replace the mechanical compressor of a conventional system. High-pressure and high-temperature refrigerant vapor is evolved in a boiler to produce the primary fluid for the ejector. The ejector draws vapor refrigerant from the evaporator as its secondary. This causes the refrigerant to evaporate at low pressure and produce useful refrigeration. The ejector exhausts the refrigerant vapor to the condenser where it is liquefied. The liquid refrigerant accumulated in the condenser is returned to the boiler via a pump whilst the remainder is expanded through a throttling valve to the evaporator, thus completing the cycle. As the working input required to circulate the fluid is typically less than 1 % of the heat supplied to the boiler, the COP may be defined as the ratio of evaporator refrigeration load to heat input to the boiler as follows:
Recently, Aphornratana et al. (2001) have developed a new jet ejector refrigeration system using R-ll as the refrigerant as shown in Figure 3.68. All vessels in the systems were constructed from galvanized steel. The boiler was designed to be electrically heated, two 4 kW electric heaters being located at the lower end. At its upper end, three baffle plates were welded to the vessel to prevent liquid droplets being carried over with the refrigerant vapor. The evaporator design was similar to that of the boiler. A single 3 kW electric heater was used to simulate a cooling load. A water-cooled plate type heat exchanger was used as a condenser. Cooling water was supplied at 32°C. The boiler was covered with a 40 mm thickness of glass wool with aluminum foil backing. The evaporator was covered with a 30 mm thickness of neoprene foam rubber. A diaphragm pump was used to circulate liquid refrigerant from the receiver tank to the boiler and the evaporator. The pump was driven by a variable speed 1/4 hp motor. One drawback of using the diaphragm pump is cavitation of the liquid refrigerant in the suction line due to pressure drop through an inlet check-valve. Therefore a small chiller was used to sub-cool the liquid R-11 before entering the pump. Figure 3.68c shows a detailed schematic diagram of the experimental ejector. The nozzle was mounted on a threaded shaft, which allowed the position of the nozzle to be adjusted. Two different mixing chambers with throat diameter of 8 mm were used: in mixing chamber no.l, the mixing section is constant area duct: in mixing chamber no.2, the mixing section is convergent duct.
Aphornratana et al.’s experiments showed that an ejector-refrigeration system using R-11 proved to be practical and could provide reasonably acceptable performance. It can provide a cooling temperature as low as -5°C. The cooling capacity ranged from 500 to1700 W with COP ranging from 0.1 and 0.25.