A Seminar Report On
Working Principal of Steam jet refrigeration system Submitted by Amit Prakash Roll No-1126004
Department of Mechanical Engineering National Institute of Technology, Patna Patna-800005, Bihar (India)
Candidate Declaration
This is to certify that seminar report entitled Working principle of Steam jet refrigeration system has been prepared by me under the supervision of
Dr. Amarnath Sinha Professor of Mechanical Engg. Department N.I.T, Patna has not been submitted by me elsewhere for award of any degree
Amit Prakash Roll.No.1126004 Student of second semester M.Tech (Mechanical Engg.) N.I.T., Patna
CONTENTS
Introduction Principle Working Steam jet ejector Advantage and Disadvantage Application Recent Development in Steam jet refrigeration system Conclusion Refrence
INTRODUCTION
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. here water is used as the refrigerant and the cooling effect is produced in the steam jet refrigeration continuous cycle by vaporization of a part of the water in the evaporator at a low absolute pressure level. The steam jet refrigeration system (also known as ejector system refrigeration system) is one of the oldest methods of producing refrigeration effect. The basic components of this system are an evaporator, a compressor device, a condenser and a refrigerant control device. This system employs a steam injector or booster (instead of mechanical compressor) to compress the refrigerant to the required condenser pressure level. In this system, water is used as the refrigerant. Since the freezing point of water is 0°C, therefore, it cannot be used for applications below 0°C. The steam jet refrigeration system is widely used in food processing plants for precooling of vegetables and concentrating fruit juices, gas plants, paper mills, breweries etc.
Principle of steam jet refrigeration system: -
The boiling point of a liquid changes with change in external pressure. In normal conditions, pressure exerted on the surface of a liquid is the atmospheric pressure. If this atmospheric pressure is reduced on the surface of a liquid, by some means, then the liquid will start boiling at lower temperature, because of reduced pressure. This basic principal of boiling of liquid at lower temperature by reducing the pressure on its surface is used in steam jet refrigeration system. The boiling point of pure water at standard atmospheric pressure of 760 mm of Hg is 1 00°C. It may be noted that water boils at 12′C if the pressure on the surface of water is kept at 0.014 bar and at 7′C if the pressure on the surface of water is 0.01 bar. The reduced pressure on the surface of water is maintained by throttling the steam through the jets or nozzles.
WORKI NG OF STEAM JET REFRIGERATION SYSTEM 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 fastmoving 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. Steam flows through the ejector (or sometimes called an eductor or venturi) and creates a low pressure zone in the branch nozzle of the ejector. The low pressure nozzle is connected to a chamber filled with water.As the pressure continues to reduce in the water filled chamber the water begins to boil. This provides the cooling action. 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.
DIAGRAM OF STEAM JET REF. SYSTEM
Steam jet ejector
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. Highpressure primary fluid (P) enters the primary nozzle, through which it expands to produce a lowpressure 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. In the diverging portion of the diffuser the velocity head is recovered as pressure head and finally high pressure steam is condensed in condenser.
Figure shows steam jet ejector from refrigers.com
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, COP may be defined as the ratio of evaporator refrigeration load to heat input to the boiler.
ADVANTAGES AND DISADVANTAGES Advantages :
1. Simple construction 2. Vibration free system 3. Highly reliable 4. Low production cost 5. Water is used as a refrigerant which is perfectly safe. 6. Low running cost 7. Load variation quickly adjusted 8. It is suitable for places where steam is available such as process industry, steel plant, petroleum plants, thermal power plants etc. Disadvantages
1. Not suitable for water temperature below 4C. 2. The maintenance of high vacuum in the evaporator is necessary for proper functioning of the system. 3. Since the ejected vapour and motive steam condensed in the condenser, the magnitude of heat transfer is about 3 to 4 times that needed in the vapour compression system per tone per hour. Thus large size condenser is needed. 4. COP is very low 5. Eff. of ejector is not good.
APPLICATION
1. Food processing plant 2. Gas plants 3. Breweries 4. Rubber and vulcanizing industries 5. Paper and pulp industries
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 32В°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 ex periments 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 - 5В°C. The cooling capacity ranged from 500 to1700 W with COP ranging from 0.1 and 0.25.
CONCLUSION
Steam jet refrigeration system were already being considered a few years ago for specialized applications, where their simplicity, lack of lubrication, low production cost, low maintenance and their use of environmentally harmless working fluids as steam were adequate compensation for their lower efficiencies. This latest breakthrough, steam jet refrigeration using R11 refrigerant. the ejected vapour and motive steam condensed in the condenser, the magnitude of heat transfer is about 3 to 4 times that needed in the vapour compression system per tone per hour. Thus large size condenser is needed. Eff. of ejector is not good and it is not used where water temperature is below 4 C.Thus conclusion of steam jet refrigeration system it is used where steam is available such as thermal power plant, process industry, petroleum industry etc. This latest breakthrough, steam jet refrigeration using R11 refrigerants The latest achievements of the former are certainly encouraging, but there are still much left to be done.
REFRENCE
1. Refrigeration and air conditioning by R.K.Rajput 2. Refrigeration and air conditioning by R.S.Khurmi 3. Figures and steam ejector to pics from refrigers.com
