This chapter gives a brief explanation of the ejector working principle and shows how a refrigeration system can be built on it. A short history of the ejector follows. Finally, some applications within and outside of the refrigeration field are reviewed.
steam ejector working principle pdf download
Not all condensers maintain vacuum but definitely those working in boiler system do. Water is used to condense steam back to liquid water in any form of boiler system, whether on board ship or in power plants. As water and steam has a volume ratio of 1:1600; that means steam is condensed to the size of 1/1600 times than before. In other words, the specific volume of water at a given pressure is many times lower than that of steam. This enormous reduction in volume creates the vacuum inside the condenser. This partial vacuum will draw in more gas to be cooled. Now to ensure proper and continuous flow of steam to condensers at all times, vacuum must be maintained externally. In other words, it is impossible to prevent the entrance of air and other non-condensable gases into the condenser. A vacuum is maintained in the condenser so that steam can easily flow and more work can be extracted from the steam in the turbine; this is the reason why vacuum is maintained in condensers. Therefore, some methods must exist to initially cause a vacuum to exist in the condenser. This necessitates the use of vacuum-producing devices to establish and help maintain condenser vacuum.
Although the working pressure of a condenser do have many advantages including faster condensation, better heat exchange, avoid air to get dissolved in water and increased overall efficiency of the unit, there is a limit to which it is feasible and when it starts to make more trouble than it solves. Generally, maintaining perfect sealing condition in a condenser as the pressure reduces below one atmosphere is a hard goal to accomplish. This increase on the cost and maintenance requirement makes it ineffective in long run. Furthermore, as we reduce the pressure; the steam starts to condense at lower and lower temperature. As the temperature of condensing steam decreases with decrease in pressure, different problems emerge to maintain the same heat exchange between the medium. Ideally, the greater the temperature difference between the two medium, the better is the heat exchange efficiency of the condenser. Similarly, if we remove the vacuum from the system it not only increases the amount of heat energy lost unused; but also causes a rapid drop in PH of the boiler water which will lead to sudden drop in system efficiency in short run and complete damage of the turbine assembly in long run. So a condenser must never be run without vacuum unless it is safe.
The aim of this study is to reveal the complication of the flow and the mixing process of a steam ejector used in a jet refrigeration cycle by using the simulation software package (FLUENT). In Part 1 of this work, the CFD results of the steam ejector's performance were validated with the experimental values. After the validation is satisfied, this paper is able to analyze the flow phenomena inside the steam ejector when its operating conditions and geometries were varied. Using the applications provided by the CFD software, the flow structure of the modeled ejectors could be created graphically, and the phenomena inside the flow passage were explored. The CFD method was evaluated as an efficient tool to represent the flow inside a steam ejector.
An injector is a system of ducting and nozzles used to direct the flow of a high-pressure fluid in such a way that a lower pressure fluid is entrained in the jet and carried through a duct to a region of higher pressure. It is a fluid-dynamic pump with no moving parts except a valve to control inlet flow.A steam injector is a typical application of the principle used to deliver cold water to a boiler against its own pressure, using its own live or exhaust steam, replacing any mechanical pump. When first developed, its operation was intriguing because it seemed paradoxical, almost like perpetual motion, but it was later explained using thermodynamics.[1] Other types of injector may use other pressurised motive fluids such as air.
Depending on the application, an injector can also take the form of an eductor-jet pump, a water eductor or an aspirator. An ejector operates on similar principles to create a vacuum feed connection for braking systems etc.
An additional use for the injector technology is in vacuum ejectors in continuous train braking systems, which were made compulsory in the UK by the Regulation of Railways Act 1889. A vacuum ejector uses steam pressure to draw air out of the vacuum pipe and reservoirs of continuous train brake. Steam locomotives, with a ready source of steam, found ejector technology ideal with its rugged simplicity and lack of moving parts. A steam locomotive usually has two ejectors: a large ejector for releasing the brakes when stationary and a small ejector for maintaining the vacuum against leaks. The exhaust from the ejectors is invariably directed to the smokebox, by which means it assists the blower in draughting the fire. The small ejector is sometimes replaced by a reciprocating pump driven from the crosshead because this is more economical of steam and is only required to operate when the train is moving.
An empirical application of the principle was in widespread use on steam locomotives before its formal development as the injector, in the form of the arrangement of the blastpipe and chimney in the locomotive smokebox. The sketch on the right shows a cross section through a smokebox, rotated 90 degrees; it can be seen that the same components are present, albeit differently named, as in the generic diagram of an injector at the top of the article. Exhaust steam from the cylinders is directed through a nozzle on the end of the blastpipe, to reduce pressure inside the smokebox by entraining the flue gases from the boiler which are then ejected via the chimney. The effect is to increase the draught on the fire to a degree proportional to the rate of steam consumption, so that as more steam is used, more heat is generated from the fire and steam production is also increased. The effect was first noted by Richard Trevithick and subsequently developed empirically by the early locomotive engineers; Stephenson's Rocket made use of it, and this constitutes much of the reason for its notably improved performance in comparison with contemporary machines.
In practice, for suction pressure below 100 mbar absolute, more than one ejector is used, usually with condensers between the ejector stages. Condensing of motive steam greatly improves ejector set efficiency; both barometric and shell-and-tube surface condensers are used.
In parallel flow jet condensers, both the steam and water enter at the top and the mixture is removed from the bottom.The principle of this condenser is shown in the figure. The exhaust steam mixes up with the water and condensed. Condensate, cooling water and airflow downwards and are removed by two separate pumps known as an air pump and condensate pump. The condensate pump carries the condensate to the hot well.(b) Low-level Jet Condenser or Counter Flow Jet CondenserA low-level or counter-flow jet condenser is shown in the figure. In these types of steam condenser, the cooling water enters at the top and sprayed through jets. The steam enters at the bottom and mixes with the fine spray of cooling water. A separate pump removes the condensate. The air is removed by an air pump separately from the top. In a parallel flow type of this condenser, the cooling water and steam to be condensed move in the same direction. (i.e. from top to bottom).(c) High-level Jet Condenser (or) Barometric Jet CondenserA high-level jet condenser is shown in the figure. This is similar to the low-level condenser, except the condenser shell is placed at a height of 10.36 m (barometric height) above the hot well. In this condenser, the cooling water enters at the top and sprayed through jets. if(typeof ez_ad_units!='undefined')ez_ad_units.push([[300,250],'theengineerspost_com-large-mobile-banner-1','ezslot_7',692,'0','0']);__ez_fad_position('div-gpt-ad-theengineerspost_com-large-mobile-banner-1-0');The steam enters the bottom and mixes with the fine spray of cooling water. The column of water in the tailpipe forces the condensate into the hot well by gravity.
An ejector condenser is shown in the figure. In this condenser, cooling water under ahead of 5 to 6 m. enters at the top of the condenser and it is passed through a series of convergent nozzles. There is a pressure drop at the throat of the nozzle.The reduction in pressure draws exhaust steam into the nozzle through a non-return valve. Steam is mixed with water and condensed. In the converging cones, pressure energy is partly converted into kinetic energy. In diverging cones, the kinetic energy is partly converted into pressure energy. The pressure obtained is higher than atmospheric pressure and this forces the condensate to the hot well.2. Surface CondensersIn surface condensers, there is no direct contact between the cooling water and the steam that is to be condensed. The heat transfer between steam and cooling water is by conduction and convection. The condensate can be recovered for re-use as feed water.Types of Surface CondenserDownflow surface condenserCentral flow condenserRegenerative condenserEvaporative condenser(a) Downflow Surface Condenser (Two-pass surface condenser)The figure shows a two-pass downflow surface condenser. This arrangement is compact and the heat exchange is more efficient. The surface condenser has a great advantage over the jet condensers, as the condensate does not mix up with the cooling water. if(typeof ez_ad_units!='undefined')ez_ad_units.push([[300,250],'theengineerspost_com-large-mobile-banner-2','ezslot_8',675,'0','0']);__ez_fad_position('div-gpt-ad-theengineerspost_com-large-mobile-banner-2-0');As a result of this, the whole condensate can be reused in the boiler. This types of steam condenser can be used when the supply of cooling water is limited. It consists of a horizontal cast-iron cylindrical vessel packed with tubes, through which the cooling water flows.The ends of the condenser are cut off by vertical perforated type plates into which, the water tubes are attached. The condensate extraction pump, which is located at the bottom, creates suction. The exhaust steam enters from the top and flows over a nest of tubes.The cooling water enters at the bottom tubes and leaves through the upper half of the tubes. A section of tubes is screened by providing a baffle. This reduces the amount of water vapour escaping with air.(b) Central Flow CondenserIn the central flow condenser, steam enters the top of the condenser and flows downward. In this suction pipe of the air extraction pump at the centre of the tube nest.Due to this placement of the suction pipe at the centre of the tube nest, and the exhaust steam passes radially inside over the tubes towards the suction pipe. The condensate is collected at the bottom of the condenser and pumped into the hot well.(c) Regenerative CondenserIn the regenerative surface condenser, the condensate is heated using the regenerative method. In it, the condensation passes through the exhaust steam emitted from the turbine or engine. It raises its temperature and is utilised as feedwater for boilers.if(typeof ez_ad_units!='undefined')ez_ad_units.push([[300,250],'theengineerspost_com-leader-2','ezslot_9',676,'0','0']);__ez_fad_position('div-gpt-ad-theengineerspost_com-leader-2-0');(d) Evaporative condenserEvaporative condenser is another type of surface condenser. When the supply of cooling water is limited, evaporating the circulating water under small partial pressure can reduce its quantity required for condensing the steam. This principle is employed in evaporative condensers.The exhaust steam from the steam engine or steam turbine enters at the top of a series of pipes outside of which a film of cold water is falling. At the same time, a stream of air rotates above the water film, causing rapid evaporation of some of the cooled water. 2ff7e9595c
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