12-03-2011, 04:53 PM
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INTRODUCTION
Steam ejectors are designed to convert the pressure energy of a motivating fluid to velocity energy to entrain suction fluid and then to recompress the mixed fluids by converting velocity energy back into pressure energy. This is based on the theory that a properly designed nozzle followed by a properly designed throat or venturi will economically make use of high pressure fluid to compress from a low pressure region to a higher pressure. This change from pressure head to velocity head is the basis of the jet vacuum principle.
The nozzle provides controlled expansion of the motive steam to convert pressure in to velocity which creates a vacuum with in the body chamber to draw in and entrain gases or vapours. The motive steam and suction gas are then completely mixed and then passed through the diffuser or tail, where the gases velocity is converted in to sufficient pressure to meet the predetermined discharge pressure.
Vacuum Ejectors are used in a variety of applications in the process, food, steel and petrochemical industries. Typical duties involve filtration, distillation, absorption, mixing, vacuum packaging, freeze drying, dehydrating and degassing. Ejectors will handle both condensable and non-condensable gas loads as well as small amounts of solids or liquids, however accidental entrainment of liquids can cause a momentary interruption in vacuum but this will not cause damage to the ejector
Ejectors are generally categorized into one of four basic types: single-stage, multi-stage non-condensing, multi-stage condensing and multi-stage with both condensing and non-condensing stages.
Components of Steam-jet Vaccum Ejector:
The main components or basic parts that Steam-jet Vaccum Ejector consists are:
1. Nozzle
2. Mixing chamber
3. Diffuser
2. EJECTOR:
Basic Construction:
Ejectors are composed of three basic parts: a nozzle, a mixing chamber and a diffuser. The diagram below left illustrates a typical ejector. A high pressure motivating fluid (Ma) and (Mb) enters at 1, expands through the converging-diverging nozzle to 2. The suction fluid (Mb) enters at 3, mixes with the motivating fluid in the mixing chamber 4. Both Ma and Mb are then recompressed through the diffuser to 5. The pressure and velocity changes are also shown graphically for the process directly below the ejector diagram. The diagram below right shows the thermal changes on a Mollier diagram for a typical ejector using high pressure steam as the motivating fluid and saturated vapor as the suction fluid Ejectors are generally categorized into one of four basic types:
a. Single-stage
b. Multi-stage non-condensing
c. Multi-stage condensing
d. Multi-stage with both condensing and non-condensing stages.
3. EQUIPMENT ARRANGEMENT
Ejectors:
An ejector is a type of vacuum pump or compressor. Since an ejector has no valves, rotors, pistons or other moving parts, it is a relatively low-cost component is easy to operate and requires relatively little maintenance.
In a steam-jet ejector, the suction chamber is connected to the vessel or pipeline that is to be evacuated under vacuum. The gas that is to be induced into the suction chamber can be any fluid that is compatible with the steam and the components materials of construction.
The steam nozzle discharges a high-velocity jet across the suction chamber. This steam jet creates a vacuum which extracts air or gas from the adjoining vessel. As these gases are entrained in the steam, the mixture travels through the ejector into a venturi shaped diffuser. In the diffuser, its velocity energy is converted into pressure energy, which helps to discharge the mixture against a predetermined back pressure, either to atmosphere or to a condenser.
Since the capacity of a single ejector is fixed by its dimensions, a single unit has practical limits on the total compression and throughput it can deliver. For greater compression, two or more ejectors can be arranged in series. For greater throughput capacity of gas or vapor, two or more ejectors can be arranged in parallel.
In a multi-stage system, condensers are typically used between successive ejectors. By condensing the vapors before sending the stream on to the next stage, the vapor load is reduced. This allows smaller ejectors to be used, and reduces steam consumption.
Pre-condensers can be added to reduce the load on the first-stage ejector, and allow for a smaller unit. An after-condenser can also be added, to condense vapors from the final stage. Adding an after-condenser will not affect overall system performance, but may ease disposal of vapors.Ejectors may be installed at any angle. However, to keep condensate and any entrained solids from collecting, low points in the vacuum piping system should be avoided during design and installation.
Provisions should be made to ensure proper drainage of the ejector bodies, since any condensed steam or process vapors may reduce throughput capacity. Drain valves installed at low points can be either manual or automatic, depending on customer requirements, and the drain cycle must relate to the type of process: Batch systems should be drained before each cycle, while continuous processes may be drained during operation if needed.
In most cases, the ejector is an integral part of a steam-jet vacuum system, but it is not intended to provide physical support for the system. Adequate piping support should be provided to minimize external loads on the ejectors, since any misalignment will adversely affect system performance. In fact, care must be exercised during system design, so that external loads caused by thermal movement and mechanical loading are minimized. If the ejector or piping is steam jacketed to prevent ice buildup, its orientation will affect the operation and drainage of the jackets. To keep the jackets from filling with condensate, all inlet and outlet piping should be installed so that the jacket can be sufficiently drained.
In certain systems, vacuum processes produce varying amounts of solid carryover, which can deposit inside the ejector system. During ejector placement, access for cleaning must be maintained, especially if the potential for deposits exists.
3.1 EJECTOR MATERIALS OF CONSTRUCTION
Steam-jet ejectors are typically furnished in cast iron or steel with a nozzle of stainless steel. Due to the broad range of applications for steam jet vacuum equipment, the units are frequently specified in special alloys and plastics. Steam Jet vacuum ejectors are readily available in ductile iron, steel, stainless steel and, on special order, in many more materials such as Monel, Alloy 20, Hastelloy, Silicon Carbide, Titanium, Bronze and others. They can also be made from a variety of nonmetals such as Haveg, Graphite and Teflon
3.2 EJECTOR EFFICIENCY
There are many accepted formulae to express ejector efficiency. Typically, efficiency involves a comparison of energy output to energy input. This ratio is of little value in the selection and design of ejectors. Since ejectors approach a theoretically isentropic process, overall efficiency is expressed as a function of entrainment efficiency. The direct entrainment of a low velocity suction fluid by a motive fluid results in an unavoidable loss of kinetic energy owing to impact and turbulence originally possessed by the motive fluid. This fraction that is successfully transmitted to the mixture through the exchange of momentum is called the entrainment efficiency.
That proportion of the motive fluid energy which is lost is transferred into heat and is absorbed by the mixture, producing there-in a corresponding increase in enthalpy. The following formula is based on steam handling saturated fluid.
EFF=Ec x En x Ed = [ Mb + 1
Ma
] [ H5 - H4
H1 - H2
]
Where:
Ec = entrainment efficiency
En = nozzle efficiency
Ed = diffuser efficiency
Mb = suction fluid--lb./hr.
Ma = motive fluid--lb./hr.
H1 = motive fluid enthalpy--btu./lb.
H2 = enthalpy at nozzle discharge--btu./lb.
H4 = mixture enthalpy before compression--btu./lb.
H5 = enthalpy at discharge-–btu./lb.
The capacity (#/Hr.) of an ejector handling other than saturated vapor is a function of the fluid’s molecular weight and temperature. The higher the molecular weight of a fluid, the greater the ejector suction capacity, assuming equal motivating quantities. Conversely, an ejector will handle less of lower molecular weight fluids. For example, a steam ejector will handle approximately 23% more free dry air than it will be saturated vapor. The reverse of this is true where suction fluid temperatures are concerned. The ejector will handle less fluid as the temperature of that fluid increases.
Ejectors operate optimally under a single set of conditions. Ejector designs can be classified either as critical or non-critical. Critical design means that the fluid velocity in the diffuse throat is sonic. In non-critical units the fluid velocity is subsonic. A steam ejector is of critical design when the suction pressure is lower than approximately 55% of the discharge pressure. Ejectors designed in the critical range are sensitive to operating conditions other than those for which the unit was designed. The table below illustrates how changes in operation can affect ejector performance:
