Ejectors Water

There is considerable information available in the hterature on the design of ejectors (steam jet ejectors, water jet pumps, air injectors, etc.) supported by extensive experimental data. Most of this information deals with its use as an evacuator and the focus is on ejector optimization for maximizing the gas pumping efficiency. The major advantage of the venturi loop reactor is its relatively very high mass transfer coefficient due to the excellent gas-liquid contact achieved in the ejector section. Therefore, the ejector section needs careful consideration to achieve this aim. The major mass transfer parameter is the volumetric liquid side mass transfer coefficient, k a. The variables that decide k a are (i) the effective gas-hquid interfacial area, a, that is related to the gas holdup, e. The gas induction rate and the shear field generated in the ejector determine the vine of and, consequently, the value of a. (ii) the trae liquid side mass transfer coefficient, k. The mass ratio of the secondary to primary fluid in turn decides both k and a. For the venturi loop reactor the volumetric induction efficiency parameter is more relevant. This definition has a built in energy... 

EJECTOR-WATER-CHECK VAUIE RELIEF VALVE. 

Feed systems can be hybrids, combining dry or gas feed with liquid feed components. Dry and liquid systems usually include a dry-to-liquid conversion (using a dissolving tank) that subsequently combines this liquid with another liquid. Gas-liquid hybrid systems can mix the gas with water first or with another liquid chemical in a reaction chamber. Chlorine dioxide generators using chlorine gas are an example of this type of system. The chlorine dioxide formed in the reaction is ultimately mixed with ejector water, and this solution is then mixed with process water at the point of delivery. 

Humidification. For wiater operation, or for special process requirements, humidification maybe required (see Simultaneous HEAT and mass transfer). Humidification can be effected by an air washer which employs direct water sprays (see Evaporation). Regulation is maintained by cycling the water sprays or by temperature control of the air or water. Where a large humidification capacity is required, an ejector which direcdy mixes air and water in a no22le may be employed. Steam may be used to power the no22le. Live low pressure steam can also be released directly into the air stream. Capillary-type humidifiers employ wetted porous media to provide extended air and water contact. Pan-type humidifiers are employed where the required capacity is small. A water filled pan is located on one side of the air duct. The water is heated electrically or by steam. The use of steam, however, necessitates additional boiler feed water treatment and may add odors to the air stream. Direct use of steam for humidification also requires careful attention to indoor air quahty. 

The collection of particles larger than 1—2 p.m in Hquid ejector venturis has been discussed (285). High pressure water induces the flow of gas, but power costs for Hquid pumping can be high because motive efficiency of jet ejectors is usually less than 10%. Improvements (286) to Hquid injectors allow capture of submicrometer particles by using a superheated hot (200°C) water jet at pressures of 6,900—27,600 kPa (1000—4000 psi) which flashes as it issues from the nozzle. For 99% coUection, hot water rate varies from 0.4 kg/1000 m for 1-p.m particles to 0.6 kg/1000 m for 0.3-p.m particles. 

The prevacuum technique, as its name implies, eliminates air by creating a vacuum. This procedure faciUtates steam penetration and permits more rapid steam penetration. Consequendy this results in shorter cycle times. Prevacuum cycles employ either a vacuum pump/steam (or air) ejector combination to reduce air residuals in the chamber or rely on the pulse-vacuum technique of alternating steam injection and evacuation until the air residuals have been removed. Pulse-vacuum techniques are generally more economical vacuum pumps or vacuum-pump—condenser combinations may be employed. The vacuum pumps used in these systems are water-seal or water-ring types, because of the problems created by mixing oil and steam. Prevacuum cycles are used for fabric loads and wrapped or unwrapped instmments (see Vacuum technology). 

Ejector (steam-jet) refrigeration systems are used for similar apph-cations, when chilled water-outlet temperature is relatively high, when relatively cool condensing water and cheap steam at 7 bar are available, and for similar high duties (0.3-5 MW). Even though these systems usually have low first and maintenance costs, there are not many steam-jet systems running. 

Steam-Jet (Ejector) Systems These systems substitute an ejector for a mechanical compressor in a vapor compression system. Since refigerant is water, maintaining temperatures lower than the environment requires that the pressure of water in the evaporator must be... 

The condenser design, surface area, and condenser cooling water quantity should be based on the highest cooling water temperature likely to be encountered, if the inlet cooling water temperature becomes hotter then the design, the primaiy booster (ejector) may cease functioning because of the increase in condenser pressure. 

Capacity Control The simplest way to regulate the capacity of most steam vacuum refrigeration systems is to furnish several primary boosters in parallel and operate only those required to handle the heat load. It is not uncommon to have as many as four main boosters on larger units for capacity variation. A simple automatic on-off type of control may be used for this purpose. By sensing the chilled-water temperature leaving the flash tank, a controller can turn steam on and off to each ejector as required. 

Figure 1. A wide range of pressures can be achieved by using various combinations of ejectors and condensers. The same steam consumption is used for each design here. Note Curves are based on 85°F condensing water. If warmer water is used, curves shift to the left—cooler water, shift right.

Air is usually the basic load component to an ejector, and the quantities of water vapor and/or condensable vapor are usually directly proportional to the air load. Unfortunately, no reliable method exists for determining precisely the optimum basic air capacity of ejectors. It is desirable to select a capacity which minimizes the total costs of removing the noncondensable gases which accumulate in a process vacuum system. An oversized ejector costs more and uses unnecessarily large quantities of steam and cooling water. If an ejector is undersized, constant monitoring of air leaks is required to avoid costly upsets. 

The VPS overhead consists of steam, inerts, condensable and non-condensable hydrocarbons. The condensables result from low boiling material present in the reduced crude feed and from entrainment of liquid from the VPS top tray. The noncondensables result from cracking at the high temperatures employed in the VPS. Inerts result from leakage of air into the evacuated system. Steam and condensable hydrocarbons are condensed using an overhead water-cooled condenser. The distillate drum serves to separate inerts and non-condensables from condensate, as well as liquid hydrocarbons from water. Vacuum is maintained in the VPS using steam jet ejectors. 

Figure 6-3 illustrates a single-stage non-condensing ejector. In this type of installation the steam outlet from the ejector is either exhausted to atmosphere or on top of water in a sump. 

Figure 6-7B. Chilled water refrigeration unit using steam jet ejectors. By permission, Croll-Reynolds Co., Inc.

Figure 6-9C. Improving the maximum vacuum obtainabie by staging an ejector ahead of the suction of a iiquid ring pump or any other device that can handie water into the unit, uniess dry air is used. By permission, Graham Manufacturing Co., Inc.

Wet steam erodes the ejector nozzle and interferes with performance by clogging the nozzle with water droplets [16]. The effect on performance is significant and is usually reflected in fluctuating vacuum. 

Figure 6-11B. A typical relative comparison of various designs of steam jet ejectors. Based on same steam consumption, 100 psig steam pressure and 85°F water. Curves represent the capacity of ejectors designed for maximum air handling capacity at any one particular suction pressure. By permission, Graham Manufacturing Co.

Often when the non-condensable quantity is known or estimated, it is important to state whether these gases are in the presence of water or other process liquid. In this case, the amount of condensable vapor above the liquid must be considered as it also will enter the ejector suction. 

Figure 6-20B. Air and water vapor mixture data (Dalton s Law)—saturated, (continued). Reprinted by permission. Standards for Sfeam Jet Ejectors, 3rd. Ed., Heat Exchange Institute, 1956 [11].

The saturation pressure corresponds to a temperature of 79° — 7.5°F = 7l.5°F, based on condenser-ejector design practice. The pressure from steam tables = 0.78 in. Hg abs. The water vapor to saturate tire air going to the ejector is ... 

Wlien ejectors pull non-condensables and other vapors from a direct contact water condenser (barometric, low level jet, deaerator) there is also a release of dissolved gases, usually air, from water. This air must be added to the other known load of the ejector. Figure 6-22 presents the data of the Heat Exchange Insdtute [10] for the amount of air that can be expected to be released when cooling water is sprayed or otherwise injected into open qqae barometric or similar equipment. 

When the ejector system consists of one or more ejectors and intercondensers in series, the volume as pounds per hour of mixture to each succeeding stage must be evaluated at conditions existing at its suction. Thus, the second stage unit after a first stage barometric intercondenser, handles all of the non-condensables of the system plus the released air from the water injected into the intercondenser, plus any condensable vapors not condensed in the condenser at its temperature and pressure. Normally the condensable material tvill be removed at this point. If the intercondenser is a surface unit, there wall not be any air released to the system from the cooling w ater. 

Figure 6-29 is also adaptable to air and water ejector applications. 

Water ejectors and water jet eductors are also used for mixing liquids, lifting liquids, and pumping and mixing... 

Figure 6-32 illustrates ejector systems with large condensable loads which can be at least partially handled in the precondenser. Controls are used to maintain constant suction pressure at varying loads (air bleed), or to reduce the required cooling water at low process loads or low water temperatures [2]. The cooler W ater must not be throttled below the minimum (usually 30%-50% of maximum) for proper contact in the condenser. It may be controlled by tailwater temperature, or by the absolute pressure. 

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