Nozzles ejector

There are two types of two-component nozzles ejectors and injectors. The ejectors are liquid jet aspirators, which operate according to the Venturi principle and with their propulsion liquid jet in the mixing space (diffuser) attain a substantial pressure drop on the gas side. Vacuum is commonly produced by water ejectors (vacuum pumps) in the laboratory and by steam ejectors on an industrial scale. 

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. 

Ejector Performance The performance of any ejec tor is a function of the area of the motive-gas nozzle and venturi throat, pressure of the motive gas, suction and discharge pressures, and ratios of specific heats, molecular weights, and temperatures. Figure 10-102, based on the assumption of constant-area mixing, is useful in evaluating single-stage-ejector performance for compression ratios up to 10 and area ratios up to 100 (see Fig. 10-103 for notation). 

Ejectors are available in many materials of construction to suit process requirements. If the gases or vapors are not corrosive, the diffuser is usually constructed of cast iron and the steam nozzle of stainless steel. For more corrosive gases and vapors, many combinations of materials such as bronze, various stainless-steel alloys, and other corrosion-resistant metals, carbon, and glass can be used. 

In liquid ejectors or aspirators, the hquid is the motive fluid, so the gas pressure drop is low. Flow of slurries in the nozzle may be erosive. Otherwise, the design is as simple as that of the Venturi. 

The ejector is operated directly by a motive gas or vapor source. Air and steam are probably the two most common of the motive gases. The ejector uses a nozzle to accelerate the motive gas into the suction chamber where the gas to be compressed is admitted at right angles to the motive gas direction. In the suction chamber, also referred to as the mixing chamber, the suction gas is entrained by the motive fluid. The mixture moves into a diffuser where the high velocity gas is gradually decelerated and increased in pressure. 

The ejector is widely used as a vacuum pump, where it is staged when required to achieve deeper vacuum levels. If the motive fluid pressure is sufficiently high, the ejector can compress gas to a slightly positive pressure. Ejectors are used both as subsonic and supersonic devices. The design must incorporate the appropriate nozzle and diffuser compatible with the gas velocity. The ejector is one of the ( to liquid carryover in the suction gas. 

Figure 6-1. Basic ejector components and diagram of energy conversion in nozzle and diffuser. By permission, Ingersoll-Rand Co.

The motive steam design pressure must be selected as the lowest expected pressure at the ejector steam nozzle. The unit will not operate stably on steam pressures below the design pressure [16]. 

An increase in steam pressure over design will not increase vapor handling capacity for the usual fixed capacity ejector. The increased pressure usually decreases capacity due to the extra steam in the diffuser. The best ejector steam economy is attained when the steam nozzle and diffuser are proportioned for a specified performance [8]. This is the reason it is difficult to keep so-called standard ejectors in stock and expect to have the equivalent of a custom designed unit. The throttling type ejector has a family of performance curves depending upon the motive steam pressure. This type has a lower compression ratio across the ejector than the fixed-type. The fixed-type unit is of the most concern in this presentation. 

For a given ejector, an increase in steam pressure over the design value will increase the steam flow through the nozzle in direct proportion to the increase in absolute... 

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. 

Total mixture to be handled = 40 Ibs/hr Pounds of air in mixture = 14 Ibs/hr Suction pressure = 1.5 in. Hg abs Steam pressure at ejector nozzle = 150 psig... 

Steam jet thermocompressors or steam boosters are used to boost or raise the pressure of low pressure steam to a pressure intermediate bettveen this and the pressure of the motive high pressure steam. These are useful and economical when the steam balance allows the use of the necessary pressure levels. The reuse of exhaust steam from turbines is frequently encountered. The principle of operation is the same as for other ejectors. The position of the nozzle with respect to the diffuser is critical, and care must be used to properly posidon all gaskets, etc. The thermal efficiency is high as the only heat loss is due to radiation [5]. 

Discharge pressure of pump, psia = Intake pressure of pump Mth closed intake, psia = Final pressure in system, in. Hg abs Gas constant, = 1544/mol weight Pump speed, revolutions (or strokes) per second Pump speed, liters/sec Pump speed at P ", liters/sec Pump speed at P/, liters/sec Temperature, °R = 460 + °F Evacuation pump dowmtime, min Evacuation pump downtime, sec Ambient air temperature, °F Temperature of mixture at ejector suction, °F Temperature of steam on downstream side of nozzle, °F... 

The booster ejectors are usually of steel plate (or cast) with Monel steam nozzles. 

The air ejectors are usually of cast iron with Monel nozzles. The associated inter- and after-condensers are usually of cast iron shell and water boxes with Admiralty tubes (unless sea or brackish water) with Muntz metal tubesheets. Some inter- and after-condensers may also be barometric rather than tubular. 

Figure 8.46. Steam jet ejector A Steam nozzle B Mixing region C Mixed fluids D Entrained fluid...

---