Motive nozzle

The basic principle of the jet pump is that the liquid or gas jet exits the motive nozzle at a high velocity and low pressure and entrains and accelerates the surrounding liquid, gas or solid medium. The result of this action is the mixing of the driving and the entrained material at a mean velocity. In a second nozzle, this velocity of the mixture is reduced and the pressure increases to an outlet pressure that is higher than the suction pressure. 

The conversion of this principle to a practical purpose requires a simple apparatus, which normally consists of only three main parts the motive nozzle (1), the diffuser (2) and the head (3). There are three external connections to be designated the motive medium connection (A), the suction nozzle (B) and the pressure nozzle (C). 

Figure 4.4 shows the thermodynamic processes in a jet pump in a Mollier enthalpy-entropy diagram. The inlet conditions of motive flow and suction flow are assumed to be saturated steam. This is also a common situation in real processes. Besides this, it is also assumed that the expansion in the motive nozzles ends just with the suction pressure p. In the case of a non-dissipative expansion in the motive nozzle, the change of state would be described by the perpendicular from point 1 to point 2. While a real expansion with losses leads to point 2. ... 

It can be seen that the end point of the expansion (point 2 ) is in the area of wet steam. Thus small droplets of condensate will form in the motive jet. In the motive nozzle, combined with the decrease of pressure, the temperature will also decrease. In the wet steam region the steam is cooled to the boiling point temperature corresponding to the pressure. When the motive steam expands to a pressure lower than 6mbar, the corresponding temperature is below 0°C, thus ice will form. Steam jet vacuum pumps for such applications are often heated at the mixing nozzle and sometimes also at the motive nozzle, to prevent the ice crystals from adhering to the internal wall. This would cause a constriction of the cross-sectional area and adversely effect the flow. 

The motive nozzle is shaped like a Laval nozzle. This means there is an enlargement of the diameter after the smallest cross section. This is necessary to achieve velocities higher than sonic speed. For steam an expansion pressure ratio of only Pi/Po = is sufficient to just achieve sonic velocity (critical pressure ratio). At higher expansion ratios (supercritical pressure ratios), the exact critical pressure and sonic speed is achieved in the smallest cross section. In these cases in the divergent part of the motive nozzle, a supersonic velocity results from a continuing expansion. Owing to the blocking of the velocity to the sonic speed in the smallest cross section, the mass flow rate of such a supersonic nozzle only depends on the state of the motive media in front of the nozzle and of course on the diameter d.. Here the mass flow rate is proportional to the motive pressurep. ... 

The increase of the motive pressure does not only change the pressure ratio along the motive nozzle, but also the mass flow rate calculated by Eq. (4.1) rises. 

A similar effect to the increase of the motive pressure results from an increase of the motive mass flow rate, by increasing the diameter of the motive nozzle. In this case, the values for the critical discharge pressure also grow, while the suction characteristic is virtually unchanged. 

Using the calculation methods of one dimensional gas dynamic shows how different motive media behave during the expansion in the motive nozzle. Depending on the expansion ratio ( = motive vapour pressure/suction pressure), the Mach number at the motive nozzle outlet for some vapours is charted in Figure 4.18. It can be seen, that the velocity is in the supersonic range and therefore the corresponding effects (expansion, compression and shock waves) have to be considered. 

Figure 4.18 Mach number at motive nozzle outlet.

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). 

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. 

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. 

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]. 

Figure 17.3 shows a steam turbine with three, rather than one, nozzles. The single, largest, left-hand valve is called the main nozzle. It handles 60 percent of the motive-steam flow. Each of the two smaller nozzles handles 20 percent of the steam. These 20 percent nozzles can be plugged off by a device sometimes called either a horsepower valve, jet valve, speed valve, star (for the handle shape) valve, or port valve. 

The net effect of this exercise will be to save not 20 percent of the motive steam, but 10 percent. The 20 percent reduction in nozzle area is partially offset by the opening of the governor valve. The inefficient, irreversible, isoenthalpic expansion and pressure drop across the governor speed control valve are reduced. The efficient, reversible, isoen-tropic expansion and pressure drop across the nozzles are increased. 

Jet problems. These include low motive-steam pressure, excess wear on the steam nozzles, high condenser backpressure, and air leaks that exceed the jet s capacity. To determine whether a poor vacuum in a surface condenser is due to such jet problems, consult the chart shown in Fig. 18.4. Measure the surface condenser vapor outlet temperature and pressure. Plot the point on the chart. If this point is some-... 

The main variables in the operation of atomizers are feed pressure, orifice diameter, flow rate and motive pressure for nozzles and geometry and rotation speed of wheels. Enough is known about these factors to enable prediction of size distribution and throw of droplets in specific equipment. Effects of some atomizer characteristics and other operating variables on spray dryer performance are summarized in Table 9.18. A detailed survey of theory, design and performance of atomizers is made by Masters (1976), but the conclusion is that experience and pilot plant work still are essential guides to selection of atomizers. A clear choice between nozzles and spray wheels is rarely possible and may be arbitrary. Milk dryers in the United States, for example, are equipped with nozzles, but those in Europe usually with spray wheels. Pneumatic nozzles may be favored for polymeric solutions, although data for PVC emulsions in Table 9.16(a) show that spray wheels and pressure nozzles also are used. Both pressure nozzles and spray wheels are shown to be in use for several of the applications of Table 9.16(a). 

The main variables in the operation of atomizers are feed pressure, orifice diameter, flow rate and motive pressure for nozzles and geometry and rotation speed of wheels. Enough is known about these factors to enable prediction of size distribution and throw of droplets in specific equipment. Effects of some atomizer... 

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