Critical nozzles

It must be pointed out that a Code stamp does not necessarily mean that the vessel is fabricated in accordance with critical nozzle dimensions or internal devices as required by the process. The Code inspector is only interested in those aspects that relate to the pressure-handling integrity of the vessel. The owner must do his own inspection to assure that nozzle locations are within tolerance, vessel internals are installed as designed, coatings are applied properly, etc. 

Critical nozzles, 11 660—661 Critical oxygen concentration, selected organisms, l 731t... 

Figure 5.1 Impinging stream device for the measurement of droplet size distribution. 1 critical nozzle 2, 3 rotameter 4 frame 5 guider 6 apron A-A impingement plane.

Calibration Gas Generators with Super-Critical Nozzles. 854... 

One of the two other remaining alternatives is a device that attains the deHned flow rate with the help of super-critical nozzles [9, 10, 11, 12]. The flow rate through a nozzle, Fonfij, can be calculated according to the following equation ... 

The super-critical nozzle has an advantage over the subcritical nozzle in that only the inlet pressure Pi but not the back pressure, depending on the operational conditions, enters into the flow rate as a variable, thereby making further regulation of the back pressure unnecessary. The reproducibility of the proportioned flow rate is mainly defined by the pressure in front of the orifice. For this regulation extremely good long-term stability is required. 

This may be achieved with MFCs, as well as with the help of super-critical nozzles however, both alternatives require a preliminary pressure of 2-3 bar on the side of the precursory calibration gas. 

Spent Acid or Burning. Burners for spent acid or hydrogen sulfide are generally similar to those used for elemental sulfur. There are, however, a few critical differences. Special types of nozzles are required both for H2S, a gaseous fuel, and for the corrosive and viscous spent acids. In a few cases, spent acids maybe so viscous that only a spinning cup can satisfactorily atomize them. Because combustion of H2S is highly exothermic, carehil design is necessary to avoid excessive temperatures. 

Liquid fuel is injected through a pressure-atomizing or an air-blast nozzle. This spray is sheared by air streams into laminae and droplets that vaporize and bum. Because the atomization process is so important for subsequent mixing and burning, fuel-injector design is as critical as fuel properties. Figure 5 is a schematic of the processes occurring in a typical combustor. 

The hydrocarbon gas feedstock and Hquid sulfur are separately preheated in an externally fired tubular heater. When the gas reaches 480—650°C, it joins the vaporized sulfur. A special venturi nozzle can be used for mixing the two streams (81). The mixed stream flows through a radiantly-heated pipe cod, where some reaction takes place, before entering an adiabatic catalytic reactor. In the adiabatic reactor, the reaction goes to over 90% completion at a temperature of 580—635°C and a pressure of approximately 250—500 kPa (2.5—5.0 atm). Heater tubes are constmcted from high alloy stainless steel and reportedly must be replaced every 2—3 years (79,82—84). Furnaces are generally fired with natural gas or refinery gas, and heat transfer to the tube coil occurs primarily by radiation with no direct contact of the flames on the tubes. Design of the furnace is critical to achieve uniform heat around the tubes to avoid rapid corrosion at "hot spots."... 

Critical Flow Nozzle For a given set of upstream conditions, the rate of discharge of a gas from a nozzle will increase for a decrease in the absolute pressure ratio po/pi until the linear velocity in the throat reaches that of sound in the gas at that location. The value of po/pi for which the acoustic velocity is just attained is called the critical pressure ratio r. The actual pressure in the throat will not fall below even if a much lower pressure exists downstream. 

The critical pressure ratio r can be obtained from the following theoretical equation, which assumes a perfect gas and a frictionless nozzle ... 

Discharge coefficients for critical flow nozzles are, in general, the same as those for subsonic nozzles. See Grace and Lapple, Trans. Am. Soc. Mech. Fug., 73, 639-647 (1951) and Szaniszlo, ]. Eug. Power, 97, 521-526 (1975). Arnberg, Britton, and Seidl [J. Fluids Eug., 96, 111-123 (1974)] present discharge-coefficient correlations for circular-arc venturi meters at critical flow. For the calciilation of the flow of natural gas through nozzles under critical-flow conditions, see Johnson,/. Ba.sic Eng., 92, 580-589 (1970). 

Product diameter is small and bulk density is low in most cases, except prilling. Feed hquids must be pumpable and capable of atomization or dispersion. Attrition is usually high, requiring fines recycle or recoveiy. Given the importance of the droplet-size distribution, nozzle design and an understanding of the fluid mechanics of drop formation are critical. In addition, heat and mass-transfer rates during... 

When testing to estabhsh the thermodynamic performance of a steam turbine, the ASME Performance Test Code 6 should be followed as closely as possible. The effec t of deviations from code procedure should be carefully evaluated. The flow measurement is particularly critical, and Performance Test Code 19 gives details of flow nozzles and orifices. The test requirements should be carefully studied when the piping is designed to ensure that a meaningful test can be conducted. 

For critical measurement, where the drop p is more than the barometric pressure, flow should be measured with static-pressure taps upstream from the nozzle as illustrated in Figure 20-4. For exhaust measurements, differential pressure is measured at two static taps located downstream from the nozzle at the inlet as shown in Figure 20-5. 

To determine the critical pressure ratio for gas sonic velocity across a nozzle or orifice use... 

The angle of repose is defined as the angle between a line of repose of loose material and a horizontal plane. Its value depends on the magnitude of friction and adhesion between particles and determines the mobility of loose solids, which is a critical parameter in designing conical discharge and feeding nozzles and in... 

As normally designed, vapor flow through a typical high-lift safety reliefs valve is characterized by limiting sonic velocity and critical flow pressure conditions at the orifice (nozzle throat), and for a given orifice size and gas composition, mass flow is directly proportional to the absolute upstream pressure. 

Critical and Subcritical Flow - The maximum vapor flow through a restriction, such as the nozzle or orifice of a pressure relief valve, will occur when conditions are such that the velocity through the smallest cross-sectional flow area equals the speed of sound in that vapor. This condition is referred to as "critical flow" or "choked flow . 

This method employs a theoretical critical mass flow based on an ideal nozzle and isothermal flow condition. For a pure gas, the mass flow can be determined from one equation ... 

The first stage of the cycle is the flow of molten polymer into the mould cavity through a standard feed system. Before this flow of polymer is complete, the injection of a predetermined quantity of gas into the melt begins through a special nozzle located within the cavity or feed system as shown in Fig. 4.45. The timing, pressure and speed of the gas injection is critical. 

Heni, R. E. and H. K. Fauske, 1971, The Two-Phase Critical Flow of One-Component Mistuies in Nozzles, Orifices, and Short Tubes, J. Heat Transfer, pp 179-187, May. 

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