Critical vapor velocity

The maximum vapor load on the drum is based on the largest release from safety valves discharging as a result of a single contingency. Vapor velocities in the drum are based on 100% of critical velocity (refer back to Equation 1). However, a velocity of 175% of critical is permitted when one is applying the 1.5... 

The vapor space is sized to avoid water entraimnent in the flare gas. As a rule, vapor velocities in the drum should not exceed 150 % of critical. This however can be increased to 230 % critical velocity) when considering... 

Vapor-Liquid Gravity Separator Design Fundamentals The critical factors in the performance of a horizontal separator are the vapor residence time and the settling rate of the liquid droplets. However, two other factors enter into the design—the vapor velocity must be limited to avoid liquid entrainment, and there must be sufficient freeboard within the vessel to allow for a feed distributor. For vertical separators, the design is based on a vapor velocity that must be less than the settling velocity of the smallest droplet that is to be collected, with due allowance for turbulence and maldistribution of the feed. The vapor residence time is a function of the vapor flow rate (mass), vapor density, and volume of vapor space in the separator, based on the following ... 

The decrease in film burn-out heat flux with increasing mass velocity of flow at constant quality has been explained by Lacey et al. in the following way. At constant quality, increasing total mass flow rate means increasing mass flow of vapor as well as liquid. It has been shown that above certain vapor rates increased liquid rates do not mean thicker liquid layers, because the increased flow is carried as entrained spray in the vapor. In fact, the higher vapor velocity, combined with a heat flux, might be expected to lead to easy disruption of the film with consequent burn-out, which seems to be what actually occurs at a constant steam mass velocity over very wide ranges of conditions—that is, the critical burn-out steam quality is inversely proportional to the total mass flow rate. 

In film breakdown at burn-out, nucleation may be a factor together with loss by entrainment and evaporation (in excess of spray deposition), and instabilities associated with surface tension. There is evidence for the existence of a critical vapor mass velocity, independent of pressure, above which the film is easily disrupted by heat fiux it is also clear that upstream conditions, including the inlet arrangements, must strongly influence the film breakdown at the exit. 

The critical striking velocity (Vc) for impact explosions of the target is that vel at which the heat developed by inelastic collision is equal to the heat of vaporization. For single-particle impact, the equation ... 

The allowable vapor velocity and the corresponding tray diameter are represented by the work of Souders and Brown, which is cited in standard textbooks, for example Treybal (1980). Its equivalent is the Jersey Critical formula,... 

In addition to the critical design factors for finite-stage contactors of number of theoretical trays, maximum allowable vapor velocity, column efficiency, and pressure drop as discussed earlier, a number of other factors are of importance in the development of the design. These factors are discussed in the following sections. 

When the densities of the two fluids are of comparable magnitude, the velocities within the boundary layers do not differ significantly from their free stream values. Subsequently the governing equations can be linearized and solved (78, 79, 80, 81). This case is expected to be relevant for near-critical vaporization, provided the droplet can still maintain its spherical shape. 

It was supposed, that for a high vapor velocity and a thin liquid film the influence of gravity is small and the correlation for up flow was used. Total boiling suppression occurs when mass quality more than 0.3 for a film thickness less than 60 pm. That value is close to the bubble departure diameter observed for flow boiling in a film. When the film thickness is smaller than the critical one, the forced convection occurs with a small heat transfer coefficient. The crisis of the heat transfer was observed for a complete liquid evaporation on a heated wall. While the mass quality less than 0.3, we have the cell or slug flow mode, so boiling is not suppressed. 

For sloped downcomers, the critical liquid velocity is at the bottom, insofar as final disengagement of vapor is concerned. The total volume of the filled portion or lire downcomer can be used in estimating residence lime, For downcomers with bottom recesses, where the liquid must make an extra mm before entering the tray, the pressure lora under the downcomer may he estimated as twice that calculated from Eq. (5.7-30). This rule of thumb applies also to the case where an inlet weir is used io distribute the liquid after it has flowed under die downcomer baffle. 

Klausner [34] studied bubble nucleation in stratified flow of refrigerant R113 in a horizontal rectangular channel. The nucleation site density decreased with increasing vapor velocity as illustrated in Fig. 15.15 at a velocity of around 5 m/s, nucleation was totally supressed. Klausner et al. interpreted their data in terms of the relationship between critical radius of nucleation site rc and number density the data from this interpretation are shown in Fig. 15.16 and it will be seen, that for these particular conditions, a rather narrow range of site radius applied. The question of suppression of nucleate boiling is discussed further later in this chapter. 

Eventually, the velocity of vapor in the jets becomes so large that the jets themselves become unstable near the interface as a result of Helmholtz instability (of wavelength XH) as shown in Fig. 15.63). The breakup of the jets destroys the efficient vapor-removal mechanism, increases vapor accumulation at the interface, and leads to liquid starvation at the surface and to the critical heat flux phenomenon. If jet breakup occurs at a vapor velocity UH within the jets, the critical heat flux q"m is given by... 

Ejector design may be either critical or noncritical. Critical design means that the vapor velocity in the diffuser is sonic and occurs at compression ratios above 1.8. In noncritical designs the vapor velocity in the diffuser is subsonic. 

Curve B When the condenser temperature is lowered by increasing the heat rejection rate, the evaporator temperature is also lowered. The vapor velocity at the exit becomes sonic and critical, and choked flow condition exists. 

Once a liquid droplet is allowed to settle through a continuous vapor phase, the settling velocity depends on the particle size, resistance to settling (defined as drag), and densities on vapor and liquid phases. The critical settling velocity is defined as... 

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 . 

If the pressure Pj downstream of the restriction is less than the critical flow pressure, then the maximum obtainable flow which occurs at critical velocity is a function of P, and P but is unaffected by Pj. If Pj is greater than P , however, then the flow is termed "subcritical," and the rate is a function of P, and Pj. There are thus two equations for sizing PR valves in vapor service, depending on whether the flow is critical or subcritical. 

Critical or sonic flow will usually exist for most (compressible) gases or vapors discharging through the nozzle orifice of a pressure relieving valve. The rate of discharge of a gas from a nozzle will increase for a decrease in the absolute pressure ratio P2/P1 (exit/inlet) until the linear velocity in the throat of the nozzle reaches the speed of sound in the gas at that location. Thus, the critical or sonic velocity or critical pressures are those conditions... 

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