Heat transfer lower saturation temperature

Top spray systems During top-spray cooling of an overheated core, the wall temperature is usually higher than the Leidenfrost temperature, which causes water to be sputtered away from the wall by violent vapor formation and then pushed upward by the chimney effect of the steam flow generated at lower elevations (as shown in Fig. 4.25). A spray-cooling heat transfer test with BWR bundles was reported by Riedle et al. (1976). They found the dryout heat flux to be a function of spray rate and system pressure. The collapsed level required to keep the bundle at saturation for various pressures compared reasonably well with that in the literature (Duncan and Leonard, 1971 Ogasawara et al., 1973). 

Groendes and Mesler (1982) studied the saturated film boiling impacts of a 4.7 mm water droplet on a quartz surface of 460 °C. The fluctuation of the surface temperature was detected using a fast-response thermometer. The maximal temperature drop of the solid surface during a droplet impact was reported to be about 20 °C. Considering the lower thermal diffusivity of quartz, this temperature drop implies a low heat-transfer rate on the surface. Biance et al. (2003) studied the steady-state evaporation of the water droplet on a superheated surface and found that for the nonwetting contact condition, the droplet size cannot exceed the capillary length. 

In subcooled impact, the initial droplet temperature is lower than the saturated temperature of the liquid of the droplet, thus the transient heat transfer inside the droplet needs to be considered. Since the thickness of the vapor layer may be comparable with the mean free path of the gas molecules in the subcooled impact, the kinetic slip treatment of the boundary condition needs to be applied at the liquid-vapor and vapor-solid interface to modify the continuum system. 

The steam supplied to a given feedwater heater must be at a pressure hi enough that its saturation temperature is higher than the temperature of feedwater stream leaving the heater. We have here presumed a minimum ter perature difference for heat transfer of no less than 5°C, and have chose extraction steam pressures such that the tsM values shown in each feedwat heater are at least 5 C greater than the exit temperature of the feedwater strear The condensate from each feedwater heater is flashed through a throttle valv to the heater at the next lower pressure, and the collected condensate in the fin heater of the series is flashed into the condenser. Thus, all condensate retu from the condenser to the boiler by way of the feedwater heaters. 

We know from Ihemiodynamics lhai when tlie temperature of a liquid at a specified pressure is raised to the saturation temperature at that pressure, boiling occurs. Likewise, when the temperature of a vapor is lowered to condensation occurs. In this chapter we study the rates of heat transfer during such Itquid-to-vapor and vapor-to-liquid phase transformations. 

Consider a vertical plate of height L and width b maintained at a constant temperature r, that is exposed to vapor at the saturation temperature The downward direction is taken as the positive x-direction with the origin placed at the lop of the plate whete condensation initiates, as shown in Fig. 10-24. The surface temperature is below the saluratioii temperature (7 j < r <) and thus the vapor condenses on the surface. The liquid film flows downward under the influence of gravity. The film thickness S and thus the mass flow rate of the Condensate increases with x as a result of continued condensation on the existing film. Then heal transfer from the vapor to the plate must occur through the film, which offers resistance to heat transfer. Obviously the thicker the film, ihe larger its thermal resistance and thus the lower the rate of heal transfer. 

The saturation temperature at the phase interface can, depending on the gas content, lie considerably below the saturation temperature t s(p) associated with the pressure p, which would occur if no inert gas was present. The temperature difference between the phase interface and the wall is lowered because of the inert gas and with that the heat transfer is also reduced. In order to avoid or prevent this, it should be possible to remove the inert gas through valves. Large condensers are fitted with steam-jet apparatus which suck the inert gas away. In other cases, for example the condensation of water out of a mixture of steam and air or in the condensation of ammonia from a mixture with air, it is inevitable that inert gases are always present. Therefore their influence on heat transfer has to be taken into account. 

Shang and Adamek [15] recently studied laminar film condensation of saturated steam on a vertical flat plate using variable thermophysical properties and found that the Nusselt theory with the Drew [14] reference temperature cited above produces a heat transfer coefficient that is as much as 5.1 percent lower than their more correct model predicts (i.e., the Nusselt theory is conservative). 

Although sensible heat transfer coeflicients are considerably lower than condensing coeflicients, heat transfer rates are quite high in the desuperheating zones of distillation condensers. The low heat transfer coefficient in the desuperheating zone is compensated for by the higher temperature difference between the superheated vapor and the coolants (compared to the temperature difference between the saturated vapor and the coolant). In most cases, moderate variation in superheat has little effect on condenser performance and is seldom troublesome in distillation operation. 

Vulcanization can take place in hot air or in steam. Vulcanization in hot air ovens is not very efficient because of the poor heat transfer of hot air. Consequently, longer vulcanization times at lower temperatures are necessary to prevent aging caused by oxygen. Unlike hot air, saturated steam has better heat transfer and acts as an inert gas. Steam vulcanization takes place in autoclaves with a heated jacket and a closed chamber in which the articles are placed and the steam is introduced. All air in the autoclave needs to be displaced by steam before the cure time is started. This produces the best possible properties and most uniform cure. 

Saturated Steam. Generation of saturated steam is the most common form of heat recovery in coal gasification systems. The high heat transfer coefficient and low and constant metal temperatures for saturated steam generation allow the use of lower cost materials and better operability. 

A common practice is to cool the solutions by flash evaporation when a liquid at a given To temperature is transferred in a chamber at a pressure Pi such that the liquid saturation temperature Ti at Pi is lower than To, the liquid will release heat according to the temperature difference and a quantity of solvent, whose total latent heat of vaporization equals the difference in enthalpy. In simple words, the liquid is cooled by evaporating a part of it. 

With reference to Fig. 6.11, assume that this binary feed mixture enters the column as saturated vapor on the fourth plate. Ideally, the liquid on the plate above the feed inlet (point L5) has the same composition as the feed, but is at a lower temperature since the latter is at the bubble-point temperature rather than at the dew-point temperature. Within the column, vapor flows upward through the liquid layer on each plate, while the liquid flows across each plate and down to the next plate by means of a downcomer. The vapor transfers heat to the liquid on each plate as it bubbles through the liquid. This heat transfer results in the evaporation of a small amount of the more volatile component from the liquid layer and correspondingly in the condensation of a small amount of the less volatile component in the vapor. Thus, the vapor becomes richer in nitrogen as the vapor comes in contact with the liquid layer and the liquid layer becomes richer in oxygen as the liquid contacts the vapor and flows downward from plate to plate. This is illustrated in Fig. 6.12, which shows the ideal temperature-composition of the vapor and liquid above and below the feed entry. As the saturated liquid moves down the column, its composition moves to the left along the bubble-point curve (points L4, L3,... 

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