Pressure subcooling

Figure 5.8 Saw-shaped bubble layer at DNB in a high-pressure, subcooled, vertical Freon flow p = 190 psig (1.4 MPa) G = 3.2 X 106 lbm/hr ft2 (4,320 kg/m2 s) A7 ub = 29°F (16°C) subcooled (qlA)t = 1.2 x 10s Btu/hr ft2 (3.8 x 105 W/m2). (From Dougall and Lippert, 1973. Reprinted with permission of NASA Scientific Technical Information, Linthicum Heights, MD.)...

TABLE 10.6 Vapor Pressures, Subcooled Liquid Vapor Pressures (pL), Solubilities in Water (S, Cs),... 

J. J. Schroder, S. McGill, M. Dirbach, and F. Podzelny, Secondary Nucleation Ambivalent Heat Transfer and Sound Emission in Low Pressure Subcooled Boiling, in Proc EUROTHERM Semi-... 

If the liquid state (P,T) lies above the saturation curve ABC and to the left hand side of the To isotherm, then the liquid is said to be subcooled , or strictly, thermally subcooled . The term subcooled also applies if the hquid temperature remains at To while the pressure is raised above Po i.e. the hquid is pressure subcooled. 

A pressure subcooled hquid state is generated when the vapour space pressure is increased above Po at constant Tq. There is little change in enthalpy and the degree of pressure subcoohng may be specified in terms of the pressure head (P-Po). 

This can be simply achieved with adequate pressure subcooling of the liquid, by pressurising the hquid along AA before the transfer commences. 

As the pumping process continues, the liquid level falls reducing the hydrostatic pressure head and associated pressure subcooling, at the entry to the pump. Eventually, the total pressure head falls to the NPSH or below the pump begins to stall and wiU eventually no longer remove further liquid from the tank. (However, if the outlet valve of a centrifugal pump is throttled when the Uquid level falls to near this stalling point, flie flow reduction in effect reduces the NPSH and enables the liquid to be pumped further. This can be important if the tank is to be emptied completely.)... 

This is not the whole story, because, as the liquid level drops, the vapour in the vapour space expands into the volume previously occupied by the liquid removed. Generally, there is not enough liquid evaporation to maintain flie vapour pressure, and the pressure falls, causing the pressure subcooling at the pump entry to further reduce. This latter effect can be alleviated by back-filling vapour into the tank. 

Va.por Pressure. Vapor pressure is one of the most fundamental properties of steam. Eigure 1 shows the vapor pressure as a function of temperature for temperatures between the melting point of water and the critical point. This line is called the saturation line. Liquid at the saturation line is called saturated Hquid Hquid below the saturation line is called subcooled. Similarly, steam at the saturation line is saturated steam steam at higher temperature is superheated. Properties of the Hquid and vapor converge at the critical point, such that at temperatures above the critical point, there is only one fluid. Along the saturation line, the fraction of the fluid that is vapor is defined by its quaHty, which ranges from 0 to 100% steam. 

For subcooling, a liquid inventory may be maintained in the bottom end of the shell by means of a weir or a hquid-level-controUer. The subcoohng heat-transfer coefficient is given by the correlations for natural convection on a vertical surface [Eqs. (5-33 ), (5-33Z )], with the pool assumed to be well mixed (isothermal) at the subcooled condensate exit temperature. Pressure drop may be estimated by the shell-side procedure. 

Outlet Temperature and Pressure. It is important to have proper subcooling in the vent end of the unit to prevent large amounts of process vapors from going to the vacuum system along with the inerts. 

The control valve allows the Jets to pull noncondensibles out of the condenser as needed for system pressure control. In addition to requiring extra surface area for control, the vacuum condenser also needs enough surface area for subcooling to ensure that the Jets do not pull valuable hydrocarbons or other materials out with the noncondensibles. To allow proper control and subcooling, some designers add approximately 50% to the calculated length. 

Regarding two pliase flow, pressurized liquid above its noniial boiling point will start to flash when released to aUiiospheric pressure, and two pliase flow will result. Two-pliase flow is also likely to occur from depressurization of tlie vapor space above a volatile liquid, especially if the liquid is viscous (e.g., greater tlian 500 cP) or has a tendency to foam. Fauske and Epstein liave provided tlie following practical calculation guidelines for two-phase flashing flows. The discharge of subcooled or saturated liquids is described by... 

Calculate condensing pressure, with to as bubble point (if subcooling exists, and to is below bubble point, use bubble point temperature for pressure calculation only). 

Note that the method described assumes that the high-pressure condensate has not been sub-cooled. If any subcooling has taken place, then the figure taken for the enthalpy of water at the higher pressure is reduced by the amount of sub-cooling. The chart or table can still be used if the upstream pressure is taken as that corresponding to saturated steam at the same temperature as the sub-cooled condensate. 

The boiling point is limited by the critical temperature at the upper end, beyond which it cannot exist as a liquid, and by the triple point at the lower end, which is at the freezing temperature. Between these two limits, if the liquid is at a pressure higher than its boiling pressure, it will remain a liquid and will be subcooled below the saturation condition, while if the temperature is higher than saturation, it will be a gas and superheated. If both liquid and vapour are at rest in the same enclosure, and no other volatile substance is present, the condition must lie on the saturation line. 

The variable orifice of the expansion valve can be replaced, in small systems, by a long thin tube. This is a non-modulating device and has certain limitations, but will give reasonably effective control over a wide range of conditions if correctly selected and applied. Mass flow is a function of pressure difference and the degree of liquid subcooling on entry. 

Fast burn-out, Fig. 4A, occurs when the temperature rise is very rapid, for example, less than one second elapsing between the initiation of burn-out and the time at which the metal temperature becomes dangerously high. Unless the channel power is quickly interrupted, a fast burn-out will usually result in physical burn-out. Lee and Obertelli (L4) report having examined a large number of instrument traces to see whether fast burn-out could be associated with any particular ranges of flow velocity, pressure, or quality at the burn-out point, but no generalization could be made. However, it does appear that in the case of water, fast burn-out is nearly always associated with subcooled or low-quality conditions at burn-out. 

Fig. 18. Example of the linear effect of bulk fluid subcooling on burn-out for copper and iron wires with pool boiling of water at atmospheric pressure [from Farber (FI)].

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