Subcooling sufficient

A boiler bank is also included. The boiler-bank tube bundle provides sufficient heat transfer surface area to provide the rated capacity for saturated steam. Boiler-bank tube spacing and dimensions are arranged so that a steam-water circulation subsystem connects the top and bottom drums with subcooled water passing down the tubes farthest from the furnace and returning as a steam-water mixture. 

For temperatures below the vapor—liquid critical temperature, T, isotherms to the left of the liquid saturation curve (see Fig. 3) represent states of subcooled liquid isotherms to the right of the vapor saturation curve are for superheated vapor. For sufficiently large molar volumes, V, all isotherms are approximated by the ideal gas equation, P = RTjV. Isotherms in the two-phase liquid—vapor region are horizontal. The critical isotherm at temperature T exhibits a horizontal inflection at the critical state, for which... 

Common design error. Please refer back to Fig. 13.2. How can the liquid from the condenser rise to the higher elevation in the reflux drum, without being pumped Simple The pressure head of the liquid leaving the condenser is converted to elevation as the liquid flows up into the reflux drum. This works fine, as long as the liquid leaving the condenser is sufficiently subcooled. By sufficiently subcooled, I mean that when the lower-pressure liquid flows into the reflux drum, it has to be cold enough that it does not flash. 

Figure 3.10. Condensers, (a) Condenser on temperature control of the PF condensate. Throttling of the flow of the HTM may make it too hot. (b) Condenser on pressure control of the HTM flow. Throttling of the flow of the HTM may make it too hot. (c) Flow rate of condensate controlled by pressure of PF vapor. If the pressure rises, the condensate flow rate increases and the amount of unflooded surface increases, thereby increasing the rate of condensation and lowering the pressure to the correct value, (d) Condenser with vapor bypass to the accumulator drum. The condenser and drum become partially flooded with subcooled condensate. When the pressure falls, the vapor valve opens, and the vapor flows directly to the drum and heats up the liquid there. The resulting increase in vapor pressure forces some of the liquid back into the condenser so that the rate of condensation is decreased and the pressure consequently is restored to the preset value. With sufficient subcooling, a difference of 10-15 ft in levels of drum and condenser is sufficient for good control by this method.

Usually 1-2 days (for reactors on this scale) of experimental effort are required to traverse the loop as shown in Figure 3.1b. In order to avoid obtaining an erroneous dissociation temperature and pressure, the dissociation part of the loop must be performed at a sufficiently slow heating rate (about 0.12 K/h) to allow the system to reach equilibrium (Tohidi et al., 2000 Rovetto et al., 2006). The temperature difference between the temperature at Point D to that at Point B is called the subcooling [more properly the supercooling, ArSUb, where AFsub = 7eqm(D) — T (B)]. 

For crystals to form from a liquid state, the molecules of the crystallizing species must come together in sufficient number (form a cluster) to overcome the energy cost of forming a surface. Once this energy barrier is overcome, the latent heat associated with crystallization is released, and further nucleation is strongly promoted. Thus, there often is a metastable zone where a supersaturated or subcooled system may not nucleate for a very long time. 

The bypassed vapor heats up the liquid there, thereby causing the pressure to rise. WTien the bypass is closed, the pressure falls. Sufficient heat transfer surface is provided to subcool the condensate, (f) Vapor bypass between the condenser and the accumulator, with the condenser near ground level for the ease of maintenance When the pressure in the tower falls, the bypass valve opens, and the subcooled liquid in the drum heats up and is forced by its vapor pressure back into the condenser. Because of the smaller surface now exposed to the vapor, the rate of condensation is decreased and consequently the tower pressure increases to the preset value. With normal subcooling, obtained with some excess surface, a difference of 10-15 ft in levels of drum and condenser is sufficient for good control, (g) Cascade control The same system as case (a), but with addition of a TC (or composition controller) that resets the reflux flow rate, (h) Reflux rate on a differential temperature controller. Ensures constant internal reflux rate even when the performance of the condenser fluctuates, (i) Reflux is provided by a separate partial condenser on TC. It may be mounted on top of the column as shown or inside the column or installed with its own accumulator and reflux pump in the usual way. The overhead product is handled by an alter condenser which can be operated with refrigerant if required to handle low boiling components. 

Nucleation, or the formation of a crystalline phase from the liquid state, is probably the most important factor in controlling crystallization. The nucleation rate is the major determining factor in the number and size of crystals formed, their polymorphic form, and the ultimate distribution of crystalline solids. Crystallization cannot occur until the phase is supersaturated or subcooled. However, attaining the supersaturated or subcooled state is not necessarily sufficient to promote crystallization because a certain energy barrier exists to formation of nuclei. 

Finally, by sufficient subcooling of a vapour and the presence of condensation nuclei , that is tiny particles on which condensate can be deposited, a mist forms, as Fig. 4.5 shows schematically. 

If a body of vapor can be sufficiently subcooled without contact with a surface (by radiation, for example), theory predicts that a liquid phase can form spontaneously. This process is termed homogenous condensation, and fog formation is often cited as an example. However, the subcooling required is so great that it is unhkely to occur in any process situation. Fog formation does occur in process condensers, but the probable mechanism is condensation on tiny droplets carried over from a previous processing step (a distillation column, for example, where bubble collapse produces large numbers of submicroscopic droplets). 

Boiling also will occur with a liquid that is below its saturation temperature if the hot surface temperature is sufficiently above the nucleation temperature for the surface. The bubbles thus formed collapse quickly when they move from the surface into the bulk subcooled liquid, resulting in high heat-transfer rates. Moderate subcooling at the entrance to a vaporizer can be assumed to be heated by heat transfer at the same rates as for the saturated boiling process, if the saturation temperature of the fluid at the existing pressure is used in the temperature difference. 

For energy exchange equipment Supply sufficient excess of heat transfer area in reboilers, condensers, cooling jackets, and heat removal systems for reactors to be able to handle the anticipated upsets and dynamic changes. Sometimes extra area is needed in overhead condensers to subcool the condensate to prevent flashing in the downstream control valves. Too frequently, overzealous engineers size the optimum heat exchangers based on an economic minimum based on steady-state conditions and produce uncontrollable systems. 

Certain equilibrium states of thermodynamic systems are stable to small fluctuations others are not. For example, the equilibrium state of a simple gas is stable to all fluctuations, as are most of the equilibrium states we will be concerned with. It is possible, however, to carefully prepare a subcooled liquid, that is, a liquid below its normal solidiflcation temperature, that satisfies the equilibrium criteria. This is an tin-.stable equilibrium. state because the slightest disturbance, such as tapping on the. side of the containing ve.s.sel, will cause the liquid to freeze. One sometimes encounters mixtures that, by the chemical reaction equilibrium criterion (see Chapter 13). should react however, the chemical reaction rate is so small as to be immeasurable at the temperature of interest. Such a mixture can achieve a state of thermal equilibrium that is stable with respect to small fluctuations of temperature and pressure. If, however, there is a sufficiently large, but temporary, increase in temperature. so that die rate of the chemical reaction is appreciable for some period of time) and then the system... 

Unstable equilibrium states are rarely encountered in nature unless they have been specially prepared e.g., the subcooled liquid mentioned earlier). The reason for this is that during the approach to equilibrium, temperature gradients, density gradients, or other nonuniformities that exist within a system are of a sufficient magnitude to act as disturbances to unstable states and prevent their natural occurrence. 

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