Heating condensing vapor

Latent heat of vaporization decreases i.e., reboiler and condenser duties become lower. 

On compression, a gaseous phase may condense to a liquid-expanded, L phase via a first-order transition. This transition is difficult to study experimentally because of the small film pressures involved and the need to avoid any impurities [76,193]. There is ample evidence that the transition is clearly first-order there are discontinuities in v-a plots, a latent heat of vaporization associated with the transition and two coexisting phases can be seen. Also, fluctuations in the surface potential [194] in the two phase region indicate two-phase coexistence. The general situation is reminiscent of three-dimensional vapor-liquid condensation and can be treated by the two-dimensional van der Waals equation (Eq. Ill-104) [195] or statistical mechanical models [191]. 

In estimating the enthalpy of polymerization, the physical state of both starting monomer and polymer must be specified. Changes in state are accompanied by ethalpy changes. Therefore, they also affect the level of the polymerization enthalpy. The AfT forN ylylene previously mentioned is apphcable to the monomer as an ideal gas. To make comparisons with other polymerization processes, most of which start with condensed monomer, a heat of vaporization for N ylylene is needed. It is assumed herein that it is the same as that for N ylene, 42.4 kJ /mol (10.1 kcal/mol). Thus the AfT of the hquid monomer -xylylene is 192.3 kJ/mol (46.0 kcal /mol). 

When an atom or molecule receives sufficient thermal energy to escape from a Hquid surface, it carries with it the heat of vaporization at the temperature at which evaporation took place. Condensation (return to the Hquid state accompanied by the release of the latent heat of vaporization) occurs upon contact with any surface that is at a temperature below the evaporation temperature. Condensation occurs preferentially at all poiats that are at temperatures below that of the evaporator, and the temperatures of the condenser areas iacrease until they approach the evaporator temperature. There is a tendency for isothermal operation and a high effective thermal conductance. The steam-heating system for a building is an example of this widely employed process. 

The effect on the coolant temperature of latent and sensible heat transferred to the surface from the condensing vapor is as shown in equation 5 ... 

Thermal shock failures using water result from the water vapor entering the enamel layer through small, submicroscopic cracks formed at the instant of shock. The water condenses in the cracks and in the bubbles of the enamel traversed by the cracks. On subsequent heating, the vapor from the entrapped water expands to cause spalling of the enamel layer. Other quenchant Hquids, such as toluene, oils, and other organic Hquids, also cause fine, almost invisible cracks, but thermal shock failures do not result with these quenchants on subsequent heating (39). 

Other Energy Systems. Chemical plants usually require cooling water, compressed air, and fuel distribution systems. Sometimes also included are refrigeration, pressurized hot water, or specialized heat-transfer fluids such as Therrninol Hquid or condensing vapor. Each of these systems serves the process and reflabiUty is the most important characteristic. Thus a project in any of them that achieves a 10% reduction in energy cost at the expense of a 1% loss of rehabihty loses money for the operation. 

For condensing vapor in vertical downflow, in which the hquid flows as a thin annular film, the frictional contribution to the pressure drop may be estimated based on the gas flow alone, using the friction factor plotted in Fig. 6-31, where Re is the Reynolds number for the gas flowing alone (Bergelin, et al., Proc. Heat Transfer Fluid Mech. Inst., ASME, June 22-24, 1949, pp. 19-28). 

Partial condenser Condenses vapors at a point high enough to provide a temperature difference sufficient to preheat a cold stream of process fluid. This saves heat and eliminates the need for providing a separate preheater (using flame or steam). 

Vapor-Liquid Separation This design problem may be important for a number of reasons. The most important is usually prevention of entrainment because of value or product lost, pollution, contamination of the condensed vapor, or fouling or corrosion of the surfaces on which the vapor is condensed. Vapor-liquid separation in the vapor head may also oe important when spray forms deposits on the w ls, when vortices increase head requirements of circulating pumps, and when shoiT circuiting allows vapor or unflashed liquid to be carried back to the circulating pump ana heating element. 

Condensation Equipment There are two basic types of condensers used for control contact and surface. In contact condensers, the gaseous stream is brought into direct contact with a cooling medium so that the vapors condense and mix with the coolant (see Fig. 25-15). The more widely used system, however, is the surface condenser (or heat exchanger), in which the vapor and the cooling medium are separated by a wall (see Fig. 25-16). Since high removal efficiencies cannot be obtained with low-condensable vapor concentrations, condensers are typically used for pretreatment prior to some other more efficient control device such as an incinerator, absorber, or adsorber. 

Pressure can also be controlled by variable heat transfer coefficient in the condenser. In this type of control, the condenser must have excess surface. This excess surface becomes part of the control system. One example of this is a total condenser with the accumulator running full and the level up in the condenser. If the pressure is too high, the level is lowered to provide additional cooling, and vice versa. This works on the principle of a slow moving liquid film having poorer heat transfer than a condensing vapor film. Sometimes it is necessary to put a partially flooded condenser at a steep angle rather than horizontal for proper control response. 

Another example of pressure control by variable heat transfer coefficient is a vacuum condenser. The vacuum system pulls the inerts out through a vent. The control valve between the condenser and vacuum system varies the amount of inerts leaving the condenser. If the pressure gets too high, the control valve opens to pull out more inerts and produce a smaller tube area blanketed by inerts. Since relatively stagnant inerts have poorer heat transfer than condensing vapors, additional inerts... 

In addition to volume changes the effect of temperature is also important. Thus the specific latent heat of vaporization of a chemical is the quantity of heat, expressed as kJ/kg, required to change unit mass of liquid to vapour with no associated change in temperature. This heat is absorbed on vaporization so tliat residual liquid or tlie sunoundings cool. Alternatively an equivalent amount of heat must be removed to bring about condensation. Thus the temperature above a liquefied gas is reduced as tlie liquid evaporates and tlie bulk liquid cools. There may be consequences for heat transfer media and the strength of construction materials at low temperatures. 

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