Vapor condensable

The revolatilization of condensable vapors will decreases vacuum potential by replenishing the vapors in the system you were just trying to remove. This can create an artificially high maximum limit on the pump s potential vacuum. In addition to the backstreaming of vapors, the pump itself is affected when condensable vapors contaminate the pump s oil. Not only will this decrease the vapor pressure of the pump oil, but the condensed vapors can cause a reduction of lubrication and sealing properties of the oil and lead to an eventual corrosion of the pump s internal parts.16 Other condensed liquids (such as hydrocarbons) can mix, emulsify, and/or break down the pump oil. They can also directly destroy a pump by chemical attack, or indirectly, by poor pump performance, they can cause extra wear and tear on the pump parts. 

There is no one good way to prevent condensable vapors from affecting a mechanical pump. There are, however, two directions that one can take in dealing with the problem One is to limit them from getting to the pump, the other is to prevent them from affecting the pump once they are present. Neither is the best approach, and usually it takes combinations of the two to deal effectively with the problem. An alternative approach is to constantly change the pump oil. This solution however, is neither cost- nor time-effective. 

To prevent (or limit) condensable vapors from getting to a pump, traps [either of chilled or chemical design (see Sec. 7.4 on traps and foreline traps)], are used. Depending on the type of trap used, there are opportunities for vapors to pass on to the mechanical pump. Thus, one cannot depend fully on traps of any kind, and one must also deal with vapors at the pump itself. 

To prevent (or limit) condensable vapors that reach a pump from affecting the mechanical pump oil, a gas ballast (also called a vented exhaust) is used. The gas ballast allows a small bit of atmosphere (up to 10%) into the pump during the compression stage so that the gas from the system is only part of the gas in the pump at the time of greatest compression. Thus, at the time of compression, the total percentage of condensable vapor within the pump is much less than there would be otherwise. Because the gas prior to being expelled is at a lower pressure, less of the vapor can be compressed into a liquid. Then, as the veins sweep into the vacuum side of the pump, no condensed vapor can expand back into a vapor. 

Ballasting decreases the potential vacuum a pump could normally produce (about one decade of performance capability ). However, it dramatically improves its performance over the long run in the presence of condensable vapors. Plus, it helps to protect pump oils from contamination, which decreases pump breakdown possibilities and increases the longevity of the pump oils. Incidentally, running a pump with a ballast causes the pump to run a bit hotter than it otherwise would, which decreases the potential gas-carrying ability of the oil. 

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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 ... 

The system of primary interest, then, is that of a condensable vapor moving between a Hquid phase, usually pure, and a vapor phase in which other components are present. Some of the gas-phase components may be noncondensable. A simple example would be water vapor moving through air to condense on a cold surface. Here the condensed phase, characterized by T and P, exists pure. The vapor-phase description requiresjy, the mole fraction, as weU as T and P. The nomenclature used in the description of vapor-inert gas systems is given in Table 1. 

Ethanol removed by the vapor stream can be recovered by condensation, vapor recompression, or scmbbiag. Ia the first two methods, the coaceatratioa of the recovered ethanol depends on the relative humidity of the sweep stream and the ratio of sweep and permeation rates. In scmbbiag, the rate of water deflvery to the Hquid-gas coatactor affects the ethanol concentration ia the recovered stream. 

Vapor-Phase Techniques. Vapor-phase powder synthesis teclmiques, including vapor condensation, vapor decomposition, and vapor—vapor, vapor—Hquid, and vapor—soHd reactions, employ reactive vapors or gases to produce high purity, ultrafine, reactive ceramic powders. Many nonoxide powders, eg, nitrides and carbides, for advanced ceramics are prepared by vapor-phase synthesis. 

Drying is an operation in which volatile Hquids are separated by vaporization from soHds, slurries, and solutions to yield soHd products. In dehydration, vegetable and animal materials are dried to less than their natural moisture contents, or water of crystallization is removed from hydrates. In freeze drying (lyophilization), wet material is cooled to freeze the Hquid vaporization occurs by sublimation. Gas drying is the separation of condensable vapors from noncondensable gases by cooling, adsorption (qv), or absorption (qv) (see also Adsorption, gas separation). Evaporation (qv) differs from drying in that feed and product are both pumpable fluids. 

Humidf denotes the amount of condensable vapor present in a gas, expressed as weight of vapor per unit weight of dry gas ie, dry basis weight. 

Percent saturation is the ratio of the partial pressure of a condensable vapor ia a gas to the vapor pressure of the Hquid at the same temperature, expressed as a percentage. For water vapor ia air this is called percent relative humidity. 

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. 

Use of Desiccants and Chemical Means to Remove Water. Another means to remove the water of esterification is calcium carbide supported in a thimble of a continuous extractor through which the condensed vapor from the esterification mixture is percolated (41) (see Carbides). A column of activated bauxite (Elorite) mounted over the reaction vessel has been used to remove the water of reaction from the vapor by adsorption (42). 

Condensedf Vapor Condensedf Vapor Condensed Vapor Condensed Vapor Condensed Vapor Condensed Vapor Condensed Vapor ... 

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). 

For split flow (Fig. 11-35G), the longitudinal baffle may be solid or perforated. The latter feature is used with condensing vapors. 

The downflow condenser is used mainly for nonisothermal condensation. Vapors enter through a header at the top and flow downward. The refliix condenser is used for isothermal and small-temperature-change conditions. Vapors enter at the bottom of the tubes. 

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. 

Cooling of a gas containing a condensable vapor. Here the problem is that the gas cools faster than condensable vapor can be removed by mass transfer. 

A foulinghke problem may occur when condensable vapors are left in the residiie. Condensation may result which in the best case results in blinding of the membrane, and in the usual case, destruction of the membrane module. Dew-point considerations must be part of any gas-membrane design exercise. 

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. 

Combination of cooling and condensation of a mixture of gas and condensable vapor... 

This expansion of a condensing vapor is highly desirable thermodynamically, but the hquid must not bombard and erode the rotor blades, and, in particular, it must not accumulate in the rotor, since that would cause efficiency loss. 

Figure 8.8 Severe grooving on the internal surface of a copper pipe carrying condensate. Grooves were cut by condensing vapors running down pipe walls. Note the vivid blue corrosion products and deposits near the bottom.

Figure 8.9> A brass condenser tube severely wasted by condensing vapors containing ammonia.

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