Venting, of noncondensables

Condensation may be performed inside or outside tubes, in horizontal or vertical positions. In addition to the statements made in the previous section about the merits of tube side or shell side When freezing can occur, shell side is preferable because it is less likely to clog. When condensing mixtures whose lighter components are soluble in the condensate, tube side should be adopted since drainage is less complete and allows condensation (and dissolution) to occur at higher temperatures. Venting of noncondensables is more positive from tube side. 

Figure 5.22. (a) Pressure control by direct venting, (b) Venting of noncondensables after a condenser, (c) Con-... 

Second, the velocities are also reduced because of bypassing. The reduced velocities may result in excessive fouling from solids deposition or from high tube-wall temperatures. Third, venting of noncondensables may not be properly achieved with resultant reduction of condensation heat transfer rates. 

To nelp consei ve steam economy, venting is usually done from the steam chest of one effecl to the steam chest of the next. In this way, excess vapor in one vent does useful evaporation at a steam economy only about one less than the overall steam economy. Only when there are large amounts of noncondensable gases present, as in beet-sugar evaporation, is it desirable to pass the vents directly to the condenser to avoid serious losses in heat-transfer rates. In such cases, it can be worthwhile to recover heat from the vents in separate heat exchangers, which preheat the entering feed. 

The top pressure controller varies the level of liquid in the condenser, so it, like the reboiler, must have extra surface for the derating required for control. Many other control methods also require some control surface. If noncondensibles are present, a vent should be provided. Otherwise, they collect at the liquid seal. With large amounts of noncondensibles, another type of system should be considered. 

For steam side noncondensibles, a proper vent is required. A small amount of noncondensibles can greatly lower the steam side heat transfer coefficient. The improper removal of condensate is another way to reduce... 

The vapor outlet should preferably be connected to the flare system. However, when the safety valve releases and other streams tied into the drum contain only a small quantity of noncondensible hydrocarbons or inerts, and where no pollution problems are anticipated, then an atmospheric vent is acceptable, subject to the following conditions ... 

Finally, with respect to reaction products, a chemical reaction that does not generate hazardous reaction products or by-products is inherently safer than one that does. Thought must be given not only to hazardous reaction products, however. The generation of any kind of noncondensible gases can cause a vessel rupture due to internal overpressurization, if not adequately vented or relieved. 

The process includes a condenser for continuous separation of the mixture of gases and the mixture of liquid fuels. The condensed heavier fractions are collected in the accumulator and the uncondensed gases are collected and stored separately through the vent gas stream consisting of noncondensable with gases up to C4. The liquid hydrocarbons are subjected to fractional distillation. 

Most condensers used in steam power plants operate at pressures well below the atmospheric pressure (usually under 0.1 atm) to maximize cycle thermal efficiency, and operation at such low pressures raises the possibility of air (a noncondensable gas) leaking into the condensers. Experimental studies show that the presence of noncondensable gases in the vapor has a detrimental effect on condensation heat transfer. Even small amounts of a noncondensable gas in the vapor cause significant drops in heat transfer coefficient during condensation. Eor example, the presence of less than 1 percent (by mass) of air in steam can reduce the condensation heat transfer coefficient by more than half. Therefore, it is common practice to periodically vent out the noncondensable gases that accumulate in the condensers to ensure proper operation. 

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