Vacuum systems vapor condensers

In the evaporation plant especially, glycerine can be carried out with the vapor during evaporation and lost in the vacuum system s condensing of water. The amount of carryover can be minimized in several ways ... 

Metallization layers are generally deposited either by CVD or by physical vapor deposition methods such as evaporation (qv) or sputtering. In recent years sputter deposition has become the predominant technique for aluminum metallization. Energetic ions are used to bombard a target such as soHd aluminum to release atoms that subsequentiy condense on the desired substrate surface. The quaUty of the deposited layers depends on the cleanliness and efficiency of the vacuum systems used in the process. The mass deposited per unit area can be calculated using the cosine law of deposition ... 

Batch distillation equipment can range from a free-standing column with a reboiler, condenser, receiver, and vacuum system, to the use of a jacketed reactor with a condenser. Distillation often involves the generation of combustible vapors in the process equipment. This necessitates the containment of the vapor within the equipment, and the exclusion of air from the equipment, to prevent the formation of combustible mixtures that could lead to fire or explosion. 

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... 

Air is usually the basic load component to an ejector, and the quantities of water vapor and/or condensable vapor are usually directly proportional to the air load. Unfortunately, no reliable method exists for determining precisely the optimum basic air capacity of ejectors. It is desirable to select a capacity which minimizes the total costs of removing the noncondensable gases which accumulate in a process vacuum system. An oversized ejector costs more and uses unnecessarily large quantities of steam and cooling water. If an ejector is undersized, constant monitoring of air leaks is required to avoid costly upsets. 

For big vapor lines and condensers (frequent in vacuum systems) always insulate the line, condenser, and top of column. Rain or sudden cold fronts will change column control otherwise. It is possible to have more surface in the overhead line than in the condenser. 

Few vacuum systems are completely airtight, although some may have extremely low leakage rates. For the ideal system the only load for the ejector is the non-condens-ables of the process (absorbed gases, air, etc.) plus the saturated vapor pressure equivalent of the process fluid. Practice has proven that allowance must be made for air leakage. Considering the air and non-condensables. For base ejector capacity determine inert gases only by ... 

To prevent/reduce the undesirable condensation in the pump, a small hole is drilled in the pump head to admit air or other process non-condensable gas (gas ballast) into the latter portion of the compression stroke. This occurs while the vapor being compressed is sealed off from the intake port by the piston. By reducing the partial pressure of the vapor s condensables, the condensation is avoided. Obviously, this can reduce the capacity of the pump, as the leakage past the seals allows the gas ballast to dilute the intake volume of ba,se suction gas. For most process applications, the effect of this leakage is negligible, unless the vacuum system suction is below 1 torr [22]. 

When simultaneously pumping permanent gases and condensable vapors from a vacuum system, the quantity of permanent gas will often suffice to prevent any condensation of the vapors inside the pump. The quantity of vapor which may be pumped without condensation in the pump can be calculated as follows ... 

The vacuum system must be able to attain the required pressures reliably despite these high gas loads. In the example shown, the system is evacuated with a combination of a backing and Roots pump. A diffusion pump along with a cold surface forms the high vacuum pump system. The cold surfaces pump a large portion of the vapor and volatile substances emitted by the plastic parts while the diffusion pump basically removes the non-condensable gases as well as the noble gas required for the sputter process. 

Vapors can be transferred into or out of the apparatus by applying a rough vacuum to the reservoir which draws mercury out of the U and into the reservoir. After the volatile materials are condensed into the apparatus, they are isolated from the vacuum system by bringing the mercury reservoir to atmospheric pressure and slowly bleeding mercury into the U. If it is important to know the gas volume in the apparatus, the mercury level is adjusted to some reference point, such as A in Fig 9.2, for which the volume of the apparatus has previously been determined. 

A. Liquid Nitrogen and Liquid Air. Liquid nitrogen is the most commonly used refrigerant for the manipulation of condensable materials on a vacuum system because it is easy to use, inexpensive, and will not support combustion. Furthermore, its boiling point is low enough, — 196°C (77°K), to lower the vapor pressure of most materials below 10 3 torr, which is necessary for the quantitative transfer of condensible materials on the vacuum system. 

E. Vapor Pressures above Room Temperature. Since a volatile liquid will distill to the coldest point in an apparatus, it is necessary to thermostat the entire tensimeter system when vapor pressures are determined above room temperature. Two different designs are presented in Fig. 9.7 which meet this requirement alternatively an immersible glass Bourdon pressure transducer may be used. The apparatus in Fig. 9.7.b is suitable for the measurement of gas-phase equilibria as well as vapor pressures. The first and simplest design of the two (Fig. 9.7.a), called an isoteniscope,3 is operated in the following manner On a vacuum system, liquid is condensed into the terminal bulb. A few hundred torr of an inert gas is introduced, the valve is turned off, the apparatus removed from the vacuum system, and the frozen liquid is allowed to melt. The part of the liquid in the terminal bulb is now tipped into the lower U, and inert gas in the region between the bulb and the U is removed by gentle pumping on the system. 

Cold can be used in the laboratory to prevent an experiment from getting too warm, to slow the rate of a reaction, to transfer materials in a vacuum system, to allow for the separation of materials (with fractional condensation), to decrease the vapor pressure of materials so as to trap them in vacuum systems, or as a (cryogenic) vacuum pump. 

For maximum efficiency, a gas ballast should be open when first pumping on a vacuum system, or when pumping a system that has (or is creating) condensable vapors. Once sufficient vacuum has been achieved and the majority of condensable vapors have been removed from a system, the gas ballast can be closed, although it does not hurt (beyond ultimate pump performance) to leave a ballast open all the time. 

It is fairly easy to move condensable vapors from one section to another within a vacuum system by placing liquid nitrogen (within a Dewar) around a trap or sam-... 

Traps (and baffles and filters) are used on vacuum systems because of the need to remove, catch, or bind condensable vapors, particulate matter, and aerosols. To obtain a vacuum, you must remove everything from a given area. However, like the environmental adage you cannot throw anything away, you may not want everything that is removed from a vacuum system passed into the pumping sys-... 

As mentioned, a trap prevents condensable vapors from damaging pump components as they are removed from the vacuum system. Additionally, a trap can improve the quality of the vacuum by preventing condensable vapors and pump oil vapors from drifting back into the system. Remember that once a pump has achieved its potential vacuum, gas flow drops to zero leaving materials to drift... 

The movement of condensable vapors from the mechanical pump can potentially decrease diffusion pump performance. If your system has diffusion and mechanical pumps, there should be a trap between the two pumps in addition to the cold trap between the system and the diffusion pump (see Fig. 7.30). The use of properly designed and placed cold traps can allow diffusion-pumped vacuum systems to achieve vacuums in the region of 10 9 torr35 and greater ... 

Improper selection of coolant for a cold trap may artificially limit the potential vacuum of your system. For instance, the vapor pressure of water (which is often the primary condensable vapor in many vacuum systems) is quite high without any cold trapping, moderate at dry-ice temperatures, and negligible at liquid nitrogen temperatures (see Table 7.11). If your vacuum needs are satisfied within a vacuum of 5 x 10"4 torr, you can safely use dry ice (and save money because dry ice is less expensive than liquid nitrogen). Another temperature option for a coolant is the slush bath (for more information on coolants see Sec. 6.2). 

Suggestion 1 should always be practiced whenever possible. The re-entry of atmosphere into a vacuum system reintroduces copious amounts of moisture onto the vacuum system s walls and reintroduces gases back into the liquids within your system (oil and/or mercury). The next time the system is used, the walls will need to be re-dried and the liquids re-outgassed. Thus, a greater amount of time than would otherwise be necessary will be required the next time you wish to obtain a vacuum. This extra time will also place more wear and tear on your pumps and expose them to extra condensable vapors. 

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