Vacuum surface condensers

For moderate-temperature and low-pressure condensers, such as vacuum surface condensers (see Chap. 18), a single pass on the tube side is not uncommon. These exchangers are typically fixed-tubesheet... 

In refinery steam condensation services (in both reboilers and steam turbines exhausting to vacuum surface condensers), 1 have observed that small amounts of non-condensables greatly reduce the apparent heat-transfer coefficient. 

For moderate-temperature and low-pressure condensers, such as vacuum surface condensers (see Chap. 25), a single pass on the tube side is not uncommon. These exchangers are typically fixed-tubesheet designs. Such exchangers are fine from a purely process point of view. However, there is no practical way to disassemble the exchanger to clean the outside (i.e., the shell side) of the tubes. This inability to clean the shell side frequently leads to tremendous loss of efficiency after the condensers foul. 

For a steam turbine and vacuum-surface condenser, check for dissolved oxygen in the steam condensate. The oxygen would come from cooling water leaking into the shell side. 

Example 17-11. Design of Vacuum Surface Condenser. A large vacuum condenser is to handle the following materials per hour ... 

Space needs to be provided for the auxiliaries, including the lube oil and seal systems, lube oil cooler, intercoolers, and pulsation dampeners. A control panel or console is usually provided as part of the local console. This panel contains instmments that provide the necessary information for start-up and shutdown, and should also include warning and trouble lights. Access must be provided for motor repair and ultimate replacement needs to be considered. If a steam turbine is used, a surface condenser is probably required with a vacuum system to increase the efficiency. AH these additional systems need to be considered in the layout and spacing. In addition, room for pulsation dampeners required between stages has to be included. Aftercoolers may also be required with knockout dmms. Reference 8 describes the requirements of compressor layouts and provides many useful piping hints. 

When such welding is not possible, a bhnd -gasket type of construction is utilized. The blind gasket is not accessible for maintenance or replacement once the unit has been constructed. This construction is used for steam surface condensers, which operate under vacuum. 

By far the largest load on the vacuum pump is water vapor carried with the noncondensable gases. Standara power-plant practice assumes that the mixture leaving a surface condenser will have been cooled 4.2°C (7.5°F) below the saturation temperature of the vapor. This usually corresponds to about 2.5 kg of water vapor/kg of air. One advantage of the countercurrent barometric condenser is that it can cool the gases almost to the temperature of the incoming water and thus reduce the amount of water vapor carried with the air. 

Thin films of metals, alloys and compounds of a few micrometres diickness, which play an important part in microelectronics, can be prepared by die condensation of atomic species on an inert substrate from a gaseous phase. The source of die atoms is, in die simplest circumstances, a sample of die collision-free evaporated beam originating from an elemental substance, or a number of elementary substances, which is formed in vacuum. The condensing surface is selected and held at a pre-determined temperature, so as to affect die crystallographic form of die condensate. If diis surface is at room teiiiperamre, a polycrystalline film is usually formed. As die temperature of die surface is increased die deposit crystal size increases, and can be made practically monocrystalline at elevated temperatures. The degree of crystallinity which has been achieved can be determined by electron diffraction, while odier properties such as surface morphology and dislocation sttiicmre can be established by electron microscopy. 

Vacuum Distillation - Heavier fractions from the atmospheric distillation unit that cannot be distilled without cracking under its pressure and temperature conditions are vacuum distilled. Vacuum distillation is simply the distillation of petroleum fractions at a very low pressure (0.2 to 0.7 psia) to increase volatilization and separation. In most systems, the vacuum inside the fractionator is maintained with steam ejectors and vacuum pumps, barometric condensers, or surface condensers. 

There are three potential types of OTEC power plants opcii-cyclc, closed-cycle, and hybrid systems. Open-cycle OTEC systems exploit the fact that water boils at temperatures below its normal boiling point when it is under lower than normal pressures. Open-cycle systems convert warm surface water into steam in a partial vacuum, and then use this steam to drive a large turbine connected to an electrical generator. Cold water piped up from deep below the oceans surface condenses the steam. Unlike the initial ocean water, the condensed steam is desalinated (free of salt) and may be collected and used for drinking or irrigation. 

Vacuum Pump Capacities From Steam Surface Condensers... 

Example 3-6 NPSH Available in Vacuum System, 191 Example 3-7 NPSH. Available in Pressure System, 191 Example 3-8 Closed System Steam Surface Condenser NPSH Requirements, 191 Example 3-9 Process Vacuum System, 192 Reductions in NPSHr, 192 Example 3-10 Corrections to NPSHr for Hot Liquid Hydrocarbons and Water. 192 Example 3-9 Process Vacuum System, 192 Example 3-10 Corrections to NPSHr for Hot Liquid Hydrocarbons and Water, 192 Example 3-11 Alternate to Example 3-10, 194 Specific Speed,... 

These turbine units finally exhaust the steam at considerably less than atmospheric pressure to a condenser (in most circumstances a surface condenser is employed). The condenser is designed to raise turbine operating efficiency by reducing the turbine back-pressure to an absolute minimum. This is achieved by condensing the exhaust steam into a smaller volume of condensate, thus creating a substantial vacuum. 

The efficiency of the condenser is reduced by poor air removal (and the presence of other noncondensable gases), so surface condensers usually are equipped with vacuum pumps but also may incorporate older style, single or multistage multielement, steam-jet air ejectors. Under most normal operations, the residual oxygen level is below 20 to 40 ppb 02. 

Sucked into the surface condenser hot well under vacuum... 

A steam driven device fitted to surface condensers and other items of equipment that removes oxygen and other noncondensable gases, thus maintaining a vacuum. 

Condensers and vacuum pumps will be needed for evaporators operated under vacuum. For aqueous solutions, steam ejectors and jet condensers are normally used. Jet condensers are direct-contact condensers, where the vapour is condensed by contact with jets of cooling water. Indirect, surface condensers, are used where it is necessary to keep the condensed vapour and cooling water effluent separate. 

Many refineries now use vacuum pumps and surface condensers in place of barometric condensers to eliminate generation of the wastewater stream and reduce energy consumption. Reboiled side-stripping towers rather than open steam stripping can also be utilized on the atmospheric tower to reduce the quantity of sour-water condensate. 

A third waste source is very stable oil emulsions formed in the barometric condensers used to create the reduced pressures in the vacuum distillation units. However, when barometric condensers are replaced with surface condensers, oil vapors do not come into contact with water and consequently emulsions do not develop. 

Barometric condenser systems can be a major source of contamination in plant effluents and can cause a particularly difficult problem by producing a high-volume, dilute waste stream [8]. Water reduction can be achieved by replacing barometric condensers with surface condensers. Vacuum pumps can replace steam jet eductors. Reboilers can be used instead of live steam reactor and floor washwater, surface runoff, scrubber effluents, and vacuum seal water can be reused. 

Poly (p-Xylylene). it was discovered by Szwarc, researched by several companies, and finally commercialized experimentally by Union Carbide. It is prepared by pyrolyzing p-xylene at high temperature and vacuum, then condensing on a cool surface, and is... 

Steam used to drive a turbine can be extracted at an intermediate pressure, for further use of the low-pressure steam. Rarely is the steam vented to the atmosphere, as this wastes steam, and the condensate is also lost. Many turbines exhaust steam, under vacuum, to a surface condenser. The lower the pressure in the surface condenser, the greater the amount of work that can be extracted from each pound of steam (see Chap. 17). 

One such turbine, in a refinery near London, would not drain properly, In order to push the condensate out of the turbine case, the operators were forced to raise the surface condenser pressure from 100 to 250 mm Hg (i.e., 20 in of mercury vacuum, in the American system). Note that the balance line shown in Fig. 8.11 keeps the pressure in the turbine case and the condensate drum, into which the turbine case is draining, both equal at the same pressure. 

The condensing turbine does not produce exhaust steam. All the turbine exhaust steam is turned into water in a surface condenser. We will study surface condensers in Chap. 18. The surface condenser is just like the sort of vacuum condensers we discussed in Chap. 16, sections on steam jets. The exhaust-steam condenses under a deep vacuum— typically 76 mm Hg, or 0.1 atm. Basically, then, a condensing steam... 

For a surface condenser to work properly, noncondensable vapors must be sucked out of llie shell side. This is done with a two-stage jet system, as shown in Fig. 18.3. When I was first commissioned the jets, they were unable to pull a good vacuum. Moreover, water periodically blew out of the atmospheric vent. I found, after considerable investigation, that the condensate drain line from the final condenser was plugged. 

The pressure in condenser A is greater than that in the surface condenser, and less than that in the final condenser (condenser B). This means that condenser A is operating at vacuum conditions. This prevents the condensed steam formed in condenser A from draining out to atmospheric pressure, unless the condenser is elevated by 10 to 15 ft. To avoid this problem, the condensate is drained back to the lower-pressure surface condenser. To prevent blowing the noncondensable vapors back to the surface condenser as well, a loop seal is required. The height of this loop seal must be greater than the difference in pressure (expressed in ft of water) between the surface condenser and the primary jet discharge condenser (condenser A). 

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