Surface condensers exhaust steam, condensation

Utility plants primarily employ reheat condensing turbines, whereas cogeneration plants and larger process industries that produce their own electrical power tend to employ extraction turbines. Both types of turbine rely on a surface condenser to receive exhaust steam from the LP turbine stage and condense it to liquid for reuse. 

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. 

Most boiler plants with electrical power generating facilities employ surface condensers. These are shell-and-tube heat exchangers in either one-, two-, or four-pass configurations. Surface condensers typically receive cooling water on the tube-side and steam on the shell-side of the heat exchanger. The LP turbine steam generally is received at the top of the condenser and proceeds through the condenser in a downward flow, while the FW turbine exhaust steam enters at the side. 

A design of turbine whereby the driving force is provided by exhaust steam condensing in a surface condenser. 

Surface condensers, in which the cooling water and exhaust steam remain separate (see Chapter 15). Rather than using cooling water, boiler feedwater can be preheated to recover the waste heat. 

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). 

Discounting the presence of air leaks, the temperature inside the surface condenser determines the pressure of the steam exhausting from the turbine. This pressure is the vapor pressure of water at the surface condenser outlet temperature. 

Air-cooled surface condensers. Figure 8.11 shows a surface condenser elevated above the steam turbine. This creates an additional problem, in that moisture from the turbine exhaust steam will accumulate in the bottom of the turbine case. A special drain line from the turbine s case is needed to prevent condensate backup from damaging the spinning wheels. 

The turbine case pressure was increased by raising the pressure in the air-cooled surface condenser. This was accomplished by shutting off several of the air fans, which, in turn, increased the condensing temperature of the exhaust steam. But why would raising the turbine case pressure drain the turbine, anyway After all, increasing the surface condenser pressure also increased the pressure in the drum that the turbine case drained to. 

Surface condensers which condense the exhaust steam from steam... 

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... 

Take a look at Fig. 18.3. It is the vapor outlet temperature of the surface condenser, rather than the condensate outlet temperature of the surface condenser, that determines the real condensing temperature and pressure of the exhaust steam. 

Under certain conditions, such as high water temperatures, insufficient water supplies and problems of blowdown disposal, systems that depend on convection and use air as the transport medium may be preferable. The two types of dry cooling towers are the direct and indirect systems. Figures 4.21 and 4.22 show these systems in operation for nuclear station cooling. Indirect units use a surface or jet condenser at the turbine to condense exhaust steam. Water from the condenser is pumped to the dry tower for cooling and recirculation back to the condenser. In the direct system, steam is condensed in cooling coils without interfacing with a condenser. 

In direct condensing, turbine exhaust steam is conveyed through a trunk to the air-cooled coils, where cooling air passing over the finned coil surfaces condenses the steam. Steam enters the coil section and condenses as it travels downward, with steam and condensate flowing in the same direction, minimizing pressure loss and increasing the heat transfer coefficient. 

Often, there exists spare capacity in a surface condenser, one that condenses exhaust steam from a centrifugal compressor or from a steam turbine under vacuum. This spare capacity may often be over 10% of the normal operating capacity and can be attributed to one or more of the following ... 

A condenser used to condense turbine exhaust steam. Both direct contact and surface condensers are used in direct steam geothermal power plants. 

The condenser is of the surface, twin pass type. It is of a twin shell construction. There are two deaerators that utilize extraction steam from the low pressure turbines, five low pressure feedwater heaters that utilize extraction steam from the low pressure turbines, three high pressure heaters that utilize extraction and exhaust steam from the high pressure turbine, three one-half-sized condensate pumps and condensate booster pumps, and three one-half-sized feedwater pumps. Heater drains from the three high pressure feedwater heaths are cascaded to the deaerator, drains from the five low pressure heaters are cascaded to the condenser. 

Exhibit 4-1 shows a surface condenser mounted directly below the turbine. This arrangement is used when the condenser is designed to service only one steam turbine. The arrangement shown in Exhibit 4-2 is generally used when several turbines are exhausting into one condenser. Exhibit 4-3 shows the various compressor systems and their reciprocals. 

Surface condensers are used in conjunction with condensing steam turbines that drive large centrifugal compressors. As depicted in Exhibit 4-25 the exhaust steam enters the top of the condenser and passes through the shell, which is filled with tubes. Cold water is pumped through the tubes while hot exhaust steam passes around the outside. Hot water, called condensate, results and collects in the hot well at the bottom of die condenser. 

Arrangement D shows a top nozzle that allows the exhaust steam line to run to a surface condenser servicing multiple turbines, as shown in Exhibit 4-2. 

Condensing steam exhaust in surface condensers of low-pressure steam systems. 

The CAREM commercial plants will use a two-stage turbine with re-heater and exhaust steam at low pressure is condensed in a water-cooled surface condenser. The CAREM prototjipe uses a single turbine. 

Figure 12.10 shows the type of surface condenser most widely used on older steam turbines. Note that it has both a vapor and a liquid outlet. The turbine is located above the surface condenser. The wet exhaust steam flows down into the top of the condenser shell. Note that the exhaust steam from an efficient turbine will contain several percent of water. 

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. 

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