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WEEK 04: GENERATION: AUXILLIARIES Auxiliary Systems: Circulating-Water Sections: Circulating-Water | Flue & Waste | Diesel Plant d. Pumps provided to move the circulating water throughout the system are of the rotating type and maybe axial-flow, mixed flow, or centrifugal-flow in design. There are two basic types of pumpwell installations; wet pit and dry pit. Wet pit commonly refers to arrangements where the majority of the pump is located in the flooded area of the pumpwell. Dry pit commonly refers to arrangements where the majority of the pump is located in a dry area of the pumpwell. The axial-flow and mixed-flow vertical-column pumps maybe installed in either wet- or dry-pit pumpwells. Vertical suction, volute pumps and horizontal, double-suction pumps are installed in dry-pit types. Considering the types of pumps available and the installations possible, several arrangements are normally feasible for a particular system and the final decision is usually based on economics, engineering judgment and operator experience. Pumping head requirements vary from approximately 25 to 100 ft (8 to 30 m) and flows typically range from approximately 65,000 to 200,000 gal/min (4.1 to 12.6 m³/s) per pump. Most power plants use between two and four pumps, although six pumps have been used at large nuclear plants. Generally, space capacity is not provided. Horsepower requirements per pump are on the order of 750 to 5000 HP (560 to 3730 kW). Pump motors are normally 3-phase, single-speed induction machines and are connected to the pump via a direct-drive shaft. Motors are generally sized to provide some margin in horsepower (about 10%) over that required by the pumps at their normal operating point. During start-up, a pump motor must be capable of accelerating itself and the pump to full speed at reduced voltage. e. Condenser removes the waste heat from the turbine exhaust steam. The condensed steam collects in a "hot well" at the base of the condenser where it is pumped back to the steam generator as feedwater. There are two basic types of condensers: the surface type and the direct-contact type. In the direct-contact type, water is sprayed directly into the exhaust steam. Because of condensate contamination, this design is not practical when the cooling water comes in contact with the outside environment. The surface condenser is a shell-and-tube heat exchanger. The turbine exhaust steam to the shell side and the cooling water passes theough the tubes, never coming in contact with the steam. The steam condenses on the tubes and collects in a hot well below the tubes. Condenser-tube diameter generally varies from 3/4 to 1-1/4 in (19 to 32 mm) and the condenser may contain from 50,000 to 100,000 tubes. Good heat-transfer properties and resistance to corrosion and fouling are key factors in tube material selections. Materials considered include the following: copper and brass alloys, stainless steel, and titanium. Final selection is based on water quality and economics. f. Cooling Towers are designed to cool via one or two processes or a combination of both: evaporation and direct sensible heat transfer. Towers which cool primarily by evaporation are commonly referred to as evaporative or wet cooling towers. This type of tower is adaptable to most site conditions and requires considerably less space. The water is introduced to a wet tower via pipes which, in the tower, are divided into thousands of distribution nozzles. From here the water falls into a cooling (fill) section. The fill or packing section typically consists of either vertically positioned sheets of thin plastic material or of symmetrically arranged horizontal splash bars. In the former, known as counterflow type, the water spreads in a thin film over the sheets, creating thousands of square feet of contact area. In the latter, also known as cross-flow type, the area of contact is increased by the water impinging on the splash bars, thus breaking into billions of droplets. Wet cooling towers may either be natural-draft, mechanical draft, cross-flow, counterflow or a combination. Natural-draft towers are characterized by their tall hyperbolic concrete shells which reaches up to 600 ft (185 m) in height. Air flow through the tower is created by the chimney effect. Air in the tower receives heat from the cooling tower and its density is reduced. The higher-density outside air flows into the tower to replace the heated air, forcing it up and out the tower. The driving force is directly related to the height of the shell or stack. Thus, natural-draft-tower is dependent on wet-bulb temperature as well as relative humidity. Mechanical-draft towers, as the name implies, the required air flow is achieved by mechanical means, generally of induced-draft fan type. The fans are located downstream of the tower fill. Traditionally, mechanical towers are made up of cells, each cell containing a fan. The cells are arranged in line, making the tower rectangular in shape. The power requirements of the fan motor average about 200 HP (150 kW). Towers which cool by sensible heat transfer are referred to as dry cooling towers. There are two basic types of dry-cooling-tower systems. First, the direct system, in which the cooling tower also functions as the condenser. The turbine exhaust is ducted directly to the cooling tower (an air-cooled finned tubed heat exchanger). The steam is condensed and returned to the steam generator. Secondly, the indirect system, in which a portion of the condensate is cooled. The turbine exhaust steam is condensed via direct contact with the cooling water. The cooling water and condensate combine in the hotwell. A portion of this water is returned to the steam generator as feedwater and the remainder is pumped back to the dry tower. |
