The ultimate heat sink for a large thermal power station is the atmosphere. There are various options available, using different processes to achieve the most effective heat sink, and therefore meet the requirements of the condensing plant and cooling water (CW) system.
Typical atmospheric heat dissipation systems are illustrated in Fig 4.1. Those most commonly used are:
- Process (a) evaporative cooling, associated with closed systems (cooling towers) for heat dissipation.
- Process (b) heated water discharges, associated with direct cooled systems (river or eawater) for heat dissipation.
When considering a new site for a power station, it is important at the planning stages to ensure that it has adequate cooling water facilities. With increasingly high station output and unit rating, the choice of location is narrowed by the necessity to match available water resources. This along with equally important factors, such as type of fuel and selection of steam conditions, are the major features considered when assessing the suitability of any site.
In order for a steam power station to operate an efficient closed cycle, the condensing plant, CW system, and associated pumps must extract the maximum quantity of heat from the exhaust steam of the LP turbines.
The primary functions of the condensing plant are:
- To provide the lowest economic heat rejection temperature for the steam cycle.
- To convert the exhaust steam to water for reuse in the feed cycle.
- To collect the useful residual heat from the drains of the turbine feedheating plant, and other auxiliaries.
The aim of the CW system is to maintain a supply of cooling medium to extract the necessary heat, in order that the condensing plant can meet its objectives. It achieves this by the use of effective screening equipment, circulating water pumps, valves, and (where necessary) cooling towers.
The economics, design, construction and functional requirements of the above systems and associated plant components are discussed in detail in the sections which follow.
Since many pumps of different types and duties feature in the above systems, aspects of their design are collectively considered in later sections of this chapter. These include condensate extraction pumps, cooling water pumps, circulating water pumps and feed pumps.
Figure 4.2 indicates the component terminology for the two typical arrangements of the evaporative cooled and heated water systems which will be used throughout this chapter, and it is intended to be used for reference purposes. The following paragraphs briefly describe the functional requirements of some of the plant components.
The screening plant must remove any debris from the cooling water which is large enough to block the condenser or auxiliary cooler tubes. It must be easy to keep clean, even during periods of excessive debris.
The cooling water (CW) pumps must circulate the water against system resistance, or pumping head, under all conditions encountered at a particular site. To ensure efficient and flexible CW pump operation, valves are usually provided to allow any combination of pumps, condensers and cooling towers to operate together.
In the heated water discharge direct-cooled system, the cooling water (river or seawater) is used once and then discharged. In the evaporative-cooled closed cooling tower and mixed cooling systems, the cooling towers transfer heat from the plant to the atmosphere and the cooled water is recirculated. In this case the water requirements are for make-up and purge purposes only.
In addition to the condenser satisfying the primary functions, its design must also be capable of meeting the following objectives:
- To provide the turbine with the most economic back pressure consistent with the seasonal variations in CW temperature or the heat sink temperature of the CW system.
- To effectively prevent chemical contamination of the condensate either from CW leakage or from inadequate steam space gas removal and condensate de-aeration.
The aim of the designs is to ensure that these objectives are met within the framework of the following practical considerations:
- Economies of size, space and pumping power.
- Ease of maintenance and construction.