Further Reading
Section 8.3.4: Section 8.3.4: Carbon Emissions 1
9.1 Background
9.1.1 Steam Surface Condensers
Condensers for large steam‐turbine generators are designed to discard waste heat from the steam power cycle to the atmosphere, most often to water or air. The engineer optimizes the capital cost and performance of the steam‐turbine power plant to provide a system of pumps or fans, water piping, and the condensing equipment in a way that minimizes capital cost for the maximum steam turbine output. A wet surface condenser is a shell‐and‐tube design with cooling water on the tube side and steam condensing on the shell side. Cooling water may be supplied from a body of water, or from a cooling tower. Dry condensers are finned tube exchangers with steam on the inside of the tubes. Air is blown across the tubes to remove the latent heat from the steam.
The condenser’s design balances two requirements: (i) the removal of latent heat from the steam given by equation (9.1), and (ii) the heat‐transfer relationship shown in equation (9.2), where LMTD is given by (9.3) and its terms are defined in Figure 9.1. The flow from the steam turbine and its efficiency determine the mass flow (m) and inlet enthalpy (h) to the condenser giving the required energy that must be transferred to the cooling water. The heat exchange area (A) and the exchanger’s characteristic heat transfer coefficient (U) determine the satura- tion temperature (Tsat) of condensing; and thus the latent heat in the steam. The condensing
pressure, and condensate enthalpy determined by equation (9.2) balance with the requirement to condense the quantity of steam at its enthalpy from the steam turbine – equation (9.1):
Q m h
i n
i 1
(9.1)
where i = inlet stream flow1 1 through n
Q UA LMTD (9.2)
LMTD T T
T T
T T
c c
sat c
sat c
2 1
2 1
ln
(9.3)
For a wet condenser, the capital cost to install the cooling water piping, cooling water pumps condenser tubes and shell is optimized with the steam turbine output and cycle efficiency. As an example, a high rate of cooling water flow can decrease the condensing pressure, thus improving the steam turbine output but the piping capital cost, and pumping power soon overwhelm the improved steam‐turbine output.
The inlet temperature of the cooling water from a water body is a boundary condition for the designer. This temperature changes with seasons, affecting the performance of the steam turbine and the design of related systems. If a cooling tower is used, its size is optimized with the cooling water, condenser, and power generation system to provide the owner with the best combination of capital and operating costs, with steam‐turbine output.
Materials for a wet‐surface condenser are selected for performance, reliability, and corro- sion resistance. Copper alloys have good heat‐transfer characteristics, and are generally
Inlet Outlet
Temperature
Condenser LMTD
Δ T1
Tsat
Tc1
Δ T2 Tc2
Figure 9.1 Condenser LMTD definitions.
resistant to most corrosion mechanisms in cooling water. However, these alloys do not have the strength to tolerate high tube velocities and thin walls. Stainless steels, or titanium alloys, are more costly than copper alloys, and suffer from lower heat‐transfer characteristics but these materials have the strength to allow thinned wall tubes that can have high water veloc- ities. Higher tube velocity improves heat transfer, and physically scours the tube to remove scale that builds up at speeds suitable for copper‐alloy tubing. Therefore, a condenser with titanium tubes is smaller, lighter, and can cost less than a copper alloy design, even with the high‐cost material.
The tube bundle of the wet condenser is designed to guide steam around the full perimeter of the shell, and the steam flows radially towards the center. Steam flowing from the bottom serves to reheat droplets falling from tubes in the upper section of the condenser. This reduces subcooling of the condensate below the condensing temperature and improves the efficiency of the exchanger. See the tube bundle design shown in the cutaway of Figure 9.2. Noncondensable gases that may be present in the steam collect in the center of the bundle and are evacuated.
In a steam‐turbine power plant with multiple low‐pressure steam turbines, the condenser is often a multipressure design. The cooling water flows continuously from one condenser to the next, resulting in a higher condensing pressure in the downstream condensing sections. Steam condensate from a lower pressure condenser section flows to the higher pressure section through a difference in static head (see Figure 9.3). Steam is directed under the distribution platform and is condensed by direct contact with the colder condensate, reheating the conden- sate to the higher pressure condenser saturation temperature. A multipressure design improves the capital cost and use of resources.
For an air‐cooled, or dry, condenser the air flow rate and finned surface area are optimized with steam‐turbine performance in much the same way as for a wet condenser. Dry con- densers are considerably larger than a wet design due to the lower coefficient of heat transfer
Figure 9.2 Steam surface condenser. Source: Reproduced by permission of Alstom.
between the air and tube surface. Optimization of an air‐cooled condenser results in a higher condensing pressure than for a wet condenser. Due to their high cost, air‐cooled condensers are used only when conditions prevent using water‐cooled designs. For example, when there is a shortage of water, or where environmental considerations limit temperature rise or dis- charge of dissolved solids from a wet cooling tower, an engineer may specify an air‐cooled design. In Figure 9.4, the air cooled condenser is to the right of the main power station. Steam
Condenser 1 low pressure
condenser Condenser 2
higher pressure condenser Water level 1
Water level 2 Water flow through
underflow weir Steam flow for condensate
reheat To condensate pump
Figure 9.3 Steam condensate flowing to the higher pressure section through a difference in static head.
Figure 9.4 Power plant with an air‐cooled condenser. Source: Reproduced by permission of the ATCO Group.
is directed into the condensing surfaces through the four parallel white ducts on the top of the condenser. Fans are located beneath the condenser, forcing air through the unit, enhancing heat transfer.