Environmental Engineering Reference
In-Depth Information
5.2.6
Condenser
In heat engine cycles, after performing useful work, the working fluid must reject heat to a cold
body. Steam turbine electric power plants work on the principle of heat engines. They reject a
significant amount of heat into the environment. Between 1.5 and 3 times as much heat is rejected
as the plant produces work in the form of electricity. A 1000-MW electric power plant working
at 25% efficiency rejects 3000 MW of heat to the environment, whereas one working at 40%
efficiency rejects 1500 MW. Some of that heat is added to the environment by the condensing
system in a steam cycle, and the rest is added by the discharge to the atmosphere of the hot flue
gas vented through the smoke stack.
In the Rankine steam cycle, after expansion in the turbine, the steam is first condensed into
water in a condenser, then the water is recycled into the boiler by means of a feed pump. The
circulating cooling water of the condenser rejects its heat to the atmosphere by means of a cooling
tower or to a nearby surface water. The condenser serves not only the purpose of condensing the
high-quality feed water of the boiler, but also to lower the vapor pressure of the condensate water.
By lowering the vapor pressure, a vacuum is created which increases the power of the turbine.
There are two types of condenser: direct contact and surface contact condensers. A direct
contact condenser is depicted in Figure 5.4(a). The turbine exhaust passes an array of spray nozzles
through which cooling water is sprayed, condensing the steam by direct contact. The warm conden-
sate is pumped into a cooling tower where updrafting air cools the condensate that flows in tubes.
The cooled condensate is recycled into spray nozzles. Because the cooling water is in direct contact
with the steam exhaust, its purity must be maintained, just like that of the feed water. This makes the
process more expensive, and the majority of power plants use a surface contact condenser, depicted
in Figure 5.4(b). It is essentially a shell-and-tube type heat exchanger. The turbine exhaust passes an
array of tubes in which the cooling water flows. Because for large power plants very large volumes
of steam need to be condensed, the contact surface area can reach 100,000 m 2 for a 1000-MW
plant. The design of a properly functioning condenser involves complicated calculations of heat
transfer. The tubes are surrounded by fins to increase the heat transfer area. The incoming steam
needs to be deaerated of noncondensables, mainly air that leaked into the system. The oxygen of
Turbine exhaust
Turbine exhaust
Dry
cooling
tower
Cold
Cooling water
Condenser
Hot
Air
Condenser
Condensate
Condensate
To feed water
To feed water
Pump
Pump
(a)
(b)
Figure 5.4 (a) Direct contact condenser; (b) surface condenser, schematic.
 
 
 
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