The steam cycle power plants are equiped with cooled condensers where exhaust steam is
condensed under vacuum. The operating condenser pressure is 0.03 to 0.4 bar and depends
on the cooling system and medium temperature.
Three different steam condensers are used in two fundamentally different cooling systems:
1. Direct Cooling Systems:
a) One through cooling in surface condenser
b) Dry cooling air condenser
2. Indirect Cooling Systems:
a) Wet cooling tower and surface condenser
b) Dry cooling tower and direct contact condenser
1. Direct Cooling Systems
a) Once Through Cooling in Surface Condenser:
This open loop system has been achieved using water from a river, a stream or seawater.
The cold wate is pumped through the condenser tubes and the warm water is discharged back
to the water source. Surface condenser is explaned in wet cooling system.
b) Direct Dry Cooling, Air Cooled Steam Condenser:
Another form of condensing system is the air-cooled condenser. They are more environmentally
acceptable forms of condensing steam. The process is similar to that of a radiator and fan.
Exhaust steam from the low pressure section of a steam turbine runs through the condensing
tubes. The heat transfered from the process steam to the cooling air via extended surfaces
or tubes. The tubes are usually finned and ambient air is pushed through the fins with the
help of a large fan. The steam condenses to water to be reused in the water-steam cycle.
The performance of dry cooling systems is primarily dependent on the ambient temperature of
the dry air. Since the ambient dry air temperature is higher than the wet air temperature,
dry cooling systems are less efficient than wet cooling tower design.
In dry cooling systems, the turbine exhaust is connected directly to the air cooled steam
condenser (direct cooling system). The steam exhaust duct has a large diameter and is
usually as short as possible to reduce pressure losses. An optimum fin tube geometry which
give the highest heat transfer for the minimum amount of metal should be selected.
Advantages of dry cooling
Disadvantages of dry cooling
No water required
Duct pressure losses, less efficient
Can be located at fuel source
Large plot area required
No impact on environment
Generated more noise
Less permitting required
-
The condensation temperature
in the condenser section is by 2-4 °K lower than the exhaust steam temperature,
due to the steam pressure drop through the distributing duct and the heat exchanger tubes.
2. Indirect Cooling Systems:
a) Indirect Wet Cooling System, Surface Condenser:
The need to reduce the amount of water requires a closed loop or wet cooling system.
In a wet cooling system, water is circulated to condense the steam in the surface condenser.
The warm water, instead of being rejected to the water source, is cooled in cooling tower
using air as cooling medium. The wet cooling tower based on principle of evaporation.
The heated cooling water coming out of the surface condenser is cooled as it flows through
a cooling tower, where air is forced through the tower by either natural draft or mechanical.
The exhaust steam is condensed at the outside of the surface condenser tubes.
Using cold water coming from the cooling tower.
Part of the cooling water is evaporated in the cooling tower, and a continuous source of
fresh water (make-up water) is required to operate a wet cooling tower.
The Make-up requirements for a cooling tower consists of the summation of evaporation loss,
drift loss and blow-down.
Estimation of the evaporation loss:
Meva = 0.0017 x Mcw x dTcool (dT in °K)
Drift is entrained water in the tower discharge vapors. Drift loss is a function of the
drift-eliminator design. And a typical value is 0.005% of the cooling water flow rate.
Surface Condenser:
Water cooled condenser used in once through cooling system and in wet cooling system.
The steam condenser is a major component of the steam cycle in steam power and
combined cycle power plants. It is a necessary component of the steam cycle for two reasons:
- It converts the used steam back into feedwater for return to the boiler.
- It increases the cycle´s efficiency by allowing the cycle to operate with
the largest possible Temperature and pressure difference between the boiler and the condenser.
Design principle:
There are different condenser designs which are defined by suppliers. Condenser tubes are
arranged as tube bundles in condenser shell with a single-pass or two-pass. The bundle shape
and air cooler location are optimized by the supplier.
The design of single-pass condenser provides cooling water flow through straight
tubes from the inlet waterbox on one end, to outlet waterbox on the other end.
The design of two-pass condenser provides cooling water flow through straight
tubes from the inlet waterbox, reversed in the return waterbox to the outlet watebox on
the same end of Inlet waterbox.
In the condenser several thousand tubes are placed at low tube pitch in order to get
acceptable dimensions. It is not favourable to increase friction losses in the steam flow,
therefore the number of tube rows along the steam flow is limited. The separation between
the water box areas and the steam condensing area is accomplished by two tubesheet to
which the coolingwater tubes are attached. The cooling water tubes are supported within
the condenser by the tube support plates.
The condenser tubes are made of brass or stainless steel to resist corrosion from either side.
Nevertheless they may become internally fouled during operation by bacteria or algae in the
cooling water or by mineral scaling, all of which inhibit heat transfer and reduce thermodynamic
efficiency. Many plants include an automatic cleaning system that circulates sponge rubber balls
through the tubes to scrub them clean without the need to take the system off-line.
The exhaust steam condensates on the outside of condenser tubes. The saturated liquid can continues
to transfer heat to the cooling water. A few degrees subcooling prevents the condensate pump
cavitation. The condensate subcooling or depression decreases the operation efficiency of plant
because the subcooled condensate must be reheated. The condensat subcooling can be avoided by using
a suitable bundle design. The condensate is collected in the hotwell which is arranged in the
bottom area of the condenser where the condensate pump takes its suction.
Condensation heat Load:
Q = Mcw x (hwout -hwin) = Mexh x (hsteam -hcond) [kW]
Condensing area: A = Q/(k x dTlog) [m2]
Log. temperature difference: dTlog = (dTin -TTD)/(dTin/TTD)
dTin=(Tsteam-Twin)
Overall heat transfer coeffitient k-value: the value of "k" depends on the heat transfer rate
inside the tubes and condensation heat transfer rate outside, which is known worldwide
in several norms.
The condenser is maintained at a vacuum using either vacuum pumps or air ejectors. Cooling of
the steam is provided by Condenser Cooling Water pumped through the condenser by Circulating
Water Pumps, which take a suction from water supplied from the ocean, sea, lake, river, or
Cooling Tower (shown in cooling systems).
The temperature and flow rate of the cooling water through the condenser controls the saturation
pressure (vakuum) and the temperature of the condensate.
To prevent the condensate level from rising to the lower tubes of the condenser, a hotwell
level control system is employed.
The performance of the condenser is dependent on the efficiency of its air cooler section.
The condenser should be equipped with a highly effective air cooler section.
The non-condensable gases should be collected and removed by venting system.
b) Indirect Dry Cooling System (Heller System)
Direct contact condenser and dry cooling tower
The turbine exhaust steam is air-cooled by means of intermediate heat transfer circuit with
condensate quality water. The circuit water is circulated by pumps. Water films formed by the
fill of this circuit condenses the exhaust steam in the direct contact condenser
(spray condenser). A fraction of the warm circuit water, equal to the condensate stream, is
pumped forward to the water-steam cycle by means of condensat pump or by using the circuit pump.
The most of the warm circuit water is pumped via pipeline to a natural draft cooling tower where
it cools down in water-to-air heat exchangers arranged vertically around the towers
circumference.
The dry cooling system keeps approximately a constant temperature difference between
exhaust steam and ambient air. This inlet temperature difference (ITD~30°K) is in reverse
proportion to the dimension and price of the cooling system.
The Heller system offers a number of advantages:
Efficient condenser (TTD~0.3 K),
Condenser low-cost (four time less expensive than the surface condenser),
Simple and maintenance free
Lower condenser pressure in winter
No make-up is required, the system saves cooling water
The system is available with natural or mechanical draft tower
No fog is caused by the dry-cooling tower
Recovery water turbines are recommended, thus saving on pumping power
Due to reduction of the terminal temperature difference TTD and condenser pressure, the
power of the turbine is increased compered with a surface condenser with a dry cooling
tower. But the comparison is complete if the consequences on the overall economy of the
cooling systems are investigated: DC condenser with dry cooling tower and surface condenser with wet cooling tower.
Hybrid Condenser with hybrid cooling tower:
Combination of Direct contact condenser and Surface Condenser (hybrid condenser) connected
with dry and wet cooling tower (hybrid cooling tower). US Patent (US2006080964): Hybrid Condenser
