Surface condenser

Surface condenser

From Wikipedia, the free encyclopedia

A surface condenser is a commonly

used term for a water-cooled shell and

tube heat exchanger installed on the

exhaust steam from a steam turbine in

thermal power stations.[1][2][3] These

condensers are heat exchangers which

convert steam from its gaseous to its

liquid state at a pressure below

atmospheric pressure. Where cooling

water is in short supply, an air-cooled

condenser is often used. An air-cooled condenser is however, significantly more

expensive and cannot achieve as low a steam turbine exhaust pressure (and

temperature) as a water-cooled surface condenser.

Surface condensers are also used in applications and industries other than the

condensing of steam turbine exhaust in power plants.

Contents [hide]

1 Purpose

2 Why it is required

3 Diagram of water-cooled surface condenser3.1 Shell

3.2 Vacuum system

3.3 Tube sheets

3.4 Tubes

3.5 Waterboxes

4 Corrosion4.1 Effects of corrosion

4.2 Protection from corrosion

5 Effects of tube side fouling

6 Other applications of surface condensers

7 Testing

8 See also

9 References

10 External links

Purpose [edit]

In thermal power plants, the primary purpose of a surface condenser is to

condense the exhaust steam from a steam turbine to obtain maximum

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Diagram of a typical water-cooled surface condenser

efficiency, and also to convert the turbine exhaust steam into pure water

(referred to as steam condensate) so that it may be reused in the steam

generator or boiler as boiler feed water.

Why it is required [edit]

The steam turbine itself is a device to convert the heat in steam to mechanical

power. The difference between the heat of steam per unit mass at the inlet to

the turbine and the heat of steam per unit mass at the outlet from the turbine

represents the heat which is converted to mechanical power. Therefore, the

more the conversion of heat per pound or kilogram of steam to mechanical

power in the turbine, the better is its efficiency. By condensing the exhaust

steam of a turbine at a pressure below atmospheric pressure, the steam

pressure drop between the inlet and exhaust of the turbine is increased, which

increases the amount of heat available for conversion to mechanical power.

Most of the heat liberated due to condensation of the exhaust steam is carried

away by the cooling medium (water or air) used by the surface condenser

Diagram of water-cooled surface condenser [edit]

The adjacent

diagram depicts a

typical water-

cooled surface

condenser as used

in power stations to

condense the

exhaust steam

from a steam

turbine driving an

electrical generator

as well in other

applications.[2][3][4][5] There are many fabrication design variations depending on

the manufacturer, the size of the steam turbine, and other site-specific

conditions.

Shell [edit]

The shell is the condenser's outermost body and contains the heat exchanger

tubes. The shell is fabricated from carbon steel plates and is stiffened as

needed to provide rigidity for the shell. When required by the selected design,

intermediate plates are installed to serve as baffle plates that provide the

desired flow path of the condensing steam. The plates also provide support that

help prevent sagging of long tube lengths.

At the bottom of the shell, where the condensate collects, an outlet is installed.

Diagram of a typical modern injector or ejector. For a steam ejector,the motive fluid is steam.

In some designs, a sump (often referred to as the hotwell) is provided.

Condensate is pumped from the outlet or the hotwell for reuse as boiler

feedwater.

For most water-cooled surface condensers, the shell is under vacuum during

normal operating conditions.

Vacuum system [edit]

For water-

cooled

surface

condensers,

the shell's

internal

vacuum is

most

commonly

supplied by

and

maintained

by an

external steam jet ejector system. Such an ejector system uses steam as the

motive fluid to remove any non-condensible gases that may be present in the

surface condenser. The Venturi effect, which is a particular case of Bernoulli's

principle, applies to the operation of steam jet ejectors.

Motor driven mechanical vacuum pumps, such as the liquid ring type, are also

popular for this service.

Tube sheets [edit]

At each end of the shell, a sheet of sufficient thickness usually made of stainless

steel is provided, with holes for the tubes to be inserted and rolled. The inlet end

of each tube is also bellmouthed for streamlined entry of water. This is to avoid

eddies at the inlet of each tube giving rise to erosion, and to reduce flow friction.

Some makers also recommend plastic inserts at the entry of tubes to avoid

eddies eroding the inlet end. In smaller units some manufacturers use ferrules

to seal the tube ends instead of rolling. To take care of length wise expansion of

tubes some designs have expansion joint between the shell and the tube sheet

allowing the latter to move longitudinally. In smaller units some sag is given to

the tubes to take care of tube expansion with both end water boxes fixed rigidly

to the shell.

Tubes [edit]

Generally the tubes are made of stainless steel, copper alloys such as brass or

bronze, cupro nickel, or titanium depending on several selection criteria. The

use of copper bearing alloys such as brass or cupro nickel is rare in new plants,

due to environmental concerns of toxic copper alloys. Also depending on the

steam cycle water treatment for the boiler, it may be desirable to avoid tube

materials containing copper. Titanium condenser tubes are usually the best

technical choice, however the use of titanium condenser tubes has been

virtually eliminated by the sharp increases in the costs for this material. The

tube lengths range to about 55 ft (17 m) for modern power plants, depending on

the size of the condenser. The size chosen is based on transportability from the

manufacturers’ site and ease of erection at the installation site. The outer

diameter of condenser tubes typically ranges from 3/4 inch to 1-1/4 inch, based

on condenser cooling water friction considerations and overall condenser size.

Waterboxes [edit]

The tube sheet at each end with tube ends rolled, for each end of the

condenser is closed by a fabricated box cover known as a waterbox, with

flanged connection to the tube sheet or condenser shell. The waterbox is

usually provided with man holes on hinged covers to allow inspection and

cleaning.

These waterboxes on inlet side will also have flanged connections for cooling

water inlet butterfly valves, small vent pipe with hand valve for air venting at

higher level, and hand operated drain valve at bottom to drain the waterbox for

maintenance. Similarly on the outlet waterbox the cooling water connection will

have large flanges, butterfly valves, vent connection also at higher level and

drain connections at lower level. Similarly thermometer pockets are located at

inlet and outlet pipes for local measurements of cooling water temperature.

In smaller units, some manufacturers make the condenser shell as well as

waterboxes of cast iron.

Corrosion [edit]

On the cooling water side of the condenser:

The tubes, the tube sheets and the water boxes may be made up of materials

having different compositions and are always in contact with circulating water.

This water, depending on its chemical composition, will act as an electrolyte

between the metallic composition of tubes and water boxes. This will give rise to

electrolytic corrosion which will start from more anodic materials first.

Sea water based condensers, in particular when sea water has added

chemical pollutants, have the worst corrosion characteristics. River water with

pollutants are also undesirable for condenser cooling water.

The corrosive effect of sea or river water has to be tolerated and remedial

methods have to be adopted. One method is the use of sodium hypochlorite, or

chlorine, to ensure there is no marine growth on the pipes or the tubes. This

practice must be strictly regulated to make sure the circulating water returning

to the sea or river source is not affected.

On the steam (shell) side of the condenser:

The concentration of undissolved gases is high over air zone tubes. Therefore

these tubes are exposed to higher corrosion rates. Some times these tubes are

affected by stress corrosion cracking, if original stress is not fully relieved during

manufacture. To overcome these effects of corrosion some manufacturers

provide higher corrosive resistant tubes in this area.

Effects of corrosion [edit]

As the tube ends get corroded there is the possibility of cooling water leakage to

the steam side contaminating the condensed steam or condensate, which is

harmful to steam generators. The other parts of water boxes may also get

affected in the long run requiring repairs or replacements involving long duration

shut-downs.

Protection from corrosion [edit]

Cathodic protection is typically employed to overcome this problem. Sacrificial

anodes of zinc (being cheapest) plates are mounted at suitable places inside

the water boxes. These zinc plates will get corroded first being in the lowest

range of anodes. Hence these zinc anodes require periodic inspection and

replacement. This involves comparatively less down time. The water boxes

made of steel plates are also protected inside by epoxy paint.

Effects of tube side fouling [edit]

As one might expect, with millions of gallons of circulating water flowing through

the condenser tubing from seawater or fresh water, anything that is contained

within the water flowing through the tubes, can ultimately end up on either the

condenser tubesheet (discussed previously) or within the tubing itself. Tube side

fouling for surface condensers falls into five main categories; particulate fouling

like silt and sediment, biofouling like slime and biofilms, scaling and

crystallization such as calcium carbonate, macrofouling which can include

anything from zebra mussels that can grow on the tubesheet, to wood or other

debris that blocks the tubing, and finally, corrosion product (discussed

previously).

Depending on the extent of the fouling, the impact can be quite severe on the

condenser's ability to condense the exhaust steam coming from the turbine. As

fouling builds up within the tubing, an insulating effect is created and the heat

transfer characteristics of the tubes are diminished often requiring the turbine to

be slowed to a point where the condenser can handle the exhaust steam

produced. Typically, this can be quite costly to power plants in the form of

reduced output, increase fuel consumption and increased CO2 emissions. This

"derating" of the turbine to accommodate the condenser's fouled or blocked

tubing is an indication that the plant needs to clean the tubing in order to return

to the turbine's nameplate capacity. A variety of methods for cleaning are

available including online and offline options depending on the plant's site-

specific conditions.

Other applications of surface condensers [edit]

This page was last modified on 23 November 2014 at 13:51.

Text is available under the Creative Commons Attribution-ShareAlike License; additional terms may apply.

Vacuum evaporation

Vacuum refrigeration

Ocean Thermal Energy (OTEC)

Replacing barometric condensers in steam-driven ejector systems

Geothermal energy recovery

Desalination systems

Testing [edit]

National and international test codes are used to standardize the procedures

and definitions used in testing large condensors. In the U.S., ASME publishes

several performance test codes on condensers and heat exchangers. These

include ASME PTC 12.2-2010, Steam Surface Condensers,and PTC 30.1-2007,

Air cooled Steam Condensers.

See also [edit]

Condensing steam locomotive

Deaerator

Feedwater heater

Fossil fuel power plant

Jet condenser

Power station

References [edit]

1. ^ Robert Thurston Kent (Editor in Chief) (1936). Kents’ Mechanical Engineers’Handbook (Eleventh edition (Two volumes) ed.). John Wiley & Sons (WileyEngineering Handbook Series).

2. ^ a b Babcock & Wilcox Co. (2005). Steam: Its Generation and Use (41st editioned.). ISBN 0-9634570-0-4.

3. ^ a b Thomas C. Elliott, Kao Chen, Robert Swanekamp (coauthors) (1997).Standard Handbook of Powerplant Engineering (2nd edition ed.). McGraw-HillProfessional. ISBN 0-07-019435-1.

4. ^ Air Pollution Control Orientation Course from website of the Air PollutionTraining Institute

5. ^ Energy savings in steam systems Figure 3a, Layout of surface condenser(scroll to page 11 of 34 pdf pages)

External links [edit]

Overview of power plant condenser and cooling systems

Categories: Power station technology Heat exchangers

Energy conversion Steam power

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