Condenser and Cooling Water

System

By Aklilu Tesfamichael (Dr.)

Condenser

What is the purpose of condenser in a

power plant?

1. To reduce the turbine exhaust

pressure so that

The turbine specific output (thermal

efficiency of the plant) increases.

(for p=1 atm (1 bar) Tsat=100 oC and

P=0.074 bar (Tsat=40 oC) this can

reject heat to 30 oC cooling water.

Reduce the steam flow rate for a

given plant power output

2. To recover high quality feedwater in the form of condensate

and reuse it without any further treatment. Hence, only

makeup water that is required to top up the water lost in the

cycle needs treatment

Types of condensers

They are two types 1. Direct contact: where the condensate and the cooling water

directly mix and come out as a single stream

Fig: Schematic diagram of a direct contact condenser and its T-s diagram

2. Surface condensers

They are shell and tube heat exchangers.

Cooling water and condensate are separated by a solid surface. Heat

transfer is through the walls of the tubes into the cooling water.

For cleaning purpose cooling water flows inside the tubes and the steam

condenses outside the tubes.

Fig: schematic of one and two-pass surface condensers

Condensing process and design

consideration

• Steam contacts the cold surface

• The average heat transfer coefficient as given by Nusselt

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The inside heat transfer coefficient on the water side may be obtained as

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Energy balance between the steam and the cooling water gives:

The rise in cooling water temperature is limited to about 8-10 oC.

For every kg of steam condensate, 75 to 100 kg of water is required.

Hence, to meet the water demand the plant is located where water is available

in plenty.

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Condensing process and design

consideration (contd.)

Cooling Water Outlet Temperature

Calculation The surface area needed by the condenser is obtained by:

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waterofdensityρwhere

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Air removalWhat will happen if air enters to the condenser?

Affects the condenser performance badly because

1. It reduces the heat transfer considerably as air has low

thermal conductivity

2. It reduces the condenser vacuum pressure and increase the

turbine exhaust pressure thus reducing the turbine output.

Source of air leakages

Turbine gland, large diameter flanges such as the steam inlet or

turbine exhaust, open valves or steam chest on the ejectors.

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pressure saturated steam

pressureair pressure totalmeasured Shell

pressure. partial of law sDalton'by

estimated becan shell the theinto dinfiltrate pressureair The

satsatairmsh,

Fig: Turbine shaft gland

Air removal (contd.)

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Condenser performance

Cooling water

Circulating water system supplies cooling water to the turbine condenser thus it act as a medium through which heat is rejected from the steam cycle to the environment.

Cooling water can flow through the condenser in two ways

(a) One through system

(b) Closed loop system

Once through system

Used when there is alarge source of waterlike river, lake or oceanare available.

Fig: schematic of once-through circulating water system

Closed loop water circulating system

• More universal to avoid thermal pollution of river or oceans plus huge water is not every where available

• But this system needs cooling tower

Condenser

Fig: schematic of wet cooling tower operating in closed system

Cooling Towers

Cool the warm water discharged from the condenser by atmospheric air and feed it back to the condenser.

According to the main mode of heat transfer there are two types: wet (evaporative) cooling tower and dry (sensible) cooling tower.

Wet cooling towerAir entering the tower is unsaturated when it comes in contact with the water

spray, the water continues to evaporate till the air becomes saturated.

The minimum temperature to which water can be cooled is the adiabatic

saturation or wet bulb temperature of the ambient air.

Evaporation

Causing cooling

According to the draft type the wet cooling

tower is further classified as

1. Mechanical draught

a. Induced draught

b. forced draught

2. Natural draught

Fig: Natural draught cooling tower

Design parameters of cooling towers

A cooling tower is specified by

a. Approach

b. Range

c. Cooling efficiency

a. Approach (A): the difference between the exit cooling water

temperature and the wet bulb temperature of the ambient air

(minimum achievable), or

b. The cooling range or simply range(R) is defined as the

difference in temperature of the incoming warm water (tc1) and

the exiting cooled water (tc2), or

c. The cooling efficiency is defined as the ratio of the actual

cooling water to the maximum cooling possible, or

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Dry cooling towers

Advantages of dry cooling towers:

1) There is no thermal pollution and loss of water due to evaporation.

2) Power plant can be located closer to the load centre (does not large supply of cooling water)

Disadvantages:

1) they are not as effective as evaporative cooling. As their performance is dependent on the atmospheric conditions and

so turbine exhaust temperatures are much higher resulting in a substantial loss of turbine efficiency ,

most critical in warm climates.

2) Due to low heat transfer coefficient , dry cooling towers require enormous volumes of air, large surface areas and are

less effective at high natural air temperatures.

Wet Cooling Tower Analysis

• Ambient air is used to cool the warm water exiting the

condenser. Properties associated with air-water vapor mixture

• Atmospheric air (dry air plus water vapor) pressure is given by

• Relative humidity

aw ppp

s

w

p

p

etemperaturairtheatpressuresaturation

airinvaporwatertheofpressurepartialRH )(

ps

pw

td.p.

td.b.

Dew point temperature (tdw) is the

temperature at which water vapor starts to

condense when cooled at constant pressure

Dry bulb (tdb) is the temperature recorded by a

thermometer with a dry bulb.

Wet bulb (twb) is the temperature recorded by

a thermometer when the bulb is enveloped by

a cotton wick saturated with water

Wet Cooling Tower Analysis(contd.)

• Humidity Ratio (w)

• If dry and water vapor act as ideal gases

• Degree of saturation is the ratio of the actual specific humidity

to the saturated specific humidity, both at the same

temperature T,

]/[ airdrykgvaporkgm

m

m

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Wet Cooling Tower Analysis(contd.)• If is the make-up water supplied to replenish the

evaporative loss, then

• Energy balance,

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Fig: temperature relationship in counter flow cooling tower

Example 1

A surface condenser receives 250 ton/h of steam at 40oC with

12% moisture. The cooling water enters at 32oC and leaves at

38oC. The pressure inside the condenser is found to be 0.078 bar.

The velocity of circulating water is 1.8 m/s. The condenser tubes

are of 25.4 mm OD and 1.25 mm thickness. Taking the overall

heat transfer coefficient as 2600 W/m2K, determine (a) the rate of

flow of cooling water, b) the rate of air leakage into the condenser

shell, c) the length of tubes, and d) the number of tubes.

Example 2

The following readings were taken during a test on surface

condenser:

Mean condenser temperature = 35oC, Hot well temperature=

30oC, condenser vacuum=69 cmHg, Barometric reading 76

cmHg. Condensate collected 16 kg/min. Cooling water enters at

20oC and leaves at 32.5oC, flow rate being 37,500 kg/h. Calculate

(a) mass of air present per cubic meter of condenser, b) quality of

steam at condenser inlet, c) vacuum efficiency, and d) condenser

efficiency.

Example 3

Water at 30 oC flows into a cooling tower at the rate of 1.15 kg/kg

air. Moist air enters the tower at 8 m3/s volumetric flowrate, 20 oC dbt and a relative humidity of 60%. It leaves at 28oC dbt and

90% relative humidity. Makeup water is supplied at 20oC.

Determine (a) evaluate the mass flow rate of the dry air, b) the

temperature of water leaving the tower, c) the make up water,

and d) the approach and range of the cooling tower. Assume the

atmospheric pressure is 1 atm.

Example 4

Water exiting the condenser of a power plant at 45 C enters a cooling tower

with a mass flow rate of 15000 kg/s. A stream of cooled water is returned to

the condenser from the cooling tower with the same flow rate. Make-up water

is added in a separate stream at 20 C. Atmospheric air enters the cooling tower

at 30 C with a wet bulb temperature of 20 C. The volumetric flow rate of

moist air into the cooling tower is 8000 m3/s. Moist air exits the tower at 40 C

and 90% relative humidity. Assume an atmospheric pressure of 101.3 kPa.

Determine:

a) the mass flow rate of dry air,

b) the mass flow rate of make-up water, and

c) the temperature of the cooled liquid water exiting the cooling tower.