Condenser and Circulating Water System

Condenser and Cooling Water

Condenser and Cooling Water

System

System

By Aklilu Tesfamichael (Dr.)

Condenser

Condenser

What is the purpose of condenser in a What is the purpose of condenser in a power plant?

power plant? 1.

1. TTo reo reduduce tce the the tururbibine ene exhxhauaustst pressure so that

pressure so that



 The turbine specific output (thermalThe turbine specific output (thermal efficienc

efficiency of y of the plant) increases.the plant) increases. (fo

(for r p=1 p=1 atmatm (1 (1 barbar) ) TT_satsat=100=100 oo_{CC aanndd} P=0.074 bar (T

P=0.074 bar (T_satsat=40=40 oo_{C) this canC) this can} reject heat to 30

reject heat to 30 oo_{CC coocoolinling wag waterter..}



 Reduce the steam flow rate for aReduce the steam flow rate for a given plant power output

given plant power output 2

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m

maakkeeuupp wwaatteerr tthhaatt iiss rreeqquuiirreedd ttoo ttoopp uupp tthhee wwaatteerr lloosstt iinn tthhee cl

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

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.

Condensing process and design

consideration

• Steam contacts the cold surface

• The average heat transfer coefficient as given by Nusselt

4 1 2 3 4 1 4 1 4 1 725 . 0 ,  /  1 ,  /  1 o  f   fg  f   f  av  fg av w sat  av av d   N  gh k  h h h t  t  h tubes horizontal of  number   N   N  h

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

4 . 0 8 . 0 Pr Re 023 . 0 _d  d   Nu

, , ,cw cw e cw in  p s cw t  t  c h m m

_



Energy balance between the steam and the cooling water gives:

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

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 lm o o in cw ou t  cw cw  p cw ou t  s in s h m c T  T  U  A t  h _{, , , , ,} s m Q  _ e i e i lm t  t  t  t  t  ln C  t  C  t  C  o e o in o 3 17 11 range d Recommende

Condensing process and design

consideration (contd.)

Cooling Water Outlet Temperature

Calculation

The surface area needed by the condenser is obtained by:

2.5m/s)

(1.8

water

of 

velocity

V

water

of 

density

ρ

where

4

:

is

tubes

the

in

rate

flow

water

The

.

condenser)

pass

single

a

(for

tube

one

of 

length

l

and

tubes,

of 

number

n

where

2 V  d  n m l d  n T  U  h m  A i c o lm o s o





Air removal

What 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.

temp measured shell at pressure saturated the is p ; p p p pressure saturated steam pressure air pressure total measured Shell pressure. partial of  law s Dalton' by estimated be can shell the the into d infiltrate pressure air The sat sat air m sh,

Air removal (contd.)

water cooling of  rise re temperatu Maximum water cooling of  rise re temperatu Actual efficiency Condenser pressure steam exhaust at pressure saturation -pressure Barometric inlet condenser steam by produced Vacuum efficiency Vacuum shell. condenser the from removed ly continuous be to has air This equation. above the from estimated be can leakage air of  rate the Hence, kJ/kgK 0.287 air of  constant gas stic characteri R steam exhaust of  volume specfic where 273) (t R m m p pressure, low such at gas ideal an as behaves air Assuming a 2 m sh, a a 2 s air   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 a

large source of water like river, lake or ocean are available.

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 tower

Air 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

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 (t_c1) and the exiting cooled water (t_c2), or

c. The cooling efficiency is defined as the ratio of the actual cooling water to the maximum cooling possible, or

C  C  t  t   A _{c2 wb}; 6o to 8o C 10 to C 6 ; o o 2 1 c c t  t   R wb c c c cooling t  t  t  t  1 2 1 possible cooling maximum cooling actual

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 a w

p

 p

 p

s w  p  p e temperatur  air  the at   pressure saturation air  in vapor  water  the of   pressure  partial  RH ( ) p_s p_w t_d.p. t_d.b.

Dew point temperature (t_dw) is the

temperature at which water vapor starts to condense when cooled at constant pressure Dry bulb (t_db) is the temperature recorded by a thermometer with a dry bulb.

Wet bulb (t_wb) 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,

]  / 

[kg vapor  kg dry air  m m m m air  dry of   Mass air  the in vapor  water  of   Mass a w a w   w w  p  p  p 622 . 0 w s s w s  p p  p  p  p  p

Wet Cooling Tower Analysis(contd.)

• If is the make-up water supplied to replenish the

evaporative loss, then

• Energy balance, air  dry kg vapor  kg humidity specfic air  dry of  rate  flow mass m where m m a a m w  /  , ; 1 2    4 4 2 2 3 3 1 1 cw cw m w w a cw cw a h m h m h m h m h m_{    } w a a cw cw cw h h m h h m h m_{ 3 4  2 1  2 1} m w m_ w a cw  pw a cw cw h h m h m c m t  t   R  Range( ) _{3 4 2 1  2 1}   1 3 ) ( A t _cw t _wb  Approach

Example 1

A surface condenser receives 250 ton/h of steam at 40o_{C with} 12% moisture. The cooling water enters at 32o_{C and leaves at} 38o_{C. 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/m2_{K, 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 = 35o_{C, Hot well temperature=} 30o_{C, condenser vacuum=69 cmHg, Barometric reading 76}

cmHg. Condensate collected 16 kg/min. Cooling water enters at 20o_{C and leaves at 32.5}o_{C, 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 o_{C 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} o_{C dbt and a relative humidity of 60%. It leaves at 28}o_{C dbt and} 90% relative humidity. Makeup water is supplied at 20o_C.

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