Condenser and Circulating Water System

Condenser and Cooling Water System By Aklilu Tesfamichael (Dr.)

Condenser What is the purpose of condenser in a power plant? 1. To redu reduce ce the the tur turbi bine ne exh exhau aust st pressure so that 

The turbine specific output (thermal efficiency efficiency of the plant) increases. (for (for p=1 atm (1 bar) bar) Tsat=100 oC and P=0.074 bar (Tsat=40 oC) this can reject heat to 30 oC coolin cooling g water 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 fur further treatment. Hence, only makeup water that is required to top up the water lost in the cl ed tr tm t

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

hav

1 /  N 1 4 , N 

hav

1 / 

hav

h fg

hav

14

,

number  of  horizontal tubes t sat  t w

14

0.725

k  f 

3

2  f 

gh fg

14

 N   f  d o

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

 Nud 

0.8

0.023 Re d 

Pr0.4

Condensing process and design consideration (contd.) Energy balance between the steam and the cooling water gives:

 Q

 s hs ,in m t lm

hs ,ou t  t i

ln

 cw m

 cwc p ,cw T cw,ou t  T cw,in m

s m

U o Ao t lm

t e t i t e

h

c p ,cw t cw,e

t cw,in

Recommended range 11o C  t e

t in

17 o C 

3o C 

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

Cooling Water Outlet Temperature Calculation The surface area needed by the condenser is obtained by:  Ao

s m

h

n d o l

U o T lm

where n

number of  tubes, and

l length of  one tube (for a single pass condenser). The water flow rate in the tubes is :

c m

n

4

d i

2

V 

where ρ density of  water V

velocity of  water (1.8 2.5m/s)

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 .

The air pressure infiltrate d into the the shell can be estimated by Dalton' s law of  partial pressure. Shell measured total pressure

Fig: Turbine shaft gland

air pressure steam saturated pressure

p sh,m

p air

p sat ; p sat is the saturated pressure at shell measured temp

Air removal (contd.) Assuming air behaves as an ideal gas at such low pressure,

s p air m where

 a R a (t sh,m m

2 2

Ra

273)

specfic volume of  exhaust steam characteristic gas constant of  air

0.287 kJ/kgK

Hence, the rate of  air leakage can be estimated from the above equation. This air has to be continuous ly removed from the condenser shell.

Condenser performance Vacuum efficiency

Condenser efficiency

Vacuum produced by steam condenser inlet Barometric pressure - saturation pressure at exhaust steam pressure Actual temperature rise of  cooling water Maximum temperature rise of  cooling water

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.

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 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. 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

Evaporation Causing cooling

Fig: Natural draught cooling tower

The minimum temperature to which water can be cooled is the adiabatic saturation or wet bulb temperature of the ambient air.

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  A t c 2 t wb ; 6o C to 8o C  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  R t c1 t c 2 ; 6o C to 10o C c. The cooling efficiency is defined as the ratio of the actual cooling water to the maximum cooling possible, or cooling

actual cooling

t c1

t c 2

maximum cooling possible

t c1

t wb

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

 p •

 pw

pa

Relative humidity  RH ( )

 partial  pressure of  the water  vapor in air 

 pw

saturation  pressure at the air temperatur e

 ps

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

ps

td.b.

pw td.p.

Wet Cooling Tower Analysis(contd.) •

•

Humidity Ratio (w)  Mass of  water vapor in the air 

mw

w m

 Mass of  dry air 

ma

a m

If dry and water vapor act as ideal gases 0.622

•

[ kg vapor  / kg dry air ]

 pw  p  pw

Degree of saturation is the ratio of the actual specific humidity to the saturated specific humidity, both at the same temperature T,

s

 pw

 p  ps

 ps

 p

pw

Wet Cooling Tower Analysis(contd.) •

If m m wis the make-up water supplied to replenish the evaporative loss, then

 mw m

a m

a where m

2

1

mass  flow rate of  dry air ; specfic humidity, kg vapor  / kg dry air 

•

Energy balance,  a1h1 m

 cw3hcw3 m

 cw hcw3 m

hcw 4

 Range ( R) t cw3

 m whw m

 a 2 h2 m

 a h2 m

h1

t cw4

 Approach ( A) t cw3

a m

a m  cw c pwm t wb1

 cw4 hcw4 m

h2

2

1

h1

hw

a m

2

1

hw

Fig: temperature relationship in counter flow cooling tower

Example 1 A surface condenser receives 250 ton/h of steam at 40 oC with 12% moisture. The cooling water enters at 32 oC 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/m 2K, 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 = 35 oC, 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 32.5 oC, 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 28 oC dbt and 90% relative humidity. Makeup water is supplied at 20 oC. 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 m 3 /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.