Power plant cooling systems

Applied Thermo Fluids-II (Autumn 2015): Module-5 (Power plant cooling ystems)Dr. M. Ramgopal, Mechanical Engineering, IIT Kharagpur

Introduction• According to 2nd law of thermodynamics,

a continuous conversion of heat into work is possible only when some part of the heat supplied is rejected to a heat sink

• The amount of heat rejected in power plants can be very large, e.g. for 1 MW power generated about 2 MW heat is rejected

• Heat rejection in a steam power plant takes place in the condenser and through the chimney

• Efficiency of condensing type power plant is higher than the non-condensing type

• The efficiency of a steam power plant improves as the pressure at which heat is rejected in the condenser decreases

• The rate at which enthalpy drops with pressure is much higher at low pressures, compared to high pressure, i.e,

• Hence it is advantageous to operate the plant at as low a condensing temperature as possible, subject to the availability of a suitable heat sink

• Economic analysis of a power plant generally yield an optimum condensing pressure at which the overall performance of the power plant is best

0 0.2 0.4 0.6 0.8 10.36

0.38

0.4

0.42

0.44

0.46

pc, bar

hth

pb=165 bar, tb = 560oC

Condenser classification

• Depending upon the heat sink, condensers can be classified

into:

1. Water cooled condensers, or

2. Air cooled condensers

• Water cooled condensers can be further classified into:

1. Direct contact type condensers, and

2. Indirect contact type or surface condensers

Direct contact type spray condensers

From mass and energy balance for the direct contact spray condenser it can be shown that:

This implies that the amount of water to be circulated by the pump is much higher than the steam flow rate

The heat transfer rate in the dry cooling tower is:

Where cw is the specific heat of the liquid waterNot commonly used in large power plants as the purity of coolant water should be very high

Direct contact, barometric type condensers

Indirect contact or surface condensers

• These condensers, which are similar to Shell-and-tube type heat exchangers are most commonly used in power plants

• The low pressure steam from the turbine condenses on the shell side, while the cooling water flows through the tubes

• Due to the large size, the surface condensers are made out of strong plates and is in the form of a box

Indirect contact or surface condensers (contd.)

• To minimize the pressure drop on the low pressure condensing steam side, the tube spacing at the top is more and it gradually reduces towards the bottom

• Due to higher heat transfer, the number of tubes are also more at the top compared to the bottom Funnel shape arrangement

• The required heat transfer area can be as high as 1,00,000 m2 with thousands of tubes, each of length as high as 30 m!

Nippon power plant condensers Inside a power plant condenser

• Surface condensers can be further classified into:

Single pass or multi-pass

Single pass Two pass

1. For the same cooling water velocity through the tubes, the required cooling water mass flow rate decreases as the no. of passes increase

2. However, the exit cooling water temperature, and hence the condensing temperature increase with no. of passes

3. Thus a multi-pass condenser results in lower water consumption and pumping power, but higher condensing pressure, and hence lower thermal efficiency of the plant

Indirect contact or surface condensers (contd.)

Surface condenser calculations

Heat transfer rate in condenser, Qc:

Where U is the overall heat transfer coefficient and A is the heat transfer area of the condenser

The U-value of condenser depends on the heat transfer coefficients on the steam and cooling water sides, tube wall material and thickness etc, i.e,

The cooling water temperature rise across the power plant condenser is usually in the range of 6 to 14 K

The velocity of cooling water is usually in the range of 1.5 to 2.5 m/s for acceptable levels of water side heat transfer coefficient and tube erosion

1𝑈𝐴=

1h𝑤 𝐴𝑤

+1

h𝑠 𝐴𝑠+𝑅𝑤+𝑅𝑓 ,𝑤+𝑅 𝑓 ,𝑠

Considering all the resistances to heat transfer, the overall heat transfer coefficient is:

Where:

hw and Aw are heat transfer coefficient and area on coolant fluid side,

hs and As are heat transfer coefficient and area on steam side,

Rw is the resistance of the tube wall,

Rf,w and Rf,s are the resistance due to fouling on coolant and steam sides, respectively

Overall heat transfer coefficient (U-value) in condensers (Heat Exchanger Institute)

C1 = Constant that depends on tube outer diameter

(2777/2705/2582 for tube ODs of 3/4th, 7/8th and 1” respectively)

C2 = Correction factor for inlet temperature of cooling water

(0.57 for 1.7oC and 1.1 for 37.8oC)

C3 = Correction factor for tube material and thickness

(0.58 for 18 gauge and SS; 0.83 for 18 gauge and Cu-Ni etc..)

C4 = Correction factor for tube cleanliness

(0.85 for clean tubes and less for dirty tubes)

Cleaning of condenser tubes is very important as the cooling water used may contain

many impurities and may lead to scale formation and clogging of the tubes

Continuous purging of the condenser is done using a Steam Jet Air Ejector in all power plant condensers to remove the air leaking into the condenser that is under vacuum

Wilson’s PlotThis is an experimental technique for estimating heat transfer coefficients in situations with complex geometry and/or complex physics

It is assumed that the heat transfer coefficient that is being estimated remains constant when the fluid flow rate of the fluid on the other side is varied

When applied to power plant condenser, the equation for Wilson’s plot is:

Thus from the intercept and from known values of Rw, Rf,a and Rf,s, the value of hc can be estimated

1(𝑉 𝑤)0.8

1𝑈

𝑐1 }

Experimental data points

Best-fit

Macro fouling in steam power plant heat exchanger

Fouling in condenser tubes

Fouling and scale formation in power plant condensers

Fouling and scaling are serious problems in condensers

Periodic removal of the scale formed is required to ensure high performance

Chemical as well as mechanical techniques are used to remove the scale

Scale removal can be done manually or automatically

Corrosion of tubes is another problem that needs to be taken care of

Fouling and scale formation in power plant condensers

The resistance offered to heat transfer due to fouling and scale formation increases with timeProblem of scale formation is more severe on cooling water side as maintaining its purity is difficultIf descaling is not done, then the capacity and efficiency of the plant reduce significantly

However, too frequent descaling may affect the economics due to losses incurred during plant downtime

The resistance due to scale formation can be estimated by from the knowledge of U-vale of the brand new condenser (Unew), and U-value after some usage (Uold)

In recent times, several technologies are developed that either prevent or remove scale formed automatically, without shut-down

Purging of steam condensers

• Due to operation in vacuum, power plant condensers are prone to continuous leakage of atmospheric air

• Die to the presence of air, the partial pressure of water decreases leading to poor condensation

• Where, pc is the condenser pressure, pw and pa are the partial pressures of water and air, respectively.

• If pw is the partial pressure of water, then steam condenses at saturation temperature tc, corresponding to pw

• Thus for a given condenser pressure, as the amount of air inside the condenser increases, then pw and hence tc decrease, leading to reduced temperature difference for heat transfer

Purging of air in steam power plant condensers is done using steam jet air ejectors

(SJAE)

To minimize the loss of steam during purging, air+steam mixture is extracted from the

air cooler section, where the partial pressure of water is minimum

https://www.youtube.com/watch?v=8MvHplOIQCI

Problem on power plant condenser• Steam (quality = 0.92) condenses in a shell-and-tube type, power plant

condenser at 40oC and leaves the condenser as saturated liquid water. The condenser pressure as indicated by the pressure gage is 0.077 bar (abs.). The cooling water enters the condenser at 33oC and leaves at 37oC. If the flow rate of steam is 50 kg/s, find

• A) Flow rate of cooling water in m3/s, and

• B) Rate at which air leaks into the condenser in kg/s

• Given at 40oC,

• psat = 7.384 kPa, hf = 167.57 and hg = 2574.3 kJ/kg, vf = 0.001008 and

vg = 19.52 m3/kg

• Molecular weight of air = 28.97 kg/kmol

• Density of water at 35oC = 994 kg/m3

Ans.: A) 6.675 m3/s; B) 3.16 kg/s

Problem on power plant condenser

• If the U-value of the condenser based on tube side is 2100 W/m2.K, find: A) the number of tubes required if the tube ID is 25.4 mm and its length is 12 m. Assume a single pass condenser.

• If the heat transfer coefficient on water side follows the equation:

• B) Find the condensing side heat transfer coefficient neglecting the wall thickness and scale formation.

Given: = 994 kg/m3, k = 0.6106 W/m.K, = 0.0007196 kg/m.s, Pr = 4.93

• Ans.: A) 11689, B) 4982 W/m2 K

Air cooled condensers

• In recent years there is a growing interest in air cooled condensers due to the problems associated with water cooled condensers:

1. Increasing water scarcity and levels of water pollution

2. Site restrictions imposed by water cooled condensers

3. Cooling tower freeze-up and problem of tower vapour plume

4. Problems related to bio-fouling, scaling etc.

• However, even though air cooled condensers offer solutions to the above problems, the plants that use air cooled condensers suffer as :

1. Air is bad conductor of heat, hence, lower U-value and larger surface area is required

2. Water is near wet bulb temperature, whereas air is at dry bulb temperature

3. Air cooled condensers consume large fan power and also their initial cost is high (about 1.5 to 2.5 times)

4. Higher noise levels and larger land requirements

1𝑈𝐴=

1oh𝑎𝑖𝑟 𝐴𝑎𝑖𝑟

+1

h𝑠 𝐴𝑠+𝑅𝑤+𝑅 𝑓 ,𝑤+𝑅𝑓 , 𝑠

is the overall efficiency of the finned surface

Worked out example: Air cooled condenser• An air cooled condenser is used in a power plant condenser that produces 10

MW power with an overall efficiency of 36 %. The condenser used is a fin-and-tube type. The following is the data:

• Tube inner diameter: 25 mm• Length of each tube: 30 m• Face velocity of air: 3 m/s• Ratio of external-to-internal heat transfer area: 20:1• Fin efficiency: 90 %• Condensation heat transfer coefficient: 4000 W/m2.K• Air side heat transfer coefficient: 52 W/m2.K• Air side pressure drop: 25 Pa• Air density: 1.2 kg/m3

• Cp value of air: 1005 J/kg.K• Condensing temperature: 45oC• Air inlet temperature: 38oC• Minimum T for heat transfer: 3 K• Efficiency of fan: 85 %

Worked out example: Air cooled condenser

• From the given data, find:

1. Total Number of tubes required

2. Height of the condenser

3. No. of rows required

4. Fan power consumption

Ans.: 1) 21052) 40.95 m3) 54) 108.4 kW

Circulating water systems

• The circulating water system supplies the necessary cooling water to the water cooled condenser of the thermal power plant

• It also supplies cooling water for:

1. turbine cooling,

2. water for fire fighting, and

3. cooling of steam generator building, and

4. Reactor building cooling and emergency core cooling in nuclear power plants

• However, most of the cooling water (about 95 %) is required for condenser heat extraction

• The cooling water after extracting heat from the power plant condenser has to discharge the same to the environment

Circulating water systems (contd.)

• The circulating water system can be classified as:

1. Once through system

2. Closed loop system, and

3. Combination of once through and closed loop systems

Once through system

• Water is taken from a

large water body such as

an ocean, river or lake,

• Pumped to the power

plant condenser, and

• The return water is

discharged back into the

water reservoir in an

appropriate manner

Once through system (contd.)

• The return water is discharge can be:

1. Surface discharge

2. Submerged discharge, or

3. Diffuse discharge

• Though water required for once through

systems is very large, almost all of it is

returned to the water source

• Also, the power plant efficiency can be high

as the cooling water temperature is lowest

• However due to environmental regulations

and site restrictions these systems are being

replaced with closed loop systems

Closed loop system

• In this system the cooling water from the power plant condenser is

cooled in a cooling system and the cooled water is re-circulated

• The cooling system used for cooling the return water can be:

1. A cooling tower

2. A cooling pond

3. A spray pond, or

4. A spray canal

• Of the above, the cooling tower is most commonly used in large,

modern power plants

• A cooling tower can be a wet cooling tower, a dry cooling tower or a

combination cooling tower

Spray PondCooling Pond

Cooling Towers Spray Canal

Wet cooling towers – Most commonly used in power plants

Wet cooling towers rely on evaporation of water for cooling the warm return water from the condenserHence there is a continuous loss of water due to evaporationIn addition, some amount of water is also lost due to drift and bleed-off (or blow down) Hence make-up water has to be added continuously to account for the losses

Cooling towers can be:

1. Natural draft cooling towers

2. Mechanical draft cooling towers

3. Hybrid cooling towers• Mechanical draft cooling towers

can be:

1. Forced draft type, or

2. Induced draft type• Depending upon flow direction.• Cooling towers can be:

1. Parallel flow

2. Counter flow, or

3. Cross flow

Mechanical, induced draft tower

Natural draft, hyperbolic tower

Hyperbolic Cooling tower installed in Westfalen power plant in Germany

Multi-cell, induced draft cooling tower

Cross flow cooling tower (mechanical)

Theory of wet cooling tower

• In a wet cooling tower, there is an exchange of sensible and latent heat between warm water and air with which it comes in contact inside the tower

• Sensible heat transfer takes place due to temperature difference between water and air

• Latent heat transfer takes place due to vapour pressure difference between saturated air film in contact with water and the unsaturated surrounding air

Qsensible

Qlatent

Air (ta, wa)

Water (tw)

The net heat transfer, Qnet is:

Qnet = Qlatent ± Qsensible = mwcw(tw,itw,o)

• For a cooling tower, the net heat transfer, Qnet

Qnet = Qlatent ± Qsensible = mwcpw(tw,itw,o) should be +ve

Qsensible = hcA(twta) can be +ve or ve

Where:

hc is the convective heat transfer coefficient between water and air,

A is the interfacial area

tw and ta are the water and air temperatures

Qlatent = hmA(pvspva)hfg has to be +ve in a cooling tower

Where:

hm is the convective mass transfer coefficient between water and air,

pvs and pva are the saturated vapour pressure at water temperature and

water vapour pressure in air

hfg is the latent heat of vapourization of water

Using heat and mass transfer analogy, it can be shown that the net elemental heat transfer rate at any section of the cooling tower is given by:

Where

hs,w is the enthalpy of saturated air at water temperature tw (kJ/kga)

ha is the enthalpy of surrounding, unsaturated moist air (kJ/kga), and

cpm is the specific heat of moist air (kJ/kga.K), and

dA is the elemental interfacial area

The total net heat transfer, Qnet is given by:

𝑄𝑛𝑒𝑡=∫𝐴

❑

𝑑𝑄𝑛𝑒𝑡=∫𝐴

❑ h𝑐 𝑑𝐴𝑐𝑝𝑚

(h𝑠 ,𝑤−h𝑎 )=�̇�𝑤𝑐𝑝𝑤 (𝑡𝑤 ,𝑖−𝑡𝑤 ,𝑜 )

Where A is the total interfacial area between water and air that is required to obtain the heat transfer rate

Saturated air at tw

air

in out

Parallel Flow

in

out

air

Counterflow

𝑄𝑛𝑒𝑡=∫𝐴

❑

𝑑𝑄𝑛𝑒𝑡=∫𝐴

❑ h𝑐 𝑑𝐴𝑐𝑝𝑚

(h𝑠 ,𝑤−h𝑎 )

Saturated air at tw

High interfacial area is obtained through use of fills or sprays

Cooling tower Spray Components

Typical technical characteristics of a commercial cooling tower

Performance of a cooling tower depends upon the local weather and operating conditions

Air and water path in a counterflow cooling tower

• Assuming ideal gas behavior, the enthalpy of moist air ha (kJ/kga) is given by:

Where

• Wa is the humidity ratio of moist air (kgw/kga)

• ta is the dry bulb temperature of moist air (oC), and

• cpa is the specific heat of dry air (kJ/kga.K)

• cpv is the specific heat of water vapour (kJ/kgw.K)

• hfg,o is the latent heat of vapourization of water at 0oC (kJ/kgw)

• Substituting the average specific heat values and latent heat of water, h𝑎=1.005 𝑡𝑎+𝑤𝑎 (2501+1.88 𝑡𝑎 )

• From overall energy balance considering make-up water:

• Where is the mass flow rate of dry air, ha,i and ha,o

are the enthalpies of moist air at the inlet and outlet of the cooling tower (in kJ/kgda)

• are the mass flow rate and specific heat of the cooling water, respectively

• are the flow rate of make-up water and its enthalpy

• From mass balance of water across the cooling tower, the evaporation rate of water is given by:

Make-up water requirement

Make-up water is required to take care of:

1. Evaporation

2. Blowdown, and

3. Drift losses

• The performance of a cooling tower is indicated in terms of:

1. Range: Range is the difference between the cooling water temperature at the inlet and the outlet, i.e., the temperature drop undergone by the cooling water as it flows through the cooling tower

Range = (tcw,itcw,o)

2. Approach: Approach is the difference in between the cooling water outlet temperature and incoming air wet bulb temperature twbt,i, i.e.,

Approach = (tcw,otwbt,i)

• Usually the cooling towers are designed for a

range of 6 to 10 K and an approach of 6 to 8 K

• As approach decreases cooling tower size

increases, condensing temperature decreases and

plant efficiency increases

• As range increases, the cooling water flow rate

decreases, but the condensing temperature

increases

Problem on wet cooling tower

• 3 kg/s of water from condenser enters a cooling tower at 65oC and leaves at 30oC. The make-up water enters the cooling tower at 28oC. Atmospheric air at 40oC and 40 % relative humidity enters the cooling tower and exits as saturated air at 35oC. The air pressure drops by 300 Pa as it flows through the cooling tower. Cp of water = 4.2 kg/kJ.K

• Assuming ideal gas behaviour for atmospheric air find:• a) Heat transfer capacity of the cooling tower• b) Make-up water requirement per day• c) Fan power requirement assuming a fan efficiency of 80 %

• Ans.: a) 441 kW, b) 17539.2 kg/day, c) 3.885 kW

Problem on wet cooling tower

• Water from condenser enters a cooling tower at 42oC and leaves at 36oC. Atmospheric air at 40oC and 30 % relative humidity enters the cooling tower and exits as saturated air at 35oC. The convective heat transfer coefficient between water and air is 300 W/m2.K

• Find the interfacial area required for heat and mass transfer for a) Parallel flow, and b) Counterflow.

• The heat transfer capacity of the cooling tower is 200 MW. • Assume ideal gas behaviour for atmospheric air and use

logarithmic enthalpy difference in cooling tower equation.

• Ans.: a) 18628 m2, b) 11860 m2

Dry cooling towersIn dry cooling tower the return water from the condenser is cooled sensibly using atmospheric air

A dry cooling tower can be;

1. Natural draft type, or

2. Mechanical draft type, or

3. Hybrid

Natural draft type, dry cooling tower

Mechanical type, dry cooling tower

Hybrid Cooling Tower

Dry cooling towersIn dry cooling tower the return water from the condenser is cooled sensibly using atmospheric air

A dry cooling tower can be;

1. Natural draft type, or

2. Mechanical draft type, or

3. Hybrid

• Dry cooling towers are:

1. Less expensive compared to wet cooling towers

2. Easier to maintain

3. Do not depend on site requirements

• However, they are less efficient compared to wet cooling towers

Natural draft type, dry cooling tower

Hybrid cooling tower

• In a hybrid cooling tower, depending upon season, the condenser cooling water is cooled:

1. Only by sensible cooling in dry cooling tower mode,

2. Both by sensible and heat transfer modes in wet cooling tower mode

• The dry mode is used when the ambient temperature is normal or less than normal

• The wet mode is used when the ambient temperature is high

Hybrid cooling tower

Hybrid cooling tower

Hybrid cooling tower

The hybrid cooling tower offers the

following advantages:

1. The cooling tower plume

formation can be minimized as

air leaving the wet tower portion is

heated in the dry portion

2. Evaporation losses are reduced

3. In cold weather, the tower can be

operated only in dry mode

Waterproofing inside a large cooling tower

Greenpeace protests against cooling tower!

End of Module 5