P M V Subbarao Professor - · PDF filecombined-cycle plant located in an arid, ... many...

Fundamentals of Air Cooled Steam Condensers

After Coal, Water to hit India’s Power Sector – Is there

enough water to fuel India’s power expansion?

Avoid Usage of More Water and Conserve Water …..

P M V Subbarao

Professor

Mechanical Engineering Department

I I T Delhi

Introductory Remarks

• The production of electricity requires a reliable, abundant,

and predictable source of freshwater.

• A resource that is limited throughout the world.

• The process of power generation from fossil fuels such as

coal, oil, and natural gas is water intensive.

• In a country where hardly any city gets 24-hour supply of

drinking water …

• The per capita water availability is shrinking at an

incredible pace ..

• Water levels of India’s dams are falling to record lows ..

• Agriculture draws approximately 90% of domestic water ..

• It is inevitable that India will be water stressed not in the

medium term but in the immediate short term.

The Scarcity of Water

• With the present population of over 1,200 million, the per

capita water availability is around 1.170 m3/person/year.

• This translates to 1170 litres/person/per year or less than 3

litres per day per person.

• The urban area consumption is in upwards of 100-150

litres per day.

The Water Requirement

• The water requirement for coal based plant with cooling tower

used to be about 7 m3/h per MW without ash water recirculation

and 5 m3/h per MW with ash water recirculation.

• In recent past, plants have been designed with water

consumption requirement in the range 3.5 - 4 m3/h per MW.

• This is important for a country like India, which has about 16%

of the world’s population as compared to only 4 per cent of its

water resources.

• An installed capacity of 130,370 MW of thermal power plants if

assumed running even at 75% PLF would consume over 30 bcm

of water.

Water Withdrawal in Cooling Towers

Coal Power Plants

Water Withdrawal in Cooling Towers

Water consumption Vs Power Generation Technology

Alternative Cooling Systems

• Direct Dry Cooling Systems.

• Indirect Dry Cooling Systems.

• Hybrid Cooling Systems.

Direct Dry Cooling Systems.

Indirect Dry Cooling Systems.

Hybrid Cooling System

Air Cooled Steam Condensers

• Air Cooled Condensers Types

• Natural Convection: Occurs when a heater with a reduction of

dense fluid is pressed left by a cooler denser fluid.

• Forced Convection: Occurs when an outer force pushes a fluid,

such the same as water of air, to create it move about and transfer

heat.

• Geometrical Designs

• The nearly all accepted style of Air Cooled Condenser is the

modularized ('A' frame structured design), used on industries and

power plants of all sizes.

• The 'A' frame structured designs are mostly used in the power

plants.

• Coil Design : Advanced Smart Circuitry Coil Technology.

Natural Convection ACSC

Forced Convection ACSC

Mixed Convection ACSC

A Frame Mechanical Draught ACSC

Coil Type Mechanical Draught ACSC

World Market Evolution for large Power Plants with

ACSCs

Development of Design Conditions

• The minimum amount of information required to establish

the simplest design point for an ACSC is:

• Steam flow, W (tons/hr)

• Turbine exhaust team quality, x (kg dry steam/kg turbine

exhaust flow)

• Turbine backpressure, pb (mm Hga)

• Ambient temperature, Tamb (deg C)

• Site elevation, (m---above sea level)

Site Characteristics

• In addition to these basic quantities, the ACC design (and

cost) may be affected by a number of plant and site

characteristics which are listed below.

• Site characteristics

• Meteorology

• Annual temperature duration curves

• Prevailing wind speeds and directions

• Extreme conditions (hottest day; freezing conditions)

• Topography and obstructions

• Nearby hills, valleys, etc.

• Nearby structures, coal piles, etc.

• Nearby heat sources---aux. coolers, plant vents, etc.

• Other

• Noise limitations

• At ACSC

• At some specified distance---neighbors, sanctuaries, etc.

• Maximum height restrictions

• “Footprint” constraints (length, width)

• Location restrictions---distance from turbine exhaust

Basic Design Determination

• Specification of the quantities and characteristics above are

sufficient to obtain a “budget” estimate from ACC

vendors.

• Consider a case study to illustrate the considerations in

selecting an appropriate design point.

• An ACC for installation at a 500 MW (nominal), gas-fired

combined-cycle plant located in an arid, desert region

selected the following design values:

• Steam flow, W (tons/hr): 497.2

• Quality, x (kg/kg) 0.95

• Backpressure, pb (cm Hga) 10.16

• Ambient temperature, Tamb (C) 27

• Site elevation Sea level (pamb = 75.9968 cm Hga)

Number of Cells : Air-Cooled Steam Condenser

• The number of cells (also referred to as modules) is

clearly an important part of the supplier data.

• Obviously, the number of cells dictates the amount of

mechanical equipment (i.e. fans, motors, gear boxes).

• Further, many current large-scale designs use components,

whose dimensions are optimized for shipping and erection.

• For instance, use of 10 meter diameter fans and individual

tube bundle sections of approximately 11m and with 2.5m

bundle and 5 bundles per cell per side for a plan area of 11

m by 12.4 m per cell per side.

• As a result, the number of cells often dictates a number of

features of the air-cooled condenser, including the

mechanical equipment as well as the amount of heat

transfer surface.

• The total number of cells or modules is the sum of the

Primary and Secondary Modules.

• The Primary Modules are responsible for the majority of

the heat transfer and condensing, while the Secondary

Cells are responsible for residual heat transfer and

condensables collection and evacuation

• Number of Primary Modules – The number of Primary

Modules is typically about 80 percent of the total number of

modules.

• Length of Primary Modules - The length of the primary

modules is typically on the order of 10-13 m for a Single Row

Condenser type system.

• Number Of Secondary Modules – The number of Secondary

Modules is typically about 20 percent of the total number of

modules and there is typically one module per row (or street).

• Length of the Secondary Modules – these modules are

typically shorter than the primaries by about 1 – 1.5 m.

• Primary Module Dimensions – (Width) – must be greater

than the fan diameter and typically run on the order of 15-25

percent larger than the fan diameter.

• Fan Characteristics – Fan diameters for ACC’s used on

most recent power plant applications are typically 10-12m.

• The number of blades per fan will minimally be 5 but may

be as many as 8-10 depending upon the fan supplier and

the performance requirements.

• Motor Characteristics – Fan motor power must be equal

to that required by the fan shaft power divided by the

motor and gear box efficiencies.

• Often a margin of 5-10 percent if provided, in addition to

service factor margins.

Thermo-hydraulic Specifications of ACSC

• Overall Heat Transfer coefficient, U, (based on air-side

surface area)

• Total Air-Side Surface Area, A

• Total Mass Flow Rate of Air at Each Design Condition,

mair

• Fan Static Pressure (pstatic) or the total system pressure

drop.

• Log Mean Temperature Difference (LMTD)

• Steam Duct Pressure Drop

• Heat Exchanger Bundle Pressure Drop (Steam Side)

Important Global Parameters

• Thermal Duty – It is important to verify that the thermal

duty solicited (i.e. the amount of heat to be rejected) is matched

or exceeded by the supplier’s offering.

• Heat transfer Area – This is calculated knowing the total heat

transfer area of the tubes in the ACC’s.

• For a Single Row Condenser (SRC), the ratio of the airside

surface area and the total “face” area is approximately 124.

• Outlet Air Temperature – The outlet air temperature is

obviously less than the steam temperature and can be calculated

from the following equation.

• Face Velocity of the Air - The face velocity of the air, can be

calculated from the mass of air flow rate, the air density, and the

total face area of the ACC.

• Typical values will run from about 1m/s) to as much as 3 m/s)

with the average being about midway between those limits.

• Fan Static Pressure - Fan Static Pressures will vary

depending upon whether the fan is a low-noise or more

standard design.

• Fan Static Pressure, which in essence is the force required

to overcome the system resistance (with the required

design air flow rate), will run on the order of ~100 Pa +/-

20%) for a standard fan and system design.

• Fan Shaft Power or Brake Horsepower - Depending

upon the fan static efficiency, one can calculate whether

the fan system will deliver the appropriate amount of air.

• Power Requirements - Total fan power can be calculated

using the aforementioned information and assuming

nominal gear box efficiencies of ~97% and motor

efficiencies ~92-94%.

Operating characteristics of an air-cooled steam

condenser - I

Operating characteristics of an air-cooled steam

condenser - II

Operating characteristics of an air-cooled steam

condenser -III

Performance Impacts

• Wind Effects :Prevailing ambient winds can be high (>10-

20 mph) at some sites, leading to:

• Flow separation at the fan inlet and poor fan performance,

• Recirculation of the hot exit air into the air inlet of the

ACC, and

• mal-distribution of the air in the plenum and across the

heat exchange surfaces.

Local Interferences

• The location of the ACSC is necessarily closer to heat

sources such as service water cooling systems, turbine

exhaust piping, etc. than evaporative cooling towers

typically are from the Plant.

• The entrained air from adjacent sources is very likely to be

warmer than design or ambient conditions and therefore

the performance of the ACSC is negatively impacted.

ACSC Unit with 30 A frames

Streamline plot: Global flow field

Volumetric effectiveness of fans

Air inlet temperature of the fans in row 6, and the

wind in the x-direction

Volumetric effectiveness of ACSC

Thermal effectiveness of ACSC

Ambient temperature and wind effect on saturation

temperature of the turbine exhaust steam.

Ambient temperature and wind effect on turbine

back pressure

Distribution of dimensionless heat rejection rate under

different wind speeds (a) 0 m/s (b) 6 m/s (c) 15 m/s.

Schematic of the deflecting plates

Dimensionless total heat rejection versus wind speed

Identifying the Steam-Condenser Problem

• A successful air-cooled steam condenser must continuously and

completely gather and discharge all of the noncondensables in the

system.

• These are the gases that result from atmospheric air leaks into the

vacuum portions of the steam-cycle equipment, and from the

chemicals used for boiler feed water treatment.

• The noncondensables are left behind inside the tubes and headers

when the steam condenses.

• They accumulate if not removed from the system at the release

rate.

• Such trapping of noncondensables is responsible for the steam

condenser problem.

Steam-condensing system ties in with turbine and

with air-removal package

Trapping of noncondensables causes the steam-

condenser problem

Backflow Problems