1. Power Plant Steam Generators : Introduction
By
P M V Subbarao
Professor
Mechanical Engineering Department
I I T Delhi
A Two in One – Combustor and Heat Exchan
2. The Steam Power Plant
Executes a Thermodynamic Cycle using an
assembly of CVs
3. Cause – Effect Analysis of A Steam Generator
• Combustion is a primary cause.
• Steam Generation is an ultimate effect.
• Heat transfer is a mediation.
• Combustion caused the generation of heat in side furnace volume.
• Heat generation caused the production of high temperature gases.
• These high temperature gases caused the Radiation and convection
heat transfer processes.
• Heat Transfer processes carried the thermal energy to furnace wall
& steam tubes.
• Conduction through the tubes and walls caused the convection
inside the tubes.
• Convection Caused the generation of steam.
• Few hundred years of ingenious trials lead to a Scientific
technologty.
6. Water Tube Boilers: The Steam Generators
• As industry developed during 19th century, so the use of
boilers for raising steam became widespread.
• Disastrous explosions sometimes occurred.
• Boilers of that period consisted of heated pressure vessels
of large diameter.
• These are subject to internal pressure which is tensile
stresses in the walls of the enclosure.
• The value of stress, known as ‘hoop stress’ is given by
T
D
p
f
2
8. Steam generator versus steam boiler
• Opposite the principle of the steam boilers, the water in the
steam generators evaporates inside the tube winded up into
serial connected tube coils.
• The feed water is heated up to the evaporation temperature and
then evaporated.
• The intensity of the heat, the feed water flow and the
size/length of the tube are adapted, so that the water is exactly
fully evaporated at the exit of the tube.
• This ensures a very small water and steam volume (content of
the pressure vessel).
• Thus there are no buffer in a steam generator, and is it
temporary overloaded.
9. Steam generator versus steam boiler
• The advantages using a steam generator compare to
conventional steam boilers:
• Easy to operate - normally no requirement for boiler
authorization.
• Rapid start-up and establishing full steam pressure Compact
and easy to adapt in the existing machinery arrangement
• Price attractive - especially at low steam rates.
18. The Steam Generator : The water tube boiler
• As you can see, the Water Tube
Boiler (below) looks very
complicated.
• Thousands of tubes are placed in
strategic location to optimize the
exchange of energy from the heat to
the water in the tubes.
• These types of boilers are most common because of their ability
to deliver large quantities of steam.
• The large tube like structure at the top of the boiler is called the
steam drum.
• The hundreds of tube start and eventually end up at the steam
drum.
19. Steam Generator Theory
• Within the boiler, fuel and air are force
into the furnace by the burner.
• There, it burns to produce heat.
• From there, the heat (flue gases) travel
throughout the boiler.
• The water absorbs the heat, and
eventually absorb enough to change
into a gaseous state - steam.
• To the left is the basic theoretical
design of a modern boiler.
• Boiler makers have developed various
designs to squeeze the most energy out
of fuel and to maximized its transfer to
the water.
20. Steam Generator Theory
• Water enters the boiler, preheated, at the
top.
• The hot water naturally circulates
through the tubes down to the lower area
where it is hot.
• The water heats up and flows back to the
steam drum where the steam collects.
• Not all the water gets turn to steam, so
the process starts again.
• Water keeps on circulating until it
becomes steam.
• Meanwhile, the control system is taking
the temperature of the steam drum, along
with numerous other readings, to
determine if it should keep the burner
burning, or shut it down.
21. Steam Generator Theory : Water Side
• As well, sensors control the amount of
water entering the boiler, this water is
know as feedwater.
• Feedwater is not your regular drinking
water.
• It is treated with chemicals to neutralize
various minerals in the water, which
untreated, would cling to the tubes
clogging or worst, rusting them.
• This would make the boiler expensive to
operate because it would not be very
efficient.
22. Steam Generator Theory : Fire Side
• On the fire side of the boiler, carbon
deposit resulting from improper
combustion or impurities in the fuel can
accumulate on the outer surface of the
water tube.
• This creates an insulation which quickly
decrease the energy transfer from the heat
to the water.
• To remedy this problem the engineer will
carry out soot blowing. At a specified time
the engineer uses a long tool and insert it
into the fire side of the boiler..
• This device, which looks like a lance, has a tip at the end which
"blows" steam.
• This blowing action of the steam "scrubs" the outside of the water
tubes, cleaning the carbon build up.
23. Scaling of Steam Generators
• This blowing action of the steam "scrubs" the outside of the
water tubes, cleaning the carbon build up.
• Water tube boilers can have pressures from 7 bar to as high
as 350 bar.
• The steam temperature's can vary between saturated steam,
100 degrees Celsius steam with particle of water, or be as
high as 600 - 650 degrees Celsius, know as superheated
steam or dry steam
• The performance of boiler is generally referred to as tons of
steam produced in one hour.
• In water tube boilers that could be as low as 1.5 t/hr to as
high as 2500 t/hr.
30. Once Through Subcritical Steam Generator
Once-through tangential fired
Max. continuous rating: 520 kg/s
Max.Steam temperature outlet: 540°C
Live steam pressure outlet: > 18.3 MPa
31. Major Components of Coal Fired Steam
Generator
Offline
Online
~600mm
Crushed to small
pieces of about
20 mm diameter
32. Design Steps for Steam Generators
• Thermodynamic Design
– Air/fuel ratio
– Specific Size/Output.
– Efficiency
• Thermodynamic Parameters.
• Heats Transfer based Design
– Surface area of various parts.
– Materials used for various parts.
– Geometry of various parts.
• Detailed Temperature distribution.
• Constructional details.
33. Design Steps for Steam Generators
• Hydraulic Design.
– Pressure distribution/drop in various parts
– Velocity distribution.
• Hydraulic parameters.
• Design Optimization.
• Mechanical Design.
35. Steam Generator Specifications
• Pressure Drop
• Solid Waste
– Ash Deposit system
• Auxiliaries
• Site
• Personnel
• Evaluation Criteria
• Special Parameters
– Heat Recovery SG
• Flue gas amount composition
• Gas temperature at inlet
• Back pressure of Turbine Pinch point.
36. Steam Generator Design Methodology
• Preliminary Design
– Useful for estimation of budget price
• Detailed proposal Design
– Essential for making a detailed competitive bid.
• Final Design
– Used in preparation of manufacturing drawings
37. Preliminary Design
• Primitive First Law Analysis of various controls volumes.
• INPUT: Steam conditions and Fuel Analysis
• Database: Design data of previous plants.
• Procedure: Interpolation or Extrapolation of past data.
– Overall size
– Weight
– Cost of the plant
• Commercial Software Packages available –
CFBCAD for Fluidized Bed Boilers.
38. Detailed Proposal Design
• Detailed study and Review of parameters of SG w.r.t.
– Steam
– Fuel
– Environment
• Thermodynamic Design
• Heats Transfer based Design
• Hydraulic Design.
• Design Optimization.
• Mechanical Design.
39. Combustion Safeguards and Controls
• Furnace Explosion : The ignition and almost instantaneous combustion of highly
inflammable gas or vapour or dust accumulated in furnace.
• Conditions leading to Furnace Explosion:
– Accumulation of unburned fuel.
– Air and fuel in an explosive mixture.
– Source of ignition. -- hot furnace walls, improper ignition timing, faulty torch
etc.
• Types of Furnace Explosions:
– Gas explosions and coal dust explosions.
– Primary and Secondary.
• Reasons for increased number of furnace explosions:
– Large Boilers -- higher burner capacity
– Compact furnaces.
– Low fire box temperatures in water tube boilers.
– New fuels.
40. Causes of Fire Explosions
• Flame failure due to liquids or inert gases entering the boiler
fuel system.
• Insufficient purge before lighting the first burner.
• Human error.
• Faulty automatic fuel regulating controls.
• Fuel shutoff valve leakage.
• Unbalanced fuel/air ratio.
• Faulty fuel supply systems.
• Loss of furnace draft.
• Faulty pilot igniters.
41. Furnace Controls
• Manual Control
• Remote Manual Sequence Control
• Automatic Sequence Control System.
• Degree of Automation:
– Manual
– Supervised manual
– Automatic nonrecycling
– Automatic recycling.
42. Burner Flame Safeguard System
• An arrangement of flame detector, interlocks and
relays
– to sense the presence of a proper flame
– cause fuel to be shut of to the furnace during
hazardous conditions.
• Prevent Boiler and Furnace explosions.
