35 terms

Plant Theory - Water Tube Boilers

Unit 12 - Chapter 53 - A2 -
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What are the basic parts that all watertube boilers have?
-steam drum(s)
-mud drum(s)
-furnace
-watertubes
-downcomers
-risers
Describe a straight tube boiler.
-an early boiler design with straight inclined tubes which run between vertical connected to the front and rear of the steam drum
What is a straight tube boiler with the drum longitudinally positioned in relation to the tubes?
-longitudinal straight tube boilers
What is a straight tube boiler with the drum in a crossway position in relation to the tubes?
-cross drum straight tube boilers
What are the most common types of bent tube boilers?
-A,D, and O
What is sometimes a problem with D-type boilers?
-excessive vibration
When are O-type boilers most commonly used?
-when space is limited due to their compact design
How is circulation maintained in A,D, an O -type boilers?
-by water returning to the mud drums through the downcomers
Describe the first bent tube boiler design.
-known as a Stirling type
-has four drums, 3 steam, 1 mud
-the3 steam drums are interconnected by steam and water circulating pipes
-steam drums are supported by steelwork
-mud drum is freely suspended from the tubs
-furnace constructed of brickwork, not water-cooled
What is meant by 'bottom supported'?
-the mud drum is supported by a steel casing and the steam drum by the tubes
What are the advantages of packaged boilers?
-low construction costs (shop assembled)
-low shipping costs (everything sent in one package)
-pre-tested and inspected prior to delivery
-simple installation
-compact and versatile in design and can occupy relatively small floor area
What elements does a steam generating unit consist of?
-boiler
-superheater
-reheater
-economizer
-air heater
-fuel equipment
-draft fans
-ash removal equipment
What is the general purpose of all components of a steam generating unit?
-to extract heat from flue gas and transfer it to the working medium
What is a superheater?
-a bank of tubes located at the furnace outlet, or radiant area of the furnace, which give extra heat to dry saturated steam taken from the steam drum
When are superheaters used?
-when steam required for power generation or a process is at a temperature higher then that of saturation
How does superheated steam increase overall plant efficiency in driving turbines?
-by increasing the amount of energy that can be extracted from each kilogram of steam
What is a reheater?
-a bank of tubes located withing the furnace outlet or radiant area of the furnace which reheats steam drawn from the medium pressure stage of a turbine to reduce the chance of condensate and to increase the energy available to the turbine
-the reheated steam is returned to the remaining stages of the turbine
What is the purpose of an economizer?
-to maximize fuel economy of the system by using waste heat from flue gases to heat feedwater flowing through economizer tubes
What is the purpose of an air heater?
-to use heat from the flue gases leaving the economizer section to preheat combustion air for the burners which improves combustion efficiency and assists in the burning of pulverized coal
In what instance would you use an industrial type watertube boiler for low pressure heating?
Why are they not usually used for low pressure heating?
-to heat a large building complex
-cost is higher then firetube boilers and they require greater supervision and careful water treatment
Briefly describe a watertube heating boiler with serpentine-shaped tubes.
-consists of a lower and upper header connected by copper or steel serpentine-shaped tubes, which are attached with threaded connections
-the combustion chamber is below the tubes
-has good heat transfer efficiency despite short travel of gas due to turbulence of the gases caused by the tube arrangement
What are the advantages of a watertube heating boiler with serpentine-shaped tubes?
-flexibility of tubes which eliminates expansion and contraction stresses
-ease of replacing tubes since no welding or tube end expansion required
At what temperature and pressure does a low pressure hot water boiler become high temperature and high pressure?
-121*C and 1100kPa
What is the capacity of a Stirling type 4-drum boiler?
-45 000kg/h of steam at 4820kPa
What are the advantages of bent tube boilers?
-greater steam capacities, higher pressure drops
-more efficient use of the furnace
-more versatility in arrangements such as floor space and head room
-furnace can be lined with watertubes which absorbs more radiant heat from the fire
-built at a lower cost
-permits more efficient use of superheaters, reheaters and other heat recovery components
How did Stirling boilers acquire their name?
-when B&W purchased Stirling Consolidated Boiler Company of Barberton in 1906
The cross drum has been built for capacities up to _______________kg/h of steam and pressures up to _______kPa.
-about 230 000
-10 000
What is meant by "integral-furnace"?
-the furnace is water-cooled
-the tubes are an integral part of the boiler circulation circuit
What is the capacity of a packaged watertube boiler determined by?
-length
-height and width are limited by transportation limitations
Briefly describe the Hybrid WW CG boiler design.
-high pressure steam or hot water
-two pass hybrid design
-water tubed boiler-type water membrane and two-pass fire tube scotch marine vessel
-has fast demand response times and safer operation
-3450 - 60 000 lbs/hr output from 100 - 400psi
List the advantages/disadvantages of low-pressure watertube and tubular type boilers.
Advantages
-more compact then firetube boilers of similar capacity
-no special foundation required
-short warm-up period
-bent or coil tube design avoids thermal stress and distortion
-rapid response to load fluctuations
-supplied as packaged units
-low maintenance cost (contains minimum refractory)
-safer than firetube boilers in that they contain very little water

Disadvantages
-higher initial cost
-need for closely monitored water treatment
In a packaged watertube boiler, furnace pressure is often positive. How is flue gas leakage prevented?
-a steel casing forms a seal for the furnace
The bent tube boiler design allows tube to be attached radially on the outside of the ______.
-drum
High pressure watertubes _________.
a) are always smaller than firetubes
b) can be of various straight or bent configurations
c) are always straight
d) require pumps for circulation
-b) can be of various straight or bent tube configuration
List the three parts that make up the heating surface of a copper-tubular heating boiler.
Made of a continuous length of small diameter copper pipe consisting of;
-lower section = tightly wound coil surrounding combustion chamber
-intermediate section = several more layers of coil loosely wound
-upper section = fin-and-tube heat exchanger