Low-Pressure Boilers Flashcards
What happens to the water in high-pressure boilers?
The water circulates inside the tubes, which are heated by the hot flue gases from the fire.
What are the advantages of a watertube boiler?
- they are safer due to the relatively small water content
- they respond fast to load changes
Why aren’t industrial type watertube boilers used for low-pressure steam heating plants?
- Mainly because of the high cost (compared to cast-iron or steel fire tube boilers)
- needs more supervision (esp. with water treatment)
Can any water tube steam heating boilers be used for low-pressure heating applications?
Yes. Only specially designed water tube boilers
What does a Low-pressure watertube boiler consist of?
- lower & upper header
- headers connected by lots of copper / steel serpentine shaped tubes
In a low-pressure watertube boiler, where is the combustion chamber & what does it do?
Location: below the tubes & enclosed by insulated casting
Equipped with atmospheric gas burners
How do the combustion gases travel through the tubes?
Gases travel upwards between the tubes towards the flue, and give up their heat by convection
Is the heat transfer in watertube boilers efficient or no? Why or why not?
Yes, heat transfer is efficient because of the arrangement of the tubes.
Why does the arrangement of the tubes in a watertube boiler make heat transfer efficient?
Because it causes turbulent and intensive scrubbing of the hot gases around the tubes
What is an advantage of the low-pressure water tube boiler design?
Flexibility of the serpentine-shaped tubes
- gets rid of expansion and contraction stress
- tubes can be easily replaced (no welding or expanding needed)
T/F: A tubular boiler is a watertube boiler
True (in principal)
What makes a tubular boiler different from a low-pressure watertube boiler?
Tubular boilers…
- not equipped with drums or headers
- has one or more continuous coil of copper or steel tubing instead of many small tubes
- have a pump that forces water through the coil (which is exposed to the hot products of combustion)
Where are copper-tubular boilers used?
- mainly in residential and commercial hot water heating systems
- domestic hot water supply
What is the heating surface of a copper-tubular boiler?
one continuous, small diameter copper tube
Why is copper used in the copper-tubular boiler?
Because…
- copper resists corrosion
- has a faster heat transfer rate than cast iron or steel
List and describe the 3 parts of the heating surface in copper-tubular boilers.
1) the lower section
- consists of a tightly wound coil that surrounds the combustion chamber
- coil absorbs the radiant heat of the fire and transfers it to the water
2) the intermediate section
- made of several layers of loosely wound spirals
- allows hot gases to flow freely around the tubing after they leave the furnace
3) the upper part
- consists of a fin-and-tube type heat exchanger
- fins increase the heating surface so more heat is absorbed from the hot gases rising from the furnace
Where is the burner placed in the copper-tubular boiler?
in the lower part of the furnace section
How does water circulate in copper-tubular boilers?
water circulates at a high velocity through the boiler tubing from the top downwards, while the hot gases travel upwards
–> this means no cold water is pumped into the hottest section of the boiler, excessive stress is avoided, and the boiler will have a longer life
What is softened water used for?
used as a feed water to prevent scale from forming
What boiler produces steam most rapidly?
copper-tubular boilers because there is no water stored in the drums/headers
Explain the difference between the terms ‘steam boiler’ and ‘steam generator’
Boiler: when there are ‘stored’ water compartments (ex. drums/headers)
Generator: when there is more water in the tubes than in the drums
T/F: The tubular unit doesn’t require more attention when it comes to water treatment programs/
False. Tubular units DO require more attention.
Describe the water circulation in a longitudinal drum straight tube boiler.
- the feedwater enters the lower part of the drum
- feedwater flows down the downcomer into the downcomer header
- the tubes add heat to the water & decrease the water density
- steam in the tubes further decrease the water density and provides circulation in the boiler
- water then moves upwards through the tubes and into the header
- water is then forced further up into the steam drum
What is the difference between a longitudinal drum boiler and a cross drum boiler?
longitudinal boilers have the drum running front-to-back
cross drum boilers have the drum side-to-side
Describe the parts of the two-drum bent tube boilers.
- has a large upper drum, small lower drum, and multiple bent watertubes that connect the upper and lower drums
- tubes arranged to form the furnace
- furnace walls are formed by water-filled tubes called ‘waterwalls’
- waterwalls are connected to each other by steel plating to form gas-tight walls
- spaces in the back of the furnace to allow gases to reverse and travel towards the front of the unit
- tubes redirect to the back of the unit to form 3 passes / walls
- inner walls form the furnace enclosure, outer walls (convection bank) are passageways for the flue gases
Describe the heat circulation in two-drum bent tube boilers
- large amounts of radiant heat from the fire are absorbed by the furnace walls
- heat carried by the circulating water in the tubes
- combustion gases leave the furnace and travel between the inner and outer membrane walls
- combustion gases release their heat to the walls by convection
- boiler is equipped with a forced draft fan
Can a bent tube boiler be used as a steam or hot water boiler? And is it used for low or high pressure service?
Can be used as steam OR hot water
Can be used for low- OR high-pressure service
Describe the advantages of a bent tube boiler compared to a straight tube boiler.
bent tube boilers allow for better heat transfer since the tubes can form flue gas baffles and furnace waterwalls
–> straight tube boilers have limitations with designs and configurations
What is another name for the two-drum bent tube boiler?
“O” type watertube boiler
Describe the construction of the “A” type boiler.
- has 2 small lower drums or headers
- upper drum is larger to allow for the separation of water and steam
- most steam production occurs in the centre furnace wall tubes entering the drums
Describe the construction of the “D” type packaged boilers.
- has two main drums: a steam drum on top and a mud drum below, connected by water tubes
- is tile-covered to prevent overheating of the floor tubes
- furnace sits on one side, surrounded by tubes to absorb heat
- a burner supplies fuel and air for combustion
Describe the construction of an “O” type packaged boiler.
- compact steamer
- requires a longer boiler
- has a symmetrical design, exposing the least tube surface to radiant heat
What are the advantages of the packaged watertube boilers?
- more efficiently produced; built using the assembly line
- easy to install
- designed to easily fit through double doors
- one supplier provides all the components (assembly line/manufacturer)
- warranties easier to deal with
MC: One reason that low capacity watertube boilers are seldom used for low pressure heating is that they…
a) require less supervision
b) cost less
c) have the flue gas flowing inside the tubes
d) require more supervision for water treatment
d)
MC: the heating surface of a copper tubular boiler consists of…
a) drums and heaters
b) a continuous copper tube
c) straight firetubes
d) bent tubes attached to drums
b)
MC: The type of watertube boiler that has an upper drum and two lower drums is a(n) _____ type.
a) “O”
b) “B”
c) “A”
d) “D”
(c)
MC: An advantage of a packaged boiler would be…
a) ease of installation
b) constructed entirely on site
c) It can be built to any height
d) no water treatment required
(a)
MC: The purpose of attaching ‘fins’ to water tubes is to…
a) provide for a swirling action of the flue gases
b) increase the heating surface
c) increase the strength of the tubes
d) provide for better water circulation
(b)
