Boilers Flashcards
What are cast iron sectional boilers used for?
Used for steam and hot water heating systems in small - medium sized residential buildings
- to heat buildings
- provide domestic hot water
What is the assembly / disassembly like for cast iron sectional boilers?
They are made using sections which can be assembled on site
–> also means they can be disassembled, moved through tight spaces, and reassembled
How would cast iron sectional boilers be used in larger buildings?
You would need multiple cast iron boilers that are designed to operate in parallel
What are the benefits of using multiple cast iron boilers to meet the heating load for larger buildings?
allows for greater fuel efficiency, because each boiler is only brought online to meet its demand
Describe the general construction of cast-iron sectional boilers.
- built up from multiple hollow cast-iron sections
- sections are connected to each other by push nipples in the upper and lower parts of the sections
- sections held together with tie rods/screw jacks
- water & steam flow freely from one section to the other through the push nipples
- sections are place side by side OR stacked one above the other
What are the 2 arrangements of the sections in cast-iron boilers?
1) vertical sectional boilers (bread slices ; side by side)
2) horizontal sectional boiler (pancakes ; one above the other)
How are cast-iron boilers able to give maximum heat transfer from the gases to the heating surface?
- by exposing as large a surface as possible to the hot gases
- by lengthening the time of contact between the gases and the heating surface
How does gas travel/transfer in cast-iron boilers?
- sections are placed on a base that contains the gas burners
- hot combustion gases travel upwards between the sections to the collector hood
- collector hood guides gases to the draft hood & into the uptake
What do flue gas passages do in cast-iron sections?
increase the heat transfer
Can cast-iron boilers have fins instead of gas passages?
Yes. There can be multiple different designs.
What do fins do in boilers?
Fins cause the hot gases to swirl about causing a scrubbing action on the surface –> more effective heat transfer
What part differs in different types of cast-iron boilers?
The front and rear sections of cast-iron boilers differ in design from the intermediate sections (“??? check this, pg. 2-5)
What’s different about water leg & wet-bottom type boilers?
The front and rear sections have a burner opening (water leg) & cleanout opening (wet-bottom)
T/F: Gas flow in water leg & wet-bottom boilers is considerably longer than the flow in boilers with sections suspended above the burners.
True
How does hot gas travel in water leg boilers?
- gas travels from the furnace up between the water legs of the sections to the outer gas passages
- gas travels forward from the upper part to the front
- gas flow gets reversed at the front and goes back through the centre passages
- gas the exits the boiler through the rear
T/F: Do some types of boilers with water legs require extra support?
Yes, it depends on the internal boiler pressure
What are push nipples?
Short pieces of pipe that are smooth & tapered so that they can enter the holes in the boiler sections easily and form a tight seal
What are cleanout slots in wet-bottom boilers?
Small openings / access points that are designed to allow the removal of slag & clean out the furnace bottom.
- closed off during operation
T/F: Horizontal sectional cast-iron boilers need more than one nipple to connect the two sections.
False, only need one nipple to connect.
How does hot gas travel through horizontal boilers?
Hot gas travels upwards in a zig zag pattern through the gas passages
What are the advantages of cast-iron boilers?
- higher resistance to corrosion (compared with steel)
- ease of assembly (doesn’t require much skill)
- choice of assembly location (boiler can be shipped as a unit or by pieces to assemble on site)
- possibility of enlarging boiler capacity (can add more boilers)
What is a disadvantage of cast-iron boilers?
Limited in the maximum pressure they can endure
When do heating boilers attain maximum efficiency?
When they are operated continuously at full load
–> during 90% of the heating season, only 60% or less of the heating plant capacity is required
T/F: A single, large capacity boiler will operate near full capacity only 40% of the time.
False, it will only operate near full capacity 10% of the time
What do large capacity boilers do when it’s not operating at full capacity?
It will operate at reduced load intermittently or continuously
T/F: Maximum efficiency of boilers are hardly ever achieved.
True
What is efficiency in terms of boiler capacity?
the percentage of the heat released during combustion that is actually transferable to the boiler water
What is more efficient: a single large capacity boiler or several small boilers
several small boilers
What are the advantages of having multiple heating boiler plants?
1) improved thermal efficiency
2) if one boiler fails, the other ones can continue to carry the load
3) parts (burners/controls/etc.) are more simple and cheap
4) smaller water content (less destructive if there’s a failure)
5) less space required
6) relatively light (no need for a strong foundation)
7) easily moveable
8) easy installation / disassembly
T/F: Each boiler in multiple boiler plants is tied with the primary / main hot water circulating system
True
T/F: Each boiler in a multiple boiler plant does not have its own secondary circulating pump.
False - Each boiler will have a secondary circulating pump
How are multiple boilers operated / controlled?
operated in sequence by a step controller, which is controlled by an indoor-outdoor thermostat
What does an indoor-outdoor thermostat do?
Senses the outdoor temperature & the heating system supply water temperature
–> so that heat supplied by boilers to the water balances the heat lost from the building
What is a cast-iron modular boiler?
A type of multiple small use capacity boilers, but instead use multiple horizontal sectional cast-iron boilers to for a single unit (the modular boiler)
How are modular boilers constructed?
They are connected side-by side between the supply header and return header of the hot water heating system
- flow through all modules is continuous
- each module has its own burner system and controls
How are cast-iron modular boilers controlled?
By a step controller which starts or stops modules in sequence as load demand increases or decreases.
- they are individually operated sections that are controlled in sequence
What was a main reason why condensing boilers were not popular before the 1990’s?
Because of the high cost
Why are condensing boilers more costly?
Because….
- large heat exchangers (this would cause moisture in the gases to condense out)
- higher corrosive actions (due to the condensed moisture) // need for materials that can withstand the corrosion
MC: Cast-iron sectionals have flue gas passages which…
a) are often termed ‘breeching’
b) increase heat transfer
c) receive radiant heat only
d) act as boiler water legs
(b)
MC: a machined and slightly tapered piece of pipe found in sectional heating boilers is referred to as a:
a) tie rod
b) tapered brushing
c) push nipple
d) cleanout
(c)
MC: An advantage of a cast iron boiler is….
a) its large capacity
b) light weight resulting in lesser foundation requirements
c) its ability to withstand high pressure
d) the choice of assembly location
(d)
MC: Small capacity heating boilers operate most efficiently at….
a) continuous full load
b) any type of load
c) approx. 75% load
d) approx. 65% load
(a)
MC: A modular cast-iron heating boiler….
a) has a number of burners which start and stop simultaneously
b) consists of a number of individually operated sectional boilers
c) cannot be enclosed in a single casting
d) does not have continuous water flow through each module
(b)
MC: Cast-iron boiler sections maximize heat transfer by…
a) using for push nipples per section
b) vertical firing
c) exposing as large a surface as possible to the hot gases
d) utilizing an external furnace
(c)
What is the maximum allowable working pressure (MAWP)?
The pressure that the boiler has been designed to withstand while in operation.
- it is stamped on the boiler and must not be operated above this pressure
What is a dry-back boiler?
A firetube boiler that has brickwork or insulated metal at the rear reversing chamber to redirect the flue gases.
What is an externally fired boiler?
A boiler that has the combustion chamber located outside of the shell or drum and surrounded by brickwork.
What are firetubes?
These are the smaller diameter cylindrical tubes through which the flue gas travels inside and are surrounded by water on the outside.
T/F: The distinction between power boilers and heating boilers is determined by code.
True
–> ASME code states they are distinguished by the pressure and temperature limitations for each type of boiler
What are the factors involved in the process of boiler designation?
- pressure limitations
- temperature limitations
- energy input
- size
- capacity
- design
- construction
- application of the boiler
What can make hot water supply boilers exempt from the regulations according to the Canadian Standards Association (CSA) Boiler Code?
if they are less than a given size and below a given energy input
What are the differences between Power boilers and Heating boilers?
Power Boilers: built to ASME Section 1 code requirements
- steam boiler: pressure EXCEEDS 103 kPa (14.7 psi)
- hot water boiler: pressure EXCEEDS 1100 kPa (160 psi), and temperature EXCEEDS 121 degrees C
Heating Boilers: built to ASME Section 4 code requirements
- steam boiler: pressure DOES NOT EXCEED 103 kPa (14.7 psi)
- hot water boiler: pressure DOES NOT EXCEED 1100 kPa (160 psi), and temperature DOES NOT EXCEED 121 degrees C
Are hot water supply boilers (aka water heaters / hot water tanks) generally considered to be heating boilers?
Yes (generally, unless one of three requirements are exceeded)
What would make a hot water supply boiler be considered as a power boiler?
If it exceeds one of the following:
- capacity 450 litres
- temperature 99 degrees C
- energy input 58.6 kW
What classifies a boiler as a firetube boiler?
If it has a shell containing straight tubes through which hot gases travel from the fire to the stack
How does the water heat up in firetube boilers?
The water surrounds the firetubes and the heat from the hot gases inside the tubes passes through the tube walls to the water
T/F: The horizontal return tubular (HRT) boiler was one of the earliest forms of firetube boilers.
True
How do horizontal return tubular (HRT) boilers work?
Hot gases are created by the burning coal that was under the boiler drum, and returned through the firetubes to the boiler stack
–> two-pass boiler
–> encased in bricks to contain the flame (externally fired boiler)
Why are horizontal return tubular (HRT) boilers no longer made?
Because they were labour intensive to make and inefficient to operate –> due to the bricks
- heat loss through the bricks was too high
What was the evolution of the HRT boiler?
HRT –> internally fired boiler –> Scotch marine boiler
Internally fired boiler: enlarged the return flue (of the HRT), put the firing grate inside, and made the furnace surrounded by water
Scotch Marine boiler: combined both boilers… internally fired, had a large heating surface, long gas travel, no external brickwork
Describe a locomotive boiler.
- internal furnace (almost completely surrounded by water)
- hot gases travel from the furnace –> firetubes –> smokebox (barrel) –> smoke stack
- uptake area has a damper for combustion control
- top of furnace has a crownsheet (under stretch force during operation)
What was an advantage of the locomotive boiler?
- absence of brickwork, refractory, or ‘setting’
- led to portable boilers
What is the difference between a locomotive boiler and a firebox boiler?
Similar configuration, but the firebox boiler had a
‘pass’ that was added –> firebox more self-supporting than locomotive
–> the pass led to more support above the furnace and eliminated need for the crownsheet
What were the types of stays that firetube boilers required for extra support (due to use of endplates & eliminating crownsheets)?
- through or longitudinal stay (rods attached internally, tubesheet-tubesheet)
- diagonal stays (rods attached at corners of the shell)
- stay bolts (attached to each plate to add strength)
- firetubes
What are the types of Scotch Marine boilers?
1) wet-back boiler
2) dry-back boiler (more popular because of the simple construction / design)
T/F: Passes in firetube boilers rarely exceed 4.
True
What are the advantages and disadvantages of firetube boilers?
Advantages:
- simple and rugged construction
- relatively low cost
- large amount of stored heat in water makes it possible to meet load changes quickly
Disadvantages:
- relatively slow in coming up to operating pressure & temperature because of the large water capacity
- disastrous if there is a pressure failure (lots of water will turn to steam)
What are vertical firetube boilers popular for?
The supplier of small quantities of high-pressure steam
Describe the construction & gas flow of a vertical firetube boiler.
- cylindrical shell with tubes connecting the upper and lower tube sheets
- hot gases travel through the tubes from the combustion chamber –> uptake and give off eat to the surrounding water
- upper part is used as a steam collector space before It leaves the boiler
–> upper part of the tubes get cooled by the steam
How should you take care of / fire up a vertical firetube boiler?
special care should be taken during the warm-up period….
boiler should be fired up very slowly until steam is being produced in order to avoid overheating the tubes & upper sheet
What are steel plate heating boilers?
A type of small capacity, vertical firetube boilers, used for hot water heating systems in residences / small apartments.
Describe the construction and gas flow of a steel plate heating boiler.
- divided into two parts: lower (combustion chamber / furnace) and upper (boiler filled with water)
- heat passes from the hot combustion gases to the water in the boiler
- hot gases travel upwards through the firetubes –> give heat to tube walls
- circulating pump in the lower part pushes the water upward around the tubes, collects the heat, and leaves the boiler through the top outlet
T/F: Vertical firetube boilers are a single pass boiler.
True. The gases travel through the boiler in one direction only.
T/F: Vertical firetube boilers have a disadvantage but horizontal firetube boilers have an advantage when floor space is limited.
False.
- Vertical boilers have advantage with limited floor space because they only occupy a small area
- horizontal boilers take up more floor space but have an advantage when there’s less ceiling height
What are vertical tubeless boilers used for?
dry-cleaning and laundry applications
What are the advantages of vertical tubeless boilers?
- simple and packaged design
- ready for hook-up with automatic controls
- occupies little floor space
Describe the construction and gas flow in a vertical tubeless boiler.
- vertical cylindrical shell thats closed at the top and bottom
- central pipe inside the shell that is used as a furnace
- forced air burned at the top of the boiler (usually)
- secondary pipe contains the water and is outside (concentric / surrounding) the inner furnace pipe
- firing occurs in the furnace (central) pipe in a DOWNWARD direction
- hot combustion gases create a swirling motion as the pass down the flame retainer rings
- gas returns up for a second pass outside of the outer (water) pipe
- gas bases through the fins on the outer water pipe for more heat transfer
- gas reaches a steam space at the top and exit from a chimney
T/F: Vertical tubeless boilers can come in electric, oil, or gas applications.
True.
- gas is most popular in Western Canada
What are the steam pressure and power ranges for vertical tubeless boilers?
steam pressure: 101.3 kPa - 1050 kPa
Power: 4.5 kW - 75 kW
Why are modern firetube boilers marketed as packaged units now?
Previously, the basic boiler was manufactured at the factory and shipped to site….. Required refractory, insulation, boiler fittings, controls, and firing equipment to be installed.
- local tradesmen didn’t know how to build properly
- often used incorrect pieces / not suitable
- boiler didn’t work up to expectations
What is a packaged firetube boiler?
A boiler supplied by the manufacturer and comes as a completely equipped unit that’s mounted on it’s own base, and ready for operation as soon as it’s placed in the boiler room / properly hooked up.
- however, operation and control system is most often completed at the buyer’s site
MC: The maximum pressure for a low-pressure steam boiler…
a) 103 kPa (14.7 psi)
b) 160 kPa (23.2 psi)
c) 250 kPa (36.2 psi)
d) 1100 kPa (160 psi)
(a)
MC: The main part of a high-pressure firetube boiler is called the….
a) windbox
b) furnace
c) shell
d) drum
(c)
MC: The pressure and temperature limiting factors for steam units used for heating and hot water heating boilers are…
a) 121 kPa or 103 degrees C for steam units, and 1100 kPa for hot water units
b) 1100 kPa for steam units, and 103 kPa or 121 degrees C for hot water units
c) 103 kPa for steam units, and 1100 kPa or 121 degrees C for hot water units
d) 1100 kPa or 121 degrees C for steam units, and 1100 kPa for hot water units
(c)
MC: In a vertical firetube boiler, the steam space is located…
a) in a separate chamber
b) in a compartment on the side
c) at the lower part of the shell
d) at the top of the shell
(d)
MC: The forerunner of the modern packaged firetube heating boiler was the…
a) internally fired horizontal return tubular (HRT) boiler
b) internally fired scotch marine boiler
c) externally fired scotch marine boiler
d) externally fired horizontal return tubular (HRT) boiler
(b)
MC: The chief advantage of a packaged boiler is…
a) its compactnesss
b) mass production ability
c) higher pressure rating
d) its higher factor of safety
(b)
MC: In a vertical firetube boiler…
a) the water passes through the inside of the tubes
b) the steam accumulates in the lower part of the boiler shell
c) the horizontal shell has vertical firetubes at each end
d) the upper part of the tubes are cooled by the steam
(d)
MC: The ‘pipe’ is a common term for…
a) the combustion chamber of a vertical tubeless boiler
b) the drain on an immersion boiler
c) the water jacket of a vertical boiler
d) the steam outlet of an electrode boiler
(a)
MC: In which of these boilers does the firing occur in a downward direction?
a) immersion electrode
b) vertical electric
c) vertical firetube
d) vertical tubeless
(d)
MC: One feature of a vertical tubeless boiler that would not be found on the vertical firetube boiler is…
a) water level control
b) burners
c) refractory
d) water column
(c)
What is a main difference between firetube / watertube boilers and electric boilers?
Firetube / watertube boilers: burn fuel in a boiler furnace to heat water or to produce steam
Electric boilers: uses energy from an electrode or an element to generate heat
Why / where are electric boilers generally used?
used in hospitals, schools, hotels
–> provide steam for heating, sterilizing, laundry, and kitchen equipment
–> not usually used in hot water heating systems
