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
