Low-Pressure Boilers

Question 1

What happens to the water in high-pressure boilers?

Answer 1

The water circulates inside the tubes, which are heated by the hot flue gases from the fire.

Question 2

What are the advantages of a watertube boiler?

Answer 2

• they are safer due to the relatively small water content
• they respond fast to load changes

Question 3

Why aren’t industrial type watertube boilers used for low-pressure steam heating plants?

Answer 3

• Mainly because of the high cost (compared to cast-iron or steel fire tube boilers)
• needs more supervision (esp. with water treatment)

Question 4

Can any water tube steam heating boilers be used for low-pressure heating applications?

Answer 4

Yes. Only specially designed water tube boilers

Question 5

What does a Low-pressure watertube boiler consist of?

Answer 5

• lower & upper header
• headers connected by lots of copper / steel serpentine shaped tubes

Question 6

In a low-pressure watertube boiler, where is the combustion chamber & what does it do?

Answer 6

Location: below the tubes & enclosed by insulated casting

Equipped with atmospheric gas burners

Question 7

How do the combustion gases travel through the tubes?

Answer 7

Gases travel upwards between the tubes towards the flue, and give up their heat by convection

Question 8

Is the heat transfer in watertube boilers efficient or no? Why or why not?

Answer 8

Yes, heat transfer is efficient because of the arrangement of the tubes.

Question 9

Why does the arrangement of the tubes in a watertube boiler make heat transfer efficient?

Answer 9

Because it causes turbulent and intensive scrubbing of the hot gases around the tubes

Question 10

What is an advantage of the low-pressure water tube boiler design?

Answer 10

Flexibility of the serpentine-shaped tubes

• gets rid of expansion and contraction stress
• tubes can be easily replaced (no welding or expanding needed)

Question 11

T/F: A tubular boiler is a watertube boiler

Answer 11

True (in principal)

Question 12

What makes a tubular boiler different from a low-pressure watertube boiler?

Answer 12

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)

Question 13

Where are copper-tubular boilers used?

Answer 13

• mainly in residential and commercial hot water heating systems
• domestic hot water supply

Question 14

What is the heating surface of a copper-tubular boiler?

Answer 14

one continuous, small diameter copper tube

Question 15

Why is copper used in the copper-tubular boiler?

Answer 15

Because…

• copper resists corrosion
• has a faster heat transfer rate than cast iron or steel

Question 16

List and describe the 3 parts of the heating surface in copper-tubular boilers.

Answer 16

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

Question 17

Where is the burner placed in the copper-tubular boiler?

Answer 17

in the lower part of the furnace section

Question 18

How does water circulate in copper-tubular boilers?

Answer 18

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

Question 19

What is softened water used for?

Answer 19

used as a feed water to prevent scale from forming

Question 20

What boiler produces steam most rapidly?

Answer 20

copper-tubular boilers because there is no water stored in the drums/headers

Question 21

Explain the difference between the terms ‘steam boiler’ and ‘steam generator’

Answer 21

Boiler: when there are ‘stored’ water compartments (ex. drums/headers)

Generator: when there is more water in the tubes than in the drums

Question 22

T/F: The tubular unit doesn’t require more attention when it comes to water treatment programs/

Answer 22

False. Tubular units DO require more attention.

Question 23

Describe the water circulation in a longitudinal drum straight tube boiler.

Answer 23

• 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

Question 24

What is the difference between a longitudinal drum boiler and a cross drum boiler?

Answer 24

longitudinal boilers have the drum running front-to-back

cross drum boilers have the drum side-to-side

Question 25

Describe the parts of the two-drum bent tube boilers.

Answer 25

- 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

Question 26

Describe the heat circulation in two-drum bent tube boilers

Answer 26

- 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

Question 27

Can a bent tube boiler be used as a steam or hot water boiler? And is it used for low or high pressure service?

Answer 27

Can be used as steam OR hot water Can be used for low- OR high-pressure service

Question 28

Describe the advantages of a bent tube boiler compared to a straight tube boiler.

Answer 28

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

Question 29

What is another name for the two-drum bent tube boiler?

Answer 29

"O" type watertube boiler

Question 30

Describe the construction of the "A" type boiler.

Answer 30

- 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

Question 31

Describe the construction of the "D" type packaged boilers.

Answer 31

- 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

Question 32

Describe the construction of an "O" type packaged boiler.

Answer 32

- compact steamer - requires a longer boiler - has a symmetrical design, exposing the least tube surface to radiant heat

Question 33

What are the advantages of the packaged watertube boilers?

Answer 33

- 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

Question 34

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

Answer 34

d)

Question 35

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

Answer 35

b)

Question 36

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"

Answer 36

(c)

Question 37

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

Answer 37

(a)

Question 38

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

Answer 38

(b)