Watertube Boilers

Front 1

What are the parts of a Watertube Boiler

Back 1

1) Steam Drum: the top drum(s)

2) Mud Drum: the bottom drum(s)

3) Furnace: where the fuel is burned

4) Watertubes: tubes which contain boiler water

5) Downcomer: watertube conducting cooler water down to the mud drum

6) Riser: watertube conducting hotter water and steam up to the steam drum

Front 2

What is a Straight Tube Boiler

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Early watertube boiler design with straight, inclined tubes which run between vertical headers connected to the front and rear of the steam drum. No longer built for industry.

Front 3

What type of boiler is this

Back 3

Straight Tube Boiler (Longitudinal) - Watertube

Front 4

What type of boiler is this?

Back 4

Straight Tube Boiler (Cross Drum) - Watertube

Front 5

Describe Bent Tube Boilers

Back 5

Most common watertube boiler design. Capable of large steam capacities, high pressures and versatile arrangements. They also allow efficient use of superheaters, reheaters and other heat recovery components. The furnace is often lined with watertubes to absorb more radiant heat from the fire. Differentiated both by their type and number of drums. Most common types are A, D and O: Steam bubbles form in the hottest tubes (risers) and rise to the steam drum, where the steam is separated out of the water and steam mixture. Circulation is maintained by water returning to the mud drum through the cooler tubes (downcomers).

Front 6

Describe A-Type Boilers

Back 6

A bent-type boiler design with 2 small mud drums. The steam drum is larger to permit separation of water and steam. Bent tubes running from the upper drum to the two mud drums form the furnace enclosure.

Front 7

What type of boiler is this

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A-Type Boiler Design - Watertube-Bent Tube

Front 8

Describe D-Type Boilers

Back 8

Watertube Bent-Tube Boiler design with two drums. Bent tubes, on one side of the boiler, form a "D" shape which creates a water-cooled furnace. The back wall, which receives radiant and convection heat, is usually protected by refractory plus it has some wider spaced water tubes. The burner may be located in the front wall or in the side wall.

Front 9

What type of boiler is this

Back 9

D-Type Boiler Design - Watertube Bent-Tube

Front 10

Describe O-Type Boilers

Back 10

Watertube Bent-Tube Boiler design with two drums. The tubes connecting these two drums are arranged in an "O" shape to form a water-cooled furnace. Exposes less tube surface to radiant heat than A or D types, it's compact design makes it a popular choice where space is limited.

Front 11

What type of boiler is this

Back 11

O-Type Boiler Design - Watertube Bent-Tube Boiler

Front 12

Describe Four-Drum Boilers (Stirling Type)

Back 12

First bent-tube type to be developed with three upper drums and a lower mud drum. 3 drums are connected by watertubes to the mud drum which is provided with a blowoff connection. The space above the water level in the three upper drums serve as a steam space and the three drums are interconnected by both steam and water circulating pipes. The steam outlet is from the rear upper drum which has a safety valve and feedwater inlet connected. Combustion gas is directed across the tube banks by means of brick baffles. Steelwork supports the three upper drums while the mud drum is freely suspended from the tubes. The furnace is constructed of brickwork and is not water-cooled.

Front 13

What type of boiler is this

Back 13

Four-Drum (Stirling Type) Boiler Design - Watertube Bent-Tube

Front 14

Describe Packaged Watertube Boilers

Back 14

Have water-cooled furnaces and the air for combustion is supplied from a forced draft fan. A steel casing which prevents combustion gas leakage into the boiler room covers the outside of the boiler and furnace. A skid-type steel foundation is included and the packaged boiler is bottom-supported, meaning the mud drum is supported by a steel casing and the steam drum by the tubes. This support requires special concrete footings or piers to eliminate excessive vibration in the boiler setting which can cause failures of the insulation, casing and supports. Available for both high- and low-pressure applications.

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Pros of Packaged Boilers

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1. Relatively low cost – construction costs are lower since the entire boiler
is built and assembled in the factory, rather than in the field. The term
“shop assembled” is often used.
2. Shipping costs are lower, since the unit is transported as a single unit,
complete with fuel system, draft equipment, and controls.
3. The boiler can pre-tested in the shop before delivery to site, ensuring
quality and inspections are approved.
4. On-site installation is reduced to a minimum, with only positioning of the
boiler and connection of external auxiliary systems and piping required.
5. Compact and versatile in design. Can be made to occupy relative small
floor areas.

Front 16

What are all the elements of a steam generating unit that contribute to the efficient production of steam

Back 16

- Boiler

- Superheater

- Reheater

- Economizer

- Air heater

- Fuel equipment

- Draft fans

- Ash removal equipment

Front 17

Describe the superheater

Back 17

Consists of 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. Used when the steam required for power generation or a process is at a temperature higher than that of saturation. They minimize the the chance of condensation of the steam in the latter stages of a steam turbine. Superheated steam also increases the overall plant efficiency in driving turbines by increasing the amount of energy that can be extracted from each kg of steam.

Front 18

Reheater

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Consists of a bank of tubes located within the furnace outlet, or radiant area of the furnace. It reheats the steam drawn from a medium pressure stage of the turbine to further reduce the chance of condensation and increase the energy available to the turbine. The reheated steam is returned to the remaining stages of the turbine.

Front 19

Economizer

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Used to remove waste heat from the flue gas and transfer it to feedwater flowing through the economizer tubes. This process maximizes the fuel economy of the system.

Front 20

Air Heater

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Preheats the combustion air for the burners, which improves combustion efficiency and assists in the burning of pulverized coal. They get their heat from flue gases leaving the economizer section.

Front 21

Describe the flow of air and flue gases, from inlet to outlet

Back 21

- Combustion air is drawn in from the atmosphere by a Forced Draft (FD) Fan

- The air is preheated in the Air Heater, where it receives heat by transfer from the flue gases before they enter the exhaust stack

- The preheated air is sent through the windbox to the burners where it mixes with incoming fuel in the burners, producing combustion in the furnace

- The hot combustion gases leave the furnace and are now referred to as flue gas

- The flue gases are then progressively cooled as they transfer (i.e. lose) heat to each of the major components in the following order: radiant superheater, reheater, convection superheater, economizer, and air heater

- The coldest flue gases are then drawn out of the air heater by the Induced Draft (ID) Fan, which discharges them up the Stack to atmosphere

Front 22

Describe Watertube Heating Boilers with Serpentine-Shaped Tubes

Back 22

Popular type of boiler consisting of a lower and upper header connected by copper or steel serpentine-shaped tubes, which are attached with threaded connections. The combustion chamber of this boiler is below the tubes, thereby exposing the lower part of the tubes to the radiant heat of the fire. The combustion gases travel between the tubes upwards to the flue. Even though the gas travel is short, heat transfer is efficient due to the arrangement of the tubes which causes turbulent and intensive scrubbing of the gases around them.

Front 23

Pros of the Watertube Heating Boilers with Serpentine-Shaped Tubes

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- Flexibility of the serpentine-shape tubes which eliminates expansion and contraction stresses

- Ease of replacement of a defective tube since no welding or tube end expanding is needed

Front 24

Describe Bent-Tube Watertube Heating Boilers

Back 24

A special version of the industrial "O" type packaged watertube boiler. It consists of a large upper drum, a small lower drum and a number of bent watertubes which connect the upper and lower drums. The tubes are arranged in such a way as to form the furnace enclosure, thus, the furnace walls are formed by water-filled tubes and therefore called "waterwalls". The tubes in these walls are spaced apart but connected to each other by steel plating welded to the tubes so that gastight walls (membrane waterwalls) are formed. 2 such walls are used in each side of the boiler, the inner walls form the furnace enclosure, the outer walls passageways for the flue gases. Front and rear of the boiler are closed by single waterwalls. Large amounts of radiant heat from he fire are absorbed by the furnace walls. This heat is carried off by rapidly circulating water in the tubes. The combustion gases, after leaving the furnace, travel between the inner and outer membrane walls on their way to the stack, giving up their heat by convection. The boiler is enclosed by an insulated steel casing. It is either oil or gas fired and equipped with a forced draft fan.

Front 25

Describe Tubular Steam Heating Boilers

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In principle, a watertube boiler but without drums or headers. Instead of a large number of tubes, it has one or more continuous coils of copper or steel tubing, the number depending on the capacity of the boiler. A pump forces water through the coil which is exposed to the hot products of combustion. Hardness-forming salts in the water is no longer a problem so used as a steam boiler.

Front 26

Where are Copper Tubular Heating Boilers popular

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Residential and commercial hot water heating systems.

Front 27

Describe Copper Tubular Heating Boilers

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Heating surface consists of one continuous, small diameter copper tube. Copper is used because it resists corrosion and it has a better heat transfer rate than cast iron or steel. The boiler is either gas or oil fired, with the burner placed in the lower part of the furnace section.

The heating surface is divided into 3 parts:

1. Lower section consisting of a tightly wound coil which surrounds the combustion chamber.

2. An intermediate section made up of several layers of loosely wound spirals, allowing the hot gases to flow freely around the tubing after they leave the furnace.

3. The upper section consisting of a fin-and-tube type heat exchanger. Fins are crimped or bonded on the tubes, increasing the heating surface so more heat is absorbed from the hot gases rising from the furnace on their way to the chimney.

Smaller heating systems use a single, copper-tubular boiler. Larger heating systems use several smaller units in parallel.

Front 28

Describe Packaged Tubular Steam Heating Boilers

Back 28

The boiler is supplied as a packaged unit and is equipped with its own feedwater pump and a steam separator. The heating surface consists of one continuous steel tube forming a waterwall of tightly wound coils around the furnace in the lower section, and several layers of loosely wound spirals in the upper section. The tubing in the waterwall is protected is protected from the erosive actions of the fales by steel sheathing. Water circulates at high veolocity through the boiler tubing from the top downwards while the hot gases travel upwards. In this way, no cold water is pumped into the hottest section of the boiler, excessive stresses are prevented, and longer life is assured. The boiler can be fired by oil or gas. A blower, driven by the same motor driving the feedwater pump, supplies the combustion air.

Front 29

Describe the operation of the Packaged Tubular Steam Heating Unit

Back 29

Make-up water and returned condensate enter the feedwater inlet and flow to the feedwater pump headers. The water is then pumped through the mixing chamber to the heating coil. In the single pass heating coil the water is heated to equivalent steam temperature by the combustion gases flowing upward through the coil assembly. The steam-water mixture is then passed into the thermostat tube and counterflows back across the outside of the thermostat tube to the accumulator. Here steam and water are centrifugally seperated by the separating nozzle. Steam is discharged through the steam discharge valve. The water is drawn from the accumulator by the recirculating pump heads and pumped to the mixing chamber where it blends with the feedwater. Excess water in the accumulator is returned to the condensate tank through a steam trap. A liquid flow control is incorporated to prevent burner operation in case of a low water condition. Automatic half-fire modulation is provided to reduce "on-off" cycling during periods of light steam demand. Softened water is used as feedwater to prevent scale forming. Since any salt carried into the boiler with the feedwater stays behind and would concentrate in the boiler water, an automatic blow-down is provided which drains sufficient water off the accumulator to keep the concentration within safe limits.

Front 30

Pros of Low-Pressure Watertube and Tubular Type Boilers

Back 30

- Compact - considerably smaller and lighter than firetube boilers of equal capacity

- No special foundation required

- Very short warm-up period required

- Bent or coil tube design avoids themal stress and distortion

- Rapid response to fluctuating loads

- Supplied as packaged units completely equipped with firing equipment, automatic controls and safety devices

- Safer than firetube boilers with respect to tube failure since boilers contain very little water and no disastrous explosion could occur

- Maintenance cost low. Boiler has a minimum of refractory.

Front 31

Cons of Low-Pressure Watertube and Tubular Type Boilers

Back 31

- Higher initial cost

- Need for closely monitored water treatment