Basice Structure of boiler | for mechanical engineer begineers

boiler structure
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I will explain the structure of boiler.

Steam (vapor) is used as a matter of course in chemical plants, so it is better to know it as knowledge.

In recent years, due to the miniaturization of boilers, the type that can be used without a boiler qualification is useful.

Isn’t the knowledge of boilers a bit dubious even for mechanical and electrical engineers? There are more people who say.

Boilers are used not only in chemical plants, but also in towns, so it would be a waste not to study them just because you are not currently involved in boilers.

It’s better to know as basic knowledge so that you don’t panic at that time.

Boiler configuration

When studying for the 2nd class boiler engineer qualification, the first thing you want to keep in mind is the “configuration of the boiler.”

The “big picture” of a complex mechanical device called a boiler.

It’s important to have a composition that keeps the big picture in check and goes into the details.

Please check the article below for the overall composition of the 2nd class boiler engineer.

Surprisingly, there is no material that simply shows and explains the configuration of the device.

I’ve seen textbooks for various qualification exams, but I’ve never seen a material that shows the system of the equipment…

Structurally, the boiler can be divided into the following two types.

  • main unit
  • Ancillary equipment

main unit

Introduce the basics in the basics of the main equipment of the boiler.

As shown below.

Boiler configuration foundation (the structure of boiler)

Eh, that’s all? Didn’t you think?

It’s the foundation of the foundation. Let’s keep it simple.

Put in water, put in fire, make steam.

Here is the boiler.

The significance of a boiler lies in how safely and stably this can be done automatically.

Let’s take a closer look at the boiler device itself.

boiler body

Boilers use many “bulks” to transfer fire energy to water.

For chemical plant engineers, it is better to say that it is the same as a heat exchanger.

The area of ​​fire is often called the furnace, and the area of ​​water is often called the main body.

The boiler main unit is now finished.

Ancillary equipment

The boiler equipment itself is extremely simple.

If you line up the specific devices for stable operation of this, it becomes complicated at once.

Attachment configuration (boiler)

All of a sudden it became complicated and hard to understand.

heating system

Consider breaking down the above process.

For this kind of device, the first thing to consider is the flow of things.

Let’s divide it into two, the heating system and the steam system.

Let’s take a look at the heating system.

The red line in the figure below is the heating system.

Heating system (boiler)

The flow of the heating system is simply written as follows.

  • inject fuel and air
  • Create a flame by mixing in a burner
  • transfer heat from combustion to water
  • Exhaust gas is discharged outside
  • Overheating the generated steam

Follow along to see where each one points in the diagram above.

steam system

Let’s follow the flow in the same way for the steam system.

steam system

The flow of the steam system is simply written as follows.

  • send water into the boiler body
  • Receiving the heat of the furnace to generate steam
  • Supply part of the steam heat to the fuel and air

The steam system is simpler than the heating system.


Let’s take a quick look at the accessories.


Instruments are concentrated in the boiler body.

  • Liquid level indicator
  • thermometer
  • pressure gauge
  • safety valve
  • blow
  • alarm

Instruments that are familiar to chemical plant engineers will appear one after another.

Conversely, equipment in chemical plants is equipped with instruments similar to boilers.

It means that it is not limited to boilers.

A device is also installed on the side of the material flow.

  • Water lines: pumps, flow meters
  • Steam lines: pipes, holders, etc.
  • Air line: blower
  • Fuel lines: storage tanks, etc.
  • Exhaust gas line: anemometer

As the flow side of things gets closer, it goes further and further away from the boiler itself.

Handling the boiler is important, but in the sense of a qualification test, it may have to be postponed.

Type of boiler

We will introduce the types of boilers that fall within the scope of class 2 boilers.

  • Fire chamber inside round boiler / water outside
  • Water tube boiler inside is water / outside is firebox 
  • cast iron boiler 
  • special boiler 

These four are major categories.

It is especially important to hold down the structure of round boilers and water tube boilers . Let’s consider the water tube boiler to be the most important.

round boiler

There are the following types of round boilers.

  • vertical boiler
  • smoke tube boiler
  • fire tube boiler

The structure is in order as shown in the figure below.

round boiler

What they have in common is that water is stored in the outer cylindrical part called the body.

Is it a round boiler because the torso is round?

This can be clearly seen by looking at a water tube boiler.

water tube boiler

A water tube boiler has a fire chamber on the outside and a water tube on the inside.

round boilerfirewater
water tube boilerwaterfire

Vertical boiler (furnace boiler)

A vertical boiler has a very simple structure.

Is it an early model boiler?

There is a firebox in the middle and water on the outside.

vertical boiler

It is an image of “arranging water around the chimney”.

There is a chimney in the center that burns the fire and discharges the exhaust gas, and in order to use that heat to turn the water into steam, we should place the water outside the chimney, right?

I think it’s such a simple idea.

The features of the vertical boiler are as follows.

  • Heat transfer area is small ← Because there is a restriction on the diameter of the barrel that stores water
  • Low facility cost ← Small size
  • Not suitable for high pressure ← Due to restrictions on the diameter of the barrel that stores water
  • Little pressure fluctuation ← (relatively) large amount of water
  • There is a lot of water in the steam ← Because the water surface is narrow
  • Difficult to inspect and maintain ← Small size

The essential problem with vertical boilers is that the barrel diameter for storing water is large.

The larger the barrel diameter, the stronger the tensile stress at the same pressure. This means that the strength of the equipment will be weakened.

Boilers are more efficient when they can supply high-pressure steam, but the fate of vertical boilers is that they impose a large restriction on equipment dimensions such as the barrel diameter in order to supply high pressure.

Since the barrel diameter has to be reduced, the firebox becomes smaller and the heat transfer area becomes smaller.

Although it has the advantage of reducing equipment installation costs, it has the disadvantage of being difficult to maintain.

Since the barrel diameter is small, the area of ​​the water surface is small, and the speed of the part where the steam passes through the water surface is relatively fast.

This causes moisture to be mixed into the steam.

In addition, it seems that the type with a round firebox is especially called “furnace tube”.

smoke tube boiler

A smoke tube boiler is made by dividing the part (fire chamber) of a vertical boiler through which fire passes into finely divided smoke tubes.

The structure is as shown below.

smoke tube boiler

Rather than using a single chimney like a vertical boiler, dividing the chimney into small pieces has the advantage of increasing the heat transfer area.

The size can be reduced accordingly, and the degree of design freedom can be increased.

fire tube boiler

A fire tube boiler is literally a combination of a fire tube boiler and a fire tube boiler.

fire tube boiler

This type is more efficient than a smoke tube boiler.

Compared to a vertical boiler with only a furnace tube, the heat transfer area is naturally larger due to the presence of the smoke tube.

  • For industrial use and heating ← Because it is easy to manufacture and handle
  • Package type ← Because bricks are not required for installation
  • Can be automated
  • Requires water treatment ← Complex structure, difficult to clean and inspect
  • spiral tube

If you look at the features, the fire tube boiler looks like a standard boiler.

My workplace uses a once-through boiler, so it feels a little strange.

Brickwork can be determined by whether or not there is a firebox at the bottom.

A vertical boiler has a firebox at the bottom, which must be insulated with bricks due to the high temperature.

The fire tube boiler is placed horizontally, and the bottom part is filled with water, so you can install the package unit as it is without getting hot.

water tube boiler

Let’s look at the water tube boiler in the same way as the round boiler.

  • natural circulation
  • forced circulation
  • flow through

The characteristics of the water tube boiler compared to the vertical boiler are as follows.

  • Wide range of pressure and capacity ← Because the number of water tubes can be adjusted arbitrarily
  • Compatible with various combustion methods ← Combustion chamber can be designed freely
  • High thermal efficiency ← Due to large heat transfer area
  • Free arrangement of heaters and economizers ← Water pipes can be arranged freely
  • Start-up time is short ← Because the amount of water is small
  • Water treatment is necessary ← Because the amount of water is small

The point is that there is a high degree of freedom in design because there is no need to make a large body for the part through which water passes, and all you have to do is combine small pipes.

It’s very versatile.

However, compared to a vertical boiler, the amount of water is sacrificed, so it is slightly weaker against fluctuations in usage conditions.

This is why water treatment is strictly required. This is because even a small change in water quality can cause damage to equipment.

The small amount of water also has the advantage of shortening the start and stop time.

Natural circulation water tube boiler

The basics of water tube boilers are natural circulation.

The structure is as shown below.

Natural circulation (boiler)

There is a part that stores water at the top and bottom of the water pipe. This is called a drum.

The reason for storing water in the drum is to withstand sudden load changes.

When you suddenly increase the load, there is no end to replenishing water one by one.

The point is that the natural circulation boiler is installed upright.

Hot water becomes less dense and tends to rise , while cold water tends to sink.

This is what is meant by natural circulation.

The natural circulation type can inevitably be used only at low pressure.

This is because at high pressure, the difference in density between water and steam disappears, weakening the natural circulation force.

Forced circulation water tube boiler

Forced circulation water tube boilers have a pump for forced circulation.

The structure is as shown below.

forced circulation (boiler)

It is used for high pressure steam.

High-pressure steam is hot steam.

As the temperature increases, the density difference between water and steam decreases.

At atmospheric pressure of 100 kPaA, the water:steam ratio is about 1000:1.

When this exceeds 20 MPa, the water:steam ratio approaches 1:1.

Then natural circulation will not work.

If the water does not come down to the lower drum and the steam does not go up, the efficiency will be drastically reduced.

This is solved by forcibly creating a flow with a pump.

Attaching a pump reduces restrictions on the shape of the water pipe.

In the natural circulation system, vertical and shortest water pipes are often used in order not to reduce the circulation force.

If you can use a forced circulation pump, you can freely design the water pipe.

As a result, it has the advantage of being compact.

once-through boiler

Once-through boilers do not place the drum in contact with the firebox.

The structure is as shown below.


There are many merits because the drum is removed and it is composed only of tubes.

  • Resistant to high pressure ← Because it is only a pipe with a small diameter
  • Size can be reduced ← Because it is only a combination of piping
  • Start-up is short ← Because the amount of retained water is small
  • Requires good quality water ← Because it evaporates quickly

In order to reduce the amount of water, it is necessary to have a mechanism that can withstand load fluctuations.

But this is commonly done as an automatic control.

It was a problem in the days when there was no automatic control. This is the story of the world.

A once-through boiler uses a series of water tubes to raise the temperature, evaporate, and superheat to produce steam.

Flow is consistent.

Since no circulation line is installed, relative attention must be paid to water treatment and load fluctuations.

Since the pressure of the water flowing through the once-through line is increased by a pump, it has the same advantages as the forced circulation type, such as flexibility in piping and compactness.

cast iron boiler

Cast iron boilers are literally cast iron.

No steel plate is used.

There are merits and demerits because it is cast iron.

Since this is a technical part of the exam, the details are omitted.

cast iron is weak

That’s all I want to comment on.

Cast iron is often used for mechanical equipment in chemical plants, so I would like to keep it down as general knowledge.

special boiler

There are several types of special boilers, but the explanation is almost omitted because they are special.

  • waste heat boiler
  • special fuel boiler
  • Fluidized bed combustion boiler
  • heat medium boiler

It seems that there are these types.


Evaporation rate is defined in the order of t/h.

A batch chemical plant consumes about 2 to 5t/h per plant.

This is surprisingly little known for such an important indicator.

Evaporation (t/h)Boiler classification
50 to 300Natural circulation water tube boiler
10 to 30Round boiler (furnace tube smoke tube boiler)
10-15once-through boiler
~4cast iron boiler
0.01 to 7small capacity reflux boiler

Evaporation is not important for boiler engineer testing, but it is extremely important in practice.

Things to consider such as optimization of operating conditions, selection of pipe diameter, and selection of ancillary equipment are based on the amount of evaporation.

Some plant designers of factories who are not involved in boilers do not know how much steam they can supply in their own factories.

I’m kind of like that too…

evaporation pressure

Evaporation pressure is expressed in MPa.

In the boiler engineer test, relative pressure and absolute pressure are used subtly, so you need to be careful.

Evaporation pressure (MPa)Boiler classification
15-18Forced circulation water tube boiler
8-17Natural circulation water tube boiler
1 to 3round boiler
~1small once-through boiler
~0.1cast iron boiler

Steam pressure is also less of an issue in the Boiler Technician exam.

On the boiler classification, there is a description about pressure in the classification of simple boilers and once-through boilers, but it will not be asked much.

Most batch chemical plants use steam of 1 MPa or less. Easy, isn’t it?

Even if a pressure higher than 1 MPa is used, the limit is 5 MPa or less.

Even round boilers, which are said to have low pressure, are within the acceptable range for batch-type chemical plants.

0.1 MPa is too low, but 1 MPa is almost enough.

Glass lining and fluorine resin lining cannot withstand steam over 200°C.

180°C for saturated steam with absolute pressure of 1 MPa, and 310°C for absolute pressure of 10 MPa.

I would like to have some sense of this area.

Heat transfer area

The heat transfer area that appears most in the boiler engineer examination.

This is difficult to understand intuitively in practice.

Heat transfer area (m2)Boiler classification
20 to 150fire tube boiler
~50One safety valve is enough for a steam boiler
~10Two water test cocks for water level measuring devices that are not glass water level gauges
~3Steam boilers that can be handled by those who have completed the boiler handling skill course

The heat transfer area is also somewhat questioned in relation to laws and regulations, but there are not many types that appear.

That’s why the sense of the heat transfer area is also low.

Looking at the flue and smoke tube boiler with the concept of heat transfer surface evaporation rate kg (m2 hr),

20~150/10~30 = 0.7~15kg(m2hr)

It will be in the range.

The heat transfer surface evaporation rate of a round boiler is 15 to 30 kg (m2hr), so there is no big mistake.

Assuming 20 kg (m2hr), if we calculate the evaporation amount/heat transfer surface evaporation rate

It will be on the order of 100-1000m2 .

500m2 if the heat transfer area per system is 50m2 in a batch chemical plant and there are 10 systems

The fact that the order of the user’s heat transfer area and the heat transfer area per boiler is almost the same.

It can also be confirmed from the fact that the amount of evaporation consumed by the user and the amount of evaporation supplied by the boiler are almost on the order of t/h.

Strictly speaking, the heat transfer area consumed by the user is.

Since the heat transfer area of ​​the part that generates steam in the boiler is a system in which one side is liquid and the other is radiant heat.

The overall heat transfer coefficient for the user’s heat transfer area is higher, though.

boiler strength

The boiler is pressure vessel construction standard.

Strictly speaking, it is legally defined by the expression “Boiler and Pressure Vessel Safety Regulations”.

Ever heard of a pressure vessel?

Same as pressure cooker.

Containers with pressure are dangerous, so let’s limit them properly by law.

There are many chemical plant equipment that correspond to pressure vessels. (Although it is less in the batch system.)

A boiler is a typical pressure vessel, as is equipment in a chemical plant.

This is because it is a facility that performs a dangerous act of changing water into steam and storing it.

Even within the scope of second-class boiler engineers, there are discussions from the perspective of the boiler as a pressure vessel.

However, the problem is that it is difficult to understand which part this is pointing to.

Looking at the overall configuration of the boiler, the red line below corresponds to this.

pressure vessel

Round boiler (vertical boiler)

Let’s start with a simple round boiler.

The points for strength calculation are shown in the figure below.

Strength point_boiler

It might be hard to understand what you’re referring to, but…

The points for strength calculation are as follows.

  1. torso
  2. head plate
  3. Firebox
  4. Gusset stay
  5. Tube stay
  6. hole

I will explain the details of each.

① Torso

The torso has a cylindrical shape.

Since it is a cylinder, it looks like the figure below.


The important thing is that the cross-sectional shape is circular.

Rectangles are no good.

A circular cross section has the following characteristics in terms of strength.

  • Uniform pressure is applied at any position
  • Minimize plate thickness
  • Minimize dimensions.

The body contains water, and steam is generated by receiving the heat of the fire.

When water turns into steam, its volume increases 1000 times.

The torso is required to have the strength to suppress this volumetric expansion.

In order to provide strength, a structure that distributes pressure evenly is required.

When it comes to structures that can withstand high pressure, the most rational shape is a sphere.

A sphere has very limited design freedom.

Therefore, a cylinder, which is weaker than a sphere but reasonably strong, is chosen as the shape of the torso.

② Head plate

Even if the trunk has a cylindrical shape, the shape of the end must be considered.

A disc with a cylindrical shape and simple upper and lower end faces would be extremely weak.

The red part in the figure below is weak.

weaker (boiler)

To solve this problem, the trunk and upper and lower extremities are smoothly connected.

It looks like the picture below.


In order to realize such a shape, there is a shape called a “head plate”.

The shape of the end plate is semi-elliptical or dish-shaped.

In the figure below, the left side is semi-elliptical and the right side is dish-shaped.

Mirror shape (boiler)

The semi-elliptical shape has a higher degree of roundness.

The higher the degree of roundness, the stronger the strength.

③ Firebox

The firebox attaches a device to absorb elongation.

It’s called an expansion joint.


The firebox heats up to the temperature of the combustion gases, while the outer shell heats up only to the temperature of the steam.

A temperature difference occurs here.

Things expand when the temperature rises.

If only certain parts are stretched greatly and other parts are not stretched, the stretched parts will have no place to escape and will be destroyed.

In order to avoid this, expansion joints are installed to provide a “relaxation point for elongation.”

④ Gusset stay

Gusset stays are installed to reinforce the flat plates at the top and bottom ends.

Gusset stay (boiler)

It is the same idea as inserting diagonal materials for seismic reinforcement in schools.

I think that it is good to the extent that there is such a thing.

⑤Pipe stay

A tube stay is a reinforcement for a flat plate with a hole.

It looks like the image below.


If pipes for smoke pipes or water pipes are passed through a part with a flat plate or panel, the strength of that part will decrease.

This is because the area of ​​the flat plate and the head plate is reduced .

Drilling holes reduces strength.

Of course.

Increase the board thickness around the perforated area to reinforce the dropped strength.

Since the tube stays are used to reinforce the tube plates for the smoke tubes, the schematic drawing of a vertical boiler as an example is not strictly correct, but the idea is the same.

⑥ Hole

The hole is the opening introduced in the tube stay section.

The strength is reduced by the amount of this opening.

This includes manholes and cleaning holes.

water tube boiler

Let’s look at the water tube boiler in the same way as the round boiler.

The points in the strength design of the water tube boiler are as shown in the figure below.

water tube boiler

Water pipes, drums, headers, tube stays, etc. are important points.

The circled numbers at the end of the above diagram correspond to the round boilers ① to ⑥.

  • Water pipe … Same as cylinder body
  • Drum … Same as the cylindrical body (including ② of the end plate)
  • Header … same as drum
  • Tube stay … ⑤ itself

Since the water pipe is basically a pipe structure, it can be calculated as a cylinder.

Rounding so that there are no ends or discontinuities, reinforcing openings…

This kind of care is the point of strength design.

boiler accessories

I will explain the boiler accessories while being aware of their position in the whole.

Use the figure below as an image of the big picture.

boiler configuration

Measuring equipment

Measuring instruments are the most popular accessories for boilers.

Usually, when we say accessories, we mean measuring instruments.

The following four points are important for boiler measurement equipment.

Measuring instrument (boiler)

pressure gauge

A pressure gauge is for checking the pressure in the boiler.

Boiler pressure is the most important index for showing the performance of generated steam, which is steam pressure and temperature.

If the boiler pressure is too high, the boiler will explode, so it is an important regulatory device for safety management.

Chemical plants use similar pressure gauges.

water gauge

A water level gauge is for checking the water level in the boiler.

If the water level is too low or too high, the operation of the boiler will be affected, so it is an important measuring instrument.

So-called “glass type liquid level gauge” is major.

From the point of view of a chemical plant, it is an old-fashioned liquid level gauge.


The flow meter is for checking the flow rate of feed water to the boiler.

It is also used to check the flow rate of the supplied fuel.

Chemical plants use a wide variety of flowmeters.

In boilers, the differential pressure type, positive displacement type, and area type are major.

From the point of view of a chemical plant, it is an old-fashioned flow meter.


An aerometer is for checking the pressure in the furnace and the pressure in the duct.

A manometer checks the pressure of steam, while an aerometer checks the pressure of gas.

The difference is “the magnitude of the pressure”

The steam side shows a high pressure on the order of 1 MPa, but the aerometer is on the order of kPa or less.

Gas pressure or air pressure is not high.

It is difficult to check this with a Bourdon tube pressure gauge.

Use a primitive U-shaped manometer.

Safety device

Boilers have high pressure, so safety equipment is important.

Important safety devices for boilers are:

safety device (boiler)

safety valve

A safety valve is a valve that releases the boiler pressure to the outside.

When the pressure of the boiler rises due to some abnormality, the boiler will burst and explode if nothing is done.

this is horrible…

To prevent this from happening, there is a safety valve that opens when a certain pressure is exceeded.

The expression “bent” has become famous.

Chemical plants also use safety valves as a matter of course.

Boilers are based on primitive, reliable, and reliable spring-type safety valves.

It’s the same with chemical plants.

alarm device

This is an alarm device for the boiler water level.

Water level gauges are important, but they are meaningless unless people monitor them.

Attach an alarm device so that the machine can monitor even when a person is not monitoring.

It gives an alarm when the water level is too high or too low.


A blower is a device for discharging impurities in the boiler.

Water always contains impurities.

As the water turns to steam, the impurities continue to build up inside the boiler.

Accumulated impurities deteriorate the performance of the boiler.

  • It becomes an obstacle to transfer the heat of the boiler to the water
  • Advancing boiler corrosion
  • blockage in boiler

Gate valves and Y-type valves are major manual valves.

It may be considered as a separate accessory category as a blow device, rather than as a safety device.

air supply system

An air supply system is a steam piping system.

The air supply system in the boiler is as follows.

air supply system (boiler)

steam piping

The air supply system is the steam piping system.

If you are a chemical plant engineer, you are not afraid of anything.

This is the general steam piping design itself.

steam valve

A steam valve is also one of the steam piping systems.

Angle valves and globe valves are common.

There are gate valves and check valves, but they are only for reference.

steam trap

A steam trap (steam trap) is a device specific to steam.

A device that automatically discharges only the drain that accumulates while using steam, and does not discharge steam.

If the drain is not discharged, the steam cannot be properly supplied and operated.

Simply opening the valve to drain the condensate will also release usable steam.

This is a very big problem from the viewpoint of energy saving.

When and how much drain accumulates is also case-by-case.

It is troublesome for people to go to open the valve one by one.

A steam trap solves these problems.

Water supply device

Since the boiler is a device that converts water into steam, the water system is also an accessory.

The water supply system for the boiler is as follows.


water pump

A pump is needed to send water to the boiler.

General centrifugal pumps are also very useful in chemical plants.


Use the injector instead of the pump.

It is sometimes used as a backup pump only when the boiler pressure is low.

It uses boiler steam to supply water with the ejector effect.

In chemical plant equipment, they are sometimes called ejectors and scrubbers.

feed water heater

A feedwater heater is a device that preheats water before it enters the boiler.

It is too famous as a heat exchanger in chemical plants.

Boilers have the role of stably supplying steam.

If you send cold water to the boiler, the temperature of the boiler will drop momentarily.

In order to keep the boiler temperature constant and supply steam with stable pressure.

It is better to warm the temperature of the water before sending it to the boiler.

water supply valve

If you are a chemical plant engineer, there is almost nothing to talk about valves.

While using angle valves and globe valves like steam valves.

I also use a check valve.

boiler auxiliary equipment

Ancillary equipment increases the efficiency of the boiler.

The following locations are auxiliary equipment for the entire boiler system:


The difference between accessories and accessories can be roughly classified as follows.

  • Accessories are things that the boiler cannot operate without.
  • The boiler can be operated to some extent without the auxiliary equipment.

Strictly speaking, I think the feed water heater is also a category of accessory equipment.

Superheater (steam heating)

The superheater is for making the steam generated in the boiler into superheated steam.

Superheated steam is saturated steam that has been further heated.

The ratio of water: steam = 100:0 is the state of ordinary water.

As water is heated, steam is generated, and the ratio of steam increases to 90:10, 80:20, and so on.

And the moment when water:steam=0:100 is saturated steam.

This saturated steam is determined by a 1:1 relationship between pressure and temperature.

From the point of view of a user who uses steam, it is OK if it is considered as saturated steam.

Saturated steam that is further heated is called superheated steam.

For users using saturated steam, the supplier boiler will provide superheated steam.

This is because they are more thermally efficient.

As for the scope of the second class boiler, it would be good to understand that “that’s what it is”.

Economizer (exhaust gas cooling)

The economizer uses the heat of the flue gas to preheat the feed water.

Boiler flue gas is very hot.

If you just dump it into the environment, you’re just wasting heat.

Can’t we use this heat for something?

With that in mind, I tried to use the economizer to preheat the water supply.

Even raising the temperature of water from room temperature to 100°C requires heat.

It is a waste to use the heat of the combustion of the boiler.

Let’s use the heat of the surplus exhaust gas!

That is the purpose of the economizer.

The economizer is economy because it is economical.

The equipment is structurally the same as a heat exchanger.

Air preheater (air heating)

Air preheaters literally preheat the air.

The water supply heater and the idea are naoji.

When using room temperature air when mixing fuel and air for combustion.

Part of the combustion heat is used to warm the air.

Oh, what a waste.

So we arrive at the idea of ​​preheating the air as well.

There are two ways to preheat the air:

  • Use steam heat (use a superheater)
  • Using the heat of the exhaust gas (using an economizer)

This is also structurally the same as a heat exchanger.

The idea of ​​warming the air naturally makes you think of the idea of ​​warming the fuel.

Soot blower (boiler cleaning)

A sootblower is a cleaning device in a boiler.

  • Dust on boiler heat transfer surfaces
  • Soot on the inner walls of the furnace

As these accumulate, not only does the heat transfer efficiency decrease.

Gas draft resistance also increases.

In addition to heat energy loss, there is also electrical energy loss.

To prevent this, regular cleaning is required.

That device is the soot blower.

There are two patterns , a method that injects steam and a method that injects air.

Boiler flue and order of heat exchange parts

I will explain the order of the boiler flue and heat exchange parts.

It’s surprisingly hard to understand, but it’s a pretty important story.

This is because they do not understand the positional relationship of the main components of the boiler, which is “transferring the heat generated by burning fuel to water and converting it to steam”.

You can pass the test just by knowing.

As a chemical plant engineer, I would like to be able to understand things from a process perspective.

If possible, to the level where you can verbalize it.

In conclusion, the order of the heat exchange components is as follows:

Evaporation tube → Heating tube → Economizer → Air preheater

It is OK if you imagine a water tube boiler.

flue flow (boiler)

Evaporation tube

Along the flue flow, the first part to exchange heat is the evaporator tube.

You will understand this without thinking about it again.

This is because the evaporator tube is the most important part that converts water into steam.

This is because we want to make the temperature difference Δt as large as possible and increase the amount of heat transfer as much as possible.

It’s not about maximizing heat transfer! Be careful!

If this water is normal temperature water, most of the evaporator tube will be used for sensible heat and not for latent heat, which is a waste.

That’s why you want the water to be as warm as possible.

The temperature difference Δt will decrease accordingly.

So it’s not about maximizing the amount of heat transfer, but about optimizing the heat transfer area and the amount of heat transfer that contribute to the latent heat.

heating tube

Place the heating pipe immediately after the evaporator pipe.

This is to make the steam superheated rather than saturated.

Evaporation tube and heating tube are separated by steam separator in water tube boiler.

  • Evaporation tube → Separation of drain with steam separator
  • Heated to superheated steam by steam separator → heating tube

That’s the flow.

If there is no drain at all, the flow will be one water pipe with the evaporating pipe on the side closer to the flue and the heating pipe on the far side.

In the world of heat exchangers in chemical plants, there is the concept of countercurrent heat transfer and parallel current heat transfer, but it means that they are arranged on the parallel heat transfer side.

Rather than the countercurrent flow in which Δt is constant, parallel flow in which Δt varies is selected in order to focus on the heat transfer performance of the evaporator tube.


Install an economizer after the heating pipe.

An economizer is a feed water preheater.

It’s hard to imagine because it’s in katakana, but if you remember the existence of the air preheater for the economizer.

It’s not air, so it’s water supply!

can be associated with

The reasons for installing the economizer before the air preheater are as follows.

  • High heat transfer performance
  • low pressure drop

heat transfer performance

Heat transfer performance refers to the performance of “heat transfer from the flue in the economizer to the water supply” and “heat transfer from the water in the evaporator tube to the flue”.

When considering “heat transfer from the flue of the economizer to the water supply” and “heat transfer from the flue of the air preheater to the air”, which one is more likely to conduct heat?

Naturally, it is “heat transfer from the flue in the economizer to the water supply”.

This is because water has a higher heat transfer coefficient than air.

It is more effective to transfer heat as much as possible by adding a temperature difference service to water, where heat is easily transferred.

The water warmed by the economizer receives heat from the flue in the evaporator tube.

Let’s assume that the air preheater is placed in front of the economizer to actively heat the air.

Air with low latent heat originally comes into direct contact with the burner and quickly becomes hot.

On the other hand, the efficiency of heat transfer to water decreases due to the sandwiched heat transfer tubes.

It means that you don’t have to give priority to warming the air.

Giving priority to raising the feed water temperature over warming the air contributes to reducing the area of ​​the evaporator tube , which is efficient.

pressure loss

In terms of pressure loss, the air preheater is more disadvantageous than the economizer.

This is because the air preheater has a larger heat transfer area required for heat exchange.

Because air has a lower density and a lower heat transfer coefficient, it has a lower heat transfer performance than water in the same area.

For that reason, let’s say that the air preheater increases the heat transfer area more than the economizer.

This means that the air preheater has higher resistance and pressure loss when viewed from the flue side.

When pressure loss occurs , the pressure in that part decreases.

As the pressure drops , the air expands and its specific gravity drops.

As a result, the heat transfer coefficient tends to decrease.

Equipment with a large disadvantage has to be given priority.

air preheater

Place the air preheater last.

This is clear from the economizer chapter.

Low priority, but I want to warm up.

Isn’t this the position? I personally think.

priming pipe

First, let’s talk about the priming pipe.

The principle of air-water separation is physical separation.

As a mechanism for physically separating air and water, there is the idea of ​​changing the flow.

The priming pipe changes the flow with the composition below.

Priming pipe (boiler)

Water is contained in the steam that has been evaporated in the boiler.

Let this steam flow in from the top of the priming pipe.

Steam escapes from the middle part of the priming pipe to the top.

The steam flow has several stages of “bending”.

Air-water separation occurs when the flow changes at a bend.

Gases can change direction suddenly, but liquids cannot.

When steam enters the priming pipe from above and turns sideways, the liquid falls downward.

If you just change the direction and separate the air and water, the effect is not very high.

Controlling the size of the hole by drilling a hole at the pipe inlet, or devising the direction of discharge of the separated water.

It is possible to increase the separation effect by mechanical ingenuity.


The cyclone also aims for the same physical separation as the priming pipe.

A cyclone has the following structure.

Cyclone (boiler)

It is a structure in which a pipe is stuck in the upper part of a container shaped like a triangular pyramid.

Steam mixed with air and water flows in from the top of the triangular pyramid.

This entrance is tangential to the triangular pyramid . This is the point.

Steam that flows in from the tangential direction flows downward along the triangular pyramid.

Centrifugal force is generated here.

Because the liquid has a higher centrifugal force than the gas, the liquid flows along the walls of the pyramid while the gas flows inside.

While doing so, the gas escapes from the middle pipe to the top.

Compared to a simple physical separation such as a priming pipe, a cyclone has a higher separation effect due to the use of centrifugal force.


A demister is like a net.

The image is below.

Demister (boiler)

When air and water pass through the thin part of the mesh, the liquid collides with the mesh and stops.

It is an idea that makes use of the surface tension of liquids.

When a certain amount of liquid adhered to some part of the net gathers, it falls under its own weight and the water droplets can be automatically separated.

Since a physical part called a net will be added, inspection will be required.

It is more susceptible to deterioration due to corrosion than priming pipes and cyclones.

The same effect can be expected with a flame arrester. However, it is better to consider the functions separately.

automatic boiler control

Stable operation of the boiler is important.

When operating a boiler, various situations change.

Boilers are required to discharge steam stably even with these fluctuations.

Controlled amount and manipulated amount

In the concept of control, there are controlled amount and manipulated amount.

We are trying to distinguish between the values ​​that we want to control in boiler operation and the values ​​that humans can manipulate.

The main boiler control and operation variables are as follows.

Control variable (boiler)
  • Control amount: pressure, temperature, water level, flow rate
  • Manipulated amount: water, air, fuel

Parameters such as pressure, temperature, water level, and flow rate are familiar to chemical plants.

To put it the other way around, the values ​​that you want to control with mechanical equipment are common.

Control point in boiler

The position of the place you want to control with the boiler as pressure, temperature, and water level is as follows.

Automatic control (boiler)

It’s not all-inclusive, so I think it’s hard to understand.

If you can only imagine that you want to see pressure, temperature, water level, etc., it will be OK for the time being.

pressure control

Pressure control includes the following points.


The control image is as follows.

Pressure control (boiler)

steam pressure

Steam pressure is the index that boilers want to control directly.

it depends on steam temperature.

The amount of heat required by the user is determined by the steam temperature.

If the steam pressure is too high or too low, the user’s request cannot be satisfied.

This requires severe control.

The following controls are performed by detecting steam pressure.

  • Insufficient steam pressure: Increase combustion volume (increase air/fuel)
  • Excessive steam pressure: Reduce combustion amount (reduce air/fuel)

Furnace pressure

Furnace pressure is one of the few indicators for controlling the combustion environment.

If the pressure in the furnace is too high or too low, the amount of combustion will be affected.

The amount of combustion is controlled within the range of keeping the steam pressure constant.

Monitor the pressure inside the furnace when controlling the amount of combustion.

  • Insufficient pressure in the furnace: Increase the amount of combustion (increase air/fuel)
  • Excessive pressure in the furnace: Reduce the amount of combustion (reduce the amount of air and fuel)

fuel pressure

Fuel pressure is important for energy efficiency.

High fuel pressure wastes fuel and low fuel pressure wastes air.

  • Insufficient fuel pressure: increase fuel pressure (increase flow rate)
  • Excessive fuel pressure: reduce combustion pressure (reduce flow)

Temperature control

Temperature control includes the following points.

Steam/superheater/fuel/air preheating

The control image is as follows.

Temperature control (boiler)

steam temperature

The steam temperature and steam pressure have a 1:1 relationship, and are an important index.

It has the same relationship as steam pressure.

  • Insufficient steam temperature: Increase combustion volume (increase air/fuel)
  • Excessive steam temperature: Reduce combustion amount (reduce air/fuel)

Monitors both temperature and pressure instead of monitoring and controlling only temperature and pressure.

That’s the point.

overheating temperature

The superheat temperature is for making superheated steam at the steam outlet.

Considering the case where combustion heat is used to increase the degree of superheat, control is based on the same concept as steam temperature.

  • Insufficient steam superheat: increase combustion amount (increase air/fuel)
  • Excessive steam superheat: Reduce combustion (reduce air/fuel)

fuel temperature

Fuel temperature affects the heat of combustion.

As the fuel temperature rises and falls, the heat of combustion fluctuates, thus changing the temperature of the steam.

Assuming that the fuel is preheated with steam, the control is based on the following ideas.

  • Insufficient fuel temperature: increase steam
  • Excessive fuel temperature: reduce steam

air preheat temperature

Air preheating is the same idea as fuel preheating.

  • Insufficient air temperature: Increase steam volume
  • Excessive air temperature: reduce vapor volume

Combustion control

Combustion control includes the following points.


The control image is as follows.

Combustion control (boiler)

Combustion control means controlling the air-fuel ratio , which is the amount of air and fuel.

Monitor steam pressure → adjust fuel amount → adjust air amount

We will control it in this way.

The fuel flow rate is varied to control the amount of combustion, followed by the air amount to keep the air/fuel ratio constant.

water level control

Water level control includes the following points.

The control image is as follows.


Water level control (boiler)

As the water level in the boiler changes, the steam flow will change.

A low water level results in low steam flow, and a high water level results in high steam flow.

This is because even if the temperature inside the furnace is constant, the heat transfer area for transferring heat to the water changes.

Depending on the water level in the boiler, the water flow rate is controlled as follows.

  • Insufficient water level: Increase water flow
  • Excessive water level: reduce water flow

Combustion safety device

Combustion safety control includes the following points.


The control image is as follows.

Combustion safety device (boiler)

A combustion safety device is a device that monitors whether fuel is burning properly.

It’s more like a safety device than a control.

Detect heat and light when fire is burning  → adjust fuel flow rate → adjust air flow rate

Unfamiliar instruments for chemical plants, such as flame detectors and phototubes, will appear.


There are a great many books on boiler engineers.

In extreme terms, almost anything is OK, but I dare to think that the following two are good in terms of cost performance.


I explained what you need to know about the structure of the second class boiler.

It explains the structure and types of boilers from the basics such as round boilers and vertical boilers, and also explains accessories, attached parts, and control devices.

Please feel free to post your worries, questions, and questions about the design, maintenance, and operation of chemical plants in the comments section. (The comment section is at the bottom of this article.)

*I will read all the comments and answer them seriously.