Industrial Steam Boilers: Which one to choose?

Francesco Z.

Francesco Z.

Marketing Manager

Francesco Z.

Francesco Z.

Marketing Manager

Boilers that produce steam

Ever since the industrial revolution, steam production has increasingly become a key aspect of our lives.

From the first steam trains of the late 1800s to steam boilers that we now use in ironing shops, sterilizing machines, pasta production lines, and even electrical power plants.

Since each specific application requires precise parameters and characteristics, the field of steam generation has become somewhat large and complex.

Let’s see if we can sort it all out.

1. What is an Industrial Steam boiler?

Let’s start by defining what a steam boiler is.

A steam boiler is a machine designed to heat a liquid until it boils, thus creating steam.
The resulting steam can be saturated, dry saturated, or superheated, depending on its final temperature, pressure, and humidity.

And while there are thousands of models of boilers and steam boilers out there, the one thing they all have in common is that they all have a burner and a body.

The burner is responsible for the combustion, while the boiler body acts as a heat exchanger, transmitting the heat from the flame to the fluid.

The way in which this occurs depends on the specific application and the system’s requirements.

The characteristics needed at a pasta factory, for example, will be quite different from those required at an electrical power plant or ironing shop.

A vast range of products has therefore been developed over time in order to meet all of these needs.

2. The characteristics of steam boilers

Given the numerous configurations and characteristics available, it often seems impossible to make any headway in choosing the right steam boiler.

Let’s see if we can provide some guidelines.

Let’s start by defining the main characteristics of a generic steam boiler:

  1. The type of partition: fire tubes or water tubes.
  2. The fuel used to heat the water;
  3. The type of fluid circulation, which can be assisted, combined, or forced;
  4. The total volume of fluid (water) that the boiler body is able to contain;
  5. The type of draught (atmospheric, pressurized, vacuum);
  6. The available steam flow rate;
  7. The maximum operating pressure.

 

Some of these parameters are easier to understand, while some are a bit more technical.

Let’s take a look at each one individually to eliminate any doubts we might have.

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a. Fire tube or water tube boilers

As previously mentioned, the most important factor for a steam boiler’s classification is its type of partition.

Or rather whether it’s a water tube or fire tube boiler.

Each boiler is made up of tube bundles, which provide for the necessary heat exchange.

Each boiler falls into either the first or second category, depending on whether the tubes have combustion flue gas circulating inside them with water on the outside, or vice versa.

b. Types of fuel used in steam boilers

Another important distinction is the type of fuel used to heat the fluid to the point that it produces steam.

The most commonly used fuels are the following:

  1. Fuel oil;
  2. Natural gas;
  3. Coal;
  4. Fossil fuels.

 

The use of fuel oil is becoming less widespread, and over the last few decades, it was mostly used for thermal engines used to produce electricity.

The use of natural gas, on the other hand, is quite widespread, above all in fire tube boilers, where the power and pressure values are limited, typically below 20 bar, as opposed to water tube boilers, where pressure values of up to 160 bar can be reached.

In fact, fire tube boilers are widely used in industrial production processes in areas like the food industry, as well as in the textile and hospital sectors.

Natural gas is the ideal fuel for these applications, as it guarantees high efficiency and low costs.

As previously mentioned, water tube boilers are generally larger and can produce steam by burning coal (a fuel that’s gradually becoming less common) or else by burning fossil fuels, as is the case with electrical power plants, where they’re used to power large turbines.

c. The circulation of the steam inside the boiler’s body

Boilers are also distinguished based on the way that the internal fluid is made to circulate.

In fact, there are 3 different types of circulation:

  1. Natural
  2. Accelerated;
  3. Forced.

 

Natural circulation occurs when the fluid is kept in motion as a result of the density gradients created within the system.

This type of situation typically occurs in fire tube boilers or in low-pressure water tube boilers.

Accelerated or forced circulation is used, on the other hand, whenever the density differences between the intake and the output are no longer sufficient.

In these cases, pumps are used to accelerate or force the passage of the fluid, depending on the system’s specific configuration.

Do you need further details?

Choose the Right Steam Boiler – Ultimate Guide

d. The water content of steam boilers

All boilers are classified based on their fluid content in relation to the heated surface.

One common classification scheme is as follows:

  1. Large: volume 130 to 250 L/m^2;
  2. Medium: 70 to 130 L/m^2;
  3. Small: <70 L/m^2.

 

The greater the value the larger the body, and therefore the internal volume, of the boiler itself.

The very first boilers were designed to keep this number high, with the volume of available water even reaching 150-200 kg/m^2.

These boilers with large quantities of water require very long start-up times to bring all the water to temperature.

However, once the system is up and running, the volume of heated water creates a thermal flywheel effect, rendering the boiler extremely stable and less susceptible to fire irregularities.

Such irregularities were due to the lack of available technology during that time.

As new systems were gradually developed and brought to market, medium-volume steam boilers (ranging from 70 to 100 L/m^2) became more common, especially for industrial production processes.

The start-up times were thus drastically reduced, while at the same time maintaining the same stability thanks to burners of the latest generation.

Flexibility increased even further, dropping below the threshold of 70 L/m^2.

Here, the available thermal flywheel effect is residual, and for each steam variation required it is essential to have a burner with a low response time and a high modulation range.

e. Combustion chamber pressure values

Let’s get back to the basics for a moment.

Every boiler needs a combustion chamber in order to function.

This is where the fuel is burned, generating the heat that will be transmitted to the fluid (normally water) through the boiler’s body.

Combustion chambers were initially designed to make use of the natural draught or rather using exclusively the atmospheric pressure to circulate the flue gas inside the boiler.

This resulted in a slight vacuum inside the combustion chamber with respect to the external atmospheric pressure.

The evolution of steam boilers has led to increasingly complex flue gas pathways at higher speeds.

All aimed at increasing the heat exchange coefficients, while at the same time decreasing the surface area required.

As the complexity increased, a simple natural draught was no longer sufficient. Fans, therefore, had to be added in order to ensure the proper circulation of the flue gas.

Depending on whether they have 2 fans (one at the intake and one at the output) or just 1 fan, they are respectively referred to as either balanced or forced draught boilers.

The large water tube boilers in thermoelectric power plants or in waste-to-energy plants are normally of the former type, while fire tube boilers are normally of the latter type.

The use of a single inlet fan entails considerable savings in terms of electricity consumed, as well as in terms of the life span of the unit’s components, which are not exposed to the dirty flue gas emitted by the chimney.

f. Steam boiler flow rates

One of the steam boiler’s most important characteristics is its steam flow rate.

The size of the boiler can be determined based on the flow rate required by the system.

Depending on the application, the flow rate can vary considerably, from a few hundred kg/h with a small fire-tube boiler to 200-300 thousand kg/h with a water tube boiler.

For example, when purchasing a new boiler, the flow rate is undoubtedly one of the first parameters to be taken into consideration.

The steam flow rates necessary to power each machine in the production line must be taken into account, even considering the factor of simultaneous use.

This applies, for example, for the processes in which certain machines are only used at specific times of the day.

It is therefore essential to calculate the daily steam requirements in order to avoid choosing the wrong steam boiler.

Finally, an additional 20-30% should be added in order to play it safe.

g. Steam boiler pressure values

The pressure value is just as important as the flow rate.

In terms of pressure, steam boilers can be classified as:

  1. Low pressure <1 bar;
  2. Medium pressure 1 to 15 bar;
  3. High pressure 15 to 100 bar;
  4. Extra high pressure >100 bar.

 

As mentioned above, fire tube boilers usually operate at lower pressure values, while water tube boilers tend to operate at higher pressure values.

The boiler must therefore be chosen keeping in mind any pressure spikes that may occur during the boiler’s lifespan, with the appropriate model being chosen accordingly.

3. Industrial Steam Boiler Applications

As we have seen so far, steam boilers differ greatly in terms of their characteristics and applications.

We’ve seen that a boiler used at an electrical power plant is not at all the same as a boiler used at a hospital.

Let’s look at several examples of steam boilers in use.

a. Steam boilers for pasta production

The use of steam in the food production sector is widespread.

The case of pasta production lines is perhaps the most comprehensive because steam is used on various levels.

At pasta factories, everything begins with pasteurization, or rather the process to which most of the raw materials are subjected before the actual production begins.

Here, the steam (which must be dry) reaches temperatures of around 120 degrees in order to eliminate the bacterial load.

It is also later used in the finished product packaging process in order to eliminate the moisture present on the surface, thus preventing the product from sticking to the external package.

As is the case with all food processing lines, pasta production machinery needs to be continuously sterilized, and here the “dry” high-temperature steam comes into play once again. 

b. Steam boilers for dairies

Remaining within the “food & beverage” industry, let’s see how steam boilers are used in dairies.

Here, steam is widely used to carry out the heat treatments that the milk must undergo prior to being processed.
The 2 main treatments are pasteurization and sterilization.

Both are aimed at eliminating all the pathogens, enzymes, and microbes that can alter the milk’s properties.

Steam is especially widely used for the sterilization process, where the milk must be brought to high temperatures (140-150 degrees) for several seconds.

Dairy production plants normally aren’t large enough to justify the use of water tube boilers.

In fact, most dairies use fire tube boilers, which guarantee excellent flexibility of use, high efficiency, and, consequently, low consumption.

c. Steam Boilers in Hospitals

Another place where steam has multiple applications is in hospitals.

Like with certain industrial processes, steam is often used in hospitals for sterilization purposes.

In fact, steam sterilization is one of the most widespread types of sterilization in the medical field.
This is because it guarantees extremely low costs, excellent ease of use, and high levels of effectiveness.

Since the volumes and pressure values required by hospitals are normally quite low, the use of fire tube boilers is generally preferred.

d. Steam Boilers for Electricity Production

Another industry that makes extensive use of steam boilers is that of energy production.

In this sector, the steam is used as a carrier to convey energy to a turbine that generates electricity.

The water-tube steam boiler is typically used due to the large dimensions and the high-pressure values required.

4. What does the future hold for steam generation?

Steam will certainly remain a major source of energy for industrial production processes.

It will, therefore, become increasingly crucial to produce it at lower and lower costs, while at the same time guaranteeing quality and reliability.

We have learned this by asking those who use steam every day.

We asked them about their needs, their problems, and their suggestions.

They responded that they would like to see greater ease of use and plant management autonomy, and, above all, a major reduction in costs.

From here, we began developing a new steam boiler concept from scratch.

We call it the EcoVapor.

The EcoVapor is the first steam boiler to have the burner completely integrated within the boiler body, and an innovative control system connected remotely.

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