Steam is an invisible gas generated by adding energy (heat) to water inside a boiler.

Sufficient heat must be added to raise the temperature of the water to boiling temperature. From this point on, if energy continues to be added, the temperature will not change, but the water will be converted to steam. That is, this energy is invested for the phase change and not for a temperature increase.

Steam is a very efficient and easily controllable means of transporting energy. In industrial environments, this transport takes place between a central location (boiler) to a variable number of locations where that energy is used in the form of heat to increase the temperature of air, water or other industrial processes.

As indicated above, the additional energy provided to convert water to steam is what will be used later to heat air, cook tomatoes, iron sheets or heat a paper drying roller.

The heat required to get boiling water to steam is called the latent heat of vaporization. The amount is different for each combination of pressure and temperature, as shown in the steam tables.

Heat flows from areas with higher temperatures to areas with lower temperatures in a process called heat transfer.

Starting in the combustion chamber of the boiler, heat flows through the tubes and walls of the furnace to the surrounding water. It is at this point that steam is generated.

When the high pressure in the boiler displaces the steam to the outside of the boiler, a temperature rise occurs in the distribution pipes. At this point, heat flows from the steam into the air surrounding the pipes through the metal walls of the pipes. This is why some of the steam turns back into water, which is called condensate. To prevent this, steam pipes must be properly insulated to minimize these unwanted heat transfer losses to the surrounding air.

Steam can be used directly, where the steam comes into direct contact with the product to be heated, or indirectly, where the steam does not come into direct contact and systems are used that allow adequate heat exchange.

When steam reaches these steam exchangers or the various energy consuming machines, the story is different. In this case, heat transfer between steam and other points is desirable. Nothing should interfere with the heat transfer between steam and air, between steam and water, or between steam and any element to be heated.

Calorie:

The calorie (symbol cal) is a unit of energy in the Technical System of Units, based on the specific heat of water. It is defined as the amount of heat energy required to raise the temperature of one gram of pure water by 1 °C at a normal pressure of one atmosphere.

July:

is the unit derived from the International System used to measure energy, work and heat. 1 kilocalorie (kcal) is equivalent to 4186.8 Joules (J).

Temperature:

It is the magnitude referred to the common notions of heat measurable by means of a thermometer. It is defined as the thermal degree or level of a body. It does not imply quantity of energy in the form of available heat.

Heat:

It is the energy transferred from one system to another (or from one system to its surroundings) due in general to a temperature difference between them.

Saturated steam:

It is pure steam at the temperature corresponding to the vaporization temperature of water at the existing pressure.

Absolute pressure and gauge pressure:

Absolute pressure is the pressure in bar indicated above perfect vacuum. The gauge pressure (barg) is indicated above atmospheric pressure.

Specific enthalpy of saturated steam:

Total energy required to generate saturated steam from water at 0ºC

As indicated in the first chapter, if water continues to be heated beyond its vaporization point, it becomes steam (or gaseous water). However, there are several types of steam and its physical properties will vary depending on the pressure and temperature at which it is found.

As can be seen in the graph, saturated steam occurs at pressures and temperatures where liquid and gaseous water can coexist together.

The use of dry saturated steam has several practical advantages. For example, it is possible to control the boiler temperature by regulating the pressure.

However, for practical purposes, in industrial plants it is more common to obtain wet saturated steam. This liquid part comes from unvaporized water particles that are entrained in the distribution lines. Generally, industrial boilers produce steam containing around 3% to 5% moisture.

Steam title

The mass percentage of vapor in a vapor-liquid mixture is called the “vapor titer”. For the above data, we would speak of a vapor titer of between 0.95 and 0.97.

Saturated steam (100% dry) contains 100% of the latent heat available at the given pressure.

It is therefore necessary to remove as much moisture as possible from the steam system, since heat transfer efficiency can be significantly reduced the higher the moisture content of the steam.

In addition, the condensate generated in the piping makes the wet steam even wetter, forming more condensate that significantly worsens performance.

In a properly sized steam piping system it is necessary to have water separator equipment to avoid the above mentioned.

Another form of steam is superheated steam (see diagram). This steam is created by superheating saturated steam to a point above the saturation point. This results in steam at a higher temperature than saturated steam at the same pressure. This steam is not usually used in heat exchange applications and is more suitable for motion or impulse applications such as turbines.

Finally, so-called flash steam is the steam that forms from hot condensate when there is a reduction in pressure. This high-temperature condensate contains a large amount of energy that cannot be maintained in the liquid state at lower pressures, since the saturated steam line has been crossed. The result is that in a system with condensed water, a percentage of flash steam is generated when the pressure is reduced.

Steam boilers are vessels in which water is heated to produce steam. They are equipment where energy from any source (generally solid, liquid or gaseous fuels) is transformed into usable and transportable energy.

Heat is transferred to the water not only by direct contact between the flame and the boiler body containing the water, i.e. by conduction, but there is also an exchange by radiation from the flame to the walls of the furnace and another by convection, since the fumes produced during combustion and which have high temperatures heat the metal parts bathed by the water.

The boiler is a particular case in which a heat exchanger is raised to high temperatures, in which a phase change occurs. In addition, it is a pressure vessel, so it is mainly made of carbon steel.

There are a multitude of boiler designs, although in general they can be grouped into:

• 
  
  Water-tube boilers:
  
  These are boilers in which the working fluid moves through the interior of tubes during heating. They are the most commonly used in thermoelectric power plants, since they allow high outlet pressures and have a large generation capacity.
• 
  Pyrotubular:
  In this type, the fluid in liquid state is in a vessel crossed by tubes, through which high temperature gases circulate, product of a combustion process. The water evaporates on contact with the hot tubes, due to the circulation of the exhaust gases inside them.

The figure above shows a boiler with a burner, in which the following parts can be seen:

 

Burner (1):

Is the one in charge of burning a liquid, gas or solid fuel producing a flame.

Firebox or combustion chamber (2): this is where the fuel is burned and where the highest temperatures, close to 2,000 ºC, are reached.

Smoke circuit (3): it fulfills the double mission of conducting the fumes produced in the combustion to the smoke box and taking away as much heat as possible and then transferring it to the water (letting them go directly to the atmosphere would entail, among other inconveniences, a great loss of energy by wasting the heat they possess).

In order to maximize the heat exchange between the gases and the water, the flue gas circuit shall have the largest possible surface area and shall be designed to reduce as much as possible the exit velocity of the gases.

This is achieved by forcing the gases to make several turns before they are released and by inserting certain elements called turbulators in their path, which make their passage more difficult by slowing them down.

Smoke box (4):

is the area where all the fumes come together to be sent to the outside through the chimney.

Steam outlet (5):

once the water has entered the boiler and has vaporized it is sent to the installation.

Pump: It is the equipment in charge of introducing water in liquid state into the boiler to replace the volume of steam that has left the boiler for its consumption.

Instrumentation:

Centralized in the electrical panel, automatic control of the boiler is performed, including levels, pump start and stop, burner actuation, safeties, etc.