A boiler is a closed pressure tank in which fluid is heated for use outside. This heat is generated by the combustion of fuel (solid, liquid, gas) or by nuclear energy or electricity.

A high-pressure boiler is a boiler that produces water vapor at a pressure higher than 15 psig.

Below the mentioned pressure, the boiler is placed in the low-pressure boiler group. High-pressure small boilers are in the group of small boilers.

According to Section 1 of the American Society of Mechanical Engineers (ASME), the Small Pressure Boiler is referred to as a high-pressure boiler, which does not exceed the following limits:

Shell internal thickness, total volume without coating and insulation 5Ft3, and pressure are respectively 16in, 5Ft3, and 100 psig.

If the boiler exceeds any of the above limits, it is called a power boiler. The regulation of welding in such small boilers is not as difficult as a large boiler.

A power boiler is a water or steam boiler that operates above a pressure of 15 psig and its dimensions exceed the dimensions of a small boiler. This definition includes hot water boilers or hot water boilers operating at pressures above 160 psig and temperatures of 2500 ºF.

A hot water boiler is a boiler in which no water vapor is produced, but its hot water circulates in a circuit for heating and returns to the boiler. The water pressure in such boilers at its outlet point should not exceed 160 psig and its temperature should not exceed 2500 ºF. Such boilers are called low-pressure heating boilers, which are made according to Section 5 of the boilers of the ASME regulations. If the pressure or temperature exceeds this range, the boiler should be designed like high-pressure boilers according to ASME regulations.

Consumption spa boiler is a boiler that is completely full of water, and for external use, produces a spa. (Spa no longer returns to the boiler) Its pressure does not exceed 160 psig and its temperature does not exceed 2500F. These types of boilers are also included in the category of low-pressure boilers and they are made according to section four (heating boilers) of ASME regulations. If the pressure or temperature exceeds this limit, these boilers should be designed according to high-pressure boilers.

Thermal waste boilers use thermal waste that is a by-product of some industrial processes, such as hot gases from a blast furnace or products from the combustion of a gas turbine, or by-products of an industrial process. Slowly Heat wastes pass through heat exchanger surfaces and produce hot water or steam.

To build this type of boiler, the same ASME regulations are used for fire boilers. Auxiliary and safety parts related to these boilers are usually in accordance with the parts procedure in other boilers.

An integrated boiler is a boiler that is completely made and mounted in the factory. This boiler has a variety of water pipes and smoky or cast iron pipes and also has combustion devices, control, and safety equipment. A factory-built boiler is cheaper than a similar boiler that has the same steaming capacity and is installed and mounted outside the factory and on-site. Although the boiler built and mounted in the factory is not normally present and ready for delivery, but it has less construction and delivery time than the boiler that is built and mounted at the place of operation. Its installation time is also relatively less. In general, it can be said that working in a workshop is usually better and more manageable and costs less.

A supercritical boiler is a boiler that operates at a pressure higher than the critical pressure of 3206.2 psig and saturation temperature of 705.40 ºF. Water vapor and water itself have a critical pressure of 3206.2 psig. At this pressure, water vapor has the same specific gravity, which means that the steam is compressed to the point of water. When the mixture is heated above a saturation temperature of 705 ۰ 40 ºF, super-hot dry steam is produced which is suitable for working at high pressures. This dry steam is especially suitable for moving turbine generators.

Supercritical boilers are divided into two types: full and open rotary. Both types operate in the supercritical range above 3206.2 psig and 705.4 F. In this range the properties of liquid and saturated vapor are the same. There is no change in the liquid-vapor phase, so there is no such thing as a water surface, and there is no need for a steam drum.

Boilers can also be grouped according to the nature of the materials used. Common groupings include stationary boilers, portable boilers, locomotives (construction of such boilers is not common today) and sea boilers, which are defined as follows:

  • A stationary boiler is a boiler that is permanently installed on the ground.
  • A portable boiler is a boiler that is mounted on a truck, small river boat or any type of vehicle.
  • A locomotive boiler is a boiler designed to tow a vehicle onto a railroad track.
  • A boiler is a boiler that is typically low in height and is designed for ocean-going passenger and cargo ships. The evaporation rate of this type of boiler is high.

The type of boiler building can also be grouped in the following order:

Cast iron boilers are low-pressure heating units whose pressure components are made by casting from cast iron, bronze, or brass. These boilers are mostly grouped according to the way the cast chambers are stacked. These chambers are held together by compression pacifiers, outer nests, and screw pacifiers. Three examples of cast iron boilers are:

1- Vertical blade boilers where the blades are placed vertically on top of each other and are connected to each other by pressure or screw pacifier

2- Horizontal blade boilers where the blades are placed horizontally side by side. In this situation, the way the blades are positioned relative to each other is like placing slices of a rectangular cube of bread in a row.

3- Small cast iron boilers that are made with integrated casting. These boilers were used in the past to provide hot water.

Steel boilers can be high-pressure or low-pressure and are typically welded today. These boilers are divided into the following groups:

  1. Smoke pipe boiler in which the products of combustion pass through the pipes while the water surrounds the pipes.
  2. Water pipe boiler in which water will pass through the pipes and combustion products around them.

Smoke pipe boilers are typically made up of a capacity of 70,000 lb/hr and up to a pressure of 300 psig. In conditions higher than this limit, water pipe boilers are used. Smoke tube boilers are also known as shell boilers. Here, water and water vapor are trapped inside the shell.

This type of boiler limits the volume of heat that the boiler can produce. In relation to the pressure of large shells, it will require a lot of thickness, and this makes them expensive to build.

  • Fire Tubes

In these boilers, combustion gases are flowing inside the pipes and the water behind the pipes, which is converted to steam due to the transfer of water heat and leaves the system.

The diameter of the pipes is between 1.2 to 4 inches.

  • Water Tubes

These boilers are similar to fire tube boilers, with the difference that water is flowing inside the pipes and hot gases are turned outside into the steam due to the transfer of heat from the water to the steam.

The diameter of water pipes is 5 to 6 in. When the amount of steam produced is more than 10 tons/hrs. With the steam pressure higher than 20 bar is used, this type of boilers must be used.