Indirect Fired Heater

An indirect Fired Heaters can be understood as a boiler. The energy produced by the combustion of liquid or gas fuels in the boiler, is transferred to a heat-transmitting intermediate fluid – basically water in liquid or gas form or thermal fluids, which is responsible for transporting the energy to other equipment – reactors, exchangers, etc. – to feed the productive process.

Therefore, under this term we can group steam, hot water and thermal fluid boilers.

Obviously their applications are infinite, as heating is indirect and the final product of the process is heated by the fluid transmitting heat in exchangers, reactors, press plates, etc.

So, we can consider:

  • Petrochemical industry
  • Food industry
  • Textile industry
  • Flexography and rotogravure
  • Industrial laundry
  • Heating of all kinds of presses
  • Automobile industry
  • Industrial adhesives and glues
  • Solar energy
  • Etc, etc


Technical features

Powers between 0.1 and 15 MW
Codes of design: AD-2000, DIN 4754, ASME VIII Div. 1
Marked CE
Design pressure: 8 bar_g
Max. working pressure: 7 bar_g
Design temperature: 400º C
Max. working temperature: 350º C
Number of coils: 2
Number of smoke passes: 3
Thermal efficiency: 87 – 91% (*)
Material’s quality of coils: ASTM A106 Gr. B

Optional executions

Horizontal / vertical execution
Different delta T
Particular applications
Communication with PC
Service temperature till 400 ºC
Opening front and rear doors for cleaning
Other service pressures
Heat recovery from combustion gases
Polishing stainless steel finish

Applications of a thermal fluid system

Heating asphalt / bitumen
Storage tanks
Plastics & Rubber
Oils and fats
Drying processes
Distillation columns
Solar energy
Flexography and rotogravure
Research Centers
Etc …

Scheme of an indirect fired heater

In the following figure we can see the usual layout of a thermal fluid boiler.

Execution of the equipment can be horizontal or vertical The most common design is that of two concentric coils (8) and (9), within which the temperature of the heat transfer fluid increases by absorbing the energy  supplied by the burner (1), attached to the lid of the boiler (17).

The inner coil performs the contour functions of the combustion chamber (5), establishing its diameter. The burner flame is projected from the burner to the combustion chamber, reaching, depending on the combustion adjustment, to just make contact with the ceramic tile (rear closure of the combustion chamber (13)) which delimits the length of the hearth. This is what is colloquially known as the first smoke pass.

Upon reaching the rear closure of the combustion chamber, the gases change direction and circulate at high speed and turbulence between the two concentric coils (second smoke pass (6)) to the front cover, where they change direction again until evacuated by the flue (14), through the passage between the outer coil and the inner casing (11) (third smoke pass).

In order to achieve the airtightness of this smoke circuit, which is necessary to ensure the anticipated energy yields of the boiler, there are closures (13) and (18) which force the combustion gases to travel the path planned initially during the design of the equipment.

To promote heat exchange, the circulation of the heat transfer fluid is initially through the outer coil to then pass to the inner coil, thus being a counter-current exchange of temperatures with respect to the flue gases and achieving excellent energy yields.

Unlike Direct Fired Heaters, the combustion chamber is closed by concentric coils that prevent the interior casing from reaching elevated temperatures (11). Therefore, the use of refractory cement is not necessary in this type of equipment in lateral walls, only in some cases and in small amounts like the support of the ceramic slab closing the combustion chamber (13).

The insulation (10) and (16) is usually rockwool, enough to minimize structural energy losses into the atmosphere, while avoiding possible burns by inadvertent contact with the surface of the boiler.

In steam boilers, and given that the equipment’s casing is bathed in water, it is also possible to reduce the amount of refractory cement, although to a lesser extent that in thermal fluid boilers, as the combustion chamber must be protected, and there are no concentric coils carrying out this function.

Indirect Fired Heater

Figure 5. Heat Transfer Fluid Heater. Indirect Fired Heater


1.- Burner
2.- Fuel supply
3.- Heat transfer fluid Output to consumer/system points
4.- Heat transfer fluid Return from consumer/system points
5.- Combustion chamber. Combustion gases, first pass
6.- Combustion gases, second pass
7.- Combustion gases, third pass
8.- Heat transfer fluid Interior coil
9.- Heat transfer fluid Exterior coil
10.- Thermal insulation of the boiler body
11.- Inner casing
12.- Base of the boiler
13.- Combustion chamber bottom closure. Ceramic tile/refractory concrete
14.- Chimney
15.- Output of combustion gases
16.- Thermal insulation of boiler and combustion chamber
17.- Boiler cover
18.- Combustion chamber top closure


Expressions such as Direct Fired Heater, Process Heater, Indirect Fired Heater frequently appear both in the usual language of energy engineering projects, and when we carry out an Internet search for boilers or heating methods.

Given there is slight confusion regarding the meaning of these terms, I believe the best option would be to start this document by defining each one, in order to then compare the advantages and disadvantages of each element and the particular applications of each one.

Heater: A source of heat. The energy source can be electricity, solid or liquid fuel, gas recovery, etc. It does not matter whether the energy is transferred to the product of the process or a heat transmitting fluid. Therefore, this includes a wide range of equipment, from fires to domestic hot water or heating boilers, to industrial boilers (steam or thermal fluid).

  • Fired Heater: Term that encompasses heaters where energy is provided by combustion and then transferred to fluid that circulates through pipes inside the equipment. Exceptionally, and for very specific applications – heating – the energy can be transferred to the fluid – air – in a fully direct manner – Based on this general definition, we can differentiate between two groups:
  • Direct Fired Heater: Basically, we can understand this as the definition of an oven. The heat is directly transferred to the product of the process, basically hydrocarbons or chemical solutions. For this reason, in petrochemical plants or petrol industry refineries the expression Process Heater is sometimes used. Depending on the fuel used, we can find a Direct Gas Heater or Direct Fuel Heater

Indirect Fired Heater: We could consider Indirect Fired Heaters, as boilers. The energy produced by the combustion of liquid or gas fuels in the boiler, is transferred to a heat-transmitting intermediate fluid – basically water in liquid or gas form or thermal fluids, which is responsible for transporting the energy to other equipment – reactors, exchangers, etc. – to feed the productive process.

However, common industrial language uses the expression Heater exclusively for Direct Fired Heaters, whereas for Indirect Fired Heaters we hear the term Boilers for steam boilers, and Heat Transfer Fluid Heaters or Thermal Oil Heaters for those with thermal fluids -.

Advantages and disadvantages

The attached table summarizes the main characteristics of each type of Heater.

We believe that it is evident that Direct Fired Heaters are especially suitable for very specific operations, high power and almost always derived from processes of the petrochemical sector and refineries, whereas Indirect Fired Heaters have very diverse and varied fields of application.

A Direct Fired Heater is almost specifically designed for specific operation, with high technical sophistication.

A substantial change in product or operation specifications can make the equipment ineffective for the new process, whereas with Indirect Fired Heaters and due to the use of intermediate heat transmitting fluid, variations in the processes do not affect the heating equipment to a great extent.

Concept Direct Fired Heater Indirect Fired Heater
Fuels Liquids and gases Liquids and gases
Powers Very high (>6000 kW up to 100000 kW) Medium/high (<8000 kW)
Efficiency Satisfactory Satisfactory
Needs for space High Normal
Combustion control Very high Normal
Flexibility Little Very high
Complexity High Little
Applications Specific Almost all
Financial cost Very high Moderate

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