The heat transfer fluid circuit

Most productive systems, in any of the industrial sectors, at some stage of their process require heating, either from intermediate components or final product, with this heating being also one of the basic, if not critical, points of the system.

Basically, a distinction can be made between two types of heating.

Direct heating

the product is directly heated by means of combustion gases, flame radiation or electric heating elements, without any intermediary fluid.

It is undoubtedly the most simple and understandable system, which we use daily in the kitchen at home when the stove burner directly heats the vessel containing the food. It can also be compared to heating something in an oven or microwave.

Industrially, the diagram is that shown in figure 1, attached. A natural gas, diesel, etc. combustion burner (1), at the base of the vessel which contains the product (2), and transmits the heat by flame radiation or by convection of the gases which are a product of that combustion.

During the process, the combustion gases are expelled to the outside by means of the flue (3).

When the product reaches the desired temperature, the burner is deactivated.

Calentamiento directo

Figure 1. Direct heating

Figure 2. Heat exchange

Indirect Heating

An intermediate medium is used, which circulates in a controlled manner between the heater and the consumer of heat, known as a heat transfer fluid.

The term “transfer fluid” is decisive for understanding the system.

The system diagram shown in figure 2, (in which the entire assembly contains a heat transfer fluid (3), a heating element – an electric element (1), and one of the boundary walls of this fluid is also a heat exchange surface with the heat consumer (2)), should be considered as a heat exchange system without an intermediate system or circuit, and in which there does not strictly exist any carrier fluid that performs only energy “transfer” functions, but a fluid as a contact medium and which, therefore, shares more similarities, especially regarding difficulties and disadvantages, with direct heating.

Heat transfer circuit diagram

A heat transfer circuit is one in which the heat carrier flows from the heater to the heat consumer and then returns again to the heater or boiler and in which, between the boundary walls of the system, heat is neither added nor eliminated, with the exception of losses into the environment.

An example of a typical heat transfer system, with everyday items in mind, is the domestic central heating system installed in many homes.

The basic diagram is shown in figure 3. A boiler (1), to which a burner (4) is fitted and which has a flue or chimney (3) to eliminate combustion gases, heats the heat transfer fluid (in the case of domestic central heating – water), which, by means of pipes (5), reaches the consumer appliance (2), (in this example – radiators), where the energy is given out and it then returns to the boiler, closing the cycle.

Sistema de calentamiento indirecto

Figure 3. Indirect heating system

Indirect heating advantages

Because of the significant advantages it has over direct heating, indirect heating by means of heat transfer fluid is undoubtedly the most used system in industrial sectors.

The main advantages are:

  • The boiler can be installed in the most convenient place, not necessarily close to any of the consumers, avoiding risks and increasing safety conditions.
  • The need for a fuel supply to each point of consumption and a combustion gas flue for each consumer appliance, increase the inflexibility of the direct heating system, making it necessary to dismiss convenient locations due to the production flow.
  • Being a centralized system, the number of elements susceptible to maintenance and/or breakdowns is much smaller than in the case of direct heating, with a burner for each consumer appliance.
  • The performance of the boiler and, therefore, the energy efficiency is much higher in indirect heating, since the equipment is designed with this in mind. Direct heating has to conform to the characteristics of the consumer appliance to achieve combustions which are rarely optimal.
  • Local overheating of the product to be heated is avoided and, therefore, there is high uniformity of temperatures, it can be controlled with precision and the final quality of the process is better. Each consumer appliance can have its own operating temperature, regulated independently as if it had its own individual heating.
  • The heating and cooling processes, if required, can be carried out with the same heat carrier and with the same system.
  • It allows the formation of sub-networks of hot water, hot air or steam, by means of heat exchangers.
  • The thickness of the insulation in the consumer is more economical, since the only place where high temperatures are reached is in the boiler. This is especially important in the event that there are a large number of consumers.

With this analysis of heating methods, we have practically defined a heat transfer oil circuit, given that, as in the case of indirect heating, it has the main components we have previously discussed and shown in figure 3: boiler, burner, flue, pipework, consumer appliance and, of course, the heat transfer fluid.

To properly complete the heat transfer fluid circuit, we have two basic elements: the recirculation pump and the expansion tank.

Indeed, in a domestic hot water heating system, a pump is also required to circulate the fluid from the boiler to the consumer appliance and guarantee its return to the heater. A tank is also required to absorb the expansion of the carrier fluid as the temperature increases.

In the case of domestic central heating, both the pump and the expansion tank, due to their small size, are, in most cases, built into into the boiler, which can lead to the misunderstanding that they do not exist.

The expansion tank is connected to the system using a pipe, known as a compensation pipe, which allows us to send the increased volume produced by heating the whole circuit to the tank and, in the cooling or end of day phase, to compensate for the lower level produced due to an increase in the fluid’s density upon cooling.

The final items that can be added are the small basic extras, such as: fittings that allow us to isolate any appliance or consumer from the system, both for maintenance as well as safety purposes; a pipe to be used for filling and emptying the system; and a filter for protecting the recirculation pump from possible impurities that exist in the pipework. The basic circuit, however, is already fully specified.

Obviously, we must keep in mind variations of this basic framework depending on the actual requirements of each productive process, which we will also discuss in this document.

Our basic diagram of a complete heat transfer fluid circuit:

  • Heat transfer fluid
  • Boiler (1)
  • Consumer appliance (2)
  • Chimney (3)
  • Burner (4)
  • Recirculation pump (5)
  • Expansion tank (6)
  • Pipes (7), (8), (9)
  • Valves (10), (11), (12)
Heat transfer fluid diagram

Figure 5. Basic diagram of a heat transfer fluid circuit

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