Chapter 1

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.

Sections of this chapter:
Types of heating

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

Chapter 2

Heat transfer fluids

In thermal fluid systems, the same fluid is a crucial part that will condition the performance and specifications of the entire system. It is important to know thoroughly the characteristics of each fluid.

Sections of this chapter:
Thermal fluid characteristics

A heat transfer fluid must possess specific characteristics to be able to carry out its function of transporting energy with technical proficiency and at a moderate cost.

These characteristics are:

  • Must have good heat transfer properties
  • Have good thermal stability that allows long periods in operation with stable serviceability
  • Low viscosity across the entire working range, especially in start-up conditions, avoiding high electrical consumption
  • Low solidification temperature allowing safe prolonged stoppages
  • Low corrosion to the elements included in the system
  • Technically suitable to satisfy the individual and specific characteristics involved in each process and especially the required operating temperature if this is high.
  • Must have low toxicity and be environmentally friendly, making its disposal easier when it has finished its work cycle
  • Moderate acquisition and maintenance costs
  • Low risks for personnel and machinery, ensuring safety and avoiding high costs in the event of possible leaks

The heat transfer fluid that can perfectly fulfil all of the above conditions does not exist but, without a doubt, those known as heat transfer fluids or oils, perfectly meet most of the above requirements and surpass other heat transfer fluids such as steam, in crucial aspects.

Thus, their high technical performance, such as high operating temperatures, high precision and uniformity in the final product temperatures and high versatility and flexibility together with high safety levels, the absence of corrosion and low maintenance costs, make heat transfer fluids the heat transfer medium par excellence at the present time, in all industrial sectors and for all type of applications.

When selecting the optimum heat transfer fluid for each system, it is advisable to use specialists, either the heat transfer fluid manufacturers themselves or the boiler manufacturers, who can give advice on the most suitable heat transfer fluid within the wide range available on the market.

The final selection will be the one that best adapts to both the technical and functional requirements of the production process, achieving not only high technical performance at a good economic cost, but also a long useful life for the heat transfer fluid charge.

Under normal working conditions, with the appropriate maintenance operations and moving within the operation parameters of the chosen heat transfer fluid, it is considered that the useful life of a charge should be in the order of 35000-40000 effective hours.

This useful life may be higher if the characteristics of the heat transfer fluid are much greater than those required by the system.

Selecting the appropriate heat transfer fluid

As already mentioned, an actual understanding of the operational requirements of the system is fundamental and will help to create a set of criteria that can be used to compare various fluids and allow the rapid elimination of those that are not the most suitable for the application. However, before individually comparing and contrasting various heat transfer fluids, considerable time and effort can be saved in the selection process by comparing and contrasting the types of fluids.

Once the type of fluid (also known as “the chemical”) that best meets the criteria required for the application has been selected, the resulting list of potential fluids becomes significantly more manageable for making more detailed comparisons.

Heat transfer fluids can be classified, according to their chemical structure, into three main types:

  • Synthetic oils, with two large subgroups: mid-range and high-end
  • Mineral oils
  • Others, including silicones.

The term synthetic or mineral refers to the method of obtaining the main component of the heat transfer fluid, the base oil.

When this base oil is obtained via chemical synthesis processes or other processes other than conventional refining, the heat transfer fluid is known as synthetic or synthetic technology.

Synthetic heat transfer oils, also called aromatic oils, consist of a benzene-based structure and include diphenyl oxide / biphenyl oxides, diphenylethanes, dibenzyltoluenes and terphenyls. Depending on the specific product, the operating temperature range for these types of fluids is around -20ºC to 400ºC.

When the base oil is obtained from conventional oil refining it is known as mineral oil. This is formed by a base obtained directly from the distillation of petroleum and the majority of these consist of paraffinic and/or naphthenic hydrocarbons, to which some additives are added to give them properties that improve their performance, basically to obtain low viscosities and raise their resistance to oxidation. The general operating range is around -10°C to 315°C

Silicone-based fluids and, to a greater extent, glycol hybrid fluids, are primarily used in specialized applications requiring process/product compatibility in the event of a heat exchanger leak.

The disadvantages of this group in terms of performance and cost in the comparative temperature ranges of synthetic and mineral oils, means that these types of fluids are options exclusively for this type of application and, therefore, unlikely to be selected for the vast majority of processes.

Types of systems

We can also differentiate heat transfer oils according to the type of system they use. They can be classified into three types of systems:

  • Non-pressurized liquid phase systems
  • Pressurized liquid phase systems
  • Pressurized steam or pressurized natural circulation phase systems

Non-pressurized liquid phase systems

Non-pressurized liquid phase systems are the most suitable for processes with operating temperatures in the order of 300°C or below (the working temperature of the fluid must be below its boiling range) since they are the simplest to design and operate. Both mineral oils and synthetic oils can be used with this type of system.

In this type of system, the expansion tank does not need to have inert gas applied in order to maintain positive pressure on the circulation pump.

To reduce the likelihood of fluid oxidation, a specific expansion tank is designed to ensure that the fluid is below temperatures in the order of 150°C for any possible contact with the atmosphere, in order to avoid premature oxidation of the fluid which would shorten its useful life.

Pressurized liquid phase systems

They use both mineral and synthetic oils and are similar in design to non-pressurized systems except that inert gas is applied through the expansion tank when the required operating temperature of the heat transfer fluid is above its boiling range.

The pressurized inert gas, nitrogen, allows the heat transfer fluid to always be kept in a liquid phase. The inert gas also acts as a buffer in the expansion tank between the surface of the hot fluid and the atmosphere, eliminating any possibility of oxidation of the fluid.

Most liquid phase synthetic heat transfer fluids and all mineral oils have no requirement for pressurized inert gas to maintain the liquid phase at the upper end of its recommended operating temperatures; only multiphase fluids such as biphenyl / diphenyl oxide have an obligatorily requirement for this system, with it being optional for the other fluids.

Its main advantage over a non-pressurized liquid phase system is the total oxidation guarantee, allowing the useful life of the charge of heat transfer fluid to be prolonged.

The increase of complexity and cost make it necessary to carefully evaluate the characteristics of the heat transfer fluid and those of the process in order to determine its suitability.

Steam phase systems

The pressurized steam phase systems used are only possible due to a group of very specific synthetic heat transfer fluids, especially biphenyl / diphenyl oxide.

A simple steam phase system can be designed using hydrostatic pressure for gravity to return the user’s condensate to the vaporizer, eliminating the need for a condensate pump. More complex systems require an evaporation tank, a condensate return tank and a condensate return pump.

The disadvantages of the cost of equipment and the complexity of the steam phase systems are compensated by the possibility of working at very high temperatures and by the increase in the user temperature control, which is important for those processes sensitive to deviations in set point.

Example of expansion tank with pressurized liquid phase system

Example of expansion tank with pressurized liquid phase system

Nota: LG, Visual level. LS Electrical level. PI Manometers. PCV Pressure Control Valve. RV Safety Valve

Main Criteria

Thermal stability. Maximum operating temperature

The thermal stability of the fluid is the primary factor for determining its maximum operating temperature.

Thermal stability is defined simply as the ability of a heat transfer fluid to withstand the molecular cracking of the thermal stress. The relative thermal stability test for heat transfer fluids measures the molecular bond strength of a fluid at a specific temperature compared to another fluid at the same temperature and under identical test conditions.

The tests are carried out under ideal laboratory conditions and do not take into account operational stresses such as mechanical failures, design defects, oxidation, etc. and, therefore, the data generated are only useful for comparative purposes. The exact predictions for the life of the fluid in actual processes should not be taken from the thermal stability data.

The maximum operating temperature is the maximum temperature recommended by the fluid manufacturer at which it can be used in continuous use while still maintaining an acceptable level of thermal stability. Because the degradation rates of the fluid are closely linked to the temperature, habitually working above the maximum operating temperature of the fluid will exponentially increase the degradation rate.

Potential problems caused by excessive degradation and the subsequent formation of degradation by-products include increased coking and dirt, mechanical difficulties and a decrease in heat transfer efficiency.

Therefore, as you could probably imagine, the first step in the process of selecting a particular type of thermal oil is to establish the maximum operating temperature. As mentioned above, most mineral oils have a maximum recommended temperature of between 270°C and 315°C, whereas the synthetic or aromatic thermal fluids are especially recommended for maximum fluid temperatures of between 315°C and 400°C.

Given that the molecular structures of aromatic compounds are significantly more thermally stable than mineral oils above 280°C, synthetic fluids are also recommended in applications above this temperature.

Process applications that require fluid temperatures of between 150°C and 280°C can specify either synthetic or petroleum based fluids, with other characteristics being used to determine the choice of thermal fluid.

Heat Transfer Efficiency

When assessing this property, it should be noted that greater efficiency in heat transfer does not, in most cases, represent an economic saving in fuel (which basically depends on the design of the boiler) as may initially appear, but this property will result in less time in obtaining the energy objectives of our process. Thus, with equal exchangeable surfaces in our consuming apparatus, we will reach the required operating temperature faster if the heat transfer fluid has a high heat transfer efficiency.

Comparisons of heat transfer efficiency between different heat transfer fluids are made using heat transfer coefficients. At a specific temperature, the overall heat transfer coefficient of a fluid can be calculated using its density, viscosity, thermal conductivity and specific heat (see spreadsheet on properties of thermal fluids) at a given flow rate and pipe diameter.

The resulting heat transfer coefficients can be evaluated and compared. At a given temperature, the heat transfer coefficients of the various types of heat transfer fluids may differ by 25%. Depending on the thermal resistance factors of the other components of the system, a fluid with a considerable heat transfer coefficient advantage may permit a reduction in the size of the system equipment.

Most synthetic thermal fluids have a significant advantage over mineral oils in heat transfer efficiency at temperatures between 150°C and 260°C. Above this temperature range (up to 300°C), some mineral heat transfer fluids narrow the gap with highly refined paraffinic / naphthenic white oils.

It must be remembered that the heat transfer coefficient is calculated using the factory supply properties of the heat transfer fluid. Fluid which has been in service for an extended period of time and which has undergone thermal degradation may have a lower coefficient due to changes in the viscosity of the fluid and the presence of less efficient fluid degradation by-products. Therefore, the thermal stability of a fluid plays an important role in maintaining its thermal efficiency over time.

Minimum pumping temperature

This temperature, and not the freezing point, is the temperature at which a heat transfer fluid can operate. It is defined as the temperature at which the viscosity of the fluid reaches a value, typically 2000 cps, at which centrifugal pumps cannot circulate the fluid.

Although most process applications are run at temperatures well above this point, system designs may encounter problems during emergency shutdowns or maintenance shutdowns if we have not taken this requirement into account in the acquisition of our system’s heat transfer fluid.

Generally speaking, most mineral heat transfer oils and mid-range synthetic fluids have start-up values in the range of -20ºC to -5ºC. High-end synthetic fluids – aromatic synthetic fluids with biphenyl / diphenyl oxide and with maximum operating temperatures of 370°C – 400°C, have values of 5°C – 15°C as their minimum pumping temperatures.

Processes that utilize a heat transfer fluid that may potentially have cold start-up problems will require a heat source in their pipework, using either steam or electrical resistance.


A comparison of the environmental and personal safety guidelines is important when selecting a specific chemical fluid. NONE of the heat transfer fluids present a significant health hazard when used in accordance with sound handling practices.

Most thermal fluids are non-toxic in terms of both contact with the skin and ingestion. Only a few aromatic synthetic fluids with biphenyl / diphenyl oxides have some different characteristics in these respects.

Investment, Economic cost

As a general rule, the higher the maximum temperature of use of the fluid, the greater the economic cost.

Medium-range synthetic heat transfer oils with operational temperatures of up to 340°C are between one and a half to two times more expensive than mineral oils, while high-end aromatic synthetic heat transfer oils for working temperatures of up to 400°C are up to five or six times more expensive.

Within this criterion of economic cost, it is important to include operating costs such as maintenance, replacements, etc. In order to minimize these, we recommend following the recommended heat transfer fluid sample analysis program, both in terms of frequency and in terms of the parameters evaluated (see analysis of heat transfer fluids).

This allows the user to be perfectly informed on the condition of the current heat transfer fluid charge and to minimize maintenance stoppages and excess costs due to energy inefficiency as a result of degradation.


Which type of heat transfer oil is the most appropriate? Which chemical one is the best?

It is most likely that no one specific chemical oil will be better than another in all the criteria required by a new process.

With the exception of those cases where a technical requirement necessarily requires a specific type of heat transfer oil, e.g. an operating temperature of more than 315°C, or compatibility with the product in case of leaks, both types have advantages: synthetic heat transfer oils provide better heat transfer efficiency and stability at high temperatures, while mineral oils have a lower cost and environmental advantages.

Identifying the primary criteria required by a new process or the main objective of the desired improvement will help prioritize the criteria by importance. By first selecting the type of heat transfer fluid that best suits the overall scenario, comparisons of the individual fluids within that group should resolve the specific objective.

Table I, attached, provides a short summary of the above reasons, with a scaled estimate for each of the properties evaluated.

Criterios de fluido térmico

Using an interpretation of the graph, we can quickly determine that high-performance synthetic heat transfer oils are the most cost-effective, the least environmentally friendly, require higher pumping temperatures, have a higher heat transfer efficiency and greater thermal stability, while mineral oils are the cheapest and the most environmentally friendly but have lower thermal stability and lower heat transfer efficiency.

Mid-range synthetic heat transfer oils have reasonably satisfactory intermediate values across all the criteria.

Chapter 3


Thermal fluids, commonly called thermal oils, provide satisfactory service for long periods of time and do not require extensive maintenance or special supervision when compared to other energy transfer systems such as steam.

However, precisely because of this fact, some of the few basic preventive maintenance operations to ensure the equipment is reliable and safe are sometimes forgotten or minimized.

Sections of this chapter:
Heat transfer fluid degradation

It should not be forgotten this is an essential component of the entire heating system and, at the same time, it is undoubtedly critical to ensure the thermal fluid in a production system is in a satisfactory service condition, as it has a major impact on such important matters as equipment safety and energy costs.

In effect, a degradation of the load implies a decrease in the heat transfer capacities of the thermal fluid. As they are not able to properly absorb the energy supplied by the fuel through the burner, the temperature of the combustion gases in the flue or chimney outlet is higher, the boiler energy efficiency is therefore lower and fuel costs will increase. Also, the productive capacity of consumption devices, such as heat exchangers or reactors, will be reduced for the same reason.

This also has a substantial financial impact, as equipment down time due to faults or more frequent corrective maintenance, must be done with the thermal fluid in a poor condition, thus preventing satisfactory and regular production.

Among these breakdowns or more frequent maintenance operations, are cleaning filters, partial replacement of fluid and interruptions due to fluid circulation faults and high smoke temperature, for example.

While the increase in financial costs is obviously important, a reduction in equipment safety due to the thermal fluid load being in poor condition is, however, more concerning. In effect, the decreased heat transfer capacity also means the thermal fluid does not cool the coil tubing properly and overheating points will result in pores in these coils. The equipment fire risk also increases, especially due to another of the properties that decreases in a degraded thermal fluid, which is the flash point.

Interestingly, and in the opposite direction, the verification of the importance of the thermal fluid in a production system often leads to a tendency to blame its poor state for every problem in the process, whether perceived or real. For example, a decrease in the performance of the heat exchanger will often be caused by a reduction in flow due to a dirty filter, a malfunctioning control valve or expansion in the equipment leading to unsatisfactory redistribution of flow rates.

Regular maintenance must therefore be established to monitor the state of the fluid load and its evolution to prevent it from degrading and to provide a proper perception of equipment operation.

The best method to determine this evolution is by chemically analyzing a sample taken from the equipment at predetermined time intervals to check the values of some of the basic properties; these serve as effective markers to properly evaluate the thermal fluid load status.


The required time interval can be based on previous experience with the system and the service conditions, such as the service temperature, actual annual production hours and the manufacturer and installer recommendations for the boiler.

Around 95% of cases of thermal fluid degradation are not due to service hours and therefore to the extinction of the useful life of the load, but due to usage errors, a bad design or initial or subsequent extensions or modifications. These are called “external conditions”.

Under these conditions, most of these problems can be identified and corrected in time if the fluid is analyzed within the first 3-6 months from its start-up or any modification made. The sampling and routine analysis frequency are then usually adjusted from the results of these initial analyses.

These routine or preventive maintenance analyses are regulated in Spain for at least once a year. Section 19.2 of the UNE 9310 standard specifies that the proper condition of the thermal fluid load must be checked annually. This standard is mandatory according to the current Pressure Equipment Regulations.

This same minimum frequency is also recommended in the German standard DIN 4754, which is a reliable international benchmark for thermal fluid installations.

In addition to these routine analyses, the user of the equipment should note any variations in the daily operation. An increase in the time required to reach operating temperature, in fuel consumption or the repetitive operation of some of the equipment safety components should also be considered, as this may indicate the thermal fluid status becoming deficient which must be contrasted with non-routine analysis.

Early identification prevents subsequent surprises which may result in significant costs due to interruptions in the production system.

Sample extraction

An active point in the equipment must be selected for the analysis sample extraction to be representative; the recommendation is for it to be close to the boiler with an extraction method that does not affect the properties to be subsequently evaluated.

Samples should especially not be taken from expansion or collection tanks.

Analysis of heat transfer fluids | Pirobloc

Fig. 1. Sample cooler

To extract a thermal fluid sample safely, the equipment must be able to maintain its temperature and pressure conditions. This is important so as not to influence the analytical data, for example, the flash point,

For safe and reliable sample extraction, the use of a cooler like the one shown in figure 1 is recommended.

This is a simple, safe and effective method for obtaining representative samples without interrupting the process. This system significantly minimizes the potential risks derived from performing this operation inadequately.

Its operation is very simple. Initially, valves A and C are closed. When extracting the thermal fluid sample, valve A connected to the equipment is opened until the expansion bottle in this system is partially filled.

The fluid is allowed to cool in the bottle, then valve B is opened to expel the gases resulting from the expansion. Then, making sure that valve A is closed and the thermal fluid is cool, valve C is opened and the sample container is filled.

This valve system and small accumulation vessel are sufficient to extract the samples necessary to evaluate the equipment fluid status cleanly and safely.

In accordance with its zero accidents policy, PIROBLOC offers this system in both new and existing facilities.

The container should be clean and dry with an approximate capacity of 0.5-1 L and preferably made of glass, rather than metal, so an immediate visual evaluation can be performed.

Water should NOT be used to clean the container, unless it can be properly dried afterwards. Thermal fluid extracted from the installation itself is often used as the cleaning liquid at the beginning of the extraction, and then discarded.

The sample must be placed directly in the sample bottle. No other container should be used for sampling if the liquid is very hot. Wait for the system to cool before taking the sample.

This operation must always be done wearing safety gloves

Only in equipment with diphenyl/biphenyl synthetic thermal fluids, should the sample be extracted at temperatures such that the fluid vapor pressure does not lead to its immediate evaporation.

Secondary circuits that work at moderate temperatures of approximately 280°C for this type of fluid are a good option. Also, installing a cooling exchanger before the collection bottle may be a satisfactory solution.

The extracted sample should be sent to a qualified laboratory or to PIROBLOC, SA for analysis, indicating the thermal fluid type and brand, the normal operating temperature, the approximate hours / years of equipment operation and the time in years the current thermal fluid has been used.

Any type of problem in the normal equipment operation should also be included, e.g. lack of temperature or frequent refilling, that may have led to the analysis or if it is a routine operation to monitor the load status.

Parameters tested

Visual inspection

Many problems with the fluid can be detected by its appearance and smell.

For example, fine black sediments at the bottom of the sample container generally indicate the accumulation of solids. Liquid contaminants may appear as a separate layer at the bottom of the sample.

Water has a very low solubility in most thermal fluids and can be noticed in most cases with the naked eye. Water in the equipment causes circulation problems, cavitation of the pumps, excessive pressurization and premature oxidation of the thermal fluid.

Contaminants soluble in synthetic, aromatic based thermal fluids will affect the odor of the sample.

A quick, practical test to determine if there are carbon particles in the fluid is to turn the sample container upside down after 24 hours and look for soot at the bottom of the container.

Fine carbon particles, similar in appearance to dirt, form from oxidation decomposition. They are produced inside the boiler and under normal operating temperatures.

These particles remain suspended in the thermal fluid while it flows, but can coalesce in sufficient quantities to form blockages. These carbon particles settle when the liquid is not circulating.

Expansion tanks have a higher risk of this type of sediment formation, when it is usually known as sludge.

Chemical analysis

All the properties tested are intended to indicate the thermal fluid load status to determine if it needs substituting to ensure proper operation.

The parameters tested to establish the thermal fluid load status are different from those for checking lubricating oils or hydraulic fluids. In addition, thermal fluids operate in closed loop systems without any continuous exposure to air, unlike lubricating oils or hydraulic fluids which operate in open systems with continuous exposure to air.

Although there are many parameters providing indications of a thermal fluid load status, usually a visual inspection as indicated above and 3 more provide a fairly accurate view.

These 3 parameters are: the acid number, viscosity and flash point.

A more extensive analysis may be necessary only if contradictory or doubtful values are found for the indicated parameters.

Acid or neutralization number

This measures the amount of acid present in the liquid from the mass (in milligrams) of potassium hydroxide (KOH) required to neutralize one gram of the sample; this indicates the amount of oxidation that has occurred. The higher the acid number, the more oxidation will have occurred.

When the thermal fluid reacts with oxygen (oxidation), organic acids are produced. This occurs as a result of the equipment not being perfectly sealed, allowing the ingress of air (mainly) and water.

Sludge deposits and a high viscosity are symptoms of oxidation and are the most common reasons for fluid degradation. However, unless there is water present, these acids are not corrosive in the traditional sense.

Most oxidation products are soluble in the thermal fluid and reactions take place between them, forming sludge, especially at points where sedimentation is favorable; for example, in the expansion tank, as previously indicated. However, other oxidation products are insoluble and can lead to deposits, partial clogging of pipes and accelerated mechanical deterioration of seals, valves and pumps, for example.


This measures the fluidity of the fluid. The value obtained is usually compared with that of new fluid, with the results sometimes being shown as a percentage in relation to the change. The comparison with new fluid values is necessary, since viscosity is proportional to the average molecular weight, and can therefore have very different values according to the composition of the fluid. This test alone, however, is not sufficient to establish the fluid status with certainty.

The property of it being proportional to molecular weight is what makes any change in viscosity suggest changes in fluid composition. However, it must be remembered that only extreme changes are significant. These extreme changes can be caused either by oxidation (confirmed by acid number values) or chemical cracking, overheating or contamination.

Flash point

This gives an indication of the presence of volatile compounds in the fluid. As with the viscosity determination, its value should be compared with that of the new fluid, as there are significant differences in the flash point, depending on the type of thermal oil.

It is one of the characteristic temperatures of thermal fluids (see ‘Characteristic temperatures of thermal fluids’).

There is no standard or regulation covering the allowed change of flash point in a thermal fluid. However, this parameter is indicative of the amount of more volatile fractions (low boilers) found.

A fluid whose flash point is significantly less than its original value will have a higher concentration of lower boiling point molecules.

Significant decreases in test results may indicate that degradation has occurred.

Other usual parameters

Although it is difficult to determine them in routine analyses, the boiling range and carbon residue are also parameters that are often evaluated.

Boiling range

Because thermal fluids are made up of different components and additives, each with its own boiling point, the liquid will evaporate over a range of temperatures called the boiling range.

The higher that range is, the greater the degradation.

Carbon residue

This indicates the tendency of the fluid to form carbon deposits when subjected to high temperatures.

The analysis values found are from residues of the product under standardized test conditions, which evaluates their quantity and appearance to judge the degree of refining and nature of the oil.

Obviously, the higher the value, the greater the degradation.


Laboratory data provide only a snapshot of the fluid status. This is why trends that become apparent after several routine samples have been taken are important.

The data must be placed in a time perspective together with the equipment operational history and knowledge about it to correctly interpret the results, obtain a complete analysis of the system and determine if the analyzed load can continue in service or has degraded to a point that a change is necessary.

Under normal working conditions, with the appropriate maintenance operations and moving within the operation parameters of the chosen heat transfer fluid, it is considered that the useful life of a charge should be in the order of 35000-40000 effective hours.

This useful life may be higher if the characteristics of the heat transfer fluid are much greater than those required by the system.

If a load change is not due to the fluid reaching the end of its useful life but to so-called “external conditions” resulting in accelerated degradation, changing the fluid could become routine unless these conditions or problems are corrected.

Obviously, if it has reached this point, it is due to a lack of routine control. As, if it were not the case, analytical results would have provided the necessary information to identify problems and their origin. Thus, the necessary corrective actions would have been implemented before the continued use of the fluid would have compromised the equipment efficiency or service.

Along with the analytical results, it is customary to refer to the standard under which the different tests have been carried out; as the values obtained may differ according to the standard and lead to misunderstandings in their interpretation.

For example, the flash point determined by the so-called closed cup or Pensky-Martens method gives values approximately 20°C lower than that determined with the so-called open cup or Cleveland method.

Also, specifying the standard under which the analyses have been carried out implies a rigor in their implementation and therefore a reliability in the results obtained.

DIN (Germany)ASTM (USA)IP (GB)NF (France)
Flash point “open cup” (Cleveland)51376D 9236T60118
Flash point “closed cup” (Pensky-Martens)51578D 9334M07019
Kinematic viscosity51550D 44571T60100
Acid or neutralization number51558D 974139T60112
Boiling range51581D 2887480M07002
Conradson carbon residue51551D 18912T06116
Table of the most general analytical standards for the values tested
Boundary values

“Boundary” values for each parameter are given below, for information only. As mentioned a number of times, knowledge of the operating process, its history and trends observed are determinant in most occasions for a proper evaluation of the load status.


Since viscosity varies with temperature, the reference value is considered for a fluid at 40°C.

An increase of 10% on the original, new fluid value may indicate oxidation or contamination with a less thermally stable fluid, lubricating oil or hydraulic fluid.

A decrease of 15% is within the normal range while a value of 30% indicates overheating has occurred and the value must be compared with other parameters, especially the flash point. The larger the decrease, the more likely a load change is.

If the original viscosity is not known exactly, this basic rule for changing the load can be applied: when the viscosity is lower than 15 cSt or higher than 100 cSt at 40°C.

Acid number (TAN)

Boundary valuesStatusObservations
< 0.05ExcellentNew fluid
> 0.3The load has begun to show symptoms of oxidation, but it can continue to be used in normal operation if the other parameters are acceptable.This is understandable if service hours are significant and the increase is progressive. Otherwise, the equipment design and its degree of sealing must be reviewed.
> 0,6More frequent fluid analyses should be done, paying special attention to other parameters.
> 0.9Immediate change of the existing thermal fluid load.End of fluid life.

Flash point

The flash point decreases as smaller, more volatile “low boilers” molecules are formed. A load change should be considered when the flash point decreases more than 50°C compared to the original, new fluid value.

If the original flash point is not known, values lower than 130°C for the Cleveland open cup method analysis would not be acceptable.

Causes, precautions and advice

Thermal fluids degrade over time due to thermal cracking and oxidation. As already mentioned, the degradation speed can be influenced by inadequate operating procedures, bad design or contamination.

Therefore, the first suggestion is obviously to have proper equipment design, adapted to the needs of the process with the thermal fluid that best meets the technical requirements being selected.

The second step is to operate under the design conditions, without extreme temperatures or design power, and with efficient preventive and corrective maintenance procedures.

Below is a list of good practices to ensure the useful life of the thermal fluid load is as expected.


As already indicated, thermal fluids react with air to form organic acids. The oxidation rate is low at ambient conditions but increases rapidly with temperature.

These acids can be subjected to the polymerization of free radicals that increase the viscosity of the fluid and, ultimately, give rise to sludge deposits.

Minimizing oxidation and lengthening the load deterioration process due to this reason is relatively simple:

  1. The equipment must be designed to be properly sealed, either by the so-called “hydraulic cushion” method or by other systems, including the use of inert gases. Check that whatever system used in the equipment design works correctly, with enough hydraulic cushion and the proper nitrogen pressure.
  2. The design should also ensure that the temperature in the expansion tank is kept below 70°C.  – Do not insulate the expansion tank or the pipes connected to it.
  3. Perform maintenance operations with the fluid at temperatures below 60°C.
  4. Replace mechanical pump seals and gaskets only if leaks are detected. Whenever the equipment stops, the entry of air may be facilitated by a vacuum occurring at those points.
  5. Before any modification or expansion of the equipment, it must be re-commissioned by repeating the initial dehydration process.


Contaminants can lead to the degradation of the fluid, as well as cause operational problems.

Contaminants can enter the system in several ways:

In new equipment

New equipment requires parts to be manufactured and assembled in the field, usually tubing, with a degree of cleanliness at least equivalent to the components to be joined and which have been supplied clean and protected from the factory.

A frequent problem in thermal fluid equipment is contamination of the fluid during assembly, due to cutting and welding operations, dust and dirt, the use of protective lacquers, poor component storage or improper handling during the load filling process.

The method of cleaning and purifying a thermal fluid in new equipment is called “Flushing”

Once the equipment is completed, it can undergo similar contamination during the fluid operational phase, with solid particles or water.

This contamination affects both the system pipes and components, so the initial cleaning level must be recovered.

Solid particles can be removed by using filters that have to be cleaned regularly, while verifying the pump suction pressure does not fall below normal service levels. If so, it is indicative of dirt in the filter.

The presence of water almost certainly means there are leaks in the heat exchangers, which must obviously be repaired.

Pressure testing

Another reason for water in the equipment may be due to pressure testing of any part of the equipment with water. This would be detected immediately after re-connecting the equipment involved, and can be removed by purging the system.

These cases should be avoided by requiring the equipment suppliers to carry out pressure testing with the thermal fluid at installation, as it is extremely difficult to dry properly.

Water may also be present from condensation in the expansion tank, which would be due to inadequate design of the sealing system and the presence of air might also be quite feasible.

Thermal cracking or overheating

Chemical cracking is decomposition of a product, in this case thermal fluid, into high and low boiling point components, due to exceeding the maximum recommended thermal fluid film temperature (see Characteristic temperatures of thermal fluids).

Another, “more graphic” description, is to say the different product component bonds break and it no longer behaves uniformly. The decomposition is divided into “low” and “high” boilers.

The first group consists of the most volatile fractions in the fluid, which evaporate and are eliminated to the outside by the expansion tanks.

This causes non-uniform warming and pump cavitation.

The second group is composed of thermal fluid substances which are carbonized and adhere to the walls of the coils.

Initially, this incipient cracking decreases the boiler performance as these products become pseudo-insulators with the increase in fuel that this represents with the obvious production difficulties.

However, the next step is even worse. As the fluid degrades, a smaller amount of heat passes through the coil pipes and the temperature of the wall increases.

This burns out and destroys the coils and the boiler has to be replaced.

Cracking can be minimized or prevented:

  1. Attention must be paid to the safety devices installed and maintenance of this instrumentation performed. Cracking is not possible using regulatory safety devices which function properly. Using high temperature and/or low flow alarms which indicate overheating can avoid cracking if they are considered and evaluated
  2. Consider a properly designed installation from the start, with thermal fluid according to your needs, but be very careful in the selection of a pump change for any circumstance, as cracking can be caused by a lack of thermal fluid flow in the boiler.For any modification or extension of the initial circuit, resort to a professional hydraulic redesign of the equipment, as this may require replacing the initial pumps. The equipment must be re-commissioned.
  3. Do not start the system at full power. Heating progressively until reaching the temperature of 100°C minimizes the thermal stress of the fluid.
  4. Also, avoid sudden stops, and let the fluid circulate until the outlet boiler temperature is around 100°C.
  5. The burner must provide the design power of the existing boiler and be properly adjusted.
  6. Power failures are common, so you should consider connecting the pump to an auxiliary power supply.
  7. Perform routine analysis of thermal fluid samples taken from your equipment and follow the recommendations of the results. Cracking can occur because you want to extend the useful life of a load that should have been replaced earlier due to another cause, e.g. oxidation.
Chapter 4


Heat transfer fluids have a series of characteristic temperatures that are indicative of their working range.

Some of these are of vital importance in the design of heat transfer fluid boilers, e.g. film temperature, while others allow technical criteria to be established in order to correctly select the most suitable heat transfer fluid for each production process, e.g. maximum operating temperature, minimum pumping temperature.

Lastly, some of these, e.g. the flash point, serve the purpose of establishing base patterns that allow us to evaluate the operating conditions of a heat transfer fluid that is used, by removing a sample and performing a subsequent analysis while comparing it with those of the specification of this new fluid.

Sections of this chapter:
Types of temperatures

The values of the characteristic temperatures are the result of laboratory tests, under specific standards that establish the parameters of the test – tangential incidence of spark, strict environmental control, specific temperature ramp up, etc., which are not present in normal circumstances but which allow baseline values to be established.

According to the specifications and, therefore, the standards used, there may be variations in the values obtained, although all of them will be very similar.

For this reason, the results obtained must include the standards under which the test was performed.

Maximum operating temperature

This is the maximum temperature recommended by the fluid manufacturer at which it can be used in continuous use while still maintaining an acceptable level of thermal stability.

Because the degradation rates of the fluid are closely linked to the temperature, habitually working above the maximum operating temperature of the fluid will exponentially increase the degradation rate.

Potential problems caused by excessive degradation and the subsequent formation of degradation by-products include increased coking and dirt, mechanical difficulties and a decrease in heat transfer efficiency.

Obviously, it is of extreme importance when selecting the most suitable heat transfer fluid for each of the production processes.

Maximum film temperature

In a boiler, the coil walls reach temperatures that are higher than the operating temperature of the heat transfer fluid.

The operating temperature of the heat transfer fluid is considered to be the one given at the section centre of the coils and is also often referred to as the mass temperature, while the film temperature is the temperature reached by the heat transfer fluid which is in contact with the wall of the coils.

This temperature always has a higher value than the mass temperature and if, under operating conditions, it exceeds the temperature defined by the manufacturer of the heat transfer fluid, the fluid will become thermally degraded. – see figure 1 -.

This temperature is of vital importance in the design of the boilers.

Under operating conditions, with a correctly designed boiler and with the appropriate flow of heat transfer fluid, the film temperature is usually between 5 and 10°C higher than the mass temperature.

Temperatura de masa Tm, Temperatura de película Tp, Temperatura de pared de tubo Tt

Figure 1. Mass temperature Tm, Film temperature Tp, Pipe wall temperature Tt

Freezing Point – Pour Point

It is the temperature at which the heat transfer fluid is not able to flow by simple gravity because the viscosity has become infinite.

To reduce this point, heat transfer fluids are added, with most of the heat transfer fluids on the market having values around -30ºC.

Minimum pumping temperature

This temperature, and not the freezing point, is the temperature at which a heat transfer fluid can operate and must, therefore, be taken into consideration when selecting the heat transfer fluid.

It is defined as the temperature at which the viscosity of the fluid reaches a value, typically 2000 cps, at which centrifugal pumps cannot circulate the fluid.

Although most process applications are run at temperatures well above this point, system designs may encounter problems during emergency shutdowns or maintenance shutdowns if we have not taken this requirement into account in the acquisition of our system’s heat transfer fluid.

Generally speaking, most mineral heat transfer oils and mid-range synthetic fluids have start-up values in the range of -20ºC to -5ºC.

High-end synthetic fluids – aromatic synthetic fluids with biphenyl / diphenyl oxide and with maximum operating temperatures of 370°C – 400°C, have values of 5°C – 15°C as their minimum pumping temperatures.

Processes that utilize a heat transfer fluid that may potentially have cold start-up problems will require a heat source in their pipework, using either steam or electrical resistance.

Flash Point

This is the lowest temperature at which the vapours produced by the heat transfer fluid will ignite, resulting in a sudden flash on the surface of the heat transfer fluid when there is a flame nearby or a spark occurs in the presence of oxygen. It does not matter if the resulting flash is distinguished immediately afterwards – see figure 2.

It is a very important indication of the condition of the heat transfer fluid. Normal values are around 190ºC.

It must be remembered that a heat transfer fluid circuit is closed and there is, therefore, no presence of oxygen and combustion cannot occur without a leak.

It is very important not to confuse this temperature with the autoignition temperature or with the combustion temperature.

Fire Point

This is the temperature at which, when a flame or spark is nearby and in the presence of combustion air (oxygen), the flame that forms remains alight for at least 5 seconds. Normal values are around 210ºC.

Autoignition Point

This is the lowest temperature at which the heat transfer fluid ignites itself, without the presence of a flame or spark to initiate the combustion. Obviously, the presence of combustion air is required.

Thus, the temperature to be examined in the face of a possible leakage of heat transfer fluid is the autoignition temperature.

A leak would be necessary, since without it there would not be any oxygen in the circuit and, therefore, even if autoignition point temperatures were reached in a closed circuit, combustion would not be possible.

Testing standards
SpecificationDIN (Germany)IP (GB)NF (France)United States
Freezing point or solidification5159715T0105D 97/ D 445Z 11.5
Open cup flash point (Cleveland)5137636T60118D 92Z 11.6
Closed cup flash point (Pensky-Martens)5175834M07019D 93Z 11.7
Autoignition Point51794E 659-78Z 11.189
Determination of flash point according to standard ASTM D 92, known as Open cup or Cleveland

Figure 2. Determination of flash point according to standard ASTM D 92, known as Open cup or Cleveland

Temperature tables

The following table shows the characteristic temperatures of the main heat transfer fluids on the market. It can be seen that the maximum operating temperature (indicated by the manufacturer) and the flash and autoignition temperatures are not directly related.

Heat transfer fluidCharacteristic temperatures (ºC)
Maximum operatingMaximum filmFlashMinimum pumpingFreezingAutoignition
BP TRANSCAL N3203402210-12350
Calflo HTF325343231-1-18355
Diphyl DT330340135-25-54545
Dowtherm A4004301131212615
Dowtherm Q330355120-30-35412
Essotherm 65032034030047-9350
Marlotherm SH350380200-5-34450
Mobiltherm 603280300190-8-15340
PIROBLOC HTF Mineral305320215-5-12340
Therminol SP315335177-10-40365
Therminol 66345375178-3-32374
Therminol 75380400132-10-18585
Therminol VP-14004251241312621
Shell Thermia Oil E310340208-2-18340
Chapter 5

The hot oil boiler

The purpose of a boiler in a heat transfer fluid system is obvious: it must provide the energy demanded by the consuming appliances at the temperature required by the production system.

We can thus define it as a source of heat and it is, therefore, the point of the system in which the heat content, or enthalpy, of the transfer medium (in our case, heat transfer fluid) increases, and, consequently, the temperature of the said medium.

Sections of this chapter:
Types of boilers

According to the nature of the supply of heat to the boiler, we can consider three types of configuration:

  1. By means of traditional fuels, whether liquid or gaseous.
  2. Heating by means of electric resistive heating elements.
  3. The energy is provided by the recovery of sensible heat from gases originating from the combustion of a furnace or from a production process.

Basic requirements

  • It must be designed in accordance with the appropriate codes for this type of equipment (ASME design codes, AD-Merkblätt), not only with regard to calculations, but also for materials and their traceability, testing (hydraulic tests, non-destructive testing, etc.), execution (welding processes), safety and environment.
  • The documentation and records generated in the design and manufacture of the equipment, should be formed into a file that will allow the final certification of the equipment in accordance with the international regulations – ASME, CE marking, etc. -.
  • Adapt to the technical requirements specified by the user, especially with regard to power supplied and maximum operating temperature.
  • The performance and energy efficiency values should be optimized and, therefore, the fuel consumption should be moderate.
  • It must be highly reliable and easy to maintain. It must be remembered that, in general, the boiler is a critical part of the equipment in almost all production systems and a stoppage or breakdown can imply costly production stoppages.
  • It must be sufficiently flexible to be able to work with the widest possible range of heat transfer oils, allowing a long useful life of the fluid used, due to a correctly designed heat exchange – see characteristic temperatures of heat transfer fluids -.
  • Obviously, another requirement is a competitive price, which not only allows easy marketing by the manufacturer but also relatively short repayment terms for the user.

The fulfilment of these basic requirements to a great extent determines the design of the equipment, in as much as some of the not so obvious technical and functional parameters are important but not critical.

Within these functional parameters, requirements to be considered include convenient and fast access to the heating equipment, whether these be burners or electric resistive elements, inspection inside of the boiler, the possibility of emptying the boiler, sufficient thermal insulation to allow safe temperatures on the outside surfaces without risk of burns.

Under the more technical headings and for boilers with traditional fuels, the correct dimensioning of the combustion chamber, which allows, on the one hand, high heat transfer by radiation without exceeding the maximum permissible temperatures in the materials of the chamber and, on the other hand, allows the installation of low index NOx burners, requires a well-adjusted and exact design.

Furthermore, losses in the charge, whether in the flue gas or in the heat transfer fluid circuits, should not be high, thereby allowing the use of standard burners and pumps with low electrical consumption.

Boiler with liquid fuels: fuel oil, gas, natural gas or LPG

This is undoubtedly the type of boiler that you will most frequently come across and its design, with some variations and details, is very similar among many of the manufacturers in the industry.

Its assembly can be vertical or horizontal, depending on the needs of the user, but, in both cases, the concept and, therefore, the design are the same.

A boiler with a horizontal assembly allows it to be located in rooms that are relatively low in height and permits convenient and easy access to the burner and to the various parts of the equipment. By contrast, the greater needs for floor space with respect to a vertical boiler, can sometimes be a decisive element in the final decision.

As can be deduced from the previous paragraph, the location of the heat transfer fluid boiler is key in the selection of its orientation. The reason for this is that heat transfer fluid boilers, in almost all circumstances (only boilers with aromatic synthetic heat transfer fluids are excluded from this premise – see heat transfer fluids), can be installed, in accordance with the regulations, as close as the user wants to the consumer appliances, offering the possibility to avoid long and expensive installations.

PIROBLOC heat transfer fluid boilers with liquid or gaseous fuels

Figure 1. PIROBLOC heat transfer fluid boilers with liquid or gaseous fuels.
Horizontal or vertical assembly


The basic diagram for a heat transfer fluid boiler using liquid and gaseous fuels is shown in Figure 2.

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). With just a single coil (and, therefore, two smoke passes) it is difficult to obtain good performance due to insufficient heat exchange surface, whereas using three or more coils, while guaranteeing high energy efficiency, also implies a high economic cost. Thus, the equipment with two coils and three smoke passes can be considered as the “optimum design” that combines satisfactory yields with a moderate cost.

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 the vast majority of cases, the two coils are connected in series. Only specific designs for large flows and low heat differentials, require the coils to be connected in parallel (see technical concepts).

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.

The entire assembly is thermally insulated (10), (12) and (16), in order to minimize structural energy losses into the atmosphere, while avoiding possible burns by inadvertent contact with the surface of the boiler.

Heat transfer fluid boiler for liquid or gaseous fuels. Basic diagram

Figure 2. Heat transfer fluid boiler for liquid or gaseous fuels. Basic diagram


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/flue
15.- Output of combustion gases
16.- Thermal insulation of boiler and combustion chamber
17.- Boiler cover
18.- Combustion chamber top closure


Heat exchange

For the purposes of heat exchange, the configuration described can be divided into three parts in accordance with the heat transfer method and in relation to the technical constraints that are required at each point, in order to achieve the energy efficiency and durability results from the heat transfer fluid charge and from the equipment materials. (see Heat transfer).

In Figure 3, the three zones are clearly distinguished:

1. Radiation

It encompasses practically the entire combustion chamber, more specifically, the inner face of the interior coil, with it being decisive in this area, from a technical point of view, to know the exact values of the maximum temperature reached by both the heat transfer fluid and the material of the coil because, although it is the area with the greatest exchange capacity, it is also at risk of exceeding the maximum permitted values. – Figure 4 -.

Areas of the boiler according to heat transfer method. In relation to the mass and film temperatures reached

Figure 4. Areas of the boiler according to heat transfer method. In relation to the mass and film temperatures reached –see Temperatures-.

The characteristics of the thermal fluid used, the fuel, the combustion regulation, the flame diameter, the exchange requirements, the minimum circulating flow of heat transfer fluid required and, therefore, its velocity and the diameter of the coil tube are all parameters that determine what must be considered as critical in the design – the dimensioning of the diameter and the length of the chamber.

A too small diameter for the combustion chamber would allow an optimum transfer of heat but would jeopardize the useful life of the charge of heat transfer fluid as well as of the boiler itself and would also cause a loss in the smoke circuit charge which may be an excessive burden for a standard burner.

On the other hand, a combustion chamber with an over-sized diameter, will decrease the energy efficiency of the equipment.

The length of the combustion chamber is also of great importance with respect to the reliability of the equipment. A combustion chamber that is too short for the power required would involve unusually high temperatures in the bottom closure and in the upper closure of the chamber, which could lead to the partial destruction of these elements.

2. Transition zone

This comprises the inner faces of the ends of the inner and outer coils. Depending on the adjustment of the burner, it may partially include the outer face of the inner coil. In this area, radiation and convection coexist as heat transfer processes and, therefore, with regard to the heat, both the precautions for exchange by radiation and the constraints due to exchange by convection must be taken into account.

Particular attention should be paid to the design for the change in direction of the combustion gas circuit in the bottom closing of the combustion chamber, since complete airtightness must be achieved (otherwise the combustion gases would pass directly from the 1st pass to the flue outlet, giving very poor performance and worse, with extremely high temperatures in the flue that could cause its destruction) together with a low loss of charge in the change of direction of the flue gases.

3. Convection zone

This corresponds to both faces of the outer coil and the inner face of the interior coil.

Although there may a slight risk of exceeding the maximum temperatures of use of heat transfer fluid and materials (see Figure 4), the main concern when designing this area is that of achieving a high level of heat transfer by means of a considerable velocity of combustion gases but without producing significant contamination risks in smoke passes 2 and 3 owing to under sizing in these passages or a high loss of charge in the smoke circuit (known as boiler overpressure) making it difficult to use standard market burners.

Distinct areas in a heat transfer fluid boiler for heat exchange purposes

Figure 3. Distinct areas in a heat transfer fluid boiler for heat exchange purposes

In addition to all of the parameters discussed above, the coils should also be carefully designed so that, from the hydraulics point of view, the heat transfer fluid circuit charge losses are not high, which would result in non-standard pumps and high electricity consumption and which, at the same time, guarantees sufficient heat transfer fluid velocity in order to provide satisfactory heat transfer coefficients – see Figure 5.

Heat transfer fluid velocity / heat transfer coefficient. Values for BP Transcal N. heat transfer fluid Temperature 290°C. Other factors are excluded for a better understanding of the importance of velocity

Figure 5. Heat transfer fluid velocity / heat transfer coefficient. Values for BP Transcal N. heat transfer fluid Temperature 290°C. Other factors are excluded for a better understanding of the importance of velocity

Heat differential. Passes in the coils

Heat differential also known as heat jump, is the maximum increase in temperature of the heat transfer fluid that a boiler is able to obtain in its nominal heat power, at the design flow rate of heat transfer fluid.

The most common thermal jumps are 20°C and 40°C, although these values have some margins depending on the heat transfer fluid used and the operating temperature, thus, we should actually talk about intervals of between 18-22°C in the first case and 36-42°C in the second case.

It is important to keep in mind that one boiler is not better or worse than another boiler with the same heat power but a different jump. With the correct design, both types of boiler will have similar energy performances and similar operating functions.

The reason for having boilers with different heat differentials is to obtain the best adaptation of the boiler to the characteristics of the production process and, more specifically, to the system’s consumer appliances.

Initially, a boiler with a 20ºC heat jump can give a greater uniformity of temperature in the consuming appliances due to having a greater circulating flow, although with an initially more expensive installation due to a larger pipe diameter, more heat transfer fluid capacity in the system and a higher electrical consumption in the main pump. However, a boiler with a 40°C heat differential can also achieve the same results by means of recirculation circuits with secondary pumps which provide a greater flow rate in consumer appliances and, thus, greater uniformity. In the latter case, however, the installation cost of the heat differential boiler is considerably higher which is not a positive factor.

Heat differentials higher than 40 or 50ºC are not common given that the useful life of the heat transfer fluid is affected by such high and abrupt changes of temperature and the design of the boiler must anticipate measures for absorbing additional expansions, which makes the design more specialized and more expensive. However, in applications for solar thermal power plants, heat transfer fluid boilers with heat differentials of 100°C can be found.

Our recommendation is that the user contact the boiler manufacturer, authorized installer or in-house or external engineer to discuss what heat differential would be the most suitable for their process.

We have already seen that determining the heat differential, basically by the characteristics of the consuming devices, determines the circulating flow rate of heat transfer fluid required in the system. But this flow must also meet certain requirements marked on the boiler.

The velocity of the heat transfer fluid in the coils must be high enough to ensure a good heat exchange while not exceeding the film temperature of the heat transfer fluid used in order to avoid its rapid degradation. But these high circulation speeds that are required also imply significant charge losses (pressure losses) since the charge loss is proportional to the high velocity squared, with the possibility of having to resort to very large pumps with inordinately high electricity consumption in order to achieve hydraulic stability in the circuit.

Reconciling the factors of high velocity and acceptable charge losses is only possible with a precise heat and hydraulic study of the coils, the diameter of their tubes, the length of these and their connection.

With the help of the diagrams in Figure 6 and a short example, we will try to clarify a little all these issues. We have simplified the possible hydraulic options exclusively in these three cases. In reality, the parallel passes of the coils can be from only 1 pass or up to 6, 7 or 8.

The operating temperature T1 and its kW heat output are the same in all three diagrams in Figure 6. Also, the total length of the component pipe of the coils is the same – 4L.

The differences relate to the boiler inlet temperatures (return temperature from the consuming appliances after supplying the required energy), T2, T3 and T4. The circulating flow rates Q, Q1 y Q2 and the charge losses ΔP1, ΔP2 and ΔP3 are also different.

Real numeric example

We have a heat transfer fluid boiler with 40ºC heat differential and with 1100 kW of heating power. Its exchange surface is 54 m2 with yields in the order of 86-89%, depending on operating temperature.

Its design outline is A) in Figure 6, with two coils in series and two parallel passes per coil. The design flow rate for these conditions is 52 m3/h, with a charge loss of 2.37 bar at 260ºC operating temperature.

If we try to operate this boiler with a heat jump of 20°C, the flow rate would have to be 104 m3/h and the expected charge loss at the same temperature as before, 260°C, would be 8.17 bar. We would have to resort to very sophisticated and expensive pumps, with very high electricity consumption.

On the other hand, if we use design outline B) in Figure 6 (two coils in series with three parallel passes per coil) with the same flow rate, 104 m3/h, and exchange surface, 54 m2, the charge loss would be 2.62 bar, which is acceptable for conventional pumps.

This type B) design outline would not be feasible for a boiler with a 40ºC heat differential since, with the low flow rate required, 52 m3/h, there would be no problems of pressure drop (only 0.71 bar) but, instead, the problem would be overcoming the fluid film temperature, since this would be approximately 44°C higher than the operating temperature.

As can be seen in Temperatures, the maximum film temperature is usually in the order of 10-20°C above the maximum operating temperature so, in this hypothetical case, we would either suffer a rapid degradation of the heat transfer fluid charge or we would be forced to work at low temperatures, which may not be acceptable for our production system.

Design C), with two coils connected in parallel, each one with three passes of heat transfer fluid, corresponds to a fairly unusual assembly and one typical of boilers requiring very small heat differentials, in the order of 10 or 15ºC. Under these conditions, the flow rate, 205 m3/h, is very high and if this configuration were not chosen, the heat transfer fluid charge loss would be excessively high, even with the three-pass configuration in design outline B), given that it would be around 8.45 bar.

Types of coil connection

Figure 6. Types of coil connection. A) In series, two passes per coil in parallel. B) In series, three passes per coil in parallel. C) In parallel, two passes per coil in parallel

We can see, therefore, that the heat jump required greatly influences the design of the boiler and it must, therefore, be considered as a key factor in the installation project of a heat transfer fluid system.

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