Thermal engineering and heat transfer fluids solutions

When designing a thermal oil circuit there are a set of variables that must be taken into account to ensure its correct operation. Among all these variables, there are a number of parameters that are of vital importance in determining the final design:

• The amount of heat that will be transferred
• Inlet and outlet temperatures at the boiler and at the consumers
• The maximum allowable pressure drop on both sides
• The maximum operating temperature
• The maximum operating pressure
• Circulation flows
• The load losses of the circuit

Proper design is of vital importance to ensure that the plant’s production capacity will always be what is ideal, while maximising energy efficiency and reducing production costs.

1. Purpose and size of the installation

The first element to consider is the application of the thermal fluid boiler, i.e. what it’s function is and the expected results. The purpose desired by the customer is the starting point and the key element that will determine how the rest of the variables have to be adjusted in the final configuration of the thermal oil circuit. Understanding the customer’s needs is of vital importance to ensure maximum efficiency of the installation.

2. Configuration of the coils

Most direct thermal oil heating coils employ a helical (spiral) design. Several helical coils in a single boiler can be more efficient than a single coil, as a larger heating surface is obtained, usually resulting in more efficient heat transfer.

For example, a double coil system allows three flue gas passages through the surfaces of the coils.

3. Volume of the combustion chamber

The combustion chamber of a thermal oil boiler must be designed according to the flame dimensions of the installed burner, in addition to other variables. Proper combustion chamber sizing prevents the burner flame from getting too close to the oil coils, reducing any possible damaging impact of the flame. Sizing the combustion chamber correctly also keeps film temperatures relatively low, helping to extend the life of the thermal oil and protect the boiler itself.

4. Film temperature

The film temperature refers to the surface temperature of the boiler coils, calculated as a function of the flue gas temperature, the oil temperature and the film coefficient.

The highest temperature to which the thermal oil is exposed is its film temperature. Therefore, if the combustion chamber is too small, the film temperature can end up being between 100°F and 200°F (38°C to 93°C), higher than the total oil temperature. This situation can lead to rapid degradation of the heat transfer fluid and shorten its service life. It is for this reason that the maximum film temperatures of different boiler designs must be carefully analysed and compared.

5. Calculate the heating surface area

Although two boilers may be similar in size, this does not imply that they share the same heating surface, so correctly calculating the heat transfer surface area is of vital importance when designing a thermal oil boiler. It is important to consider the fouling factor when sizing the heating surface and to remember that the size of the heating surface has a significant impact on the flue gas temperature as well as the thermal efficiency.

6. Operating temperature

The operating temperatures of the thermal oil must be defined according to the temperature required by the production process and its heat demand. Operating at unnecessarily high temperatures can significantly shorten the service life of thermal oil, resulting in increased costs and unnecessary production downtime. The permissible temperature range of the thermal oil must be above the maximum operating temperature to minimise thermal oil degradation.

Design temperatures and pressures also merit close inspection in the design of a thermal oil boiler. Pumps, valves, filters and accessories must guarantee efficient operation under maximum process conditions.

7. Safety

Unlike steam systems, thermal oil boilers do not require continuous supervision by an operator, but a number of preventive and maintenance measures must be taken for safe operation.

• Flow control. Every thermal oil boiler must include the most important safety device for its operation: a flow controller, which is the tool that allows us to detect when the flow is insufficient, possibly leading to overheating of the thermal oil that ends up damaging the boiler. Poor circulation will trigger an alarm that will stop heat input if the flow rate drops below a preset percentage. This safety is tri-redundant.
• Burner control. A burner controller helps to maintain the oil temperature. However, excessive thermal oil or flue gas temperatures must be detected by temperature probes or thermocouples that allow the heat source to be shut down immediately if required. The burner must also be equipped with a flame scanner and flame arrester to detect and monitor possible flame faults.
• Expansion tank. Located at the highest point of the circuit, the function of the expansion tank is to contain any excess thermal oil caused by expansion of the fluid at high temperatures. To prevent oxidation, the oil in the expansion tank should be kept at a relatively low temperature (below 150°F (66°C). In addition, the expansion tank must be equipped with at least one low level switch to detect any loss of oil. When the fluid does not reach a preset level, the switch should trip immediately and shut down the system. Although most systems operate with the expansion tank open to atmosphere, a relief valve on the boiler outlet is a useful protection in case the boiler becomes isolated from the expansion tank and therefore pressurised.
• Deaerator. The function of the deaerator is to separate entrained air or water vapour from the circulating oil. This can be critical during system start-up when there is often a considerable amount of air and water in the thermal oil.

8. Compliance with environmental regulations

There are various standards specifying a number of requirements and safety features that must be integrated into a thermal fluid system. These regulations vary depending on the geographical area and must be complied with. In all cases these specifications also contribute to extending the service life of the installation.

9. Fuel type and cleanliness

The most common fuels used in thermal fluid boilers are natural gas, propane, diesel and heavy fuel oil. With relatively clean fuels, most thermal oil heating coils will accept the combustion products without additional cleaning. However, if you choose to burn low quality heavy oils, you must take the necessary measures to clean the combustion chamber.

In projects where a potential customer contacts us to request a new thermal fluid installation, it is advisable that both parties collaborate in all phases of the design, as well as in the commissioning of the installation.

Our recommendation is always to involve the technology supplier in the engineering and commissioning processes of the installation, as the long-term benefits will far outweigh the short-term benefits.

We consider that it has many advantages for the customer to involve the supplier of the installation in its commissioning, as this will allow the installation to operate at its optimum level, achieving the best efficiency both in terms of production and in cost savings derived from energy consumption. While a customer’s own commissioning may result in a quality product, it may not reach its maximum efficiency.