Certain industrial applications require fluid temperatures (of steam, water, or thermal fluid) of more than 410°F. With water, the boiler would need to have a design pressure of more than 300 PSIG to achieve these temperatures, rendering a fire tube design inadequate and a water tube design extremely expensive.
For such applications, thermal fluid can be used. Thermal fluids (or thermal oils) are special types of oil that remain liquid at very high temperatures and as such work without needing to be pressurized.
If thermal fluid is to be used, then a thermal fluid system is required and must include the following basic components:
A heating system by means of a closed thermal fluid circuit requires strict operational controls and the selection of all materials by a technical professional. The components interact with each other so that a failure of one of the parts damages the entire system.
The thermal fluid heater controls the temperature of the system via the exit temperature (while the entry temperature is variable).
There are different types and manufacturers of thermal fluids for lower temperatures (482°F), intermediate temperatures (590°F), and high temperatures (680°F).
The temperature of the process is the average temperature of the thermal liquid throughout the process, however within the walls of the thermal fluid heater, where the fluid flows in tubes which are then exposed to the boiler flame, a fine layer of thermal fluid is formed at a temperature much higher than the average – this temperature is called the film temperature.
The correct thermal fluid choice is based on this higher film temperature, not on the average process temperature.
A multitubular heater distributes its load in a uniform fashion via multiple parallel tubes. The pressure drop along these tubes is relatively small (requiring less power in the recirculation pump). The film temperature is lower than a heater with only one or two parallel tubes.
In both multitubular heaters and their single or parallel tube counterparts, as the maximum film temperature recommended by the thermal fluid manufacturer approaches the film temperature, the thermal fluid becomes degraded and no longer functional, carbonizes and blocks the heater tubes. Because of this, thermal fluid heaters require control tests be conducted every six months. Any increase in the pressure drop along the tubes indicates the start of the degradation of the thermal fluid and may also be a sign of possible carbonization and/or incrustation of the tubes.
Calderas Powermaster manufactures and sells thermal fluid heaters from 12,000,000 BTUH only within Mexico. They are not available for export.