Normal System OperationDuring normal operation, the thermal fluid heater provides both radiant and convective heat to heater tubing located in the firebox. This heat is transmitted through the tube wall to the fluid's film layer. High flow inside the tube minimizes the difference between the wall (film) temperature and the average (bulk) temperature by maintaining a high film coefficient as well as sufficient fluid mass to remove the heat transferred through the tube wall. The amount of heat transferred to the tubing per unit area in the fire box (commonly called the heat flux) varies substantially with the type of fuel (gas has lower heat flux than wood or liquid fuels) and tubing configuration.
If the flow rate decreases while the heat input remains constant, both film and bulk temperatures will increase. Less fluid mass is available to remove the heat transferred to the tubing. And because the film coefficient is almost linear with velocity, the film temperature will increase due to the decreased heat transfer efficiency.
Improper ShutdownAt normal operating temperatures, the heater's refractory and structural metal members may be almost as hot as the flame itself.
If the entire system goes down at once -- whether from a power interruption or someone shutting off the main switch -- the heater will stop firing and fluid flow will stop. However, heat stored in the heater's refractory and structural metal will be released into the firebox. In addition, certain types of heaters such as solid fuel units come equipped with more refractory than other types. Although firebox temperature may be the same, more time is required to bleed off the additional stored heat.
With the combustion blower off, this residual heat will transfer into the now stagnant fluid, rapidly increasing the temperature. Depending on the amount of heat stored in the firebox, the fluid temperature easily can exceed both the boiling and cracking temperatures of the fluid.
Improper StartupMost medium- to high-temperature thermal fluids have high viscosity at ambient temperatures. This affects the pumpability of a fluid as well as significantly reduces the fluid film coefficient. This combination can cause serious fluid degradation if a heater is started up too quickly. The low film coefficient causes high film/bulk differential temperature while the low flow rate reduces the rate at which heat is removed. Because the pump is circulating the fluid and the actual temperature is well below the heater setpoint, the high temperature situation can exist for extended periods of time and affect the entire charge of fluid.
Proper ShutdownWhen shutting down the system, turn off the heater but keep the circulating pump and, if possible, the combustion air blower running to remove the residual heat from the firebox. Ask your heater manufacturer to specify how much pump run-time is required to exhaust excess stored heat in your type of heater.
One rule of thumb is to keep the pump running until the heater outlet temperature has decreased to 250oF (121oC) or lower. However, in well-insulated systems, cool down can take an extended period of time. Consider installing a thermostat and relay designed to automatically shut down the circulating pump when a predetermined temperature is reached.