Heat transfer fluids are vital to many processes worldwide. To ensure their ongoing, safe operation, it is important to protect the fluid not only during regular operation but also during system startup and shutdown. Here are some guidelines to ensure a smooth startup and shutdown of your heat transfer system.
Characterization of Fluid Flow in Industrial Processes
The two primary characterizations of fluid flow in a pipe are laminar flow and turbulent flow. Laminar flow is described as a smooth, layered flow in which the quickest flow happens at the center of the pipe. Turbulent flow is choppy and tumultuous with no specific pattern. The point at which fluid flow transitions from laminar to turbulent flow is defined by the Reynolds number, a dimensionless number characterized by the length of pipe through which the fluid is traveling, and the fluid’s density, viscosity and velocity.
In a heat transfer fluid heating system, when the pump and heater are turned on, the velocity of the fluid increases and the viscosity decreases. The velocity and viscosity changes make the Reynolds number increase. Once the Reynolds number is greater than 2000, the fluid flow is considered turbulent.
The transition from laminar flow to turbulent flow is important during startup due to the layered nature of laminar flow. As noted, during laminar flow, the fluid at the center of the pipe flows most quickly. The fluid at the outside of the flow forms a stagnant layer along the pipe wall where the heat transfer happens. If heated too quickly, this layer will inhibit heat transfer and the fluid will exceed its bulk film temperature, causing thermal cracking and carbon and sludge buildup.
Turbulent flow of the heat transfer fluid allows for superior heat transfer and maximum thermal efficiency along the wall of the pipe. For this reason, the most critical thing to remember during startup of a heat transfer system is to begin circulation of the fluid before raising the system temperature.
Startup of Heat Transfer Systems for Industrial Processes
Many steps are required to properly start up a heat transfer system. Whether the system is brand new or has been previously used, all systems should be flushed before startup to remove any debris or residue trapped in the piping. After flushing, the heat transfer fluid must be charged into the system. Finally, the system should be started slowly and carefully to avoid damage to the heat transfer fluid. Here is a closer look at each of these key steps.
Flushing the System. To complete a flush of a heat transfer system, the system should be filled to 80 percent of its total volume with flushing fluid, according to the filling instructions (see below), and run for eight hours at 400°F (204°C). It is important to use the correct flushing fluid for your system, whether it is a hot oil system or a synthetic heat transfer system. Your heat transfer fluid supplier can help you choose the correct flushing fluid. Drain the flushing fluid before proceeding to the next step.
Filling the System. The system should be charged with heat transfer fluids through low points in the system. One main objective in starting up a heat transfer system should be to remove any additional moisture or other light-end fluids trapped in the system. (Light-end fluids are contaminants with a boiling point lower than that of the heat transfer fluid.) To do this, the fluid should be run through the expansion tank during startup. Any moisture or light-ends escaping may cause a brief increase in system pressure or puffs of vapor leaving the expansion tank vent.
The expansion drum should be filled and drained continuously to aid in venting nitrogen and to fill the system. Once the system has been filled, nitrogen and any additional moisture or light-ends can be vented through bleeder valves at high points of the system. After the system is completely filled and nitrogen has been vented, the circulation pump can be turned on.
Starting the System. The pump always should be started before the heater is turned on to ensure that the fluid is circulating and reaches turbulent flow before being heated. Once the fluid is circulating, the following steps should be taken to bring the system up to the desired operating temperature:
Increase the heat at a rate of 1°F (~0.5°C) per minute or less until it reaches 200°F (93°C). Run one cycle.
Increase the heat at a rate of 1°F per minute or less until it reaches 230°F (110°C). Run one cycle.
Increase the heat at a rate of 1°F per minute or less until it reaches 260°F (127°C). Run one cycle.
Increase the heat at a rate of 1°F per minute or less until it reaches 300°F (149°C). Run one cycle. Bypass the expansion tank if the system has been running normally.
Increase the heat at a rate of 50°F (~25°C) per minute or less until it reaches the desired operating temperature. Run one cycle under observation.
Note that after each step, the system should be run for one cycle before moving on to the next step. The time it takes to complete one cycle can be determined with the pump output and the total system volume. If an excessive amount of vapor generation is noticed at a certain temperature, that temperature should be maintained until the rate of venting decreases. The saturation of moisture is highest at the boiling point of water, 212°F (100°C). Therefore, the temperature should be increased slowly through that point to allow as much moisture as possible to vent out of the system.
Shutdown of Heat Transfer Systems
As previously mentioned, it is critical that the heat transfer fluid be circulating when the system is operating at high temperatures. Therefore, during shutdown of a heat transfer system, the heat should be turned down before the pump is shut off. (In other words, the procedure follows the opposite order in which the system is started.) The fluid should circulate until the system temperature drops below 200°F (93°C), and the residual heat has been removed from the system. At this point, the system pump can be shut off.