Reasons for adding flow control after the pump in a forced-circulation reboiler
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I’ve seen some PID systems that include flow regulation installed after the pump used for forced circulation; what is the reason for this? Can a frequency converter be used in place of the pump, considering that the pipelines for forced circulation are quite large in size?
The flow rate in a forced-circulation reboiler is usually fixed; it may be adjusted when the load changes. For cost reasons, a globe valve is generally added for manual control.
It’s generally to prevent expansion effects, right? Flashing behind the valve
However, all the control valves I’ve seen are located on the inlet pipeline of the reboiler; if they were on the outlet pipeline, they would presumably serve to facilitate flashing behind the valve. Have I remembered that wrong? :dizzy:
If it’s an imported pipeline, then it’s really better to install an inverter
Thank you for your reply. If it is installed at the outlet, it still only controls the amount of flashing in the pipeline; after all, the pressure inside the tower remains constant. What is the difference from installing it on the inlet pipeline?
If the valve is installed at the inlet of the reboiler, two-phase flow may occur, which can affect heat transfer efficiency. If it is installed at the outlet of the reboiler, a single-phase liquid flow exists inside the reboiler, resulting in a higher overall heat transfer coefficient
The valve is installed at the inlet of the reboiler, so there shouldn’t be a two-phase flow – after all, the fluid doesn’t enter the reboiler. Even if there is a pressure drop downstream of the valve, it isn’t high enough to cause flashing. What do you think?
It facilitates the adjustment of the flow rate; however, using an inverter-driven motor is not very economical； Additionally, material components that are typically used in forced circulation are relatively heavy or have a high viscosity, and no flashing occurs downstream of the valve.
Thank you for your reply. Furthermore, does adjusting the flow rate of a variable-frequency pump change its head? If the opening degree of the pump’s outlet valve is changed, does the pump’s operating point shift according to its performance curve, or does changing the pump frequency alter its performance curve?