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All the reboilers I have worked with have had the flow control valve in the steam line feeding the reboiler, and a steam trap in the condensate exit line.
However, I recently saw a P&ID which showed the flow control valve in the condensate line, but with the flow element still in the steam line. I suppose this has the advantage of making the valve smaller and saving a steam trapping station, but I would be interested to know if this works without blowing steam into the condensate line?
I guess the control valve would cause the condensate in the reboiler to back up making the reboiler a “variable surface area” device, whereas if the valve is in the steam line it is a “variable pressure (or temperature)” device.
I would be most grateful to hear from anyone who has operated a reboiler in this way.
I haven’t operated any reboilers designed this way but have heard about them. Yes, as you suspect they are operated as variable surface to control the heat input compared to adjusting the temperature difference (which is what a control valve on the inlet gives you).
Response time is typically slower as you have to drain or raise the condensate level to change the portion of heat transfer surface in the steam zone. Also, you can have corrosion problems at the steam/condensate interface depending on how well you control your steam/condensate water chemistry.
Typically, there is a low level override on the condensate valve to avoid blowing steam into your condensate return system.