Method: Vary condensing area.
Process: Net vapor rate zero.
Advantages: No liquid control system for condensate drum required.
Disadvantages: Control action to vary surface area varies distillate product rate. May cause problems due to unsteady feed to downstream units. May be difficult to vent inert gas or non-condensable material from upsets from this configuration.
Application: Often used with very large and heavy condensers or with equipment requiring recurring cleaning or maintenance (exchangers at grade).
Variants: No condensate drum. Reflux rate may be controlled instead of distillate product.
Configuration notes: A liquid level must be maintained in the exchanger at all times. The purpose of the drum, where present, is often to decant a second phase from the overhead: for example, water from a hydrocarbon system. This has not been shown on the figure.
Operation: Variable product rate changes level in condenser.
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