Method: Vary condensing area.
Process: Net vapor rate zero.
Disadvantages: Liquid level control in drum affects pressure balance. Relatively large control valve required. Extra exchanger area required.
Application: Generally a stable and effective control scheme for many processes.
Configuration notes: Pressure in the drum and column overhead are the same. Equalizing line needed for stable control. Liquid from condenser must enter drum in the liquid layer so that control valve changes can affect liquid level in condenser. Preferred application is liquid entry into drum from below (shown). If air-fins are used for condenser, they should be angled to allow for gradual surface area changes with liquid level changes. Cooling done at lowest possible utility temperature (potential problem for cooling water, potential advantage for heat integrated cryogenic systems).
Operation: Control valve imposes a variable DP in main flow line through condenser. Liquid level in control line up to exchanger varies with DP changes. This exposes or submerges tubes, shifting area from condensation to subcooling.
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