Method: Vary condensing area.
Process: Net vapor rate zero.
Advantages: Simple. Relatively small control valve.
Disadvantages: Condensate from condenser to drum must allow for gravity flow of liquid. Great attention must be paid to layout and control valve sizing. Extra exchanger area required.
Application: Generally a stable and effective control scheme for many processes.
Variants: Liquid may enter condensate drum in top.
Configuration notes: Pressure in the drum and column overhead are the same. Equalizing line needed for stable control. If air-fins are used for condenser, they should be angled to allow for gradual surface area changes with liquid level changes.
Operation: Control valve imposes a variable DP in main flow line after 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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