Method: Vary condensing area.
Process: Net vapor rate zero.
Advantages: Simple.
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.
Variants:
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.
Warnings:
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