Method: Vary heat flux.
Process: Net vapor rate zero.
Advantages: Simple, fast response.
Disadvantages: Control valve is bigger than for other options.
Condenser and condensate drum pressure variable. This may create
a problem with condensate pump head and drum level control.
Application: Pressure towers. Good system for towers with
large quantities of non-condensable gas in overhead vapor .
Variants:
Configuration notes: Cooling done at lowest possible utility
temperature (potential problem for cooling water, potential advantage
for heat integrated cryogenic systems).
Operation: Direct control of overhead pressure. DP varies
to control condensation temperature in condenser, this varies
condenser LMTD.
Warnings: In vacuum systems the minimum pressure drop across
the control valve may have significant utility and equipment costs.
To a lesser extent this applies to any system with a downstream
compression on a vapor product. In liquid-ring vacuum pump systems
suction throttling can create the conditions for vaporizing the
seal fluid.
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