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 .
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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