Method: Vary condensing area.
Process: Net vapor rate zero. Condensate drum runs at a lower pressure than tower pressure.
Advantages: Condenser may be mounted below condensate drum.
Disadvantages: Two control valves required. Condenser requires subcooling area.
Application: Often used with very large and heavy condensers or with equipment requiring recurring cleaning or maintenance (exchangers at grade). Preferred system to use with cooling boxes as condensers. Cooling boxes have too high an internal heat capacitance on the cooling water side to allow for rapid changes in cooling water level.
Configuration notes: A liquid level must be maintained in the exchanger at all times.
Operation: To maintain pressure on the tower overhead, the control valve in the condenser line DP varies. As control valve DP changes, pressure balance between the bypass and the condenser flows varies the liquid level in the condenser. This changes the allocation of condensing versus subcooling surface available. To maintain drum pressure, the control valve in the bypass line varies flow through the bypass.
Warnings: Mounting condenser below drum requires sub-cooling in condenser for stable operation
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