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STEADY UNDER PRESSURE

Method 2: 1B: Control exchanger bypass rate. 'Hot vapor bypass.'

Method: Vary condensing area.
Process: Can operate at zero vapor product rate. Should not be used in systems with large quantities of non-condensable gas.
Advantages: Condenser may be mounted below condensate drum.
Disadvantages: Concept is not straightforward. Vapor product not in equilibrium with the condensate drum liquid. Vapor product will have very close to the same composition as the tower overhead. Control valve must respond to both condensate liquid level and product vapor rate (composition) disturbances. Requires subcooling area in condenser.
Application: Often used with very large and heavy condensers or with equipment requiring recurring cleaning or maintenance (exchangers at grade).
Variants: Condensate drum pressure instead of tower overhead pressure may be controlled. Most common application of this method has exchanger below drum to simplify piping and gravity flow hydraulics, see Figure 15. General method often referred to as 'hot vapor bypass.'
Configuration notes: A liquid level must be maintained in the exchanger at all times. Careful attention to piping is required, especially if the exchanger will be mounted above the condensate drum. The author prefers to mount the exchanger below the condensate drum or use the two valve method in Figure 22. Liquid must enter the drum without mixing with the drum's vapor space. Either have the liquid enter the drum from the bottom or have an internal pipe down from the top. The author prefers the bottom entry as this creates less liquid turbulence. The control valve and bypass line must be sized so that the bypass flow rate changes allow for a DP change that corresponds to the liquid level range available in the condenser. Exchanger must be sized to allow for sub-cooling condensate. Often, 25% extra surface area above a total condenser is added.
Operation: To maintain pressure, the control valve 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.
Warnings: Some units have worked very well with this scheme. Others have failed. Selecting bypass rates and exchanger surface required is mostly empirical. Some general problem areas for this method:
High purity products: This method does not work well with high purity products that have narrow boiling ranges. The liquid insulating layer between the bulk condensate pool and the vapor space fails to adequately insulate the liquid. Control is erratic.
Self refluxing condensers: Heavy material in the overhead vapor condenses first. Some liquid falls to the bottom of the exchanger and runs along it to the outlet. This may change the composition of the vapor sufficiently that at the outlet the vapor is no longer fully condensable.


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This page updated 03 April 2000.
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