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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