Many factors specific to a unit's circumstances weigh into the decision of which control method to use. However, as a starting point, the following recommendations are made for common situations:
Net vapor always > 0 or Vapor product only: Method
1 is usually the simplest and best.
Vapor product to compression system: Method 3 is recommended.
Steam Jet Ejector Systems: Method 5 is recommended for
systems with precondensers or with the vacuum system taking vapor
from the condensate drum instead of directly from the tower. Method
9 is recommended for systems with the ejector connected directly
to the tower. Method 7 should never by specified as the main control
method unless allowance for a backup method is made.
Large Quantities of non-condensable gas requiring venting from
system: Method 1 is preferred. Method 26 as an alternate.
Net Vapor Rate > or = zero: Both Method 10 and many
of the variants of Method 11 work well. Method 11 plus Method
14 works well. Method 11 plus Method 23 can be used to maintain
the condensate drum pressure if cooling utility fouling can be
avoided.
Narrow Condensing Temperature Range on Process Side: Method
15 should be avoided.
Wide Condensing Temperature Range on Process Side: Method
15 works well in many applications.
Heavy Exchangers, Exchangers Requiring Cleaning Often, Cooling
Boxes: Method 17 is recommended for exchangers sited below
the reflux drum. Method 15 works well for wide condensing temperature
range processes.
Vaporizing Coolant: Method 18 is used most often. Method
19 is rarely used.
Solidification Possible on Process Side: Method 20, its
variants, and Method 21 are recommended.
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