A well designed shell and tube heat exchanger will
handle its rated load under the conditions for which it was specified.
Emergencies occur in some processes, and it may be
advantageous to have some extra margin of heat exchanger capacity to
take care of them. The cost of such a safety factor can be modest
compared to the protection it will afford to valuable material in
process or to other expensive equipment in the system.
Fouling affects capacity, and this factor sometimes
accounts for a certain amount of confusion when ratings are compared.
There is no economy in assuming an optimistically
low fouling factor even if it seems to make a smaller heat exchanger
feasible. Later on, difficulties with reduced capacity; low process
yields frequent shutdown for cleaning and extra maintenance can
dissipate this saving very quickly.
