Overloaded heat exchanger equipment may also
contribute to non-optimal stream outlet conditions
for best operating plant efficiency and a
corresponding influence toward limiting production
capacity.  This is a somewhat grey area best
established by the engineering specialist for the
plant site or facility to assess.  With such existing
limitations, replacement of certain limiting heat
exchanger equipment can result in improved plant
efficiency, energy savings and greater plant
capacity.
New heat exchanger configurations can sometimes
be used to expand existing plant production, while
simultaneously improving efficiency.  For example,
the use of cross-flow (TEMA X) heat exchanger
designs, in place of conventional designs that use
segmental baffles, can dramatically lower shell
side fluid pressure drop at given plant rate, which
may be a utility stream, or a process stream.  
(New TEMA X type exchanger designs may often
be suited to replace existing TEMA E, F, G, H, or
J types)
When high plant production rate causes heat
exchanger failure as shell side vibration damage
and resultant leaks, then redesign of replacement
heat exchanger equipment using NTIW (No Tubes
In baffle Windows) designs can insure reliable
expanded plant production, without the risk of
future heat exchanger failures and leaks.  A new
NTIW heat exchanger design requires slightly
greater capital investment than a replacement
conventional segmental baffle design for given
higher plant production, but has a greater
reliability, with negligible risk of future tube failures
caused by shell side fluid flow induced tube
mechanical vibration at expanded future plant
rates.
Furnace Equipment

Process furnaces include radiant heat transfer coils, and convection section coils, optimally designed to efficiently provide
the necessary process and utility heat input to operate the plant.  Equipment designs evolve over time, and what was an
efficient furnace design decades ago, can now be improved upon today, with energy reduction included, when plant
expansion is desired.  From the Base Case plant data for expansion studies, the furnace equipment can be modeled with
both process simulation software, further combined with rigorous furnace design, evaluation and rating software.  Generally,
the operating plant does not have all of the ideal instrumentation for both the inlet and outlet conditions of furnace coils, and
also the flue gas conditions.  Key assumptions for some of the process data usually have to be made.  By the use of both
process simulation software and furnace design/evaluation software, missing or incorrect process or flue gas temperatures
and pressures can be determined which best fit the operating data and reconcile actual performance of the furnace
equipment.

At higher plant production rates, convection coil outlet temperatures generally decrease because of higher duty with the fixed
coil surface area.  The effect of plant operation at extensively higher than original design production capacity causes flue gas
temperature throughout the furnace convection section to increase, resulting in lower furnace efficiency.