Page 401 - Handbook of Energy Engineering Calculations

External Steam Tracing Heat-Transfer Design for Pipelines

Selecting Air-Cooled Heat Exchangers

Quick Design and Evaluation of Heat Exchangers

HEAT-TRANSFER CALCULATION PARAMETERS

In the world of energy, heat transfer is of critical importance because the
greater the efficiency of heat transfer, the more heat can be recovered from
the process being considered. And, in general, the more heat recovered, the
more attractive the project is, provided the cost of recovering the heat is not

excessive. Greater efficiency in heat recovery is producing more energy
savings while reducing carbon-dioxide emissions worldwide. As a result, the
overall energy field is benefiting, as is the atmosphere.
To save energy in the heat-transfer process, several requirements must be

met: (a) a sufficient temperature difference must exist to provide a “heat
head” to make heat transfer economical; (b) there must be a need and use for
the recovered heat; (c) the heat transfer must result in a fuel saving that is
enough to pay for the needed equipment in an acceptable time frame—

usually 3 years, or less.
Factors important in providing effective heat transfer at an economical cost
include: (a) type of heat exchanger selected for the application, with the type
ranging from shell-and-tube, direct-contact with mixing, regenerative-plate

type, to double-tube exchanger; (b) type of heat transfer occurring—heating,
cooling, boiling, or condensing; (c) overall heat-transfer coefficient, U, which
2
2
can vary from 1 to 1000 Btu/(h · °F · ft )[W/m · °C], depending on the fluid,
type of heat exchanger, relative flow direction, and other factors; (d) size of
exchanger chosen relative to the amount of heat being transferred—whether
the exchanger is over-, under-, or right-sized; (e) type of fluid flow selected
for the heat exchanger—parallel or counterflow; and (f) obtaining, and using,
the actual logarithmic mean temperature difference (LMTD) in the heat–

transfer calculations. This section of the handbook focuses on energy
conservation using heat exchangers of various types to reduce heat losses
while recovering the maximum amount of usable heat possible.