Page 316 - Mechanical Engineers' Handbook (Volume 4)

3 Rating Methods 305

cost picture. Pressure drop is roughly related to the individual heat-transfer coefficients by
an equation of the form,

m
P Ch EX (11)
where P shellside or tubeside pressure drop
h heat-transfer coefficient
C coefficient depending on geometry
m exponent depending on geometry—always greater than 1.0, and usually about
3.0
EX extra pressure drop from inlet, exit, and pass turnaround momentum losses
See Section 3 for actual pressure drop calculations.
Pressure drop is sensitive to the type of exchanger selected. In the final design it is
attempted, where possible, to define the exchanger geometry so as to use all available pres-
sure drop and thus maximize the heat-transfer coefficient. This procedure is subject to some
2
constraints, however, as follows. The product of density times velocity squared v is limited
2
to minimize the possibility of erosion or tube vibration. A limit often used is v 4000
2
lbm/ft sec . This results in a velocity for liquids in the range of 7–10 ft/sec. For flow
entering the shellside of an exchanger and impacting the tubes, an impingement plate is
2
recommended to prevent erosion if v 1500. Other useful design recommendations may
be found in Ref. 1.
For condensing vapors, pressure drop should be limited to a fraction of the operating
pressure for cases with close temperature approach to prevent severe decrease of the MTD
owing to lowered equilibrium condensing temperature. As a safe ‘‘rule of thumb,’’ the pres-
sure drop for condensing is limited to about 10% of the operating pressure. For other cases,
‘‘reasonable’’ design pressure drops for heat exchangers roughly range from about 5 psi for
gases and boiling liquids to as high as 20 psi for pumped nonboiling liquids.

3 RATING METHODS
After the size and basic geometry of a heat exchanger has been proposed, the individual
heat-transfer coefficients h and h may be calculated based on actual velocities, and the
o
i
required surface may be checked, based on these updated values. The pressure drops are
also checked at this stage. Any inadequacies are adjusted and the exchanger is rechecked.
This process is known as ‘‘rating.’’ Different rating methods are used depending on exchanger
geometry and process type, as covered in the following sections.

3.1 Shell and Tube Single-Phase Exchangers
Before the individual heat-transfer coefficients can be calculated, the heat exchanger tube
geometry, shell diameter, shell type, baffle type, baffle spacing, baffle cut, and number of
tubepasses must be decided. As stated above, lacking other insight, the simplest exchanger—
E-type with segmental baffles—is tried first.

Tube Length and Shell Diameter
For shell and tube exchangers the tube length is normally about 5–8 times the shell diameter.
Tube lengths are usually 8–20 ft long in increments of 2 ft. However, very large size ex-
changers with tube lengths up to 40 ft are more frequently used as economics dictate smaller