Page 306 - Fluid mechanics, heat transfer, and mass transfer

HEAT EXCHANGERS 287

for a given shell diameter, tube sheet becomes TABLE 10.3 Maximum Unsupported Span for the Tubes
too weak.
Maximum Unsupported
- Access for maintenance of tube bundles be- Span (mm)
comes more difficult.
. What is the recommended range of optimum tube pitch Approximate O.D. Group A Group B
to tube diameter ratio for conversion of pressure drop to of Tube (mm) Materials Materials
heat transfer? 19 1520 1321
25 1880 1626
& Pressure drop–heat transfer relationships: Higher
32 2240 1930
turbulence is achieved at the expense of higher 38 2540 2210
pressure drops. 50 3175 2794
& Increased turbulence increases heat transfer rates.
Group A: Carbon and high alloy steel, low alloy steel, nickel–copper, nickel,
& In the design of exchangers, these two variables are to nickel–chromium–iron. Group B: Aluminum and aluminum alloys, copper
be optimized in order to achieve the best possible and copper alloys, titanium and zirconium.
energy economy in terms of heat transferred and
energy expended on pumping to overcome pressure
drops. ➢ To enable a desirable velocity to be maintained for
the shell side fluid.
& The optimum tube pitch to tube diameter ratio for
conversion of pressure drop to heat transfer is typ- ➢ To increase turbulence and reduce stagnant pock-
ically 1.25–1.35 for turbulent flow and around 1.4 for ets in the exchanger.
laminar flow. . Show by means of a diagram how a baffled shell and
. What are the reasons for not increasing tube pitch to tube heat exchanger look like.
reduce pressure drop? & Figure 10.18 is a cut section view of a baffled shell
& There are two reasons: and tube heat exchanger.
(i) It increases shell diameter that is the single . Why is it necessary to use baffles in a condenser?
variable for increased equipment costs. & In a condenser, necessity to increase turbulence on
(ii) Reducing pressure drop by modifying the baffle the condensing side is not there as heat transfer
spacing, baffle cut, or shell type will result in a coefficients for condensing vapors are quite large.
cheaper design. But it will be necessary to support tubes.
. Name different types of baffles used in a heat
exchanger.
10.1.2 Baffles
& (i) Segmental, (ii) disk and doughnut, (iii) orifice, and
(iv) rod baffles.
. What are the functions of baffles used in a heat
exchanger? & Segmental, disk and doughnut, and orifice baffles are
plate-type baffles.
& First Function:
➢ To support tubes.
➢ Must be supported at intervals of not more than
1.5 m (5 ft) depending on diameter and material.
➢ Shorter intervals where flow-induced vibrations
occur.
➢ To prevent mechanical vibrations, sagging that
results in tubes touching each other and giving
rise to leakages and failures especially near tube
sheets.
➢ Supporting distances depend on outside diameter
of the tubes and materials of construction as
illustrated in Table 10.3.
& Second Function:
➢ To direct flow through the shell in the desired FIGURE 10.18 Photograph of a cutaway of a baffled shell and
pattern. tube heat exchanger.