Page 431 - Handbook of Energy Engineering Calculations

lb/h)(specific heat) (1.8 conversion factor). Or, temperature decrease =
(2,391,120)/(307,500)(0.72)(1.8) = 6°C (10.8°F). Then, the shellside outlet
temperature = 105 – 6 = 99°C (210.2°F). Then, the LMTD = (54 – 15)/ln
(54/15) = 30.4°C (86.7°F) = Δ T .
m

3. Make a first-trial calculation of the surface area of this exchanger
For a first-trial calculation, the approximate surface can be calculated using
2
an assumed overall heat-transfer coefficient, U, of 250 Btu/(h) (ft ) (°F) (44.1
2
W/m °C). The assumed value of U can be obtained from tabulations in texts
and handbooks and is used only to estimate the approximate size for a first
trial:
2
A = 2,391,000/(250× 30.4×1.8) = 175ft (16.3m 2
Since the given conditions specify a maximum tube length of 12 ft and a
minimum tube diameter of 5/8 in, the number of tubes required is:

n = 175(12×0.1636) = 89 tubes
and the approximate shell diameter will be:
D = 1.75 ×0.625 ×89 0.47 = 9 in (228.6mm)
a
With the exception of baffle spacing, all preliminary calculations have
been made for the quantities to be substituted into the dimensional equations.
For the first trial, we may start with a baffle spacing equal to about half the

shell diameter. After calculating the shellside pressure drop, we may adjust
the baffle spacing. Also, it is advisable to check the Reynolds number on the
tubeside to confirm that the proper equations are being used.

4. Find the maximum number of tubes for this heat exchanger
To find the maximum number of tubes (n max ) in parallel that still permits

flow in the turbulent region (N = 12,600), a convenient relationship is n max
Re
= W /(2d Z ). In this example, n max = 307.5/(2× 0.495 × 1.7) = 183. For any
i
i
i
number of tubes less than 183 tubes in parallel, we are in the turbulent range
and can use Eq. (1)
From Table 6, the appropriate expressions for rating are: Eq. (1) for
tubeside, Eq. (11) for shell-side, Eq. (18) for tube wall, and Eq. (19) for
fouling. Eq. (21) and (25), respectively, are used for tubeside and shellside

pressure drops.