Page 223 - Chemical Process Equipment - Selection and Design

8.6. TYPES OF HEAT EXCHANGERS 193

8.6
EXAMPLE Find 11 from the chart. Tube length, L = AJ0.2021 ft.
Sizing an Emhanger with Radial Finned Tubes
A liquid is heated from 150 to 190°F with a gas that goes from 250 ~ x Q A , L .
to 200°F. The duty is 1.25MBtu/hr. The inside film coefficient is
26 1.149 0.59 121.8 602
200, the bare tube outside coefficient is h, = 20 Btu/(hr)(sqft)("F). Steel 60 0.756 0.76 96.5 477
Brass
The tubes are 1 in. OD, the fins are 2 in. high, 0.038 in. thick, and AI 120 0.535 0.86 86.1 426
number 72/ft. The total tube length will be found with fins of steel,
brass, or aluminum:
LMTD = (60 - 50)/1n(60/50) = 54.8,
u5 = 61/20 + 1/200)-l= 18.18.
Fin surface:

A, = 72(2)(~/4)[(2.25'- %)/I441 = 3.191 sqft/ft.
Uncovered tube surface:

A, = (n/12)[1- 72(0.038/12)] ~0.2021 sqft/ft,
A,/Ab = 3.1'91/0.2021 = 15.79,
y, = half-fin thickness = 0.038/2(12) = 0.00158 ft.
Abscissa of .the chart:

x = (re - r,).dm= [(2.25 - 1)/24]~20/0.00158k
= 5.86/<i,
re/rb = 2.25,
A5 = Q/ubaa(I f VAe/Ab)
+
= 1.25(1~16)/a8.18(54.8)(1 15.79~) sq ft.

EXAMPLE 8.7
Pressure Drop on the Tube Side of a Vertical Thermosiphon
Reboiler
Liquid with the properties of water at 5 atm and 307°F is reboiled at
a feed rate of 2800 Ib/(hr)(tube) with 30 wt % vaporization. The
tubes are 0.1 ft ID and 12 ft long. The pressure drop will be figured
at an average vaporization of 15%. me Lockhart-Martinelli,
method will be used, following Example 6.14, and the formulas of
Tables 6.1 and 6.8:

Liquid Vapor
m (Ibihr) 2380 420
p (IbJft hr) 0.45 0.036
p (Ibl'cuft) 57.0 0.172
Re 67340 148544 Average density in reboiler tubes is
f 0.0220 0.0203
AP/b (psi/R) 0.00295 0.0281 2800 = 1.13 lb/cuft.
X2 = 0.0029.5/0.0281= 0.1051, pm = 2380/57 + 420/0.172
c = 20, Required height of liquid in tower above bottom of tube sheet
4: = 1 f 20/X f 1/X2 = 72.21,
(hP/L) two phase = 72.21(0.00295) = 0.2130, pLh = 2.56(144) + 1.13(12),
AP = 0.2130(12) = 2.56 psi, 5.90 ft water. h = 382.2/57 = 6.7 ft.