Page 246 - Applied Process Design For Chemical And Petrochemical Plants Volume III

66131_Ludwig_CH10G 5/30/2001 4:38 PM Page 209

Heat Transfer 209

Table 10-35
Typical Fouling Factors—Cast Iron Cooling Sections

Inside Outside
Service Fouling Fouling
Concentrated sulfuric acid 0.002 —
Ammonia liquor 0.002 —
Clean water 0.001 0.005
Clean oil 0.002 —
Dirty oil 0.005 —
Tar 0.010 —
Dirty water 0.01
Sea water, brackish water 0.01—0.05
Used by permission: Cat. HT-23, National U.S. Radiator Corp. Existence
of company not confirmed (1998). Value for sea water provided by this
author.

Table 10-36
Figure 10-129. MTD correction factors for drip type coolers. (Used by Wall Resistance Factors—Cast Iron Cooling Sections
permission: Bowman, R. A., Mueller, A. C., and Nagle, W.M. Transac-
tions of ASME, No. 62, ©1940. American Society of Mechanical Engi- Metal Thickness, in. Wall Transfer, h w , Btu/hr (ft )(°F)
2
neers. All rights reserved.)
1 1,800
/ 4
3 1,350
/ 8
1 900
/ 2
Used by permission: Cat. HT-23, National U.S. Radiator Corp. Existence
of company not confirmed (1998).
5. Assume a unit for the service or assume a pipe size and
length and determine the number of lengths required.
6. Determine outside film coefficient for spray or drip
81
cooling using the equation of McAdams as presented
by Kern. 70 8. Assume fouling factors. Inside tube factors can be
selected from Table 10-12 or 10-13 or by referring to
1>3
G d Table 10-15. Because the water rate is low over these
h o 65a b (10-205)
D o coolers, they may develop salt crusts, scale, algae, etc.;
therefore, the values of fouling will be high, see Table
2
where h o outside film coefficient, 25%, Btu/hr (ft )(°F) 10-35.
G d W/2L, lb/hr (ft) 9. Determine overall coefficient U in the usual manner.
W lb cooling water/hr flowing over length of tube 1
L length of each pipe in bank, ft U
1 1 1
D o O.D. of pipe, ft r i r o
For most purposes, an estimated value of h o 500 550 is con- h io h w h o
servative.
L w >k 1>h w , wall resistance
For a submerged unit, handle as natural convection on
For cast iron sections, see Tables 10-36 and 10-37.
outside of pipes; values usually range from 40–130
10. Calculate area required:
Btu/hr (ft )(°F) for h o .
2
7. Determine the inside film coefficient using Equation Q
A
10-41 and Figure 10-46 for tube-side heat transfer. If two U 1corrected LMTD2
or more coils are in parallel, be certain that the flow
rate per pipe is used in determining h i . Correct h i to If this area is considerably different than the assumed
outside of tube, giving h io . Note that Figure 10-46 also unit, reassume a new unit and recalculate the
applies to cast iron cooling sections. preceding steps.