Page 308 - Applied Process Design For Chemical And Petrochemical Plants Volume III

66131_Ludwig_CH10G 5/30/2001 4:41 PM Page 267

Heat Transfer 267

1scfm211.082 film coefficient is greater, thus “approaching” a balance for
C min C air
R (10-294) the two sides. The film coefficients on the tube side are cal-
3Q>1T 1 T 2 24
C max C hot
culated in the same manner as described in an earlier topic
for conventional exchangers. Mukherjee 265 suggests pres-
1FV2L W11.0821T 1 T 2 2>Q
sure drop ranges in tubes:
QR
W width of exchanger, ft = (10-295)
1.081FV2L1T 1 T 2 2
a. For gases and condensers, allowable pressure drop is
AU AU nNaWLU
Nta (10-296) 0.7–2.84 psi.
C min C air 1.08WLFV
Lower pressure systems require lower pressure drops.
nNa b. For liquids, allowable pressure drop is 7.11–9.95 psi,

1.081FV21r i r air r f r m 2 except when viscosity is high requiring higher pressure
drops. The air-side calculations require the specific data
T 1 T 2
t 2 t 1 (10-297)
R of the manufacturer and can be estimated or
approximated only by some published data.
3
Air flow is expressed as standard ft per min (scfm). It is
determined by the effective width of the exchanger, W, times C air C cold Q/ t Q/(t 2 t 1 )
air side heat capacity rate, Btu/(hr) (°F) (10-298)
the length, L, times the face velocity, FV, in standard ft per
1.08(FV)(L)(W)
min (sfm). From reference 251:
C tube C hot Q/ T Q/(T 1 T 2 )
FV(ft/min) Rows of Tubes tube-side heat capacity rate Btu/(hr)(°F) (10-299)
Mc p
650 4 C min minimum heat capacity rate, Btu/(hr) (°F)
600 5 C max maximum heat capacity rate, Btu/(hr) (°F)
550 6 CMTD corrected mean temperature difference
400–450 8–10 °F(LMTD), °F
E exchanger thermal effectiveness, dimensionless
Because it is not practical for the design engineer to
expect to specify all fabrication features (including size, C hot 1T 1 T 2 2
number of tubes, etc.) the foregoing provides an exposure ;
C min 1T 1 t 1 2
to the topic, but relies on contact with a competent
design/manufacturing firm for the final details.
Note, see previous information; C hot C tube (10-300)
C cold 1t 2 t 1 2
where (from reference 251 by permission) (10-301)
a heat transfer surface area per unit length of tube, C min 1T 1 t 1 2
2
ft /ft
A total exchanger bare tube heat transfer surface, ft 2 F MTD correction factor, dimensionless
c p specific heat, Btu/(lb)(°F) FA face area, ft 2
t air temperature, °F FV standard air face velocity, sfm
2
T hot fluid temperature, °F G mass velocity, lb/(sec) (ft )
U overall heat transfer coefficient (rate), h individual heat transfer coefficient,
2
2
Btu/(hr) (ft )(°F) Btu/(hr)(ft )(°F)
k parameter nNa/[(1.08)(FV)(I/U)]
LMTD log mean temperature difference, °F
Subscripts
M mass flow rate, lb/hr
air air side
Ntu number of heat transfer units, dimensionless
cold cold fluid air
N number tubes/row in direction of air flow
f tube-side fouling
n number tubes/row, per ft of exchanger width, 1/ft
hot hot fluid tube-side fluid
Q total exchanger heat load (duty), Btu/hr
i inside tube
R C min /C max heat capacity ratio, dimensionless
max maximum
min minimum
m tube metal Mean Temperature Difference
1 inlet
2 outlet These units are pure cross-flow and require the use of spe-
cific data not found in the TEMA Standards, 266 but are avail-
The lower outside film coefficient (air side) makes use of able in references 251 and 206. See Figures 10-187A,
finned tubes beneficially, while the inside (usually liquid) 10-187B, and 10-187C.