Page 249 - Chemical Process Equipment - Selection and Design

8.12. INSULATION OF EQUIPMENT 219
EXAMPLE 8,15--(continued) From (Eq. (4),
in the radiant section. Repeat the calculation with an
estimate of 60(E6) T, = 920°F.
HI =: 248 + (77.14 - 60)(E6)/195,394 = 335.7, 28-31.
TI = 542, LMTD = 735.6
T, = 100 + 0.5(542 + 900) = 821,
T, = 1550.5, mean gas film temp is
F = 0.16498,
4 = 0.5(400 + 547 + 735.6) = 841.3.
QR = 3190(0.6498)(28,727) = 59.55(E6).
Since G = 0.325 lb/(sec)(sqft),
Interpolating,
? 8, T, V, = 5.6 Btu/(hr)(sqft)("F) [(Eq. (1091,
~Bss"PII.3Kl Qcalsd Q/A
57.86 565 832.5 1553.7 59.43 17'64(E6) 4282 sqft,
60.00 642 821 1550.5 59.55 = 735.60 =
Interpolation [547 1551.2 59.50 99001
4282 = 64.1 bare tubes
1.7344(38.5)
26-27.
Q,,,, = (7'7.14 - 59.50)(E6) or 16 rows of 4 tubes each. Spacing the same as of the shield
= 17.54(E6). tubes.
Beyond the first two rows, extended surfaces can be
Fraction lost in stack gas installed.
Qs/Q, = 1 - 0.02 - 0.75 = 0.23. Total rows = 2 + 1412 = 9.

bank, and is called the equivalent cold plane area. Evaluation of the is found with the trial calculation described in Steps 22-23 of Table
exchange factox F is explained in item 9 of Table 8.16. It depends 8.17 and may utilize the computer program of Table 8.18.
on the emissivity of the gas and the ratio of refractory area A,,, to
the equivalent cold plane area aA,. In turn, A,,, =A -A,, where 8.12. INSULATION OF EQUIPMENT
A is the area of the inside walls, roof, and floor that are covered by
refractory. Equipment at high or low temperatures is insulated to conserve
In the convection zone of the heater, some heat also is trans- energy, to keep process conditions from fluctuating with ambient
fenred by direct radiation and reflection. The several contribu- conditions, and to protect personnel who have occasion to approach
tions to overall heat transfer specifically in the convection zone of fired the equipment. A measure of protection of the equipment metal
heaters were coirrelated by Monrad [Ind. Eng. Chern. 24,505 (1932)]. against atmospheric corrosion also may be a benefit. Application of
The combined effects are approximated by item 10 of Table 8.16, insulation is a skilled trade. Its cost runs to 8-9% of purchased
which is adequate for estimating purposes. The relation depends on equipment cost.
the temperature of the gas film which is taken to be the sum of the In figuring heat transfer between equipment and surroundings,
average process temperature and one-half of the log mean it is adequate to take account of the resistances of only the
temperature difference between process and flue gas over the entire insulation and the outside film. Coefficients of natural convection
tube bank. The temperature of the gas entering the convection zone are in Table 8.9 and properties of insulating materials at several

EXAMPLE 8.16 1 + 0.0722vm
-
Applieaticaln of the Wilson-Lobo-Hottel Equation - 1.8327
In the case of Example 8.15, 25% excess air was employed,
corresponding to 19.0 lb/air/lb fuel, the heat release was :. Q, = 95.82(106) Btu/hr,
Qf = 102.86(10') Btu/hr, and cuA, = 3036. The effect will be found
of changing the excess air to 10% (16.721b air/lb fuel) on the which is the heat release with 10% excess air.
amount of fuel to be fired while maintaining the same heat
absorption.
Ratioing Eq. (8.45) to yield the ratio of the releases at the two With 25% excess air, Q/Q, = 111.8327 = 0.5456,
conditions, With 10% excess air, Q/Q, = 0.5456(102.86/95.82) = 0.5857,
Qf2 .= 1 + (16.72/4200)a,/3036 which shows that approximately 7% more of the released heat is
102.86(106:) 1 + (19.O/4200)'d102.86(1O6)/3036 absorbed when the excess air is cut from 25% down io 10%.