Page 371 - Analysis, Synthesis and Design of Chemical Processes, Third Edition

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8. Heat transfer coefficients for estimating purposes, W/m °C (Btu/hr ft °F): water to liquid, 850
(150); condensers, 850 (150); liquid to liquid, 280 (50); liquid to gas, 60 (10); gas to gas 30
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(5); reboiler 1140 (200). Maximum flux in reboiler 31.5 kW/m (10,000 Btu/hr ft ).
When phase changes occur, use a zoned analysis with appropriate coefficient for each zone.
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9. Double-pipe exchanger is competitive at duties requiring 9.3–18.6 m (100–200 ft ).
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10. Compact (plate and fin) exchangers have 1150 m /m (350 ft /ft ), and about 4 times the heat
transfer per cut of shell-and-tube units.
11. Plate and frame exchangers are suited to high-sanitation services and are 25–50% cheaper in
stainless steel construction than shell-and-tube units.
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12. Air coolers: Tubes are 0.75–1.0 in. OD., total finned surface 15–20 m /m (ft /ft bare
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surface), U = 450–570 W/m °C (80–100 Btu/hr ft (bare surface) °F). Minimum approach
temperature = 22°C (40°F). Fan input power = 1.4–3.6 kW/(MJ/h) [2–5 hp/(1000 Btu/hr)].
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13. Fired heaters: radiant rate, 37.6 kW/m (12,000 Btu/hr ft ); convection rate, 12.5 kW/m (4000
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Btu/hr ft ); cold oil tube velocity = 1.8 m/s (6 ft/sec); approximately equal transfer in the two
sections; thermal efficiency 70–90% based on lower heating value; flue gas temperature 140–
195°C (250–350°F) above feed inlet; stack gas temperature 345–510°C (650–950°F).

(Adapted from S. M. Walas, Chemical Process Equipment: Selection and Design, Stoneham, MA:
Butterworth, 1988. Copyright © 1988 by Butterworth Publishers, adapted by permission of Butterworth
Publishers, Stoneham, MA. All rights reserved)

Table 11.12 Heuristics for Thermal Insulation

1. Up to 345°C (650°F), 85% magnesia is used.
2. Up to 870–1040°C (1600–1900°F), a mixture of asbestos and diatomaceous earth is used.
3. Ceramic (refractory) linings at higher temperature.
4. Cryogenic equipment –130°C (–200°F) employs insulation with fine pores of trapped air, e.g.,
Perlite.
5. Optimal thickness varies with temperature: 1.27 cm (0.5 in) at 95°C (200°F), 2.54 cm (1.0 in) at
200°C (400°F), 3.2 cm (1.25 in) at 315°C (600°F).
6. Under windy conditions 12.1 km/h (7.5 miles/hr), 10–20% greater thickness of insulation is
justified.

(Adapted from S. M. Walas, Chemical Process Equipment: Selection and Design, Stoneham, MA:
Butterworth, 1988. Copyright © 1988 by Butterworth Publishers, adapted by permission of Butterworth
Publishers, Stoneham, MA. All rights reserved)

Table 11.13 Heuristics for Towers (Distillation and Gas Absorption)

1. Distillation is usually the most economical method for separating liquids, superior to extraction,
absorption crystallization, or others.
2. For ideal mixtures, relative volatility is the ratio of vapor pressures .
3. Tower operating pressure is most often determined by the temperature of the condensing media,
38–50°C (100–120°F) if cooling water is used, or by the maximum allowable reboiler

temperature to avoid chemical decomposition/degradation.