more multiples of the minimum fluidizing velocity. In reactors, the entrained material is recovered with cyclones and returned to process. In driers, the fine particles dry most quickly so the entrained material need not be recycled.
HEAT EXCHANGERS
1 1. For conservative estimate set F = 0.9 for shell and tube exchangers with no phase changes, q = UAF∆Tlm. When ∆T at exchanger ends differ greatly then check F, reconfigure if F is less than 0.85.
2 2. Take true countercurrent flow in a shell-and-tube exchanger as a basis.
3 3. Standard tubes are 19.0 mm (3/4 in.) outer diameter (OD), 25.4 mm (1 in.) triangular spacing, 4.9 m (16 ft) long.A shell of 300 mm (1 ft) diameter accommodates 9.3 m2 (100 ft2);600 mm (2 ft) diameter accommodates 37.2 m2 (400 ft2);900 mm (3 ft) diameter accommodates 102 m2 (1100 ft2).
4 4. Tube side is for corrosive, fouling, scaling, and high-pressure fluids.
5 5. Shell side is for viscous and condensing fluids.
6 6. Pressure drops are 0.1 bar (1.5 psi) for boiling and 0.2–0.62 bar (3–9 psi) for other services.
7 7. Minimum temperature approach is 10°C (20°F) for fluids and 5°C (10°F) for refrigerants.
8 8. Cooling water inlet temperature is 30°C (90°F), maximum outlet temperature 49°C (120°F).
9 9. Heat-transfer coefficients for estimating purposes, W/m2°C (Btu/h-ft2-°F): water to liquid, 850 (150); condensers, 850 (150); liquid to liquid, 280 (50); liquid to gas, 60 (10); gas to gas, 30 (5); and reboiler 1140 (200). Maximum flux in reboiler is 31.5 kW/m2 (10,000 Btu/h-ft2). When phase changes occur, use a zoned analysis with appropriate coefficient for each zone.
10 10. Double-pipe exchanger is competitive at duties requiring 9.3–18.6 m2 (100–200 ft2).
11 11. Compact (plate and fin) exchangers have 1150 m2/m3 (350 ft2/ft3), and about four times the heat transfer per cut of shell-and-tube units.
12 12. Plate and frame exchangers are suited to high sanitation services and are 25–50% cheaper in stainless steel construction than shell-and-tube units.
13 13. Air coolers: Tubes are 0.75–1.00 in. OD., total finned surface 15–20 ft2/ft2 bare surface, U = 450–570 W/m2°C (80–100 Btu/h-ft2 (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/h)].
14 14. Fired heaters: radiant rate, 37.6 kW/m2 (12,000 Btu/h-ft2), convection rate, 12.5 kW/m2 (4000 Btu/h-ft2); cold oil tube velocity = 1.8 m/s (6 ft/s); approximately equal heat transfer in the two sections; thermal efficiency, 70–75%; flue gas temperature, 140–195°C (250–350°F) above feed inlet; and stack gas temperature, 345–510°C (650–950°F).
INSULATION
1 1. Up to 345°C (650°F), 85% magnesia is used.
2 2. Up to 870–1040°C (1600–1900°F), a mixture of asbestos and diatomaceous earth is used.
3 3. Ceramic refractories at higher temperatures.
4 4. Cryogenic equipment −130°C (−200°F) employs insulations with fine pores of trapped air, for example, PerliteTM.
5 5. Optimum thickness varies with temperature: 12.7 mm (0.5 in.) at 95°C (200°F), 25.4 mm (1.0 in.) at 200°C (400°F), 32 mm (1.25 in.) at 315°C (600°F).
6 6. Under windy conditions, 12.1 km/h (7.5 miles/h), 10–20% greater thickness of insulation is justified.
MIXING AND AGITATION
1 1. Mild agitation is obtained by circulating the liquid with an impeller at superficial velocities of 30.48–60.9 mm/s (0.1–0.2 ft/s), and intense agitation at 213.4–304.8 mm/s (0.7–1.0 ft/s).
2 2. Intensities of agitation with impellers in baffled tanks are measured by power input, hp/1000 gal., and impeller tip speeds:Operationhp/1000 gal.Tip speed (ft/min)Tip speed (m/s)Blending0.2–0.5Homogeneous reaction0.5–1.57.5–100.038–0.051Reaction with heat transfer1.5–5.010–150.051–0.076Liquid–liquid mixtures515–200.076–0.10Liquid–gas mixtures5–1015–200.076–0.10Slurries10
3 3. Proportions of a stirred tank relative to the diameter D: liquid level = D; turbine impeller diameter = D/3; impeller level above bottom = D/3; impeller blade width = D/15; four vertical baffles with width = D/10.
4 4. Propellers are made with a maximum of 457.2-mm (18-in.) turbine impellers to 2.74 m (9 ft).
5 5. Gas bubbles sparged at the bottom of the vessel will result in mild agitation at a superficial gas velocity of 0.0051 m/s (1 ft/min), severe agitation at 0.02 m/s (4 ft/min).
6 6. Suspension of solids with a settling velocity of 0.009 m/s (0.03 ft/s) is accomplished with either turbine or propeller impellers, but when the settling velocity is above 0.05 m/s (0.15 ft/s) intense agitation with a propeller is needed.
7 7. Power to drive a mixture of a gas and a liquid can be 25–50% less than the power to drive the liquid alone.
8 8. In-line blenders are adequate when a second contact time is sufficient, with power inputs of 0.1–0.2 hp/gal.
PARTICLE SIZE ENLARGEMENT
1 1. The chief methods of particle size enlargement are compression into a mold, extrusion through a die followed by cutting or breaking to size, globulation of molten material followed by solidification, agglomeration under tumbling or otherwise agitated conditions with or without binding agents.
2 2. Rotating drum granulators have length-to-diameter ratios of 2–3, speeds 10–20 rpm, pitch as much as 10°. Size is controlled by speed, residence time, and amount of binder; 2–5 mm diameter is common.
3 3. Rotary disk granulators produce a more nearly uniform product than drum granulators: fertilizer, 1.5–3.5 mm diameter; iron ore 10–25 mm diameter.
4 4. Roll compacting and briquetting is done with rolls ranging from 130 mm diameter by 50 mm wide to 910 mm diameter by 550 mm wide. Extrudates are made 1–10 mm thick and are broken down to size for any needed processing, such as feed to tableting machines or to dryers.
5 5. Tablets are made in rotary compression machines that convert powders and granules into uniform sizes. The usual maximum diameter is about 38.1 mm (1.5 in.), but special sizes up to 101.6 mm (4 in.) diameter are possible. Machines operate at 100 rpm or so and make up to 10,000 tablets/min.
6 6. Extruders make pellets by forcing powders, pastes, and melts through a die followed by cutting. A 203.2-mm (8-in.) screw has a capacity of 907.2 kg/h (2000 lb/h) of molten plastic and is able to extrude tubing at 0.76–1.52 m/s (150–300 ft/min) and to cut it into sizes as small as washers at 8000/min. Ring pellet extrusion mills have hole diameters of 1.6–32 mm. Production rates are in the range of 30–200 lb/h-hp.
7 7. Prilling towers convert molten materials into droplets and allow them to solidify in contact with an air stream. Towers as high as 60 m (196.9 ft) are used. Economically the process becomes competitive with other granulation processes when a capacity of 200–400 tons/day is reached. Ammonium nitrate prills, for example, are 1.6–3.5 mm diameter
