and are suited to a range of 12 mm–50 µm.5 5. Air classification is preferred for fine sizes because screens of 150 mesh and finer are fragile and slow.
6 6. Wet classifiers mostly are used to make two product size ranges, oversize and undersize, with a break commonly in the range between 28 and 200 mesh. A rake classifier operates at about 9 strokes/min when making separation at 200 mesh and 32 strokes/min at 28 mesh. Solids content is not critical, and that of the overflow may be 2–20% or more.
7 7. Hydrocyclones handle up to 600 ft3/min and can remove particles in the range of 300–5 µm from dilute suspensions. In one case, a 20-in. diameter unit had a capacity of 1000 gpm with a pressure drop of 5 psi and a cutoff between 50 and 150 µm.
UTILITIES, COMMON SPECIFICATIONS
1 1. Steam: 1–2 bar (15–30 psig), 121–135°C (250–275°F); 10 barg (150 psig), 186°C (366°F); 27.6 barg (400 psig), 231°C (448°F); 41.3 barg (600 psig), 252°C (488°F) or with 55–85°C (100–150°F) superheat.
2 2. Cooling water: For design of cooling tower use, supply at 27–32°C (80–90°F); from cooling tower, return at 45–52°C (115–125°F); return seawater at 43°C (110°F); return tempered water or steam condensate above 52°C (125°F).
3 3. Cooling air supply at 29–35°C (85–95°F); temperature approach to process, 22°C (40°F).
4 4. Compressed air at 3.1 (45), 10.3 (150), 20.6 (300), or 30.9 barg (450 psig) levels.
5 5. Instrument air at 3.1 barg (45 psig), −18°C (0°F) dew point.
6 6. Fuels: gas of 37,200 kJ/m3 (1000 Btu/SCF) at 0.35–0.69 barg (5–10 psig), or up to 1.73 barg (25 psig) for some types of burners; liquid at 39.8 GJ/m3 (6 million British Thermal unit per barrel).
7 7. Heat-transfer fluids: petroleum oils below 315°C (600°F) Dowtherms below 400°C (750°F), fused salts below 600°C (1100°F), and direct fire or electricity above 232°C (450°F).
8 8. Electricity: 0.75–74.7 kW (1–100 hp), 220–550 V; 149–1864 kW (200–2500 hp), 2300–4000 V.
VESSELS (DRUMS)
1 1. Drums are relatively small vessels to provide surge capacity or separation of entrained phases.
2 2. Liquid drums are usually horizontal.
3 3. Gas/liquid phase separators are usually vertical.
4 4. Optimum length/diameter = 3, but the range 2.5–5.0 is common.
5 5. Holdup time is 5 min half-full for reflux drums and gas/liquid separators, 5–10 min for a product feeding another tower.
6 6. In drums feeding a furnace, 30 min half-full drum is allowed.
7 7. Knockout drums placed ahead of compressors should hold no less than 10 times the liquid volume passing through per minute.
8 8. Liquid/liquid separators are designed for a setting velocity of 0.85–1.27 mm/s (2–3 in./min).
9 9. Gas velocity in gas/liquid separators, m/s (ft/s), with k = 0.11 (0.35) for systems with a mesh deentrainer and k = 0.0305 (0.1) without a mesh deentrainer.
10 10. Entrainment removal of 99% is attained with 102–305 mm (4–12 in.) mesh pad thickness; 152.5 mm (6 in.) thickness is popular.
11 11. For vertical pads, the value of the coefficient in step 9 is reduced by a factor of 2/3.
12 12. Good performance can be expected at velocities of 30–100% of those calculated with the given k; 75% is popular.
13 13. Disengaging spaces of 152–457 mm (6–18 in.) ahead of the pad and 305 mm (12 in.) above the pad are suitable.
14 14. Cyclone separators can be designed for 95% collection of 5-µm particles, but usually only droplets greater than 50 µm need be removed.
VESSEL (PRESSURE)
1 1. Design temperature between −30 and 345°C is 25°C (−20° F and 650°F if 50°F) above maximum operating temperature; higher safety margins are used outside the given temperature range.
2 2. The design pressure is 10% or 0.69–1.7 bar (10–25 psi) over the maximum operating pressure, which-ever is greater. The maximum operating pressure, in turn, is taken as 1.7 bar (25 psi) above the normal operation.
3 3. Design pressures of vessels operating at 0–0.69 barg (0–10 psig) and 95–540°C (200–1000°F) are 2.76 barg (40 psig).
4 4. For vacuum operation, design pressures are 1 barg (15 psig) and full vacuum.
5 5. Minimum wall thickness for rigidity: 6.4 mm (0.25 in.) for 1.07 m (42 in.) diameter and under, 8.1 mm (0.32 in.) for 1.07–1.52 m (42–60 in.) diameter, and 9.7 mm (0.38 in.) for over 1.52 m (60 in.) diameter.
6 6. Corrosion allowance 8.9 mm (0.35 in.) for known corrosive conditions, 3.8 mm (0.15 in.) for noncorrosive streams, and 1.5 mm (0.06 in.) for steam drums and air receivers.
7 7. Allowable working stresses are one-fourth the ultimate strength of the material.
8 8. Maximum allowable stress depends sharply on temperatureTemperature (°F)−20–6507508501000(°C)−30–345400455540Low-alloy steel, SA 203 (psi)18,75915,65095502500(bar)12901,070686273Type 302 stainless (spi)18,75018,75015,9506250(bar)129012901100431
VESSELS (STORAGE TANKS)
1 1. For less than 3.8 m3 (1000 gal.), use vertical tanks on legs.
2 2. For 3.8–38 m3 (1000–10,000 gal.), use horizontal tanks on concrete supports.
3 3. Beyond 38 m3 (10,000 gal.) use vertical tanks on concrete foundations.
4 4. Liquids subject to breathing losses may be stored in tanks with floating or expansion roofs for conservation.
5 5. Freeboard is 15% below 1.9 m3 (500 gal.) and 10% above 1.9 m3 (500 gal.) capacity.
6 6. A 30-day capacity often is specified for raw materials and products but depends on connecting transportation equipment schedules.
7 7. Capacities of storage tanks are at least 1.5 times the size of connecting transportation equipment; for instance, 28.4-m3 (7500 gal.) tanker trucks, 130-m3 (34,500 gal.) rail cars, and virtually unlimited barge and tanker capacities.
Source: The above mentioned rules of thumb have been adapted from Walas, S.M., Chemical Process Equipment: Selection and Design, copyright 1988 with permission from Elsevier, all rights reserved.
Physical Properties Heuristics.
| Units | Liquids | Liquids | Gases | Gases |
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