13
Rules of Thumb-Summary

13.0 Introduction

An engineering Rule of Thumb is an outright statement regarding suitable sizes or performance of equipment that avoids all requirements for extended calculations. These are safely applied by engineers who are substantially familiar with the topics. However, such rules should be of value for approximate design and cost estimation, and should not provide the inexperienced engineer with perspective and a foundation where detailed and computer-aided results can be determined.

Experienced engineers often know where to find information and how to make accurate calculations; they also retain a minimum body of information in mind, which is made up largely of shortcuts and heuristics. The compilation below may fit into such a minimum body of information that boosts to the memory or extension in some instances into less often encountered areas.

COMPRESSORS, FANS, BLOWERS, AND VACUUM PUMPS

1. 1. Fans are used to raise the pressure by about 3% [12 in. (30 cm) water], blowers raise to less than 2.75 barg (40 psig), and compressors to higher pressures, although the blower range is commonly included in the compressor range.
2. 2. For vacuum pumps use the following: Reciprocating piston type down to 133.3 Pa (1 torr) Rotary piston type down to 0.133 Pa (0.001 torr) Two lobe rotary type down to 0.0133 Pa (0.0001 torr) Steam jet ejectors 1 stage down to 13.3 k Pa (100 torr) 3 stage down to 133.3 Pa (1 torr) 5 stage down to 6.7 Pa (0.05 torr)
3. 3. A three-stage ejector needs 100 kg steam/kg air to maintain a pressure of 133.3 Pa (1 torr).
4. 4. In-leakage of air to evacuated equipment depends on the absolute pressure (torr) and the volume of the equipment, V in m3 (ft3), according to W = kV2/3 kg/h (lb/h), with k = 0.98 (0.2) when P > 90 torr, k = 0.39 (0.08) when P is between 0.4 and 2.67 kPa (3 and 20 torr), and k = 0.12 (0.025) at p less than 133.3 Pa (1 torr).
5. 5. Theoretical adiabatic horsepower

   where T1 is inlet temperature in Rankine, R = °F + 460 and a = (k - 1)/k, k = Cp/Cv. Theoretical reversible adiabatic power = m?1RT1[({P2/P1}a - 1)]/a, where T1 is inlet temperature, R = Gas Constant, ?1 = compressibility factor, m = molar flow rate, a = (k - 1)/k and k = Cp/Cv. Values of °R = 8.314 J/mol K = 1.987 Btu/lb mol R = 0.7302 atm ft3/lb mol° R.

6. 6. Outlet temperature for reversible adiabatic process
7. 7. To compress air from 37.8° C (100°F), k = 1.4, compression ratio = 3, theoretical power required = 62 hp/million ft3/day, outlet temperature 152.2°C (306°F).
8. 8. Exit temperature should not exceed 167-204° C (350-400° F); for diatomic gases (Cp/Cv = 1.4), this corresponds to a compression ratio of about 4.
9. 9. Compression ratio should be about the same in each stage of a multistage unit, ratio = (Pn/P1)1/n, with n stages.
10. 10. Efficiencies of reciprocating compressors: 65% at compression ratio of 1.5, 75% at 2.0, and 80-85% at 3-6.
11. 11. Efficiencies of large centrifugal compressors, 2.83-47.2m3/s (6000-100,000 acfm) at suction, are 76-78%.
12. 12. Rotary compressors have efficiencies of 70%, except liquid liner type which have 50%.

CONVEYORS FOR PARTICULATE SOLIDS

1. 1. Screw conveyors are suited to transport of even sticky and abrasive solids up inclines of 20° or so. They are limited to distances of 3.81 m (150 ft) or so because of shaft torque strength. A 304.8 mm (12 in.) diameter conveyor can handle 28.3-84.95 m3/h (1000-3000 ft3/h), at speeds ranging from 40 to 60 rpm.
2. 2. Belt conveyors are for high capacity and long distances (a mile or more, but only several hundred feet in a plant), up inclines of 30° maximum. A 609.6-mm (24 in.) wide belt can carry 84.95 m3/h (3000 ft3/h) at a speed of 0.508 m/s (100 ft/min), but speeds up to 3.048 m/s (600 ft/min) are suited to some materials. Power consumption is relatively low.
3. 3. Bucket elevators are suited to vertical transport of sticky and abrasive materials. With 508 × 508-mm (20 × 20-in.) buckets, capacity can reach 28.3 m3/h (1000 ft3/h) at a speed of 0.508 m/s (100 ft/min), but speeds up to 1.524 m/s (300 ft/min) are used.
4. 4. Drag-type conveyors (Redler) are suited to short distances in any direction and are completely enclosed. Units range in size from 19.4 × 10-4 to 122.6 × 10-4 m2 (3-19 in.2) and may travel from 0.15 m/s (30 ft/min) (fly ash) to 1.27 m/s (250 ft/min) (grains). Power requirements are high.
5. 5. Pneumatic conveyors are for high capacity, short distance (122 m (400 ft)) transport simultaneously from several sources to several destinations. Either vacuum or low pressure 0.4-0.8 barg (6-12 psig) is used with a range of air velocities from 10.7 to 36.6 m/s (35-120 ft/s); depending on the material and pressure and air requirements, 0.03-0.2 m3/m3 (1-7 ft3/ft3) of solid is transferred.

COOLING TOWERS

1. 1. Water in contact with air under adiabatic conditions eventually cools to the wet bulb temperature.
2. 2. In commercial units, 90% of saturation of the air is feasible.
3. 3. Relative cooling tower size is sensitive to the difference between the exit and the wet bulb temperatures: ?T, °F 5 15 25 Relative volume 2.4 1.0 0.55
4. 4. Tower fill is of a highly open structure so as to minimize pressure drop, which is in standard practice a maximum of 497.6 Pa (2 in. of water).
5. 5. Water circulation rate is 48.9-195.7 L/min m2 (1-4 gpm/ft2) and air rate is 6344-8784 kg/h m2 (1300-1800 lb/h ft2) or 1.52-2.03 m/s (300-400 ft/min).
6. 6. Chimney-assisted natural draft towers are hyperboloidally shaped because they have greater strength for a given thickness; a tower 76.2 m (250 ft) high has concrete walls 127-152.4 mm (5-6 in.) thick. The enlarge cross section at the top aids in dispersion of exit humid air into the atmosphere.
7. 7. Countercurrent-induced draft towers are the most common in process industries. They are able to cool water within 2°F of the wet bulb.
8. 8. Evaporation losses are 1% of the circulation for every 10°F of cooling range. Windage or drift losses of mechanical draft towers are 0.1-0.3% Blowdown of 2.5-3.0% of the circulation is necessary to prevent excessive salt buildup.

CRYSTALLIZATION FROM SOLUTION

1. 1. Complete recovery of dissolved solids is obtainable by evaporation, but only to the eutectic composition by chilling. Recovery by melt crystallization also is limited by the eutectic composition.
2. 2. Growth rates and ultimate sizes of crystals are controlled by limiting the extent of supersaturation at any time.
3. 3. The ratio S = C/Csat of prevailing concentration to saturation concentration is kept near the range 1.02-1.05.
4. 4. In crystallization by chilling, the temperature of the solution is kept almost 1-2°F below the saturation temperature at the prevailing concentration.
5. 5. Growth rates of crystals under satisfactory conditions are in the range of 0.1-0.8 mm/h. The growth rates are approximately the same in all directions.
6. 6. Growth rates are influenced greatly by the presence of impurities and of certain specific additives, which vary from case to case.

DISINTEGRATION

1. 1. Percentages of material greater than 50% of the maximum size are about 50% from rolls, 15% from tumbling mills, and 5% from closed-circuit ball mills.
2. 2. Closed-circuit grinding employs external size classification and return of oversize for regrinding. The rules of pneumatic conveying are applied to the design of air classifiers. Closed circuit is most common with ball and roller mills.
3. 3. Jaw crushers take lumps of several feet in diameter to 102 mm (4 in.). Stroke rates are 100-300/min. The average feed is subjected to 8-10 strokes before it becomes small enough to escape. Gyratory crushers are suited to slabby feeds and makes a more rounded product.
4. 4. Roll crushers are made either smooth or with teeth. A 610-mm (24-in.) toothed roll can accept lumps of 356 mm (14 in.) diameter. Smooth rolls affect reduction ratios up to about 4. Speeds are 50-90 rpm. Capacity is about 25% of the maximum, corresponding to a continuous ribbon of material passing through the rolls.
5. 5. Hammer mills beat the material until it is small enough to pass through the screen at the bottom of the casing....