Rules Of Thumb : Filtration

1. Processes are classified by their rate of cake buildup in a laboratory vacuum leaf filter: rapid, 0.1–10.0 cm/sec; medium, 0.1–10.0 cm/min; slow, 0.1–10.0 cm/hr.
2. The selection of a filtration method depends partly on which phase is the valuable one. For liquid phase being the valuable one, filter presses, sand filters, and pressure filters are suitable. If the solid phase is desired, vacuum rotary vacuum filters are
   desirable.
3. Continuous filtration should not be attempted if 1/8 in. cake thickness cannot be formed in less than 5 min.
4. Rapid filtering is accomplished with belts, top feed drums, or pusher-type centrifuges.
5. Medium rate filtering is accomplished with vacuum drums or disks or peeler-type centrifuges.
6. Slow filtering slurries are handled in pressure filters or sedimenting centrifuges.
7. Clarification with negligible cake buildup is accomplished with cartridges, precoat drums, or sand filters.
8. Laboratory tests are advisable when the filtering surface is expected to be more than a few square meters, when cake washing is critical, when cake drying may be a problem, or
   when precoating may be needed.
9. For finely ground ores and minerals, rotary drum filtration rates may be 1500 lb/(day)(sqft), at 20 rev/hr and 18–25 in. Hg vacuum.
10. Coarse solids and crystals may be filtered by rotary drum filters at rates of 6000 lb/(day)(sqft) at 20 rev/hr, 2–6 in. Hg vacuum.
11. Cartridge filters are used as final units to clarify a low solid concentration stream. For slurries where excellent cake washing is required, horizontal filters are used. Rotary disk filters are for separations where efficient cake washing is not essential. Rotary drum filters are used in many liquid- solid separations and precoat units capable of producing
    clear effluent streams. In applications where flexibility of design and operation are required, plate-and-frame filters are used.

Rules Of Thumb : Extraction, Liquid–Liquid

1. The dispersed phase should be the one that has the higher volumetric rate except in equipment subject to backmixing where it should be the one with the smaller volumetric rate. It should be the phase that wets the material of construction less well. Since the holdup of continuous phase usually is greater, that phase should be made up of the less expensive or less hazardous material.
2. Although theory is favorable for the application of reflux to extraction columns, there are very few commercial applications.
3. Mixer–settler arrangements are limited to at most five stages. Mixing is accomplished with rotating impellers or circulating pumps. Settlers are designed on the assumption that droplet sizes are about 150 mm dia. In open vessels, residence times of 30–60 min or superficial velocities of 0.5–1.5 ft/min are provided in settlers. Extraction stage efficiencies commonly are taken as 80%.
4. Spray towers even 20–40 ft high cannot be depended on to function as more than a single stage.
5. Packed towers are employed when 5–10 stages suffice. Pall rings of 1–1.5 in. size are best. Dispersed phase loadings should not exceed 25 gal/(min) (sqft). HETS of 5–10 ft may be realizable. The dispersed phase must be redistributed every 5–7 ft. Packed towers are
   not satisfactory when the surface tension is more than 10 dyn/cm.
6. Sieve tray towers have holes of only 3–8 mm dia. Velocities through the holes are kept below 0.8 ft/sec to avoid formation of small drops. At each tray, design for the redistribution of each phase can be provided. Redispersion of either phase at each tray
   can be designed for. Tray spacings are 6–24 in. Tray efficiencies are in the range of 20–30%.
7. Pulsed packed and sieve tray towers may operate at frequencies of 90 cycles/min and amplitudes of 6–25 mm. In large diameter towers, HETS of about 1 m has been observed. Surface tensions as high as 30–40 dyn/cm have no adverse effect.
8. Reciprocating tray towers can have holes 9/16 in. dia, 50–60% open area, stroke length 0.75 in., 100–150 strokes/min, plate spacing normally 2 in. but in the range 1–6 in. In a 30 in. dia tower, HETS is 20–25 in. and throughput is 2000 gal/(hr)(sqft). Power requirements are much less than of pulsed towers.
9. Rotating disk contactors or other rotary agitated towers realize HETS in the range 0.1–0.5 m. The especially efficient Kuhni with perforated disks of 40% free cross section has HETS 0.2 m and a capacity of 50 m3 =m2 hr.

Rules Of Thumb : Evaporators

1. Long tube vertical evaporators with either natural or forced circulation are most popular. Tubes are 19–63 mm dia and 12–30 ft long.
2. In forced circulation, linear velocities in the tubes are 15–20 ft/sec.
3. Film-related efficiency losses can be minimized by maintaining a suitable temperature gradient, for instance 40–458F. A reasonable overall heat transfer coefficient is 250 Btu/(h)(ft2).
4. Elevation of boiling point by dissolved solids results in differences of 3–108F between solution and saturated vapor.
5. When the boiling point rise is appreciable, the economic number of effects in series with forward feed is 4–6.
6. When the boiling point rise is small, minimum cost is obtained with 8–10 effects in series.
7. In countercurrent evaporator systems, a reasonable temperature approach between the inlet and outlet streams is 308F. In multistage operation, a typical minimum is 108F.
8. In backward feed the more concentrated solution is heated with the highest temperature steam so that heating surface is lessened, but the solution must be pumped between stages.
9. The steam economy of an N-stage battery is approximately 0.8N lb evaporation/lb of outside steam.
10. Interstage steam pressures can be boosted with steam jet compressors of 20–30% efficiency or with mechanical compressors of 70–75% efficiency.

Rules Of Thumb : Drying Of Solids

1. Drying times range from a few seconds in spray dryers to 1 hr or less in rotary dryers and up to several hours or even several days in tunnel shelf or belt dryers.
2. Continuous tray and belt dryers for granular material of natural size or pelleted to 3–15 mm have drying times in the range of 10– 200 min.
3. Rotary cylindrical dryers operate with superficial air velocities of 5–10 ft/sec, sometimes up to 35 ft/sec when the material is coarse. Residence times are 5–90 min. Holdup of solid is 7–8%. An 85% free cross section is taken for design purposes. In countercurrent flow, the exit gas is 10–208C above the solid; in parallel flow, the temperature of the exit solid is 1008C. Rotation speeds of about 4 rpm are used, but the product of rpm and diameter in feet is typically between 15 and 25.
4. Drum dryers for pastes and slurries operate with contact times of 3–12 sec, produce flakes 1–3 mm thick with evaporation rates of 15–30 kg/m2 hr. Diameters are 1.5–5.0 ft; the rotation rate is 2–10 rpm. The greatest evaporative capacity is of the order of
   3000 lb/hr in commercial units.
5. Pneumatic conveying dryers normally take particles 1–3 mm dia but up to 10 mm when the moisture is mostly on the surface. Air velocities are 10–30 m/sec. Single pass residence times are 0.5– 3.0 sec but with normal recycling the average residence time is
   brought up to 60 sec. Units in use range from 0.2 m dia by 1 m high to 0.3 m dia by 38 m long. Air requirement is several SCFM/lb of dry product/hr.
6. Fluidized bed dryers work best on particles of a few tenths of a mm dia, but up to 4 mm dia have been processed. Gas velocities of twice the minimum fluidization velocity are a safe prescription. In continuous operation, drying times of 1–2 min are enough, but batch drying of some pharmaceutical products employs drying times of 2–3 hr.
7. Spray dryers are used for heat sensitive materials. Surface moisture is removed in about 5 sec, and most drying is completed in less than 60 sec. Parallel flow of air and stock is most common. Atomizing nozzles have openings 0.012–0.15 in. and operate at pressures of 300–4000 psi. Atomizing spray wheels rotate at speeds to 20,000 rpm with peripheral speeds of 250–600 ft/sec. With nozzles, the length to diameter ratio of the dryer is 4–5; with spray wheels, the ratio is 0.5–1.0. For the final design, the experts say, pilot tests in a unit of 2 m dia should be made.

Rules Of Thumb : Drivers And Power Recovery Equipment

1. Efficiency is greater for larger machines. Motors are 85–95%; steam turbines are 42–78%; gas engines and turbines are 28–38%.
2. For under 100 HP, electric motors are used almost exclusively. They are made for up to 20,000 HP.
3. Induction motors are most popular. Synchronous motors are made for speeds as low as 150 rpm and are thus suited for example for low speed reciprocating compressors, but are not made smaller than 50 HP. A variety of enclosures is available, from weather-proof to explosion-proof.
4. Steam turbines are competitive above 100 HP. They are speed controllable. They are used in applications where speeds and demands are relatively constant. Frequently they are employed as spares in case of power failure.
5. Combustion engines and turbines are restricted to mobile and remote locations.
6. Gas expanders for power recovery may be justified at capacities of several hundred HP; otherwise any needed pressure reduction in process is effected with throttling valves.
7. Axial turbines are used for power recovery where flow rates, inlet temperatures or pressure drops are high.
8. Turboexpanders are used to recover power in applications where inlet temperatures are less than 10008F.

Rules Of Thumb : Distillation And Gas Absorption

1. Distillation usually is the most economical method of separating liquids, superior to extraction, adsorption, crystallization,or others.
2. For ideal mixtures, relative volatility is the ratio of vapor pressures a12 1⁄4 P2 =P1 .
3. For a two-component, ideal system, the McCabe-Thiele method offers a good approximation of the number of equilibrium stages.
4. Tower operating pressure is determined most often by the temperature of the available condensing medium, 100–1208F if cooling water; or by the maximum allowable reboiler temperature, 150 psig steam, 3668F.
5. Sequencing of columns for separating multicomponent mixtures: (a) perform the easiest separation first, that is, the one least demanding of trays and reflux, and leave the most difficult to the last; (b) when neither relative volatility nor feed concentration vary widely, remove the components one by one as overhead products; (c) when the adjacent ordered components in the feed vary widely in relative volatility, sequence the splits
   in the order of decreasing volatility; (d) when the concentrations in the feed vary widely but the relative volatilities do not, remove the components in the order of decreasing concentration in the feed.
6. Flashing may be more economical than conventional distillation but is limited by the physical properties of the mixture.
7. Economically optimum reflux ratio is about 1.25 times the minimum reflux ratio Rm.
8. The economically optimum number of trays is nearly twice the minimum value Nm .
9. The minimum number of trays is found with the Fenske–Underwood equation
   Nm 1⁄4 log {[x=(1 À x)]ovhd =[x=(1 À x)]btms }= log a:
10. Minimum reflux for binary or pseudobinary mixtures is given by the following when separation is essentially complete (xD ’ 1) and D/F is the ratio of overhead product and feed rates:
    Rm D=F 1⁄4 1=(a À 1), when feed is at the bubblepoint,
    (Rm þ 1)D=F 1⁄4 a=(a À 1), when feed is at the dewpoint:
11. A safety factor of 10% of the number of trays calculated by the best means is advisable.
12. Reflux pumps are made at least 25% oversize.
13. For reasons of accessibility, tray spacings are made 20–30 in.
14. Peak efficiency of trays is at values pffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi of the vapor factor
    pffiffiffiffiffi Fs 1⁄4 u rv in the range 1.0–1.2 (ft/sec) lb=cuft. This range of Fs establishes the diameter of the tower. Roughly, linear velocities are 2 ft/sec at moderate pressures and 6 ft/sec in vacuum.
15. The optimum value of the Kremser–Brown absorption factor A 1⁄4 K(V =L) is in the range 1.25–2.0.
16. Pressure drop per tray is of the order of 3 in. of water or 0.1 psi.
17. Tray efficiencies for distillation of light hydrocarbons and aqueous solutions are 60–90%; for gas absorption and strip- ping, 10–20%.
18. Sieve trays have holes 0.25–0.50 in. dia, hole area being 10% of the active cross section.
19. Valve trays have holes 1.5 in. dia each provided with a liftable cap, 12–14 caps/sqft of active cross section. Valve trays usually are cheaper than sieve trays.
20. Bubblecap trays are used only when a liquid level must be maintained at low turndown ratio; they can be designed for lower pressure drop than either sieve or valve trays.
21. Weir heights are 2 in., weir lengths about 75% of tray diameter, liquid rate a maximum of about 8 gpm/in. of weir; multipass arrangements are used at high liquid rates.
22. Packings of random and structured character are suited especially to towers under 3 ft dia and where low pressure drop is desirable. With proper initial distribution and periodic redistribution, volumetric efficiencies can be made greater than those of tray towers. Packed internals are used as replacements for achieving greater throughput or separation in existing tower shells.
23. For gas rates of 500 cfm, use 1 in. packing; for gas rates of 2000 cfm or more, use 2 in.
24. The ratio of diameters of tower and packing should be at least 15.
25. Because of deformability, plastic packing is limited to a 10–15 ft depth unsupported, metal to 20–25 ft.
26. Liquid redistributors are needed every 5–10 tower diameters with pall rings but at least every 20 ft. The number of liquid streams should be 3–5/sqft in towers larger than 3 ft dia (some experts say 9–12/sqft), and more numerous in smaller towers.
27. Height equivalent to a theoretical plate (HETP) for vapor– liquid contacting is 1.3–1.8 ft for 1 in. pall rings, 2.5–3.0 ft for 2 in. pall rings.
28. Packed towers should operate near 70% of the flooding rate given by the correlation of Sherwood, Lobo, et al.
29. Reflux drums usually are horizontal, with a liquid holdup of 5 min half full. A takeoff pot for a second liquid phase, such as water in hydrocarbon systems, is sized for a linear velocity of that phase of 0.5 ft/sec, minimum diameter of 16 in.
30. For towers about 3 ft dia, add 4 ft at the top for vapor disengagement and 6 ft at the bottom for liquid level and reboiler return.
31. Limit the tower height to about 175 ft max because of wind load and foundation considerations. An additional criterion is that L/D be less than 30.

Rules Of Thumb : Disintegration

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. Closed circuit grinding employs external size classification and return of oversize for regrinding. The rules of pneumatic conveying are applied to design of air classifiers. Closed circuit is most common with ball and roller mills.
3. Jaw and gyratory crushers are used for coarse grinding.
4. Jaw crushers take lumps of several feet in diameter down to 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 for slabby feeds and make a more rounded
   product.
5. Roll crushers are made either smooth or with teeth. A 24 in. toothed roll can accept lumps 14 in. dia. Smooth rolls effect reduction ratios up to about 4. Speeds are 50–900 rpm. Capacity is about 25% of the maximum corresponding to a continuous ribbon of material passing through the rolls.
6. Hammer mills beat the material until it is small enough to pass through the screen at the bottom of the casing. Reduction ratios of 40 are feasible. Large units operate at 900 rpm, smaller ones up to 16,000 rpm. For fibrous materials the screen is provided with cutting edges.
7. Rod mills are capable of taking feed as large as 50 mm and reducing it to 300 mesh, but normally the product range is 8– 65 mesh. Rods are 25–150 mm dia. Ratio of rod length to mill diameter is about 1.5. About 45% of the mill volume is occupied by rods. Rotation is at 50–65% of critical.
8. Ball mills are better suited than rod mills to fine grinding. The charge is of equal weights of 1.5, 2, and 3 in. balls for the finest grinding. Volume occupied by the balls is 50% of the millvolume. Rotation speed is 70–80% of critical. Ball mills have a length to diameter ratio in the range 1–1.5. Tube mills have a ratio of 4–5 and are capable of very fine grinding. Pebble mills have ceramic grinding elements, used when contamination with metal is to be avoided.
9. Roller mills employ cylindrical or tapered surfaces that roll along flatter surfaces and crush nipped particles. Products of 20–200 mesh are made.
10. Fluid energy mills are used to produce fine or ultrafine (sub-micron) particles.

Rules Of Thumb : Crystallization From Solution

1. The feed to a crystallizer should be slightly unsaturated.
2. 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.
3. Growth rates and ultimate sizes of crystals are controlled by limiting the extent of supersaturation at any time.
4. Crystal growth rates are higher at higher temperatures.
5. The ratio S 1⁄4 C=Csat of prevailing concentration to saturation concentration is kept near the range of 1.02–1.05.
6. In crystallization by chilling, the temperature of the solution is kept at most 1–28F below the saturation temperature at the prevailing concentration.
Growth rates of crystals under satisfactory conditions are in the range of 0.1–0.8 mm/hr. The growth rates are approximately the same in all directions.
Growth rates are influenced greatly by the presence of impurities and of certain specific additives that vary from case to case.
Batch crystallizers tend to have a broader crystal size distribution than continuous crystallizers.
To narrow the crystal size distribution, cool slowly through the initial crystallization temperature or seed at the initial crystallization temperature

Rules Of Thumb : Cooling Towers

1. Water in contact with air under adiabatic conditions eventually cools to the wet bulb temperature.
2. In commercial units, 90% of saturation of the air is feasible.
3. Relative cooling tower size is sensitive to the difference between the exit and wet bulb temperatures: DT (0F) 5 15 25 ; Relative volume 2.4 1.0 0.55
4. Tower fill is of a highly open structure so as to minimize pressure drop, which is in standard practice a maximum of 2 in. of water.
5. Water circulation rate is 1–4 gpm/sqft and air rates are 1300–1800 lb/(hr)(sqft) or 300–400 ft/min.
6. Chimney-assisted natural draft towers are of hyperboloidal shapes because they have greater strength for a given thickness; a tower 250 ft high has concrete walls 5–6 in. thick. The enlarged cross section at the top aids in dispersion of exit humid air into the atmosphere.
7. Countercurrent induced draft towers are the most common in process industries. They are able to cool water within 28F of the wet bulb.
8. Evaporation losses are 1% of the circulation for every 108F 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.

Rules Of Thumb : Conveyors For Particulate Solids

Part 1

1. Screw conveyors are used to transport even sticky and abrasive solids up inclines of 208 or so. They are limited to distances of 150 ft or so because of shaft torque strength. A 12 in. dia conveyor can handle 1000–3000 cuft/hr, at speeds ranging from 40 to 60 rpm
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 308 maximum. A 24 in. wide belt can carry 3000 cuft/hr at a speed of 100 ft/min, but speeds up to 600 ft/min are suited for some materials. The number of turns is limited and the maximum incline is 30 degrees. Power consumption is relatively low.
3. Bucket elevators are used for vertical transport of sticky and abrasive materials. With buckets 20 Â 20 in. capacity can reach 1000 cuft/hr at a speed of 100 ft/min, but speeds to 300 ft/min are
   used.
4. Drag-type conveyors (Redler) are suited for short distances in any direction and are completely enclosed. Units range in size from 3 in. square to 19 in. square and may travel from 30 ft/min (fly ash) to 250 ft/min (grains). Power requirements are high.
5. Pneumatic conveyors are for high capacity, short distance (400 ft) transport simultaneously from several sources to several destinations. Either vacuum or low pressure (6–12 psig) is employed with a range of air velocities from 35 to 120 ft/sec depending on the material and pressure. Air requirements are from 1 to 7 cuft/cuft of solid transferred.
   
   

Rules Of Thumb : Compressors And Vacuum Pumps

Although experienced engineers know where to find information and how to make accurate computations, they also keep a mini mum body of information readily available, made largely of shortcuts and rules of thumb. This compilation is such a body of information from the material in this book and is, in a sense, a digest of the book.

Rules of thumb, also known as heuristics, are statements of known facts. The word heuristics is derived from Greek, to discover or to invent, so these rules are known or discovered through use and practice but may not be able to be theoretically proven. In practice, they work and are most safely applied by engineers who are familiar with the topics. Such rules are of value for approximate design and preliminary cost estimation, and should provide even the inexperienced engineer with perspective and whereby the reasonableness of detailed and computer-aided design can be appraised quickly, especially on short notice, such as a conference.

Everyday activities are frequently governed by rules of thumb. They serve us when we wish to take a course of action but we may not be in a position to find the best course of action. Much more can be stated in adequate fashion about some topics than others, which accounts, in part, for the spottiness of the present coverage. Also, the spottiness is due to the ignorance and oversights on the part of the authors. Therefore, every engineer undoubtedly will supplement or modify this material (Walas, 1988).

1. Fans are used to raise the pressure about 3% (12 in. water), blowers raise to less than 40 psig, and compressors to higher pressures, although the blower range commonly is included in the compressor range.
2. Vacuum pumps: reciprocating piston type decrease the pressure to 1 Torr; rotary piston down to 0.001 Torr, two-lobe rotary down to 0.0001 Torr; steam jet ejectors, one stage down to 100 Torr, three stage down to 1 Torr, five stage down to 0.05 Torr.
3. A three-stage ejector needs 100 lb steam/lb air to maintain a pressure of 1 Torr.
4. In-leakage of air to evacuated equipment depends on the absolute pressure, Torr, and the volume of the equipment, V cuft, according to w 1⁄4 kV 2=3 lb/hr, with k 1⁄4 0:2 when P is more than 90 Torr, 0.08 between 3 and 20 Torr, and 0.025 at less than 1 Torr.
5. Theoretical adiabatic horsepower (THP) 1⁄4 [(SCFM)T1 /8130a] [(P2 =P1 Þa À 1], where T1 is inlet temperature in 8F þ 460 and a 1⁄4 (k À 1)=k,k 1⁄4 Cp =Cv .
6. Outlet temperature T2 1⁄4 T1 (P2 =P1 )a
7. To compress air from 1008F, k 1⁄4 1:4, compression ratio 1⁄4 3 theoretical power required 1⁄4 62 HP/million cuft/day, outlet temperature 3068F.
8. Exit temperature should not exceed 350–4008F; for diatomic gases (Cp =Cv 1⁄4 1:4) this corresponds to a compression ratio of about 4.
9. Compression ratio should be about the same in each stage of a multistage unit, ratio 1⁄4 (Pn =P1 )1=n , with n stages.
10. Efficiencies of fans vary from 60–80% and efficiencies of blowers are in the range of 70–85%.
11. Efficiencies of reciprocating compressors: 65–70% at compression ratio of 1.5, 75–80% at 2.0, and 80–85% at 3–6.
12. Efficiencies of large centrifugal compressors, 6000–100,000 ACFM at suction, are 76–78%.
13. Rotary compressors have efficiencies of 70–78%, except liquid liner type which have 50%.
14. Axial flow compressor efficiencies are in the range of 81–83%.