Intuitive technical hints, provided by a team of Australian and New Zealand based engineers
Selecting and designing equipment
Hints and Rules can be provided for many types of equipment – from compressors to distillation columns, to heat exchanges, to vessels. Such guidelines are useful in preliminary design calculations and cost estimates.
These hints may be valuable for preliminary design and cost-estimating. They have been obtained from a variety of sources and publications and are intended as a helpful first estimate only.
The hints given here are subject to various qualifications.
Accordingly, most of the rules are best applied by experienced engineers familiar with their application. Glyde would like to emphasise whilst helpful the use of these hints is at the complete risk of the user and professional advice should be obtained for specific designs.
- Drying and Solids
- Conveyors for Particular Solids
- Cooling Towers
- Compressors and Vacuum Pumps
- Crushing and Grinding
- Extraction, Liquid
- Crystallization from Solution
- Heat Exchangers
- Mixing and Agitation
- Particle Size Enlargement
- Size Separation of Particles
- Utilities, Common Specifications
- Vessels (Storage Tanks)
Drying and Solids
- Rotary, cylindrical dryers employ superficial air velocities of 5-10 ft./s, sometimes to 35 ft./s when the material is coarse. Residence times are 5-90 min. Holdup of solids is 7-8%. An 85% free cross section is taken for design purposes. In counter current flow, the exit gas is 10-20ƒC above the solid temperature; in parallel flow, the temperature of exit solids is 100ƒC. Rotation speeds of about 4 rpm are used, but the product of speed (rpm) and diameter (ft.) is typically between 15 and 25.
- Drying times range from a few seconds in spray dryers, to one hour or less in rotary dryers, and up to several hours (or even days) in tunnel shelf or belt units.
- Continuous tray or belt dryers for granular materials of natural size or pelleted to 3-15 mm have 10-200 min drying times.
Conveyors for Particular Solids
- Belt conveyors are for high capacities and long distances (a mile or more, but only several hundred feet in a plant), up inclines of 30 deg. maximum. A 24 in. wide belt can carry 3,000 ft., /h at 100 ft./min, but speeds up to 600 ft./min are suitable for some materials. Power consumption is relatively low.
- Screw conveyors are suited to transport of even sticky and abrasive solids up inclines of 20 degrees or so. They are limited to distances of about 150 ft. because of shaft-torque strength. A 12 in. diameter conveyor can handle 1,000-3,000 ft., /h at speeds ranging from 40 to 160 rpm.
- Bucket elevators are used for vertical transport of sticky and abrasive materials. With buckets 20 in. square, capacity can reach 1,000 ft., /h at a speed of 100 ft./min but speeds to 300 ft./min are used.
- Relative cooling tower size is sensitive to the difference between the exit and wet bulb temperatures (see the table):
- In commercial units, 90% saturation of the air is feasible.
- Tower fill has a highly open structure to minimise pressure drop, which, typically, is a maximum of 2 in. in H²O.
- Water in contact with air under adiabatic conditions eventually cools to the wet-bulb temperature.
Compressors and Vacuum Pumps
- Relative cooling tower size is sensitive to the difference between the exit and wet bulb temperatures
- Theoretical adiabatic horsepower:
Phpt = (FT1/8,130a)((P2/P1)a-1)
where Phpt = power, hp; F = flowrate, scfm; T¹ = inlet temperature, R; P2 and P1 = outlet and inlet pressures, psia; a = (k-1)/k, where k = ratio of specific heats.
- Outlet temperature: T2 = T1(P2/P1)a
Crushing and Grinding
- Weight percentages of material greater than 50% of the maximum size are about 50% for roll mills, 15% for tumbling mills and 5% for closed circuit ball mills.
- Roll crushers are made either smooth or with teeth. A 24 in. toothed roll can accept lumps of 14 in. diameter. Smooth rolls have reduction ratios of up to about Speeds are 50-900 rpm. Capacity is about 25% of the maximum corresponding to a continuous ribbon passing through the rolls.
- Hammer mills beat the material until it is small enough to pass through the screen at the bottom of the casing. A reduction ratio to 40 is feasible. Large units operate at 900 rpm, smaller ones up to 16,000 rpm. For fibrous materials, the screen has cutting edges.
- The dispersed phase should be the one that has the higher volumetric flowrate, except in equipment subject to back mixing, where it should be the phase with the smaller rate. It should be the phase that wets the material of construction less well. Since the holdup of the continuous phase usually is greater, that phase should be the less expensive or less hazardous material.
- Mixer settlers are limited to five stages. Mixing is done with rotating impellers or circulating pumps. Settlers are designed assuming that droplet sizes are about 150 µm diameter. 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%.
- Spray towers even as high as 20-40 ft. cannot be depended on to function as more than a single stage.
- Packed towers are employed when 5-10 stages will suffice. Pall rings of 1-1.5 in. are best. Dispersed phase loadings should not exceed 25 gal/(min)(ft2). An HETP of 5-10 ft. may be realisable. The dispersed phase must be redistributed every 5-7 ft. Packed towers are not satisfactory when surface tension is more than 10 dynes/cm.
- Sieve tray towers have holes of 3-8 mm diameter. Hole velocities are kept below 0.8 ft./s to avoid formation of small drops. Re-dispersion of either phase at each tray can be designed for. Tray spacings are 6-24 in.; efficiencies are 20-30%.
Crystallization from Solution
- Complete recovery of dissolved solids is possible by evaporation, but only to the eutectic composition by chilling. The eutectic also limits recovery by melt crystallisation.
- Growth rates and ultimate sizes of crystals are controlled by limiting the extent of supersaturation at any time.
- Continuous filtration should not be attempted if 1/8 in. cake thickness cannot be formed in less than 5 min..
- Processes are classified by their rate of cake build-up in a laboratory vacuum-leaf filter. Rapid, 0.1-10.0 cm/s; medium, 0.1-10.0 cm/min; slow, 0.1-10.0 cm/h.
- Take true counter current flow in a shell and tube exchanger as the basis for comparison.
- Tubeside is for corrosive, fouling, scaling and high pressure fluids.
- Shellside is for viscous and condensing fluids.
- Pressure drops are 1.5 psi for boiling liquids and 3-9 psi for other services.
- Double pipe exchangers are competitive at duties requiring 100-200 ft¾.
- Plate and frame exchangers are suited to sanitary service, and, in stainless steel, are 25-50% cheaper than shell and tube units.
Mixing and Agitation
- Mild agitation is obtained by circulating the liquid with an impeller at superficial velocities of 0.1-0.2 ft./s and intense agitation at 0.07-1.0 ft./s.
- Intensity of agitation using impellers in baffled tanks is measured by power input and impeller tip speeds, as given in the table.
- In-line blenders are adequate when a contact time of 1-2 s is sufficient, with power inputs of 0.1-0.2 hp/gal.
- Power and Tip Speed requirements for agitation in baffled tanks
Particle Size Enlargement
- The chief methods are: compression into a mold, and 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.
- Tablets are made in rotary compression machines that convert powders and granules into uniform size pieces. Usually maximum diameter is about 1.5 in., but special sizes up to 4 in. are possible. Machines operate at 100 rpm or so and make up to 10,000 tablets/min.
- Extruders make pellets by forcing powders, pastes and melts through a die, followed by cutting. An 8 in. screw with a capacity of 2,000 lb/h of molten plastic is able to extrude tubing at 150-300 ft./h and cut it into sizes as small as washers, at 8,000 pieces/min. Ring pellet extrusion mills have hole diameters of 1.6 to 32mm. Production rates are 30-200 lb/(h)(hp).
- Flanges and fittings are rated for 150, 300, 600, 900, 1,500 or 2,500 psig.
- Line velocities and pressure drops, with line diameter, D, given in in.; Liquid pump discharge (5 + D/3) ft./s, 2.0 psi/100 ft.; liquid pump suction (1/3 + D/6) ft./s, 0.4 psi/100 ft.; steam or gas 20D ft./s 0.5 psi/100 ft.
- Control valves require at least a 10 psi drop for good control.
- Screwed fittings are used only on sizes 1.5 inch; smaller, flanged or welded fittings are used otherwise.
- Power for pumping liquids:
Power, hp = (gpm) (psi difference) / (1,714) (fractional efficiency)
- If pump damage is to be avoided, normal pump suction head (NPSH) must be in excess of a certain number, depending on the kind of pump and conditions, as given by the supplier.
NPSH = (pressure at the eye of the impeller – vapour pressure) / (density) The common range is 4-20 ft.
- Centrifugal pumps: Use single stage for 15-5,000 gpm, 500 ft. maximum head; multistage for 20-11,000 gpm, 5,500 ft. maximum head. Efficiency is 45% at 100 gpm, 70% at 500 gpm, 80% at 10,000 gpm.
- Axial pumps are for 20-100,000 gpm, 40 ft. head, 65-85% efficiency.
- The rate of reaction in every instance must be established in the laboratory.
- Catalyst particles are 0.1 mm dia in fluidised beds, 1 mm in slurry beds and 2-5 mm in fixed beds.
- The optimum proportions of stirred tank reactors are liquid level equal to tank diameter; at high pressures, slimmer proportions are more economical.
- One ton of refrigeration equals the removal of 12,000 Btu/h of heat.
- CAt various temperature levels; 0 to 50ƒF, use chilled brine and glycol solutions; -50 to 40ƒF, ammonia, Freons, butane; -150 to -50ƒF, ethane or propane.
- In single stage compression, the compression ratio is limited to about 4.
- In multistage compression, economy is improved with interstage flashing and recycling, i.e.: economiser operation.
- Absorption-refrigeration (employing ammonia to -30ƒF, lithium bromide to 45ƒF) is economical when waste steam is available at 12 psig or so.
Size Separation of Particles
- Flat screens are vibrated, or shaken, or impacted with bouncing balls. Inclined screens vibrate at about 600-7,000 strokes/min and are used for down to 38 µm, although capacity drops off sharply below 200 µm. Reciprocating screens operate at 30-1,000 strokes/min and handle sizes down to 0.25 mm at the higher speeds.
Utilities, Common Specifications
- Compressed air at 45, 150, 300 or 450 psig.
- Instrument air at 45 psig, 0ƒF dewpoint.
- Steam: 15-30 psig, at 250-275ƒF; 150 psig, at 366ƒF; 400 psig, at 448ƒF; 600 psig, at 488ƒF, or with 100-150ƒF of superheat.
- Cooling water: supply from a cooling tower is at 80-90ƒF; return is at 115-125ƒF; return seawater at 110ƒF, return tempered water or steam condensate above 125ƒF.
- Cooling air supply at 85-95ƒF; temperature approach to process, 40ƒF.