Fluid handling

• Fans are suitable for raising gas pressures moderately (for instance, by 3%, or by 12 in. of water); for higher pressures up to about 40 psig, blowers are suitable; for yet higher pressures, employ compressors (however there is overlap between the operating ranges of blowers and compressors)
• Typical polytropic efficiencies for large centrifugal compressors are about 76 to 78%; rotary compressors normally have efficiencies around 70%, except for liquid-sealed ones, which have efficiencies around 50%
• For pipe lines of diameter D in inches, typical fluid velocities and pressure drops are as follows:
  1. for pump discharge (liquid): (5 + D/3) ft/s, and 2 psi/100 ft;
  2. at pump suction (liquid): (1.3 + D/6) ft/s and 0.4 psi/100 ft;
  3. for steam or gas, 20D ft/s and 0.5 psi/100 ft
• Control valves function best if the pressure drop through them is at least 10 psi
• Single-stage centrifugal pumps can operate at rates of up to about 5,000 gal/min, (and to maximum heads of 500 ft); multistage pumps can operate to about 11,000 gal/min.

Conveying of particulate solids

• Screw conveyors:
  1. Can transport solids that are abrasive or sticky
  2. Typical incline is about 20 deg
  3. Most are 150 ft or less in length
  4. With a conveyor of 12-in. diameter, throughputs of up to about 3,000 ft3/h are feasible; typically, screw rotation rates are up to about 60 rev/min
  5. Power consumption relatively low
• Bucket elevators:
  1. Vertical transport of abrasive or sticky materials is feasible
  2. Typically, speeds can reach 100 to 300 ft/min; at 100 ft/min, bucket elevators with 20X20-in. buckets can convey about 1,000 ft3/h
• Drag type conveyors:
  1. Can convey for relatively short distances in any direction
  2. Have high power requirements
  3. Typical speeds are 30 ft/min (for,e.g., fly ash) to 250 ft/min (for grains)
• Pneumatic conveyors:
  1. They offer high capacity
  2. Usually employed with convey-ing distances of 400 ft or less
  3. Can transport simultaneously to several destinations
  4. Operate under vacuum or low pressures
  5. Typical conveying-gas velocities are 35 to 120 ft/s

Cooling towers

• In full-scale units, air saturation can reach 90%
• To minimize pressure drop (ordinarily a maximum of 2 in. water), employ an open structured material for the tower fill
• Typical water circulation rates are 1 to 4 gal/min per square foot, whereas the air rates are 1,300 to 1,800 lb/h per square foot, or 300 to 400 ft/min
• Countercurrent induced-draft towers, which can cool water to about 2°F above the wet-bulb temperature, are the most prevalent version of tower used in the process industries
• For a given service, the required size (volume) of a given tower is a function of the difference between the wet-bulb and the exit temperatures; the smaller the difference, the larger the required volume
• Evaporation losses are typically 1% of the circulation for every 100°F of cooling range. Windage or drift losses in mechanical-draft towers typically amount to 0.1 to 0.3%. To keep salt from building up exces-sively, it is typical to blow down 2.5 to 3% of the circulation Heat exchangers; refrigeration
• In a shell-and-tube exchanger, the tube side is for corrosive, fouling, scaling and/or high-pressure fluids; the shell side is for viscous and/or condensing fluids
• Typical minimum temperature approaches are 20°F with normal coolants, or 10°F or less with refrigerants
• Ordinarily, the maximum heat transfer area for shell and tube heat exchangers is about 5,000 ft2
• When refrigerating to temperatures below about – 80°F, it is customary to use cascades of two or more refrigeration stages

Evaporators

• The maintaining of a suitable temperature gradient (for instance, about 45°F) can minimize film-related efficiency losses. From an efficiency standpoint, about 250 Btu/(h)(ft2) is a suitable overall coefficient of heat transfer
• In countercurrent evaporation systems, a suitable temperature approach between the inlet (hot) and output (cold) streams is about 30°F. In multistage operation, the typical minimum value is 10°F
• In a well-designed evaporator system, it should be possible to achieve heat recoveries of more than 75%