Jumat, 17 Juli 2009

Heuristics Rules for Process Equipment

When we were asked by our manager or supervisor to make process design or asked by our lecturer to make a preliminary plant design, we often spend so much time reading a bunch of literatures. This is the summary of article that I found in chemical engineering magazine (www.che.com). If applied with thought and care, heuristics like these can make life much easier during project scoping, preliminary plant design, process design, equipment specification and similar tasks.

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%

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