PE 3

• Tube wall thickness depends on the cor-rosiveness of the fluids and their operating pressure & temperature and is specified by Birmingham wire gauge (B.W.G)-a number which varies from 8 to 18 with the numbers 14 and 16 being more commonly used. Outside diameter of tubes varies from about 15 to 50 mm; however a tube of less than 19 mm outside diameter is generally not recommended for fouling fluids.
• Angled baffles are preferred in the agitation tank to

  control the swirling action; if the viscosity of the liquid to

  be mixed is about 200 poise. However, the baffle may be

  completely eliminated for mixing liquids having viscoisty

  above 600 poise.
• Vapor phase reactor


  The safest from operation point of view
• In a bag filter, the pressure drop increases directly as the density of the gas and inversely as viscosity of the gas
• The LMTD correction factor (FT) is applied in all multipass heat exchangers
• Movement of pipeline caused by thermal expansion is taken care by

  • Providing expansion joint
  • Changes in its direction
  • Change in the shape of pipeline
• For a cylindrical shell, the thickness of the courses of shell decreases upwards (subject to the thickness of uppermost course being more than the minimum for diameter of the tank in question)
• The design stress, which is more than the damaging stress, is the least unit stress that will render a member unfit for service before the end of its normal life. The design stress factor or factor of safety indicates the margin between design stress and the damaging stress.
• Hole diameter of the sieve trays in the distillation column ranges from 3 to 12.5 mm
• Factor of safety (i.e., overdesign factor) in heat transfer equipment is about 15 - 20 percent more than the actual/theoretical design factor
• For annular flow of a fluid, the ratio of the equivalent diameter for pressure drop calculation to the equivalent diameter for heat transfer calculation is d1/d1+d2
• In actual operation of distillation column, the vapour is not distributed uniformly among the bubble caps, primarily because of the liquid gradient on the tray
• The minimum shell thickness for tank ≤ 15 metres diameter is limited to 5 mm, for reasons of elastic stability
• If 'D' is the inside diameter of the shell of a shell and tube heat exchanger, then the baffle spacing is usually in the range of D/5 or minimum 2" to 5D
• Brass valves are usually made for pipe sizes ≤2 inches
• At a given point for a given packing and set of fluids, the pressure drop per metre of packed height remains the same with variation in fluid rates and operating pressure
• In a shell and tube heat exchanger, triangular pitch arrangement as compared to square pitch arrangement results in higher shell side pressure drop
• pick out the wrong about routing of fluids in a S&T HE: Higher pressure fluid stream should be routed through the shell side in a shell and tube heat exchanger
• The necessary wall thickness for a metallic storage vessel is a function of

  • Ultimate tensile strength (or yield point) of the material and operating temperature
  • Operating pressure and welding/joint efficiency
  • Diameter of the vessel
• In a multiple effect evaporator system having 'n' effects, the amount of water evaporated per unit surface area is approximately equal to (1/n)th of that in a single effect
• wrong statement: Flat head covers are the most suitable for larger vessels operating at very high pressure
• wrong statement pertaining to the design of a

  horizontal tube evaporator.

  ANSWER: Liquor flows inside the tube, while the steam is outside submerging the tube.
• Weep holes provided in the plates of a distillation column

  • Facilitate draining out liquid from a tray when the unit is not in operation
  • Are normally located near the overflow weir so that any delivery of liquid during operation follows approximately the same path as the overflow fluid
  • Must be large enough (usually 1/4" to 5/8" dia) to prevent plugging but should not deliver excessive amount of fluid during operation
• Steam is preferred to be used as a heating medium in heat exchangers, because of its high latent heat
• Conical shaped roof is the most commonly used roof for cylindrical storage tanks
• wrong statement pertaining to 'Horton sphere' used for the

  storage of liquid ammonia:

  Answer: Horton sphere is used for the storage of liquid at sub-zero temperature at very high pressure up to 200 kg/cm2
• Value of Peclet number = ∞, is the representative of plug flow
• Value of K for a Tee (used as elbow, entering branch)

  • Is more than that for Tee (used as elbow, entering run)
  • Is around 90
• In most of the shell and tube heat exchangers, the tube pitch is generally 1.25-1.50 times the tube diameter
• In a distillation column, bubble caps are located on trays with a pitch of 1.3 to 2 times the outside diameter of the caps
• 1.2 to 3.8 cms is the value of

  • Downcomer liquid seal in case of a balanced tray design
  • Static submergence in case of a bubble cap tray for column operating under pressure (≥ 1 atm.)
  • Skirt clearance for bubble caps
• Accidents in chemical plants are mostly due to faulty operating procedures
• For identical operating conditions, the pressure drop over bubble cap tray is the highest out of the following
• The ligament efficiency of a perforated plate is given by (ph - dt)/ph, where ph is the hole pitch and dt is the tube ID passing through the hole
• Permissible slot velocity is the determining factor for the number of bubble caps to be used per tray
• wrong statement about distillation operation:

  Answer: Flash distillation, differential distillation & steam distillation are all continuous distillation processes:
• Low pressure steam (with saturation temperature between 105 to 120°C) is usually used in the vacuum evaporators, which normally employ a vacuum of about 650 mm Hg (referred to a 760 mm mercury barometer)
• Copper is the best tube material from thermal conductivity point of view alone
• Normal screwed fittings are used for pipes up to a diameter of 3 inches
• In a shell and tube heat exchanger, the clearance of the tube is generally not less than one-fourth of the tube diameter or 3/16"