Experienced-Based Rules of Chemical Engineering

Electric Motors and Turbines

A. Efficiencies range from 85-95% for electric motors, 42-78% for steam turbines 28-38% for gas engines and turbines
B. For services under 75 kW (100 hp), electric motors are almost always used. They can be used for services up to about 15000 kW (20000 hp)

C. Turbines can be justified in services where they will yield several hundred horsepowers. Otherwise, throttle valves are used to release pressure.
D. A quick estimate of the energy available to a turbine is given by:

where: Delta H = Actual available energy, Btu/lb
Cp = Heat Capacity at constant pressure, Btu/lb 0F

T1 = Inlet temperature, 0R

P1 = Inlet pressure, psia

P2 = Outlet pressure, psia

K = Cp/Cv

Evaporation

A. Most popular types are long tube vertical with natural or forced circulation. Tubes range from 3/4" to 2.5"
(19-63 mm) in diameter and 12-30 ft (3.6-9.1 m) in length.
B. Forced circulation tube velocities are generally in the 15-20 ft/s (4.5-6 m/s) range.
C. Boiling Point Elevation (BPE) as a result of having dissolved solids must be accounted for in the differences between the solution temperature and the temperature of the saturated vapor.
D. BPE's greater than 7 �F (3.9 �C) usually result in 4-6 effects in series (feed-forward) as an economical solution. With smaller BPE's, more effects in series are typically more economical, depending on the cost of steam.
E. Reverse feed results in the more concentrated solution being heated with the hottest steam to minimize surface area. However, the solution must be pumped from one stage to the next.
F. Interstage steam pressures can be increased with ejectors (20-30% efficient) or mechanical compressors (70-75% efficient).

Filtration

A. Initially, processes are classified according to their cake buildup in a laboratory vacuum leaf filter :
0.10 - 10.0 cm/s (rapid), 0.10-10.0 cm/min (medium), 0.10-10.0 cm/h (slow)
B. Continuous filtration methods should not be used if 0.35 sm of cake cannot be formed in less than 5 minutes.
C. Belts, top feed drums, and pusher-type centrifuges are best for rapid filtering.
D. Vacuum drums and disk or peeler-type centrifuges are best for medium filtering.
E. Pressure filters or sedimenting centrifuges are best for slow filtering.
F. Cartridges, precoat drums, and sand filters can be used for clarification duties with negligible buildup.
G. Finely ground mineral ores can utilize rotary drum rates of 1500 lb/dat ft2 (7335 kg/day m2) at 20 rev/h and 18-25 in Hg (457-635 mm Hg) vacuum.
H. Course solids and crystals can be filtered at rates of 6000 lb/day ft2 (29,340 kg/day m2) at 20 rev/h and 2-6 in Hg (51-152 mm Hg) vacuum.

Mixing and Agitation

A. Mild agitation results from superficial fluid velocities of 0.10-0.20 ft/s (0.03-0.06 m/s). Intense agitation results from velocities of 0.70-1.0 ft/s (0.21-0.30 m/s).
B. For baffled tanks, agitation intensity is measured by power input and impeller tip speeds:

C. Various geometries of an agitated tank relative to diameter (D) of the vessel include:
Liquid Level = D
Turbine Impeller Diameter = D/3
Impeller Level Above Bottom = D/3
Impeller Blade Width = D/15
Four Vertical Baffle Width = D/10
D. For settling velocities around 0.03 ft/s, solids suspension can be accomplished with turbine or propeller impellers. For settling velocities above 0.15 ft/s, intense propeller agitation is needed.
E. Power to mix a fluid of gas and liquid can be 25-50% less than the power to mix the liquid alone.

Pressure and Storage Vessels

Pressure Vessels

A. Design Temperatures between -30 and 345 �C (-22 to 653 �F) is typically about25 �C (77 �F) above maximum operating temperature, margins increase above this range

B. Design pressure is 10% or 0.69 to 1.7 bar (10 to 25 psi) above the maximum operating pressure, whichever is greater. The maximum operating pressure is taken as 1.7 bar (25 psi) above the normal operation pressure.
C. For vacuum operations, design pressures are 1 barg (15 psig) to full vacuum
D. Minimum thicknesses for maintaining tank structure are: 6.4 mm (0.25 in) for 1.07 m (42 in) diameter and under 8.1 mm (0.32 in) for 1.07-1.52 m (42-60 in) diameter 9.7 mm (0.38 in) for diameters over 1.52 m (60 in)
E. Allowable working stresses are taken as 1/4 of the ultimate strength of the material

F. Maximum allowable working stresses:

Temperature	-20 to 650 �F	750 �F	850 �F	1000 �F
	-30 to 345 �C	400 �C	455 �C	540 �C
CS SA203	18759 psi	15650 psi	9950 psi	2500 psi
	1290 bar	1070 bar	686 bar	273 bar
302 SS	18750 psi	18750 psi	15950 psi	6250 psi
	1290 bar	1290 bar	1100 bar	431 bar

G. Thickness based on pressure and radius is given by:

where pressure is in psig, radius in inches, stress in psi, corrosion allowance in inches.

**Weld Efficiency can usually be taken as 0.85 for initial design work

H. Guidelines for corrosion allowances are as follows: 0.35 in (9 mm) for known corrosive fluids, 0.15 in (4 mm) for non-corrosive fluids, and 0.06 in (1.5 mm) for steam drums and air receivers.