Thumb rules are not precise calculations but serve as useful approximations.

Some may be incorrect as per your process applications. Here are some common chemical engineering thumb rules:

​​Rule of 70:

To estimate the doubling time for a chemical reaction or process, divide 70 by the growth rate percentage. For example, if a population is growing at 7% per year, it will double in approximately 10 years (70/7 = 10).

 Rule of 15:

To estimate the final temperature of a mixture after two substances at different temperatures are mixed, take the average of their initial temperatures and add 15 degrees Fahrenheit or 8 degrees Celsius. This is a rough approximation for heat exchange.

 Rule of 6-10-50:

In distillation, the first 6% of the distillate is typically the most volatile component, the next 10% is intermediate, and the last 50% is the least volatile component.

 Rule of 3:

For batch reaction times, roughly one-third of the reaction occurs in the initial reaction time. Two-thirds of the reaction occurs during the remaining time.

​​Rule of 1.5:

In heat exchangers, the heat transfer area required is approximately 1.5 times the product of the overall heat transfer coefficient (U), the log mean temperature difference (LMTD), and the heat load (Q).

​Rule of 25:

In chemical reactor design, if the volume of a reactor is increased by a factor of 25, the time required for a chemical reaction to reach a specified conversion will be increased by a factor of 10.

Rule of 2:

For pump selection, the total dynamic head (TDH) required is usually around two times the elevation difference between the inlet and outlet, plus additional losses due to friction and other factors.

Rule of 100:

To estimate the number of theoretical plates required for a distillation column, use the McCabe-Thiele method, and typically aim for a reflux ratio of 2 to 5 times the minimum reflux ratio. 

 Rule of 4-6-8:

For crystallization processes, typically, it takes 4 hours for nucleation, 6 hours for crystal growth, and 8 hours for further crystallization.

​​Rule of 20:

In filtration, the filtration rate typically decreases to about 20% of its initial rate during a batch operation.

Remember that these thumb rules provide quick estimates and approximations and should not be relied upon for precise engineering calculations. They are valuable for initial design considerations and as a starting point for more detailed engineering analysis and calculations. Actual results may vary based on specific conditions and factors.​​

• Rate of reaction doubles per 10 deg increase in temperature.

• Steam temp. is √√(steam pressure) x 100.

• Pore size of sieve is( 16750 / Sieve Mesh).

• Cooling tower : Evaporation losses are about 1% by mass of the circulation rate for every 10 °F (5.5 °C) of cooling.  Drift losses are around 0.25% of the circulation rate.  A blowdown of about 3% of the circulation rate is needed to prevent salt and chemical treatment buildup.

• Crystallization : 

• A.  During most crystallizations, C/Csat (concentration/saturated concentration) is kept near 1.02 to 1.05

• B.  Crystal growth rates and crystal sizes are controlled by limiting the degree of supersaturation.

• C.  During crystallization by cooling, the temperature of the solution is kept 1-2 °F (0.5-1.2 °C) below the saturation point at the given concentration.

• D.  A generally acceptable crystal growth rate is 0.10 - 0.80 mm/h

• Drying :

• Spray dryer have drying times of a few seconds.  Rotary dryers have drying times ranging from a few minutes to up to an hour.

• Continuous tray and belt dryers have drying times of 10-200 minutes for granular materials or 3-15 mm pellets.

• Drum dryers used for highly viscous fluids use contact times of 3-12 seconds and produce flakes 1-3 mm thick.  Diameters are generally 1.5-5 ft (0.5 - 1.5 m).  Rotation speeds are 2-10 rpm and the maximum evaporation capacity is around 3000 lb/h (1363 kg/h).

• Rotary cylindrical dryers operate with air velocities of 5-10 ft/s (1.5-3 m/s), up to 35 ft/s (10.5 m/s).  Residence times range from 5-90 min.  For initial design purposes, an 85% free cross sectional area is used.  Countercurrent design should yield an exit gas temperature that is 18-35 °F (10-20 °C) above the solids temperature.  Parallel flow should yield an exiting solids temperature of 212 °F (100 °C).  Rotation speeds of 4-5 rpm are common.  The product of rpm and diameter (in feet) should be 15-25.

• Pneumatic conveying dryers are appropriate for particles 1-3 mm in diameter and in some cases up to 10 mm.    Air velocities are usually 33-100 ft/s (10-30 m/s).  Single pass residence time is typically near one minute.  Size range from 0.6-1.0 ft (0.2-0.3 m) in diameter by 3.3-125 ft (1-38 m) in length.

• Fluidized bed dryers work well with particles up to 4.0 mm in diameter.  Designing for a gas velocity that is 1.7-2 times the minimum fluidization velocity is good practice.   Normally, drying times of 1-2 minutes are sufficient in continuous operation.

• Evaporation :

• 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.

• Forced circulation tube velocities are generally in the 15-20 ft/s (4.5-6 m/s) range.

• 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.

• 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.

•   Interstage steam pressures can be increased with ejectors (20-30% efficient) or mechanical compressors (70-75% efficient).

• Filtration :

• 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)

• Continuous filtration methods should not be used if 0.35 sm of cake cannot be formed in less than 5 minutes.

• Belts, top feed drums, and pusher-type centrifuges are best for rapid filtering.

• Vacuum drums and disk or peeler-type centrifuges are best for medium filtering.

• Pressure filters or sedimenting centrifuges are best for slow filtering.

• Cartridges, precoat drums, and sand filters can be used for clarification duties with negligible buildup.

• 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.

• 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.

• Storage Tanks :

• For less than 3.8 m3 (1000 gallons) use vertical tanks on legs

• Between 3.8 m3 and 38 m3 (1000 to 10,000 gallons) use horizontal tanks on concrete supports

• Beyond 38 m3 (10,000 gallons) use vertical tanks on concrete pads

• Liquids with low vapor pressures, use tanks with floating roofs.

• Raw material feed tanks are often specified for 30 days feed supplies

• Storage tank capacity should be at 1.5 times the capacity of mobile supply vessels.