Heat Exchangers Design Rules of the thumb: 

1. Take true counter current flow in a shell-and-tube exchangers as a basis.

2. Standard tubes are ¾ in. OD, 1 in. triangular spacing, 16ft long; a shell 1ft dia accommodates 100 sqft; 2 ft dia, 400 sqft, 3ft dia, 1100 sqft.
3. Tube side is for corrosive, fouling, scaling, and high pressure fluids.

4. Shell side is for viscous and condensing fluids.

5. Pressure drops are 1.5 psi for boiling and 3-9 psi for other services.

6. Minimum temperature approach is 20% with normal coolants, 10˚F or less with refrigerant.
7. Heat transfer  coefficients for  estimating purposes, Btu/(hr)(sqft)(F˚): water  to liquid, 150: condensers, 150; liquid to liquid, 50; liquid to gas, 5; gas to gas, 5; reboiler, 200. Max flux in reboilers, 10.000Btu/ (hr)(sqft).
8. Double-pipe exchanger is competitive at duties requiring 100-200 sqft.

9. Compact (plate and fin) exchangers have 350 sqft/cuft, and about 4 times the heat transfer per cuft of shell-and-tube units.
10. Plate and frame exchangers are suited to high sanitation services, and are 25-50% cheaper in stainless construction than shell-and-tube units.

11. Air coolers: Tubes are 0.75-1.00 in. OD, total finned surface 15-20 sqft/sqft bare surface, U=80―100 Btu/(hr)(sqft bare surface)(˚F), fan power input 2―5 HP (MBtu/hr), approach 50˚F or more.
12. Fired heaters: radiant rate, 12,000 Btu/(hr)(sqft); convection rate, 4000cold oil tube velocity, 6ft/sec; approx equal transfers of heat in the two sextions; thermal efficiency 70―75%; flue gas temperature 250―350˚F above feed inlet; stack gas temperature 650-950˚F.