Kern [2] recommends that t he maximum allowable vaporizing film coefficients:

•  300 Btu/h.ft² °F for natural or forced circulation vaporizing organics.

•  1000 Btu/h.ft² °F for natural or forced circulation vaporizing aqueous solution of low concentration.

The maximum allowable heat flux:

•  20000 Btu/(h)ft²) for forced circulation reboilers and 12000 Btu/(h)ft²) for natural circulation reboilers vaporizing organics.

•  30000 Btu/(h)ft²) for both forced or natural circulation reboilers vaporizing aqueous solution.

Assume that h_(assm) = 300 Btu/h.ft²°F for organics or 1000 Btu/h.ft²°F for water. With this assumed value, calculate the tube wall temperature ( T_w):

(1.17)

Where, ( d_i tube ID and d_o tube OD)

T_h(avg)= Average temperature of the hot fluid

T_hc=Calorific temperature of the hot fluid

Now, re-determine h_cal (latent heat transfer) from the Figure 1.12 corresponding to (T_w - t ) . ( t is the cold fluid boiling temperature).

Continue the calculation till, T_cal ≈ h_(assm) .

If the calculated h_cal is greater than the maximum heat transfer co-efficient of 300 Btu/h.ft²°F for organics and 1000 Btu/h.ft²°F for water, take h_cal= 300 Btu/h.ft²°F for organics and h_cal = 1000 Btu/h.ft² °F for water.

Calculate the overall heat transfer-coefficient ( U_d ) including the dirt factors.

Figure 1.12 . Natural circulation boiling and sensible heat transfer [3] .