2 replies to this topic

[Hawkz1600]

I am an final year undergraduate in chemical and process engineering. I am designing a falling film evaporator for black liquor evaporation.

 

I came across the following two equations in The Handbook of Evaporation Technology by Paul E. Minton (1986)

 

ɼ_min = 19.5*(µ_1*s_1*σ^3)^0.2  - (EQ 1)

 

where 

ɼ_min = minimum tube loading, lb/(hr)(ft)

µ_1 = liquid viscosity, centipoise

s_1 = liquid specific gravity (referred to water)

σ = surface tension, dynes/cm

 

and

 

ɼ_T = 2.4*(µ_1*s_1*σ^3)^0.2 - (EQ 2)

 

where

ɼ_T = Terminal flow rate, lb/(hr)(ft)

and other terms are the same as defined in the previous equation.

 

According to this if the minimum rate as given in (EQ 1) is not achieved, the film will not form. After that the flow rate should be maintained above the terminal rate as given in (EQ 2).

 

I could not find the definition of the flow rate that we compare with here. The rate here should be in the units  lb/(hr)(ft)  (i.e. in metric terms something like kg/sm)

 

I'm wondering whether the flow rate is the mass flow rate of liquid through the tube per unit tube length.

i.e. [Mass flow rate(lb/hr)/tube length(ft)]

 

Or it could be the same as the flow rate defined in film Reynolds number (4*Γ/η_f) as mass flow rate of liquid through the tube per unit wetted perimeter. i.e. [Mass flow rate(lb/hr)/π*tube inner diameter(ft)]

 

Or is it defined in a different manner? I cannot seem to find a definition for the flow rate here. 

 

Thanks in advance.

 

Regards

Edited by Hawkz1600, 29 December 2013 - 01:09 PM.

[breizh]

Consider this resource to support your study . References in the paper mentioned HTRI and Perry .

 

Hope this helps.

 

Breizh

[Hawkz1600]

@Breizh: Thanks for the resource. I managed to confirm my suspicion based on the definition of "Minimum wetting rate" which is defined as "minimum mass flowrate per unit circumference that is required to maintain of complete falling film of liquid on a surface". So I need to divide the mass flow rate by the wetted perimeter and not by the tube length.

 

References

• http://onlinelibrary...140113/abstract
• http://ir.canterbury...is_fulltext.pdf