Hot Vapour Bypass Line (Equalizing line)

[Biggel]

I need to size a hot vapour bypass line for the OVHD system of a depropanizer column. In winter time when the ambient temperature is very low, liquid leaving the air cooler will be sub-cooled. I have to calculate how much hot vapour needs to bypass the air cooler to maintain the pressure in the reflux drum.
Could someone please tell me how to calculate the required vapour flow rate?

Thank you in advance for your comments.

 

[dcasto]

just enough so that the sub-cooled energy equals the heat of condensation of the by-pass.