thermal movement calculation

Which is the right method of calculating thermal movement of an equipment when there are more than one incoming or outgoing flow in the equipment.

my problem is which should be followed either on the basis of average temperarature
  or average alpha (coefficient of thermal expansion).

for very tall columns like crude or vacuum column in cdu/vdu unit which would be the governing fluid (either incoming or outgoing or both) for thermal movement calculation
at particular height.

please give a rightful idea regarding this.

thanks

[Non-text portions of this message have been removed] Received on Sat Sep 20 01:54:00 2003