LMTD, Log mean temperature difference is the temperature driving force for heat transfer in flow systems. When you are calculating heat transfer by overall heat transfer coefficient (U), you should not simply multiply temperature difference of hot stream or cold stream. Multiplying this temperature difference doesn't account the overall driving force.
You can calculate the heat lost by a hot stream or cold stream by applying (Thin – Thout ) or (Tcout - Tcin) as mCp∆T. Then you can calculate the overall heat transfer coefficient (U) or area (A) if you know LMTD by the following formula.
mCp∆T = UA∆TLMTD
Whereas LMTD =( ∆T1 - ∆T2)/ln ( ∆T1 /∆T2)
∆T1 = Temperature approach at one end Thin – Tcin for co current flow and Thin – Tcout for counter current flow.
∆T2 = Temperature approach at one end Thout – Tcout for co current flow and Thout – Tcin for counter current flow.
Note: LMTD is not applicable for boilers or condensers where latent heat involves.
For the excel calculator click the following link.