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Log Mean Temperature Difference (lmtd) Calculator

Log mean temperature difference (LMTD) in the logarithmic average between the hot and cold flows at each end of a heat exchanger. The LMTD is used in heat exchangers to determine the temperature-driving force of heat transfer. This LMTD calculator is used specifically for heat exchangers operating in parallel flow.

Calculation

Inputs



THO
:70K​



TCO
:25K​



THI
:90K​



TCI
:20K​


Output



LMTD
:56.5824961K​



Explanation

In parallel flow heat exchangers the hot and cold fluids enter at the same end, and flow in the same direction to leave together at the other end of the exchanger. This means that the greatest temperature difference occurs as the fluids enter. This difference decreases along the length of the exchanger as heat is transferred between the fluids. Log mean temperature difference is is used to describe this difference using the following the equation.
Log Mean Hot Temperature

Log Mean Hot & Cold Temperature



ΔTLM=(THo−TCo)−(THi−TCi)ln(THo−TCoTHi−TCi)\Delta T_{LM} = \frac {(T_{H_o} - T_{C_o})-(T_{H_i} - T_{C_i})}{ln(\frac {T_{H_o} - T_{C_o}}{T_{H_i} - T_{C_i}})}
Where:

ΔTLM=Log Mean Temperature Difference (K)THO= Outlet Temperature of the Hot Fluid (K)TCO= Outlet Temperature of the Cold Fluid (K)THi= Inlet Temperature of the Hot Fluid (K)TCi= Inlet Temperature of the Cold Fluid (K)\Delta T_{LM} = Log \: Mean \: Temperature \: Difference \: (K) \\ T_{H_O} = \: Outlet \: Temperature \: of \: the \: Hot \: Fluid \:(K) \\ T_{C_O} = \: Outlet \: Temperature \: of \: the \: Cold \: Fluid \: (K) \\ T_{H_i} = \: Inlet \: Temperature \: of \: the \: Hot \: Fluid \:(K) \\ T_{C_i} = \: Inlet \: Temperature \: of \: the \: Cold \: Fluid \: (K) \\

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