The logarithmic mean temperature difference for parallel flow heat exchangers is
1
\(\rm \frac{\theta_1 - \theta_2}{ \ln (\theta_1 - \theta_2)}\)
2
\(\rm \frac{\theta_2 - \theta_1}{ \ln (\theta_1 / \theta_2)}\)
3
\(\rm \frac{\theta_1 - \theta_2}{ \ln (\theta_1 + \theta_2)}\)
4
\(\rm \frac{\theta_1 - \theta_2}{ \ln (\theta_1 / \theta_2)}\)