# d8

Chris Nkinthorn, $\texttt{20190717}$

**Prompt:** Please discuss 4 major regions for phase change heat transfer

**Discussion:** Parallel and counterflow heat exchangers (HX), are both thermodynamic systems, where a hot line donates thermal energy to a cool line. In order to maximize heat transfer, they are often coiled together but simply because these are two lines adjacent to each other, heat energy will move from high to lower temperature, along the length of contact . Relative to the direction of the hot fluid in the line, the coolant can either flow in the same direction or opposite. This is the difference between parallel and counterflow HX. In parallel flow, the cool fluid will always remain cooler than the end of the hot line, which is lower in temperature than the maximum temperature. In counter flow, the cool fluid can exit closer to the maximum temperature. Counter flow is also more efficient than parallel flow HX, and so generally preferable.

**Response to:** Joe Bostick

Great discussion in bringing in the log mean temperature difference (LMTD), where the difference in temperature is normalized by the difference in logarithmic values. Due to log transformation rules, the denominator can be written as [ ln(T_a/T_b) = ln(T_a) - ln(T_b) ]. This gives insight to how the temperature difference's along the absolute scale affects the role of temperature, as each additional increment in the higher temperature drives exponentially reduces in contribution.

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