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The effectiveness NTU method is used for determining the heat transfer rate for a heat exchanger, with known inlet temperatures and surface area.

It makes use of effectiveness which is defined as

$$\epsilon = \frac{Q_{actual}}{Q_{max}}$$

What the term in the denominator truly represents?

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  • $\begingroup$ The term represents the maximum possible heat transfer for the heat exchanger. This occurs when one of the fluids undergoes the largest possible temperature change (which is determined by the difference between the inlet temperatures of the two fluids), constrained by the fluid with the smaller heat capacity rate (the product of mass flow rate and specific heat). $\endgroup$
    – mcodesmart
    Commented Sep 4 at 23:47

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$Q_{max}$ is the maximum rate of heat transfer possible, and is equal to either $\dot{m_h}C_h(T_{h,in}-T_{c,in})$ or $\dot{m_c}C_c(T_{h,in}-T_{c,in})$ whichever is less, where $\dot{m}$ is the mass flow rate of a stream and C is its heat capacity. The first expression assumes that the hot stream is cooled to the inlet temperature of the cold stream, and the second expression assumes that the cold stream is heated to the inlet temperature of the hot stream. One of these rates will be limiting.

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  • $\begingroup$ Ohh Ok. I read that maximum rate of heat transfer is possible in a counter flow heat exchanger with some given inlet temperatures, mass flow rates and specific heats of fluids. Could you explain why is that the case? Why not some other heat exchanger say cross flow. $\endgroup$
    – Harshit Rajput
    Commented Aug 16, 2021 at 4:15
  • $\begingroup$ Instead of cross flow, why don't you consider the answer to this question first for co-current flow? $\endgroup$
    – Chet Miller
    Commented Aug 16, 2021 at 11:41
  • $\begingroup$ Yes it makes sense when I look from the perspective of a co-current flow HX. Is it like that no matter what is the type of HX, we will obtain maximum heat transfer rate when the fluids flow anti parallel to each other( for given inlet temp., flow rates and specific heats) So if I have a double pipe HX, then for this construction type, I can have parallel flow or counter flow, and maximum heat transfer will be obtained when I assume counter flow, even though it may be the case my actual double pipe HX is a parallel flow one? $\endgroup$
    – Harshit Rajput
    Commented Aug 16, 2021 at 13:26

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