Number of heat transfer (NTU)

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Hi,

What does NTU indicate about a water cooing coil with fins?
The higer the better?

I am trying to figure how to calculate the NTU for two type of cooling coil with flat and corrgulated fins. what formula do I adopt? Please advice, on hand readings are on coil and off coil temp. (dry and wet bulb).

Please advice .

Thanks