Question

# What is the physical significance of the number of transfer units $\mathrm{NTU}=h A_{s} / \dot{m} c_{p} ?$ What do small and large NTU values tell about a heat transfer system?

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NTU or Number of Transfer Units method is a combination of overall heat transfer coefficients, transfer area, fluid flow rate, and heat capacity. It is a summary of all these dimensional parameters into one dimensionless parameter used to measure the effectiveness of a heat transfer system. It signifies how many units of heat, the given heat transfer system is capable of transferring from one fluid to another.

A small NTU value means that the mass flow rate of a particular fluid is low assuming the specific heat capacity remains constant which means that less heat will be transferred into the fluid before its temperature shoots up. A larger NTU value indicates that the heat transfer limit has reached.

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