NTU method

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The Number of Transfer Units (NTU) Method is used to calculate the rate of heat transfer in heat exchangers (especially counter current exchangers) when there is insufficient information to calculate the Log-Mean Temperature Difference (LMTD).

The method proceeds by calculating the heat capacity rates (i.e. flow rate multiplied by specific heat) Ch and Cc for the hot and cold fluids respectively, and denoting the smaller one as Cmin.

A quantity

q_{max}\equiv C_{min} (T_{h,i}-T_{c,i})

is then found. This value of qmax is the maximum heat which could be transferred between the fluids.

The effectiveness, E is then defined in terms of that maximum:

E \equiv \frac{q}{q_{max}}

where

q = Ch(Th,i − Th,o) = Cc(Tc,o − Tc,i)

is the actual heat transferred between the fluids..

For given geometries, E can be calculated using correlations in terms of the 'heat capacity ratio'

C_r \equiv \frac{C_{min}}{C_{max}}

and the number of transfer units, NTU

NTU \equiv \frac{U A}{C_{min}}

where U is the overall heat transfer coefficient and A is the heat transfer area.

[edit] References

[1] heat- exchanger performance by the LMTD and -NTU methods.

2. F. P. Incropera & D. P. DeWitt 1990 Fundamentals of Heat and Mass Transfer, 3rd edition, pp. 658–660. Wiley, New York

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