NTU method - NTU number

Description

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). In heat exchanger analysis, if the fluid inlet and outlet temperatures are specified or can be determined by simple energy balance, the LMTD method can be used; but when these temperatures are not available The NTU or The Effectiveness method is used.

For any heat exchanger it can be shown that:
the effectiveness (epsilon) = f (NTU,Cr)

For a given geometry, the number of transfer units (NTU) can be calculated by the shown formula.

The heat capacity ratio (Cr) is calculated by the formula:

NTU method – heat capacity ratio

Related formulas

Variables

NTUNumber of transfer units (dimensionless)
UOverall heat transfer coefficient (W/m2.K)
AHeat transfer area (m2)
CminThe smaller heat capacity rate between the hot and cold fluids (W/K)