NTU method - heat capacity ratio
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, epsilon can be calculated using correlations in terms of the “heat capacity ratio” (Cr) which is calculated using the shown formula.
The NTU is calculated by the formula:Related formulas
|Cr||Heat capacity ratio (dimensionless)|
|Cmin||The smaller heat capacity rate between the hot and cold fluids (W/K)|
|Cmax||The larger heat capacity rate between the hot and cold fluids (W/K)|