NTU method - the effectiveness of a counter-current flow heat exchanger
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 example, the effectiveness of a counter-current flow heat exchanger is calculated with the shown formula.
Note that the case that there is a phase change condensation or evaporation is occurring in the heat exchanger. Hence the heat exchanger behavior is independent of the flow arrangement. Therefore the Cr=0.
The heat capacity ratio (Cr) is calculated by the formula:
NTU method – heat capacity ratio
The NTU is calculated by the formula:
NTU method – NTU number
|ϵ||Heat exchanger effectiveness (dimensionless)|
|NTU||Number of transfer units (dimensionless)|
|Cr||Heat capacity ratio (dimensionless)|