The number of transfer units (NTU) method is used to calculate the rate of
heat transfer in
heat exchangers (especially parallel flow,
counter current, and cross-flow exchangers) when there is insufficient information to calculate the
log mean temperature difference (LMTD). Alternatively, this method is useful for determining the expected heat exchanger effectiveness from the known geometry. 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 either the NTU or the effectiveness NTU method is used.
The effectiveness-NTU method is very useful for all the
flow arrangements (besides parallel flow, cross flow, and counterflow ones) but the effectiveness of all other types must be obtained by a numerical solution of the partial differential equations and there is no analytical equation for LMTD or effectiveness.
Defining and using heat exchanger effectiveness
To define the effectiveness of a heat exchanger we need to find the maximum possible heat transfer that can be hypothetically achieved in a counter-flow heat exchanger of infinite length. Therefore ''one'' fluid will experience the maximum possible temperature difference, which is the difference of
(the temperature difference between the inlet temperature of the hot stream and the inlet temperature of the cold stream). First, the
specific heat capacity of the two fluid streams, denoted as
, must be known. By definition
is the derivative of enthalpy with respect to temperature:
This information can usually be found in a thermodynamics textbook, or by using various software packages. Additionally, the mass flowrates (
) of the two streams exchanging heat must be known (here, the cold stream is denoted with subscripts 'c' and the hot stream is denoted with subscripts 'h'). The method proceeds by calculating the
heat capacity rates (i.e. mass flow rate multiplied by
specific heat capacity)
and
for the hot and cold fluids respectively. To determine the maximum possible heat transfer rate in the heat exchanger, the minimum heat capacity rate must be used, denoted as
:
: