Number of Transfer Units (NTU):

• Effectiveness ε is a function of several variables and as such it is inconvenient to combine them in a graphical or tabular form.
• Also, in calculation of effectiveness, we require three temperatures, two out of which are inlet temperatures and one is outlet temperature. It is difficult to predict outlet temperature of fluid before installation of Heat Exchanger.
• However, by compiling a non-dimensional grouping, ε can be expressed as a function of three non-dimensional parameters. This method is known as NTU method.
• Here ε can be determined with the help of only inlet temperatures.
• This method facilitates the comparison between the various types of heat exchangers which may be used for a particular application.