It is expected that the Poppe method will lead to
more accurate results than those obtained by employing
the Merkel and e-NTU methods, as it is the more rigorous
method. The results of the Merkel and e-NTU methods
of analysis are generally very close to each other as
these methods are based on the same simplifying
assumptions. The comparison between the Poppe and
Merkel methods is shown on the psychrometric charts
in Figs. 5 and 6. The conventional psychrometric chart
is extended to accommodate air that is in the supersaturated
state. The enthalpy of supersaturated air on the
psychrometric chart is given by Eq. (29).
Fig. 5 shows the heating path of the state of the air in
a wet-cooling tower for relatively cold inlet air which is
saturated with water vapor in this particular example.
The path of the air according to the Merkel method is
shown as a broken straight line in Fig. 5. The line for
the Merkel method is presented as a broken line because
straight lines can only be used on psychrometric charts if
the temperature of the water surface is constant. The line
according to the Merkel method is presented as a
straight line because no other information is given by
the Merkel method about the humidity of the air except
that it is saturated at the air outlet side. That is why the
air at the outlet of the cooling tower is assumed to be on
the saturation line as shown in Fig. 5.
The Poppe method, on the other hand, gives the state
of the air for the entire evaporative process. Since the