For this section we suggest that you review Potter Example 12.14, and attempt to solve Supplementary Problems 12.64 and 12.66.
In the following we show a schematic diagram
of a cooling tower in the context of a steam power plant:
mass flow:
Referring to the diagram above the mass flow rate of the makeup water is given by the difference in specific humidity
at the inlet and outlet air streams. Thus the mass flow balance equations for the cooling tower become:
energy:
Combining the mass flow equations with the cooling tower energy equation leads to the final equation relating the mass flow rate of the dry air to the circulating cooling water of the condenser, as follows:
The mass flow rate of the liquid water at station
(3) is normally provided from the condenser energy equation of
the steam power plant. Recall from Chapter
12 that the specific humidity
is related to the various pressures and the relative humidity
by the following relations:
The pressure Pv is the partial pressure of the vapor, Pg is the saturation pressure at temperature T, and P is the total pressure (air + vapor), usually taken as one atmosphere (101.325 kPa). All of these relations can be most conveniently evaluated on a Psychrometric Chart.
Note that the enthalpies of the vapor (h1 and h2) and those of the liquid (h3, h4, hw) can be conveniently evaluated as follows:
The temperature T is in degrees Celsius, and the specific heat capacity of dry air CP is approximately 1.00 [kJ/kg°C] and that of liquid water approximately 4.18 [kJ/kg°C]. In the above analysis we have assumed that the temperature of the makeup water Tw equals the temperature of the cooled circulating water T4. Alternatively the values of enthalpy for the vapor (h1 and h2) can also be conveniently evaluated on the Psychrometric Chart.
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