Page 225 - Process Equipment and Plant Design Principles and Practices by Subhabrata Ray Gargi Das

7.3 Design illustration 223

Steps of calculation
The saturated air enthalpy curve (AB) is drawn in Fig. 7.7 for the range 20e50 C using data from

Table P7.1.

Saturated air enthalpy (on line AB) at T amb;WBT ¼ 29 C, is read; h c ¼ 22.479 kcal/kg dry air.
1. Point C, the air entry location on the operating line is located on the graph corresponding to

(T c ¼ 33 C, h c ¼ 22.479 kcal/kg dry air)
0
2. A straight line CD is drawn that touches the curve AB. Slope of this line corresponds to the
theoretically minimum air requirement, i.e., ðL=GÞ .
max
The slope of the operating line must be lower than ðL=GÞ .
max
3. Referring to Table 7.2, we note that for “Film Fill” 1.5 < ðL=GÞ < 2 is recommended.
Accordingly, a value of ðL=GÞ is assumed as say 1.5.
4. An operating line CD is drawn with a slope of ðL=GÞ. The end point D of the operating line is

located corresponding to water temperature T h ¼ 45 C. Corresponding enthalpy (h D ) of air
exiting the tower is: h D ¼ h C þðL=GÞðT h T c Þ e.g., for ðL=GÞ¼ 1.5,
h D ¼ 22:479 þ 1:5 ð45 33Þ¼ 40:479 kcal/kg dry air
5. The KaV corresponding to the ðL=GÞ value assumed is the area CDD’B that represents R Th dT .
L Tc ðh hÞ
0
The integral can be evaluated by the Chebeyshev’s method as suggested by CTI. For ðL=GÞ¼ 1.5,
KaV
L ¼ 1.7858.
6. For different values of ðL=GÞ, the corresponding KaV is calculated following Step 5e7. The results
L
are presented in Table P7.2.
Table P7.2 KaV [ R Th dT , from Merkel equation.
L Tc ðh LhÞ
0
ðL=GÞ 1.50 1.55 1.60 1.65 1.70 1.75 1.80 1.85 1.90 1.95 2.00
KaV 1.7858 1.8558 1.9326 2.0181 2.1138 2.2216 2.3445 2.4861 2.6515 2.8482 3.0869
L

ðL=GÞ corresponds to the operating condition when the chosen fill provides the required KaV
op L
i.e., the KaV from the Merkel equation and that from the fill characteristics match. ðL=GÞ is therefore
L op
found by drawing the lines for ðL=GÞ vs. KaV from the fill characteristics equation and from Table P7.2
L
on the same graph (Fig. P7.1) and locating the intersection point. The lines intersect at
ðL=GÞ op ¼ 1.587 in this design example.
Note: Although earlier, it has been mentioned that the tower characteristics are plotted with log-log
axes, Fig. P7.1 has been drawn with the linear axis as the range of variation is small.
Fan power calculation
3 2 2
For ðL=GÞ ¼ 1.587 and C ¼ 6000 mt/hr, L ¼ 15 m /m hr, G ¼ 15/1.587 ¼ 9.45 mt/m hr dry
op
airflow.