Page 227 - Process Equipment and Plant Design Principles and Practices by Subhabrata Ray Gargi Das

7.3 Design illustration 225

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In this design: H ¼ 1.5 m; L ¼ 15 m /m hr.
Exit air enthalpy (h D;op ) at the operating point,
h D;op ¼ h C þðL=GÞ ðT h T c Þ¼ 22:479 þ 1:587 ð45 33Þ¼ 41:523
op
Corresponding to h D;op , the saturated air temperature interpolated from Table P7.1 is 40.865 C.

Average temperature of air inside the fill ¼ (40.865 þ 29)/2 ¼ 34.933 C
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Interpolating data from Table P7.1, r air (saturated at 34.933 C) ¼ 1.0827 kg dry air/m saturated

air and velocity of air
V air ¼ 1000 945=ð3600 10 10 1:0827Þ¼ 2:4249 m=s
3 1:771 0:77
DPðmmWCÞ¼ 0:63644 exp 2:445307 10 15 ð2:4249Þ ð1:5Þ ð1.0827Þ
¼ 4:7 mmWC
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Fan suction flow rate ¼ 945/(4 1.0827) ¼ 218 m /hr.
Considering head loss in the fill zone to be 70% of total head loss.
Head to be developed by the fan ¼ 4.7/0.7 ¼ 6.71 mmWC, Say, 7 mmWC.
Interpolation of data in Table P7.1 corresponding to saturated air at exit temperature (40.865 C)

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gives specific volume ¼ 0.96357 m /kg dry air.
G op (per cell) ¼ 945 mt/hr dry air ¼ (945/3600) mt/s.
Power required by fan ¼ (1000 945/3600) 0.96357 (7/1000) 1000 9.8 ¼ 17351 W.
i.e., (17351/746 ¼ ) 23.3 HP
Assuming the efficiency of fan, gearbox and the motor to be 69%, 96% and 94%,

Motor power ¼ 23.3/ (0.69 0.97 0.94) ¼ 37 HP.

Estimating make up water (M) requirement
Evaporation loss (E)

Total air inflow to tower (G op ) ¼ 3780 mt/hr dry air

From Table P7.1, corresponding to the exit air temperature (40.865 C) and the ambient wet-bulb
temperature (T amb WB ), the absolute humidity values are 0.05,141 and 0.025,365 kg H 2 O/kg dry air
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E ¼ Evaporation loss (all cells) ¼ 3780 (0.05141e0.025365) ¼ 98.467 mt/hr, i.e., 98.467 m /hr
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(1.64% on 6000 m /hr)

Drift loss (D)
This is based on an empirical estimation of 0.02% of circulation rate, assuming efficient drift elimi-
nators. (Refer 7.2.3)
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D ¼ 0:0002 L ¼ 1:2m =hr