Question

(a) In a typical water-cooling tower, warm water flows countercurrent to an air stream. The warm water enters the top of a packed tower and cascades down through the packing, leaving at the bottom. Air enters at the bottom of the tower and flow upward through the descending water. Briefly explain why usually adiabatic mass transfer and constant water flow can be assumed in calculation of tower size. (5 Marks) (b) A typical countercurrent water-cooling tower is to be designed for to cool water from 46.1 to 29.4 $^\circ$C. Air enters at the bottom of the tower at 23.9 $^\circ$C (with a wet bulb temperature of 21.1 $^\circ$C) and flow upward through the descending water. Estimate the tower height needed if 1.5 times the minimum air rate is used. The height of gas enthalpy transfer unit is 0.5 m. (20 Marks) The enthalpies of saturated air-water vapor mixtures are shown in Table 1. State clearly your assumptions. Table 1 Temp ($^\circ$C) Enthalpy (J/kg dry air) 15.6 $43.68 \times 10^3$ 26.7 $84.0 \times 10^3$ 29.4 $97.2 \times 10^3$ 32.2 $112.1 \times 10^3$ 35.0 $128.9 \times 10^3$ 37.8 $148.2 \times 10^3$ 40.6 $172.1 \times 10^3$ 43.3 $197.2 \times 10^3$ 46.1 $224.5 \times 10^3$ 60.0 $461.5 \times 10^3$

          (a) In a typical water-cooling tower, warm water flows countercurrent to an air stream. The
warm water enters the top of a packed tower and cascades down through the packing, leaving
at the bottom. Air enters at the bottom of the tower and flow upward through the descending
water.
Briefly explain why usually adiabatic mass transfer and constant water flow can be
assumed in calculation of tower size.
(5 Marks)
(b) A typical countercurrent water-cooling tower is to be designed for to cool water from 46.1
to 29.4 $^\circ$C. Air enters at the bottom of the tower at 23.9 $^\circ$C (with a wet bulb temperature of
21.1 $^\circ$C) and flow upward through the descending water. Estimate the tower height needed
if 1.5 times the minimum air rate is used. The height of gas enthalpy transfer unit is 0.5 m.
(20 Marks)
The enthalpies of saturated air-water vapor mixtures are shown in Table 1.
State clearly your assumptions.
Table 1
Temp ($^\circ$C) Enthalpy (J/kg dry air)
15.6 $43.68 \times 10^3$
26.7 $84.0 \times 10^3$
29.4 $97.2 \times 10^3$
32.2 $112.1 \times 10^3$
35.0 $128.9 \times 10^3$
37.8 $148.2 \times 10^3$
40.6 $172.1 \times 10^3$
43.3 $197.2 \times 10^3$
46.1 $224.5 \times 10^3$
60.0 $461.5 \times 10^3$

(a) In a typical water-cooling tower, warm water flows countercurrent to an air stream. The
warm water enters the top of a packed tower and cascades down through the packing, leaving
at the bottom. Air enters at the bottom of the tower and flow upward through the descending
water.
Briefly explain why usually adiabatic mass transfer and constant water flow can be
assumed in calculation of tower size.
(5 Marks)
(b) A typical countercurrent water-cooling tower is to be designed for to cool water from 46.1
to 29.4 ^∘C. Air enters at the bottom of the tower at 23.9 ^∘C (with a wet bulb temperature of
21.1 ^∘C) and flow upward through the descending water. Estimate the tower height needed
if 1.5 times the minimum air rate is used. The height of gas enthalpy transfer unit is 0.5 m.
(20 Marks)
The enthalpies of saturated air-water vapor mixtures are shown in Table 1.
State clearly your assumptions.
Table 1
Temp (^∘C) Enthalpy (J/kg dry air)
15.6 43.68 × 10^3
26.7 84.0 × 10^3
29.4 97.2 × 10^3
32.2 112.1 × 10^3
35.0 128.9 × 10^3
37.8 148.2 × 10^3
40.6 172.1 × 10^3
43.3 197.2 × 10^3
46.1 224.5 × 10^3
60.0 461.5 × 10^3

Added by Thomas P.

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