Question 13.5: Select a cooling tower using Fig. 13-9 and Table 13-2 for th......
Select a cooling tower using Fig. 13-9 and Table 13-2 for the conditions of Examples 13-3 and 13-4. Compute the cooling range, approach, and heat-transfer rate.
| Table 13-2 Performance Data for Some Factory-Assembled Cooling Towers | ||||||
| Nominal Rating | No. of | No. of | Motor | |||
| Model | tons | gpm | Cells | Fans | cfm | hp |
| A | 50 | 120 | 1 | 1 | 10,500 | 5 |
| B | 100 | 240 | 1 | 2 | 21,000 | 10 |
| C | 100 | 240 | 1 | 2 | 21,000 | 2-5 |
| D | 150 | 360 | 1 | 3 | 31,500 | 15 |
| E | 200 | 480 | 1 | 4 | 42,000 | 20 |
| F | 200 | 480 | 1 | 4 | 42,000 | 2-10 |
| G | 250 | 600 | 1 | 5 | 52,500 | 25 |
| H | 300 | 720 | 1 | 6 | 63,000 | 30 |
| I | 300 | 720 | 1 | 6 | 63,000 | 2-15 |
| J | 350 | 840 | 1 | 8 | 84,840 | 2-20 |
| K | 400 | 960 | 1 | 8 | 84,000 | 2-20 |
| L | 500 | 1200 | 1 | 10 | 105,000 | 2-25 |
| M | 600 | 1440 | 1 | 12 | 126,000 | 2-30 |
The entering water temperature and air wet bulb temperature are 100 F and 75 F, respectively, with a water flow rate of 1000 gpm. Referring to Fig. 13-9, a model L would be the obvious choice (or two model G’s). With a cooling range of 15 F the capacity of the towers is
\dot{q} = \dot{m}_{w} c_{pw} Δt_{r} = 500 gpm × Δt
\dot{q} = 500 (1000) 15 = 7.5 × 10^{6} Btu/hr
or about 500 tons (Table 13-2).
Suppose that the heat exchangers (condensers) in the circuit could be changed so that the water would enter the tower at 103 F with a flow rate of 835 gpm. This is still the same duty of about 7.5 × 10^{6} Btu/hr. Referring back to Fig. 13-9, a model K tower fits this situation. The cooling range is
t_{l1} – t_{l2} = 103 – 85 = 18 F
and the approach is
t_{l2} – t_{wb1} = 85 – 75 = 10 F