Page 228 - Process Equipment and Plant Design Principles and Practices by Subhabrata Ray Gargi Das

226 Chapter 7 Industrial cooling systems

Blowdown, including loss from the process plant (B)
COC for cooling tower normally remains between 3 and 7. We assume COC ¼ 5. Accordingly,

3
3

B ¼ E=ðCOC 1Þ D ¼ 98:467=ð5 1Þ 1:2 ¼ 23:417 m =hr 0:39% on 6000m=hr
3
3
Hence, make up water, M ¼ B þ D þ E ¼ 23.417 þ 1.2 þ 98.467 ¼ 123.08 m /hr, say 124 m /hr,
3
3
amounting to 31 m /hr per cell (2.07% on 6000 m /hr), assuming negligible leakage loss.
Pump calculations
Selection of pump: We choose centrifugal pumps as per industrial practice for high flow rates, i.e.,
3
2 nos. 3000 m /hr capacity. The operating point of the centrifugal pump is chosen to have maximum
3
efficiency. Selecting 2 3000 m =hr pumps will give us additional benefit of operating only 2 cells
with a single pump running if need arises. Since centrifugal pump capacity can be turned down to 50%,
even a single cell operation would be possible.
00
The cooling water header pipe size is chosen to be 20 NB, Schedule 40, having ID 18:812 . This
00
would entail a reasonable velocity of 2.3236 m/s, and the pressure loss estimated to be 0.0776 kg/cm 2
per 100 m length.
At the tower, the header climb is the total of air inlet height (taken to be half the cell width, i.e.,
5 m), and fill and distributor height, i.e., 2 m.
Hence, the total static head of 7 m is considered.
For a static climb of 7 m, the header pressure at the base of the cooling tower, if kept at 1 kg/cm 2
(g) is sufficient.
Pump discharge pressure ¼ Pressure at CT base þ Pressure drop in process plant þ Pressure drop in
2
2
header to and from CT (assumed 1 kg/cm ) ¼ 1 þ 2 þ 1 ¼ 4 kg/cm (g), i.e., 40 mWC.
Assuming pump and motor h to be 85% and 95%,
Pump motor power ¼ (3000/3600) 40 1000 9.8/(1000 0.85 0.96) ¼ 400 kW

Cooling tower sump

2
Base area of tower ¼ 4 10 10 ¼ 400 m

Sump water depth ¼ 1.75 m, Sump freeboard ¼ 0.25 m (assumed)

Sump holdup ¼ 400 1.75 ¼ 700 m 3

This corresponds to holding time of ð60 700=6000Þ¼ 7 min
Note: In case the holding time needs to be increased, additional sump may be provided. The depth
of the sump may be kept 2 m, and a space adjacent to the tower may be used in constructing the
additional sump (Table P7.3).