CHAPTER 9 | Pumps

Every cooling tower requires at least one pump to deliver water. Pump selection is based on:

-Flow Rate

- Total Head, and

- Ancillary issues such as type, mounting, motor enclosure, voltage, efficiency, price, etc..

GPM is dictated by the manufacturer of the equipment being cooled.

The total head is calculated for the unique characteristics of each project:

Total head =

Net vertical lift (ft.). Plus, Pressure drop at the cooling tower exit. Plus, Pressure drop in the piping to the pump. Plus, Pressure drop from the pump to the item being cooled. Plus, Pressure drop through the item being cooled. Plus, Pressure drop from the cooled item back to the tower. Plus, Pressure drop for the tower's water distribution system. Plus, Velocity pressure (For open systems- the pressure necessary to cause the water to attain its velocity... V2/2g.) The total head is tabulated in feet- the height of a vertical water column. Values expressed in psi are converted to feet by the formula:

Headft = psi x 2.31 The vertical lift is the distance the water must be ‘lifted’ before it is ‘let go’ to fall through the cooling tower. It is the distance the water falls. Typically, this is the distance between the operating level and the water inlet. (The value is zero for closed circuit systems.)

Pressure drop at the cooling tower outlet consists of the strainer and the outlet connection. The presence of a large area strainer and vortex breaker make the value low enough that it may be ignored or assigned a value of about 1psi..

Pressure drop in the piping to the pump consists of friction losses as the water passes through the pipe, fittings and valves. Fittings and valves are converted to equivalent lengths of straight pipe (from piping manual) and added to the actual run to get an equivalent length of suction piping. Then, the tabulated pressure drop (from the piping manual again) for 100’ of pipe is compared to the equivalent pipe length and the pressure drop calculated by proportion:

Pressure drop = pressure drop for 100’ of pipe x equivalent pipe length/100’
The remaining pipe segments are calculated the same way. [Actually, all piping of the same size can be lumped together and calculated at one time.] Pressure drop through the equipment is provided by its manufacturer.

Pressure drop at the inlet is zero for towers where the water is delivered into an open hot water basin. (A very small pressure drop can be added for flow control valves.)

Towers with a pressurized header and spray nozzles will have the spray pressure tabulated on the spec sheet. A typical value would be 2psi..

Velocity pressure can be calculated as V2/2g but is typically picked from a chart.

Finally, the pump is selected based on flow and total head.


Lift = 12’ minus 18” or,
10.5 ft.
PD at tower exit assume 1psi
PD in the suction piping:

Linear ft. of pipe = 40

Equivalent linear ft. of fittings:

six elbows + one gate valve

6 (4.7) + 1 (2.6) = 28.2 + 2.6 = 30.8

Total equivalent linear ft. 70.8’

PD = (5.05 ft./100 lin. ft.) x 70.8 lin. ft.= 3.6 ft.
PD in all discharge piping:

Linear ft. of pipe = 60

Equivalent linear ft. of fittings

four elbows

4 (4.7) = 18.8

Total equivalent linear ft. 78.8

PD = (5.05 ft./100 lin.ft.) x 78.8 lin. ft. = 4.0ft.

Separator pres. drop = 3psi x 2.31’/psi = 6.9ft

Cooling tower spray pressure 2psi x 2.31ft./psi = 4.6ft.
PD thru heat exchanger:

10psi x 2.31 ft./psi = 23.1 ft.
Velocity pressure (from chart):


The required spray pump is selected at 900gpm and 57ft. of head.

Manufacturers of cooling towers with pressurized water distribution systems can give the ‘spray pressure’ in psi at the cooling tower inlet (as in the example) but it is more common to find the cooling tower head requirement expressed in feet at the base of the tower. This figure is merely the elevation to the inlet for towers with hot water basins or this same elevation plus the spray pressure in feet for towers utilizing spray nozzles. It is important that the distinction be understood.

Typically, end suction pumps are selected and are of the close coupled type (where the pump impeller fastens directly to the motor shaft and the pump housing bolts directly to the motor) for up to about the 10hp size and of the base mounted type (where the separate pump and motor fasten to a base and are connected by a coupling) for the larger sizes. Where suction lift is high- as when concrete basins are below grade- turbine pumps are used. Cooling tower pumps should be brass fitted with mechanical seal. A TEFC motor enclosure is a good idea- and necessary for outdoor installations. If ozone water treatment is used, the pump seal must be Viton. A good pump sales engineer can help the specifier with the required features.

The static lift is typically the distance between the operating level in the cold water basin and the tower inlet near the top- a figure close to the height of the tower. So close, in fact, that using the height of the tower can become common place. This practice will cause a problem, however, when a Remote Sump is used. The distance between the remote sump operating level and the tower inlet must be used in this case for proper pump sizing.

The final check when selecting a pump is to make sure the net available suction head exceeds the required net suction head. This insures the application will not cause water to vaporize inside the pump causing a phenomenon called cavitation. Vaporization inside the pump occurs when small water particles essentially ‘boil’ on the suction side of the pump. These ‘bubbles’ collapse as they pass into the high pressure side producing the classic ‘marbles in the pump’ sound. The pump can be damaged when made to operate under this condition and it must be avoided. Fortunately, cavitation is rare in cooling tower applications because of the relatively low water temperatures and the fact that pumps are generally located next to the towers so there is little pressure drop in the suction line. They are installed below the cooling tower’s cold water basin such that they are flooded. (Long, restrictive suction lines with strainers and suction lift invite cavitation.)

The pump’s required net positive suction head (NPSH) is obtained from the pump manufacturer- typically, on a small curve just below the pump (GPM Vs Head) curve.

The available NPSH equals:

- Barometric pressure - less friction losses in the suction line - less vapor pressure of the water being pumped - less suction lift (or, plus suction head depending if the pump is above or below the basin)
Example: From the previous example, available NPSH equals...

Barometric pressure @ 5,000ft.(see Glossary) 28.2ft.
Less suction line losses 3.6ft.
Less vapor pressure @ 100 Degrees F. (see Glossary) 2.2ft.
Plus suction head 2.0ft.
Total....... 36.0ft.


If the required NPSH (from ther pump mfgr.) is less than the available 36.0ft., cavitation will not be a problem.

Cavitation and air entrainment are often confused but are not the same. Cavitation has water vaporizing and collapsing inside the pump while air entrainment is the pumping of small air bubbles along with the water.

Be sure to read and follow the pump manufacturers recommendations as to mounting, pipe support, required length of straight pipe before the inlet, etc..

The previous example demonstrates the need to keep suction losses low- but not so low that the suction line becomes an effective air separator (Ch.5). In line suction strainers, if used, should not have fine mesh baskets.

It also shows why very large industrial towers tend to be of the crossflow design (zero spray pressure) and built as low as possible to save pumping costs.

The system designer should consult with his local pump rep(s) for recommendations. This is also an excellent place to secure pipe sizing charts and pump selection software.

Check the Glossary for pump equations.
More information can be found at www.mcnallyinstitute.com