BELL & GOSSETT BULLETIN NO. TEH-275Revision 4June, 2004

Sizing Cooling TowerPumps and Piping

FLUID HANDLINGTRAINING & EDUCATION DEPT.

COPYRIGHT 1975,1980 BY ITT CORPORATION

The following six steps must be taken in sizing the pump and piping for small cooling towerinstallations:

Step 1. Determine the amount of water in gallons per minute, (GPM), to be circulated to thetower.

Step 2. Make a layout drawing showing the complete piping system.

Step 3. Determine the total static head of the system and the pressure drop through all units.The sum of these static and friction head losses is an important component of the requiredpump head.

Step 4. Size the pipe for the required flow rate. This determines the friction loss rate, andtherefore the total piping system friction loss. Add the piping system friction loss to thestatic head and friction loss of the system components to determine the pump headrequired.

Step 5. Select the pump for the required flow and head.

Step 6. Double check pump and pipe sizes.

This simple method is accurate for general applications of centrifugal pumps to cooling towers. Itwill enable the designer to select pumps with minimum installation and operating cost, yet assureproper circulation through the entire system. More detailed discussion of cooling tower pumping isavailable in ITT Fluid Handling Publication TEH 1075.

ExampleTo illustrate the procedure, assume that a cooling tower circulating system is to be designed for anair conditioning installation of 15 tons.

STEP 1. Determine the flow rate of water in GPM to be circulated to the tower.

In air conditioning and refrigeration projects, the required flow can usually be determined from thetower manufacturer's data. The typical cooling tower requires 3 to 5 GPM per ton. For applicationsother than air conditioning and refrigeration, the required flow must be determined from the coolingload or tower manufacturer.

In this case, assume that 3 GPM per ton or 45 GPM total is required.

STEP 2. Make a layout showing the complete piping system.

Sketch all the components of the cooling tower circuit, (see Figure 1). If drawn to scale, it is easy todetermine the lengths of pipe used to connect the pump and other components to the tower. It isextremely important that the condensers and all other high pressure drop devices be installed onthe discharge side of the pump only.

STEP 3. Determine the static head of the system, and the friction head loss through all unitsexcept the pipe and fittings.

Figure 1The total static head of a cooling tower system is the vertical distance, in feet, between the freewater level of the cooling tower pan or storage tank and the point of water discharge at the nozzlesof the tower.This distance is either of two values:

1. The vertical distance from the level of the water in the tower pan to the nozzles. Thecooling tower manufacturer sometimes adds this distance to friction losses in the tower to get anoverall pressure drop figure for the tower. If it is not included in the overall figure, it must be added.See Total Static Head in Figure 1.

2. When a storage tank is used, the vertical distance between the free water level in thestorage tank and the nozzles of the tower. See Figure 2.

If the free surface of the water in the tower basin is lower than the pump suction, a Static SuctionLift exists. The pump must develop a partial vacuum at its suction in order to lift water againstgravity and overcome suction piping friction head loss. Special care is required in designingsuction lift systems. See Step 4 for details.

The pressure drop through each unit in the system can be determined from manufacturers' data.The items usually considered are condensers or air conditioning units, control valves, if used, andthe cooling tower. Insure that the units head loss is calculated at the required flow rate. A Bell &Gossett System Syzer calculator can be used to find actual head loss at design flow from themanufacturers data.

StaticDischarge

Head

StaticSuctionHead

TotalStaticHead

Tower

Condenser

Triple Duty ValveA

B

C

D

Figure 2

If more than one condenser or air conditioning unit is used, the piping should be made in paralleland only the largest pressure drop considered in determining pump head requirements.

It is advisable to check system flow conditions through all units. For example, a condenser mayrequire a flow of 3 GPM per ton to provide the desired cooling effect, whereas the cooling towermay require 4 GPM per ton to reject the total heat load. If the pressure drop through the condenseris calculated at 3 GPM flow and the full 4 GPM system flow is circulated through it, the totalpressure drop in the system will be greater than expected. It is necessary therefore to open theby-pass valve to allow a by-pass flow of 1 GPM per ton, so that the pump head remainsapproximately as originally calculated.

After the total static head and other pressure drops have been determined, they should beconverted to feet of head, and totaled.

In this problem, assume the following pressure drops from the manufacturer's data:

Condenser manufacturers data:7.5 psi at 50 GPM

Using the System Syzer, find that the actual pressure drop at 45 GPM equals 6 psi.

StaticSuctionHead

TotalStaticHead

Condenser

Triple Duty Valve

Tower

Gravity Head

IndoorTank

X

Y

Converting to feet of head loss:6 psi pressure drop x 2.3= 13.8 ft.

Cooling tower data: 24.2 feet at 45 GPM= 24.2 ft.Assume that this includes both static head and tower friction losses.

STEP 4. Size the pipe

Size the pipe to carry the required flow within the following velocity limits: Minimum of 2 ft/s in order to carry entrained air bubbles to the purge point in the tower. Maximum of 5 ft/s in the suction piping to avoid excessive head loss.According to the enclosed friction loss chart, a 2 pipe can carry the required flow of 45 GPM ata velocity of about 3 ft/s, and a friction loss rate of 1.6 ft/100 feet. A 2 pipe at the same flow ratewould have a velocity of about 4.5 ft/s, and a friction loss rate of about 4 ft/100 feet. For thisexample, assume all pipe and fittings are 2 .Fittings usually have greater resistance to flow than a straight length of pipe. There are severalways to account for this fitting head loss. For typical small systems, it is safe practice to considerthe resistance of the fittings to be 50% of the resistance offered by the straight lengths of pipealone. The sum of the lengths of straight pipe plus 50% is called the total equivalent length.

To find the total equivalent length in this example assume the pipe lengths as shown in Fig. 1 to beas follows:

Section Length(feet)

AB 60CD 45DA 5Measured length 110

Adding 50% for the resistance of the fittings:

Total equivalent length = 110+55=165 ft.

Determine the friction head loss in pipes and fittings by multiplying the total equivalent length infeet, by the friction loss rate in feet of head loss per 100 feet of pipe length.Instead of friction loss tables, a System Syzer, could be used.The total head loss in the piping is:

165 feet x 1.6 feet head loss/100 feet pipe length = 2.7 feet head loss.

A multi-purpose valve called a "Triple Duty Valve" is often used in cooling tower circuits to providethe operator with the means to adjust flow to the design value. Triple Duty Valves also act as checkvalves and as service valves for the pump. The Triple Duty Valve is installed at the pump dischargewhere the check valve is shown in the figure. An open 2 1/2" Triple Duty Valve would add about1.6 feet of head loss to the circuit at 45 GPM.Some systems may benefit by using a special fitting called a suction diffuser. If a suction diffuseris required, it must be specially built to withstand the dirty, highly oxygenated water typical of

cooling tower systems. Consult your Bell & Gossett dealer to determine if a suction diffuser isrequired.

All the piping in the circuit is sized at 2 1/2" If the pump, condenser, or cooling tower nozzles arenot 2 1/2", use reducers with close nipples to make the connections.

Total pump head required:

Static head and manufacturers data 13.824.2

Pipe, fitting, and valve head loss 4.3Total head required 42.3

Suction PipingIt's important to check the piping on the suction side of the pump to be sure that an excessivevacuum will not be created.

It is good practice to never permit the pressure on the suction side of a centrifugalpump to go below a vacuum of approximately 13" Hg. This is the equivalent of a negative waterpressure of 15 ft. measured at 85F. That is to say, if there is a vertical lift on the suction side of apump, the sum of the vertical distance from the level of the water up to the pump plus the headloss through the suction piping must not exceed 15 ft. This is often referred to as a maximum TotalDynamic Suction Lift of 15 ft.

This is a standard in the pump industry because a centrifugal pump cannot produce its establishedperformance of head and capacity if total dynamic suction lift exceeds 15 feet.

To check the suction piping in this example, determine its Total Equivalent Length. On the sketchthe pipe length from point C to the pump at A is 50 ft. Adding 50% for the resistance of the fittings,the Total Equivalent Length is 75 ft.

It was determined earlier in this step that the pipe was to be sized at a head loss rate of 1.6 feet ofhead loss per 100 feet of pipe. The total head loss through the suction piping is therefore:

1.6 feet/100 feet x 75 feet = 1.2 feet of friction head loss in the suction piping

As shown on the sketch, there is a positive static head of at least 45 ft. between the cooling towerand pump. Deducting the head loss of 1.2 ft. in the piping, the head on the suction side of thepump is at least 45 ft. - 1.2 ft. = 43.8 ft.This is 57.8 ft. above the negative 15 ft. of water head allowable in the industry.

Step 5. Select the Pump

The pump can be identified from the sample range charts. Other methods exist, but for thisexample, we'll use the range charts. The pump selected must be able to deliver the 45 GPMdetermined in Step 1 at a head equal to or slightly greater than the 42.3 feet calculated above.

It's best to select a pump which can provide the design flow near the middle of its curve becausethe pump will be more efficient, and operating problems are less likely to occur.

The 3500 rpm Series 1535 Model 352 is near the end of its curve and the 1750 rpm Series 1522provides only 29 feet of head at 45 GPM, and the. Therefore, we refer to the Series 1531 1750 rpmrange chart. Read upward from 45 GPM into the area covered by the first pump, a 1 1/4AC.Reading to the left from this point shows the available head to be well within the 43 ft required.The exact impeller diameter and motor size can be determined from the actual pump curve.Detailed pump curves are available in Bell & Gossett pump curve booklets.An even better way of selecting the pump, Triple Duty Valve, and Suction Diffuser is by usingESP-Plus software, available from your Bell & Gossett representative.

Step 6. Check Pump and Pipe Sizes

To be sure that the pump selected is large enough to permit the most economically sized pipes, thedesigner should recalculate, using the next larger sized pump. In some cases a slightly largerpump will permit the use of smaller pipes and fittings, resulting in savings. Be careful to observethe recommended velocity limits, particularly in the suction piping.

Sizing the gravity return from the tower to the storage tank.A storage tank is sometimes employed within the building connected by a gravity line from thetower basin. The purpose of this tank is to allow cool water to flow rapidly away from the towerduring the winter months to prevent freezing. It is not generally necessary to use such anarrangement where the cooling tower is closed down during the winter.

ExampleRefer to Figure 2. Assume that the pipe length from tower (X) to tank (Y) is 50 ft. Adding 50%, forthe resistance of the fittings, the total equivalent length is 75 ft. Assume further that the verticaldrop (gravity head) from the surface of the water in the tower basin to the pipe exit is 2 ft. and thatthe flow requirement is 45 GPM.

At 45 GPM, a 2 pipe has a friction loss rate of 1.6 ft/100 feet. For a total equivalent length of 75feet, the total head loss in the pipe would be:

75 feet x 1.6 ft/100 feet = 1.2 feet

According to the sketch, 2 feet of gravity head exists between the tower sump and the indoor tank.Therefore, the 2 pipe will be adequate.

Work SheetThis work sheet simplifies the design process as described in this manual. It lists all requiredcalculations in the correct sequence.

Step 1. Flow Required1. Tower manufacturer requires _____GPM per ton.2. Design tonnage = _____tons.3. _____GPM per ton X _____tons = _____GPM Required.

Step 2. Piping Layout.

Step 3. Determine Static Head and Component Head Loss1. Pressure drop through condenserFrom unit manufacturer, _____PSID X 2.3 = _____feet2. Pressure drop in towerFrom tower manufacturer, _____PSID X 2.3 = _____feet3. Total static head, if not already included above _____feet

Step 4. Size the Pipe1. Use tables or System Syzer, to select pipe at acceptable velocity.Friction head loss rate = _____feet per 100 feet.2. Determine equivalent length of the piping.Measured length of piping circuit = _____feetAdd 50% for fitting head loss = _____feet.Total equivalent length, (TEL) = _____feet3. Calculate piping friction head lossTEL _____feet X friction rate _____ft/100 ft = _____feet

Check Suction Pipe1. Suction pipe sizeFriction head loss rate = _____feet per 100 feet.2. Determine equivalent length of the suction piping.Measured length of suction piping = _____feetAdd 50% for fitting head loss = _____feet.Total equivalent length, (TEL) = _____feet3. Calculate suction piping friction head lossTEL _____feet X friction rate _____ft/100 ft = _____feet4. Dynamic suction liftStatic suction lift _____ft + Friction loss _____ft = _____feet5. Dynamic suction headStatic suction head _____ft Friction loss_____ft = _____feetNote: Either a suction lift, (#4) or suction head (#5) exists. Compare thecalculated value to the 15 ft of dynamic suction head limit.

Gravity Pipe1. Pipe size for design flow rate.Friction loss rate = _____ft per 100 ft.2. Determine equivalent length of gravity pipe.Measured length of gravity pipe, Figure 2.,= _____feet Add 50% for fitting head loss = _____feetTotal equivalent length = _____feet3. Calculate gravity pipe friction head lossTEL _____ft X Friction loss rate _____ft per 100 ft = _____feet4. Gravity head availableFrom drawing, _____feet of vertical drop = _____feet

Step 5. Select the PumpUse range charts to select the pump style, size, and rpm. Larger pumps often requireadditional decisions about impeller diameter and motor size. Use printed pump curvesor ESP Plus to select these pumps.

Step 6. Double check the design to determine if alternate selections will result insavings.

Bell & Gossett Range Charts for Typical Cooling Tower Pumps

Bell & Gossett Series 1535

Bell & Gossett Series 1522

Range Charts for Bell & Gossett Series 1531 Pumps