1Chapter 49Chapter 49

Cooling Towers and Cooling Towers and PumpsPumps

Cooling Tower FunctionCooling Tower Function

Rejects heat absorbed by the chiller to the atmosphere.

Water is moved by a pump from the condenser to the cooling tower and back to the condenser.

Cooling Tower FunctionCooling Tower Function

Compression System Example:1. The cooling tower must reject more heat

than the chiller can absorbs.2. Heat of compression is added to the heat

absorbed by the chiller.3. The compressor adds approximately

25% additional heat4. If a chiller absorbs 1,000 tons of heat,

the cooling tower would reject about 1,250 tons of heat

Cooling Tower FunctionCooling Tower Function Design Temperature of water leaving the cooling tower

is 85*F. The tower can cool the water down to within 7*F of the

wet bulb temperature of the ambient air. Just because the pump is running doesnt mean the

fan is running. Cools the water by evaporation1. As air passed over the water, some of it evaporates2. This evaporating water cools the remaining water

Types Of Cooling TowersTypes Of Cooling Towers

Natural Draft Towers Forced / Induced Draft Tower Closed Loop Hybrid Towers Dry / Wet Mode Adiabatic Mode Dry Mode

Natural Draft TowersNatural Draft Towers1. Rely on prevailing winds2. Cool the water better when the winds are stronger3. Operate with an approach temperature of about

10*F.4. Spray ponds can be located at ground level or on

a rooftop.5. Rooftop spray ponds help to cool the roof and

reduce the air-conditioning load6. Use pumps to atomize the water into droplets7. Spray nozzles must be kept clean.

2 Mechanical draft Cooling Towers have long piping runs that spray the water downward. Large fans pull air across the dropping water toremove the heat. As the water drops downward onto the "fill" or slats in the cooling tower, the drops break up into a finer spray. On colder days, tall plumes of condensation can be seen. On warmer days, only small condensation plumes will be seen.

This photo shows a single natural draft cooling tower as used at a European plant. Natural draft towers are typically about 400 ft (120 m) high, depending on the differential pressure between the cold outside air and the hot humid air on the inside of the tower as the driving force. No fans are used. Whether the natural or mechanical drafttowers are used depends on climatic and operating requirement conditions.

The green flow paths show how the water is taken from a river (yellow) to an intake supply basin (green) that the Circ Water Pumps take a suction from. The water is then pumped to the Condenser where the water is heated. The water is then sent to an exit distribution basin where the water then can be returned to the river and/or pumped by the Cooling Tower Pumps to the Cooling Towers then the water returned to the intake supply basin where the water can be reused.

The green flow paths show how the warm water leaves the plant proper, is pumped to the natural draft cooling tower and is distributed. The cooled water, including makeup from the lake to account for evaporation losses to the atmosphere, is returned to the condenser.

Forced / Induced Draft TowerForced / Induced Draft Tower

1. Use a fan to move air through the tower.

2. Majority use centrifugal fans3. Can be located indoors or outdoors4. Indoor installations require

ductwork and centrifugal blowers.

Induced draft cooling towers, illustrated in Figure 11, are constructed such that the incoming circulating water is dispersed throughout the cooling towervia a spray header. The spray is directed down over baffles

that are designed to maximize the contact between water and air. The air is drawn through the baffled area by large circulating fans and causes the evaporation and

the cooling of the water

3 The most economical and most common style air cooler GEA Rainey Corporation builds, the forced draft ACHE, uses axial fans to force air across the fin tube bundle. The fans are positioned below the bundle thus not exposing the mechanical sections to the hot exhaust airflow.

The forced draft aircooler also simplifies future plant expansion by providing direct access to bundle for replacement. Structural disassembly is not required

Closed Loop Hybrid TowerClosed Loop Hybrid Tower

1. Equipped with a finned coil, prime surface coil, and a wet deck surface.

2. Three modes: Dry / Wet Mode, Adiabatic mode, and Dry Mode

Approach Temperatures in Cooling Approach Temperatures in Cooling TowersTowers

Approach is the difference between the leaving water (where does it pump from) and the wet bulb temperature of the cooling towers entering air.

(Talk About ) The lowest the water from the cooling tower can be is

the wet bulb temperature of the entering air.

Stop

Dry / Wet ModeDry / Wet Mode

1. Fluid to be cooled is fed first to the dry finned coil.

2. Fluid then fed to the prime surface coil.3. Fluid then leaves the tower4. Water in the tower flows over the prime

surface coil and wet deck surface.5. Air is drawn through the prime surface

coil and wet deck surface.

4Adiabatic TowerAdiabatic Tower1. Condenser water is cooled by evaporating tower

water.2. No heat is added to or removed during the process3. Fluid to be cooled passes only through the finned

coil.4. Spray water is used to help cool the air passing

through the tower.5. Plume is the saturated discharge air.

Dry ModeDry Mode

1. Fluid to be cooled passes through the finned coil and the prime surface coil.

2. No spray water is used.3. No plume results.4. Fluid is cooled by air passing over

the coil.

Fire ProtectionFire Protection The off season can create a fire hazard. Tower components may be flammable. A tower wetting system may be required

1. Usually consists of sprinkler heads2. Water is sprayed over the tower periodically

during the off season Some towers are kept wet whenever the

temperature is above freezing

Fill MaterialsFill Materials Designed to slow the flow of trickling water through

the tower Splash method

1. Uses wood slats, PVC pipe, or FRP plastic.2. Both have a slow burn rate.3. Tower has a framework to support slats at the correct

angle. Film or wetted surface

1. Fill is usually plastic or fiberglass2. The water is spread out over the fill as air passes over it.

Flow PatternsFlow Patterns Crossflow

1. Air enters from the side and is discharged from the top or the other side

2. Tower water must be contained since the chemicals can be corrosive

Counterflow1. Air enters from the bottom and is exhausted at the top.2. The water flows down as the air moves up

Water that is blown out of the tower is called drift Eliminators reduce the amount of drift.

5Tower MaterialsTower Materials Must withstand the environment Must withstand fan and drive mechanism vibrations Usually made of galvanized steel, fiberglass, or FRP Larger towers may have a concrete base The sides of the tower can be made of wood,

fiberglass, corrugated FRP, or corrugated asbestos-cement panels

(older towers) Fiberglass is the preferred material for safety reasons

because they arent susceptible to fires like the wood towers.

Fan SectionFan Section Belt-driven fan

1. Primarily found on small towers2. Requires a great deal of maintenance

Gear box transmissions1. Motors are usually mounted at a 90-degree angle to the

fan2. Mat be designed to reduce the fan speed3. Motor, gear box, and bearings must be accessible for

servicing

Fan blade is located in a housing

Tower AccessTower Access Tower fill must be accessible for cleaning or

replacement Sludge needs to be cleaned from the tower basin Garbage, bird feathers, and other pollutants

accumulate in the sump There is a strainer to prevent this garbage from

entering the pump and water circuit Stairs or ladders provide access to fans and drive

mechanisms on all towers

Tower SumpTower Sump Area where tower water collects Sump water must not freeze

1. Thermostatically-controlled heater2. Larger units may use a hot-water coil or low-pressure

steam3. Antifreeze can be added to the water

May be installed underground1. Will not freeze in southern climates2. Must be protected from freezing in northern climates

Should be accessible for cleaning Is usually equipped with a strainer to protect the

pump

MakeMake--up Waterup Water Water continuously evaporates from the system Fresh (normal )water must be supplied to the

system as needed Float valve

1. As the water level drops, the float ball drops2. The valve will open and supply water

Solenoid controlled valve1. Solenoid valve operation controlled by a float switch2. When float switch falls, the valve is energized

Electrodes1. Uses to sense the water level2. Valves open and close to maintain proper water level

Blow downBlow down Process of bleeding off a portion of the system water This water is replaced with fresh water Designed to reduce the amount of solid materials in the water

1. As the water evaporates, it leaves solids behind2. Over time, the amount of solids increases3. Solids form a cement like substance4. Can act as an insulator in the condenser

Blow down allows the dilution of solids in the water circuit Without blow down

1. High head pressure2. High approach temperatures3. Condenser would need to be cleaned

Must be done correctly1. Excessive amounts of water can be lost2. Excessive amounts of water treatment chemicals can be lost

6Balancing the Water Flow for a TowerBalancing the Water Flow for a Tower

Water flow to each of the tower cells must be equal

Distribution pans1. Receives water returning from the

condenser2. Have calibrated holes to distribute water3. Holes must be clean

Balancing valves must be adjusted properly

Water PumpsWater Pumps Responsible for moving water

through the condenser and cooling tower circuit

Usually a centrifugal pump Made from cast iron a majority of

the time Close coupled pump

1. Impeller is mounted to the motor shaft2. Used in small applications3. Shaft seal prevents water leakage

Base Mounted PumpBase Mounted Pump Motor and pump are connected by a

flexible coupling Can have a single or double sided

impeller Motor and pump are mounted on a

steel or cast iron base Base is usually cemented to the floor Motor and pump are factory aligned

Water PumpsWater Pumps Double Inlet Impeller

1. Allows water to enter the pump from both sides2. Can be disassembled from the ends or from the top

Pump must have a shaft seal1. Stuffing box

a. Packing gland type sealb. Must be tightened with a wrenchc. Used on pumps with pressure up to 150 psig

2. Mechanicala. Used on higher pressures up to about 300 psigb. Uses a carbon ring, O-ring, and a ceramic ring

Water PumpsWater Pumps Can be used in a vertical position Most pumps are made of cast iron Most centrifugal pump impeller are

made of bronze The eye of the impeller must be

under water during startup1. They cannot pump air to pull water into the impeller2. A free flow of water from the sump to the pump is

desired

Water PumpsWater Pumps If the pump is located higher than

the sump, the pump must be filled with water before starting

1. Water hose can be used2. A foot or check valve is used to keep water from flowing

back to the sump during shutdown Whirlpool action in the pump is called vortexing

1. Introduces air into the system2. Can be caused by poor design or installation3. Can be eliminated by cross shaped device

7VortexingVortexing A whirlpool action in the sump or tower basin of a cooling tower. Vortexing is caused by insufficient surface tension on the

liquid Symptoms 1. Pump makes a growling sound 2. Whirlpool usually visible on liquid surface 3. Loss of Flow Causes 1. There is not enough liquid height above the suction line

entrance 2. The velocity at the suction line entrance is too high Remedy Decrease liquid velocity and/or increase submergence in

accordance with the following table

Water PumpsWater Pumps Strainers are located between the

sump and the pump1. Prevents trash from entering the pump2. A coarse strainer is located at the outlet of the tower to

the sump3. A finer strainer is usually located at the inlet to the pump4. A blocked strainer at the pump inlet will cause a

pressure drop and may cause cavitation

Water PumpsWater Pumps Tower Bypass Valve1. Helps to maintain correct water pressure during

startup and low ambient conditions2. Water from the pump outlet is re-circulating to the

pump inlet3. Prevents cold water from the condenser4. Effects of cold water entering the condenser

a. Reduced head pressureb. Evaporator can starvec. Can cause oil migrationd. Can cause salt crystallization (absorption

systems)

Water PumpsWater Pumps Mixing Valve

1. Has 2 inlets and 1 outlet2. Located in the pumps inlet line

Diverting Valve1. Has one inlet and 2 outlets2. Usually located at the condenser outlet (return line

to the tower)3. Water can be piped to either the tower or the pump

inlet

Water PumpsWater Pumps Pump motor and pumps are joined

together by a flexible coupling in most cases.

Pumps can have sleeve bearings or ball bearings

1. Sleeve bearings are used in small pumps2. Ball bearings are used in larger pumps

Pumps and shafts must be properly aligned

Centrifugal Water PumpsCentrifugal Water Pumps Use 2 types of seals:

1. Stuffing box2. Mechanical types

8Cooling Towers Cooling Towers

No matter the design all cooling towers need routine maintenance.

Angular & Parallel AlignmentAngular & Parallel Alignment

When installing or changing a pump it must be aligned to operate properly.

Angular alignment ensures that both shafts are aligned with each other and are mounted on the same angle

Parallel alignment ensure that the shafts are end to end.

Dial indicators are used to ensure proper aligment.