SECTION 10

CHILLED WATER AIR-CONDITIONING SYSTEMS

UNIT 49

COOLING TOWERS AND PUMPS

UNIT OBJECTIVES After studying this unit, the reader should be able to

• Describe the function of the cooling tower in a chilled water system

• Identify the relationship between the wet-bulb temperature of the

ambient air and the temperature of the water leaving the tower

• Describe three types of cooling towers

• Describe various fan and drive types used on cooling towers

• Describe the function of the fill in a cooling tower

• Explain the purpose of makeup water and blowdown

COOLING TOWER FUNCTION• Water is moved by a pump from the condenser to the

cooling tower and back to the condenser • Tower must reject more heat than the chiller absorbs• The compressor adds approximately 25% additional heat• Design temperature of water leaving the tower is 85°• The tower can cool the water down to within 7° of the

wet-bulb temperature of the ambient air (approach)• Cools the water by evaporation

– As air is passed over the water, some of it evaporates– This evaporating water cools the remaining water

Cooled water out (85°F)

Hot water in (95°F)

Air in 95°F dry bulb, 78°F wet

bulb

Air out

Fan motor

Condenser

Evaporator

Load85°F water

95°F water

Pump45°F 55°F

Pump

TYPES OF COOLING TOWERS

• Natural draft towers rely on prevailing winds• Operate with an approach temperature of

about 10°• Forced/induced draft towers use a fan to

move air through the tower – Fans can be gear-driven or belt-driven

• Closed-loop hybrid tower– Dry/wet mode, adiabatic mode, and dry mode

Prevailing winds

Make-up water

Float Valve

Warm water from condenser Slats on all four

sides of the tower Spray nozzles

Water to pump

DRY/WET MODE

• Fluid to be cooled is fed first to the dry finned coil

• Fluid then fed to the prime surface coil

• Fluid then leaves tower

• Water in the tower flows over the prime surface coil and wet deck surface

• Air is drawn through the prime surface coil and wet deck surface

ADIABATIC MODE• Condenser water is cooled by evaporating the

tower water

• No heat is added or removed during from the process

• Fluid to be cooled passes only through finned coil

• Spray water is used to help cool the air passing through the tower

• Plume is the saturated discharge air

DRY MODE• Fluid to be cooled passes through the finned

coil and the primed surface coil

• No spray water is used

• No plume results

• Fluid is cooled by air passing over the coil

FIRE PROTECTION

• The off season can create a fire hazard

• Tower components may be flammable

• A tower wetting system may be required

• Some towers are kept wet whenever the temperature is above freezing

FILL MATERIAL

• Designed to slow the flow of trickling water through the tower

• Splash method – Uses wood slats, PVC pipe, or FRP plastic

– Tower has framework to support slats at the correct angle

• Film or wetted surface – Fill is usually plastic or fiberglass

– The water is spread out over the fill as air is passed over it

FLOW PATTERNS• Crossflow

– Air enters from the side and is discharged from the top or the other side

• Counterflow – Air enters from the bottom and is exhausted at the top

– 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

TOWER 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

FAN SECTION • Belt-driven fan

– Primarily found on smaller towers

• Gear box transmissions – Motors are usually mounted at a 90 degree

angle to the fan – May be designed to reduced the fan speed– Motor, gear box, and bearings must be

accessible for servicing

TOWER ACCESS

• Tower fill must be accessible for cleaning or replacement • Sludge needs to be cleaned from the tower basin • Garbage, bird features, and other pollutants accumulate

in the sump • There is a strainer to prevent garbage from entering the

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

mechanisms on tall towers

TOWER SUMP

• Area where tower water collects

• Sump water must not freeze

• May be installed underground

• Should be accessible for cleaning

• Is usually equipped with a strainer to protect the pump

Prevailing winds

Make-up water

Float Valve

Warm water from condenser Slats on all four

sides of the tower Spray nozzles

Water to pumpThermostatically controlled heater

MAKE-UP WATER • Water continuously evaporates from the system • Fresh water must be supplied to the system as

needed • Float valve

– As the water level drops, the valve will open and add supply water

• Solenoid controlled valve – Solenoid valve operation controlled by a float switch

• Electrodes – Used to sense the water level

To pump Strainer

Make up water

Float ball

Float valve

Water level

Float and Valve

Water level

To pump Strainer

Make up water

Float ball

Float Switch and Solenoid

Float switch

Solenoid

To pump Strainer

Electronic Water Level Control

Solenoid

Control Sensors

BLOWDOWN • 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 • Blowdown allows the dilution of solids in the water

circuit • Blowdown reduces head pressure and approach

temperature • Must be done correctly

BALANCING THE WATER FLOW FOR A TOWER

• Water flow to each of tower cell must be equal

• Distribution pans – Receives water returning from the condenser– Have calibrated holes to distribute water – Holes must be clean

• Balancing valves must be adjusted properly

WATER PUMPS

• Responsible for moving water through the condenser and cooling tower circuit

• Usually a centrifugal pump

• Close coupled pump – Impeller is mounted to the motor shaft– Used in small applications– Shaft seal prevents water leakage

WATER PUMPS• Base 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 base – Base is usually cemented to the floor– Motor and pump are factor aligned

WATER PUMPS• Pump must have a shaft seal • Most pumps are made from cast iron • Most centrifugal pump impellers are made of

bronze • The eye of the impeller must be under water

during startup • If the pump is located higher than the sump, the

pump must be filled with water before starting• Whirlpool action in the pump is called vortexing

WATER PUMPS

• Strainers are located between the sump and pump

• Tower bypass valve– Helps to maintain correct water pressure during start-up

and low-ambient conditions– Water from the pump outlet is recirculated to the pump

inlet

• Pumps can have sleeve bearing or ball bearings

• Pumps and shafts must be properly aligned

SUMMARY - 1• Heat laden water from the condenser is cooled in the

tower so the water can be reused• Tower must reject more heat than the chiller absorbs• The tower can cool the water down to within 7° of the

wet-bulb temperature of the ambient air (approach)• Natural draft towers rely on prevailing winds• Forced/induced draft towers use a fan to move air

through the tower• Closed-loop hybrid tower• Tower components may be flammable

SUMMARY - 2• Fill material is designed to slow the flow of trickling

water through the tower• Airflow pattern can be counterflow or crossflow• Tower materials must withstand the environment• The fan section often has a gear box transmission• Tower fill, sump and strainer must be accessible • Fresh water must be supplied to the system as water

evaporates• Blowdown is the process of bleeding off a portion of the

system water• Water flow to each of tower cell must be equal