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SECTION 10
CHILLED WATER AIRCONDITIONING 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
Hot water in (95°F)
Fan motor
Air in 95°F dry
bulb, 78°F wet
bulb
Air out
Cooled water out (85°F)
95°F
water
85°F
water
Pump
Load
45°F 55°F
Condenser
Evaporator
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
Warm water from
condenser
Spray nozzles
Slats on all four
sides of the
tower
Prevailing winds
Float
Valve
Make-up water
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
Warm water from
condenser
Spray nozzles
Slats on all four
sides of the
tower
Prevailing winds
Float
Valve
Make-up water
Thermostatically
controlled heater
Water to pump
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
Float and Valve
Float ball
Water level
Make up water
Float valve
Strainer
To pump
Float Switch and Solenoid
Solenoid
Float switch
Float ball
Water level
Make up water
Strainer
To pump
Electronic Water Level Control
Control
Sensors
Solenoid
Strainer
To pump
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