section 10 chilled water air- conditioning systems unit 49 cooling towers and pumps
TRANSCRIPT
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SECTION 10
CHILLED WATER AIR-CONDITIONING SYSTEMS
UNIT 49
COOLING TOWERS AND PUMPS
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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
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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
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Cooled water out (85°F)
Hot water in (95°F)
Air in 95°F dry bulb, 78°F wet
bulb
Air out
Fan motor
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Condenser
Evaporator
Load85°F water
95°F water
Pump45°F 55°F
Pump
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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
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Prevailing winds
Make-up water
Float Valve
Warm water from condenser Slats on all four
sides of the tower Spray nozzles
Water to pump
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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
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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
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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
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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
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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
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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
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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
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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
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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
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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
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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
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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
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To pump Strainer
Make up water
Float ball
Float valve
Water level
Float and Valve
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Water level
To pump Strainer
Make up water
Float ball
Float Switch and Solenoid
Float switch
Solenoid
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To pump Strainer
Electronic Water Level Control
Solenoid
Control Sensors
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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
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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
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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
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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
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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
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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
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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
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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