CHILLED WATER AIR-CONDITIONING SYSTEMS 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

Air in 95°F dry bulb, 78°F wet bulb Fan motor Hot water in (95°F) Air in 95°F dry bulb, 78°F wet bulb Air out Cooled water out (85°F)

95°F water 85°F water Load 45°F 55°F Condenser Pump 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 Slats on all four sides of the tower Spray nozzles Prevailing winds Valve Float 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 Counterflow 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 Gear box transmissions 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 Slats on all four sides of the tower Spray nozzles Prevailing winds Valve Float 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 Water level Float ball Make up water Float valve Strainer To pump

Float Switch and Solenoid Water level Float ball Make up water Strainer To pump

Electronic Water Level 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