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Chiller System Annual Maintenance – Part Three

Cooling Tower – Annual Maintenance

The Cooling Tower is the final device that is used to reject the heat energy absorbed from the building and transferred into the condenser water via the chiller.  It also must reject the energy input by the chiller motor.  The cooling tower functions to reject the total heat by taking advantage of the water evaporation process.  It is dependent upon the relative humidity and barometric pressure.  The average temperature, humidity and barometric pressure in the central Florida summertime equates to around 86°F wet bulb temperature, which even the best functioning cooling towers cannot get below under those conditions.  Since many centrifugal chiller manufacturers select their impellers very close to the surge limit for maximum efficiency, a slightly underperforming tower can cause the chiller compressor to surge which impacts the life of the chiller bearings.  Consequently, it is very important to keep the cooling tower in its best operating condition.  Towers with loose fan drive-belts, clogged fill or clogged dispersion nozzles will fail to produce design sump condenser water.  Dirty cooling towers can also be conducive to biological contagions like Legionella pneumophila, a bacterium named after the Legionnaires that contracted and perished from the subsequent disease due to a dirty hotel cooling tower.

Cooling Tower Annual Maintenance Tasks

1. Tower Cleaning

a. Prior to cleaning, a water treatment company should “shock” the tower with the appropriate chemical to protect the worker from contagions while cleaning or servicing.

b. Hot Deck/Dist Nozzles – Pressure wash, remove and scale deposits, inspect and clean every nozzle.

c. Fill – High pressure washing can damage fill.  Moderate pressure can be used to remove soft scale blockages and algae at the leading edge.  Impacted fill not responding to chemical cleaning needs replacing.

d. Sump – Flush and remove all sediment, algae and scale build-up.

e. Strainer – remove and clean to allow unrestricted drainage to the pump inlet.

2. Tower Drive/Fan System

a. Belt Drive – Replace belt at a minimum annually & set tension as per manufacturer’s recommendation, inspect sheaves for excessive wear, lubricate motor bearings, lubricate fan shaft bearings, check run-out, check for imbalance.

b. Gear box/transmission: Test oil for wear metals, viscosity, excess moisture, etc. prior to the annual shutdown; check/adjust oil level or replace as per test; check shaft seals for leakage; check run-out, thrust, excessive wear; check fill hoses and vent tubing for cracks, leaks, etc.; and test vibration levels.

c. Inspect fan for cracking, tip wear, proper positioning and excess imbalance.

d. Motor – perform insulation resistance test as per IEEE43 and record

3. Tower Water Make-up System

a. Check mechanical or electrical level control: proper level (tower level too high can reduce capacity, too low can cause air/pump cavitations); check for valve leakage; inspect float mechanism; inspect/test pressure regulator where applicable.

b. Inspect back-flow prevention assembly (should be done by a certified tester)

4. Tower Starter/VFD

a. Electromechanical – check electrical connections; inspect, clean and test contactors, interlocks, staging controls.

b. Variable Frequency Drive – (Be sure to follow the manufacturer’s suggested maintenance procedures, especially the safe discharge of the DC Bus prior to inspecting) Check cooling fans, clean heat exchange surfaces & cabinet; check connections for overheating, tightness, etc.; check capacitors for leakage, bulging; check control parameters and temperature response to set point.

5. Water Treatment (May be performed by water treatment provider)

a. Clean strainers in water treatment blow-down loop(bleed) and circulation loops

b. Check hoses, pumps, tanks, etc. for leakage

c. Check controller calibration and function

d. Check chemical containment system for apparent compromise

Water Circulating Pump/Hydronic System  – Annual Maintenance

Water circulating pump/Hydronic Systems are critical to the chiller systems they serve.  It interconnects directly with each portion of the overall system to transmit heat between devices.  Depending on the design, there may be more than one type of chill water pump, the primary circulating water through your chiller and the secondary through the building.  Unless you have a variable system, the volume through the chiller should match the chiller design, typically 2.4 GPM per ton and at the primary pump nameplate Total Dynamic Head.  The condenser pump should do the same except typically at 3.0 GPM per ton.  Either chill water or condenser water pumps should have good seals, bearings, rotating components and drive motors.  Both loops have controls, bypasses and valves that are often neglected for inspection and maintenance. The pump strainers should also be cleaned.

Water Circulating Pump/Hydronic System  – Annual Maintenance Tasks

1. Strainers – Isolate, drain and clean

2. Seals, Bearings and Couplings – Inspect for seal leakage; lubricate bearings only as per manufacturer’s recommendation intervals; check for excess vibration; check pump/motor alignment; check coupling insert for cracks and excess wear.

3. Pump Starter/VFD

a.electromechanical – check electrical connections; inspect, clean and test contactors, interlocks, staging controls.

b. Variable Frequency Drive – (Be sure to follow the manufacturer’s suggested maintenance procedures, especially the safe discharge of the DC Bus prior to inspecting) Check cooling fans, clean heat exchange surfaces & cabinet; check connections for overheating, tightness, etc.; check capacitors for leakage, bulging; check control parameters and temperature response to set point.

4. Pump Motor – Lubricate bearings as per manufacturer’s recommendations; clean windings with air as possible; perform insulation resistance test as per IEEE43 and record.

5. Total Dynamic Head and Flow Testing – With the appropriate procedures and instrumentation, the pressure and flow performance of the pump can be tested to reveal any internal wear or insufficiencies prior to the summer season when pump failure could mean a building unable to be occupied.

6. Water Make-up, Air Removal and Expansion System

a. Check make-up pressure regulator and relief system for proper pressure and against leakage; inspect back-flow prevention assembly (should be done by a certified tester)

b. Check the air separator and automatic air vent for operation and against continual leakage

c. Check expansion tank level (and safety controls where applicable)

7. Bypass Systems

a. Systems with two-way chill water valves at the air handlers will typically have a bypass circuit valve varied by a pressure controller.  Testing during the Annual shutdown season can help to prevent dead-heading during the moderate cooling periods or over-bypassing, limiting capacity during the hottest months.

Energy Management System – Annual Maintenance

The HVAC/Chiller system depends upon integration with the Building or Energy Management System to control capacity, notify the operator of failures or potential problems and achieve maximum efficiency for the HVAC system overall.  However, it is often ignored during the Annual Maintenance season.  All system data and program routines should be backed up to a secondary source.  Disk drives should be defragmented and de-bugged as necessary.  Electrical connections at primary and secondary devices should be checked for overheating, tightness, etc.  Sensors should be tested and calibrated.  Repeat alarms should be investigated.  The overall control scheme should be reviewed periodically to ensure the controls are being adapted to alterations in building layout and function.

 <– Read Part II

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