Guide for servicing screw chillers, compressors
Building owners now view hvac equipment as a long-term investment. As with any investment, careful planning must occur before and after they purchase that equipment.
An important part of that planning is the development of a service plan. The hvac service technician plays a significant role in the design and execution of such a plan.
Some of the benefits of a service plan to building owners include the safety of the building and its occupants, minimal equipment downtime and discomfort, and reduced operating costs.
The first step is to set up a maintenance schedule for all equipment in operation. With screw chillers, the schedule should be based on actual requirements such as chiller load, run hours, and water quality.
In addition, the plan should include one or more of the following preventive maintenance services: routine operating inspection, major operating inspection, and annual or multiyear inspection.
Routine operating inspections are typically performed during the cooling season to adjust, lubricate, and perform minor repairs in order to keep the chillers in good operating condition. In comparison, a major operating inspection is performed at least during the first seasonal inspection each year and expands on services included in the routine operating inspection.
Annual maintenance inspections consist of prescheduled maintenance determined mainly by equipment operating hours.
Annual maintenance inspections are designed to prepare the equipment for effective, reliable, efficient operation during peak demand months.
In addition to annual maintenance services, each multiyear inspection should include repair or replacement of parts as needed. The frequency of multiyear inspections should be determined by manufacturer recommendations, equipment operating hours, and inspection results.
Specific plansAll hvac equipment should have a service plan specific to it. Screw compressors, for example, require different service plans than centrifugal compressors.
The following describes what services should be included for screw chillers and compressors. The time intervals and actions listed should be viewed as a guide. Always check manufacturer recommendations.
- Vibration analysis — With hermetic compressors, take baseline vibration measurements upon initial startup and at typical operating conditions. To ensure accurate readings, take successive measurements at the same location and operating conditions each time a vibration test is performed.
Contact the equipment manufacturer for the acceptable vibration displacement and peak velocity values.
- Motors — The screw compressor type, open or hermetic, determines the frequency of vibration checks. Open-drive compressors require frequent vibration measurements to check the alignment between the motor and compressor.
Vibration readings should be taken on the motor and compressor, and adjustments made accordingly. With hermetic compressors, vibration measurements should be taken every five years.
Both hermetic and open-drive motor windings should be checked annually for insulation integrity, using a megohm meter.
- Rotors — Some compressors are field-serviceable. If the compressor is disassembled for service work, visually inspect the screw rotors. With each inspection, evaluate and log the contact band width over time, looking for pitting or other surface deformities. Degradation of the rotor contact surface will result in decreased chiller efficiency over time.
- Capacity control — If the compressor is disassembled for service work, the capacity control assembly should be removed and pressure checked for internal oil seal leakage. Verify that the capacity control solenoid valves are operational.
Check the integrity of the supply-return oil lines for signs of crimping or oil leakage and all internal oil passages for oil blockage. Inspect assembly locknuts for any sign of looseness, and inspect the spring and piston. The seals and O-rings/gaskets should be replaced when reassembling.
- Bearings — Most screw compressors contain rolling-element bearings. If the compressor is disassembled for service work, visually inspect the bearings. Always check vibration readings and excessive chiller noise, which may indicate excessive bearing wear.
- Oil — The key to good bearing maintenance is proper lubrication. Ensure that the proper type and grade of oil, maintained at the recommended level, temperature, and pressure, is used. Change the oil after the first year of operation and at least every three years thereafter.
However, if a continuous oil monitoring system is present and/or a yearly oil analysis is performed, the time between oil changes may be extended. (Note: Check with the equipment manufacturer for the recommended oil change frequency.)
- Oil filters — The lubrication system should be inspected annually. Oil filters and oil reclaim system strainers (if present) should be changed once a year — sooner if the strainer condition indicates a need for more frequent replacement.
While the chiller is shut down, observe and log the oil level. If the level falls below the sight glass, check the oil reclaim system for proper operation.
- Condenser — For water-cooled condensers, the water circuit is usually an open-type, and the tubes may be subject to contamination and scale. Clean the condenser tubes with a rotary tube-cleaning system at least once a year — more often if the water is contaminated.
For tube cleaning, do not use wire brushes; use brushes specially designed to avoid scraping and scratching tube walls.
Inspect the entering and leaving condenser water sensors for signs of corrosion and scale. Remove any scale found or replace sensors if corroded. Higher-than-normal condenser pressures, together with the inability to reach full refrigerant load, usually indicate dirty tubes or air in the machine.
If the operating log indicates a rise above normal condenser pressure, check the condenser refrigerant temperature against the leaving condenser water temperature. If the reading is more than what the design difference should be, the condenser tubes may be dirty or the water flow may be incorrect.
- Water treatment — Untreated or improperly treated water may result in corrosion, scaling, erosion, or algae formation. The services of a qualified water treatment specialist should be obtained to develop and monitor a treatment program.
- Eddy current testing — Perform an eddy current tube test every five years, depending on the water treatment. Eddy current testing is the process of inducing small electrical currents into a heat exchanger tube and observing the interaction between the tube and current. The testing will detect tube degradation such as pits, cracks, or wall loss over time.
- Air-cooled condenser — If the condenser is air-cooled, straighten the fins and power wash the coils. Check the fan motors, pulleys, and belts, and lubricate the bearings as needed. Clean the fan blades if necessary and tighten all electrical connections. Units installed in corrosive environments should have coil cleaning as part of a planned maintenance schedule.
- Evaporator — Inspect and clean the evaporator tubes at the end of the first operating season. Since the evaporator is typically a closed system, inspection results will determine the scheduled frequency for cleaning and whether water treatment is adequate in the chilled water-brine circuit. Follow the same procedure for condenser tube cleaning and sensor inspection.
- Refrigerant filters — Change the refrigerant filter-drier once a year — sooner if needed. A moisture indicator will indicate the presence of moisture in the refrigerant. If moisture is present, locate the source of water immediately by performing a thorough leak check. Whenever the indicator shows a presence of moisture, replace the filter-drier.
- Relief valves — Relief valves protect the system against potentially dangerous effects of over-pressure. To ensure against damage to the equipment and possible injury to personnel, these devices must be kept in peak operating condition.
At least once a year, disconnect the vent piping at the valve outlet and carefully inspect the valve body and mechanism for any evidence of internal corrosion or rust, dirt, scale, leakage, etc. If corrosion or foreign material is found, do not attempt to repair or recondition; always replace the valve. If the chillers are installed in a corrosive atmosphere, or the relief valves are vented into a corrosive atmosphere make valve inspections at more frequent intervals.
- Float valve — Inspect the float valve once every five years, or when the machine is open for service. Remove the float valve access cover, clean the chamber and valve assembly, and check that the valve moves freely. Ensure that the openings are free from obstructions, examine the cover gasket, and replace if necessary.
- Leak testing — The entire chiller should be leak tested annually. Because HCFC-22 and HFC-134a are above atmospheric pressure at room temperature, leak testing can be performed with refrigerant in the machine. Be sure the room is well ventilated and free from refrigerant to keep false readings to a minimum.
Before making any necessary repairs to a leak, transfer all refrigerant from the leaking vessel. If the chiller has been opened up for service, the machine or the affected vessels should be pressurized and leak tested.
- Storage tank — For routine maintenance, refrigerant isolation valves allow refrigerant storage in the chiller. If absent, storage tanks and transfer procedures must be used. If a storage tank is used to hold refrigerant during service, leak test the tank on an annual basis.
If the storage tank contains a pumpout compressor, the oil should be checked annually and changed as needed. Also, run the compressor to check for proper operation. Check pressure switches and calibrate if necessary.
- Controls — Control maintenance is generally limited to cleaning and tightening of connections. Vacuum all cabinets to eliminate dust buildup and debris from components. With microprocessor controls, verify that the most recent control software version is installed.
Check the service history for abnormal values or alarms and make adjustments as necessary. If condenser water control is used, verify that the pneumatic or electronic controls are operating correctly to ensure bypass when needed. Annually check all pressure transducers against a good pressure gauge.
- Motor starters — For air-cooled mechanical starters, follow manufacturer instructions for contact replacement, lubrication, and other maintenance requirements. Periodically vacuum or blow off accumulated debris on the internal parts with a high-velocity, low-pressure blower. For water-cooled starters, in addition to normal maintenance, starter jackets must be inspected and cleaned as required.
Other servicesAlways speak with the facility personnel that have daily exposure to the equipment. They can provide valuable information regarding chiller operation and servicing needs.
Keep a log of all basic data, such as compressor amps and volts, suction and discharge temperatures and pressures, oil pressure, etc. This provides a baseline of comparison for future data.
A well-planned service program will return dividends to the building owner and manager in the form of comfort, reduced operating costs, and peace of mind.