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CHILLER MAINTENANCE
Following are some of the recommended performance monitoring and maintenance tasks for Chillers & DX units.
The frequency of these tasks should be as indicated below or based on operating experience -
- External visual Inspections - weekly
- Refrigerant Analysis - annual
- Oil Analysis - annual
- Vibration Analysis - annual
- Calibration of controls and instrumentation - annual
- Change Oil and Oil Filter - semi annual
- Change Refrigerant Filter or Filter/Drier - annual
- Clean Condenser Tubes (water cooled) - annual or based on water quality
- Clean Condenser Fins (air cooled) - annual
- Clean Oil Cooler Tubes (water cooled) - annual
- Clean Evaporator Tubes (chillers) - bi annual
- Clean Evaporator Fins (DX units) - annual
- Condenser Tubes Eddy Current Test (raw water cooled) - bi annual or based on water quality
- Internal Inspections & overhaul - Every 5 yrs
The internal inspection/overhaul should include the following -
- Compressor Journal &Thrust Bearings, Compressor drive gears, Compressor & Motor Oil Seals, Motor End
Bearing , Compressor Motor Megger, Controls/electrical connections, Relief Valve /Rupture Disc, Hot gas
bypass valve, thermostatic expansion valves, solenoid valves.
Additional items for centrifugal machines - Refrigerant Float Valve System, Economizer damper,
Dehydrator/Purge Unit, Guide Vane Linkage Assembly and Drive Mechanism, Guide Vane control shaft seal
Additional items for reciprocating machines - Compressor suction and discharge valves.
Periodic Replacement of vulnerable components - Every 5 yrs
- Following components should be considered for periodic replacement -
Oil Heater Thermostat, Capacity Controls, Load Recycle switch, motor bearing and oil temperature
switches, Start/Stop Program Timer, solenoid valves, thermostatic expansion valves.
Additional items for centrifugal machines - Guide vane cycle timer, Guide vane solenoid valve.
Visual Inspection Checklist
Note1: Refrigerant level in the sight glass only provides a rough indication of the charge. The correct
refrigerant charge should be based on performance parameters per vendor manual.
Performance Evaluation
- Performance evaluation should involve review of the actual operating parameters observed during the
visual inspections, against the normal condition parameters, to interpret any problems based on the
guidelines provided in the table below. The normal operating conditions should be based on the design
data provided by the Original Equipment Manufacturer (OEM) for a specific chiller. In case design data is
not readily available, an approximate idea for the normal operating conditions can be based on past
normal operation.
This performance evaluation provides a good pulse of the chiller, and based on the trend it can provide
valuable input on any maintenance/ repair needed, before a minor problem manifests into a chiller failure.
- LTD: Leaving Temperature Difference.
Notes
- The normal operating conditions should be based on the design data provided by manufacturer for a
specific chiller. In case design data is not readily available from the manufacturer, it may be based on past
normal operating conditions. - One of the five potential problems given above can be identified, if comparison of the actual operating
parameters with normal conditions is the same as above for the corresponding problem. The comparison
symbols given above are as follows
- > Operating parameter higher than the normal condition
< Operating parameter lower than the normal condition
= Operating parameter equal to the normal condition
Vacuum Dehydration Guidelines
Vacuum dehydration is an important step for the recovery process of a refrigeration unit after it has been
opened, since any moisture remaining in the system can result in major problems (see Refrigerants section for
the effects of moisture). Following are the guidelines for vacuum dehydration best practices.
A- Minimize exposure of system internals during maintenance.
This will reduce moisture access to the refrigeration unit internals. Applications found to be effective are:
- Use of Foreign Material Exclusion (FME) plates fabricated from gauge metal with at least 4-bolt holes and
secured on all openings on the refrigerant side. - Use of FME magnetic sheets cut to size on inspection hatches and openings.
- Use of dry nitrogen to break vacuum following refrigerant evacuation and prior to start of work, and
maintaining nitrogen blanket for as much of the work as practical.
- Ensure recent refrigerant and oil samples are analyzed for moisture content, prior to taking the
equipment out of service to identify tube leak issues that might be present. Following are the typical
tests done on condenser and evaporator tubes to locate leaks -
a. Eddy current testing -
This is performed to trend and detect wall thinning and general tube condition.
b. Pressure testing -
Pressure testing has been found to be a better and more reliable indicator for known tube leaks. This
testing does not replace eddy current testing but can be used to pinpoint known leaks. Examples of
methods used are:
· Pressure test the tube bundle and then check with an ultrasound probe to identify leaking tubes.
· Pressure test with nitrogen and a tracer gas.
- The use of external heat is recommended during vacuum dehydration for high moisture situations and in
instances where high humidity is present, such as with tube leaks. External heat is not typically used for
routine preventive maintenance. When required, the following recommendations should be considered:
· The area to apply heat will depend on the type of chiller and the locations that have been exposed to
high moisture.
· Band heaters can be used independently, or in series to fit various application sizes.
· Quartz lights have been used for specific local heating when pinpointed areas of moisture are known.
However, quartz lighting will not focus the heat as well as band heating, and it can heat the surrounding
work area, which would make it uncomfortable for the work crew.
- a. Vacuum pump sizing -
Manufacturers information and/or past experience should be used to determine the optimum size of the
vacuum pump to be used for a particular application. Too small a vacuum pump will increase the
dehydration time, while too large a vacuum pump may cause trapped water to freeze. Current practices
in this area are:
· Use of vacuum pumps in the 8 scfm range.
· Use of multiple pumps to improve performance and allow continued draw down, if the oil has to be
changed on one of the pumps
· Matching pump size to fitting size. The size of access fittings will restrict the amount of flow.
b. Use of ice traps or cold traps -
If the chiller is known to contain a large amount of moisture, the use of ice trap between the chiller and
vacuum pump will minimize moisture contamination of the vacuum pump oil. This technique is not usually
employed during routine preventive maintenance.
c. Moisture Contamination of pump oil –
Frequent monitoring of the vacuum pump oil should be performed and the oil changed when it turns
milky. Practice has shown oil changes to be necessary during initial pull down after approximately two
hours, and then after every six to eight hours of run time.
d. Acceptable Vacuum micron values and hold times -
Manufacturers recommendations should be used to determine the vacuum level and hold time to be
achieved for the purpose of dehydration. In case the Manufacturers recommendations are not available,
following guidelines should be considered.
WARNINGS –
- Do not start or megger the compressor motor or oil pump motor, even for a rotation check, if the chiller
is under dehydration vacuum. Insulation breakdown and severe damage may occur. - To avoid the possibility of freezing water in the evaporator or condenser tubes when charging an
evacuated system, only refrigerant vapor from the top of the drum or cylinder must be admitted to the
system in the beginning until the system pressure is raised above the point corresponding to freezing
temperature.
Vacuum Dehydration Time -
Vacuum Dehydration time is typically a function of the following –
- Physical size of the refrigeration unit.
- Duration of exposure of the system internals to the atmosphere.
- Ambient humidity during the exposure.
- Moisture intrusion in the system due to tube leaks.
- Leaks missed prior to starting the vacuum process.
- Level of vacuum to be achieved and hold-time.
- Ambient temperature during the vacuum process.
- Size of vacuum pump and suction lines.
There is a range of values currently being used for vacuum and hold time in the industry. It ranges from 5000
microns vacuum and 2 hrs of hold time (per Carrier recommendations for some chillers), to 500 microns vacuum
and 24 hrs of hold time. Following is a table giving the boiling or vaporization pressures for free water.
Water Vaporization Pressure Table -
- Note: 1 Atmosphere = 29.921” mercury (Hg) = 760 mm Hg
1” Hg = 25400 microns, or 1 mm Hg = 1000 microns
It might be construed from the above information that 5000 microns would be adequate if the chiller
temperature is warm enough and there is loose moisture in the machine, but a deeper vacuum may be required
for colder conditions and if the moisture is embedded in the motor insulation and other surfaces inside the
chiller. Therefore, to ensure thorough dehydration a deeper vacuum of 1000 microns and 12 hr hold time, or 700
microns and 8 hr hold time should be targeted. If deeper vacuum cannot be achieved due to time limitations, a
minimum of 5000 microns with a hold time of 2 hrs may be used with application of external heat to the chiller,
on the basis that the system is provided with a filter/dryer to remove any remaining moisture, and the
following actions would be taken subsequently
- Monitor moisture indicator (if provided)
- Test refrigerant and oil sample for moisture.
- Change filter/dryer core periodically as necessary.
temperature to prevent freezing of any moisture in the remote sections of the system.
Other recommended practices
- Vacuum lines should be equal to or larger than the vacuum pump intake connection.
- Piping connection to the vacuum pump should be as short a length and as large in diameter as possible.
- System vacuum gauge should be connected as far as possible from the vacuum pump.
- Measuring a system vacuum should be done with the system isolated and vacuum pump turned off.
- Ensure that no section of the refrigeration system is isolated from the vacuum pump.
- Use the more accurate and preferred method of measuring deep vacuum in microns (1” of Hg = 25400
microns).
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