The screw compressor is the heart of large and medium-capacity chillers. A well-maintained screw compressor runs for many years, but not forever. Bearings wear, seals harden, rotor clearances open up and efficiency slowly declines. Doing the overhaul at the right time is the difference between losing the compressor to an unexpected failure and renewing it in a planned shutdown.
In this article we explain what an overhaul is, which signs show that it is time for one, and how to decide between overhaul and replacement.
What is a screw compressor overhaul?
A screw compressor overhaul is a comprehensive repair in which the compressor is disassembled, worn parts are renewed, rotor and housing tolerances are measured, and the compressor is brought back to the manufacturer's values. Unlike a simple parts replacement, the whole compressor is addressed.
The typical scope of an overhaul is as follows:
- Replacement of the main and thrust bearings
- Renewal of shaft seals, O-rings and gaskets
- Measurement of wear, scoring and clearance on the rotors and housing
- Inspection of the capacity control slide valve and its piston
- Inspection or replacement of the oil filter, oil separator element and check valves
- Megger and resistance tests of the motor winding
- After reassembly: leak test, vacuum, oil and refrigerant charging, and commissioning measurements
Running hours: the manufacturer's recommended interval
The first criterion that determines overhaul timing is the compressor's total running hours. Manufacturers give bearing and general overhaul intervals for screw compressors in running hours. This interval varies with the model, operating conditions and oil maintenance; it is often in the tens of thousands of hours.
However, hours alone are not enough. A compressor that starts and stops frequently, runs at high condensing pressure or has missed oil changes may need an overhaul well before the recommended interval. Conversely, a compressor that runs in ideal conditions and is monitored regularly can stay healthy beyond the recommended interval. So hours are a planning criterion, not a definitive decision criterion.
The number of starts is at least as important as hours. At every start the motor draws high current, and the bearings come under load before the oil film has fully formed. A compressor that makes many starts per hour, that is, one that short-cycles, wears much faster than another compressor with the same running hours. If the number of starts is high, the cause should be found first: an oversized unit, low water volume or a wrong set value are the most common causes.
The chiller control panel records each compressor's running hours and number of starts. Noting these two values at every maintenance visit is the simplest and cheapest way of planning an overhaul.
What are the signs that it is time for an overhaul?
The need for an overhaul shows itself as deterioration over time in the compressor's mechanical and electrical values. If one or more of the signs below are seen, the compressor needs to be examined regardless of running hours.
01Increased vibration and noise
The earliest sign of bearing wear is an increase in vibration level. Vibration measurement shows the change long before it is noticed by ear. A metallic sound, a harsher hum or vibration concentrating at a particular frequency points to a bearing or rotor problem.
02Metal particles in oil analysis
Oil analysis is the most valuable test showing the compressor's condition without opening it up. A rise in wear metals such as iron, chromium or copper points to bearing and rotor wear, while rising moisture and acid values point to motor and insulation risk. The analysis should be done at least once a year, and results should be compared with previous years.
03Rising current and discharge temperature
If the current drawn by the compressor increases and the discharge line temperature rises under the same load and conditions, compression efficiency has dropped. Opening rotor clearances lets gas leak back, and the compressor uses more energy to do the same work. When this picture combines with high condensing pressure, wear accelerates; we explained the causes in our high pressure alarm article.
04Oil pressure and oil return problems
Oil pressure differential alarms, a falling oil level or rapid clogging of the oil filter are also signs of an overhaul need. In a screw compressor, oil does lubrication, sealing and cooling at once; a problem in the oil system quickly turns into bearing and rotor damage.
Is your compressor due for an overhaul?
Let us measure its condition on site with vibration, current, megger and oil analysis, and present overhaul and replacement options side by side.
05Capacity loss
If the chiller cannot reach the chilled water set point at full load and capacity drops even though the condenser and evaporator are clean, the compressor itself should be examined. The capacity slide failing to open fully or rotor wear causes this. To track efficiency loss numerically, see our COP and EER article.
Overhaul or replacement?
The decision between overhaul and replacement is made according to the condition of the motor winding, the degree of rotor damage, the age of the compressor and the availability of spare parts. The general framework is as follows:
| Situation | General tendency |
|---|---|
| Winding sound, bearings worn, no rotor damage | Overhaul is usually economical |
| Light scoring on rotors, tolerances borderline | Overhaul is considered with rotor machining or replacement |
| Winding burned, acid spread into the system | Motor rewinding or replacement, followed by system cleaning |
| Rotor fracture, housing damage | Replacement in most cases |
| Older model with no parts supply | Replacement or unit renewal |
The decision should not be made by looking at repair cost alone. Replacement time, parts lead time, how long the facility can go without cooling and the remaining life of the unit also come into the calculation. If the chiller itself is nearing the end of its life, discussing unit renewal may make more sense than an expensive compressor job.
Timing also directly affects cost. An overhaul planned when the signs appear can be done in winter or in a low-load period, with parts prepared in advance. When a compressor breaks while running, however, metal particles spread into the oil system and sometimes into the refrigerant circuit; the repair scope grows, the circuit needs to be cleaned, and the facility may be left without cooling in the hottest period.
How does the overhaul process work?
A planned overhaul generally proceeds in the following steps. Because part of the time depends on parts supply, the parts list is finalized before the overhaul.
- Preliminary inspection: evaluation of vibration, current, megger, oil analysis and operating records
- Shutdown plan: choosing the period when the facility is at its lowest load, with temporary cooling if needed
- Recovery of the refrigerant and removal of the compressor
- Post-disassembly measurement and report: confirming which parts will be replaced
- Parts replacement, assembly, leak test and vacuum
- Oil and refrigerant charging, commissioning, first-run measurements
- Follow-up measurement in the first weeks and repeat oil analysis
Because an overhaul involves refrigerant handling and electrical connections, it should be done only by a qualified, F-gas certified team. The facility staff's role is to keep records and take part in the shutdown plan.
Can we postpone the need for an overhaul?
The need for an overhaul cannot be eliminated entirely, but it can be noticeably postponed with correct operation. Keeping the condenser and evaporator clean, preventing short cycling, changing oil and filters on time and keeping condensing pressure at a reasonable level reduce the load on the compressor. Mistakes made in winter preparation also affect the compressor indirectly; we covered this in our chiller winter preparation article.
How do we work at YOİ?
We overhaul screw compressors of all brands, and before an overhaul we measure the condition with vibration, current, megger and oil analysis. In our quotes we write overhaul and replacement options side by side with cost and duration; the price is determined during the survey. For details, see our compressor overhaul service.
Frequently asked questions
After how many hours does a screw compressor need an overhaul?
The interval varies by manufacturer, model and operating conditions, and is generally in the tens of thousands of hours. For a firm decision, oil analysis, vibration and current measurements should be reviewed in addition to running hours.
How long does the chiller stop during an overhaul?
The duration depends on parts supply and the extent of damage. In a planned overhaul with parts prepared in advance, downtime is noticeably shorter; in overhauls after a breakdown, supply time becomes the deciding factor.
Does the compressor run like new after an overhaul?
When bearings, seals and tolerances are returned to manufacturer values, compressor efficiency and reliability are largely restored. If the rotors have permanent damage, the result depends on the work done and the parts replaced.
Are scroll compressors also overhauled?
Scroll compressors are generally hermetic and are not overhauled; they are replaced when they fail. Overhaul applies mainly to semi-hermetic screw and reciprocating compressors.
How often should oil analysis be done?
At least once a year is recommended. In critical facilities, or for compressors that showed borderline values in the previous analysis, it can be done every six months.