Centrifugal Chiller Compressor Maintenance

Grey centrifugal chiller compressor being maintained

Centrifugal Chiller Compressor Maintenance

Centrifugal chiller compressor maintenance keeps your critical plant up and running and your customers happy. The work can be scheduled to be carried out during factory shutdown, so as not to disrupt your production. We can also carry out this work to your compressors while the factory is in production. This is achieved by isolating the compressor that needs to be worked on when it is in an off cycle. The compressors on remaining machines can carry on running.

Tasks during Centrifugal Chiller Compressor Maintenance

Here are some of the tasks that we carry out…

Oil Changes

The oil becomes dirty over time by picking up contaminants that have formed in the system. Two of these contaminants are…

Acid

Compressor discharge is the hottest part of the refrigerant cycle. Acid can be formed from the refrigerant in this part of the system.

Refrigerant Types

A popular refrigerant for centrifs is R134a. Because it consists only of one kind of refrigerant, it does not fractionalise into different component refrigerants. This would be no good for a flooded system because one or more of the refrigerants would end up in the bottom of the evaporator and condenser. The remaining refrigerant would circulate and the whole plant would not function as it should. Refrigerants popular for other kinds of system are zeotropic HFC refrigerants. This means that the different component refrigerants have different boiling points- R407c is a good example.

Metal

White metal from the compressor and copper from the heat exchangers end up in the oil. They will eventually be caught by the system oil filters…

Oil Filter Change

After completing the above, now is a good time to change the filter as the compressor is valved off and has been broken into. It is also when an oil sample is taken depending on the schedule…

Oil Samples during Centrifugal Chiller Compressor Maintenance

Our oil samples are transported using our specialist kits which include the bottles and labels. This saves mix ups in our laboratory during the analysis.

Leak Rectification

The drawback of carrying out the above are leaks because the pressure has been pumped out of the compressor into another part of the system. The ‘o’ rings and shaft seal can now leak, this kind of failure can be rectified whilst still on site by the knowledge of our engineers.

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Oil Pre Lubrication

When the compressor starts, the oil pressure is built up first using an oil pump. This is so the internal components such as the high and low speed shaft are properly oiled before they start to rotate. They can run at 10,000 RPM and are very expensive to replace. Therefore, we check the oil pressure gauges and the system controls to ensure optimum ‘pre lubing’ of your compressors.

Volumetric Efficiency

The ratio between the volume actually compressed and the theoretical volume derived from compressor design calculations. This kind of compressor has a lower volumetric efficiency than positive displacement compressors. It is because the refrigerant is compressed off the tip of the rotating impellor or impellors. The refrigerant moves outwards in a circular path due to centrifugal force. A centrif more than makes up for this lower volumetric efficiency by the high mass and volume of the refrigerant that it circulates around the system.

Design 

The refrigerant is sucked from the evaporator into the centre of the impellor which is a disc of radial blades positioned to direct the refrigerant outwards. Due to the low differential of pressure, multiple stages of impellors are often arranged in series with the discharge being directed onto the suction of the next impellor. This is simple in design with less moving parts than some other compressor types. Modern centrifs can have magnetic, levitating bearings and so remove the need for oil in the system altogether. The faster the speed, the better the efficiency- so high speed rotation of the impellor is achieved at full load. The compressed refrigerant is discharged into the condenser.

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Electric Drive Centrifugal Chiller Compressor Maintenance

Most modern centrifs are driven by variable speed drives. This is for efficiency as the load can be exactly matched. Another reason for this is to soft start the compressor. 415v and 3.3kv are popular volt inputs, although other voltages can be made available by the onsite transformer.

Open

An open drive electric motor is the most popular design. It has the drawback, however, of needing a shaft seal which needs to be replaced at periodic intervals. This shaft seal is also prone to leaking refrigerant and oil.

Semi Hermetic

The photo is of a semi hermetic design of this compressor type. The motor is contained inside the suction housing and so has the benefit of being cooled by the refrigerant. No shaft seal is needed and therefore it has none of the associated maintenance drawbacks.

Steam Drive Centrifugal Chiller Compressor Maintenance

On oil rigs there can be an abundant supply of steam that can be used to drive the compressor. It is often used on multi stage compressors which are used for the liquefaction of natural gas. The steam goes through a turbine which is connected to a shaft- this drives the impellors. The steam flow and pressure can be tested and adjusted during the visit.

Woops Something Went Wrong

When you suffer a centrifugal compressor failure- don’t worry. You are in safe hands with the team here at Maximus Chillers. We have a team who can get the compressor out and lift it to the workshop on site, or transport it to our Head Office. We have another team who are experts in the remanufacturing of this kind of compressor. A fast supply chain is in place for delivery of the internal moving parts, gaskets and bearings. We are so confident that you will be happy with us- we offer a 12 month warranty on all of our compressor rebuilds.

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6 green Bitzer scroll compressors being maintained in a chiller

Scroll Chiller Compressor Maintenance

Here at Maximus Chillers, we carry out scroll chiller compressor maintenance to extend the lifespan and reduce downtime to your critical plant. This planning ahead is central to how we do things- we resolve small problems before they become big problems. Having the capability to do anything is part of what we call the MAXIMUS ADVANTAGE™ Any Chiller- Any Problem- Any Part- Any Refrigerant- Anywhere.

How they Work

The refrigerant vapour passes in between two scrolls (spirals) One of these is fixed, the other orbits backwards and forwards against the fixed scroll using a swing link. This creates a series of crescent shaped gas pockets in between the scrolls. These gas pockets get smaller in size as the refrigerant travels from the suction at the edge, down the spirals to the discharge at the centre. It then leaves downwards through a port. There are check valves to prevent back feeding of refrigerant during off cycles when other compressors on the same system are still running.

Advantages

There are several gas pockets occurring at any one time through the scroll, therefore giving a smooth and continuous compression cycle. Other advantages are low internal friction resulting in a quiet operation and low vibration levels. These low vibration levels help to lower the occurrence of leaks around the compressor.

Oil Level during Scroll Chiller Compressor Maintenance

The correct oil level is noted on our detailed Tick Sheets for all of the compressors. This record helps with the diagnosis when things start to go wrong. We can look for a pattern where the level starts to drop off in the compressors on a particular system, then look at the other readings which may be the cause.

Oil Samples during Scroll Chiller Compressor Maintenance

At periodic intervals, we take oil samples which we analyse in our laboratory at Head Office in Droylsden. We look for white metal, coloured metal, refrigerant composition, acid and sludge. This means that we can notice, then resolve system issues before a costly compressor failure occurs.

Oil Changes during Scroll Chiller Compressor Maintenance

This kind of compressor is fitted with a service port for the draining and filling of oil. The correct procedure is followed due to system pressure being present inside. We then carry out the oil changes using our specialist oil handling tools. Our skilled engineers carry the correct PPE for the work being carried out. We also we provide Risk Assessment Method Statements for each job.

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Crank Case Heaters

In the photo you can see the silver band of the crank case heaters at the bottom of the compressors. They keep the oil at the required temperature so as to not cause compressor wear on start up. The heaters also have the effect of ensuring that the refrigerant cannot condense into liquid during cold weather. As a compressor cannot compress a liquid, compressor failure would occur. A normally closed contact on the delta compressor starter contactor usually runs the heater: it drops out when the compressor starts. The function of all of these heaters is checked on each maintenance visit.

Discharge Temperature

It is critical that the discharge temperature is not too high, as this is the cause of the refrigerant breaking down into acid and sludge as previously mentioned. The acid rots the insulation on the copper windings inside the compressor. When this has occurred, an electrical failure will result in the compressor, causing the fuses to blow and a fault condition on the chiller. Compressor swap out is the cure which is expensive and inconvenient. We take the discharge temperature readings during the maintenance so as to fault find the cause.

Suction Pressure

Adequate suction pressure is needed to ensure good oil return to the compressor and prevent low pressure trips. As the refrigerant entrains the oil around the system, a good mass flow rate is needed, or the oil will just ‘pool’ in the bottom of the evaporator. Poor oil return will result in a seized compressor. Therefore, particular attention is taken to the low pressure gauge by our engineers. System adjustments or recommendations are made to ensure the seamless operation of your plant. If system overhaul is required, a Quote will be submitted at the end of the visit.

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Compressor Holding Down Bolts

There are usually four of these on each compressor. They fit though a steel sleeve which, in turn, fits through a rubber mount. The whole assembly is bolted into the frame of the chiller. The function is to securely hold the compressor in place and to dissipate vibration. We check the tightness is correct with our torque wrenches and change the rubbers periodically. A compressor rattling around uncontrollably, especially during start up and stopping causes catastrophic leaks around the compressor. This usually leads to the whole, or the most part of the refrigerant charge being lost to atmosphere.

Motor Protection Module

A resistance sensor embedded in the compressor windings plugs into this module on the S1 and S2 terminals. The fault feedback to the controls is on the M1 and M2 terminals. The power supply to the module is on the L and N terminals. When the windings start to get hot, the module detects this after a pre determined level of resistance is reached. The M2 terminal opens and volts drop out to the relay board, then a lower volt control signal drops out to the controller. We check the sensor resistance and compare it to a chart to ensure it is within the allowable range on each visit.

Wire Tightening during Scroll Chiller Compressor Maintenance

We waggle the motor connectors and check for any cracking to the wires which may lead to an earth sort out. We tighten the wiring on the compressor starter contactors and check the condition of the contactor contacts. These become ‘splattered’ causing an uneven amount of amps to the compressor. This volt drop causes the contactors to fail and will eventually cause compressor motor failure. We change the contactor contacts from our range of parts at Head Office.

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Air cooled chiller planned maintenance showing both economizers with solenoid valves and expansion valves

Air Cooled Chiller Planned Maintenance

An air cooled chiller planned maintenance visit for a long standing customer of ours. They have just signed up to another 3 year contract. We know at Maximus Chillers that if we want to keep our customers, we must continue to offer the Maximus Service. Maximus is Latin for the highest and the greatest.

Critical Air Cooled Chiller Planned Maintenance

Planned maintenance is essential for these chillers as they are critical plant. They feed a large conference room which is a major part of the end user’s business. They cannot afford to loose the conference room when there is an event on. There is redundancy in the plant, but the customer always makes sure that the issues highlighted on the maintenance are acted on promptly. Also, the chillers feed a water cooled condenser for a pack which cools £50,000 of products used during the conferences.

Air Cooled Chiller Oil Planned Maintenance

There is 1mw of cooling across the 2 chillers. Our engineer noticed that the oil level was slightly below the sight glass on one of the 250kw Bitzer compressors. The machine had previously been extensively run tested to ensure no oil return issues were present. Therefore, he decided to adjust the level with the oil available onsite.

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Economizer

The economizers, in the picture, were tested to ensure the solenoids were opening when the program sent the run signal. The electronic expansion valves functioned correctly and subcooled the liquid further after the condenser.

Head Pressure Control

The head pressure control was functioning correctly, keeping a steady 8 bar of pressure across all stages of loading and water system temperatures. This is because these chillers use a fan speed controller for each system. The fans cut in at the minimum running speed, which prevents stalling and over heating. Also, all fans running together are more reliable. Fans fail on systems using relays or pressure switches as the higher pressure fans don’t often come on. When they do, moisture has ingressed into them causing them to fail.

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